CHANNEL DEFORMATION, TURBULENCE STRUCTURE AROUND SPUR DIKE,
AND REDUCTION OF LOCAL SCOUR AT BRIDGE ABUTMENTS USING SPUR
DIKES UNDER ICE-COVERED CONDITIONS - AN EXPERIMENTAL STUDY

by

Rahim Jafari
B.Sc., Islamic Azad University of Mashhad, Iran 2010
M.Sc., Kerman Graduate University of Advance Technology, Iran, 2013

DISSERTATION SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR
THE DEGREE OF DOCTOR OF PHILOSOPHY IN NATURAL RESOURCES AND
ENVIRONMENTAL STUDIES

UNIVERSITY OF NORTHERN BRITISH COLUMBIA
July 2024

© Rahim Jafari

Abstract
Local scour around bridge abutments and piers presents a significant challenge in hydraulic
engineering, threatening the structural integrity of bridges. Scour refers to removing sediment
around bridge foundations due to high-velocity flow and turbulence from water passing around
these structures. This process can undermine the stability of bridges by exposing and weakening
their foundations, potentially leading to failures and catastrophic collapses. Several factors
contribute to scour, including water flow characteristics, surface conditions, hydraulic structure
features, and riverbed geomorphology.
Effective mitigation of scour is essential to ensure bridge safety and longevity. Traditional
methods, such as riprap, concrete aprons, and various hydraulic structures, aim to alter flow
patterns and reduce erosive forces. However, these methods can be constrained by environmental
conditions and site-specific characteristics. This research explores using spur dikes, hydraulic
structures extending from the riverbank to redirect the flow, to mitigate local scour at bridge
abutments, especially under ice cover conditions. The study utilizes a large-scale outdoor hydraulic
flume at the Quesnel River Research Center in British Columbia, Canada. The flume measures
38.5 meters in length, 2 meters in width, and 1.3 meters in depth, with a longitudinal bed slope of
0.2% to replicate natural flow conditions with non-uniform flow characterized by longitudinal
variations in water depth. Two sandboxes are filled with natural sediments of different median
grain sizes (0.48 mm, 0.60 mm, and 0.90 mm) to replicate riverbed conditions.
Spur dikes made from marine plywood were positioned upstream of the abutment (25 cm and
50 cm) and at different alignment angles (45º, 60º, 90º) in the flume. Abutments constructed from
galvanized plates were installed in the sandboxes. Styrofoam panels simulated smooth and rough
ice cover conditions, with smooth panels representing natural sheet ice and rough panels
ii

mimicking ice jams through attached Styrofoam cubes. Flow rate and water depth were measured
using a SonTek-IQ Plus, an advanced instrument with six sensors for comprehensive flow field
coverage and high-accuracy data collection. Acoustic Doppler Velocimetry (ADV) captured
detailed 3D velocity components and turbulence intensities, measuring the velocity of scattering
particles in the flow to provide insights into complex flow dynamics around the spur dikes and
abutments.
This experimental study aims to enhance understanding of scour dynamics by investigating
the interactions between different spur dike configurations, flow conditions, and ice cover types.
It provides detailed insights into how these factors influence local scour and sediment transport
processes. Additionally, the study seeks a comprehensive understanding of the flow field and 3D
velocity distribution around spur dikes under various conditions, analyzing the effects of different
alignment angles and ice cover on flow patterns and turbulence structure, which are critical for
predicting and mitigating scour. Another goal is to develop effective scour mitigation strategies,
identifying optimal configurations that provide maximum protection under various hydraulic and
environmental conditions. Overall, the combined studies aim to advance the field of hydraulic
engineering by offering practical solutions for mitigating scour-related risks, thereby ensuring the
stability and safety of bridge abutments in diverse hydraulic environments.

iii

Table of Contents
Abstract……………………………………………………………….…………………………...ii
ACKNOWLEDGEMENT…………………………………………………...………………..….xi
1

CHAPTER ONE: Introduction and Literature Review .......................................................... 1
1.1

Scour ............................................................................................................................... 5

1.1.1

The Role of Vortex Dynamics in the Scour Process............................................... 5

1.1.2

Local Scour Evolution and Mechanism .................................................................. 7

1.1.3

Previous Research on Scour.................................................................................... 8

1.1.4

Scour Mitigation Measures ................................................................................... 10

1.2

Spur Dike ...................................................................................................................... 16

1.2.1

Design and Orientation ......................................................................................... 16

1.2.2

Spur Dike Types ................................................................................................... 18

1.2.3

Previous Researches Overview ............................................................................. 21

1.3

Riverbed Material and Sediment Transportation .......................................................... 24

1.3.1

Riverbed Material ................................................................................................. 24

1.3.2

Sediment Transportation ....................................................................................... 25

1.3.3

Literature Review.................................................................................................. 26

1.4

Ice Cover ....................................................................................................................... 30

1.4.1

Ice Cover and Its Impact on Flow Dynamics........................................................ 30

1.4.2

Roughness Coefficient of Ice Cover ..................................................................... 31

1.4.3

Scour Phenomena Under Ice Cover ...................................................................... 32

1.4.4

Previous Researches Overview ............................................................................. 33

1.5

Research Objectives ...................................................................................................... 35

1.5.1
Dike

Objective One: Analysis of Flow Fields and 3D Velocity Distribution Around Spur
36

1.5.2
Objective Two: Investigation of the Impact of Ice Cover and Roughness Coefficient
on the Scour Process ............................................................................................................. 36
1.5.3
Objective Three: Investigation of Local Scour at Spur Dikes with different
alignment angles and flow surface condition........................................................................ 37
1.5.4

Objective Four: Analysis of Effects on Sediment Removal and Deposition ........ 37

1.5.5
Objective Five: Development of a Predictive Formula for Maximum Scour Depth
at Spur Dike .......................................................................................................................... 38
1.5.6
1.6

Objective Six: Optimization of Spur Dike Configuration to Protect Abutment ... 38

Research Innovation...................................................................................................... 39

1.6.1

Empirical Formula for Scour Depth ..................................................................... 39
iv

1.6.2

Comprehensive Parameter Analysis ..................................................................... 39

1.6.3

Application of Non-Submerged Spur Dikes ......................................................... 40

1.6.4

Optimal Spur Dike Design .................................................................................... 40

1.6.5

Impact of Ice Cover on Scour Processes............................................................... 40

1.6.6

Innovative Experimental Setup ............................................................................. 41

1.7
2

Thesis Structure ............................................................................................................ 41

CHAPTER TWO: Materials and Experimental Procedures ................................................. 43
2.1

Site Description ............................................................................................................. 43

2.2

Experimental Setup ...................................................................................................... 46

2.3

Measurement Equipment .............................................................................................. 50

2.4

Data Collection Procedures.......................................................................................... 52

2.5

Bibliography ................................................................................................................. 54

3 CHAPTER THREE: Velocity Field and Turbulence Structure around Spur Dikes with
Different Angles of Orientation under Ice-Covered Flow Conditions ......................................... 65
3.1

Introduction ................................................................................................................... 65

3.2

Materials and Methods .................................................................................................. 69

3.2.1

Site Description ..................................................................................................... 69

3.2.2

Apparatuses for Measurements ............................................................................. 72

3.2.3

Data Collection ..................................................................................................... 74

3.3

Results and Discussions ................................................................................................ 77

3.3.1

Streamwise Velocity Component (Ux) ................................................................. 79

3.3.2

Lateral Velocity Component (Uy) ........................................................................ 83

3.3.3

Vertical Velocity Component (Uz) ....................................................................... 85

3.3.4

Turbulence Intensities and Reynolds Shear Stress ............................................... 88

3.4

Conclusions ................................................................................................................... 92

3.5

Bibliography ................................................................................................................. 95

4 CHAPTER FOUR: Channel deformation around non-submerged spur dikes with different
alignment angles under ice cover .................................................................................................. 98
4.1

Introduction ................................................................................................................... 98

4.2

Methodology and Materials ........................................................................................ 101

4.2.1

Experimental setup and equipment for measurements ....................................... 101

4.2.2

Data Acquisition Procedures............................................................................... 105

4.3

Results ......................................................................................................................... 106

4.3.1

Vortex system at the dike .................................................................................... 108

4.3.2

Scour morphology ............................................................................................... 109
v

4.3.3

Maximum scour depth ........................................................................................ 119

4.4

Conclusions ................................................................................................................. 126

4.5

Bibliography ............................................................................................................... 127

5 CHAPTER FIVE: Evaluating the Impact of Spur Dike Alignment and Ice Cover Roughness
on Local Scour at Bridge Abutments .......................................................................................... 130
5.1

Introduction ................................................................................................................. 130

5.2

Experimental Procedures ............................................................................................ 134

5.2.1

Research Site and Flume Setup........................................................................... 134

5.2.2

Data Collection and Analysis Procedures ........................................................... 138

5.3

Results and Discussions .............................................................................................. 139

5.3.1

Scour at Abutment .............................................................................................. 140

5.3.2

Application of Spure Dike .................................................................................. 143

5.3.3

Comparative Assessment of Scour Depth Variability Around Bridge Abutments
…………………………………………………………………………………..158

5.3.4
Comparative Analysis of Maximum Scour Depth Around Abutments with
Different Alignment Angles of Protective Spur Dikes ....................................................... 160
5.3.5
Quantitative Analysis of Maximum Scour Depth Around Abutments with
Protective Spur Dikes under Ice-Covered Conditions ........................................................ 162

6

5.4

Conclusions ................................................................................................................. 168

5.5

Bibliography ............................................................................................................... 172

CHAPTER SIX: General Conclusions ............................................................................... 175
6.1

Conclusions of Chapter 3 ............................................................................................ 175

6.1.1

Impact of Ice Cover on Flow Dynamics ............................................................. 175

6.1.2

Orientation Angle of Spur Dikes ........................................................................ 176

6.1.3

Analysis of Velocity Components ...................................................................... 177

6.1.4

Turbulence Intensities and Reynolds Shear Stress ............................................. 179

6.2

Conclusions of Chapter 4 ............................................................................................ 181

6.2.1

Scour Hole Geometry and Influence of Ice Cover .............................................. 181

6.2.2

Impact of Dike Alignment Angle ....................................................................... 181

6.2.3

Effect of Flow Conditions ................................................................................... 182

6.2.4

Development and Dynamics of Vortex Systems ................................................ 182

6.2.5

Equations for Maximum Scour Depth ................................................................ 183

6.3

Conclusions of Chapter 5 ............................................................................................ 184

6.3.1

Scour at Abutments ............................................................................................. 184

6.3.2

Application of Spur Dikes .................................................................................. 185
vi

6.3.3

Sediment Deposition Patterns ............................................................................. 186

6.3.4

Influence of Flow Depth and Velocity................................................................ 187

6.4

Limitations of this experimental study ....................................................................... 188

6.5

Future Directions ........................................................................................................ 188

APPENDIX……………………………………………………………………………….183

vii

List of Figures
Figure 1. 1:CN Rail Bridge over Fraser River, Prince George, BC, Canada. ................................. 3
Figure 1. 2:Development of a vortex system around a bridge pier (Ranjbar-Zahedani et al., 2019)
......................................................................................................................................................... 6
Figure 1. 3:A vortex system around the pier with a bed sill as a scour mitigation measure (Grimaldi
et al., 2009) ................................................................................................................................... 14
Figure 1. 4:Types of spur dikes in terms of the orientation angle (Zhang & Nakagawa, 2008) .. 19
Figure 1. 5:Different shapes of spur dikes (Zhang & Nakagawa, 2008) ...................................... 20
Figure 1. 6:The Shield's Diagram (Schwimmer, 2006 ) ............................................................... 27
Figure 1. 7:Comparison of time development for clearwater and live-bed scour (Kiraga & Popek,
2016) ............................................................................................................................................. 29
Figure 2. 1:Grain size distribution of the bed materials................................................................ 45
Figure 2. 2: Overview of Flume .................................................................................................... 46
Figure 2. 3:Experimental Flume, Dike layout: (a) Plan view; (b) Vertical view ......................... 48
Figure 2. 4:Experimental flume, Protective Dike layout: (a) Plan view, (b) Vertical view ......... 48
Figure 2. 5:Different setups of the experiment ............................................................................. 49
Figure 2. 6:An experimental run under smooth cover condition .................................................. 50
Figure 3. 1:The layout of the experimental flume: (a) Plan view; (b) vertical view .................... 69
Figure 3. 2:The view of the original site (experimental flume) .................................................... 71
Figure 3. 3:3D Velocity data collection using the 10-MHz SonTek ADV ................................... 76
Figure 3. 4:Plan view of the scour hole ........................................................................................ 77
Figure 3. 5:Development of the vortex system around the spur dike (Ghaderi & Abbasi, 2019) 77
Figure 3. 6:Deposition ridge on the downstream side of the spur dike ........................................ 78
Figure 3. 7: ux under different surface cover conditions, spur dike with an orientation angle of 90°
....................................................................................................................................................... 80
Figure 3. 8: ux inside and outside the scour holes under different surface conditions with a 45° dike
....................................................................................................................................................... 81
Figure 3. 9: ux inside and outside the scour holes for different dike orientation angles (water depth:
31 cm) ........................................................................................................................................... 82
Figure 3. 10: uy inside and outside the scour holes under different surface conditions with a 90°
dike ................................................................................................................................................ 83
Figure 3. 11: uy inside and outside the scour holes under different surface conditions, water depth:
35 cm ............................................................................................................................................. 84
Figure 3. 12: uz inside and outside the scour holes under different flow conditions with a dike of
90° ................................................................................................................................................. 85
Figure 3. 13:Scour hole profiles around the dike with different angles (water depth: 11 cm, open
channel flow) ................................................................................................................................ 86
Figure 3. 14: uz inside and outside the scour holes under different flow rates with a dike of 90° 87
Figure 3. 15: uz inside and outside the scour holes under different surface conditions; water depth:
21.5 cm .......................................................................................................................................... 88
Figure 3. 16:Turbulence intensity in three directions with the 90° dike (water depth: 35 cm) .... 90
Figure 3. 17:Reynolds stress, under different surface conditions for the dike with 90° (water depth:
31 cm) ........................................................................................................................................... 92
viii

Figure 4. 1:Flume for experiments: (a) Overview of the flume, (b) Plan view, (c) Vertical view
..................................................................................................................................................... 102
Figure 4. 2:A cover condition experiment under smooth ice cover with a 45º inclination angle104
Figure 4. 3:Scour holes under various water surface conditions (Fr=0.19, and 90º-dike alignment)
..................................................................................................................................................... 110
Figure 4. 4:Scour holes under different dike alignment angles and an open flow condition
(Fr=0.19) ..................................................................................................................................... 110
Figure 4. 5:Plan view of the scour hole ...................................................................................... 111
Figure 4. 6:Effect of experimental conditions on the upstream length of a scour hole (a/L) ..... 113
Figure 4. 7:Effect of experimental conditions on the downstream length of a scour hole (b/L) 114
Figure 4. 8:Effect of experimental conditions on the maximum outward scour width (w/L) .... 116
Figure 4. 9:Effect of experimental conditions on the maximum frontal inward scour width (c/L)
..................................................................................................................................................... 118
Figure 4. 10:Effect of experimental conditions on the maximum rear inward scour width (d/L)
..................................................................................................................................................... 118
Figure 4. 11:Cross-section profile of scour holes around a dike with the alignment angle of 90º for
different Froude numbers under an open flow condition............................................................ 120
Figure 4. 12:Calculated maximum depth of scour holes using proposed equations compared to
results of laboratory experiments: (a) ice-covered flow conditions, (b) open channel flow
conditions. ................................................................................................................................... 125
Figure 5. 1:Experimental flume: (a) Overview of the flume, (b) Plan view, (c) Vertical view .. 136
Figure 5. 2:A cover condition experiment under smooth ice cover with a 60º alignment angle 137
Figure 5. 3:Scour patterns around the abutment under different cover conditions (flow depth=22
cm) .............................................................................................................................................. 142
Figure 5. 4:Scour patterns around the abutment under different cover conditions and protective
dike with 90 alignment angle (Flow depth =22 cm) ................................................................... 145
Figure 5. 5: Scour patterns around the abutment under different cover conditions and protective
dike with 60 alignment angle (low velocity, flow depth=22 cm) ............................................... 148
Figure 5. 6:Scour patterns around the abutment under different cover conditions and protective
dike with 60º alignment angle (high velocity, flow depth=22cm) .............................................. 150
Figure 5. 7:Scour patterns around the abutment under different cover conditions and protective
dike with 45º alignment angle (low velocity, flow depth=22cm) ............................................... 153
Figure 5. 8:Scour patterns around the abutment under different cover conditions and protective
dike with 45º alignment angle (high velocity, flow depth=22cm) .............................................. 155
Figure 5. 9:Scour patterns around abutment scour under different cover conditions and protective
dike with 90ºalignment angle, 25cm distance (flow depth=22cm) ............................................. 157
Figure 5. 10:Comparison of calculated maximum scour depth around the abutments with a
protective spur dike to those of experiments under ice-covered flow conditions....................... 167

ix

List of Tables

Table 5. 1: Comparison of maximum scour depth around abutment with and without protective
dike, flow depth=11cm ............................................................................................................... 159
Table 5. 2: Comparison of maximum scour depth around abutment with and without protective
dike, flow velocity=23cm/s......................................................................................................... 159
Table 5. 3:Comparison of maximum scour depth around abutment with different alignment angles
of protective spur dike, flow depth=22cm .................................................................................. 162
Table 5. 4:Comparison of maximum scour depth around abutment with different alignment angles
of protective spur dike, flow velocity=28 cm/s .......................................................................... 162
Table I. 1:Experimental data and the results of Chapter 4 .......................................................... 190
Table I. 2:Experimental data and the results of Chapter 5 .......................................................... 193

x

ACKNOWLEDGEMENT
I would like to express my deepest gratitude to my supervisor, Dr. Jueyi Sui, for his
invaluable guidance, support, and encouragement throughout my PhD journey. His expertise,
patience, and unwavering trust in my capabilities have played a fundamental role in shaping the
outcome of this thesis. His mentorship and constructive feedback not only enhanced the quality of
my research but also enriched my academic growth. I consider myself incredibly fortunate to have
had him as my supervisor.
I am also immensely grateful to all my committee members, Dr. Faran Ali, Dr. Liang Chen,
and Dr. Wenbo Zheng, for their indispensable input, which significantly influenced the direction
of my research. Their astute observations played a crucial role in shaping this thesis.
I extend my heartfelt appreciation to Dr. Ellen Petticrew and Dr. Phil Owens, whose
support made this research possible. Furthermore, the provision of resources and facilities at the
Quesnel River Research Center (QRRC) has profoundly enriched the calibre and depth of my
research endeavours. Additionally, I am thankful for the kindness and support of John Orlowsky,
Doug Thompson, Vincent Aubichon and Todd French, whose passing was a great loss. A special
thanks to two of my dear friends, Parima Sattari and Mahboubeh Barahimi, who provided
continuous motivation and encouragement throughout my research.
Finally, I would like to express my deepest gratitude to my family for their love, patience,
and encouragement. To my beloved wife, Saede, for her unconditional love, constant support, and
understanding, and for being my rock throughout this challenging yet rewarding journey. To my
parents for their steadfast belief in me and for instilling in me the values of perseverance and hard
work.
This dissertation is dedicated to my family, whose sacrifices and support have been the
foundation of all my achievements.

xi

1 CHAPTER ONE: Introduction and Literature Review
Bridges are essential structures facilitating road connectivity, significantly easing
transportation and reducing traffic congestion. They play a crucial role in both economic activities
and the daily lives of commuters. Given their extensive use, ensuring the safety of bridges is
paramount. A bridge collapse represents one of the most hazardous forms of structural failure.
Several factors can contribute to such a collapse, including natural disasters such as earthquakes
and floods, fires, construction and design defects, manufacturing flaws, inadequate maintenance,
and hydraulic issues like local scour around bridge abutments or piers (Chang, 1973; Kattell &
Eriksson, 1998; Richardson & Davis, 1995).
According to Kattell and Eriksson (1998), the most common cause of bridge failures on
the highways in the USA is local scour. Scour is an engineering term referring to a specific form
of erosion around bridge piers and abutments in riverbeds. It involves the removal of soil around
the abutments and piers of a bridge or the granular bed material in natural watercourses. Scour is
a dynamic and complex process that can be categorized into three types: general scour, contraction
scour, and local scour (Brandimarte et al., 2012).
General scour refers to the removal of sediment from the main channel bed by river flow,
resulting in the lowering of the river bed and the gradual removal of large amounts of sediment
(Warren, 2011). This process can occur as either degradation, the general reduction of river bed
material by river flow and can accelerate during floods, or aggradation, defined as the deposition
of sediment eroded from the river bed or watershed upstream of the channel (Richardson et al.,
1993). Contraction scour involves the removal of sediment from the bottom and sides of the river
due to increased velocity caused by a reduction in the cross-sectional area of streamflow.
According to the continuity equation, a decrease in flow areas results in increased average velocity

1

and bed shear stress in the contraction section. These contractions can be caused by natural
phenomena such as earthquakes and landslides or by human-made structures like bridges (Umbrell
et al., 1998). Local scour is caused by an increased flow velocity in the vicinity of hydraulic
structures, such as bridge piers, abutments, and other obstacles that block the flow (Chang, 1992).
In the USA, 503 bridge failures between 1989 and 2000 were reviewed by Wardhana and
Hadipriono (2003). This study concluded that these failures were mainly related to scour and flood.
Between 1969 and 1991, there were more than 1000 bridge failures, 60% of which were due to
local scour (Briaud et al., 1999). Further, in 1994, more than 500 bridges in Georgia were damaged
by scouring (Richardson & Davis, 2001). These bridge failures incur substantial financial costs.
For instance, the US Federal Highway Administration reported that major local floods between
1964 and 1972 caused approximately $100 million in damages per event to bridges over rivers
(Brice et al., 1978). New Zealand’s road administration also reported that scour caused by rivers
results in NZ$36 million per year (Macky, 1990). In another study, the US Transportation Research
Board in 1997 declared that there were over 488 thousand bridges over streams and rivers in the
USA, and scour-related bridge failures cost roughly $30 million annually (Lagasse et al., 1997).
Another critical concern associated with bridge failures is the potential loss of human life.
While the financial costs are substantial, the loss of human life is even more significant. For
example, the Schoharie Creek Bridge collapse in New York on April 5th, 1987, resulted from
scouring and tragically led to the loss of ten lives (LeBeau et al., 2007). In 1995, the failure of a
bridge over the Arroyo Pasajero River in California resulted in the loss of seven lives. A 3-meterdeep scour hole caused this tragic incident (Arneson, 2013).
Riverbeds undergo erosion to a specific depth, known as scour depth, and numerous studies
have been conducted to understand scour patterns and their maximum depth (Chang, 1992).
2

However, most of these studies have used uniform sediment, which does not accurately represent
natural river conditions. These studies primarily focused on open channel conditions, often
neglecting rivers with unique characteristics, such as those in cold northern regions. In such
regions, river surfaces can remain frozen for extended periods, impacting scour processes. Canada
is an example of a region where river surfaces freeze for about six months each year. The Fraser
River, the longest river in British Columbia, exemplifies this phenomenon. With an annual
discharge of 3,550 cubic meters per second, the Fraser River’s entire surface in the northern
regions freezes during winter (Figure 1.1).
Ice cover can introduce additional hydraulic boundaries on river flow, leading to significant
changes in velocity, flow rate, sediment transport, and scour patterns. Under ice-covered
conditions, the maximum flow velocity shifts closer to the riverbed, increasing bed shear stress
around bridge abutments and piers (Namaee & Sui, 2019a, 2019b; Sui et al., 2010; Wang et al.,
2008; Wu, Hirshfield, Sui, et al., 2014; Peng Wu et al., 2015).

Figure 1. 1:CN Rail Bridge over Fraser River, Prince George, BC, Canada.

3

Bridge failure remains a critical area of research, with local scour around bridge abutments
or piers identified as one of the primary causes of collapses. According to a study by the US Federal
Highway Administration in 1973, 75% of 383 bridge failures involved abutment damage, while
25% involved pier damage (Chang, 1973). Additionally, a study found six out of ten bridge failures
in New Zealand during Cyclone Bola were due to abutment scour (Kandasamy & Melville, 1998).
Accurately predicting scour depth is essential for the safe design of bridge foundations. Since
abutments are more vulnerable to scour than piers, further research on scour patterns around
abutments is necessary. Developing strategies to reduce or prevent local scour at abutments is
crucial for enhancing bridge safety and longevity.
One effective method for reducing scour is using spur dikes to protect abutments and piers.
Spur dikes are structures that extend from the riverbank and project a certain distance into the river
flow (Kuhnle et al., 1999). These dikes serve various purposes, including riverbank protection and
flood control, and have been suggested as a countermeasure for reducing local scour (Nazari et al.,
2017). Despite the widespread application of spur dikes, there is still a lack of published data
addressing their effectiveness as a scour reduction method. To date, no research has been reported
on using spur dikes to protect bridge abutments from local scour under ice-covered conditions.
Therefore, it is crucial to study the influence of spur dikes on local scour around bridge abutments
in ice-covered rivers to fill this knowledge gap and enhance bridge safety in such environments.
This experimental research investigates the application of spur dikes to reduce scour under
various flow surface conditions, including open flow, smooth ice cover, and rough ice cover. The
study will also consider different flume bed materials and flow properties. Additionally, the
research will examine the scour morphology and flow field around the spur dike inside and outside
the scour hole.
4

1.1 Scour
1.1.1 The Role of Vortex Dynamics in the Scour Process
Based on previous research, local scour at bridge abutments and piers is one of the leading
causes of bridge failures (Wardhana & Hadipriono, 2003). Local scour is a complex hydrodynamic
process in which an object, such as piers or abutments, interferes with the river’s flow, accelerating
the water flow and generating a system of vortices around the obstacle. In fluid dynamics, a vortex
system refers to fluid particles’ organized motion around a central axis, creating circular or spiral
patterns. This system is crucial in understanding various fluid flow phenomena, especially around
objects immersed in a fluid stream (Dey & Raikar, 2007). This vortex system, coupled with high
turbulence in the water flow, is primarily responsible for removing sediments at the base of any
object obstructing the streamflow. These vortices are categorized into three main types: Primary
Vortex, Horseshoe Vortex, and Wake Vortex (Ishigaki et al., 2000).
The primary vortex is the fluid’s initial swirling motion created by an object’s disturbance
in the fluid flow. It forms directly downstream of the object and is characterized by its relatively
stable and large-scale circulation. This vortex plays a fundamental role in determining the overall
flow pattern around the object (Bearman, 1978). The horseshoe vortex is a specific type of vortex
system that forms around the base of an object, such as a bridge pier or a ship’s bow, when it
obstructs the fluid flow. This vortex wraps around the front and sides of the object, resembling the
shape of a horseshoe. It consists of two counter-rotating vortices originating from the object’s sides
and extending downstream. The horseshoe vortex is significant in scour formation and sediment
transport around structures (Melville & Sutherland, 1988). The wake vortex is formed in the wake
region downstream of an object as the fluid flow separates and creates a turbulent trail. This vortex
system is characterized by alternating vortices shed from either side of the object, forming a vortex
5

street. The wake vortices are crucial in understanding drag forces and turbulent mixing in the flow.
Suspended sediments are subsequently transported downstream by wake vortices (Figure 1.2)
(Baker et al., 1974).

Figure 1. 2:Development of a vortex system around a bridge pier (Ranjbar-Zahedani et al., 2019)

The primary vortex, horseshoe vortex, and wake vortex are interconnected and influence
each other in various ways. The primary vortex generated by the object’s presence influences the
formation of the horseshoe vortex around its base. The interaction between these vortices can
intensify the swirling motion and increase the turbulence in the flow (Melville & Sutherland,
1988). Energy is transferred between these vortices, where the primary vortex feeds energy into
the horseshoe vortex, enhancing its strength. This energy transfer contributes to the stability and
intensity of the vortex system (Baker et al., 1974). The horseshoe vortex affects the flow patterns
around the object’s base, influencing the wake vortex’s characteristics. The wake vortex inherits
the disturbances created by the horseshoe vortex, leading to complex flow structures downstream
(Cheng et al., 2011; Melville & Sutherland, 1988). The combined action of the primary, horseshoe,
and wake vortices is critical in sediment transport and scour around structures. The horseshoe
6

vortex, in particular, is responsible for initiating scour by lifting sediment from the base, while the
wake vortex can transport sediment further downstream (Das et al., 2013).
1.1.2 Local Scour Evolution and Mechanism
As water flows around the structure, it accelerates and increases in turbulence. The initial
stages of scour are characterized by the rapid sediment removal around the structure’s base. This
initial phase is often the most intense, with high sediment transport rates due to the strong
horseshoe vortices and downward flow (Melville & Coleman, 2000). As the scour progresses, the
depth of the scour hole increases. The process continues as long as the shear stress exerted by the
flowing water exceeds the critical shear stress required to move the sediment particles. The
dimensions of the scour hole, including its depth and width, depend on several factors, such as the
flow velocity, sediment size and type, and the size and shape of the obstruction. As the scour hole
develops, the bed shear stresses are reduced (Benedict, 2016). The shape of the scour hole typically
progresses from a narrow and shallow form to a broader, deeper, and more stable configuration.
This broadening occurs as the flow continuously removes sediment from the sides of the hole and
deposits it further downstream. The process creates a more uniform and gradual slope along the
edges of the scour hole, reducing the intensity of the local flow and the associated shear stress
(Melville & Sutherland, 1988).
Eventually, under steady flow conditions, the development of scour will reach a point
where the shear stresses are no longer sufficient to remove significant amounts of bed material.
This condition is defined as the equilibrium state. This state refers to the condition where the rate
of sediment removal by water flow, such as in a river or around a structure, is equal to the rate at
which sediment is deposited or remains stable (Richardson & Davis, 1995). This balance indicates
that the depth and shape of the scour hole do not change significantly over time, signifying a stable
7

system. At this point, the depth and extent of the scour hole stabilize, meaning that the erosion and
deposition processes are in a dynamic balance. Equilibrium scour depth represents the maximum
scour for the existing river flow. This equilibrium depth is influenced by the flow characteristics,
sediment properties, and structure dimensions (Breusers & Raudkivi, 2020).
Several factors influence the evolution of a scour hole. Higher flow velocities increase the
shear stress on the sediment, accelerating the erosion process and leading to deeper scour holes
(Chiew, 1984). The sediment particles’ size, shape, and cohesiveness affect how easily they are
eroded. Coarser and more cohesive sediments are more erosion-resistant, resulting in shallower
scour holes (Breusers & Raudkivi, 2020). Larger and more streamlined structures alter the flow
patterns differently, influencing the intensity and distribution of erosive forces around the structure
(Melville & Coleman, 2000). The duration of exposure to flowing water impacts the extent of
scour. More extended exposure periods typically result in deeper and more extensive scour holes
until equilibrium is reached. Understanding the evolution of scour holes is crucial for designing
and maintaining hydraulic structures. Engineers must account for the potential maximum depth of
scour when designing foundations to ensure structural stability (Richardson & Davis, 2001).
1.1.3 Previous Research on Scour
Studies on various types of scouring have garnered significant attention, with numerous
scholars contributing extensively to this field. Research on local scour at piers, abutments, piles,
pile caps, and pile groups has been particularly intriguing to researchers from diverse disciplines
(Froehlich, 1989; Garde et al., 1961; Laursen & Toch, 1956; Liu et al., 1961; Melville, 1992;
Raudkivi, 1986). Bridge scour is related to many aspects, such as the river bed material, channel
form, flow type, velocity profile, and the size and shape of bridge piers or abutments (Deng et al.,
2009). Coleman et al. (2003) suggested that the relationship between scour depth and various
8

factors of the river system can be represented within dimensionless frameworks. This approach
has facilitated the development of models demonstrating how local scour depth varies with
parameters such as flow profile, sediment grain size distribution (D50), and flow depth. Melville
and Coleman (2000) indicate that a typical way to predict the total erosion depth for any bridge is
to consider the combination of general, contraction, and local scour. A complete review of local
scour formulae is provided by Dey and Barbhuiya (2005). However, the development of these
formulas has been a complex process due to the discrepancies between natural river conditions
and laboratory settings and the limitations in data collection (Ataie Ashtiani et al., 2006; Lu et al.,
2008).
Several scholars studied the obstruction ratio, defined as the ratio between the abutment
length and the width of the river Melville and Coleman (2000). Breusers (1991) reported that this
factor does not significantly affect the local scour depth if it is calculated to be less than 40%.
Accordingly, the obstruction ratio is neglected in many types of research on scouring topics
(Coleman et al., 2003; Kothyari & Ranga Raju, 2001; Melville, 1997). In the case of channel
contraction, Melville and Coleman (2000) stated that if a contraction is significant due to the
blockage ratio of an obstacle, the resulting scour depth would be greater than that caused by the
same abutment in a wider channel. This phenomenon, known as contraction scour, occurs due to
the increase in average velocity over the cross-section rather than the vortices developing at the
obstacle.
Time is another crucial factor in the study of scour. Investigating the transient
characteristics of the scour process is necessary to determine the time scales for the initial scour
development and the duration required to reach a specific scour depth and equilibrium state. To
draw comprehensive conclusions on the role of time in the scour process, experimental research

9

should be conducted across a range of flow conditions, riverbed sediment types, and abutment
sizes and geometries. (Chiew, 1992). Ballio et al. (2004) utilized a time scale of scour progression
phases to demonstrate that this scale decreases as flow velocity increases. Once a scour hole is
formed, the scouring mechanism is governed by a vortex system known as the principal vortex.
This vortex is initiated by the pressure gradient that develops ahead of the structure (Kwan, 1988).
This vortex has been recognized as the main scouring factor at bridge piers and abutments (Kwan
& Melville, 1994). Despite extensive research to enhance the understanding of scour-related
issues, the scour of piers and abutments remains a complex and challenging subject in the field.
1.1.4 Scour Mitigation Measures
Bridges are among the most crucial components of transportation infrastructure, and their
failure can have profound social and economic impacts. Numerous bridges fail each year
worldwide due to local scour at their piers and abutments. (Johnson & Ayyub, 1996; Lagasse et
al., 1995). The application of rocks and stones for scour protection has a long history in hydraulic
engineering. Breusers et al. (1977) suggested using boulders with high critical entrainment velocity
around the piers to protect them from floods and local scours. Traditional engineering methods of
solving local scour problems, such as placing rocks or flexible mattresses around the pier
foundations, were ineffective (Chiew, 1992). Therefore, several methods have been made to
control scouring around bridge sites, for example, using an array of small piles in front of the pier
(Chabert & Engeldinger, 1956), applying a collar around the pier (Ettema, 1980; Tanaka & Yano,
1967; Thomas, 1967), a delta-wing-like fin in front of the pier (Gupta & Gangadharaiah, 1992), a
slot through the pier (Chiew, 1992), a limited number of pier-groups (Vittal et al., 1994),
submerged vanes (Odgaard & Wang, 1987), and various other efforts such as caissons and footings

10

below the pier shaft, using riprap, streamlined piers, mats and bags, gabions, and reno mattresses
(Kumar et al., 1999).
All these efforts and methods for preventing or reducing local scour can be classified into
three categories. The first method is geometrical, investigating length, width, nosing shape,
alignment of structure, etc. The second one is the physical features of an obstacle, such as
permeability, porosity, density, etc. The third and most effective method is to use protective means
and structures to prevent or minimize scour (Singh et al., 2022). This last technique can be divided
into direct methods (bed armouring countermeasures) and indirect methods (river flow altering).
1.1.4.1 Direct Method of Controlling Scour
The direct method of scour reduction involves controlling scour by enhancing the stream
bed's resistance to erosion. This can be achieved by installing protective layers of coarse materials
or solid structures around bridge piers and abutments to improve the bed material's ability to
withstand erosive forces (Kumar et al., 1999; Parola, 1993; Wörman, 1989; Yoon et al., 1995).
Various scholars have investigated these methods, using an armouring structure to reduce local
scour (Chiew & Lim, 2000; Dey & Raikar, 2007; Lauchlan & Melville, 2001). Experimental
research was conducted to reduce scour by using submerged vanes (Odgaard & Wang, 1987) and
fins in the form of delta wings in front of the pier (Gupta & Gangadharaiah, 1992). One of the
common methods to reduce local scour at bridge piers and abutments is to place riprap as an
armour layer around them (Chiew, 1995; Chiew & Lim, 2000; Parola, 1993; Richardson & Davis,
2001; Unger & Hager, 2006). Many researchers focused on the design of riprap size around
circular piers. They investigated how the volume and sizing of riprap for rectangular piers affected
their aspect ratios (Chiew & Lim, 2000; Lagasse, 2007; Lauchlan & Melville, 2001). However,

11

riprap can enhance piezometric gradients at the riverbed, increasing sediments' erosion potential
(Koloseus, 1984).
Another countermeasure for local scour is to install a collar around the pier. The collar
shields the sediment particles and protects the riverbed from direct impact (Zarrati et al., 2006).
Research on circular piers has demonstrated collars' effectiveness in reducing scour depth. The
efficiency of a collar is reported to depend on its size and vertical placement on abutments and
piers (Alabi, 2006; Kumar et al., 1999). Wider collars installed at the lower position of piers are
more effective for scour reduction (Zarrati et al., 2004). An experimental study on collars at
vertical face wing wall abutments shows that collars can protect the bridge abutments considerably
by eliminating secondary vortex systems (horseshoe vortex) that generally could cause local scour
(Li et al., 2006). However, another study on this topic revealed that scour first occurs downstream
of the collar, then slowly advances upstream and erodes the collar, accelerating local scour.
Therefore, collars mostly postpone scour rather than reduce it long-term (Zarrati et al., 2004). To
solve this problem, Zarrati et al. (2006) used a combination of collars and riprap, considerably
reducing the size of scour holes. However, it is difficult and relatively expensive to install collars
on piers with large spacing (Zarrati et al., 2006).
1.1.4.2 Indirect Method of Controlling Scour
In this method, the flow pattern around bridge sites is changed to reduce shear stresses and
decrease the power of the eroding vortex system around piers and abutments. These efforts can be
cost-effective compared to direct methods, mainly when the required amount of stones and other
materials are unavailable near the bridge site (Zarrati et al., 2004). Indirect reduction of scouring
is possible by modifying the flow pattern around piers. For example, using slots in piers reduces
the downflow strength and decreases the large pressure field around the piers, preventing the
12

formation of the horseshoe vortex. However, if a slot is jammed by debris, the effectiveness is
reduced, making hydraulic flow conditions around piers more challenging to control (Chiew, 1992;
Kumar et al., 1999; Vittal et al., 1994).
Another common indirect countermeasure for local scour is bed sills. A bed sill, also known as a
submerged sill, is a hydraulic structure placed on the riverbed downstream of a pier to reduce local
scour. Its primary function is to stabilize the bed material and dissipate the energy of the flowing
water, thereby minimizing the erosive forces that contribute to scour (Pagliara et al., 2010). A bed
sill alters the flow pattern downstream of the pier. Raising the bed level induces a backwater effect,
which reduces the flow velocity near the bed. The reduced velocity decreases the shear stress
exerted on the sediment, thereby limiting the sediment's potential to be eroded. This change in
flow dynamics helps to mitigate the depth and extent of the scour hole around the pier. The sill
creates a wake vortex zone at piers, which decreases velocity and reduces local scour (Figure 1.3).
The bed sill acts as an energy dissipator by breaking the flow's energy. The turbulence and kinetic
energy of the water are partially absorbed by the structure of the sill, which reduces the energy
available for sediment entrainment and transport. This energy reduction is crucial in preventing
the initiation and progression of local scour (Pagliara et al., 2010).
The bed sill provides a protective layer that resists erosion by stabilizing the sediment near
the pier. The presence of the sill reduces the likelihood of sediment particles being lifted and
transported by the flow. This stabilization effect is especially significant in maintaining the
integrity of the riverbed around the pier (Tafarojnoruz et al., 2010). The effectiveness of a bed sill
in reducing local scour depends on its placement and dimensions. The sill should be placed at an
appropriate distance downstream of the pier, where it can effectively influence the flow pattern.
Its height and width must ensure adequate flow redistribution and energy dissipation without
13

causing excessive backwater effects or flow obstruction (Sanadgol et al., 2021). While bed sills
effectively reduce local scour, their environmental impact must also be considered. The design
should ensure minimal disruption to aquatic habitats and maintain natural sediment transport
processes. Environmental assessments and mitigation measures may be necessary to address
potential adverse effects (Sohrabi et al., 2019). Several laboratory experiments were conducted on
bed sills to control scour at bridges. They showed that a bed sill installation a short distance
downstream of the bridge site causes a reduction of scour depth, area, and volume. The shorter the
distance between the two structures is, the greater the efficiency of decreasing the scour hole.
However, sometimes bed sills take effect after the scour is initiated around bridge piers, reducing
the effectiveness of the countermeasure vortex system they create (Grimaldi et al., 2009).

Figure 1. 3:A vortex system around the pier with a bed sill as a scour mitigation measure (Grimaldi et al., 2009)

Applying caissons and footings below the pier shaft is another standard indirect method of
preventing local scour. Caissons and footings provide stable foundations for bridge piers and help
mitigate scour by altering flow patterns and reducing erosive forces. The caisson averts the
14

upstream flow and vortex system from directly impacting sediment particles around piers and
abutments (Chiew, 1992). Depending on the construction method and site conditions, they can be
open, pneumatic, or floating. Open Caissons are box-like structures that sunk into place by
excavating soil from within. They stabilize the pier base, reducing scour (Wei et al., 2022).
Pneumatic caissons are used in deeper water, which keeps water out with compressed air, allowing
precise foundation placement, which helps resist scour (Kodaki et al., 1997). Floating Caissons
are constructed on land and floated to the site, creating a solid barrier against erosive forces
(Chakrabarti et al., 2006). Spread Footings extend laterally from the pier base and distribute loads
over a larger area, reducing localized erosion (Kumar & Chatterjee, 2023). Pile-Supported
Footings transfer loads to deeper, stable layers, while the footing protects against scour by
stabilizing the soil around the piles (Tu et al., 2022).
While caissons and footings are commonly used to provide stable foundations for bridge
piers, they can also present specific problems when used to reduce scour. Installing caissons and
footings significantly increases the complexity of bridge construction. The process requires
specialized equipment and skilled labour, which can complicate the construction schedule and
increase the risk of construction delays. This added complexity, particularly in deep water or
difficult soil conditions (Breusers & Raudkivi, 2020). Caissons and footings can significantly alter
the flow patterns around the pier. The presence of large submerged structures can create complex
flow dynamics, including increased turbulence and vortex formation. These altered flow patterns
can exacerbate local scour instead of mitigating it (Chiew, 1984). If not correctly designed and
installed, caissons and footings can be undermined by scour. Scouring can erode the material
around and beneath these structures, potentially compromising their stability (Breusers &
Raudkivi, 2020). Another effective indirect method for preventing or controlling various types of

15

scour is the application of spur dikes. A comprehensive literature review on this topic is presented
below.
1.2 Spur Dike
Spur dikes, or groynes, are extended structures in which one end is at the riverbank, and
the other is projected toward the river flow. The application of spur dikes is one of the most
efficient means of stabilizing the river flow (Kuhnle et al., 1999). Regardless of the different types
of spur dikes, they redirect flow from the river bank and affect the flow regime, flow velocity,
sediment transportation, and the scour process (Giglou et al., 2018). They have been widely used
for many purposes, such as river bank protection, flood control, landscape improvement,
ecosystem restoration, and scour process control. Different aspects of spur dikes, including their
design, types, materials, and environmental impact, play crucial roles in their effectiveness (Sadat,
2015).
1.2.1 Design and Orientation
The design and orientation of spur dikes are crucial factors that determine their
effectiveness in reducing scour and protecting riverbanks or bridge piers. These factors include the
angle of orientation relative to the flow, the length and height of the dikes, and the spacing between
multiple dikes (Kehe, 1984). Spur dikes are typically oriented at an angle to the flow direction.
Common orientations include perpendicular (90º) and upstream-angled (between 30º and 60º). The
angle of orientation affects how the structure redirects the flow and impacts sediment deposition
patterns. Upstream-angled dikes are more effective at diverting flow away from the protected area
(Kuhnle et al., 2002). The length of a spur dike is a critical design parameter. Longer spur dikes
extend further into the river or stream, affecting a significant portion of the flow. However, they
can also cause more significant changes in the overall flow pattern and may increase the risk of
16

unwanted sediment deposition or erosion in other areas (Esmaeli et al., 2022). The height of a spur
dike should be designed to influence the flow effectively without causing overtopping during highflow events. Typically, the height is kept below the water surface during normal flow conditions
to minimize disruption to navigation and reduce visual impact (Esmaeli et al., 2022).
When multiple spur dikes are used, close spacing can provide continuous protection along
a riverbank or around a bridge pier. However, it may lead to excessive sediment deposition
between the dikes, reducing the flow capacity of the channel. Optimal spacing ensures that each
dike effectively influences the flow without causing adverse effects such as excessive turbulence
or sediment build-up. The spacing should be determined based on hydraulic modelling and sitespecific conditions to balance protection and flow efficiency (Sadat, 2015). Numerous field studies
and practical applications have demonstrated the effectiveness of well-designed spur dikes in
reducing local scour. For example, using spur dikes in the Missouri River has shown significant
reductions in bank erosion and improved stability of bridge piers. Hydraulic models are often used
to optimize the design and orientation of spur dikes. These models help predict flow patterns,
sediment transport, and potential scour around the structures, ensuring that the dikes are designed
to maximize their protective benefits while minimizing negative impacts (Rajaratnam &
Nwachukwu, 1983)
Spur dikes can be constructed from various materials, including bamboo or timber piles,
tree trunks, rock, soil, gravel, sandbags, riprap, concrete elements, and steel. Stone and riprap are
commonly used due to their availability and effectiveness in dissipating flow energy. These
materials are also flexible, adapting to minor shifts in the riverbed without losing effectiveness.
Concrete spur dikes are durable and provide strong resistance against high-velocity flows. They
are often used in locations with severe hydraulic conditions. Timber is used for permeable spur
17

dikes, providing a cost-effective and environmentally friendly option. Timber structures are more
easily constructed and effective in less aggressive hydraulic environments (Karami et al., 2011).
1.2.2 Spur Dike Types
Spur dikes can be categorized into two main types based on their construction and
interaction with water: permeable and impermeable spur dikes. Each type has distinct features,
advantages, and applications. A spur dike is considered permeable if it allows river flow to pass,
whereas it is impermeable if it blocks and redirects the flow. In rivers with impermeable spur dikes,
the energy gradient steepens, and water velocity increases due to the reduction in river width. To
recover its original cross-sectional area, the river may erode the riverbed, reducing velocity and
returning to its original state. The extent of this process is determined by the shape and size of the
spur dike and the strength of the bed material (Pandey et al., 2018).
Permeable spur dikes are constructed with materials that allow water to pass through them,
reducing the impact on flow patterns and minimizing turbulence. They are often made of timber
piles, steel, or rock-filled gabions, creating water flow openings. Permeable spur dikes usually
slow down sediment transportation by reducing the velocity on their upstream side. Allowing
water to flow through the structure reduces the turbulence and vortex formation downstream,
which helps mitigate local scour around the dike and the protected area. They can also reduce
scour by redirecting or creating a laminar flow. The partial obstruction of flow causes sediment to
deposit in the low-velocity zones around and behind the dike, helping to stabilize the riverbed and
banks (Gu et al., 2011). On the other hand, impermeable spur dikes can benefit flood control. By
deflecting turbulent flow in certain parts of the river, they help prevent riverbank erosion. These
structures can reduce flow energy during floods, redirecting and controlling the rapid flows to
mitigate flood impact (Klingeman et al., 1984).
18

Based on the flow conditions and water levels in rivers, spur dikes can either be submerged
or non-submerged. Impermeable spur dikes are typically designed to be non-submerged because
overflow over solid obstacles creates a vertical jet behind the dike. This vertical jet forms a layer
along the longitudinal axis of the dike, which moves toward the downstream bank and can cause
significant erosion (Yossef & Klaassen, 2002). Based on the orientation angle of spur dikes relative
to the river flow, they can be classified into three categories: attracting, deflecting, and repelling
(Figure 1.4). Attracting spur dikes point downstream, directing the river flow toward the center of
the channel and protecting the area on their downstream side. Repelling spur dikes are oriented
upstream, diverting the river flow from the banks. By deflecting rapid and turbulent currents away
from the riverbank, they offer adequate protection against upstream bank erosion. However, these
structures are prone to significant scour, particularly around their tips (Sadat, 2015). Deflecting
spur dikes are well-known for altering the direction of river flow without repelling it. These dikes
locally protect banks against erosion by contracting the river width and increasing water velocity.
Deflecting dikes facilitate sediment transportation management by enhancing the velocity in the
middle of the river. In other words, they generate turbulent flow, promote greater sediment
transport in the river's center, and deepen the river channel (Sadat, 2015).

Figure 1. 4:Types of spur dikes in terms of the orientation angle (Zhang & Nakagawa, 2008)

19

According to their appearance in a plan, they can have many different shapes such as
straight, mole-head, T shape, L shape, J shape, hockey shape, inverted hockey shape, etc
(Figure 1.5). These shapes with different lengths can have vast applications in the river. For
example, L-head spur dikes, where the outer tip is towards downstream flow, have less scour
around their tips than straight ones. However, they can generate more sediment deposition inside
the spur dike area. The L-head dikes can function better in widespread streams. Using them as the
first and last spur dike is usually recommended when there are several dikes as a series (Mansoori,
2014).

Figure 1. 5:Different shapes of spur dikes (Zhang & Nakagawa, 2008)

The design of spur dikes is influenced by numerous critical parameters, including river
width and depth, mean velocity, channel sinuosity, bed material type and size, sediment transport
rate, bank cohesiveness, spur dike length and shape, orientation angle relative to the flow, and
construction materials. Given the unique conditions of each engineering project, the design of a
spur dike often relies heavily on practical experience and informed decision-making (E.
Richardson et al., 1975).

20

1.2.3 Previous Researches Overview
Numerous studies have investigated the behaviour of water flow when encountering
obstacles. Mushtaq (1953) performed one of the earliest studies on spur dikes, which examined
scour around a spur dike using sand as the bed material. His research yielded significant findings
on various parameters influencing the maximum scour depth. Derrick et al. (1989) studied spur
dikes, pile dikes, and similar constructions widely used in large river development projects. Some
researchers focus on the flow and the sediment transportation pattern downstream of a dike and
measure the velocity profile behind a permeable spur dike (Tominaga et al., 2000). Furthermore,
Ishigaki et al. (2000) investigated the vortex system around the spur dike and examined how it
affects the local scour. They found that the development of the vortex takes place aside from the
dike. Therefore, the deepest hole is created at some distance from the obstacle.
Sukhodolov et al. (2002) applied flow velocity measurement on the Elbe River, one of the
major rivers in Central Europe, to find the number and the location of vortexes inside the spur dike
field. Muto et al. (2002) conducted two experimental studies about the effects of opening ratio and
water depth on velocity distribution profiles. Their first experiment was a large-scale particle
image velocimetry in the Yodo River in Japan, and the second experiment was conducted in a
laboratory using downscaled spur dike. Based on the field study experiment, they illustrated that
the flow was highly unsteady inside the scour hole area. However, the collected data in the
laboratory did not show the same unsteady characteristics. The authors concluded that because of
the complex bathymetry of the natural river and lower Reynolds numbers, the results are different
in field and laboratory studies. Furthermore, with experimental and computational techniques
improvement, several researches have been performed on the flow field, velocity profile, and
sediment transport for different obstacles in a river (Namely, spur dikes, bridge piers, and

21

abutments) (Barbhuiya & Dey, 2004; Nasrollahi et al., 2008; Zhang & Nakagawa, 2008; Zhang et
al., 2005).
The permeability of spur dikes has been extensively investigated as a technique for
managing sediment deposition transportation as well as the countermeasures methods against
scour (Alauddin et al., 2011; Baba et al., 2010; Koken & Constantinescu, 2011; Rajaratnam &
Nwachukwu, 1983; Richardson et al., 1975; Shields Jr et al., 1995; Teraguchi et al., 2010).
Michioku et al. (2004) researched permeable spur dikes made of stone gabion to reduce local scour.
The results illustrate the steady bed material formed near the spur dike tip. Zhang et al. (2005)
studied bed morphology, sediment distribution, and flow fields in a channel featuring a series of
impermeable spur dikes on both sides. Their findings indicated that the flow field and scour are
most significantly impacted around the upstream pairs of spur dikes. Teraguchi et al. (2008)
studied flow field, scour, and sediment patterns around permeable and impermeable spur dikes
using experimental and simulation methods. They described that the local scour hole on the
upstream side of impermeable spur dikes is more significant and deeper than permeable ones.
Mizutani et al. (2011) examined the effects of spur dike height and the grain size distribution of
riverbed material on the morphology around an impermeable spur dike. They found that increasing
the median particle size of the bed material reduces the maximum scour depth. Further research
on the permeability of spur dikes, specifically on the morphodynamics around stone-lined spur
dikes in the Akashi River, revealed that the maximum scour depth for permeable spur dikes is
reduced by 40% compared to impermeable ones (Kanda et al., 2019).
Few studies have employed spur dikes as a countermeasure for local scour around bridge
sites. Karaki (1959) conducted an experimental study to protect bridge abutments by setting a spur
dike perpendicular to the abutment at the highway embankment. His findings indicated that bridges

22

with smaller openings require longer dikes to protect abutments from scour effectively. He also
noted that the efficiency of spur dikes depends on the geometry of the roadway embankments and
the size of the bridge opening. Herbich (1966) demonstrated that curved spur dikes reduce scour
around abutments. His results showed that using a dike to redirect flow reduces local scour and
promotes sediment deposition around the abutment, whereas, without dikes, scour would occur.
Li et al. (2005) investigated various configurations of spur dikes using two different materials,
solid thin walls and rock walls, to protect wing wall abutments. The findings indicated that a single
solid spur dike with the same or shorter protrusion length as the abutment, when placed upstream,
is insufficient for protection. Additionally, rock wall dikes proved more effective than solid spur
dikes in safeguarding the abutment.
The researchers concluded that the most effective configuration for preventing local scour
at abutments involves using three rock wall spur dikes: one placed upstream of the abutment and
the others positioned at the upstream and downstream corners. Zhang and Nakagawa (2008)
employed a protective spur dike upstream of a set of parallel spur dikes, significantly reducing
scour depth around the main spur dike. They also noted that the farthest upstream spur dike should
be constructed more robustly, as it would be subjected to the most destructive flow influences.
Karami et al. (2011) demonstrated that critical parameters for a protective spur dike include its
length and distance from the obstacle, such as an abutment or another spur dike. Basser et al.
(2014) proposed a numerical approach to predict the optimal parameters of a protective spur dike.
Their findings indicated that the support vector regression (SVR) model provides better accuracy
than the ANFIS and ANN approaches. Additionally, the radial basis function of the SVR model
outperformed the polynomial-based SVR approach, making it the recommended model for
predicting the parameters of protective spur dikes.

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1.3 Riverbed Material and Sediment Transportation
1.3.1 Riverbed Material
Sediment transportation, bed material, and grain size distribution of riverbeds are other
essential and effective scour-related criteria. Understanding riverbed material and sediment
transportation is crucial in predicting and mitigating scour phenomena, particularly around
hydraulic structures such as bridge piers and abutments. Sediment is defined as solid particles
created by the disintegration process of organic and inorganic materials (Bortone, 2006). These
particles can be transported by water, wind, glaciers, and other natural causes and can be found in
diverse shapes and sizes. The type and characteristics of riverbed material play a significant role
in the scour process. Riverbeds typically comprise sediments ranging from fine silt and clay to
larger sand, gravel, cobbles, and boulders. The composition and cohesion of these materials
determine their susceptibility to erosion(Montgomery et al., 2000).
Fine Sediments (Silt and Clay) are small and cohesive, making them more resistant to initial
erosion, but they can be transported over long distances once entrained. Fine sediments can form
a protective layer that can resist scouring until the shear stress exceeds a certain threshold. Coarse
Sediments (Sand, Gravel, and Cobble) are non-cohesive and more easily eroded and transported
by flowing water. The critical shear stress for these particles' erosion is lower than cohesive
sediments. Larger particles like gravel and cobbles are more resistant to movement but can cause
significant scour when mobilized (Breusers & Raudkivi, 2020). Riverbeds often have stratified
layers of different sediment types. The top layers might be finer sediments, while deeper layers
consist of coarser materials. This stratification affects how scour develops, as finer materials are
eroded first, exposing coarser, more resistant layers underneath (Melville & Coleman, 2000).

24

1.3.2 Sediment Transportation
Study sediment transportation includes terms such as entrainment, deposition, critical shear
stress, suspended load transport, bed shear stress, settling velocity, incipient motion, and sediment
transportation rate. Entrainment is when turbulent flow removes the sediment from the top of the
riverbed and carries it into suspension. It happens when the bed shear stress exceeds a threshold
value (critical shear stress). After the entrained phase, the sediment is carried by the water flow
within a particular height overhead of the riverbed, known as the suspended load transport.
Deposition occurs when suspended sediments are settled in the riverbed due to gravity, buoyancy,
and friction (Bortone, 2006).
Bed shear stress provides an index of fluid force per unit area on the stream bed, which has
been related to sediment transport in several theoretical and empirical researches. Bed shear stress
and shear velocity are essential parameters in river investigation to estimate the transport rate,
scour, and deposition (Wilcock et al., 1996). The moment where the shear forces overcome
protective forces (inertia, friction), known as the moment of incipient motion, is the start of
sediment movement (Kanellopoulos, 1998). The threshold is the two forces equivalent to the
directive forces, balancing the resisting forces (Wiberg & Smith, 1987). Generally, it can be
categorized into bed load, suspended load, and dissolved load, each interacting differently with
hydraulic structures (Yang et al., 2003).
Bed load consists of particles that roll, slide, or bounce along the riverbed. This type of
transport is dominant for larger, heavier particles such as gravel and cobbles. The movement of
bed load is a primary mechanism of local scour around bridge piers and abutments, where high
flow velocities can mobilize these particles and create scour holes (Chiew, 1984). The suspended
load consists of finer particles like silt and clay in the water column. These particles are transported
25

over longer distances and can contribute to sedimentation downstream. Suspended load plays a
crucial role in the overall sediment budget of a river system and can influence the extent of scour
by altering sediment availability (Lewis, 1998). The dissolved load includes minerals and organic
matter dissolved in the water. While it does not directly contribute to scour, changes in dissolved
load can affect water chemistry and the deposition of sediments, indirectly influencing scour
processes (Liu, 1998).
1.3.3 Literature Review
Shields (1936) was one of the pioneering scholars who identified the relationship between
hydraulic parameters and sediment characteristics in explaining the conditions for incipient motion
of bed material and established that sediment is about to be mobilized by water flow. This
condition introduces the Critical Shields Value (߬ܿ) and the Shear Reynolds Number (Re). The
relationship between these two parameters is depicted in the Shields diagram (Figure 1.6).

26

Figure 1. 6:The Shield's Diagram (Schwimmer, 2006 )

Shear stress and shear velocity in the riverbed are essential parameters in river research to
calculate sediment transportation, deposition, and different types of scouring (Wilcock, 1996).
Accurate estimation and calculation of these variables are complex processes, especially in the
compound flow of natural rivers. Therefore, many scholars have focused on this topic, such as
Prandtl tubes (Ahmed & Rajaratnam, 1998) and shear plates (Rankin & Hires, 2000). Dietrich and
Whiting (1989) conducted an experimental study to measure shear stress directly. Afterward, many
others also investigated this topic (Cardoso et al., 1991; Haizhou & Graf, 1993; Tsujimoto et al.,
1990).
It is noted that one of the main reasons for bridge failure is local scour around its piers and
abutments (Dey & Barbhuiya, 2005; Melville et al., 2002; Namaee & Sui, 2019b; Wu, Peng, et al.,
2014). Local scour can be categorized into clearwater and live-bed scour groups (Chabert &
27

Engeldinger, 1956). Clearwater scour occurs when the upstream flow approaching the bridge piers
or abutments carries no sediment. In contrast, live-bed scour happens when the flow contains a
significant sediment load transported into the scour holes, while vortices within the scour remove
sediments from the hole. Distinguishing between these two types of scour is essential because the
temporal development of scour holes and the relationship between maximum scour depth and flow
velocity depend on whether clearwater or live-bed scour occurs (Raudkivi & Ettema, 1983).
Chabert and Engeldinger (1956) illuminated that the clear water maximum scour depth is
10% greater than live-bed sour (Figure 1.7). Researchers observed that the equilibrium condition
in clearwater scours is reached more slowly than the live bed scour (Kiraga & Popek, 2016).
Moreover, after the complete development of the scour hole under clearwater conditions, the flow
can no longer remove sediment from inside the hole. The armour layer (a layer of bigger particle
size that protects the smaller ones beneath them from getting eroded) protects the river bed material
from more erosion (Chee, 1982; Melville, 1984; Richardson & Davis, 2001). In investigating livebed scour, scholars mainly focused on estimating and predicting maximum scour depth rather than
the temporal development of the scouring equilibrium conditions (Chiew & Melville, 1987; Lim
& Cheng, 1998; Melville, 1997).

28

Figure 1. 7:Comparison of time development for clearwater and live-bed scour (Kiraga & Popek, 2016)

More research on live-bed scour specifies that the mean equilibrium value of scour holes
is related to different types of river bedforms (Melville & Coleman, 2000; Sheppard & Miller Jr,
2006). There have been several studies on the incipient motion of uniform sediment (Aguirre-Pe
et al., 2003; Beheshti & Ataie-Ashtiani, 2008; Buffington & Montgomery, 1997; Shvidchenko &
Pender, 2000; Vollmer & Kleinhans, 2007). The movement of nonuniform sediment is more
complex because it is affected by variables other than the approaching flow, such as grain shape,
the median grain size of bed material, interactions between grains, and the formation of the river
bed (Wu, Peng, et al., 2014). Accordingly, there are limited studies on nonuniform sediment
transportation and incipient motion.

29

Haitao et al. (2008) investigated the incipient velocity of nonuniform sediment on a
riverbed. The findings indicate that the incipient velocity of fine and coarse particles varies based
on the type of bed material. For fine particles in nonuniform sediment, the incipient velocity is
higher than in uniform sediment. Conversely, the incipient velocity for coarse particles is higher
in uniform sediment than in nonuniform sediment. Additionally, they developed a formula for
determining the incipient velocity of nonuniform sediment in rivers. Hirshfield (2015)
demonstrated that sediment nonuniformity could significantly affect local scour, as an armour
layer forms within the scour hole for nonuniform sediments. This armouring reduces the scour
depth compared to that in uniform sediments.
1.4 Ice Cover
1.4.1 Ice Cover and Its Impact on Flow Dynamics
Ice formation on river surfaces and the associated challenges are complex topics. Many
rivers freeze during winter, especially in northern regions, with ice cover persisting for several
months. Ice cover on rivers and streams significantly affects hydraulic conditions and scour
phenomena. The presence of ice changes the flow dynamics, alters sediment transport, and
influences the stability of riverbanks and hydraulic structures (Shen, 2010). Ice cover can create
several issues, such as ice blocking, flooding, interrupting the generation of a hydropower station,
blocking rivers, and interfering with ecosystem balance. The river hydrology will be dramatically
changed compared to open channel conditions such as velocity profile, bed shear stress
distribution, and sediment transportation (Sui et al., 2000).
Ice cover can form in various ways, including frazil ice, anchor ice, and surface ice. The
type and extent of ice cover affect the flow characteristics in several ways. Ice cover reduces the
cross-sectional flow area of a river, leading to higher flow velocities under the ice. This increase
30

in velocity can enhance the shear stress on the riverbed, potentially increasing scour around
structures such as bridge piers (Hicks, 2009). The presence of ice forces the water to flow in a
more confined space beneath the ice cover. Flow redistribution can change the velocity profile and
turbulence characteristics, impacting sediment transport and deposition patterns. Ice jams occur
when floating ice accumulates and obstruct the flow. This obstruction can cause water levels to
rise upstream and increase flow velocities downstream once the jam is released, potentially leading
to significant scour events (Prowse & Beltaos, 2002).
1.4.2 Roughness Coefficient of Ice Cover
One of the critical factors in understanding the impacts of ice cover is the roughness
coefficient of the ice cover, which plays a pivotal role in determining flow resistance and
subsequent scour. The roughness coefficient, often represented as Manning's coefficient quantifies
the resistance exerted by the ice cover on the flowing water. The roughness of ice cover depends
on several factors, including the type of ice and surface texture. The two common types of ice
cover are smooth and rough. Smooth cover is formed under calm conditions, and smooth ice has
a lower roughness coefficient, leading to less flow resistance. However, it can still significantly
alter flow dynamics due to the reduced flow area (Sui et al., 2005). Rough cover is formed under
turbulent or windy conditions, and Rough ice includes features such as pressure ridges and
hummocks. These features increase the roughness coefficient, leading to higher flow resistance
and more complex flow patterns beneath the ice (Hicks & Beltaos, 2008).
The roughness coefficient of the ice cover increases the overall flow resistance, which can
reduce flow velocities under the ice but increase water levels upstream. The increased resistance
can lead to higher shear stress on the riverbed, affecting sediment transport and potentially
enhancing scour around structures (Sui et al., 2006). The roughness coefficient of ice cover is
31

typically measured through field observations and empirical correlations. Various studies have
provided different values for Manning's coefficient based on ice type and conditions, emphasizing
the need for site-specific assessments. The flow under ice cover conditions can be divided into two
portions: the upper part, mainly influenced by the ice cover, and the lower part, which is affected
by the riverbed (Sui et al., 2010). The maximum velocity location depends on the roughness
coefficient of the two boundaries and shifted toward the smoother edge. Therefore, the roughness
coefficient is essential in the flow velocity profile under ice cover (Ashton, 1986; Smith & Ettema,
1995; Sui et al., 2010; Sui et al., 2000).
1.4.3 Scour Phenomena Under Ice Cover
The increased flow velocities under ice cover can enhance local scour around bridge piers
and abutments. The confined flow and increased shear stress can further erode the riverbed
material, deepening scour holes. Ice cover can also contribute to general scour by altering the
sediment transport capacity of the river (Sui et al., 2010). The changed flow dynamics can lead to
uneven sediment deposition and erosion patterns, affecting the stability of the riverbed over
broader areas. Ice jams can cause sudden changes in water levels and flow velocities. When a jam
releases, the downstream surge can significantly increase scour, posing risks to hydraulic
structures and riverbanks. Rapid changes in flow conditions during ice jam events are critical to
understanding and mitigating scour risks (Prowse & Beltaos, 2002).
Ice cover and the roughness coefficient of ice cover are essential factors in understanding
and managing scour phenomena in rivers and streams. The presence of ice alters flow dynamics,
increases flow resistance, and impacts sediment transport, leading to changes in velocity structure
and distribution. This change in velocity profile increases kinetic energy at the riverbed and causes
extra bed shear stress (Sui et al., 2010; Wang et al., 2008). These alterations would affect the
32

hydraulic structures and local scour around bridge sites and can damage them significantly.
Accurate assessment and consideration of these factors are crucial for designing and protecting
hydraulic structures in cold regions. Collapsing of a bridge over the White River in Ontario was
one of the first bridge failures due to the scour under the ice cover (Zabilansky, 1996). Since Icecovered rivers play significant roles in the safety of bridges and other hydraulic structures, the
following section will present some of the most critical research on this topic.
1.4.4 Previous Researches Overview
Lau and Krishnappan (1985) investigated sediment transport under ice-covered flows using
the k-ε model, demonstrating that this model effectively predicts average flow properties with
sufficient accuracy. The results indicate that ice formation on the river surface can lead to increased
depth, reduced average velocity, and significant changes in bed shear stress, significantly
impacting sediment transport. Subsequently, many scholars have considered ice-covered flows to
have a double-wetted perimeter, where the flow is divided at the point of maximum velocity.
However, Smith and Ettema (1997) discovered that the assumption of two layers is inadequate for
describing flow resistance and sediment transportation, especially in dune regimes.
Zabilansky (1996) investigated ice force at bridge settlement and compared it to open flow
conditions. His research shows that although the change in velocity profile affected the scour, a
considerable amount of scouring happens in the initial phase of a breakup of the ice on the surface,
emphasizing the importance of the roughness coefficient. Muste et al. (2000) declare that a rough
cover in a flume reduces the overall sediment transport rates but amplifies the sediment suspension
loads. Ackermann et al. (2002) conducted an experimental study on local scour around bridge piers
under three different flow surface conditions (open, Smooth, and rough) with uniform sand. They
explained that the cover condition generally creates a larger scour depth for different flow
33

velocities. However, the scour development pattern remains unchanged regardless of water surface
conditions.
As different scholars have done more research, Hains (2004) tried to find the relation
between different ice cover roughness and shear stress. He concluded that the roughness of the
cover considerably affects bed shear stress, and the increase of shear stress will result in amplified
bed erosion and scour depth. Furthermore, Hains and Zabilansky (2004) illustrated that a rougher
cover can push the maximum velocity toward the bed, increasing shear stress on the river bed and
enlarging the scour hole. They also presented that ice cover can increase scour depth at bridge
piers and abutments by 10 to 35 percent. Zabilansky et al. (2006) explained that ice cover
roughness can be increased under different conditions, such as dynamic evolution in steep or wave
streams, which occurs in narrow rivers while ice cover still exists on the surface. In this regard,
Beltaos (2007) concluded that differences in the thickness of an ice jam can lead to variations in
sediment transport amount and significantly affect general and local scour around bridge piers,
abutments, and other hydraulic structures.
Laboratory experiments on the incipient motion under ice cover demonstrated the role of
flow velocity and critical shear Reynolds stress (Wang et al., 2008). It was concluded that
increasing water depth under the ice cover requires a higher flow velocity for the incipient motion
of bed material. They explained that by increasing the roughness coefficient of ice cover, the
location of maximum flow moves even more toward the river bed. Also, it should be considered
that the roughness of ice cover changes with the melting down of ice at the beginning of the spring
(Wang et al., 2008). Sui et al. (2010) conducted experiments on the incipient motion of frazil
particles under ice cover, demonstrating that smaller sediment sizes in bed material require less
critical dimensionless shear stress to initiate motion. Ettema and Kempema (2012) illuminate that

34

suspended sediment loads during breakup are nearly 10 times bigger than ice cover conditions.
Moreover, they explained that the magnitude of sediment transportation and deposition is the
function of water depth, ice thickness, and ice roughness.
1.5 Research Objectives
One of the most critical river structures is the bridge. Scour is a significant issue affecting
the stability and longevity of bridges. The leading cause of bridge failure is local scour at bridge
abutments and piers (Wardhana & Hadipriono, 2003). Local scour at bridge abutments is
particularly concerning as these structures are more vulnerable than piers (Kandasamy & Melville,
1998). According to the US Federal Highway Administration (1973), 383 bridges failed over
various rivers, with 72% of these failures involving damage to abutments and 25% to piers
(Richardson & Davis, 2001). Consequently, further investigation into local scour around
abutments is essential. Among the various countermeasures to mitigate scour, spur dikes have
shown promising results. Spur dikes, by altering flow patterns and reducing flow velocities, can
effectively minimize erosion around abutments. This experimental research project aims to
enhance the understanding of how spur dikes influence scour phenomena and to develop optimized
configurations for their implementation under diverse hydraulic conditions.
The study will address critical aspects such as the velocity profile around spur dikes, local
scour at dikes and abutments, and the optimal design and configuration of spur dikes to reduce
scour at the downstream abutment. By combining physical flume experiments with advanced
measurement techniques, the research seeks to provide practical insights into effective scour
mitigation strategies. The following objectives elaborate on the specific goals of the study and the
methodologies to be employed. These objectives aim to enhance the understanding of how spur

35

dikes influence scour phenomena around bridge abutments to develop more effective scour
mitigation strategies, ensuring the stability and longevity of hydraulic structures.
1.5.1 Objective One: Analysis of Flow Fields and 3D Velocity Distribution Around Spur
Dike
Understanding the Flow fields, velocity profile, and 3D velocity distributions around spur
dikes is crucial for predicting how they influence flow patterns and sediment transport. Velocity
profiles indicate how water speed changes at different points around the dike, directly affecting
the erosion and deposition of sediments. This objective involves capturing detailed flow dynamics
using advanced measurement techniques like ADV. The study will focus on identifying the
vortices and turbulence patterns generated by spur dikes, which are critical in determining scour
zones. By examining these flow characteristics under different flow rates and sediment conditions,
the research aims to comprehensively understand flow interactions that influence scour. This
information is vital for improving the design and placement of spur dikes to maximize their
protective benefits. This objective will be assessed in chapter Three of this experimental research.
1.5.2 Objective Two: Investigation of the Impact of Ice Cover and Roughness Coefficient
on the Scour Process
Ice cover significantly impacts hydraulic conditions and scour processes by altering flow
dynamics, increasing flow resistance, and affecting sediment transport. This objective aims to
study how ice cover and its roughness coefficient influence the velocity distribution and sediment
movement around spur dikes and abutments. The research will employ two different models of Ice
cover to analyze the effects of different surface roughness (smooth and rough) on scour depth and
patterns. Understanding these impacts is essential for designing effective scour mitigation

36

strategies in cold regions with common ice cover. This experimental research will assess this
objective in chapters three, four, and five.
1.5.3 Objective Three: Investigation of Local Scour at Spur Dikes with different
alignment angles and flow surface condition
Local scour refers to erosion at the base of structures due to flowing water. This objective
focuses on understanding how spur dikes' different configurations (Alignment angles) affect the
local scour around them. The study will use scaled physical models in flume experiments to
replicate real-world conditions. Three different bed materials will be tested to observe how they
influence scour depth and extent. The research aims to develop predictive models for scour depth
and distribution by analyzing scour patterns under different flow rates and water levels. This
information is crucial for designing effective scour protection measures. This objective will be
assessed in chapter four of this experimental research.
1.5.4 Objective Four: Analysis of Effects on Sediment Removal and Deposition
To assess the effects of spur dikes on sediment removal and deposition throughout the
flume. Spur dikes influence sediment transport dynamics, leading to changes in sediment removal
and deposition patterns. This objective focuses on understanding how spur dikes alter these
patterns across the flume. The study will model the sediment deposition dunes using the Surfer 2D
& 3D data modelling and mapping to plot the deposition patterns and locations. The findings will
provide insights into how spur dikes can be designed to promote beneficial sediment deposition
and reduce harmful erosion, ensuring the stability and longevity of riverbanks and hydraulic
structures. This experimental research will assess this objective in chapters three, four, and five.

37

1.5.5 Objective Five: Development of a Predictive Formula for Maximum Scour Depth at
Spur Dike
To develop a predictive formula that estimates the maximum scour depth at spur dikes
under different alignment angles, flow surface conditions, and flow characteristics. Accurately
predicting the maximum scour depth is essential for designing effective scour protection measures.
This objective involves conducting comprehensive experiments and data analysis to derive a
formula incorporating various factors influencing scour depth. The study will consider different
alignment angles of spur dikes, flow surface conditions (such as water depth and velocity), and
sediment characteristics. Dimensional analyses will be employed to develop a robust predictive
model. This formula will help engineers and researchers design spur dikes that mitigate scour risks
under diverse hydraulic conditions. This experimental research will assess this objective in
chapters four and five.
1.5.6 Objective Six: Optimization of Spur Dike Configuration to Protect Abutment
To determine the optimal distance and orientation angle of non-submerged impermeable
spur dikes for minimizing local scour at bridge abutments under various flow conditions, water
surfaces, and bed materials. The configuration of spur dikes, including their distance from the
abutment and their orientation angle, significantly affects their effectiveness in reducing scour.
This objective involves conducting a series of experiments to test different configurations. The
research will explore a range of distances and angles to identify the setup that provides maximum
protection against scour at abutment. The goal is to develop design guidelines that specify the best
practices for installing spur dikes to protect abutments under various environmental conditions.
This objective will be assessed in chapter five of this experimental research.

38

1.6 Research Innovation
This experimental research focuses on using spur dikes as a countermeasure for local scour
at abutments. The study aims to determine the optimal setup for the dike, including the best
distance and orientation angle, to minimize local scour at the abutment. Additionally, the research
seeks to investigate methods to reduce local and contraction scour at the dike, ensuring sustained
protection for the abutment. A comprehensive analysis of the flow field, 3D velocity distribution,
and scour patterns around dikes with varying alignment angles has been conducted to achieve this
goal. The innovations of this experimental research are as follows:
1.6.1 Empirical Formula for Scour Depth
One of the significant innovations of this study is the development of an empirical formula
to estimate scour depth at the spur dike under ice-covered conditions. This formula was derived
using dimensional analysis, considering parameters like flow velocity, sediment size, alignment
angles, and ice cover characteristics. The empirical formula enhances predictive capabilities,
allowing engineers to estimate potential scour depths more accurately in cold regions with
prevalent ice cover. This predictive tool is crucial for designing and implementing effective scour
mitigation strategies.
1.6.2 Comprehensive Parameter Analysis
The study uniquely combines the analysis of multiple parameters simultaneously to
understand the scour process in detail. The parameters investigated include velocity profiles, which
are used to understand how flow velocity changes in the presence of spur dikes and ice cover.
Reynolds Shear Stress: measuring the shear stress exerted by the flowing water on the riverbed,
which is a critical factor in sediment transport and scour. Turbulent Kinetic Energy (TKE):
Evaluating the turbulence levels in the flow, which influence the erosion and deposition processes.
39

Non-uniform bed material: Modeling real-life river conditions using three different non-uniform
sediments to investigate the impact of varying sediment sizes. Identifying specific scour patterns
and extents around the dike and abutment using Surfer 3D modelling software.
1.6.3 Application of Non-Submerged Spur Dikes
The research demonstrated the effectiveness of non-submerged impermeable spur dikes as
a countermeasure for local scour at bridge abutments. Unlike submerged dikes, which can be less
effective under high-flow conditions, non-submerged dikes maintain their protective function by
altering flow patterns and reducing the flow velocity near the abutment. This study showed that
non-submerged dikes are particularly effective in diverse flow conditions, including those with ice
cover. This finding is critical for designing more resilient and effective scour protection measures.
1.6.4 Optimal Spur Dike Design
Identifying the optimal distance and orientation angle of spur dikes relative to the abutment
was another critical contribution of this research. By systematically varying these parameters, the
study found configurations that minimized local scour at the abutment. This knowledge helps
design spur dikes that protect abutments and do so efficiently with minimal impact on the
surrounding environment. Optimal design parameters ensure that spur dikes perform their
protective role without causing excessive turbulence or sediment transport issues upstream or
downstream.
1.6.5 Impact of Ice Cover on Scour Processes
The study's investigation into the impact of ice cover on protective spur dikes represents a
novel contribution to the field. Ice cover significantly alters flow resistance and sediment transport
dynamics. The research provides insights into how ice affects scouring by comparing open channel
40

conditions with smooth and rough ice-covered conditions. Understanding these impacts is crucial
for regions with seasonal ice cover, and infrastructure must be designed to withstand such
conditions. The findings aid in predicting and managing channel bed erosion in cold regions,
improving the design of structures to mitigate the impacts of ice cover effectively.
1.6.6 Innovative Experimental Setup
Using a large-scale outdoor flume, which resembles a natural stream environment, is an
innovative aspect of this research. The dimensions of the flume (2 meters wide, 1.3 meters deep,
and 38 meters long) allowed for realistic simulation of flow conditions and provided valuable data
that small-scale laboratory experiments might not capture. This realistic setup enhances the
findings' validity and applicability to real-world scenarios.
1.7 Thesis Structure
Chapter One provides a comprehensive overview of the research problem, underscoring
its critical importance in the fields of hydraulic engineering and bridge safety. It delineates the
specific objectives of the study, followed by an exhaustive review of the existing literature on
scour, spur dikes, riverbed material, sediment transport, and ice cover. The chapter concludes by
identifying gaps in the current body of knowledge and introducing the novel contributions of this
research, including the development of new empirical formulas, in-depth parameter analysis, and
the innovative experimental setup employed in the study.
Chapter Two details the experimental site and setup, focusing on the large-scale hydraulic
flume and the specific configurations of spur dikes and abutments utilized in the research. It further
elaborates on the sophisticated measurement equipment employed, such as instruments for
quantifying flow rate, water depth, velocity components, and turbulence intensities. The chapter

41

concludes with a meticulous description of the data collection procedures, ensuring the
repeatability and transparency of the research methods.
In bridge construction, a Fracture Critical Member (FCM) is a structural element whose
failure could precipitate a catastrophic collapse or significant structural compromise of the bridge.
The integrity of FCMs is paramount, as they are critical to the structure's overall stability.
Consequently, these members are subject to stringent design criteria, rigorous inspection protocols,
and ongoing maintenance to ensure their resilience. The primary objective of this research is to
investigate the efficacy of spur dikes in mitigating scour at bridge abutments. In this context, the
spur dike is identified as FCM. Given that in this research, the spur dike is designed to protect the
abutment from scour and prevent potential failure. The failure of the spur dike could intensify
scour at the abutment, compromising its foundation and potentially leading to structural failure.
Consequently, ensuring the stability and longevity of the spur dike is of paramount importance.
Chapters Three and Four are dedicated to a comprehensive analysis of the turbulence
structure and scour patterns around the spur dike to ensure its stability and durability. Chapter
Three investigates the velocity fields and turbulence structures in relation to scour and hydraulic
structures. It provides a detailed account of the experimental setup and data collection methods
specific to this phase of the research. The results section presents an analysis of the streamwise,
lateral, and vertical velocity components under varying conditions, along with a discussion on
turbulence intensities and Reynolds shear stress, and their implications for scour dynamics.
Chapter Four focuses on the study of channel deformation and scour morphology under different
alignment angles and ice cover conditions. It provides a thorough description of the experimental
methodology and materials, followed by the presentation of results concerning vortex systems,

42

scour morphology, and maximum scour depth. The chapter summarizes the insights gained
regarding channel deformation and their implications for scour mitigation.
Chapter Five examines the critical factors influencing local scour at abutment, with
particular attention to the alignment of spur dikes and the roughness of ice cover. It details the
experimental procedures, including site description, flume setup, and data collection methods. The
results section offers a comparative analysis of maximum scour depth around abutments with and
without the application of protective spur dikes, discussing how varying alignment angles and ice
cover conditions impact scour patterns. The chapter concludes by highlighting key findings on the
effectiveness of different scour mitigation strategies.
Chapter Six is the final chapter, and it synthesizes the key findings of the research,
emphasizing their contribution to understanding scour dynamics and developing mitigation
strategies. It addresses the limitations encountered during the study, such as those related to
experimental design or data collection, and proposes directions for future research, identifying
potential areas for further exploration based on the findings of this thesis.
2 CHAPTER TWO: Materials and Experimental Procedures
2.1 Site Description
This experimental study was conducted at the Quesnel River Research Center (QRRC) in
Likely, British Columbia, Canada. The QRRC provides a unique setting for research due to its
access to a large-scale outdoor hydraulic flume. This flume is specifically designed to replicate the
natural dynamics of river systems, allowing for studying complex hydrodynamic and sediment
transport processes under controlled conditions. Experimental studies are typically conducted in
small indoor hydraulic flumes due to the high construction costs associated with large outdoor

43

flumes. Small-scale flumes generally offer a limited range of flow depths and discharge, whereas
larger flumes provide various hydraulic parameters that closely resemble actual river conditions.
However, working with equipment and collecting accurate data in large-scale flumes presents
significant challenges (Namaee et al., 2017). The QRRC flume, designed to replicate the natural
dynamics of river systems, measures 38.2 meters in length, 2.0 meters in width, and 1.3 meters
deep (Figure 2.1). The flume's longitudinal bed slope is set at 0.2%, closely mimicking the gentle
gradients in natural riverbeds. This slight slope ensures a realistic flow velocity and sediment
transport behaviour, which is crucial for accurately simulating riverine conditions.
An essential feature of the flume is the upstream holding tank, which has a capacity of 90
cubic meters (dimensions: 40 meters in length, 2 meters in width, and 1.3 meters in depth). This
tank ensures a consistent and controlled water supply throughout the experimental runs. The tank
is equipped with three valves that allow for precise control of flow rates, facilitating a range of
experimental conditions. The study can simulate different discharge scenarios by adjusting these
valves, from low-flow conditions to high-flow events. Additionally, a roof covers the entire flume
section, protecting it from environmental factors such as precipitation, wind, and debris, thereby
ensuring the consistency and reliability of the experimental conditions.
Within the flume, two sandboxes are constructed to simulate riverbed conditions. These
sandboxes are placed 10.2 meters apart and provide a controlled environment for observing
sediment dynamics and scour development around spur dikes and abutments. The upstream
sandbox measures 5.6 meters in length, while the downstream sandbox is slightly longer at 5.8
meters. Both sandboxes are 2 meters wide and 0.3 meters deep (Figure 2.2). The sandboxes are
filled with three types of natural sediments, each with different median grain sizes (D50) of 0.48

44

mm, 0.60 mm, and 0.90 mm, with respective standard deviations of 1.97, 2.39, and 1.41. Figure
2.1 illustrates the grain size distribution of the bed materials.

Figure 2. 1:Grain size distribution of the bed materials

Plexiglass sidewalls are installed on one side of each sandbox to enable detailed visual
observation and monitoring of the scouring process. The average density of the sediments used is
approximately 1.7 g/cm³, closely matching the properties of natural riverbed materials.

45

Figure 2. 2: Overview of Flume

2.2 Experimental Setup
The flume is equipped with a tailgate at the downstream end to control water depth and
flow velocity. Three different tailgate heights (10 cm, 20 cm, and 30 cm) are used to create varying
flow depths, directly influencing flow velocities and scour patterns around the dike and abutment.
Adjusting the tailgate can generate various hydraulic conditions with different flow depths. This
ability to control water depth and flow velocity is critical for simulating real-world conditions and
understanding how different hydraulic parameters affect sediment transport and scour.
The experiment involves installing spur dikes and abutments in each sandbox to investigate
the local scour surrounding the spur dike and the influence of the dike on scour reduction abutment.
The spur dikes are constructed from marine-grade plywood, chosen for its durability and resistance
to water. Each dike measures 80 cm in height, 5 cm in width, and 50 cm in length, with 30 cm of

46

its height buried in the sand bed, leaving 50 cm exposed to the flow. This setup allows the dikes
to interact directly with the flowing water, replicating real-world conditions where such structures
are used to manage river flow and prevent erosion. The abutments are fabricated from galvanized
plates with dimensions of 20 cm in width, 80 cm in height, and 40 cm in length. Abutmets are
placed downstream of the dikes at 25 cm and 50 cm distances. The dikes are tested at three
alignment angles relative to the downstream abutments: 90º, 60º, and 45º. These configurations
correspond to blockage ratios of 25%, 20%, and 15%, respectively, allowing researchers to study
how different orientations and positions of the spur dikes affect flow patterns and sediment
transport around the abutments.
Three distinct experimental layouts were employed. The first setup aimed to investigate
the flow field, 3D velocity distribution, and local scour around the spur dike. In this configuration,
the dike was installed in the center of each sandbox at varying alignment angles (90º, 60º, and 45º)
(Figure 2.3). The second setup focused on exploring the application of spur dikes in reducing scour
at abutments. For this purpose, the dike was positioned upstream of the abutment at distances of
25 cm and 35 cm, with different alignment angles (Figure 2.4). The final setup examined abutment
scour without protective measures against rapid flow and local scour. In this scenario, the abutment
was installed in the middle of the sandbox without any upstream dike. All experimental layouts
are depicted in Figure 2.5.

47

(a) Plan view

(b) Vertical view
Figure 2. 3:Experimental Flume, Dike layout: (a) Plan view; (b) Vertical view

(a) Plan view

(b) Vertical view
Figure 2. 4:Experimental flume, Protective Dike layout: (a) Plan view, (b) Vertical view

48

Figure 2. 5:Different setups of the experiment

Styrofoam panels are used to simulate the effects of ice cover on flow dynamics and scour
processes. Two types of ice covers are tested: smooth and rough. Untreated Styrofoam panels
represent the smooth ice cover, mimicking the sheet ice common in natural rivers during winter.
The rough ice cover is created by attaching 2.5 cm³ Styrofoam cubes to the underside of the panels,
replicating the roughness and complexity of ice jams. Sixteen panels, each measuring 1.99 meters
by 2.4 meters, are employed to cover the entire flow surface within the flume (Figure 2.6). This
setup allows for a detailed examination of the impact of ice cover roughness on hydraulic
conditions and sediment transport. The smooth ice cover simulates the uniform surface of natural
sheet ice, while the rough ice cover replicates the irregular surface of ice jams formed by
accumulated ice floes. The distinction between these two ice cover types is crucial for
understanding how different surface roughnesses affect flow dynamics and scour processes. With
its irregular surface, the rough ice cover introduces additional complexity to the flow patterns,
providing valuable insights into how natural ice formations impact river hydraulics.

49

Figure 2. 6:An experimental run under smooth cover condition

2.3 Measurement Equipment
Accurate and continuous data collection is crucial for understanding the interactions
between hydraulic structures, flow dynamics, and scour processes. The SonTek-IQ Plus is a key
instrument used for flow monitoring in the flume. This advanced acoustic Doppler device is
specifically designed for precise flow measurement in natural rivers and controlled environments
such as hydraulic flumes. It features six sensors: four velocity beams and two additional beams for
water level and vertical acoustic measurements. The four-velocity beams are oriented to measure
flow velocities along the longitudinal and latitudinal axes, ensuring comprehensive coverage of
the flow field. This multi-beam setup allows for accurately determining flow velocity profiles,
which is essential for understanding spatial variations in flow speed and direction. The vertical
acoustic beam and the pressure sensor measure water depth accurately, providing critical data for
calculating flow discharge and depth-averaged velocities (SonTek, 2017). The data collected by

50

the instrument are processed using SonTek software, which utilizes advanced post-processing
functions to offer detailed insights into the flow and volume of data. According to the SonTek-IQ
Series User’s Manual, the data collected by the SonTek-IQ Plus has a measurement error margin
of 1% within the specified measurement range (SonTek, 2017).
An Acoustic Doppler Velocimeter (ADV) measures the three-dimensional flow velocities
around the spur dikes and abutments. The ADV is ideal for capturing detailed flow structures and
turbulence characteristics, essential for understanding the dynamics within scour holes and around
the spur dike. This instrument provides high-frequency velocity measurements, which is crucial
for analyzing fine-scale variations in flow speed and direction (Sontek, 1997).
The ADV setup includes a probe with a transmitter and three receivers, creating a sampling
volume located 10 cm beneath the probe head. This configuration allows for precise measurements
of instantaneous velocity components within a cylindrical sampling volume. To ensure data
reliability, the signal-to-noise ratio (SNR) and correlation (COR) are maintained above 15 and
70%, respectively. These parameters are vital for filtering out noise and ensuring the consistency
of the measured velocity data (Wahl, 2000).
Data collected by the ADV are processed using WinADV software, which filters the data
based on SNR and COR values to remove spikes and ensure accuracy. The sampling rate of the
ADV is set at 25 Hz, with measurements taken for 120 to 150 seconds at each location to capture
a representative dataset. The ADV is positioned at various points within the flume, including the
bottoms of scour holes, around the spur dikes, and within the main flow paths, to provide a
comprehensive view of the flow field (Martin et al., 2002; Rehmel, 2007; Wahl, 2000).

51

2.4 Data Collection Procedures
Before each experimental run, several preparatory steps were undertaken to ensure that the
conditions within the flume were consistent and controlled. The surfaces of the sandboxes are
carefully levelled to ensure uniform starting conditions for the experiments. This is crucial for
preventing any initial sediment movement and scour patterns that could arise from uneven
surfaces. For ice cover experiments, Styrofoam panels are gently placed on the bottom of the
flume. These panels simulate natural sheet ice or ice jams, depending on whether smooth or rough
panels are used. These panels are placed carefully to avoid disturbing the sediment bed and to
ensure they float uniformly on the water surface once the flume is filled.
Once the preparatory steps were completed, Water was gradually fed into the flume from
the upstream holding tank to prevent initial scouring around the spur dikes and abutments. This
slow filling process helps maintain the stability of the sediment bed and the ice cover (if applicable)
as the water level rises. The water flow was monitored continuously as it filled the flume. The flow
depth is measured using staff gauges positioned at the midpoint of each sandbox and by using
SonTek-IQ. After achieving the desired flow depth, the valves were adjusted to attain the target
flow rate, ensuring alignment with the planned experimental conditions. Flow characteristics such
as average approach velocity, Reynolds number, and vertical velocity profile were then calculated
and monitored to confirm the development of fully turbulent flow. These steps were essential in
maintaining a steady and consistent flow rate throughout the experiment, which is critical for
accurately studying equilibrium scour processes.
Continuous monitoring during the experimental runs is essential for capturing the dynamic
flow and sediment transport processes. Flow parameters, including depth and velocity, are
continuously monitored using the SonTek-IQ Plus. This device provides real-time data on the flow
52

conditions, ensuring the flow remains within the desired parameters throughout the experiment.
The SonTek-IQ Plus is strategically positioned in front of each sandbox to capture the flow
conditions directly affecting the experimental setups (Figure 2.7). The Acoustic Doppler
Velocimeter (ADV) was used to measure the flow velocities around the spur dikes and abutments.
The ADV is positioned at various locations within the flume to capture a comprehensive dataset
of velocity profiles. Measurements are taken at different heights within the water column and
various distances from the spur dike to provide a detailed view of the flow dynamics. Plexiglass
sidewalls installed on one side of each sandbox allow for detailed visual observation and
monitoring of the scouring process. These visual observations complement the quantitative data
collected by the SonTek-IQ and ADV, providing a holistic view of the experimental conditions.
After each experimental run, several post-experiment procedures are carried out to preserve
and analyze the scour patterns and sediment deposition. Upon completion of the experimental run,
the flume was carefully drained to preserve the scour patterns. The draining process is done
gradually to avoid disturbing the sediment bed and to maintain the integrity of the scour holes and
deposition dunes formed during the experiment. The Styrofoam panels are carefully removed from
the water surface for ice cover experiments. This step is performed cautiously to prevent any
disruption of the scour patterns and sediment deposition. The geometries of the scour holes and
deposition dunes are measured using precision callipers. These measurements are critical for
quantifying the extent and dimensions of the scour hole and sediment deposition, providing
valuable data for subsequent analysis. The data collected during the experimental run were
processed using Surfer Golden Software to generate detailed 3D profiles and contour maps of the
scoured regions. These visualizations help to understand the spatial distribution of scour and
sediment deposition dunes, allowing for a comprehensive analysis of the experimental results.
53

To ensure the consistency and accuracy of the data, supplementary experimental runs are
conducted with extended observation periods of 48 and 72 hours. These extended experiments
confirm that equilibrium conditions are typically achieved within 24 hours, as no significant
variations in scour depth are observed beyond this timeframe. The extended runs also help validate
the robustness of the observed scour patterns and flow dynamics. To further validate the data, some
of the experimental runs are repeated to ensure repeatability and consistency. The repeated
experiments help verify that the observed scour patterns and sediment transport behaviours are
consistent across different runs under the same conditions. This validation step is crucial for
ensuring the experimental results' reliability and building confidence in the conclusions drawn
from the data.
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3

CHAPTER THREE: Velocity Field and Turbulence Structure around Spur Dikes with
Different Angles of Orientation under Ice-Covered Flow Conditions
(Published @ Water (2021)13(13)-1844)
3.1 Introduction
When multiple spur dikes are used, close spacing can provide continuous protection along

a riverbank or around a bridge pier. Spur dikes are extended structures where one end is at the
riverbank, and the other is projected toward the river flow (Kuhnle et al., 1999). These
constructions have been widely used for many purposes, such as river bank protection, flood
control, improvement of a navigational course, control of scour process, landscape improvement,
and ecosystem restoration (Sadat, 2015). Regardless of the different types of spur dikes, they
redirect flow from the river bank and affect the flow regime, flow velocity, sediment
transportation, and, consequently, the scour process (Giglou et al., 2018).
If a spur dike allows the flow to pass through it, that is called permeable. It is impermeable
if the dikes block and then repel the river flow. Since impermeable spur dikes reduce the channel
width, the energy gradient becomes steeper, and flow velocity increases. Consequently, the
riverbed will be eroded. The shape and size of the spur dike will affect the scouring process and
flow conditions (Pandey et al., 2018). Zhang et al. (2005) investigated bed morphology, sediment
distribution, and flow field in a channel with a series of impermeable spur dikes on both sides of
the channel. They claimed that the flow field and local scour around the most upstream pairs of
spur dikes are affected the most. Teraguchi et al. (2008) studied flow field and scour patterns
around spur dikes using laboratory experiments and numerical simulations. They pointed out that
the scour holes on the upstream side of the impermeable spur dikes are larger and deeper than
those of permeable ones. Mizutani et al. (2011) investigated the impact of the spur dike height and
grain size of riverbed material on the morphology and topography around an impermeable spur
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dike. It was found that the maximum scour depth is reduced with the increase of the median particle
size of bed materials. Furthermore, field observations regarding the bed deformation around stonelined spur dikes in the Akashi River indicate that the maximum scoured depth around the
permeable spur dikes is reduced to 40% compared to the impermeable ones (Kanda et al., 2019).
Clearly, the local scour depth around the impermeable spur dikes is much deeper than the
permeable ones.
The spur dikes in channels can be classified as submerged and non-submerged, depending
on the flow conditions and water depth. Normally, the impermeable spur dikes are designed as nonsubmerged spur dikes because the overflow on the top of a dike can form a vertical jet just behind
the spur dike. This vertical jet moves downstream and causes a significant erosion of the downstream
bank along with the body of the spur dike itself (Yossef & Klaassen, 2002). Moreover, in terms of
the orientation angle of a spur dike, defined as the angle between the dike axis and the river flow
direction, spur dikes are divided into three groups: Attracting, deflecting, and repelling dikes. The
attracting spur dike, which points downstream, averts the river flow toward the middle of the channel
and protects the region on its downstream side. Deflecting spur dike is a well-known type that
changes the direction of the river flow by not repelling it; this type is usually used to protect the
banks locally against erosion. They create a turbulent flow, cause more sediment transportation in
the middle of the rivers and make the main channel deeper (Sadat, 2015). Muto et al. (2002)
conducted two experimental studies about the effects of the opening ratio and water depth on the
velocity distributions. Their first study was based on experiments in the Yodo River in Japan using
large-scale particle image velocimetry. The second one was conducted in a laboratory flume with a
downscaled spur dike. They pointed out that the flow was highly unsteady inside the scour hole
based on the experiment in the field. However, laboratory results did not show the same
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characteristics. The authors concluded that the difference in the result of the field experiment from
that of the laboratory was caused by the complex bathymetry of the natural river and lower Reynolds
numbers of flows in the laboratory.
Further research shows that models developed from a steady, uniform flow cannot be
applied to nonuniform flows (Jennifer et al., 2011; Nelson et al., 1995). Duan et al. (2009) construe
that dikes increase shear stress and turbulence intensities at the riverbed by converging water flow,
which initiates the local scour process around the dikes. A two-dimensional (2D) experimental
model has been used to evaluate the effect of turbulence intensities on the entrainment of bed
material (coarse sand and gravel). The instantaneous streamwise velocity has a higher rate than the
instantaneous vertical velocity (Papanicolaou et al., 2001). An experimental study on a threedimensional (3D) flow illuminates that the local scour around the dike causes more complexity of
the flow field (Huai et al., 2009). Dey and Barbhuiya (2006) assessed the turbulent flow field
around a vertical-wall abutment. They reported that the near-bed Reynolds stress played a
significant role in transporting sediment and the scour process.
Many researchers have investigated the turbulence structure in an open channel flow since
it is essential to the flow characteristics and the local scour process around the dike. However,
nearly all reported research has been conducted under the condition of open channel flow. The
impact of an ice cover on the local scour process around the spur dikes has not been investigated.
In cold regions, such as Canada, ice covers appear on rivers’ surfaces and may last six months,
such as the Fraser River's upper and middle reaches. During an ice-covered flow condition, the
river hydrology will be dramatically changed compared to an open flow condition. An ice cover
adds an extra hydraulic boundary to the flow in a river, which leads to considerable changes in the

67

velocity profile, flow rate, bed shear stress distribution, sediment transport, and consequently scour
pattern (Sui et al., 2000).
Under an ice-covered flow condition, the location of the maximum flow velocity gets closer
to the channel bed, which increases the bed shear stress (Namaee & Sui, 2020; Sui et al., 2010; P.
Wu et al., 2014). In conclusion, the presence of ice on the water surface makes the flow condition
much more complicated than an open flow condition. Namely, the existence of an ice cover leads
to an increase in the turbulence kinetic energy at the riverbed. Thus, the Reynolds shear stress
under an ice-covered flow condition differs from that under an open flow condition. This change
affects the incipient motion of bed material, rate of sediment transportation, and sediment
suspension loads, making evaluating (predicting) local scour more complex (Sui et al., 2010). To
our knowledge, there is no research regarding the effect of an ice cover on the local scour process
around the spur dikes. Moreover, measurements of the turbulence flow fields inside scour holes
around the spur dikes have never been conducted under an ice-covered flow condition.
Many factors affect the design of a spur dike, such as the river width and depth, flow
velocity, channel sinuosity, grain size of bed material, sediment transport rate, bank cohesiveness,
the length and shape of a spur dike, the orientation angle of the spur dike to the flow, and
construction materials. Understanding the characteristics of the flow in the vicinity of a spur dike
under an ice cover will help to develop formulas that can accurately estimate the scour depth and
elaborate models for the design of spur dikes. Additionally, it can provide a better understanding
of the turbulence structure around obstacles equivalent to spur dikes, such as boulders. The specific
objectives of the present experimental study are summarized as follows:
•

The impact of an ice cover, including ice cover roughness, on the turbulence intensities,

3D flow fields, shear velocity, and Reynolds stress around the spur dikes.
68

•

The dependence of the scour morphology and turbulence structure around spur dikes on

the orientation angle of the spur dike, ice cover roughness, and hydraulic condition.
3.2 Materials and Methods
3.2.1 Site Description
The present experimental study was conducted in a large-scale outdoor flume in the
Quesnel River Research Center, BC, Canada. The flume was 38.2 m long, 2.0 m wide, and 1.3 m
deep. The longitudinal slope of the flume bed was 0.2%. A holding tank feeds water into the flume
to have a constant discharge throughout each experimental run. The holding tank upstream of the
flume had a volume of 90 cubic meters and was 40 m long, 2 m wide, and 1.3 m deep. Three valves
provided a wide range of flow rates for various experimental runs (Figure 3.1). By adjusting three
valves, the following three different flow rates were generated in this experimental study: 0.055
m3/s, 0.105 m3/s, and 0.12 m3/s.

(a) Plan view

(b) Vertical view
Figure 3. 1:The layout of the experimental flume: (a) Plan view; (b) vertical view

69

Due to the slope of the flume, water levels varied gradually from upstream to downstream.
Therefore, the flow was classified as a nonuniform flow. Another important feature that influences
the flow is the aspect ratio B/H (where B is the channel width and H is the flow depth), which is
used to classify the channel (flume) either as a narrow (B/H < 5) or wide (B/H > 5) channel. A
flow in a narrow channel is affected by the secondary currents from the banks (flume side walls).
It causes the dip phenomena (the maximum velocity occurs below the flow surface). However, in
the wide channel, the strength of secondary currents is reduced in the flume's lateral direction (Sui
et al., 2010). In this experimental study, since the deepest flow depth was 35 cm and the flume
width was 200 cm, all flows in this flume belong to wide channel flows (B/H > 5).
Two sandboxes were spaced 10.2 m from each other. These sandboxes were 2 m wide and
0.3 m deep. The upstream sandbox was 5.6 m long, and the downstream was 5.8 m long (Figure
3.2). Three types of nonuniform sands with a median grain size (D50) of 0.9 mm, 0.6 mm, and 0.48
mm were used in the sandboxes. One side of the flume wall along each sandbox was made of
plexiglass to have a clear view of the scour process. A staff gauge was located in the middle of
each sandbox to measure the water depth during experimental runs. The impermeable model spur
dike, made of marine plywood (had a dimension of 80 cm height, 5 cm width, and 50 cm length),
was installed in the middle of each sandbox. The model dike was placed at the bottom of each
sandbox (30 cm buried in the sand) so that 50 cm of the dike was exposed to the flow. Of note, the
model spur dike was non-submerged for all experimental runs in this study.

70

Figure 3. 2:The view of the original site (experimental flume)

The tailgate at the downstream end of the flume was used to control the water depth and
flow velocity (Figure 3.2). Since the flume was very long with a longitudinal slope of 0.2%, water
depths differed in these two sandboxes for each experimental run, creating a wide range of water
depths for this experimental study. Since the roughness of the ice cover was one of the main factors
affecting the local scour process, two types of model ice cover (smooth and rough) were used.
Styrofoam panels were used to model the smooth cover, while the rough cover was made by
attaching small 2.5 cm Styrofoam cubes to the bottom of the Styrofoam panels. Sixteen panels
with 1.99 × 2.4 m dimensions covered almost the entire flow surface. For the experiments with ice
cover, before each experimental run, the sixteen Styrofoam panels were put side by side on the
flume bed. By opening the valves very slowly, water feeds inside the flume gradually. The
Styrofoam started to float on the surface of the water. There were ropes at the flume's end to keep
the Styrofoam panels floating side by side on the water surface throughout the experiment. Li
71

(2012) proposed the equation for calculating the roughness coefficient of an ice cover. He found
that Manning’s coefficient for ice-covered rivers averages from 0.013 to 0.04. To calculate the
roughness coefficient of the ice cover, a reverse engineering approach was employed. The effect
of the ice cover on the vertical velocity profile and the ice-affected layer was analyzed, and
Equation 3.2 was applied to determine the roughness coefficient, which was found to average 0.02
for the rough ice cover. The roughness coefficient for the smooth ice cover was considered 0.013,
derived from Manning’s value for smooth concrete (Mays, 1999).
(3.1)

k ୱ = 30 y୧ exp[−(1 − v୧ /v୫ୟ୶ )]

where, y୧ is the thickness of the ice-affected layer, v୧ is the average velocity of the ice-affected
layer, v୫ୟ୶ is the maximum velocity of the velocity profile.
(3.2)

ଵ/଺

݊௜ = 0.039 k ୱ

where, ݇௦ is the average roughness height of the ice underside in meters.
3.2.2 Apparatuses for Measurements
There are many different apparatuses or devices for measuring flow velocities, such as
Laser Doppler velocimeter (LDV), Particle Image velocimeter (PIV), Particle Tracking
velocimeter (PTV), Acoustic Doppler velocimeter (ADV), Electro Magnetic velocimeter (EMV),
Pitot Static Tube (PST), etc. (Kadota & Suzuki, 2010; Kuhnle et al., 2008; Weitbrecht et al., 2002).
LDV, ADV, EMV, and PST are all point instruments. Some of them, such as the LDV and PIV,
have limitations in their working range (Telionis et al., 2009). As pointed out by researchers in the
literature, the ADV is accurate for measuring the turbulence properties of flows (Martin et al.,
2002; Rehmel, 2007; Wahl, 2000). Thus, for 3D measurements of instantaneous velocity
components, a 10-MHz SonTek ADV (SonTek-A Xylem Brand, San Diego, CA, USA) was used

72

in this research. The ADV includes one probe (transmitter) and three receivers. It acquires the
instantaneous velocity components by the sampling volume positioned at the intersection of the
transmitted and received acoustic beams located 10 cm beneath the probe head. The sample
volume was a cylinder with a diameter of 0.61 cm and a height of 0.72 cm. The ADV’s sampling
volume is larger than those of the Laser-based velocimetry devices, such as the LDV and PIV
(Duan et al., 2009). The ADV cannot acquire the velocity very close to the flow surface, and the
velocity profiles were not continuous to the top layer of the flow. However, this limitation does
not affect the velocity analysis because the significant change in the velocity distribution occurs
from the mid-water depth toward the flume bed, particularly inside the scour hole.
The ADV measures the scattering particles’ velocities in the flow. Therefore, the accuracy
of the measurements depends on the quality and quantity of particles inside the sampling volume.
Two auxiliary parameters provided in the ADV files, the signal-to-noise ratio (SNR) and the
correlation (COR), should be evaluated to acquire robust data from ADV measurements (Rehmel,
2007). The SNR represents the relative density of the particulate matter in the flow. The COR,
which varies from 0 to 100, indicates the relative consistency of the particle velocity scattering
within the sampling period (Wahl, 2000). According to the user manual for ADV and previous
studies, to achieve the most accurate data in this experimental study, these two parameters were
set as follows: SNR > 15 and COR > 70% (Martin et al., 2002; Rehmel, 2007). The Water
Resources Research Laboratory of the US Bureau of Reclamation developed a software program
known as Win ADV for filtering the ADV data files. This study used Win ADV for data filterings
like SNR and COR. Regarding previous research, to obtain the most accurate data at each
measuring location, the data sampling rate of ADV was set with the highest frequency (25 Hz)
with a duration of 120 to 150 s (Martin et al., 2002; Rehmel, 2007). Following the proper setting
73

of ADV and filtering data spikes, the velocity measurements were reliable within the range of 0.25
cm/s, with an error of ±1.5% of the measurement scale (Namaee & Sui, 2020; Sontek, 1997).
The present study used a SonTek-IQ Plus (SonTek-A Xylem Brand, San Diego, CA, USA)
to measure the approaching flow rate, average velocity, and water depth. There are two types of
SonTek-IQ (standard and plus); the Plus edition contains advanced post-processing functions,
providing deeper insight into approaching flow and volume data, making it very precise and robust
(Manual, 2017). The SonTek-IQ Plus contains six measuring beams (sensors); four are velocity
beams, which monitor the flow velocity along the longitudinal and latitudinal axis to secure the
best possible coverage and most accurate depiction of the velocity field. The remaining two
measuring beams, the pressure sensor and vertical acoustic beams work together to measure the
water level precisely (Manual, 2017). According to the SonTek-IQ Series User’s Manual, the data
collected using the SonTek-IQ Plus is subjected to a 1% error in the range of measurement scale.
The SonTek-IQ Plus was installed on the flume bed in front of each sandbox (the location of the
SonTek-IQ is depicted in Figure 3.1). The flume shape, size, and exact location of the device were
set in the SonTek-IQ software. Then, based on these inputs and using the data collected by sensors,
the software calculates the average flow velocity, water depth, and flow rate during each
experimental run.
3.2.3 Data Collection
Before each experimental run, the surfaces of the sandboxes were levelled. Then, the flume
was slowly filled with water to avoid initial scouring around the spur dikes. The valves opened
completely after reaching the desired water levels, and the experimental run began. The initiation
of the local scour process at the dike tip was observed from the first minute of each run. A scour

74

hole slowly develops and surrounds the upstream side of the dike to the flume wall. Then, as time
passes, the scour hole becomes deeper and wider and extends downstream of the dike.
One of the critical factors affecting the scour process is the time needed to reach the
equilibrium condition. Equilibrium scour is defined as the condition when the dimensions of the
scour hole do not change with time. Zhang et al. (2009) studied the local scour process around a
spur dike and reported that 90% of the equilibrium depth happened about 2 hours after the
experiment started. Other researchers claimed that 80% of the maximum scour depth occurred
during the experiment's first two hours (Vijayasree et al., 2019). Namaee and Sui (2020) conducted
experimental research on local scour around side-by-side piers. They indicate that the equilibrium
depth of a scour hole could be achieved within the first 6 hours, although all their experimental
runs lasted 24 hours. In the present study, no change in scour depth was observed after about 12
hours, and the scour hole reached its equilibrium condition. Therefore, each experimental run
lasted for 24 hours to make sure that the scour process was entirely completed. Some experiments
were conducted for 48 and 72 hours to investigate the time needed to achieve the equilibrium
condition. Results confirm that after 24 hours, there was no change in the scour hole around the
spur dike comparing to those of experiments that lasted for 48 and 72 hours.
After the scour process reached equilibrium, velocity components in all three dimensions
around the dike and inside the scour hole were recorded using a 10-MHZ ADV. Regardless of the
dike layout, water surface condition, and flow depths, the velocity measurements were taken from
the bottom of the scour hole to the water surface with intervals of 1 cm for shallower flow depths
and 2 cm for other flow depths (Figure 3.3).

75

Figure 3. 3:3D Velocity data collection using the 10-MHz SonTek ADV

Results showed that the highest turbulence intensity of the flow occurred in the center of
the scour hole, where the maximum scour depth occurred. Therefore, the ADV locale for the
velocity measurements of all experimental runs was at the deepest part of the scour hole, which
was very close to the tip of the spur dike. For this experimental study, Ux (x-axis) is used to describe
the streamwise velocity in the downstream direction, Uy (y-axis) is used to denote the lateral
velocity in the transverse direction pointing to the left bank, and the vertical velocity is expressed
as Uz in the z-axis (towards the water surface). Under an ice-covered flow condition, a small part
of the Styrofoam panel around the spur dike was cut to place the ADV probe in the flow to acquire
velocity data. After 24 h, all valves were closed completely, and the flume was drained gradually.
Then, the scour pattern was measured (Figure 3.4). Some of the experimental runs were repeated
to validate the recorded data.

76

Figure 3. 4:Plan view of the scour hole

3.3 Results and Discussions
Throughout the scouring process, several horseshoe vortexes were detected inside the scour
hole. This vortex system has been observed in a clockwise direction. Some small bow waves on
the surface of the upstream side of the dike were also observed (Figure 3.5). Since bow waves had
opposite rotation directions to those of horseshoe vortexes, these two eddy systems were
interfering with each other. Results indicate that these interferences become less noticeable with
the increase in water depth. Moreover, decreasing the dike orientation angle inclined to the
downstream direction makes the bow waves smaller and less frequent. Moreover, these waves can
hardly be observed for a dike with an orientation angle of 45° (toward downstream), and their
effect on the horseshoe vortex was negligible.

Figure 3. 5:Development of the vortex system around the spur dike (Ghaderi & Abbasi, 2019)

77

The horseshoe vortexes are created because of the flow separation at the dike tip and the
formation of a powerful downflow at the upstream side of the dike. Consequently, a scour hole
around the spur dike will be developed. The wake vortex system develops behind the spur dike,
which results in the extension of the scour hole downstream of the dike. As pointed out by other
researchers, wake vortexes are smaller and weaker than horseshoe vortexes, and they cannot carry
sediment load that eroded from the scour hole (Melville et al., 2002; Namaee & Sui, 2020). This
fact explains the development of the deposition ridge downstream of the dike (Figure 3.6).

Figure 3. 6:Deposition ridge on the downstream side of the spur dike

Inside the scour hole, horseshoe vortexes were created by the intense turbulence and a high
level of instantaneous velocity fluctuation. Therefore, the bottomless hole that developed around
the dike tip was the one where the horseshoe vortex flow and downflow were stronger. By studying
the 3D velocity inside a scour hole, the effects of different flow conditions and the dike setups on
the local scour pattern should be assessed.
The relationships between different parameters should be expressed using dimensionless
variables to effectively translate laboratory findings to practical applications, such as rivers, spur
dikes, and bridge piers. Furthermore, normalizing variables (making them dimensionless) will
78

enable us to compare results under different experiment conditions. Therefore, in all figures in the
present study, both velocity components and water depth (h, the vertical distance at which 3D
velocity data was collected) were normalized by average approaching flow velocity (U) and total
water depth in the sandbox (H), respectively.
3.3.1 Streamwise Velocity Component (Ux)
Among all 3D velocity components, the streamwise velocity (Ux) plays a crucial role in
developing the scour hole and the turbulence structure. The streamwise velocity has the maximum
value and highest fluctuation compared to the other two velocity components (Uy, Uz). Results
show that regardless of the flow conditions and the orientation angle of the dike, velocity
distributions inside the scour hole are less regular (or completely irregular). Moreover, the
magnitudes of the streamwise velocity components inside the scour hole are smaller than those
outside. However, in most cases, the highest level of fluctuation and the maximum velocity
magnitude occur in the center of a scour hole, which is close to the dike tip.
In terms of the boundary conditions for the water surface, there were three different types:
open flow, smooth covered condition, and rough covered condition. As indicated in Figure 3.7,
the streamwise velocity (Ux) inside the scour hole is minimal at the bottom of the scour hole and
increases with the distance from the scour hole bottom. The maximum streamwise velocity
happens at the mid-water depth, and the streamwise velocity profile shows a convex shape. Of
note is that the ADV measuring volume is located at 0.10 m from the probe head. Due to this
limitation, the velocity profile cannot fully cover up to the water surface, as shown in Figure 3.7.
Moreover, when a high level of turbulence existed at the measuring point using the ADV, the
Doppler noise often appeared (McLelland & Nicholas, 2000). These noises decrease with the data
collection process and create data spikes. Moreover, sediment movement near the scour hole bed
79

interferes with data collection during the ADV measurements (Muste et al., 2000). By using Win
ADV software, these ambiguous data were filtered. Therefore, velocity profiles can sometimes not
cover the bottom of the hole.

Figure 3. 7: ux under different surface cover conditions, spur dike with an orientation angle of 90°

Regardless of the orientation angle of the dike and flow properties, under an ice-covered
flow condition, the maximum velocity is located at the mid-depth of water. By increasing the
cover's roughness coefficient, the maximum velocity's location is further shifted toward the
channel bed. Additionally, cover conditions not only influence the location of Umax but also affect
the magnitude of the velocity (Figure 3.7). Results indicate that rough ice cover can increase
streamwise velocity values by nearly 25%. These findings agree with previous studies (Namaee &
Sui, 2020; Sui et al., 2010; Sui et al., 2000). One can conclude that regardless of the shape and
location of the barriers in an ice-covered river, the locale and magnitude of the maximum velocity
depend on the features of an ice cover.
Under an ice-covered flow condition, the streamwise velocity (Ux) depends on the flow
properties, such as water depth, cover roughness, and approaching velocity. In the present study,
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velocity profiles have been evaluated for all experiments. The flow Froud number (Fr) is one of
the most essential dimensionless parameters, and the effect of Fr on Ux was examined.

‫ܨ‬௥ =

ܷ

(3.3)

ඥ݃‫ܪ‬

where, U is the average approaching velocity, ݃ is the gravitational acceleration, and H is the water
depth.
The result indicates that with the decrease in the flow Froud number, the streamwise
velocity distributions under different surface conditions (open channel, smooth cover, and rough
cover) become closer, especially for velocity distributions under both open channel and smooth
covered flow conditions. Figures 3.7 and 3.8 show that the Froud numbers for water depths of 14
cm, 24.5 cm, and 35 cm are 0.19, 0.15, and 0.10, respectively. The effect of ice cover condition on
the velocity distributions appears to intensify with the increase in the flow Froud number.
Moreover, as the dike orientation angle decreases from 90° to 45°, this effect becomes more
tangible (Figures 3.7 and 3.8).

Figure 3. 8: ux inside and outside the scour holes under different surface conditions with a 45° dike

81

As the dike orientation angle decreases, the magnitude of the streamwise velocity
diminishes, and the scour hole becomes smaller. As indicated in Figures 9 and 10, regardless of
the water depth and surface cover conditions, decreasing the dike orientation angle from 90° to
45° shifts the velocity profiles upward, implying a decrease in the scour hole depth. These
findings are consistent with the continuity theory (Equation (3.4)). Since the spur dike reduces
the cross-section area of the flow, the velocity should increase. As the dike orientation angle
decreases, the cross-section area increases, and velocity decreases.
(3.4)

(ߩ޿ܷ)௜௡௟௘௧ = (ߩ޿ܷ)௢௨௧௟௘௧
where, ߩ is the mass density of water, ޿ is the cross-section area of the flow, and ܷ is the
average approaching velocity.

Figure 3. 9: ux inside and outside the scour holes for different dike orientation angles (water depth: 31 cm)

82

Figure 3. 10: uy inside and outside the scour holes under different surface conditions with a 90° dike

With the decrease in the dike orientation angle (from 90° to 45°), the blockage ratio of the
flow cross-section decreases. Therefore, both the downflow and horseshoe vortex become
weaker. This leads to the formation of a smaller scour hole. Of note, these effects are
independent of the surface cover condition, water depth, and flow rate (Figure 3.9).
3.3.2 Lateral Velocity Component (Uy)
When multiple spur dikes are used, close spacing can provide continuous protection along
a riverbank or around a bridge pier. Two significant characteristics of the lateral velocity
component (Uy) are their irregularity and much smaller values than the streamwise velocity
component (Ux). After scrutinizing Uy profiles (under different conditions of surface cover, flow
rate, water depth, and dike orientation angle), results reveal that the vertical distribution of the
lateral velocity component was nonmonotonic. Compared to the streamwise velocity component
(Ux), the lateral velocity component (Uy) is considerably smaller and primarily positive. Some
non-negligible effects have been observed in the vertical distribution profiles of the lateral velocity
component (Uy). It is noted that the presence of an ice cover increases the magnitude of the lateral
83

velocity regardless of the flow rate, water depth, and dike orientation angle. With the increase in
the cover roughness coefficient, the value of Uy increases (Figure 3.10). This result indicates that
the ice cover plays an essential role in the value of Uy.
To determine the effects of the dike orientation angle on Uy, the vertical distribution
profiles of the lateral velocity component (Uy) were evaluated. As indicated in Figure 11, with the
decrease in the dike orientation angle, the magnitude of Uy decreases, similar to the effect of the
dike orientation angle on the streamwise velocity. Furthermore, results indicate that only for the
dike orientation angle of 90°, a semi-consistent pattern has been noticed for the vertical distribution
profiles of the lateral velocity component (Figure 3.11). No clear or meaningful trend was detected
for the dike orientation angles of 60° and 45° (despite different flow rates, surface conditions, and
bed materials). Nevertheless, in all cases, the vertical distribution profiles of the lateral velocity
component (Uy) were positive inside and outside the scour holes with an equivocal pattern. Thus,
the locales of the maximum lateral velocity component in the vertical distribution profiles remain
unknown (Figures 3.10 and 3.11).

Figure 3. 11: uy inside and outside the scour holes under different surface conditions, water depth: 35 cm

84

3.3.3 Vertical Velocity Component (Uz)
To evaluate the vertical velocity component (Uz) under different experimental conditions,
its vertical distribution in the center of the scour hole (the location of the maximum scour depth)
has been examined. Results show that the values of the vertical velocity component are mostly
negative inside and outside the scour holes (Figure 3.12). These negative values signify the
existence of the powerful downflow around the dike, created due to the obstruction of the flow by
the spur dike. This downflow plays an essential role in the local scour process because the
downward velocity increases and strengthens the horseshoe vortex system, which leads to a deeper
scour hole. This fact is consistent with the local scour morphology around the spur dike. As
illustrated in Figure 3.13, for the dike perpendicular to the flume wall (with an orientation angle
of 90°), the scour hole is the deepest compared to those of dikes with other orientation angles (60°
and 45°). Thus, it can be concluded that by increasing the blockage ratio of the obstacle (such as
piers and spur dikes), downward velocity will increase and create a more powerful vertical velocity
component inside the scour hole.

Figure 3. 12: uz inside and outside the scour holes under different flow conditions with a dike of 90°

85

Figure 3. 13:Scour hole profiles around the dike with different angles (water depth: 11 cm, open channel flow)

Considering the absolute value of the vertical velocity, the Uz values reach their minimum
at the bottom of the scour hole and increase with the distance from the bottom. The maximum
value of Uz occurs around the initial level of the sand bed (before the scouring process starts), and
then it reduces toward the flow surface. Uz becomes very small (close to zero) close to the water
surface, and in some cases, it turns positive. The vertical velocity distribution shows a parabolic
shape (Figure 3.12). Results show that vertical velocity vectors change their direction near the flow
surface.
As shown in Figure 3.12, the ice cover significantly impacts the value of Uz. The presence
of an ice cover on the surface of the flow results in apparent changes in the shape of the vertical
velocity distribution. One can also notice that under open flow conditions, the minimum values of
Uz are always negative. However, by adding an ice cover to the flow surface, the minimum values
of Uz are closer to zero and are positive in some cases. Furthermore, the absolute magnitude of the
Uz increases with the increase in the roughness coefficient of the cover. Therefore, it can be
concluded that under an ice-covered flow condition, the vertical velocity component (Uz) is the
most critical velocity component for developing scour holes compared to other velocity
86

components (Ux and Uy). Moreover, results indicate that the presence of an ice cover influences
the locale of the maximum vertical velocity. As the ice cover becomes rougher, the location of the
maximum vertical velocity moves closer toward the channel bed. However, compared to the
streamwise velocity component (Ux), this effect on the location of the maximum vertical velocity
component is less noticeable (Figures 3.7 and 3.12).
Results reveal that by increasing the flow rate, the value of Uz increases, and velocity
profiles shift toward the water surface. The water depths are 14 cm, 24.5 cm, and 35 cm, and the
average approaching velocity (U) is 21, 24, and 17 cm/s, respectively. One can see that the
differences in the average approaching velocity are small. However, as shown in Figure 3.14, the
difference in the vertical velocity distribution is remarkable. Therefore, one can say that the flow
rate is one important factor responsible for the change in the vertical velocity component (Uz)
profile. Results also showed that the maximum value of Uz under rough-covered flow conditions
is higher than that under open and smooth-covered flow conditions.

Figure 3. 14: uz inside and outside the scour holes under different flow rates with a dike of 90°

87

Further studies have been conducted to assess the impact of the dike orientation angle on
the Uz distributions. Results reveal that by reducing the blockage ratio of the dike (namely, by
decreasing the dike orientation angle), the effect of the ice cover on the vertical velocity profile
became less noticeable. The presence of an ice cover on the flow surface leads to a considerable
change in the shape of the Uz profile. However, by reducing the dike orientation angle from 90°
to 45°, the vertical velocity profiles for a flow under an open channel become more similar to that
under an ice-covered flow condition. Likewise, by reducing the dike orientation angle, the absolute
value of Uz decreases (Figure 3.15). Results indicated that when the dike orientation angle is
reduced, the vertical velocity component will be significantly affected in two ways: (1) the effect
of an ice cover will be counteracted, and (2) the downward velocity vectors will be weakened. Of
note, this effect is more noticeable regarding scour hole depth than the surface area.

Figure 3. 15: uz inside and outside the scour holes under different surface conditions; water depth: 21.5 cm

3.3.4 Turbulence Intensities and Reynolds Shear Stress
It is noted that inside the scour holes, the flow is a combination of the downflow and
horseshoe vortexes, which cause a complex turbulence structure. Turbulent eddies generate
88

velocity fluctuations, referred to as turbulence strength (urms). urms was defined as the standard
deviation (root mean square (RMS)) of the instantaneous velocity fluctuations. As Equation (3.5)
described, urms were calculated based on 3D velocity components measured by ADV. A larger urms
signifies a higher level of turbulence.

ே

1
‫ݑ‬௥௠௦ = ඩ ෍(‫ݑ‬௜’ )ଶ
ܰ

(3.5)

௜ୀଵ

where, urms is the root-mean-square of the turbulent velocity fluctuations, ui´ is the instantaneous
fluctuations of velocity components.
Because turbulent bursts are the main mechanism that entrains sediment and initiates the
scouring process (Jennifer et al., 2011; Sui et al., 2010), the distribution patterns of the turbulence
strength of 3D velocity components (ux´, uy´, uz´) have been examined. To accurately assess
different instantaneous velocity fluctuations, the turbulence strength has been normalized by the
average approach velocity (U). Results reveal that in most cases, the highest level of fluctuation
occurs in the center of the scour holes (very close to the dike tip) and the exit of the hole (flume
bed) due to the exposure of a larger flow field. As indicated in Figure 3.16, the streamwise
turbulence intensity (ux´) is generally the largest, and the vertical turbulence intensity is the least,
namely, ux´ > uy´ > uz´. Moreover, the streamwise and vertical turbulence intensity generally
followed a distinct pattern—they are small (close to zero) at the scour hole bottom and increase
with the distance from the scour hole bottom. Usually, ux´ and uz´ become slightly above the scour
hole (flume bed) and decrease toward the water surface, creating the reverse C shape profiles.
Nonetheless, no consistent trend has been observed for the lateral turbulence intensity (Figure
3.16).

89

Figure 3. 16:Turbulence intensity in three directions with the 90° dike (water depth: 35 cm)

In terms of the impact of an ice cover on the turbulence strength, results indicate that the
presence of an ice cover increases the turbulence intensities. With the increase in the roughness
coefficient of an ice cover, the maximum fluctuation amount increases (Figure 3.16). Additionally, the
approaching velocity plays an important role in the turbulence intensity. Regardless of the surface
cover condition or the dike orientation angle, the maximum instantaneous velocity fluctuations
happen when the approaching velocity is the highest. Experiments clearly show that with a high
approaching velocity, the maximum depth of the scour hole will be reached in a shorter amount of
time. Moreover, the scour hole becomes larger with a higher approaching velocity.
The local scour process starts with the increase in the shear stress resulting from the
accelerating flow around the spur dike. The shear stress refers to the Reynolds stress (߬), which
can be illuminated as the transport of the streamwise momentum through a surface normal to the
z-axis. Based on instantaneous velocity fluctuations collected using the ADV, the Reynolds shear
stress can be calculated (Equation (3.6)) (Zhang et al., 2009). The Reynolds stress plays a key role
in the entrainment and movement of sediment particles.
90

(3.6)

߬ = −ߩ < ‫ݑ‬௫´ ‫ݑ‬௭´ >

where, ߬ is the Reynolds stress, ߩ is the mass density of water, ‫ݑ‬௫´ is the streamwise turbulence
strength and ‫ݑ‬௭´ is the vertical turbulence strength.
To better understand the Reynolds stress distribution inside scour holes, the Reynolds stress
values were normalized by the shear velocity (friction velocity) ‫∗ݑ‬ଶ . The Shear velocity was
calculated based on the boundary layer characteristic method (BLCM) (Afzalimehr et al., 2019),
as illustrated in Equation (3.7).
ఛ
ఛ್

=

ିழ ௨ೣ´ ௨೥´ வ

(3.7)

௨మ∗

where, ߬ is the Reynolds stress, ‫ݑ‬௫´ is the streamwise turbulence strength, ‫ݑ‬௭´ is the vertical
turbulence strength, ߬ ௕ is the bed shear stress, and ‫ ∗ݑ‬is the shear velocity or friction velocity.
‫= ∗ݑ‬

(ߜ∗ − ߠ) ‫ݑ‬௠௔௫
‫∗ߜܥ‬

(3.8)

where, ߜ∗ is the displacement thickness, ߠ is the momentum thickness, which is defined by
(Schlichting & Gersten, 2016), ‫ ܥ‬is a constant which was estimated as 4.4 for Canadian rivers
(Afzalimehr & Rennie, 2009).
As shown in Figure 3.17, the Reynolds stress is zero at the scour hole bottom and gradually
decreases to the least negative value, and it becomes zero again in the middle of the scour holes;
then, it increases with the distance from the scour hole bottom. Afterward, it decreases toward the
water surface. Inside the scour hole, the distribution of the Reynolds stress has a parabolic shape.
Outside the scour hole, the Reynolds stress value becomes positive, reaching its maximum slightly
above the initial level of the sand bed (flume bed). Then, it reduces again toward the flow surface
and becomes negative close to the surface, creating a convex shape distribution (Figure 3.17).
91

Figure 3. 17:Reynolds stress, under different surface conditions for the dike with 90° (water depth: 31 cm)

The negative Reynolds stress inside the scour hole demonstrates an upward vertical
momentum transport caused by a negative velocity gradient (

ௗ௨
ௗ௭

< 0). Similarly, the negative

Reynolds stress close to the flow surface indicates the impact of the adverse pressure gradient at the
upper portion of the flow, which also has a negative velocity gradient.
Results show that the value of Reynolds stress under an ice-covered flow condition is
greater than that under an open flow condition. With an ice cover's roughness increase, the
Reynolds stress's absolute value rises (Figure 3.17). It can be concluded that higher shear stress at
the sand bed will be generated under the ice-covered flow conditions. This increase in shear stress
leads to more sediment movement around the dike, creating a larger scour hole.
3.4 Conclusions
The three-dimensional velocity components and turbulence structure inside and outside
scour holes around the spur dikes in a large-scale flume have been investigated. The model spur
dikes are impermeable and non-submerged, with orientation angles of 90°, 60°, and 45°,
92

respectively. Experiments have been conducted under open channel, smooth-covered, and roughcovered flow conditions to comprehensively study flow characteristics near a dike. Based on data
collected from laboratory experiments, the impacts of the spur dike on the 3D velocity
distributions, Reynolds shear stress, and turbulence intensities have been investigated. Overall, the
following results were obtained in this experimental study.
The presence of an ice cover on the water surface causes a considerable change in the bed
shear stress. It raises the turbulence intensities inside the scour holes, significantly affecting
sediment transportation. To be more specific, the presence of an ice cover increases the maximum
values of the 3D velocity components average by 10% to 25% for smooth and rough ice cover,
respectively. The rough ice cover shifts the location of the maximum velocity further close to the
sandbed, which leads to the increase in the Reynolds shear stress inside the scour hole and,
accordingly, results in a deeper scour hole. These effects are independent of flow rates and dike
orientation angle.
The dike with an orientation angle of 90° generates the strongest downflow around the dike
compared to those resulting from the orientation angles of 45° and 60°. Thus, the dike with an
orientation angle of 90° creates high turbulence and powerful horseshoe vortexes inside the scour
holes generate the deepest scour hole compared to those around the dike with the smaller
orientation angles. Results show that reducing the dike orientation angle from 90° to 45° shifts the
velocity profiles upward, and scour hole depth decreases by 5–10% for each 10° angle decrease.
These results imply that changing the dike orientation angle will reduce the blockage ratio of the
cross-section area. Consequently, the vortex system in the vicinity of the spur dike will become
weak. Thus, the turbulence kinetic energy inside the scour hole is reduced.

93

Generally, the 3D velocity distributions are less regular inside the scour holes. Besides, the
velocity components inside the hole are smaller than those outside the scour holes. The highest
velocity fluctuation and turbulence intensity level appear immediately above the scour holes. The
maximum turbulence intensity increases with the increase in the roughness coefficient of an ice
cover. The smooth and rough ice cover raised turbulence intensity averages by 15% and 30%,
respectively. Moreover, the instantaneous velocity fluctuation increases with the increases in the
approaching velocity. The higher the flow velocity, the more influential the turbulence kinetic
energy around the spur dike, and thus, the deeper the scour holes.
The streamwise velocity (Ux) is the highest among all 3D velocity components, implying
that Ux contributes more to the turbulence intensities, Reynolds shear stress, and, consequently,
the development of the scour holes. The lateral velocity component (Uy) has the highest level of
irregularities inside and outside the scour hole. Unlike the streamwise and vertical velocity (Uz)
distributions, no meaningful trend has been observed for the lateral velocity component. Moreover,
the presence of an ice cover on the water surface considerably affects the lateral velocity
component. The lateral velocity increased with the increase in the roughness coefficient of an ice
cover.
Both the streamwise velocity component and lateral velocity component are almost always
positive. However, the vertical velocity component is almost negative inside and outside the scour
holes. The negative vertical velocity components indicate the powerful downflow and downward
velocity near the dike. The absolute value of Uz increases proportionately with the approaching
flow velocity. The higher the approaching velocity, the more the vertical velocity component
absolute value. This effect has been intensified with the increase in the roughness coefficient of an
ice cover. Under an ice-covered flow condition, the distribution pattern of Uz differs entirely from
94

that under an open flow condition. Moreover, the maximum vertical velocity was observed when
the dike had an orientation angle of 90°. With an increase in the dike angle of each 10° (from 45°
to 90°), the absolute value of vertical velocity relatively increases by up to almost 10%, implying
that the dike orientation angle is one of the important controlling factors for Uz.
The Reynolds shear stress is negative inside the scour holes and becomes positive toward
the flow surface. It reaches its maximum slightly above the scour holes. The negative values of the
Reynolds stress are caused by the upward vertical momentum transport generated by a negative
velocity gradient. Additionally, with the increase in the roughness coefficient of an ice cover, the
absolute value of the Reynolds stress increases. It can be concluded that the presence of an ice
cover creates more powerful shear stress at the sand bed, which causes a deeper scour hole.
3.5

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97

4 CHAPTER FOUR: Channel deformation around non-submerged spur dikes with
different alignment angles under ice cover
(Accepted for publication by the Journal of Hydrology and Hydromechanics)

4.1

Introduction
The flowing water in rivers and streams plays a pivotal role in sustaining human life while

serving as a habitat for various aquatic vegetation and animals. Human activities have modified
river flows by constructing various channels and structures (Giglou et al., 2018). The in-stream
infrastructures, such as spur dikes and bridge piers, significantly affect flow characteristics and
riverbed deformation. The flow characteristics in the presence of in-stream infrastructures have
been investigated by many researchers (Namaee et al., 2021; Wu, Hirshfield, Sui, et al., 2014).
Spur dikes are one of the in-stream infrastructures widely used in river engineering and
water ecological restoration projects for various purposes such as bank protection, the maintenance
of navigation channels, flood control, improvement of the habitat zone, and ecosystem
rehabilitation. Spur dikes are hydraulic structures extending to the river flow from the channel
bank and can be built from various materials such as riprap, concrete, and steel structures. A spur
dike is described as permeable if river flow can pass through it, and it is impermeable if it
completely blocks the flow (Pandey et al., 2018). A permeable spur dike usually slows down
sediment transportation by reducing the velocity on its upstream side, thus reducing the scour
process (Gu et al., 2011). In contrast, an impermeable spur dike can benefit riverbank protection
and flood control. An impermeable spur dike protects the riverbank from erosion by repelling a
turbulent flow in some parts of the river. In the event of a flood, by reducing the flow energy, an
impermeable spur dike redirects and controls the rapid flows (Teraguchi et al., 2008; Zhang &
Nakagawa, 2008). Spur dikes significantly change the flow patterns and bed morphology due to
the decrease in the channel width and formation of a contraction section. An obvious flow
98

separation and recirculation are generated in the vicinity of a spur dike, which raises the turbulence
kinetic energy and shear velocity surrounding the spur dike and thus causes the extreme scouring
process of the river bed (Yazdi et al., 2010).
The erosion process around a spur dike is one of the most critical issues in fluvial hydraulics
and has been investigated to predict the geometry of the scour holes (Giglou et al., 2018). Derrick
et al. (1989) studied spur dikes and pile dikes that are widely used in large river development
projects. Some research focuses on the sediment deposition pattern and velocity structures behind
the permeable spur dike (Tominaga et al., 2000). Ishigaki et al. (2000) claimed that the vortex
development occurs aside from the dike, and the deepest hole is created at some distance from the
obstacle. Also, either utilizing laboratory experiments or computational simulations, some
research works have been conducted to investigate flow structures and sediment transport in the
presence of different obstacles in channels such as spur dikes, bridge piers and abutments
(Shahmohammadi et al., 2021; Wu et al., 2016).
As (Kuhnle et al., 2002) pointed out, a dike's flow depth and orientation angle affect the
maximum erosion depth and the scour hole volume. Results showed that the ratio of the scour
volume to the maximum erosion depth is variable and rises with the decrease in flow depth in
shallow rivers and demonstrated that the blockage ratio significantly affected the geometric
characteristics of the scour hole (Zhang et al., 2018). Ishigaki and Baba (2004) found that the
downward-angled spur dike (oriented at a downstream angle) was less effective than the deflecting
one (intended to deflect the flow rather than completely block it) in protecting channel banks from
erosion. Most reported research works are about calculating the maximum scouring depth instead
of the scour hole geometry (Kuhnle et al., 2002). Based on the morphological dimensions of scour
holes, Haltigin et al. (2007) predicted the geometric features of scour holes.

99

In cold regions, such as northern Canada, it is pervasive for rivers to be covered by ice in
winter. The presence of an ice cover emerges as an essential factor that significantly influences the
morphology and scouring patterns around spur dikes (Namaee & Sui, 2019b; Peng et al., 2015).
The appearance of an ice cover on the water surface enforces different geomorphological situations
to flow characteristics, sediment transportation, and velocity distribution (Wu, Hirshfield, Sui, et
al., 2014). al. (2002) stated that an ice cover on the water surface could increase the scour hole
depth up to 35%. Sui et al. (2010) pointed out that the location of the maximum streamwise
velocity depends on the ratio of roughness coefficients of the ice cover to that of the channel bed
and will be shifted toward the smoother boundary. This change in the streamwise velocity
contributes more kinetic energy to the riverbed and increases the bed shear stress (h. Wang et al.,
2008). Therefore, the roughness coefficients of the ice cover and channel bed play an essential role
in the scour morphology under an ice-covered flow condition (Ashton, 1986; Smith & Ettema,
1995). Zabilansky et al. (2006) claimed that the roughness of an ice cover is significant for the
deformation of the channel bed. Hains (2004) concluded that the roughness of an ice cover
considerably affects the bed shear stress, and the increase in the bed shear stress will result in
amplified bed erosion and scour depth.
Numerous numerical simulations and laboratory experiments have been conducted to study
the bed morphology and local scour around spur dikes. However, to the authors' knowledge, no
research has been reported to investigate the effect of an ice cover on the geometrical features of
channel bed deformation around non-submerged spur dikes with different alignment angles. The
current research is based on laboratory experiments to investigate the geometric characteristics of
channel bed deformation around spur dikes, considering different alignment angles under smooth
and rough ice-covered flow conditions.

100

4.2

Methodology and Materials

4.2.1

Experimental setup and equipment for measurements

Experimental investigations typically occur in compact indoor hydraulic flumes due to the
higher costs of constructing larger outdoor flumes. Smaller-scale flumes often have limitations in
flow depths and discharge ranges, and considerations must be made regarding the impact of
sidewalls within the flume. On the other hand, larger flumes offer a wider range of options for
selecting hydraulic parameters and can be considered approximations of small natural rivers.
Conducting simulations of scour phenomena in larger hydraulic flumes is expected to yield more
accurate results (P Wu et al., 2015).
This experimental investigation was conducted in a large-scale outdoor flume at the
Quesnel River Research Center in Canada. As depicted in Figure 4.1, this flume is 38.2 m long,
2.0 m wide, and 1.3 m deep. The longitudinal slope of the flume bed is 0.2%. A water holding tank
with a capacity of 90 m3 and dimension of 40.0 m  2.0m  1.3 m is located upstream of the flume
and used to maintain a designated discharge for each experimental run. The tank is supplied by
three standpipes, facilitating a maximum flow rate of 0.12 m3/s. Notably, a gradual change in water
level was observed along the flume section from upstream to downstream, attributed to the flume's
slope. Consequently, the flow was classified as non-uniform. A roof covering the entire flume
section was built to mitigate the influence of environmental factors, including precipitation, wind,
and tree leaves.
Two sandboxes were meticulously constructed within the flume, with a spacing distance
of 10.2 meters. The upstream sandbox is 5.6 m long, 2.0 m wide, and 0.3 m deep, while the
downstream one spans 5.8 meters in length, 2.0 meters in width, and 0.3 meters in depth. Notably,
one sidewall of each sandbox section is made of plexiglass, facilitating precise observation of the
101

scouring process. This study selected three types of non-uniform natural sediments with median
diameters (D50) of 0.48 mm, 0.60 mm, and 0.90 mm with standard deviations of 1.97, 2.39, and
1.41, respectively, as the bed material. Moreover, the average sediment density of these three types
of sediments was 1.7 (g/cm3).

(a) Overview of the flume

(b) Plan view

(c) Vertical view
Figure 4. 1:Flume for experiments: (a) Overview of the flume, (b) Plan view, (c) Vertical view

102

A staff gauge was installed in the middle of each sandbox to monitor the flow depth. The
tailgate located at the downstream end of the flume was used to control the flow depth and,
consequently, the flow velocity within the channel. The mean approaching flow velocity ranged
from 15 cm/s to 30 cm/s. The model spur dike is made of impermeable marine plywood, standing
at a total height of 80 cm, with a width of 5 cm and a length of 50 cm. Positioned centrally within
each sandbox, the dike was partially submerged, with 30 cm of its height buried beneath the sand
bed while the remaining 50 cm protruded above the initial bed surface (Figure 4.1). For each inflow
and cover condition, the model spur dike was placed at three alignment angles (inclined to the
downstream sidewall): 90º, 60º, and 45º. Consequently, the dike positioned perpendicular to the
flume sidewall exhibited the highest blockage ratio at 25%, and the blockage ratio for the dike
alignment angles of 60º and 45º reduced to 20% and 15%, respectively.
Styrofoam panels, 1.99 m wide and 2.4 m long, were utilized to replicate the ice cover
floating on the water surface. Two distinct types of model ice covers were used to assess the impact
of the ice cover roughness coefficient on channel bed deformation, namely smooth and rough ice
covers. The model smooth ice cover mimics the sheet ice typical of natural rivers, while the model
rough ice cover replicates an ice jam formed by accumulated ice floes. The smooth ice cover was
modelled using untreated Styrofoam panels. To create the model rough ice cover, numerous
Styrofoam cubes with the dimensions of 2.5 cm × 2.5 cm × 2.5 cm were affixed to the bottom
surface of the Styrofoam panels in a staggered arrangement, with a spacing distance of 35mm
between the adjacent cubes. For experiments conducted under ice-covered flow conditions, the
entire flow surface in the flume was covered by the model ice cover (Figure 4.2).
A SonTek-IQ Plus, an optimum apparatus for flow measurement in natural rivers, was used
to measure the approaching flow velocity, water depth, and incoming discharge into the flume.

103

The data collection process of the IQ Plus is exposed to an error of 1% in the range of measurement
scale (Manual, 2017). An IQ Plus was fixed to the flume bed in front of each sandbox.

Figure 4. 2:A cover condition experiment under smooth ice cover with a 45º inclination angle

An Acoustic Doppler velocimeter (ADV) was used to measure the flow velocities. To
ensure reliable measurement data using the ADV, assessing both the signal-to-noise ratio (SNR)
and the correlation (COR) is imperative. According to the user manual for ADV, these two
parameters were set as follows: SNR > 15 and COR >70% to achieve the most accurate data in
this experimental study. The software program WinADV was used to filter the ADV data files,
such as SNR and COR. To obtain the most accurate data at each measuring location, the data
sampling rate of ADV was set with the highest frequency (25 Hz) for 120 to 150 seconds. After
configuring the ADV appropriately and implementing data spike filtering, velocity measurements
in the current study were reliable within the range of 0.25 cm/s, with an error margin of ±1.5% of
the measurement scale.

104

4.2.2

Data Acquisition Procedures

Experimental runs have been carried out in the same flume on local scour around bridge
piers and abutments under ice-covered flow conditions (Namaee & Sui, 2020; Namaee et al., 2021;
Wu, Hirshfield, & Sui, 2014; Wu, Hirshfield, Sui, et al., 2014; P Wu et al., 2015). Time is one of
the essential factors in studying channel bed deformation around in-stream infrastructures such as
spur dikes. The scouring process is complete when the scour hole reaches its equilibrium condition,
such that the dimensions of scour holes and deposition dunes remain constant over time. It was
found that the quasi-equilibrium scour process can happen within the first 6 hours of the
experiment, and there was hardly a scour hole pattern after 12 hours (M. R. Namaee & J. Sui,
2019). Thus, the duration of all experimental runs in this study was 24 hours, which ensured an
equilibrium scour condition was achieved. In this study, the following procedures for data
acquisition were followed:
1) Before initiating each experimental run, verification of the fullness of the upstream water
holding tank is essential. Subsequently, the initial step involves levelling each sandbox's
entire surface. Following this, all Styrofoam panels (or model ice cover) are systematically
and carefully arranged on the flume bed to facilitate experiments under ice-covered flow
conditions.
2) Then, the valves were slowly opened to allow the gradual inflow of water from the
upstream holding tank into the flume, preventing initial erosion at the spur dikes. The flow
depth in the flume increased gradually, and concurrently, the model ice cover began to float
on the water surface. Once the flow attained the desired depth, the valves were fully opened
to achieve the required flow rate. Throughout each experimental run, monitoring and
calculating water level, flow rate, and approaching velocity were conducted using IQ Plus.

105

In the meantime, the flow depth was also monitored using a staff gauge located at the
midpoint of the sandboxes.
3) The experiment was completed after 24 hours. Then, ADV was carefully positioned within
the flow to measure the velocity field around the spur dikes at diverse locations.
After all required measurements for flow velocity were completed, all valves were turned
off. Then, the model ice cover was carefully removed from the water surface. The flume slowly
drained by lowering the tailgate to avoid disturbance of the deformation patterns of the sand bed.
The geometry of scour holes and deposition dunes around spur dikes was carefully measured using
a calliper. The Surfer Golden Software was used to process all measured data for the geometry of
scour holes and deposition dunes to generate 3D plotting of the profile and contour lines for scour
holes and deposition dunes.
4.3

Results
Channel bed scour initiates when shear stresses around an obstacle, such as a spur dike,

exceed the critical shear stress of riverbed material. In the early stages, intense scouring
predominantly occurs at the dike toe, leading to rapid development and deepening of the scour
hole. However, as time progresses, typically after approximately 2 hours, the scouring process
gradually slows down as bed shear stresses diminish. Eventually, when shear stresses become
insufficient to displace significant bed material, the scouring process reaches an equilibrium state,
indicating the maximum scour depth (Richardson & Davis, 1995). In channels with finer bed
material, smaller flow depths and higher flow velocities tend to result in larger maximum scour
hole depths. This phenomenon occurs because higher velocities generate stronger downflow on
the upstream side of the dike, leading to intensified horseshoe vortex activity, a primary scouring
mechanism.
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Moreover, as flow depths decrease while maintaining a constant flow velocity, downflow
and vortex activity within the scour hole are enhanced. This relationship is further amplified by an
increase in the flow Froude number, which elevates turbulence kinetic energy within the scour
hole, consequently expanding the erosion process (Jafari & Sui, 2021). Consequently, the
maximum depth of scour holes tends to be lesser in deeper flows with lower flow Froude numbers.
Notably, during the evolution of a scour hole, the quantity of coarse particles within the
scour hole progressively augmented. These larger sediment particles settled on the scour hole's
surface, forming what is commonly known as an armour layer, which impedes further scouring.
The presence of an armour layer indicates that the scouring process is approaching an equilibrium
state. Additionally, the formation of an ice cover on the water surface and variations in the
alignment angle of the dike were found to influence the maximum depth of a scour hole around
the dike.
The presence of a spur dike in a river modifies the flow dynamics by obstructing the
streamflow and redirecting the incoming flow in a different direction. As a result of the contraction
caused by a spur dike, the flow velocity at the dike tip increases significantly. Additionally, the
flow is separated into two segments. One portion of the flow experienced acceleration and
displacement towards the flume's central region. The velocity can be increased by up to 60% of
the approaching flow velocity to the dike, depending on the dike alignment angle and the flow
surface cover condition (Jafari & Sui, 2021). The other part of the flow goes toward the channel
bed, creating an intense downflow at the dike's upstream side, eroding around the dike toe and
forming a scour hole. There are three crucial factors for assessing the scour: the location of the
initial scour hole, the development of the scour pattern, and the maximum depth of scour holes
after reaching the equilibrium state (Namaee et al., 2020; Valela et al., 2021; Zhang et al., 2018).

107

4.3.1

Vortex system at the dike

It was observed that the local scour was initiated at the upstream side of the dike toe once
an experiment started. Due to the local flow acceleration around the dike and strong vertical
downward flow, a primary vortex was formed within the initial scour hole. The two most important
factors influencing the primary vortex are the dike orientation angle and the approaching velocity.
For the dike with an alignment angle of 90º, the primary vortices were more substantial and lasted
longer. By reducing the dike alignment angle, the primary vortex became less noticeable. As the
approaching velocity increased, the downflow interaction strengthened, and these vortices
exhibited greater intensity. Additionally, an adverse pressure gradient formed within the scour
hole, leading to the development of a secondary vortex system known as a horseshoe vortex. These
vortices were visible within the scoured zone and developed clockwise inside the scour hole
around a spur dike.
The horseshoe vortex's strength depends on flow rate, blockage ratio, and surface
condition. By increasing the flow rate and blockage ratio (increasing the dike alignment angle from
30º to 90º), the scour hole around the spur dike became deeper, implying that the intensity of the
horseshoe vortex was increased. Under an ice-covered flow condition, flow circulations are
intensified, leading to powerful vortices inside the scour hole at a spur dike. With the increase in
the roughness coefficient of an ice cover, the scour hole around a spur dike became wider and
deeper, indicating a more powerful horseshoe vortex inside the scour hole. Thus, the bed shear
stress under an ice-covered flow condition was raised above the critical shear stress for the
incipient motion of bed materials under an open flow condition. Consequently, an increased
sediment movement was observed, leading to the further development of the scour hole under an
ice-covered flow condition.
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After a couple of hours, the primary vortex faded and was no longer visible. The horseshoe
vortex system continued to be the primary cause for the expansion of the scour hole. Sediment
particles in the scour hole were transported downstream by the vortex system and deposited in the
low-velocity zone behind the spur dike. In this low-velocity zone, the streamflow mixed with the
separated flow downstream of the dike generated another vortex system called the wake vortex.
Following the dispersal of the primary vortex, the final scour process resulted from these two
remaining vortex systems, namely, horseshoe and wake vortex. The scour hole surrounding the
dike progressively increases in width and depth over time, expanding in the upstream, downstream,
and lateral directions toward the middle of the channel. Due to the difference in strength and
rotation direction of the vortices, the shape and slope of scour holes located upstream of a spur
dike differ significantly from those downstream.
4.3.2

Scour morphology

4.3.2.1 Features of scour holes
The process of channel bed deformation around a spur dike is influenced by various factors,
including flow characteristics, grain size of bed material, water surface cover condition, spur dike
layout and alignment angle. The configurations of scour holes and deposition dunes can be
effectively depicted using contour maps, which provide two-dimensional representations of threedimensional data, as depicted in Figure 4.3. Lines of equal "Z" values in contour maps portray
channel bed deformation around the dike. The shape of scour holes and deposition dunes can be
comprehended by analyzing a contour map.
The results showed that an ice cover on the water surface leads to a wider scour hole in the
lateral direction compared to an open channel flow condition. One can see from Figure 4.3 that the
scour hole around a dike under an open flow condition exhibited a circular shape, while the scour
109

hole extended toward another side of the channel wall under an ice-covered condition. This effect
was accentuated under a rough-covered flow condition, implying that an increase in the ice cover
roughness corresponds to an enlargement in the size of the scour hole around a dike. This finding
aligns with Sui et al. (2009), who investigated the local scour caused by submerged jets under icecovered flow conditions.

Open channel

Smooth ice cover

Rough ice cover

Figure 4. 3:Scour holes under various water surface conditions (Fr=0.19, and 90º-dike alignment)

In addition to the cover roughness, the dike alignment angle also significantly influences
the local scour process around a dike. As depicted in Figure 4.4, reducing the dike alignment angle
from 90º to 45º resulted in an elongated scour hole shape in the streamwise direction. The quasisemicircular scour hole associated with the 90º alignment angle was changed to an elliptical shape.

90°-alignment angle

60°-alignment angle

45°-alignment angle

Figure 4. 4:Scour holes under different dike alignment angles and an open flow condition (Fr=0.19)

110

4.3.2.2. Scour hole geometry
The scour hole pattern around a spur dike is another essential factor for studying the
scouring process. To provide a more comprehensive portrayal of the experimental results and
enhance their applicability to real-world scenarios, this study employed the following
dimensionless variables to assess the development of scour holes and deposition dunes around spur
dikes. In this context, the dimensions of scour holes in all directions were normalized by the spur
dike length (Figure 4.5).

(a) Schematic plan view of a scour hole

(b) Actual plan view of a scour hole

Figure 4. 5:Plan view of the scour hole

1) The upstream and downstream lengths (a/L and b/L) of a scour hole are used to examine
the extent of a scour hole in the streamwise direction. Their combination, (a+b)/L,
represents the maximum scour hole length in the streamwise direction.
2) The maximum outward width, measured from the dike tip to the outer boundary of a scour
hole (w/L), is used to express the maximum outward scour width in the transverse
direction. By utilizing this variable, one can evaluate whether the maximum outward
width of a scour hole can be extended to the opposite channel sidewall.

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3) The maximum inward width of a scour hole along the front side of a spur dike (c/L) is the
distance measured from the dike tip to the inner boundary of the scour hole in the
transverse direction. Along the rear side of a spur dike, the maximum inward width of a
scour hole in the transverse direction is expressed as (d/L). These two ratios (c/L and d/L)
can be used as indicators for assessing the extension of a scour hole toward the channel
wall where the dike is attached. By examining these maximum inward scour widths, one
can evaluate whether the scouring process around the dike can induce channel bank
erosion, thereby carrying the risk of detrimental consequences such as landslides and spur
dike failure.
These ratios are influenced by various factors, including flow surface conditions (open
channel, smooth ice-covered, and rough ice-covered flow), hydraulic parameters (flow depth and
velocity), dike alignment angles and dimensions, and the grain size of bed materials. In the
following section, the influence of the distinct experimental setups on the scour hole geometry has
been evaluated by assessing these ratios.
a). Upstream length of a scour hole (a/L)
The experimental results demonstrate that hydraulic conditions substantially impact the
upstream length of a scour hole (a/L). As the approaching flow depth decreases, the upstream
length of a scour hole (a/L) increases. With every 10 cm increase in the flow depth, there is an
approximately 20% reduction in the upstream length of a scour hole (a/L). The finer the bed
material (D50), the larger the upstream length of a scour hole (a/L). Moreover, the upstream length
of a scour hole (a/L) was expanded by elevating the approaching flow velocity. This means the
experimental scenario with the largest flow Froude number and finest bed material will generate
the most significant upstream length of a scour hole (a/L).
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As depicted in Figure 4.6, the upstream length of a scour hole (a/L) exhibited marginal
expansion under a smooth-covered condition compared to an open-flow condition. However, a
rough ice cover led to a pronounced increase in the upstream length of a scour hole (a/L) by 25%.
Hence, the roughness coefficient of an ice cover plays a crucial role in the initiation and
development of a scour hole toward the upstream zone of a spur dike. Under the same covered
flow condition, an elevation in the flow depth diminishes the influence of the ice cover on the
scouring process toward the upstream of the spur dike.

Open channel

Smooth ice cover

Rough ice cover

Figure 4. 6:Effect of experimental conditions on the upstream length of a scour hole (a/L)

The findings indicate that a reduction in the dike alignment angle led to an immense
decrease in the upstream length of a scour hole (a/L), as shown in Figure 4.6. Decreasing the dike
alignment angle from 90º to 60º reduces approximately 60% in the upstream length of a scour hole
(a/L). A marginal reduction of the dike alignment from 60º to 45º exhibited a less noticeable impact
on the upstream length of a scour hole (a/L). By increasing the approaching flow velocity, the
impact of the dike alignment angle on the upstream length of a scour hole (a/L) will intensify.

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b). Downstream length of a scour hole (b/L)
Results showed that the influences of pertinent factors on the downstream length of a scour
hole (b/L) differ from those on the upstream length of a scour hole (a/L). In particular, the water
surface cover condition has a much less impact on the downstream length of a scour hole (b/L),
namely, just caused a slight increase in the downstream length of a scour hole (b/L) compared to
that under an open flow condition. Also, the impact of a rough ice cover on the downstream length
of a scour hole (b/L) is not clearly different from that of a smooth ice cover. Furthermore, by
increasing the flow depth, the downstream length of a scour hole (b/L) decreases slightly but is
observable.
Results also showed that the downstream side of a scour hole expanded as the approaching
velocity increased, but it is less noticeable compared to the impact of the flow rate. With an
increase of about 50% in the flow rate (from 55 lit/s to 120 lit/s), the downstream length of a scour
hole (b/L) increased by about 20%.

Open channel

Smooth ice cover

Rough ice cover

Figure 4. 7:Effect of experimental conditions on the downstream length of a scour hole (b/L)

By changing the dike alignment angle, the shape of scour holes changes substantially. With
the decrease in the dike alignment angle, the downstream length of a scour hole (b/L) expanded.
As shown in Figure 4.7, the reduction of the dike alignment angle from 90º to 60º resulted in a
significant change in the downstream length of a scour hole (b/L). However, a further decrease in
114

the dike alignment angle from 60º to 45º had a less conspicuous effect on the downstream length
of a scour hole (b/L). In summary, as the flow rate increases and the dike alignment angle decreases
simultaneously, the downstream length of a scour hole (b/L) will substantially increase in
streamwise and lateral directions under an ice-covered flow condition.
c). Maximum outward width of a scour hole (w/L)
The flow velocity considerably impacts the width of the scour hole around a spur dike.
Increasing the flow velocity from 0.15 m/s to 0.30 m/s increases the maximum outward scour
width (w/L) by about 8%. The depth of the approaching flow is another factor that significantly
influences the maximum outward scour width (w/L). As the approaching flow depth increases, a
noticeable decrease in the maximum outward scour width (w/L) results. An increase of each 10 cm
in the water depth leads to a decrease of 15% in the maximum outward width of a scour hole (w/L).
Given that the maximum outward width of a scour hole (w/L) increases with the
approaching flow velocity but decreases with the flow depth, the flow Froude number (Fr) can be
employed to represent the influences of flow velocity and water depth on the maximum outward
scour width (w/L). Results showed that, with an increase of each 30% in the Froude number, the
maximum outward scour width (w/L) increases by 15%~20%. The findings reveal that an ice cover
on the water surface amplifies the influence of the flow Froude number on the maximum outward
scour width (w/L). A higher Froude number under the rough ice-covered flow condition led to a
larger maximum outward scour width (w/L).
As illustrated in Figure 4.8, the maximum outward scour width (w/L) also depends on the
dike alignment angle. Decreasing the dike alignment angle changes the scour hole shape from
semi-circular to elliptical. Hence, the maximum outward scour width (w/L) decreased with the
decrease in the dike alignment angle. Depending on the cover condition and flow Froude number
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(Fr), a decrease in the dike alignment angle from 90º to 60º reduces 20%~30% in the maximum
outward scour width (w/L). A further reduction in the dike angle from 60º to 45º had a marginal
effect on the maximum outward scour width (w/L). In this study, the maximum outward scour
width (w/L) reached the smallest when the dike was placed at an alignment angle of 45º.

Open channel

Smooth ice cover

Rough ice cover

Figure 4. 8:Effect of experimental conditions on the maximum outward scour width (w/L)

It has been observed that the maximum outward scour width (w/L) increases by adding an
ice cover on the water surface. Also, the rougher the ice cover, the larger the maximum outward
scour width (w/L). As the water depth increases, the effects of the ice cover on the maximum
outward scour width (w/L) become mitigated. Furthermore, an increase in flow velocity amplifies
the impact of an ice cover on the development of a scour hole. Throughout all experimental runs,
the maximum outward scour width never extended to the opposite side wall of the flume, reaching
a maximum of 62 cm (from the tip of the dike to the outside boundary of the scour hole).
Consequently, the far sidewall does not influence the development of the scour hole and the
maximum outward scour width.

116

d). Maximum frontal inward width of a scour hole (c/L)
If the inside boundary of a scour hole extends to the channel bank to which the dike is
attached, it can induce bank erosion, landslides, and dike failure. Thus, assessing the maximum
frontal inward scour width of a scour hole is necessary for the spur dike design. Results indicate
that an increase in the flow depth decreases the maximum frontal inward scour width (c/L). The
higher the approaching flow velocity, the larger the maximum frontal inward scour width (c/L).
Combining the impact of flow depth and velocity on the maximum frontal inward scour width
(c/L), it can be concluded that an increase in the Froude number caused an increase in the maximum
frontal inward scour width (c/L). Notably, as the particle size of the bed material decreases and the
Froude number increases, a substantial increase in the maximum frontal inward scour width of a
scour hole (c/L) will result.
Based on all experimental runs conducted under different conditions, it has been found that
the maximum frontal inward scour width (c/L) is hardly close to one, namely, c/L<1. This result
indicates that the likelihood of bank erosion and landslides is extremely low when the criteria of
c/L<1 is ensured in the spur dike design. As shown in Figure 4.9, the presence of an ice cover on
the water surface is another factor that significantly affects the maximum frontal inward scour
width (c/L). The appearance of an ice cover on the water surface increases the maximum frontal
inward scour width (c/L) by 40~50% compared to that under an open flow condition. Nevertheless,
an increase in the roughness coefficient of an ice cover has little impact on the maximum frontal
inward scour width (c/L).
The dike alignment angle influences the maximum frontal inward scour width (c/L). As the
dike alignment angle decreases, the maximum frontal inward scour width (c/L) decreases
accordingly. This finding suggests that the maximum frontal inward scour width (c/L) with the 90º

117

alignment angle clearly differs from that around a dike with the 60º alignment angle. This effect
becomes less prominent by further reducing the dike alignment angle.

Open channel

Smooth ice cover

Rough ice cover

Figure 4. 9:Effect of experimental conditions on the maximum frontal inward scour width (c/L)

e). Maximum rear inward width of a scour hole (d/L)
The maximum rear inward width of a scour hole (d/L) is as crucial as the maximum frontal
inward scour width (c/L). However, as illustrated in Figure 4.10, the maximum rear inward scour
width (d/L) was consistently less than the maximum frontal inward scour width (c/L) in all
experimental runs and never reached one. Results showed that the maximum rear inward scour
width (d/L) increases with the increase in the Froude number. However, the Froude number had a
minor impact on the maximum rear inward scour width (d/L) compared to the maximum frontal
inward scour width (c/L).

Open channel

Smooth ice cover

Rough ice cover

Figure 4. 10:Effect of experimental conditions on the maximum rear inward scour width (d/L)

118

Similarly, an ice cover on the water surface causes a larger maximum rear inward scour
width (d/L) than that under an open flow condition. Results showed that the maximum rear inward
scour width (d/L) slightly increases as the cover roughness increases. In addition, under an icecovered flow condition, the maximum rear inward scour width (d/L) increases with the decrease
in flow depth and particle size of the bed material. The impact of the ice cover is more pronounced
in the case of shallow flow depth and finer bed material (D50=0.48 mm).
The dike alignment angle has an entirely reverse impact on the maximum rear inward scour
width (d/L) compared to the maximum frontal inward scour width (c/L). The maximum rear inward
scour width (d/L) exhibits a noticeable increase with the decrease in the dike alignment angle.
Furthermore, an increase in the flow velocity under an ice-covered condition magnifies the impact
of the dike alignment angle on the maximum rear inward scour width (d/L). Under the same flow
and cover conditions, the maximum rear inward scour width (d/L) with an alignment angle of 45º
is more than that with the larger alignment angles of 60º and 90º (Figure 4.10)
4.3.3

Maximum scour depth

Understanding a scour hole's maximum depth and location is crucial for designing instream infrastructures like spur dikes, bridge piers, and abutments. To ensure structural integrity
and longevity, foundations must be situated at a certain depth below the riverbed to mitigate the
detrimental effects of hydraulic forces induced by flowing water and river ice. Understanding the
influence of hydraulic factors and channel conditions, particularly in the presence of ice cover and
ice jam, is essential for accurately predicting scour holes' maximum depth and locations around
in-stream infrastructures.
Experimental observations revealed no discernible impact of different flow conditions
(flow velocity and depth) on time required to develop a scour hole to reach an equilibrium
119

condition. Notably, under rough ice-covered flow conditions, the local scour around a dike with a
90º alignment angle took the longest time to reach an equilibrium state. A significant difference in
the maximum scour depth was clearly noticed among a wide range of the flow Froude numbers
from 0.10 to 0.26. With the increase of each 15% in the Froude number, the maximum depth of a
scour hole increased by 5%~10%. For most experimental runs, the maximum scour depth was
located near the toe of the dike with an alignment angle of 90º. With the decrease in the flow
Froude number, the location of the maximum depth of a scour hole was observed to shift a marginal
amount downstream. As illustrated in Figure 4.11, the cross-section profile of the scour hole was
transformed from a narrow "V" shape to a broader "V" shape. Additionally, a substantial reduction
in the scour hole volume was observed as the Froude number decreased. For an increase of 30%
in the Froude number, the volume of a scour hole increased by approximately 12%.

Fr = 0.26

Fr = 0.19

F r = 0.10

Figure 4. 11:Cross-section profile of scour holes around a dike with the alignment angle of 90º for different Froude
numbers under an open flow condition

The augmentation of the scour area was the most prominent effect induced by an ice cover
on the development of scour holes around the dikes. An ice cover on the water surface resulted in
a broader scour hole in streamwise and transverse directions, particularly in rough ice cover. This
implies that the water surface cover conditions mainly influence the maximum depth of scour
holes. The outcomes indicate that, under a smooth ice-covered condition, the maximum depth of
a scour hole increases by about 10% compared to that under an open flow condition, and it

120

increases by about 15% under the rough ice-covered condition. The effect of an ice cover on the
location of the maximum depth of scour holes does not demonstrate a consistent trend. As the
scour hole enlarged, the slopes of the hole's entrance and exit sections became steeper, but with a
milder slope of the exit section compared to that of the entrance section, regardless of water surface
cover conditions. Under a rough ice-covered flow condition, the slope of the entrance section of a
scour hole is much steeper than that of the exit section compared to that under an open flow
condition.
The dike alignment angles significantly influence the size and shape of the scour holes. By
reducing the dike alignment angle, the geometric patterns of scour holes were different. One
significant impact is the scouring depth. Under the same flow and cover conditions, the maximum
depth of scour holes is observed at the dike with an alignment angle of 90°. The depth of the scour
hole decreases significantly as the dike alignment angle reduces. In general, regardless of the dike
alignment angle, the position of the maximum depth of scour holes appears proximate to the dike
toe. As the dike alignment angle decreases, the location of the maximum scour depth is shifted
away from the dike toe, and the scouring process progresses from upstream to downstream.
The key parameters that affect the depth of scour holes around an in-stream infrastructure
should include flow velocity, turbulence intensity, water depth, sediment characteristics (such as
grain size), structure geometry, and roughness coefficients of the riverbed and ice cover. The
following essential parameters influencing the maximum depth of scour holes (Ymax) should be
considered in this experimental study:
Ymax= f (H, V, Le, B, D50, ni, nb, sin)

(4.1)

Where, H is the approaching water depth, V is the approaching flow velocity, Le is the dike
effective length, which is defined as the vertical distance from the dike tip to the sidewall to which

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the dike is attached, B is the channel width, D50 is the median particle size of the bed material, ni
is the roughness coefficient of an ice cover, nb is the roughness coefficient of the riverbed,  is
the dike alignment angle.
To precisely assess the impact of each parameter on the maximum depth of scour holes and
obtain a more detailed depiction of the laboratory results, the essential parameters mentioned in
Equation 1 are represented as dimensionless variables as follows:
௒೘ೌೣ
ு

௅

஽

௡೔

஻

ு

௡್

= ݂ ቀ‫ܨ‬௥ , ೐ , ఱబ ,

(4.2)

, sin ߠቁ

This experimental study regarding local scour around non-submerged dikes with different
alignment angles has been conducted under ice-covered flow conditions compared to that under
open flow conditions. As discussed above, the influence of an ice cover on the maximum depth of
scour holes around the angled dike is substantial since the distribution profiles of flow velocity
and the composite roughness coefficient of the channel under an ice-covered flow condition differ
significantly from those in an open flow. Thus, it is necessary to derive distinct equations for
describing the maximum depth of scour holes under ice-covered and open-flow conditions,
respectively. Based on data obtained from laboratory experiments, the following two equations
have been derived to estimate the maximum depth of scour holes for ice-covered and open-flow
conditions, respectively.

Ice-covered flow condition:
௒೘ೌೣ
ு

௅

= 17.07(‫ܨ‬௥ )଴.଺ଷ ቀ ஻೐ ቁ

଴.ସଷ ஽
ି଴.ଶ଺
ఱబ

ቀுቁ

௡

ቀ௡ ೔ ቁ

଴.ଵହଷ

್

(sin ߠ)଴.ସ଴

ܴଶ = 0.97

(4.3)

Open channel flow condition:
௒೘ೌೣ
ு

௅

଴.ଵ଻ସ ஽
ି଴.ଷଷ
ఱబ

஻

ு

= 12.66 (‫ܨ‬௥ )ଵ.଴ସ ቀ ೐ ቁ

ቀ

ቁ

(sin ߠ)଴.ହ଻

122

ܴ ଶ = 0.963

(4.4)

Results of estimations using Equations 4.3 and 4.4 reveal that, under both open channel
and ice-covered flow conditions, a larger flow Froude number (Fr) corresponds to a deeper
maximum depth of scour holes. However, under ice-covered flow conditions, the exponent of the
Froude number in Equation 3 is approximately 60% of that under open flow conditions. This
suggests that the impact of the flow Froude number on the maximum depth of scour holes under
ice-covered flow conditions is less pronounced than that under open channel conditions.
Under both open channel and ice-covered flow conditions, the negative exponent of the
ratio of the median particle size of bed material to the flow depth (or relative particle size) indicates
that the maximum depth of scour holes increases as the relative particle size decreases. This
negative exponent of the relative grain size means the finer the bed material, the deeper the
maximum depth of scour holes. One can also see from Equations 4.3 and 4.4 that there is no
significant difference in the exponents of the relative grain size between the ice-covered flow and
open channel flow.
Equations 4.3 and 4.4 demonstrate that the blockage ratio (Le/B), representing the ratio of
the effective dike length to the channel width, is a crucial factor influencing the maximum depth
of scour holes. This influence is more pronounced under ice-covered flow than under open flow
conditions, as indicated by the positive exponents of the blockage ratio. In this study, three dike
alignment angles (90º, 60º, and 45º) resulted in the blockage ratios of 25%, 20%, and 15%,
respectively. As the blockage ratio increases, with the same approaching flow rate, the flow
velocity around the dike rises, suggesting higher kinetic energy and turbulence intensity. Results
showed that the dike alignment angle 90º resulted in the most substantial downward vertical
velocity, which created a negative velocity gradient and increased vertical momentum transport
around the dike and inside the scour hole. With the decrease in the blockage ratio, a negative

123

velocity gradient gradually reduced, thus decreasing the turbulence kinetic energy. Under icecovered flow conditions, this contributed to developing a stronger horseshoe vortex, ultimately
leading to a much deeper scour hole than that under open flow conditions. The proposed equations
under both open channel and ice-covered flow conditions suggested that the dike alignment angle
notably influences the maximum depth of scour holes. As indicated by the positive exponents of
the sin function of the dike alignment angle, the smaller the dike alignment angle, the less the
maximum depth of scour holes.
Under an ice-covered flow condition, the positive exponent of the ratio of the ice-cover
roughness coefficient to the bed roughness coefficient (or relative roughness coefficient) signifies
a direct relationship between the maximum depth of scour holes and the relative roughness
coefficient. In other words, a deeper maximum depth of scour holes will result from an increase in
the relative roughness coefficient, whether through an increase in the cover roughness coefficient
(from smooth ice cover to rough ice cover) or a reduction in the bed roughness coefficient (from
coarse bed particles to fine bed material).
The comparison between the calculated maximum depth of scour holes using the proposed
equations and the results from laboratory experiments under both open channel and ice-covered
flow conditions is presented in Figure 4.12. The substantial agreement between the calculated
results and experimental data indicates the reliability and accuracy of Equations 4.3 and 4.4.

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(a): ice-covered flow conditions

(b): open channel flow conditions
Figure 4. 12:Calculated maximum depth of scour holes using proposed equations compared to results of laboratory
experiments: (a) ice-covered flow conditions, (b) open channel flow conditions.

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4.4

Conclusions
In the present study, laboratory experiments have been carried out to examine the effect of

ice cover with different roughness and non-submerged spur dikes with different alignment angles
on the morphological features of scour holes around spur dikes by changing bed materials and
hydraulic conditions. The following results have been drawn from this study:
1) The scouring was initiated with the primary vortex, and the channel bed deformation
around a dike continued with the horseshow vortex and wake vortex. The horseshoe
vortices last the longest among different vortex systems. The orientation of spur dikes and
flow Froude number emerged as the primary determinants of the vortex system. Notably,
the system's most robust downflow and vertical velocity was generated in the flow with
a higher Froude number around the spur dike with the 90º alignment angle.
2) The upstream length of the scour hole (a/L) is significantly affected by the dike alignment
angles, flow Froude number and the presence of an ice cover. Among them, the dike
alignment angle is the controlling factor for the scour hole's upstream length (a/L) and
downstream length (b/L). On average, the upstream length of scour holes (a/L) decreased
by 60% when the dike angle was changed from 90º to 60º. The presence of a rough ice
cover on the water surface led to an increase in the upstream length of a scour hole (a/L).
For flow with high flow Froude number under the rough ice cover, the upstream length
of scour holes (a/L) and the frontal inward scour width (c/L) around the 90º-angled dike
reached the most significant values. With an increase of about 50% in the flow rate, the
downstream length of a scour hole (b/L) increased by about 20%. The change of the dike
alignment angle from 90º to 60º resulted in a significant change in the downstream length

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of a scour hole (b/L). However, a further decrease in the dike angle from 60º to 45º had
less effect on the downstream length of a scour hole (b/L).
3) An increased flow Froude number corresponds to greater scour hole depths around a dike
in both open channel and ice-covered flow scenarios. A finer bed material contributes to
deeper scour holes regardless of flow surface cover conditions. The impact of the flow
Froude number on the maximum scour depth under ice-covered conditions is less
pronounced than that under open channel conditions. Under an ice-covered flow
condition, a larger relative roughness coefficient leads to a larger maximum depth of scour
holes.
4) The proposed equations under the open channel and ice-covered flow conditions are
derived to compute the maximum depth of scour holes around angled, non-submerged
spur dikes. These equations consider factors such as dike alignment angle, flow Froude
number, dike blockage ratio, particle size of bed material, and relative roughness
coefficient (for covered flow conditions). The calculated maximum depths of scour holes
using the proposed formula agree well with those of laboratory experiments, indicating
that the performance of the proposed equations is reliable.
4.5

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5 CHAPTER FIVE: Evaluating the Impact of Spur Dike Alignment and Ice Cover
Roughness on Local Scour at Bridge Abutments

5.1

Introduction
Scouring around bridge piers and abutments is a significant and pervasive challenge in

hydraulic engineering, posing a critical threat to the stability and safety of bridge structures
worldwide (Dargahi, 1990). The phenomenon of scour refers to the removal of sediment from
around bridge foundations due to the erosive action of flowing water (Sui et al., 2006). This process
can progressively undermine the structural integrity of bridges, leading to potential failures and
catastrophic collapses, with severe implications for transportation networks, economic activities,
and public safety (Pizarro et al., 2020). Several factors, including the velocity and turbulence of
water flow, sediment transport characteristics, and the geomorphological features of the riverbed,
drive the occurrence of scour (Namaee & Sui, 2019b). During high-flow events, such as floods,
the increased water velocity and turbulence can intensify the scour process, rapidly eroding the
sediment around bridge piers and abutments. This erosion creates scour holes, which can
compromise the Bridge's stability by exposing and weakening its foundations (Arneson, 2013)
Understanding and mitigating scour have evolved significantly over the years, with
extensive research conducted to develop effective methods for reducing or preventing scour
around bridge piers and abutments (Zarrati et al., 2006). Early studies primarily focused on
understanding the mechanics of scour and the factors influencing its development (Gupta &
Gangadharaiah, 1992). These foundational studies provided valuable insights into the flowing
water and sediment interaction behaviour around bridge foundations, forming the basis for
developing mitigation measures (Pandey et al., 2023). As research progressed, various structural,
geotechnical, and hydraulic approaches were proposed and implemented to address scour
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(Lagasse, 2007). Structural measures, such as riprap and concrete aprons, aim to directly protect
the riverbed and foundations. Geotechnical measures focus on enhancing the resilience of the
foundations themselves. In contrast, hydraulic measures involve altering the flow patterns to
reduce the erosive forces acting on the bridge piers and abutments (Zdravkovic & Tsiampousi,
2023).
One of the most common structural methods involves placing rock ripraps or gabions
around bridge piers and abutments. Riprap consists of large stones that are resistant to erosion and
can dissipate the energy of flowing water, thus reducing its erosive potential (Wörman, 1989).
Gabions, which are wire mesh filled with stones, provide similar protection. Both methods have
been widely studied and implemented due to their effectiveness and simplicity (Richardson &
Davis, 1995). Early studies by Chiew (1992) highlighted the effectiveness of riprap in reducing
scour around bridge piers by disrupting the flow patterns that contribute to erosion. Concrete
aprons and mats provide a solid, erosion-resistant layer around the foundation of bridge structures.
These can be prefabricated or cast in place and are designed to protect the underlying soil from
being washed away by water flow. Studies have shown that concrete aprons effectively prevent
scour, particularly in areas with high flow velocities (Melville & Coleman, 2000). More recent
research by Briaud et al. (2011) supports the use of concrete aprons in combination with other
methods to improve scour protection.
Designing foundations that can withstand significant scour depths is a critical geotechnical
approach. This method often involves using deep foundations with piles that extend well below
the expected scour depth (Sadat, 2015). This approach maintains the Bridge's structural integrity
even if the surrounding soil is eroded. Research has demonstrated the effectiveness of deep
foundations in various riverine environments (Briaud et al., 2011). Additionally, (Ettema et al.,
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1998) studied the scale effects in pier-scour experiments, highlighting the importance of
considering foundation depth in scour-prone areas. This method involves injecting grout into the
spaces between riprap stones to lock them together, creating a more cohesive and durable
protective layer. Grouted riprap has been found to be particularly effective in areas with hydraulic
solid forces that could dislodge ungrouted stones (Ettema et al., 1998). Subsequent studies by
Heibaum and Trentmann (2010) demonstrated grouted riprap's long-term stability and
effectiveness in reducing scour.
Hydraulic Measures are another method that has been used to reduce local scour. Various
devices, such as vanes, baffles, or flow deflectors, are placed upstream of the bridge piers to alter
the water flow and reduce its velocity around the critical areas (Brice et al., 1978). These structures
help redirect or slow the flow, reducing the scour potential. Studies have shown that these flowaltering devices can significantly reduce the extent of local scour (Rajaratnam & Nwachukwu,
1983). More recent research by Melville and Coleman (2000) has further refined the designs and
placements of these structures for optimal performance. Research has shown that planting
vegetation can reduce stream bank erosion and scour around hydraulic structures. Vegetation
stabilizes the soil through root reinforcement and dissipates the flowing water's energy, reducing
erosion (Shields Jr et al., 1995). A study by the Agricultural Research Service investigated the
resistance of three riparian species in the Platte River Basin to removal by drag forces and scour.
The research found that vegetation significantly mitigates the effects of local scour, emphasizing
the importance of incorporating vegetation into river management practices (Fotherby & Randle,
2007). This method has been particularly emphasized in recent environmental engineering studies
for its dual benefits of erosion control and habitat creation.

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Spur dikes, also known as groynes, extend from the riverbank into the river. They are
designed to alter the flow patterns of the water to protect bridge piers and abutments from scour
(Li et al., 2005). Kehe (1984) provided early evidence of flow diversion's benefits in altering river
morphology through spur dikes. Spur dikes obstruct the direct water flow toward the bridge piers
and abutments, redirecting the high-velocity flow away from these structures and reducing the
erosive forces acting on the foundations (Fang et al., 2006). Kuhnle et al. (1999) demonstrated that
spur dikes reduce the flow velocity around bridge piers, thus minimizing scour. The altered flow
patterns created by spur dikes encourage sediment deposition between the dikes and the protected
structure. This sediment build-up further reinforces the riverbed and reduces the likelihood of
scouring. Studies have shown spur dikes promote sediment accumulation in the desired areas,
enhancing their protective effect (Duan & Nanda, 2006).
Spur dikes dissipate the kinetic energy of the water, lowering its capacity to erode the
riverbed and the structural foundations. This energy dissipation is crucial for reducing the impact
of flowing water on hydraulic structures (Nazari et al., 2017). Research by Markovic (2012)
supports the effectiveness of spur dikes in energy dissipation and scour reduction. The
effectiveness of spur dikes depends on their strategic placement and design. Engineers must
consider the river's hydrodynamics, sediment transport patterns, and the specific vulnerabilities of
the bridge piers and abutments when designing and positioning the dikes (Esmaeli et al., 2022).
Karami et al. (2011) provide detailed guidelines on the optimal design and placement of spur dikes
to maximize their effectiveness in scour protection. Basser et al. (2014) also contributed valuable
insights into the effects of spur dike location on flow structures, which are critical for designing
effective scour mitigation systems.

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The literature on scour prevention and reduction methods at bridge piers and abutments
reveals various effective strategies, each with strengths and suitable applications. Structural
measures like riprap and concrete aprons, geotechnical measures such as scour-resistant
foundations, and hydraulic measures including flow-altering structures and vegetative
stabilization, all contribute to mitigating the risks associated with scour (Beg & Beg, 2013; Deng
& Cai, 2010; Wang et al., 2017). Among these, spur dikes have shown considerable promise in
altering flow patterns, promoting sediment deposition, and dissipating energy, thereby providing
robust protection against scour (Saneie, 2011; Vaghefi et al., 2021). To the best of the authors'
knowledge, no previous research has investigated the application of non-submerged spur dikes
with varying alignment angles under ice cover with different roughness coefficients. This study
utilizes laboratory experiments to determine the optimal configuration of protective spur dikes for
minimizing local scour at the abutment.
5.2

Experimental Procedures

5.2.1

Research Site and Flume Setup

This experimental study was conducted at the Quesnel River Research Center in Likely,
British Columbia, Canada. The research employed a large-scale outdoor hydraulic flume
specifically designed to replicate the natural dynamics of river systems. The flume, with
dimensions of 38.5 meters in length, 2 meters in width, and 1.3 meters in depth, included a
longitudinal bed slope of 0.2%. This setup allowed for the simulation of natural flow conditions
with sufficient spatial scale to capture relevant hydrodynamic and sediment transport processes.
An upstream holding tank with a volume of 90 cubic meters (40 meters in length, 2 meters in
width, and 1.3 meters in depth) was used to maintain a steady and controlled discharge throughout
the experimental runs. This tank was equipped with three valves facilitating a wide range of flow
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rates, which is crucial for simulating various riverine conditions. These valves supplied water to
the flume, ensuring a controlled and adjustable inflow. The flume's design induced a non-uniform
flow characterized by longitudinal variations in water depth, thus creating a more realistic model
of river environments (Figure 5.1).
Within the flume, two distinct sandboxes were constructed to simulate riverbed conditions.
These sandboxes, each 2 meters wide and 0.3 meters deep, were positioned 10.2 meters apart. The
upstream sandbox was 5.6 meters long, while the downstream sandbox extended to 5.8 meters,
providing sufficient space for observing sediment dynamics and scour development around the
spur dike and the abutment (Figure 5.1). The sandboxes were filled with three types of natural
sediments to replicate riverbed conditions. These sediments had median grain sizes (D50) of 0.60
mm and 0.90 mm, with respective standard deviations of 2.39 and 1.41. The sediments' average
density was approximately 1.7 g/cm³. Plexiglass sidewalls were installed on one side of each
sandbox to enable detailed visual observation and monitoring of the scouring process.

(a) Overview of the flume

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(b) Plan view

(c) Vertical view
Figure 5. 1:Experimental flume: (a) Overview of the flume, (b) Plan view, (c) Vertical view

A significant feature of the experimental setup was the installation of a spur dike and
abutment in each sandbox. The dikes were constructed from marine-grade plywood, chosen for its
durability and resistance to water. Each dike measured 80 cm in height, 5 cm in width, and 50 cm
in length, with 30 cm of its height buried in the sand bed, leaving 50 cm exposed to the flow. The
abutments were fabricated from galvanized plates designed in a rectangular shape with dimensions
of 20 cm in width and 40 cm in length. The abutments were placed downstream of the dikes at 25
cm and 50 cm distances. This design aimed to simulate the impact of spur dikes on scour reduction
surrounding the abutments (Figure 5.1,5.2). The dikes were tested at three different alignment
angles to the downstream abutments: 90º, 60º, and 45º. These angles corresponded to blockage
ratios of 25%, 20%, and 15%, respectively, providing a range of conditions to study the influence
of dike orientation on flow patterns and sediment transport at the abutments.
Styrofoam panels were utilized to simulate the effects of ice cover on flow dynamics and
scour processes. Smooth panels represented natural sheet ice commonly found in riverine
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environments during winter. Rough panels were created by affixing 2.5 cm³ Styrofoam cubes to
mimic the roughness and complexity of ice jams. Sixteen panels, each measuring 1.99 meters by
2.4 meters, were employed to cover the entire flow surface within the flume. This setup allowed
for the examination of the impact of ice cover on hydraulic conditions and sediment transport,
providing insights into wintertime river dynamics.

Figure 5. 2:A cover condition experiment under smooth ice cover with a 60º alignment angle

Flow measurements were primarily conducted using the SonTek-IQ Plus. The IQ Plus is
an advanced instrument designed for precise flow monitoring in natural rivers and controlled
environments like hydraulic flumes. It is equipped with six sensors, including four velocity beams
and two additional beams for water level and vertical acoustic measurements (SonTek, 2017).
These multiple sensors provide comprehensive coverage of the flow field, ensuring high accuracy
and reliability in data collection. These features allow researchers to gain a deeper insight into the
approaching flow characteristics and volume data, which are critical for evaluating the hydraulic
performance of structures and understanding sediment transport mechanisms (SonTek, 2017). This

137

device was strategically positioned in front of each sandbox to ensure accurate and continuous
monitoring of the flow conditions (Figure 5.1).
5.2.2

Data Collection and Analysis Procedures

Prior to each experimental run, the surfaces of the sandboxes were carefully levelled. For
the runs under the ice cover surface, the Styrofoam panels were gently placed on the bottom of the
flume. Water was gradually introduced into the flume to prevent initial scouring around the spur
dikes and abutments. Once the desired flow depth was achieved, the valves were fully opened to
commence the experimental run. Flow parameters, including depth and velocity, were
continuously monitored using the SonTek-IQ Plus and the staff gauges positioned at the midpoint
of each sandbox. Each experimental run was conducted for 24 hours to ensure that the scouring
process reached equilibrium. Equilibrium was defined as the point at which the dimensions of the
scour holes and deposition dunes stabilized (Haltigin et al., 2007; Namaee & Sui, 2019b).
Upon completion of each run, the flume was carefully drained to preserve the scour
patterns, and the model ice cover was removed (for the experiments under surface cover
conditions). The geometries of the scour holes and deposition dunes were measured using
precision callipers. Data were processed using Surfer Golden Software to generate detailed 3D
profiles and contour maps of the scoured regions. To ensure the consistency and accuracy of the
data, supplementary experimental runs were conducted, extending the observation periods to 48
and 72 hours. These extended experiments confirmed that equilibrium conditions were typically
achieved within 24 hours, as no significant variations in scour depth were observed beyond this
timeframe.

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This comprehensive experimental setup and precise data acquisition provided a robust
framework for understanding the impact of spur dike on the scouring dynamics around the
abutment under various hydraulic and ice cover conditions, contributing valuable insights into the
interactions between flow conditions, sediment transport, and protective spur dikes in fluvial
systems.
5.3

Results and Discussions
At the start of each experimental run, localized scouring was initiated at the upstream

corner of the abutment. This phenomenon was attributed to the intensified flow dynamics at the
abutment toe and downward vertical flow velocities at the abutment's upstream side, generating a
vortex system at the abutment's left corner. The erosive force of the horseshoe vortex increased
the shear stress on the bed, surpassing the critical threshold needed to move sediment. As a result,
the erosion of sediment around the abutment started, leading to the formation of a scour hole. This
process highlights how the interaction between flow dynamics and sediment transport mechanisms
influences the development of local scour at any obstacle against the river flow (namely,
abutments, piers, and spur dikes). Over time, the scour hole at the left corner of the abutment
gradually enlarges in all directions, with notable expansion downstream.
Sediment particles moved within the scour hole were carried downstream by the vortex
system and transferred into the low-velocity zone downstream of the abutment toward the river
bank. Within this zone of reduced flow velocity, the stream flow interacted with downstream
eddies, forming an additional vortex system termed the wake vortex (Ishigaki et al., 2000; Namaee
& Sui, 2019b). Unlike the robust horseshoe vortex, these wake vortices, originating from lower
velocities, displayed weaker dynamics. Consequently, the lack of capacity to retain and transport
139

sediment causes particles to gradually settle on the channel bed, forming deposition dune patterns
(Dey & Raikar, 2007). Downstream of the right corner of the abutment, near the river bank, a
sediment dune emerged.
The morphology of scour holes around abutments and the pattern of deposition dunes
downstream are influenced by various factors, including flow characteristics and the condition of
the water surface. Contour maps are invaluable tools for rendering three-dimensional data into a
two-dimensional representation. By employing contour lines to connect points with identical
elevation values, these maps provide a detailed visualization of scour hole morphology and
sediment dune configuration, enabling a comprehensive analysis of riverbed deformation patterns.
This section has thoroughly examined the impact of diverse factors on the formation of scour holes
and deposition dunes, offering valuable insights into the complex dynamics of sediment transport
and erosion processes.
5.3.1

Scour at Abutment

Open Flow Condition: In open surface conditions, scour initiation occurs at the base of
the upstream corner of the abutments. The scour expands laterally and vertically as the experiment
progresses, characterized by evident horseshoe vortices within the scour hole. These vortices
highlight the significant role of flow dynamics in sediment transport mechanisms. Similar flow
separation occurs at the downstream corner of the abutments but with lower energy levels, leading
to slower sediment movement and the gradual formation of a scour hole. Over time, the
downstream scour hole expands, but its depth remains relatively shallow (Figure 5.3).
The strong flow separation at the upstream corner results in a semi-circular scour hole
extending downstream. Variations in flow depth have minimal influence on the scour hole, and
occasional reduced flow depth enlarges the scour hole laterally and vertically. In contrast, flow
140

depth has a negligible impact on the scour characteristics at the downstream corner of the
abutment. However, the velocity of the approaching flow significantly affects the scour holes at
both corners. Increased approach velocity expands the upstream scour hole in all dimensions,
especially laterally, while its impact on the downstream scour hole is less noticeable.
In most experimental runs, the two scour holes at the abutment corners did not converge.
However, In a few cases, the scour hole at the upstream corner extends to the opposite corner,
forming a larger scour hole along the frontal side of the abutment. The scour hole depth is maximal
at the upstream corner and gradually diminishes towards the downstream corner, resulting in a
shallower scour hole at the right corner (Figure 5.3).
Smooth-Covered Flow Condition: The presence of an ice cover significantly influences
the scour hole pattern around the abutment, leading to its enlargement and deepening in both
upstream and downstream directions. The scour hole often extends to the riverbank within 8 hours,
progressing upstream and forming a large hole near the abutment. Similar to open surface
scenarios, scouring starts at the upstream corner early in the experiment. After about 10 hours, the
scour holes at both corners converge into a large scour along the abutment's front side (Figure 5.3).
The scour extends towards the channel center, resembling patterns observed downstream of a spur
dike (Jafari & Sui, 2021).
Flow depth and approach velocity significantly influence the scour hole under smooth
cover conditions. Reduced flow depths lead to deeper scour holes, especially at the abutments'
upstream side. Higher approach velocities increase the scour hole area, extending it downstream
and towards the riverbank. Consequently, the scour hole area expands with rising approach

141

velocity and flow rate, with substantial sediment deposition forming downstream near the
riverbank.
Rough-Covered Surface: Increasing the roughness coefficient of the cover surface
significantly transformed the scour hole pattern around the abutment, with notable expansion in
all dimensions, especially along the flow direction (x direction). Under rough cover conditions,
the upstream corner scour enlarges much faster than under smooth cover, extending towards the
riverbank and downstream of the abutments. In most experimental runs, the scour surrounded the
abutment in all directions within the first 8 hours. The scour also extended towards the channel
center, similar to smooth conditions. The larger scour hole caused the deposition dune to shift
further towards the riverbank downstream of the abutment, substantially altering the sediment
deposition pattern (Figure 5.3).
Flow depth significantly influenced scour dynamics in rough cover scenarios. Reduced
flow depth extended the scour hole depth, especially at the abutment's front side. Approach flow
velocity also impacted the scour hole, similar to smooth cover conditions. Higher velocities led to
a downstream extension and overall enlargement of the scour hole around the abutment. At higher
velocities, the scour hole at the channel's center showed significant enlargement and downstream
extension.

Open

Smooth

Rough

Figure 5. 3:Scour patterns around the abutment under different cover conditions (flow depth=22 cm)

142

5.3.2

Application of Spure Dike

The findings suggest that river flow conditions can lead to significant local scour at the
abutment, potentially compromising the foundation and resulting in structural failure or bridge
collapse. This section aims to thoroughly assess the effectiveness of spur dikes in mitigating and
preventing scour at the abutment. The analysis includes evaluating various dike orientation angles
and configurations with the abutment to determine the dike impact on reducing scour. The
influence of distinct flow characteristics and ice cover conditions on local scour has also been
examined. The study's ultimate goal is to identify the optimal dike layout that offers the best
protection against scour for abutments, thereby minimizing the risk of structural damage.
5.3.2.1 90 alignment Spur Dike located 50 cm Upstream of the abutment
The initial spur dike layout is perpendicular to the riverbank, positioned 50 cm upstream
from the abutment. The 90-degree dike effectively intercepts the river flow, redirecting it to the
center of the channel and inducing turbulence flow upstream of the abutment rather than directly
impacting the abutment itself. Significantly, the surface cover condition is identified as the critical
factor affecting the spur dike's protective ability for the abutment. Consequently, three distinct
surface conditions and several flow characteristics have been examined individually to assess the
influence of ice cover and varying flow conditions on the dike's protection against local scour at
the abutment.
Open Flow Condition: A small scour was observed at both outside corners of the abutment
in open-flow conditions. Elevated approach velocity and higher flow rates resulted in the
expansion of scour holes. While flow depth exerted a less significant influence on scour size,
reductions in water depth correlated with slightly larger scour holes at the abutments. Variances in
143

flow characteristics, including flow depth and approach velocity, widened the scour holes and
expanded their area. However, scour depths at both outside corners remained constant across
different runs. Generally, the upstream corner exhibited slightly larger scour than the downstream
corner in most flow conditions. The sediment deposition dunes formed downstream of the dike
scour hole, away from the abutments (Figure 5.4). Despite the consistent presence of scour holes
at both corners of the abutment in open surface conditions, their size remained minimal, posing no
threat to abutment safety.
Smooth-Covered Flow Condition: The dike scour hole enlarges under smooth cover
conditions, leading to deeper scour holes and larger sediment deposition dunes. The sediment dune
expands and shifts closer to the abutment but never reaches it across different runs. Scour holes
form at both corners of the abutments, similar to those in open channel conditions but larger. The
upstream and downstream holes are almost identical in size and shape under various flow
conditions. However, the scour at the right corner tends to be slightly more prominent at higher
velocities and flow rates. Reduced water depth results in deeper scour holes at the downstream
corner compared to the upstream corner. As in open flow conditions, the scour holes at both corners
remain relatively insignificant, posing no immediate safety risks to the structure (Figure 5.4).
Rough-Covered Surface: Under rough cover conditions, the scour hole enlarges more
than under smooth cover, leading to more extensive and closer sediment deposition dunes. This
expansion results in greater sediment erosion and downstream deposition toward the abutments.
Consistently, deposition dunes push toward the upstream corner of the abutment. In some runs,
sediment partially buries the abutment's left side, preventing local scour (Figure 5.4). Conversely,
the right side of the abutment experiences more substantial scouring compared to open and

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smooth-covered flow conditions. Scouring at the downstream corner intensifies with increasing
velocity. Water depth significantly impacts scour depth, with lower depths leading to deeper holes.
Higher velocities increase the area and dimensions of the scour hole, while water depth primarily
dictates its vertical extent. Despite the larger scour hole under rough cover conditions, the
abutments remain structurally safe. The scour depth is insufficient to expose the abutment
foundation.

Open

Smooth

Rough

Figure 5. 4:Scour patterns around the abutment under different cover conditions and protective dike with 90
alignment angle (Flow depth =22 cm)

5.3.2.2 60 alignment Spur Dike located 50 cm Upstream of the abutment
The scour characteristics around a spur dike at a 60º angle differ significantly from those
observed around right-angle dikes. This variance extensively influences the sediment deposition
pattern, consequently impacting the scour observed at the abutment downstream of the dike. In the
case of a 60-degree orientation angle, the primary controlling factor is the velocity of the approach
flow. The high velocity of the flow affects the erosive forces of the eddies and vortex system in
the scour hole, leading to an increase in scour depth and enlarging the sediment deposition dunes.
Additionally, the flow surface condition serves as a secondary controlling factor. As a result, the
research findings were assessed within two distinct scenarios: high velocity and low velocity, each
under varying cover conditions and water depths. These parameters were analyzed to gain a

145

comprehensive understanding of the interaction between flow dynamics, scour around abutments,
and sediment transport processes.
5.3.2.2.1- Spur dike with alignment angle of 60º and low approaching flow velocity
Open Flow Condition: The scour hole displaces and expands towards the channel wall on
the abutment side, pushing deposition dunes closer to the abutment and riverbank. Sediment
deposits form extensive dunes near the upstream side of the abutment, serving as protective
barriers that mitigate scouring effects. The vortex system influences the morphology of the scour
hole and sediment deposition patterns. Horseshoe vortices persist around the dike, extending
downstream, while wake vortices form within 50 cm of the dike, initiating downstream deposition
dunes, especially near the upper left corner of the abutment. These dunes exhibit a pyramid-shaped
layout, indicating that wake vortices dissipate within half a meter.
On the downstream side, scouring occurs in all runs with low approach velocity. While
water depth affects scour hole dynamics, its impact is less significant than other factors. Reducing
water depth sometimes correlates with slightly larger scour holes on the right side of the abutments,
but the scouring depth remains relatively consistent. This highlights the complex interplay between
hydraulic factors and geomorphic processes in scour hole formation, where water depth
fluctuations influence the extent and dimensions of the scouring pattern.
Smooth-Covered Flow Condition: Under smooth cover conditions, sediment deposition
dynamics change significantly near the abutments. Deposition dunes often encroach closer to the
abutments, with the upstream corner frequently buried beneath the sediment. The highest dune
elevation shifts from the upper left side in open conditions to further downstream, around the
middle to lower abutment. This shift reflects changes in wake vortex strength and position,
146

indicating altered sediment transport dynamics. In smooth cover conditions, sediment dunes
caused by dike expand downstream, with deposition more dispersed and oriented downstream.
Despite deposition around the abutments, scour holes at the downstream corner remain evident,
more significant, and with distinct patterns compared to open conditions (Figure 5.5). While water
depth influences scour dynamics more under smooth cover conditions, its impact is secondary to
velocity and cover condition. Variations in water depth affect scour hole dimensions and depth,
with reduced water depth often increasing the scour hole size and occasionally its depth.
Rough-Covered Surface: Under rough cover conditions, sediment deposition dynamics
amplify, with dunes moving downstream and encroaching toward the abutment. Nearly half of the
upstream abutment section is submerged beneath sediment deposits caused by the dike, shielding
the left side from scour (Figure 5.5). Conversely, the right side remains vulnerable to large scour
holes, particularly at the downstream corner, exhibiting more significant scouring than in open or
smooth-covered conditions. Water depth significantly affects scour hole morphology under rough
cover conditions. Reduced water depth notably increases the size and depth of the scour hole,
highlighting the sensitivity of scour dynamics to water depth variations. Turbulence and eddy
formation analysis reveal the intricate relationship between the vortex system and scour hole
morphology. A robust horseshoe vortex is evident in rough cover conditions, especially at the
downstream corner of the abutments, indicating heightened turbulence intensity compared to open
conditions.

147

Open

Smooth

Rough

Figure 5. 5: Scour patterns around the abutment under different cover conditions and protective dike with 60
alignment angle (low velocity, flow depth=22 cm)

5.3.2.2.2-Spur dike with alignment angle of 60º and high approaching flow velocity
Open Flow Condition: With increased approach velocity, the vulnerability of the
abutment's upstream corner becomes more noticeable. The sediment dune caused by spur dike acts
as a protective barrier at lower velocities, preventing scouring. However, due to complex
turbulence altering sediment deposition patterns, higher velocities lead to evident scouring at the
upstream corner of the abutment. Despite extensive analysis, no clear trend in deposition dune
patterns has been identified.
In open surface scenarios, scour holes form at both abutment corners, with the downstream
corner exhibiting greater depth and area (Figure 5.6). Flow depth has minimal impact on scour
hole size, though slight enlargement of downstream corner scour holes is noted with reduced flow
depth. Elevated approach velocity transitions the scour hole morphology from semi-circular to
elliptical. The deposition dune caused by the dike maintains a semi-pyramidal form, positioned
away from the abutments. Higher velocity significantly impacts flow dynamics and turbulence,
especially at the abutment corners. Increased kinematic energy develops a robust vortex system,
enlarging scour holes, particularly at the downstream corner of the abutment.

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Smooth-Covered Flow Condition: Under smooth cover conditions, sediment dunes
caused by spur dike extend closer to the abutments, reducing scour hole dimensions at both
corners. The upstream corner scour hole is consistently smaller compared to open flow conditions,
and at the downstream corner, the scour hole also appears smaller, occasionally nearly the same
size but never exceeding that under open flow conditions. Flow depth variations have minimal
impact on the scour hole's dimensions, but a reduction in flow depth slightly causes an increase in
the scour hole depth at the downstream corner. Increased flow velocity causes the scouring at the
downstream corner to expand toward upstream, sometimes approaching the left corner. However,
the two scour holes remain distinct and did not merge. A quarter of the abutment's front section
consistently remains unaffected by scouring (Figure 5.6).
Detailed analysis reveals a complex interaction between flow depth, deposition dunes, and
vortex dynamics. Deposition dunes near the abutments interact with the wake vortex and horseshoe
vortex, reducing erosive forces at the upstream corner and limiting scour hole expansion. This
effect persists even with increased approach velocity.
Rough-Covered Surface: Rough surface conditions enforce more significant scouring at
spur dikes than smooth-covered and open-flow conditions. Higher flow velocities result in larger
and deeper scour holes around the dike, driving them closer to the abutment. This intensified
scouring alters sediment deposition patterns, causing dunes to extend further downstream. As the
deposition dune moves away from the abutment, distinct scour holes form at both corners,
reflecting the increased roughness coefficient's impact on scour dynamics. The scour hole at the
downstream corner consistently exhibits larger and deeper scour than that at the upstream corner.

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In most experiments, the upstream corner's scour hole is larger than under smooth conditions
(Figure 5.6).
Water depth has a more noticeable influence under rough-covered flow conditions, with
lower depths leading to larger and deeper scour holes at both corners. Similar interactions between
the wake vortex and horseshoe vortex are evident. In open-flow scenarios, the horseshoe vortex is
apparent at both abutment corners, especially downstream. In contrast, high-velocity flows under
cover conditions lack a noticeable vortex system, suggesting complex interactions between
vortices generated by the dike and abutment.

Open

Smooth

Rough

Figure 5. 6:Scour patterns around the abutment under different cover conditions and protective dike with 60º
alignment angle (high velocity, flow depth=22cm)

5.3.2.3 45º alignment Spur Dike located 50 cm Upstream of the abutment
The scour hole pattern exhibited notable divergence at the dike with an alignment angle of
45º compared to dikes with other angles. Notably, the dike scour hole extended further
downstream, displacing deposition dunes towards both the abutments and the riverbank
downstream of the abutments. The primary controlling factor driving these changes was the
approaching flow velocity, followed closely by the water surface cover condition and ice cover

150

roughness coefficient, a pattern similar to the dynamics observed at the dike with an alignment
angle of 60º.
However, a notable difference was observed concerning flow depth, particularly between
the 45º and 60º alignment dikes. While the impact of flow depth was negligible at the dike with an
alignment angle of 60º, it substantially influenced scenarios featuring a dike with an alignment
angle of 45º. This consistent relationship between flow depth and velocity highlighted the
significance of the Froude number as a representative indicator of both variables. The discrete
effects of various factors are evaluated independently under different ranges of approaching flow
velocity.
5.3.2.3.1- Spur dike with alignment angle of 45º and low approaching flow velocity
Open Flow Condition: In open flow conditions, the scour hole at the dike forms a distinct
pattern, concentrates around the dike tip and extends towards the channel center. Small deposition
dunes emerge downstream of the dike and in front of the abutment but remain separated, leaving
the abutment exposed to vortex effects and scouring process. The downstream corner consistently
experiences scour hole formation, with a horseshoe vortex becoming evident within several hours,
expanding the scour hole into a semi-elliptical shape while no significant scour was observed at
the upstream corner (Figure 5.7). Changes in flow depth had minimal impact on scour hole
dynamics, though a slight increase in depth may occur with the decrease in flow depth. No
consistent correlation has been observed between flow depth and scour hole dimensions, especially
under low-velocity conditions. A small deposition dune indicates that the wake vortex was too
weak to propagate far from the dike.

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Smooth-Covered Flow Condition: Under smooth cover conditions, the scour hole around
the spur dike extends further downstream and approaches but never touches the left corner of the
abutment. A scour hole can not be developed at the abutment upstream corner. As the scour hole
expands, the deposition dune also grows and occasionally reaches the frontal side of the abutment,
although this effect was minimal (Figure 5.7). Despite forming a substantial deposition dune near
the abutment, it fails to shield the zone around the downstream corner of the abutment, resulting
in a larger scour hole similar to that around the downstream of the abutment under open flow
conditions. The scour hole morphology differs notably from that under open flow conditions, with
flow depth playing a significant role. The reduction in flow depth leads to larger and deeper scour
holes, underscoring the importance of flow depth in shaping the scour morphology around the
abutment.
Rough-Covered Surface: under conditions with low flow velocity, the scour hole around
the dike favours expansion toward the channel wall and the zone downstream of the abutment
rather than the center of the channel. These scour holes are larger and deeper than those under open
flow and smooth-covered conditions. The dike scour hole extends to the upstream corner of the
abutment, converging with the local scour around the abutment, thereby increasing the scour depth
and area upstream of the abutment (Figure 5.7). Increasing the ice cover roughness coefficient
plays a crucial role in affecting the velocity component, pushing the scour hole further
downstream. This increase in kinetic energy makes the abutment's upstream corner more
vulnerable to scour and erosion. The interaction between the dike wake vortex, and the abutment
horseshoe vortex creates a complex and inconsistent influence of flow field on the scouring process
at the abutment's upstream corner. Significant upstream scouring leads to sediment transport and
the development of the deposition dunes. Under rough covered flow conditions with low flow
152

velocities, these deposition dunes often extend downstream and thus protect the right side of the
abutment. No scour was observed at the abutment's downstream corner under rough cover
conditions. However, the upstream corner of the abutment experienced continuous scouring
regardless of changes in velocity and water depth. The extent of deposition dunes increased
significantly under rough cover conditions, indicating intensified kinetic energy and robust vortex
systems, particularly the horseshoe and wake vortices.

Open

Smooth

Rough

Figure 5. 7:Scour patterns around the abutment under different cover conditions and protective dike with 45º alignment angle
(low velocity, flow depth=22cm)

5.3.2.3.1- Spur dike with alignment angle of 45º and high approach velocity
Open Flow Condition: High flow velocity significantly impacts the scouring process
downstream of the dike. As velocity increases, the scour hole around the dike extends downstream
and approaches the abutment and channel wall, which changes the deposition dune pattern.
Sediment deposition caused by the dike moves closer to the abutment, covering a substantial
portion from the upstream corner to near the downstream corner of the abutment. The deposition
dunes effectively shield the zone around the abutment from erosive forces and rapid flow. In all
runs under open flow conditions with high velocity, about 90% of the abutment's front side was
surrounded by deposition dunes (Figure 5.8).

153

Water depth had no consistent impact on dune formation. Most experimental runs have
observed a minor scour at the downstream corner of the abutment with a semi-circular shape and
less than 1 cm deep. While water depth had little effect on scour holes, reduction in flow depth
resulted in a slight increase in their dimensions without significantly affecting their depth.
Smooth-Covered Flow Condition: Under smooth cover conditions, the dike scour hole
pattern is similar to that under open flow conditions but extends wider and closer to the abutment.
The scour hole around the dike gets very close to the upstream corner of the abutment, but it rarely
reaches the upstream corner of the abutment. Substantial scour downstream of the dike pushed the
deposition dune further downstream and away from the upstream corner of the abutment. Under
open flow conditions, sediment accumulated a considerable portion of the abutment's front side.
The highest point of the dune was shifted downstream closer to the channel center (Figure 5.8).
As the deposition dunes were shifted away, the abutment's front face, especially the zone
around the downstream corner, became more susceptible to eddies. Consequently, scour holes
consistently developed at the downstream corner of the abutment under smooth cover conditions,
with significant enlargement and deeper compared to open flow conditions. Water depth
substantially influenced the dimensions of the scour holes; decreased flow depth led to significant
horizontal expansion and vertical deepening of the scour holes.
Rough-Covered Surface: Under rough cover conditions and high approaching flow
velocity, the scour hole exhibited the largest dimensions compared to other scenarios. The scour
hole caused by the spur dike extended to encompass the entire abutment, causing severe scouring
along the upstream corner and the front side of the abutment. In most experimental runs, the
upstream corner of the abutment experienced the deepest scour, reaching approximately 4 cm. The
154

scour zone often extends across the entire front side of the abutment, including both corners (Figure
5.8). The morphology of deposition dunes indicates the impact of surface cover conditions on flow
velocity and kinematic energy. An increase in ice cover roughness redirected erosive vortices
toward the abutment, extending the scour hole. A substantial scour hole at the abutment's front
side under rough cover conditions indicates a dominant horseshoe vortex.
Flow depth critically affected the scour hole dimensions; with the decrease in flow depth,
scour depth increases more than the scour range, highlighting the enhanced effects of ice cover
roughness on the velocity gradient near the bed. Namely, the increased roughness elevated kinetic
energy at lower flow layers, creating a more robust vortex system. Thus, the deposition dune was
shifted further downstream, protecting the flum wall. The deposition dunes predominantly formed
a pyramid shape, indicating a strong wake vortex downstream of the abutment (Figure 5.8).

Open

Smooth

Rough

Figure 5. 8:Scour patterns around the abutment under different cover conditions and protective dike with 45º
alignment angle (high velocity, flow depth=22cm)

5.3.2.4 90º alignment Spur Dike located 25 cm Upstream of the abutment
Based on the results of the experiment, a 90º alignment angle dike positioned 25
centimetres upstream from the abutment emerges as the optimal configuration to effectively shield
against rapid flow and erosive eddies around the abutment, thereby preventing scour formation at
the abutment located in the downstream region of the dike. Throughout various experimental
155

setups involving diverse flow characteristics and water surface cover conditions, no scour has been
observed surrounding the abutment, either upstream or downstream. However, it must be
mentioned that sediment deposition patterns around the abutment varied significantly across
different flow conditions, indicating the significant impact of water surface cover conditions and
ice cover roughness coefficients on the interaction between the vortex system around the spur dike
and abutment.
Open Flow Condition: In experiments under open flow conditions, the dike redirected
water flow toward the channel center, developing deposition dunes away from the abutment. These
dunes consistently kept an elliptical shape downstream of the dike, with the highest point in front
of the abutment but at a certain distance from the abutment (Figure 5.9). Regardless of the flow
conditions, the sediment dunes never reached the abutment, and no identified trend was observed
in their patterns. Approaching flow velocity had minimal impact on the vortex system around the
abutment and formation of the deposition dune. Occasionally, higher velocities resulted in the
extension of the deposition dune towards the abutments and generated small wake vortices within
the dune regions.
Smooth Covered Flow Condition: The presence of a cover significantly alters the pattern
of deposition dunes compared to that under open surface conditions. Under smooth cover
conditions, deposition dunes caused by the dike are extended towards the abutments, burying the
front and both corners of the abutment, providing protection against the vortex systems and
scouring process (Figure 5.9). The deposition dunes also extended downstream, safeguarding the
channel bank. Changes in flow depth had minimal impact on the channel bed deformation
dynamics, though decreased flow depth correlated with longer dunes towards the abutment. The

156

approaching flow velocity had minimal influence; an increase in flow velocity led to slightly wider
deposition dunes.
Rough-Covered Surface: The presence of a rough cover on the water surface led to an
increase in the roughness coefficient, significantly changing the deposition dune pattern. The
rough ice cover shifted the dunes closer to the abutments, providing additional protection against
eddies. In most experiments under rough cover conditions, sediment accumulated in the upstream
corner of the abutment and much of the abutment's frontal side, with the highest dune point
occurred near the abutment's center, enhancing defence against vortices and erosion around the
abutment (Figure 5.9). An increase in the roughness coefficient resulted in intensified kinetic
energy at the lower layer, promoting a horseshoe vortex system downstream of the dike. This
vortex pushed deposition dunes closer to the abutments, creating more extensive dunes at the
frontal side of the abutment. A decrease in flow depth led to a further shift of the dunes toward the
abutment. The influence of the approaching velocity on the deposition dunes and vortex system
was observed to be similar to that of under smooth cover conditions but without a consistent trend
regarding its impact on patterns of deposition dunes.

Open

Smooth

Rough

Figure 5. 9:Scour patterns around abutment scour under different cover conditions and protective dike with
90ºalignment angle, 25cm distance (flow depth=22cm)

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5.3.3

Comparative Assessment of Scour Depth Variability Around Bridge Abutments

The comprehensive analysis of this chapter indicates a clear relationship between the angle
of the spur dike, its distance from the abutment, and the resultant scour reduction. The results
consistently show that sharper dike angles, particularly the 90° angle, are highly effective in
mitigating scour. This is especially true at shorter distances (25 cm) from the abutment, where the
scour reduction can reach 100%. The effectiveness of scour reduction decreases slightly as the
distance increases to 50 cm. At 50 cm distance, the scour reduction remains high, ranging from
69% to 97%, but the 90° angle still outperforms the 60° and 45° angles.
The 60° angle, while still effective, does not provide as much protection as the 90° angle, indicating
that the steepness of the angle plays a crucial role in the efficacy of scour reduction. The 60° dike
angle also demonstrates considerable scour reduction but is less effective compared to the 90°
angle, with reductions ranging from 68% to 89%. The 45° angle consistently offers the least
protection, particularly in rough-covered flow conditions, suggesting that more acute angles are
less effective in deflecting flow and reducing scour. The 45° dike angle provides the least
protection, particularly in rough-covered conditions, where scour reduction can be as low as 25%,
highlighting its limited efficacy compared to steeper angles. Table 5.1 and 5.2 compare the
maximum scour depth around abutments with and without protective spur dikes under varying
flow depth and flow velocity conditions.
The superior performance of the 90° dike angle, particularly at closer distances to the
abutment, suggests that this configuration should be preferred in practice to maximize scour
reduction. Conversely, the limited effectiveness of the 45° angle indicates that such configurations
should be avoided in scenarios where significant scour protection is required. These insights

158

contribute to a better understanding of how spur dikes can be optimally designed and positioned
to mitigate the erosive forces of flowing water and protect hydraulic structures.
Table 5. 1: Comparison of maximum scour depth around abutment with and without protective dike, flow depth=11cm

Dike Angle

Distance from Dik
(cm)

Flow Depth
(cm)

Velocity
(cm/s)

with Dike

without
Dike

Scour
Reduction
(%)

with Dike

without
Dike

Scour
Reduction
(%)

with Dike

without
Dike

Scour
Reduction
(%)

Maximum Scour Depth (cm)

90

50

11

28

0.2

7

97%

1.54

8.2

81%

2.9

9.3

69%

90

50

11

56

0.23

7.6

97%

1.61

9

82%

3.25

10.4

69%

60

50

11

28

0.8

7

89%

1.72

8.2

79%

2.5

9.3

73%

60

50

11

56

0.94

7.6

88%

2.46

9

73%

3.34

10.4

68%

45

50

11

28

0.58

7

92%

2.73

8.2

67%

6.1

9.3

34%

45

50

11

56

0.2

7.6

97%

3.81

9

58%

7.8

10.4

25%

90

25

11

28

0

7

100%

0

8.2

100%

0

9.3

100%

90

25

11

56

0

7.6

100%

0

9

100%

0

10.4

100%

Open Flow

Smooth-covered

Rough-covered

Dike Angle

Distance from Dik
(cm)

Flow Depth
(cm)

Velocity
(cm/s)

with Dike

without
Dike

Scour
Reduction
(%)

with Dike

without
Dike

Scour
Reduction
(%)

with Dike

without
Dike

Scour
Reduction
(%)

Table 5. 2: Comparison of maximum scour depth around abutment with and without protective dike, flow
velocity=23cm/s
Maximum Scour Depth (cm)

90

50

13.5

23

0.17

6

97%

1.27

7.3

83%

2.3

8.4

73%

90

50

26

23

0.15

5.9

97%

0.79

6.6

88%

1.565

7.5

79%

60

50

13.5

23

0.55

6

91%

1.52

7.3

79%

2

8.4

76%

60

50

26

23

0.51

5.9

91%

1.4

6.6

79%

1.67

7.5

78%

45

50

13.5

23

0.5

6

92%

2.455

7.3

66%

5.7

8.4

32%

45

50

26

23

0.2

5.9

97%

1.6

6.6

76%

4.74

7.5

37%

90

25

13.5

23

0

6

100%

0

7.3

100%

0

8.4

100%

90

25

26

23

0

5.9

100%

0

6.6

100%

0

7.5

100%

Open Flow

Smooth-covered

159

Rough-covered

5.3.4

Comparative Analysis of Maximum Scour Depth Around Abutments with
Different Alignment Angles of Protective Spur Dikes

The analysis of results provides significant insights into the effectiveness of spur dikes in
reducing scour around abutments under various hydraulic conditions. The findings highlight the
critical role of dike angle and distance from the abutment in influencing the maximum scour depth.
This section elaborates on the relationships and implications derived from the data, providing a
complete interpretation of the findings.
The angle of the spur dike is a crucial parameter that significantly affects its ability to
reduce scour. The data consistently demonstrate that the 90° dike angle is the most effective in
mitigating scour across different flow conditions. At a flow depth of 22 cm (Table 5.3), the 90°
dike angle achieves the lowest maximum scour depths, particularly at a 25 cm distance from the
abutment, where it eliminates scour under all tested conditions. This superior performance is also
evident at a distance of 50 cm, where the maximum scour depths remain significantly lower than
those observed for the 60° and 45° dike angles.
Similarly, at a flow velocity of 28 cm/s (Table 5.4), the 90° dike angle maintains its
effectiveness, achieving minimal scour depths at both 25 cm and 50 cm distances from the
abutment. These results indicate that the perpendicular alignment of the dike to the flow direction
is highly efficient in deflecting the erosive forces of the water, thereby minimizing the formation
of scour holes around the abutment. The distance of the spur dike from the abutment is another
critical factor influencing scour reduction. The data reveal that closer placement of the dike (at 25
cm) generally results in more effective scour mitigation. For the 90° dike angle, no scour is
observed at this distance under both flow depth and flow velocity conditions, indicating optimal

160

protection of the abutment. This suggests that placing the dike closer to the abutment enhances its
ability to shield the structure from the direct impact of the flow.
In contrast, the 60° dike angle, while still providing a considerable reduction in scour
depth, is less effective than the 90° angle. The scour depths for the 60° angle are consistently
higher across all conditions, indicating that as the dike angle decreases, its ability to reduce scour
diminishes. This trend is even more noticeable for the 45° dike angle, which shows the highest
scour depths, particularly in rough-covered conditions. The 45° angle's reduced effectiveness can
be attributed to its less favourable orientation relative to the flow, which likely results in less
efficient deflection and more direct impact on the abutment.
The findings provide significant practical implications for hydraulic engineering and the
design of protective structures around abutments. The clear superiority of the 90° dike angle
suggests that this configuration should be preferred in design practices to maximize scour
protection. Additionally, placing the dike closer to the abutment (at around 25 cm) offers the best
protection, effectively mitigating the erosive forces and preventing the formation of deep scour
holes. By optimizing both the angle and placement of spur dikes, engineers can enhance the
longevity and stability of hydraulic structures, reducing maintenance costs and improving safety.

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Table 5. 3:Comparison of maximum scour depth around abutment with different alignment angles of protective spur
dike, flow depth=22cm
Distance

Flow

from Dik

Depth

(cm)

(cm)

90

50

90

Dike
Angle

Maximum Scour Depth (cm)
Velocity
(cm/s)

Open Flow

Smooth-covered

Rough-covered

22

14

0.2

0.71

1.47

50

22

28

0.21

1.1

1.84

60

50

22

14

0.52

1

1.15

60

50

22

28

0.79

1.65

1.95

45

50

22

14

1.03

1.13

3

45

50

22

28

0.32

1.85

5.2

90

25

22

14

0

0

0

90

25

22

28

0

0

0

Table 5. 4:Comparison of maximum scour depth around abutment with different alignment angles of protective spur
dike, flow velocity=28 cm/s
Distance

Flow

from Dik

Depth

(cm)

(cm)

90

50

90

Dike
Angle

Maximum Scour Depth (cm)
Velocity
(cm/s)

Open Flow

Smooth-covered

Rough-covered

11

28

0.2

1.54

2.9

50

22

28

0.21

1.1

1.84

60

50

11

28

0.8

1.72

2.5

60

50

22

28

0.79

1.65

1.95

45

50

11

28

0.58

2.73

6.1

45

50

22

28

0.32

1.85

5.2

90

25

11

28

0

0

0

90

25

22

28

0

0

0

5.3.5

Quantitative Analysis of Maximum Scour Depth Around Abutments with
Protective Spur Dikes under Ice-Covered Conditions
Quantitative descriptions of the maximum scour depth around the in-stream infrastructures

are essential in hydraulic studies. The empirical formulas enable engineers and researchers to
design hydraulic structures more efficiently and provide a means to predict outcomes based on
observed data. This predictive capability simplifies the estimation of critical parameters, such as
maximum scour depth under various conditions, including ice-covered flow conditions. In the
context of this study, the empirical formula allows for estimating the maximum scour depth around
162

abutments under different layouts of the protective spur dikes, flow conditions, grain sizes of bed
material, and ice cover roughness. The following parameters, which influence the maximum depth
of scour holes (Ymax) around the abutments in the presence of both ice cover on the water surface
and a protective spur dike located upstream of the abutments, are considered in this study:
Ymax= f (H, V, Le, B, D50, ni, nb, sin, D)

(1)

Where, H is the depth of the approaching flow; V is the mean velocity of the approaching
flow; Le is the effective length of the spur dike, measured distance from the dike tip to the attached
sidewall; B is the channel width; D50 is the median particle size of the bed material; ni is the
roughness coefficient of an ice cover; nb is the roughness coefficient of the channel bed, sin is
alignment angle of spur dikes, D is the space between dike and aboutment.
To accurately evaluate the influence of each parameter on the maximum depth of scour
holes around the abutment and to provide a more detailed representation of laboratory results, the
key parameters are expressed as dimensionless variables as follows:
‫ܦ‬
ܻ௠௔௫
‫ܮ‬௘ ‫ܦ‬ହ଴ ݊௜
= ݂ ൬‫ܨ‬௥ , ,
, , sin ߠ, ൬ ൰൰
‫ܪ‬
‫ܪ‬
‫݊ ܪ ܤ‬௕

(2)

Regarding the roughness of ice cover (ni), the Manning roughness coefficient for model
smooth ice cover (smooth Styrofoam panel) can be treated as a smooth concrete-like surface with
the Manning’s roughness coefficient of 0.013 (Mays, 1999; Namaee & Sui, 2019a). The following
equation suggested by Li (2012) can be used to determine Manning’s roughness coefficient for the
model rough ice cover:
(3)

k ୱ = 30 y୧ exp[−(1 − v୧ /v୫ୟ୶ )]

163

where, yi is the thickness of the ice-affected layer, vi is the averaged velocity of the iceaffected layer, vmax is the maximum velocity of the velocity profile.
(4)

ଵ/଺

݊௜ = 0.039 k ୱ

where, ks is the average roughness height of the ice underside in meters.
By using the above equation, Manning's coefficient of 0.02 has been determined for the
rough ice cover. This value also agrees with the results of Wu et al. (2015). To calculate channel
bed roughness coefficient (nb) for non-uniform sand bed, the following equation proposed by
Hager (1999) was used:
(4)

ଵ/଺

݊௜ = 0.039 k ୱ

In this study, the roughness coefficient of the sand bed was determined to be 0.0113 for a
sand bed of D50=0.60 mm and 0.012 for a sand bed of D50=0.90 mm, respectively.
(5)

⁄

ଵ ଺
݊௕ = 0.039 ‫ܦ‬ହ଴

From the laboratory experiment data, an equation has been developed to predict the
maximum depth of scour holes around an abutment with a protective spur dike under ice-covered
flow conditions. Under open flow conditions, however, because the scour holes around the
abutment were minimal and negligible, it is difficult to derive an equation for determining the
maximum depth of scour holes under open flow conditions. Also, to ensure the safety of in-stream
infrastructures, including bridge abutments, engineers should consider the worst scenarios in
practice, namely, the maximum depth of scour holes. Thus, the following expression has been
derived based on data from laboratory experiments for calculating the maximum depth of scour
holes around an abutment with a protective spur dike under ice-covered flow conditions:

164

௒೘ೌೣ
ு

௅

ି଴.ସସ ஽
ି଴.ସ
ఱబ

஻

ு

= 0.27 (‫ܨ‬௥ )଴.ଷଵ ቀ ೐ቁ

ቀ

ቁ

௡

ቀ ೔ቁ
௡್

଴.ହଽ

஽ ଴.଼ଵ

(sin ߠ)ିଵ.ଵ଼ ቀ ቁ
ு

(6)

ܴଶ = 0.851

The exponent 0.31 indicates a less-than-linear relationship, implying that doubling the
௒

Froude number will increase ೘ೌೣ ratio by approximately 23%, suggesting that more energetic
ு

flow conditions (higher Froude number) lead to a greater maximum scour depth. Since the Froude
number describes the approach velocity and flow depth, this implies that increasing velocity while
decreasing water depth leads to larger and deeper scour holes at the abutment.
௅

The ೐ , representing the blockage ratio of a spur dike, has an exponent of -0.44, indicating
஻

a considerable negative correlation. As

௅೐
஻

increases, the dike significantly obstructs water flow,

reducing turbulence kinetic energy and consequently decreasing the size and depth of the scour
hole at the abutment. The magnitude of this exponent suggests an inverse effect on maximum scour
௒

depth. Doubling the blockage ratio results in an approximate 28% decrease in the ೘ೌೣ
ratio.
ு
The

஽ఱబ
ு

ratio, which normalizes the median grain size of sediment (D50) by water height

(H), has an exponent of -0.4. This indicates an inverse relationship between median particle size
௒

and maximum scour depth ( ೘ೌೣ ). An exponent of -0.4 suggests a nearly linear inverse relationship,
ு

௒

where doubling the median particle size results in approximately a 34% decrease in ೘ೌೣ . This
ு

reflects a moderate effect, as larger particles resist scouring more effectively, thereby reducing the
depth of the scour hole.
The

௡೔
௡್

, representing the roughness coefficient of ice cover relative to the riverbed, has an

exponent of 0.59, indicating a positive correlation. As the roughness coefficient of ice cover
increases, the maximum velocity approaches the riverbed, deepening the scour hole. The high
exponent implies that ice cover roughness significantly influences local scour, regardless of the
protective dike. Doubling

௡೔
௡್

௒

results in an approximate 50% increase in the ೘ೌೣ ratio.
ு

The sin ߠ exponent is -1.18, indicating a strong negative correlation. As sin ߠ increases, it
provides more protection against turbulence flow and erosive vortex system, thus significantly
reducing the size and depth of scour at the abutment. The large magnitude of this exponent
indicates that the sine of the spur dike angle has the most substantial negative impact on local scour
165

௒

at the abutment. Doubling sin ߠ results in an approximate 55% decrease in the ೘ೌೣ
ratio. This
ு
highlights the critical role of flow direction or the orientation of the spur dike in controlling scour
processes at the abutment.
஽
ு

has one of the most substantial positive effects on Ymax among all the variables in the

formula. Its large exponent of 0.81 indicates that variations in the distance between the dike and
abutment can lead to significant changes in maximum scour depth, making it a critical factor in
determining the extent of scouring. Doubling the ு஽ ratio results in an approximate 75% increase in
௒೘ೌೣ
ு

. This shows that ு஽ is one of the most influential variables in determining the maximum scour

depth at abutment. Compared to other variables, the space ratio (ு஽ ) plays a more dominant role in
increasing scour depth, surpassing factors such as the Froude number, sediment size, and blockage
ratio. The only variable that rivals the space ratio in its influence on Ymax is sin ߠ , which exerts an
opposite effect.
Figure 5.10 shows a strong agreement between the results calculated using the proposed
equation and those of laboratory experiments, confirming the accuracy and validity of the formula.

166

Figure 5. 10:Comparison of calculated maximum scour depth around the abutments with a protective spur dike to
those of experiments under ice-covered flow conditions

Understanding the dependence of the relative maximum depth of the scour holes (Ymax/H)
on the above-mentioned dimensionless variables provides engineers with in-depth knowledge in
designing in-stream infrastructures considering the impact of ice cover on the water surface. The
formula derived in this study can be used to predict how the maximum depth of the scour hole will
be affected by changing flow conditions, shape and dimension of structure design, as well as the
characteristics of bed material, such as grain size of sediment. By identifying the most influential
variables, resources can be allocated more effectively, focusing on controlling or measuring the
most critical variables, thereby facilitating better in-stream infrastructure design.

167

5.4

Conclusions
In this section, a comprehensive study has been conducted to investigate the effects of

various flow conditions and surface cover conditions on the scouring processes around the
abutments and the protective efficacy of different spur dike configurations. The results underscore
the complex interplay between flow dynamics, vortex formation, and channel bed deformation
mechanisms in shaping scour holes and deposition dunes. The key findings are as follows.
1. Scour Initiation and Development at the abutment:
•

Under open flow conditions, scouring was initiated at the upstream corner of the
abutment due to intensified flow dynamics and the formation of a robust horseshoe
vortex. This initial scour gradually expanded downstream, forming a semi-circular
scour hole and indicating the critical role of flow dynamics in sediment transport
mechanisms.

•

Under both smooth and rough ice-covered flow conditions, the presence of an ice
cover significantly influenced the scour hole patterns, typically enlarging and
deepening the scour holes. The presence of rough ice cover led to faster and more
extensive development of scour holes around the abutment, highlighting the
substantial impact of the ice cover roughness on scouring processes around the
abutment.

2. Application of Spur Dike and the Effects of dike alignment angle:
•

A 90º alignment dike positioned 25 cm upstream of the abutment proved to be the
most effective configuration in preventing local scour around the abutments under
168

different flow and water surface cover conditions. This configuration successfully
redirected the rapid flow, created a tranquil zone downstream of the abutment, and
diminished the erosive impact of the vortex system. Even under rough ice-covered
conditions, the shear stress remained below the critical threshold required to
remove sediment from the channel bed, thereby preventing the development of
scour holes around the abutment.
•

The 90º-alignment dike positioned 50 cm upstream of the abutment effectively
redirected river flow, minimizing the direct impact on the abutment. This
configuration consistently reduced local scour around the abutment regardless of
water surface cover conditions. Scour holes remained small and did not
compromise the abutment safety, even under higher flow velocities and lower flow
depths. Overall, this setup effectively mitigated the local scour and ensured the
stability of the abutment.

•

The 60º-alignment dike influenced scour dynamics under both low and high
approaching flow velocities. The low velocities resulted in the formation of
deposition dunes close to the upstream side of the abutment, serving as protective
barriers and effectively preventing scouring at the upstream side of the abutment.
Scour at the downstream corner of the abutment remained evident but relatively
minor, showcasing the dike's effectiveness in reducing overall scour impact. The
high velocities caused notable scouring at the abutment corners, with the greater
depth and area of the scour holes at the downstream corner. Despite the increased
scouring, the dike provided substantial protection against local scour surrounding
the abutment. Overall, the presence of the 60º-alignment dike successfully

169

redirected the erosive forces and facilitated the development of protective
deposition dunes, ensuring the stability of the abutment and reducing the risk of
structural damage.
•

The impact of the 45º-alignment dike on scour dynamics varied significantly with
respect to the magnitudes of the approaching flow velocities. At low velocities, the
dike effectively minimized the scouring process at the upstream corner of the
abutment. Scouring was consistently observed at the downstream corner of the
abutment, with the extension of the scour hole to the upstream corner of the
abutment under rough cover scenarios. In contrast, high approaching flow
velocities caused significant local scour downstream of the abutment. Sediment
deposition patterns were altered, with deposition dunes forming closer to and
sometimes accumulated in front of the abutment, providing partial protection.
However, the presence of a rough ice cover led to more pronounced local scour at
the abutment's upstream corner and frontal side. Overall, the 45º-alignment dike
mitigated scour at low velocities by maintaining a stable erosion pattern and
forming protective deposition dunes. However, at high flow velocities, increased
erosive forces required further considerations to prevent structural impact on the
abutment.

•

The spur dikes significantly reduced local scour and erosion around the abutments.
The presence of a dike created a relatively tranquil zone between the abutment and
the dike, reducing kinematic energy downstream and diminishing shear stress
levels. This effective disruption of rapid flow resulted in a smaller-scale scour zone
compared to an unaffected area by the dike.

170

3. Sediment Deposition Patterns:
•

Deposition dunes formed downstream of the abutments varied with water surface
cover conditions and flow characteristics. Under rough-covered flow conditions,
deposition dunes extended closer to the abutments, providing additional protection
against the erosive eddies and local scour. These patterns underscore the
importance of considering water surface cover conditions in sediment management
strategies.

•

The highest points of deposition dunes often occurred near the abutment's frontal
face, enhancing protection against localized erosion. This finding highlights the
role of sediment deposition in mitigating the impact of erosive forces on the
abutments.

4. Ice Cover and Surface Roughness:
•

The presence of an ice cover on the water surface leads to an increase in the
roughness coefficient and notably amplifies the erosive forces of the vortex system,
particularly the horseshoe vortex. The rough ice cover shifts the maximum velocity
closer to the channel bed, elevating shear stress levels and causing significant
enlargement of the scour hole. The result highlights the critical influence of the ice
cover roughness on the flow dynamics and sediment transport.

•

A reduction of flow depth under an ice cover intensified the formation of robust
horseshoe vortices, generating deeper scour holes. Higher approaching flow
velocities further increased the kinematic energy around the abutment, enlarging
the scour hole downstream of the abutment and towards the channel bank. These

171

findings emphasize the need to consider water surface cover conditions and flow
characteristics in designing effective scour protection measures.
5. Influence of Flow Depth and Velocity:
•

Both flow depth and approaching flow velocity significantly impacted the size and
shape of scour holes. Lower flow depths generally led to deeper scour holes, while
higher velocities expanded the scour hole dimensions. This relationship
underscores the critical role of hydraulic conditions in shaping scour patterns.

•

The interplay between flow depth and velocity influenced the extent of sediment
deposition and the development of vortex systems around the abutments. Higher
velocities and lower flow depths intensified the erosive forces, leading to more
significant scour and sediment displacement.

Overall, the study presented in this section highlights the importance of the flow dynamics
and water surface cover conditions in designing effective measures to prevent local scour around
the abutment. The findings provide valuable insights for optimizing spur dike configurations to
mitigate local scour around the downstream abutment and enhance the structural integrity of
hydraulic structures. By understanding the complex interactions between hydraulic factors,
sediment transport mechanisms, and structural features, engineers can develop more robust and
practical solutions to protect infrastructure from the erosive forces of flowing water and local
scour.

5.5

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174

6 CHAPTER SIX: General Conclusions
6.1

Conclusions of Chapter 3
This chapter explores the complex interactions between spur dikes, ice cover, and flow

dynamics. The research aims to understand how different spur dike orientation angles and varying
ice cover conditions influence velocity fields, turbulence structures, and local scour patterns.
6.1.1

Impact of Ice Cover on Flow Dynamics

A significant aspect of this study is the analysis of how ice cover affects the flow
characteristics around spur dikes. The presence of ice cover introduces an additional boundary
layer, fundamentally altering the velocity profile and increasing the complexity of the flow. Key
observations include:
•

Turbulence Intensities: The presence of ice cover significantly increases turbulence
intensities. For smooth ice cover, turbulence intensities increased by approximately 10%,
while rough ice cover increased by up to 30%. This rise in turbulence intensity is attributed to
the added roughness and friction from the ice cover, which enhances velocity fluctuations and
shear stress near the riverbed.

•

Velocity Components: The experiments revealed that the maximum values of the 3D velocity
components (streamwise, lateral, and vertical) are significantly influenced by the presence and
roughness of the ice cover. The rough ice cover increases the magnitude of these velocity
components and shifts their maximum values closer to the riverbed. This shift is critical as it
results in higher shear stress at the sand bed, leading to more extensive sediment movement
and deeper scour holes.

175

6.1.2

Orientation Angle of Spur Dikes

The study meticulously examines spur dikes set at different orientation angles (90°, 60°,
and 45°) relative to the flow direction. The orientation angle of the spur dike significantly affects
flow separation, vortex formation, and subsequent scour patterns:
•

At 90° Orientation, the spur dike oriented perpendicular to the flow generates the most
pronounced flow separation, forming strong horseshoe vortices at the upstream face of the
dike. These vortices play a significant role in sediment entrainment and transport. Moreover,
this configuration exhibits the highest turbulence intensity and bed shear stress. The strong
downflows and powerful vortices create deep scour holes closely downstream of the dike. The
perpendicular orientation results in the deepest scour holes, with a significant increase in scour
depth due to the enhanced flow blockage and increased energy dissipation around the dike.

•

At 60°, the vortices generated are less intense compared to the 90° orientation. Flow separation
occurs, but the reduced angle allows a smoother flow transition around the dike. The
turbulence intensity and bed shear stress are moderate, leading to less severe scour than the
perpendicular orientation. This angle provides a balance between flow disruption and
sediment transport. The scour holes formed are shallower than those at 90°, indicating reduced
sediment transport capacity and erosion potential.

•

The 45° orientation angle results in the least flow disruption, as the dike aligns more closely
with the natural flow direction. This reduces the formation of strong vortices and minimizes
turbulence. The turbulence intensity and bed shear stress are significantly lower, creating a
more stable flow regime around the dike. The reduced blockage ratio allows for smoother
flow and less aggressive sediment transport. The shallowest scour holes are observed at this
angle, with a 5% to 10% reduction in scour depth for each 10° decrease from 90°. This
176

demonstrates the direct correlation between orientation angle and scour depth, highlighting
the importance of angle optimization in dike design.
The orientation angle of spur dikes significantly influences the hydraulic and sediment
transport characteristics in rivers. By carefully selecting and optimizing the orientation angle,
engineers can enhance the effectiveness of spur dikes in river management. Engineers must
consider these effects when designing spur dikes to achieve optimal performance. For instance, a
90° orientation might be suitable for applications requiring significant flow diversion and sediment
entrainment, whereas a 45° orientation could be preferred for minimizing erosion and maintaining
flow stability.
6.1.3

Analysis of Velocity Components

The study presents a comprehensive analysis of the three-dimensional (3D) velocity
components, streamwise (Ux), lateral (Uy), and vertical (Uz), around spur dikes with different
orientation angles under various flow conditions, including the presence of ice cover. Key findings
include:
•

Streamwise Velocity (Ux): Ux exhibits the highest magnitude among the three velocity
components, highlighting its dominant role in flow dynamics. The streamwise velocity is
generally lower inside the scour hole than in regions outside the hole, but it shows significant
fluctuation, especially near the dike tip. Furthermore, the presence of an ice cover increases
the maximum values of Ux by approximately 10% to 25%, depending on the roughness of the
ice. Rough ice cover, in particular, enhances the velocity fluctuations, shifting the location of
maximum velocity closer to the riverbed, thereby increasing bed shear stress and sediment
transport potential. Under different surface conditions, the streamwise velocity profile inside

177

the scour hole displays a convex shape, with the minimum velocity at the bottom of the scour
hole and increasing towards the mid-water depth.
•

Lateral Velocity (Uy): Uy demonstrates high variability and irregularities in its vertical
distribution, making it less predictable than Ux. The values of Uy are generally smaller than
Ux but can be significantly influenced by surface conditions. The presence of ice cover,
particularly rough ice, increases the magnitude of Uy. This indicates that ice cover not only
affects the primary flow direction but also induces substantial lateral flow variations, which
can enhance lateral sediment transport and impact bank stability. Unlike Ux, the vertical
distribution of Uy does not follow a consistent trend across different experimental conditions.
However, Uy is observed to be mostly positive inside and outside the scour hole, indicating a
net lateral movement of water.

•

Vertical Velocity (Uz): Uz is typically negative inside and outside the scour holes, signifying
strong downflow around the spur dike. This downflow contributes to the formation and
deepening of scour holes by enhancing sediment entrainment from the bed. Ice cover
significantly modifies the vertical velocity distribution. The presence of an ice cover,
especially with higher roughness, increases the magnitude of Uz, bringing the maximum
vertical velocity closer to the bed. This results in higher sediment transport rates and deeper
scour holes. The vertical velocity profile exhibits a parabolic shape, with the minimum Uz
value at the bottom of the scour hole and increasing towards the original sand bed level before
reducing again towards the water surface. This pattern reflects the complex interaction
between downflow and the horseshoe vortex system around the spur dike.
By analyzing the velocity components, engineers can optimize the orientation angle and

structural design of spur dikes to minimize adverse scour effects and enhance stability. Insights
178

into the vertical and lateral velocity components help develop better sediment management
strategies, ensure balanced sediment transport, and reduce the risk of excessive erosion or
deposition.
6.1.4

Turbulence Intensities and Reynolds Shear Stress

This section delves into the intricate details of turbulence intensities and Reynolds shear
stress as observed in the study, providing insights into their implications for river engineering and
management.
•

Spatial Distribution and Ice Cover: The highest turbulence intensities were observed near the
spur dike tip and just above the scour hole, where the interaction between the flow and the
structure is most intense. The turbulence intensity profiles exhibited a distinct pattern, with
streamwise turbulence (ux') being the largest, followed by lateral (uy') and vertical (uz')
turbulence components, indicating a hierarchy of turbulence strength. The presence of ice
cover significantly increased turbulence intensities. Smooth ice cover raised turbulence
intensities by approximately 15%, while rough ice cover increased by up to 30%. Also, the
roughness of the ice cover amplified the velocity fluctuations, enhancing the energy
dissipation and mixing within the flow. This increase in turbulence intensities directly impacts
sediment transport and scour processes.

•

Flow Rate and Dike Orientation: Higher flow rates were associated with increased turbulence
intensities. The study found that with a higher approach velocity, the maximum depth of the
scour hole was achieved in a shorter time, and the scour hole became larger. The orientation
angle of the spur dike also influenced turbulence intensities. A 90° orientation generated the
strongest turbulence, while decreasing the angle to 45° resulted in reduced turbulence
intensity, corresponding to a decrease in scour depth.
179

•

Reynolds Shear Stress: Inside the scour holes, Reynolds shear stress was found to be negative,
indicating upward vertical momentum transport caused by a negative velocity gradient. This
phenomenon is critical for sediment entrainment from the bed. The Shear stress reached its
maximum slightly above the scour hole, aligning with the initial level of the sand bed, and
then decreased towards the flow surface, forming a parabolic distribution. Ice cover, especially
rough ice, increased the absolute value of Reynolds shear stress. This enhancement in shear
stress is attributed to the increased velocity fluctuations and turbulence intensities induced by
the ice cover. The ice cover created higher shear stress at the bed, resulting in more
pronounced sediment movement and deeper scour holes. Rough ice cover, in particular,
intensified these effects, highlighting the importance of considering ice conditions in
hydraulic design. Higher flow rates corresponded to increased Reynolds shear stress,
indicating a stronger momentum transfer and greater potential for sediment entrainment. The
orientation angle of the spur dike significantly affected the distribution and magnitude of
Reynolds shear stress. A perpendicular orientation (90°) resulted in the highest shear stress,
while a smaller angle (45°) reduced the shear stress, contributing to a decrease in scour depth
and turbulence intensity.

•

These findings provide a scientific basis for improving the design and performance of spur
dikes, contributing to more sustainable and resilient river management practices. Engineers
can optimize spur dike orientation and design parameters to effectively manage turbulence
and shear stress, minimizing adverse scour effects and ensuring structural stability. Insights
into turbulence and shear stress dynamics facilitate better sediment management practices,
enabling the control of sediment transport and deposition in river systems, particularly in
regions with seasonal ice cover. By accounting for the increased turbulence and shear stress

180

under ice-covered conditions, hydraulic structures can be designed to withstand enhanced
erosive forces, reducing the risk of structural failure and environmental impact.
6.2

Conclusions of Chapter 4

6.2.1

Scour Hole Geometry and Influence of Ice Cover

The presence of ice cover on the water surface significantly altered the geometry of the
scour holes. Under open channel conditions, scour holes around the dike exhibited a more circular
shape. However, under ice-covered conditions, especially rough ice, the scour holes expanded
laterally and became more elliptical. This indicates that the roughness of the ice cover intensifies
the scour process, leading to a wider and deeper scour hole. The rough ice cover, simulating ice
jams, led to increased turbulence and stronger vortex formation, exacerbating bed deformation.
The alignment angle of the spur dike greatly influenced the extent of scour holes. When
the dike angle was reduced from 90º to 60º, the upstream length of the scour hole decreased by
approximately 60%. This reduction in dike angle led to a decrease in the intensity of vortices and,
consequently, less scouring in the upstream direction. Conversely, the downstream length of the
scour hole increased by 20% with a 50% increase in flow rate, indicating that higher flow rates
enhance downstream erosion due to increased flow velocity and turbulence.
6.2.2

Impact of Dike Alignment Angle

The dike alignment angle was found to be a crucial factor in vortex formation and strength.
At a 90º alignment angle, the primary vortices were more substantial and lasted longer, causing
deeper and more extensive scour holes. As the dike alignment angle decreased, the primary
vortices became less intense, resulting in less severe scouring. The study also found that reducing

181

the dike alignment angle from 90º to 45º transformed the shape of the scour hole from semi-circular
to elliptical, indicating a significant change in flow patterns and bed erosion dynamics.
The dike alignment angle influenced the dike tip's flow acceleration and the downflow
creation. A 90º dike caused a more substantial flow contraction and separation, increasing flow
velocity at the dike tip by up to 60% of the approaching flow velocity. This increased velocity led
to stronger downflow and more severe scouring at the dike toe. Reducing the dike alignment angle
mitigated these effects, leading to less intense scouring and a shift in the location of maximum
scour depth downstream.
6.2.3

Effect of Flow Conditions

Higher flow velocities and Froude numbers were associated with deeper and wider scour
holes. Under high flow conditions, the kinetic energy and turbulence intensity increased, leading
to more severe bed erosion. The study found that an increase in flow Froude number by 15%
resulted in a 5% to 10% increase in maximum scour depth. Additionally, the impact of flow depth
was significant; shallower flows with higher velocities led to more scouring compared to deeper
flows with lower velocities.
The median grain size of the bed material (D50) played a significant role in scour dynamics.
Finer bed material led to deeper scour holes due to easier entrainment and transport of sediment
particles. The study observed increased bed material fineness resulted in larger maximum scour
depths regardless of flow surface cover conditions.
6.2.4

Development and Dynamics of Vortex Systems

The formation of primary vortices at the upstream side of the dike toe was a critical factor
in initiating local scour. These vortices were influenced by the dike alignment angle and
182

approaching velocity. The secondary vortices, known as horseshoe vortices, developed within the
scour hole and were the primary mechanism for ongoing scouring. The study found that under icecovered conditions, the intensity of these vortices increased, leading to deeper and wider scour
holes.
The horseshoe vortices were observed to last the longest and were the major cause of scour
hole expansion. The wake vortices, formed in the low-velocity zone downstream of the dike, also
contributed to sediment transport and deposition. The interaction between these vortex systems
and the flow dynamics around the dike was critical in determining the final shape and size of the
scour holes.
6.2.5

Equations for Maximum Scour Depth

The study derived equations to predict the maximum depth of scour holes around spur dikes
under open and ice-covered flow conditions. These equations incorporated key parameters such as
flow velocity, depth, sediment characteristics, dike alignment angle, and roughness coefficients of
the ice cover and riverbed. The derived equations demonstrated high accuracy in predicting scour
depths, with significant implications for the design and assessment of in-stream infrastructures.
The study highlighted differences in the impact of flow conditions on scour depths between
open and ice-covered scenarios. Under ice-covered conditions, the influence of the flow Froude
number was less evident than in open channel conditions. However, the presence of ice cover,
particularly rough ice, led to deeper and more extensive scour holes due to increased flow
resistance and turbulence. The findings underscore the importance of considering these factors in
the design and implementation of hydraulic structures in cold regions. The derived predictive

183

equations provide valuable tools for engineers to anticipate and mitigate the impacts of scour,
ensuring the stability and longevity of in-stream infrastructures.
6.3

Conclusions of Chapter 5

6.3.1

Scour at Abutments

Scour initiation was observed at the upstream corner of the abutment due to intensified
flow dynamics and the formation of a robust horseshoe vortex. This initial scour gradually
expanded in all directions, forming a semi-circular scour hole. Sediment particles dislodged from
the scour hole were carried downstream by the vortex system, settling in a low-velocity zone to
form deposition dunes. Higher flow velocities resulted in larger scour holes, while variations in
water depth showed minimal impact on the scour hole's dimensions.
The presence of an ice cover significantly influenced the scour hole patterns. The ice cover
led to the enlargement and deepening of the scour holes, which expanded upstream and
downstream, often reaching the river bank within eight hours. The scour holes at both corners of
the abutment converged, forming a large scour hole along the frontal side of the abutment. Flow
depth and approach velocity had notable effects on the scour hole characteristics, with reduced
flow depth resulting in deeper scour holes and higher velocities increasing the scour hole area.
Increasing the roughness coefficient of the ice cover further expanded the scour hole in all
dimensions, particularly along the flow direction. The larger scour holes prompted the deposition
dunes to shift closer to the abutment, significantly altering the sediment deposition pattern.
Reduced flow depth under rough cover conditions led to deeper scour holes, especially at the
frontal side of the abutment. Approaching flow velocity influenced the overall enlargement of the
scour hole, particularly in higher velocities, which increased the area and depth of the scour hole.
184

6.3.2
•

Application of Spur Dikes

90º Spur Dike Positioned 25 cm Upstream: This configuration proved the most effective in
preventing scour around the abutments. The perpendicular alignment of the dike effectively
redirected the rapid flow, creating a tranquil zone downstream and diminishing the erosive
impact of the vortex system. Under varying flow conditions and surface covers, this setup
consistently prevented scour holes from forming around the abutment.

•

90º Spur Dike Positioned 50 cm Upstream: This alignment effectively reduced local scour
under open, smooth, and rough ice cover conditions. The spur dike minimized the direct
impact of the flow on the abutment, leading to very small scour holes. The setup proved
effective even under higher flow velocities and lower depths.

•

60º Spur Dike: The scour dynamics varied significantly with the flow velocity. At low
velocities, sediment deposition dunes formed close to the upstream side of the abutment,
serving as protective barriers and effectively preventing scouring at the upstream side.
However, scouring at the downstream corner remained evident, though relatively minor.
Significant scouring was observed at the abutment corners at high velocities, with the
downstream corner exhibiting greater depth and area. Despite the increased scouring, the dike
provided substantial protection against local scour surrounding the abutment.

•

45º Spur Dike: The impact of this configuration varied considerably between low and high
approach velocities. At low velocities, the dike effectively minimized scouring at the upstream
corner of the abutment. However, scouring consistently occurred at the downstream corner,
particularly under rough cover scenarios, where the scour hole extended to the upstream
corner of the abutment. Significant scouring was observed downstream of the abutment at

185

high velocities, with sediment deposition patterns altering to provide partial protection. Rough
ice cover led to more scouring at the abutment's upstream and frontal corners.
6.3.3

Sediment Deposition Patterns

Sediment deposition patterns downstream of the abutments exhibited significant variability
influenced by surface conditions and flow characteristics. In open surface conditions, the
deposition dunes were generally located at a greater distance from the abutments. The absence of
surface cover resulted in a more dispersed sediment deposition, primarily driven by the less
obstructed flow dynamics. The dunes formed under these conditions were typically smaller and
positioned further downstream, offering limited abutment protection.
Under smooth cover, the deposition dunes exhibited a downstream expansion pattern.
Although the dunes extended closer to the abutments than open surface conditions, they did not
achieve the same level of protection as observed under rough cover conditions. The smooth ice
cover facilitated a more streamlined flow, leading to less turbulent interaction and a less extensive
sediment deposition pattern. The smooth cover facilitated a more uniform flow, influencing the
scour hole patterns by enlarging and deepening the holes. The smoother surface reduced the
turbulence intensity, leading to less scouring.
In scenarios with rough cover, the sediment deposition dunes extended closer to the
abutments, providing additional protection against erosive eddies and local scour. The highest
points of the deposition dunes frequently formed near the frontal face of the abutments, creating a
natural barrier that enhanced protection against localized erosion. This phenomenon was
particularly evident under rough cover conditions, where the increased roughness of the ice cover
facilitated the displacement of sediment closer to the abutments. The pyramid-shaped deposition

186

dunes, characteristic of rough cover conditions, underscored the enhanced sediment trapping
efficacy due to the intensified interaction between the wake vortex generated by the dike and the
horseshoe vortex at the abutments.
6.3.4

Influence of Flow Depth and Velocity

The interplay between flow depth and velocity was a crucial determinant of the scour hole
dimensions and sediment deposition patterns around the abutments. Lower flow depths generally
result in deeper scour holes due to the concentration of erosive forces in a reduced vertical space.
Conversely, higher flow depths tended to disperse the erosive forces over a larger vertical range,
resulting in shallower scour holes but larger in the area of the hole. This dispersion reduced the
intensity of the shear stress exerted on the bed material, leading to less scouring.
Approach velocity also played a significant role in shaping the scour morphology. Higher
velocities increased the flow's kinetic energy, thereby enhancing the erosive potential of the vortex
systems around the abutments. This increase in velocity led to larger and deeper scour holes,
particularly at the downstream corners of the abutments. The higher velocities also influenced the
sediment deposition patterns, promoting the formation of more extensive deposition dunes
downstream. In contrast, lower velocities resulted in less scouring and smaller scour holes. The
reduced kinetic energy of the flow led to a more controlled sediment transport and deposition
process, with smaller and less extensive deposition dunes forming downstream.

187

6.4

Limitations of this experimental study
The experimental investigations undertaken in this research were subject to certain

constraints. Despite implementing various strategies to mitigate these limitations, some were
inevitable.
One of the constraints was associated with the Acoustic Doppler Velocimeter (ADV) used
in the study. The ADV was unable to capture data from a depth of 10 cm below its transmitter,
resulting in the absence of data for the top 10 cm below the water surface. Additionally, the power
of the water pump limited the maximum flow rate capacity. The pump could provide a maximum
flow rate of 120 litres per second. Moreover, the study employed specific sediment sizes (0.48
mm, 0.60 mm, and 0.90 mm) to simulate riverbed conditions. However, natural riverbeds typically
exhibit a wider range of sediment sizes and compositions, potentially affecting scour processes in
ways not fully replicated in the experimental conditions.
6.5

Future Directions
Based on the results and conclusions of this experimental study, here are several

recommendations for future directions and further study:
1) Extended Field Studies: Conduct field studies in various river environments to validate the
experimental findings. Different river conditions, sediment types, and ice cover variations
should be considered to ensure the applicability and robustness of the results across diverse
natural settings.
2) Long-term Monitoring: Implement long-term monitoring of scour and sediment dynamics
around bridge abutments with spur dikes. This will provide insights into the temporal
evolution of scour and the long-term effectiveness of different spur dike configurations.

188

3) Hydrodynamic Modeling: Use advanced hydrodynamic models to simulate the flow
conditions and scour patterns observed in experiments. These models can help understand the
complex interactions between flow, sediment transport, and spur dike configurations,
providing a predictive tool for future designs.
4) Impact of Climate Change: Study the potential impacts of climate change on local scour
dynamics. Changes in river flow patterns, ice cover duration, and frequency due to climate
change could affect the performance of scour mitigation measures.
5) Integration with Other Mitigation Measures: Explore the integration of spur dikes with other
scour mitigation measures, such as riprap, collars, and bed sills. Combining different strategies
might provide enhanced protection against scour.
6) Ecological Impact Assessment: Assess the ecological impacts of installing spur dikes in rivers.
Studies should focus on how these structures affect local habitats, aquatic life, and overall
river health.
By addressing these future directions, researchers can further advance the understanding
and effectiveness of scour mitigation techniques, ensuring the stability and safety of bridge
abutments in various hydraulic environments.

189

APPENDIX
Table I. 1:Experimental data and the results of Chapter 4
Sand Size
(mm)

Dike
Angle

Surface
Condition

Open

90

Smooth

Rough

Open

0.9

60

Smooth

Rough

Open

45

Smooth

Rough

Open
0.6

90
Smooth

Water
Depth
(cm)

Approach
velocity
(cm/s)

Fr

Le/B

D50/H

ni/nb

Sin θ

Scour
Max
(cm)

11

29.08633

0.28

0.25

0.009

-

1

11.6

21

30.1414

0.21

0.25

0.0045

-

1

12.0

29

21.92688

0.13

0.25

0.003

-

1

8.2

12

28.2097

0.26

0.25

0.009

1.068

1

14.0

23

27.03777

0.18

0.25

0.0045

1.068

1

14.8

32

24.8049

0.14

0.25

0.003

1.068

1

10.0

10

29.71363

0.3

0.25

0.009

2.46

1

16.3

22

29.38163

0.2

0.25

0.0045

2.46

1

16.0

32
11

26.57668
25.96993

0.15
0.25

0.25
0.22

0.003
0.009

2.46
-

1
0.86

12.2
9.6

23

33.04617

0.22

0.22

0.0045

-

0.86

13.5

34

29.22092

0.16

0.22

0.003

-

0.86

8.8

9

21.61143

0.23

0.22

0.009

1.068

0.86

11.5

21

24.40018

0.17

0.22

0.0045

1.068

0.86

14.5

32

21.26135

0.12

0.22

0.003

1.068

0.86

10.6

11

28.04753

0.27

0.22

0.009

2.46

0.86

14.5

22

27.91255

0.19

0.22

0.0045

2.46

0.86

15.8

33

19.79175

0.11

0.22

0.003

2.46

0.86

12.8

13

27.103

0.24

0.18

0.009

-

0.7

8.3

24

30.68811

0.2

0.18

0.0045

-

0.7

8.9

33

23.39025

0.13

0.18

0.003

-

0.7

6.3

11

25.96993

0.25

0.18

0.009

1.068

0.7

10.2

24

30.68811

0.2

0.18

0.0045

1.068

0.7

11.1

34

21.91569

0.12

0.18

0.003

1.068

0.7

8.0

9

26.30957

0.28

0.18

0.009

2.46

0.7

12.4

18

29.23435

0.22

0.18

0.0045

2.46

0.7

12.7

33

23.39025

0.13

0.18

0.003

2.46

0.7

10.2

12

20.61478

0.19

0.25

0.006

-

1

9.5

26

23.9559

0.15

0.25

0.003

-

1

12.0

32

17.71779

0.1

0.25

0.002

-

1

7.2

8

15.06012

0.17

0.25

0.006

1.142

1

11.3

21

18.65896

0.13

0.25

0.003

1.142

1

14.2

33

16.19325

0.09

0.25

0.002

1.142

1

9.4

190

Rough

Open

60

Smooth

Rough

Open

45

Smooth

Rough

Open

0.9

90

Smooth

Rough

Open

0.6

90

Smooth

Rough

12

22.78476

0.21

0.25

0.006

2.636

1

13.5

24

21.48168

0.14

0.25

0.003

2.636

1

16.7

36

20.67181

0.11

0.25

0.002

2.636

1

11.7

11

20.77595

0.2

0.22

0.006

-

0.86

8.0

22

23.5053

0.16

0.22

0.003

-

0.86

9.3

32

17.71779

0.1

0.22

0.002

-

0.86

6.2

12

16.27483

0.15

0.22

0.006

1.142

0.86

9.8

23

21.02938

0.14

0.22

0.003

1.142

0.86

11.4

35

16.67674

0.09

0.22

0.002

1.142

0.86

8.3

10

16.83773

0.17

0.22

0.006

2.636

0.86

11.5

21

21.52957

0.15

0.22

0.003

2.636

0.86

13.8

34

12.78415

0.07

0.22

0.002

2.636

0.86

10.7

9

16.9133

0.18

0.18

0.006

-

0.7

7.2

23

24.03358

0.16

0.18

0.003

-

0.7

10.8

29

16.86683

0.1

0.18

0.002

-

0.7

5.4

11

17.65956

0.17

0.18

0.006

1.142

0.7

8.9

20

19.61

0.14

0.18

0.003

1.142

0.7

12.6

32

14.17423

0.08

0.18

0.002

1.142

0.7

7.2

11

18.69835

0.18

0.18

0.006

2.636

0.7

10.6

21

18.65896

0.13

0.18

0.003

2.636

0.7

14.4

32

14.17423

0.08

0.18

0.002

2.636

0.7

9.6

11

28.04753

0.27

0.25

0.009

-

1

12.7

22

27.91255

0.19

0.25

0.0045

-

1

11.5

36

26.30957

0.14

0.25

0.003

-

1

8.2

12

27.12471

0.25

0.25

0.009

1.068

1

14.8

22

29.38163

0.2

0.25

0.0045

1.068

1

13.3

34

21.91569

0.12

0.25

0.003

1.068

1

10.3

7

19.88817

0.24

0.25

0.009

2.46

1

16.5

17

23.2451

0.18

0.25

0.0045

2.46

1

15.6

28

18.23082

0.11

0.25

0.003

2.46

1

12.0

10

16.83773

0.17

0.25

0.006

-

1

7.5

20

21.01071

0.15

0.25

0.003

-

1

11.0

30

17.15517

0.1

0.25

0.002

-

1

7.2

12

16.27483

0.15

0.25

0.006

1.142

1

9.6

21

20.09427

0.14

0.25

0.003

1.142

1

13.2

32

15.94601

0.09

0.25

0.002

1.142

1

9.0

11

18.69835

0.18

0.25

0.006

2.636

1

11.8

22

24.97438

0.17

0.25

0.003

2.636

1

15.7

31

13.951

0.08

0.25

0.002

2.636

1

11.3

191

Open

90

Smooth

Rough

Open

90

Smooth

Rough
0.48
Open

60

Smooth

Rough

Open

45

Smooth

Rough

12

20.61478

0.19

0.25

0.0048

-

1

13.5

21

20.09427

0.14

0.25

0.0024

-

1

13.8

30

18.87069

0.11

0.25

0.0016

-

1

14.0

13

22.58584

0.2

0.25

0.0048

1.186

1

14.8

22

23.5053

0.16

0.25

0.0024

1.186

1

15.4

34

23.742

0.13

0.25

0.0016

1.186

1

15.7

9

19.73218

0.21

0.25

0.0048

2.737

1

16.7

22

22.03622

0.15

0.25

0.0024

2.737

1

17.5

31

19.18262

0.11

0.25

0.0016

2.737

1

17.2

8

15.94601

0.18

0.25

0.0048

-

1

11.2

18

15.94601

0.12

0.25

0.0024

-

1

8.2

29

16.86683

0.1

0.25

0.0016

-

1

9.1

12

18.4448

0.17

0.25

0.0048

1.186

1

13.1

22

19.09806

0.13

0.25

0.0024

1.186

1

10.8

31

15.69487

0.09

0.25

0.0016

1.186

1

10.9

25

32.88697

0.21

0.25

0.0048

2.737

1

15.2

24

23.01608

0.15

0.25

0.0024

2.737

1

12.5

33

14.394

0.08

0.25

0.0016

2.737

1

12.7

13

25.97371

0.23

0.22

0.0048

-

0.86

14.8

24

24.55049

0.16

0.22

0.0024

-

0.86

14.2

36

18.79255

0.1

0.22

0.0016

-

0.86

12.1

11

20.77595

0.2

0.22

0.0048

1.186

0.86

16.4

22

20.56714

0.14

0.22

0.0024

1.186

0.86

16.2

32

21.26135

0.12

0.22

0.0016

1.186

0.86

14.5

11

24.93114

0.24

0.22

0.0048

2.737

0.86

18.5

21

24.40018

0.17

0.22

0.0024

2.737

0.86

18.0

33

25.1895

0.14

0.22

0.0016

2.737

0.86

15.8

12

17.35982

0.16

0.18

0.0048

-

0.7

9.0

19

17.74821

0.13

0.18

0.0024

-

0.7

7.7

27

16.27483

0.1

0.18

0.0016

-

0.7

6.0

11

18.69835

0.18

0.18

0.0048

1.186

0.7

11.1

21

15.78835

0.11

0.18

0.0024

1.186

0.7

9.2

33

12.59475

0.07

0.18

0.0016

1.186

0.7

8.3

11

20.77595

0.2

0.18

0.0048

2.737

0.7

13.4

21

21.52957

0.15

0.18

0.0024

2.737

0.7

11.0

32

14.17423

0.08

0.18

0.0016

2.737

0.7

10.5

192

Table I. 2:Experimental data and the results of Chapter 5

without
Dike

Scour
Reduction
(%)

with Dike

without
Dike

Scour
Reduction
(%)

0.9

with Dike

0.6

Scour
Reduction
(%)

0.9

without
Dike

0.6

with Dike

0.9

Velocity
(cm/s)

0.6

Flow Depth
(cm)

0.9

Distance from Dik
(cm)

0.6

Dike Angle

Sand Size
D50
(mm)

Maximum Scour Depth (cm)

90

50

11

28

0.2

7

97%

1.54

8.2

81%

2.9

9.3

69%

90
90

50
50

11
22

56
14

0.23
0.2

7.6
6.3

97%
97%

1.61
0.71

9
7.1

82%
90%

3.25
1.47

10.4
8

69%
82%

90

50

22

28

0.21

7

97%

1.1

7.5

85%

1.84

8.5

78%

90

50

13.5

22.5

0.17

6

97%

1.27

7.3

83%

2.3

8.4

73%

90
90

50
50

13.5
26

44.3
11.7

0.2
0.15

6.8
5

97%
97%

1.55
0.43

8.5
5.8

82%
93%

9.7
6.2

71%
84%

90

50

26

23

0.15

5.9

97%

0.79

6.6

88%

7.5

79%

60

50

11

28

0.8

7

89%

1.72

8.2

79%

2.84
1
1.56
5
2.5

9.3

73%

60

50

11

56

0.94

7.6

88%

2.46

9

73%

3.34

10.4

68%

60

50

22

14

0.52

6.3

92%

1

7.1

86%

1.15

8

86%

60

50

22

28

0.79

7

89%

1.65

7.5

78%

1.95

8.5

77%

60

50

13.5

22.5

0.55

6

91%

1.52

7.3

79%

2

8.4

76%

60

50

13.5

44.3

0.72

6.8

89%

8.5

72%

2.86

9.7

71%

60

50

26

11.7

0.2

5

96%

5.8

87%

0.82

6.2

87%

60

50

26

23

0.51

5.9

91%

2.4
0.73
4
1.4

6.6

79%

1.67

7.5

78%

45

50

11

28

0.58

7

92%

2.73

8.2

67%

6.1

9.3

34%

45
45

50
50

11
22

56
14

0.2
1.03

7.6
6.3

97%
84%

3.81
1.13

9
7.1

58%
84%

7.8
3

10.4
8

25%
63%

45

50

22

28

0.32

7

95%

7.5

75%

5.2

8.5

39%

45

50

13.5

22.5

0.5

6

92%

7.3

66%

5.7

8.4

32%

45
45

50
50

13.5
26

44.3
11.7

0.12
0.92

6.8
5

98%
82%

1.85
2.45
5
3.5
0.9

8.5
5.8

59%
84%

7.5
2.67

9.7
6.2

23%
57%

45

50

26

23

0.2

5.9

97%

1.6

6.6

76%

4.74

7.5

37%

90

25

11

28

0

7

100%

0

8.2

100%

0

9.3

100%

90
90

25
25

11
22

56
14

0
0

7.6
6.3

100%
100%

0
0

9
7.1

100%
100%

0
0

10.4
8

100%
100%

90

25

22

28

0

7

100%

0

7.5

100%

0

8.5

100%

90

25

13.5

22.5

0

6

100%

0

7.3

100%

0

8.4

100%

90

25

13.5

44.3

0

6.8

100%

0

8.5

100%

0

9.7

100%

90

25

26

11.7

0

5

100%

0

5.8

100%

0

6.2

100%

90

25

26

23

0

5.9

100%

0

6.6

100%

0

7.5

100%

Open Flow

193

Smooth-covered

Rough-covered

194