LOCAL SCOUR AROUND BRIDGE ABUTMENTS UNDER ICE COVERED CONDITIONS By Peng Wu Bsc, Hefei University o f Technology, China, 2007 Msc, Hefei University o f Technology, China, 2010 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 October 10, 2014 © PENG WU, 2014 UMI Number: 3663183 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. Di!ss0?t&Ciori Publishing UMI 3663183 Published by ProQuest LLC 2015. Copyright in the Dissertation held by the Author. Microform Edition © ProQuest LLC. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code. ProQuest LLC 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106-1346 ABSTRACT Local scour refers to the sediment transport around hydraulic structures by flowing water. Excessive scour around the abutment can potentially cause damage to the bridge, which may also result in catastrophic consequences. Abutment scour refers to the local scour generated by the flow passing around bridge abutments. One o f the challenging problems for hydraulic engineers is the prediction o f maximum scour depth around abutments and pier foundations so that proper provisions can be made in the design and construction to mitigate the consequences. Despite significant research efforts to improve the understanding o f scour related problems, abutment scour is still among the more complex and challenging problems. Over the past few decades, local scour around bridge abutments has received wide attention, and many researchers have contributed various studies on the topic. The current state knowledge on local scour still has insufficiently understood aspects, for example, ice accumulation has never been addressed in the abutment scour research. The impacts o f ice cover has never been conducted. To fill this gap, the present research is conducted. The ice cover can change the channel morphology and flow field. It is well known that river ice affects the vertical and lateral distribution o f flow in a channel. Additionally, because river ice affects the flow conditions, it potentially influence sediment transport. Hence, the scour around abutments is affected. In the present research, a series o f large flume experiments are conducted. By adding different simulated ice covers in the flume, ice-covered flow can be generated. By comparing the scour profiles and maximum scour depth around two commonly used abutments in three non-uniform sediments, the ice cover impacts have been investigated. A significant increase can be noticed by adding ice cover. With the increase in ice cover roughness, the maximum scour depth increase correspondingly. Meanwhile, semi-circular abutment can generate a relatively small scour hole. Furthermore, the role o f densimetric Froude number, armor layer sediment size, M anning's roughness coefficient are all analyzed in the research. Several empirical equations are developed from present research for the estimation o f maximum scour depth around abutments. TABLE OF CONTENTS ABSTRACT....................................................................................................................................................i TABLE OF CONTENTS...............................................................................................................................ii CO-AUTHORSHIP......................................................................................................................................iv List of Tables.................................................................................................................................................v List of Figures...............................................................................................................................................vi ACKNOWLEDGEMENT............................................................................................................................ix 1 GENERAL INTRODUCTION................................................................................................................... 1 1.1 Literature review..................................................................................................................................3 1.1.1 Local Scour characteristicsaround bridge abutments in open channels....................................... 3 1.1.2 Ice Covered issues on local scour............................................................................................... 8 1.2 Research objectives............................................................................................................................ 12 1.2.1 Objective O ne............................................................................................................................. 12 1.2.2 Objective Two............................................................................................................................. 12 1.2.3 Objective Three........................................................................................................................... 13 1.3 Research innovations......................................................................................................................... 14 1.4 Outline of dissertation........................................................................................................................ 14 2 METHODOLOGY...................................................................................................................................20 2.1 Theoretical analysis...........................................................................................................................20 2.2 Experimental study............................................................................................................................22 2.2.1 Study site.....................................................................................................................................22 2.2.2 Experimental design and construction........................................................................................22 2.2.3 Measurement apparatus...............................................................................................................25 2.2.4 Experimental procedures.............................................................................................................27 3 RESULTS AND DISCUSSION...............................................................................................................29 3.1 Impacts of ice cover on local scour around semi-circular bridge abutment...................................... 30 3.1.1 Methodology...............................................................................................................................31 3.1.2 Results and discussion................................................................................................................33 3.1.3 Conclusion..................................................................................................................................46 3.2 Local scour around bridge abutments under ice covered condition: comparing of square abutment and semi-circular abutment......................................................................................................................49 3.2.1 Experimental setup......................................................................................................................50 3.2.2 Results and discussion................................................................................................................56 3.2.3 Conclusions.................................................................................................................................63 3.3 Scour morphology around bridge abutments with non-uniform sediment under ice cover..............67 3.3.1 Methodology...............................................................................................................................69 3.3.2 Results and discussion................................................................................................................73 3.3.3 Conclusions.................................................................................................................................79 3.4 Armor layer analysis of local scour around bridge abutments under ice cover................................ 82 3.4.1 Methodology...............................................................................................................................84 3.4.2 Results and discussion................................................................................................................88 3.4.3 Conclusions...............................................................................................................................100 3.5 ADV measurements of flow field along a round abutment under ice covers................................. 103 3.5.1 Methodology............................................................................................................................. 104 3.5.2 Results and Discussion.............................................................................................................. 107 3.5.3 Conclusion................................................................................................................................ 116 3.6 The incipient motion of bed material and shear stress analysis around bridge abutments under icecover....................................................................................................................................................... 119 3.6.1 Experimental setup and measurement.......................................................................................120 3.6.2 Results and discussion..............................................................................................................122 3.6.3 Conclusions............................................................................................................................... 129 4 GENERAL CONCLUSION................................................................................................................... 133 5 APPENDIX............................................................................................................................................. 135 CO-AUTHORSHIP For all chapters in this thesis, I was the primary investigator, leading: the design o f studies, collection o f data and analysis o f data. I wrote the manuscripts and was responsible for incorporating comments and feedback on into final versions o f the thesis. However, despite the use o f first-person singular in writing the thesis, I would like to acknowledge that this work was not conducted in isolation. Faye Hirshfiled is my PhD colleague who assisted in all aspects o f field work. To acknowledge her contribution, she is included in all publications that stem from my work. Chen Pangpang and Dr. Jun Wang contributed some comments and figures on the manuscripts, so they were included in some publications respectively. Finally, my supervisor, Dr. Jueyi Sui, contributed to experimental design, data analysis o f the present research. And he is included in author ship on all resulting publications. Publications and authorships stemming from this thesis (published or submitted) Wu P, Hirshfield F, Sui J, Wang J, 2014. Impacts o f ice cover on local scour around semi­ circular bridge abutment, Journal o f Hydrodynamics, 2014, 26(l):840-847. (Chapter 3.1) Wu P, Hirshfield F, Sui J, Chen P, 2014. Local scour around bridge abutments under ice covered condition- an experimental study, IJSR-D-13-00042, International Journal o f Sediment Research, accepted for publication. (Chapter 3.2) Wu P, Hirshfield F, Sui J, 2013. Scour morphology around bridge abutment with non-uniform sediments under ice cover, proceedings for the 35th IAHR World Congress, Chengdu, China, September, 2013. (Chapter 3.3) Wu P, Hirshfield F, Sui J, 2014. Armor layer analysis o f local scour around bridge abutments under ice cover, River Research and Applications, accepted for publication, published online in Wiley Online Library, DOI: 10.1002/rra.2771. (Chapter 3.4) Wu P, Hirshfield F, Sui J, 2013. ADV measurements o f flow field along a round abutment under ice covers, accepted for publication at the proceedings for 17th Workshop on River Ice, Edmonton, Canada, July 21-24, 2013. (Chapter 3.5) Wu P, Hirshfield F, Sui J, 2014. The incipient motion o f bed material and shear stress analysis around bridge abutments under ice-cover, Canadian Journal o f Civil Engineering, cjce-20130552, published on line 2014-09-09. (Chapter 3.6) List of Tables Table 3.1-1 Experimental running condition sum m ary.................................................................... 34 Table 3.2-1 Summary o f running conditions......................................................................................54 Table 3.3-1 Experimental data o f small scale flume experim ents.................................................. 70 Table 3.3- 2 Experimental data o f small scale flume experim ents.................................................. 72 Table 3.4-1 Test condition and non-uniform sediment composition o f each experim ent............ 87 Table 3.5-1 The maximum scour depth under different conditions...............................................105 Table 5-1 Experimental data collected at non-uniform sand (D 50 = 0.58 m m )............... 135 Table 5- 2 Experimental data collected at non-uniform sand ( D 5 0 = 0.50 m m )............... 136 Table 5- 3 Experimental data collected at non-uniform sand (D 50 = 0.47 m m )............... 137 Table 5- 4 Scour contours at D 5 0 = 0.58 m m ...................................................................................... 138 Table 5- 5 Scour contours at D 5 0 = 0.50 m m ...................................................................................... 144 Table 5- 6 Scour contours at D 5 0 = 0.47 m m .....................................................................................150 L ist of Figures Figure 1- 1 A typical local scour around a bridge abutment................................................................1 Figure 1- 2 Flow and local scour around a bridge abutm ent.............................................................. 3 Figure 1- 3 Time evolution o f clear-water scour and live-bed scour................................................. 5 Figure 1- 4 Comparison o f velocity and suspended sediment concentration distributions between ........................................................................................................................................................................9 Figure 1- 5 The velocity distribution o f open water, floating smooth and floating rough cover.. 11 Figure 1- 6 Bridge abutment (BA) types used in experim ents......................................................... 13 Figure 2- 1 Schematic o f force on particle on a sloping bed under ice cover................................. 21 Figure 2- 2 The modification plan for the flume at QRRC............................................................... 24 Figure 2- 3 The modification o f flume at QRRC................................................................................ 25 Figure 2- 4 The dimension o f ADV (left) and the sensor head o f a ADV (rig h t)......................... 26 Figure 2- 5 Releated parameters and Experimental procedure (BA: bridge abutm ent) .............. 28 Figure 3.1-1 Dimensions o f abutment, ice cover and rough ice cover used in the experiment... 32 Figure 3.1-2 Measuring points along the semi-circular abutm ent.................................................... 33 Figure 3.1-3 The local scour around the abutment and the measurement o f the sco u r.................34 Figure 3.1 -4 The scour profiles around the abutment under different cover conditions (D5o=0.50mm)............................................................................................................................................36 Figure 3.1-5 (a) Cross-section along the semi-circular abutment (Dso=0.50mm); (b) Crosssection along the semi-circular abutment under smooth and rough cover (D5o=0.50mm)............ 39 Figure 3.1 -6 Variation o f scour volume around bridge abutm ent....................................................40 Figure 3.1 -7 (a) Variation o f maximum scour depth with the Froude number under different sediment composition (b) The comparison o f maximum scour depth in open channel and ice covered condition (D5o=0.50mm).......................................................................................................... 42 Figure 3.1- 8 Dependence o f maximum scour depth on related variables........................................43 Figure 3.1 -9 Dependence o f maximum scour depth on related variables under ice cover........... 45 Figure 3.2-1 (a) The plan and vertical view o f the modified flume; (b) The coordinate system and abutments dim ensions.......................................................................................................................52 Figure 3.2- 2 (a) Inside view o f the flume; (b) Rough ice cover used in the research......................53 Figure 3.2- 3 Typical local scour profiles around the square abutment and semicircular abutment ......................................................................................................................................................................57 Figure 3.2- 4 The variation o f maximum scour depth with abutment m o d el.................................. 58 Figure 3.2- 5 The variation o f D 50 with scour depth under different conditions.............................60 Figure 3.2- 6 The variation o f maximum scour depth with different sediments and abutm ents.. 61 Figure 3.2- 7 The variation o f maximum scour depth with different covered conditions............. 63 Figure 3.3-1 A comparison o f flow profiles with (a) and without(b) ice cover.............................68 Figure 3.3- 2 The experimental setup o f the small scale flume (left) and large scale flume (right) ......................................................................................................................................................................69 Figure 3.3- 3 (a) The local scour around the bridge abutment in the small-scale flume a n d 74 Figure 3.3- 4 The scour contour in the large scale flu m e................................................................... 76 Figure 3.3- 5 The sediment samples LI (left) and L2 (rig h t).............................................................76 Figure 3.3- 6 The cross section o f the local scour along the abutment (left) and samples collected (right)..........................................................................................................................................................77 Figure 3.3- 7 The sand analysis o f sam ples.......................................................................................... 77 Figure 3.3- 8 The variation o f scour depth with densimetric Froude number in small-scale flume ......................................................................................................................................................................78 Figure 3.3- 9 The variation o f scour depth under smooth ice cover in large scale flum e...............79 Figure 3.4- 1 The layout o f the experimental large scale flum e........................................................ 85 Figure 3.4- 2 Dimensions and measuring points o f abutments.......................................................... 85 Figure 3.4- 3 Experimental flume set up and rough ice cover (up); Armor layer around the square abutment comer (bottom )........................................................................................................................86 Figure 3.4- 4 Typical local scour contour around square abutment (left) and semi-circular abutment (rig h t)........................................................................................................................................ 89 Figure 3.4- 5 Distribution curves for the non-uniform sediment....................................................... 91 Figure 3.4- 6 Samples o f armor layer, fine sediment ridge and related distribution curves.......... 92 Figure 3.4- 7 Variation o f maximum scour depth with F0 at square abutment (left) and semi­ circular abutment (right)..........................................................................................................................94 Figure 3.4- 8 Variation o f maximum scour depth with related variable around square abutment 94 Figure 3.4- 9 Variation o f maximum scour depth with related variable around semi-circular abutment..................................................................................................................................................... 95 Figure 3.4-10 Dependence o f maximum scour depth on related variables around square abutment..................................................................................................................................................... 95 Figure 3.4-11 Dependence o f maximum scour depth on related variables around the semi­ circular abutm ent......................................................................................................................................96 Figure 3.4- 12 The impact o f ice cover roughness on the maximum scour depth...........................98 Figure 3.4-13 Regression relationship under ice cover o f related variables around square abutment..................................................................................................................................................... 99 Figure 3.4-14 Regression relationship under ice cover o f related variables around semi-circular abutment..................................................................................................................................................... 99 Figure 3.5-1 Experimental setup..........................................................................................................104 Figure 3.5- 2 Abutment dimension and coordinate system............................................................... 107 Figure 3.5-3 Contours o f scour hole under open channel, smooth cover, and rough c o v er 109 Figure 3.5 -4 The scour profile along the round abutment under different conditions............... 111 Figure 3.5- 5 The velocity distribution along the abutment under different conditions: open channel (Left), smooth cover (Middle), rough cover (rig h t)............................................................ 116 Figure 3.6-1 Sketch o f experimental setup and abutment dim ension.............................................121 Figure 3.6- 2 Incipient motion in the scour hole under ice cover.................................................... 122 Figure 3.6- 3 Incipient motion o f different sediments with the maximum scour d e p th 125 Figure 3.6- 4 The variation o f shear Reynolds number with dimensionless shear stress 127 Figure 3.6- 5 The maximum scour depth variation with dimensionless shear stress around square abutment................................................................................................................................................... 128 Figure 3.6- 6 The maximum scour depth variation with dimensionless shear stress under ice cover and open channel (square abutment)......................................................................................... 128 Figure 3.6- 7 The maximum scour depth variation with dimensionless shear stress under smooth ice cover and rough ice cover (semi-circular abutment)................................................................... 129 ACKNOWLEDGEMENT I would like to express my first thanks to my supervisor, Dr. Jueyi Sui, who has been, and still is helpful and supportive through my entire PhD. His advice on experimental design, writing and data analysis is always helpful. Discussions about my research, but also on the academic research in general, has been really inspiring. I had a great time working with Dr. Sui and lots o f ideas for my future research are based on the conservations between us. During my three years PhD at University o f Northern British Columbia, I spent a lot time in the field and received great help from my colleagues and friends. I would like to thank Faye Hirshfield for being my most reliable and helpful colleague and friend, who spend almost two entire field seasons with me from 2011 to 2012, even when it was pouring rain or one meter snow. Anja Forster has been the best field assistant for flume construction in 2011. The work cannot be finished without her help. I would like to thank m y committee members Dr. Jianbing Li, Dr. Liang Chen, Dr. Youmin Tang, Dr. Junjie Gu for their time and support through the last few years. They brought a lot to my thesis, especially by widening my view beyond my research area. I would also express my thanks to Dr. Ellen Petticrew, Dr. Phil Owens, Dr. Neil Hanlon, Dr. Phil Burton, Dr. Joselito Arocena as my course supervisors in my first year o f PhD. Integrating my own research within a wide spectrum o f knowledge, and sharing it with people from different fields make me becoming more interested in environmental issues. I would also like to thank the Institutions that supported my research. The University o f Northern British Columbia is really welcoming and supportive o f foreign students. The Research Project Awards funding helped a lot during my hard time. The Dr. Max Blouw Quesnel River Research Center, which is the base o f my research, has the best manager and staff. Richard Holmes and Samuel Albers provided great help as managers o f the research center. Lazio Enyedy and Howard helped me a lot to finish the flume construction and experiments. I had the best life and work experience in Likely. Friends and people in Likely are greatly acknowledged. More on the personal side o f my life, I would like to say a big thank you to my parents. Also special thanks to Mr. and Mrs. Hirshfield. They always showed support and interest in my work even if it was hard to follow. Being overseas and far from home, I made m yself at home in Price George. I want to thank all the cool and wonderful friend I met there and who make my 3-year experience in Prince George so pleasant. My PhD friends, Alex Koiter, Steffi LaZerte, Adrian James, Dominic Reiffarth, Lisha Berzins, Yueting Shao et al. have been the best classmates ever. I would extend my thanks to Heidi, Leah, Ben, Dr. Youqin Wang, Guangji Hu, Lin Bai, Bo Huang for their friendship, encouragement and belief. 1 GENERAL INTRODUCTION Scour is a natural phenomenon caused by erosion on alluvial or gravel beds by a flowing stream. Local scour refers to the scour caused by an obstruction in the channel (Chang, 2002). Local scour around bridge foundation elements is one o f the most common reasons for bridge collapse and has caused huge economic loss around the world (Figure 1-1). For example, in 1987, 17 bridges were destroyed by scour during a flood in New York and New England. During the flooding in Georgia in 1994 over 500 bridges were damaged due to the scour (Richardson and Davis, 2001). According to a nation-wide study conducted by the US Federal Highway Administration, 75% o f 383 bridge failures in 1973 involved abutment damage and 25% involved pier damage (Chang, 1973). In 1978, another extensive study indicated that problems caused by local scour at bridge abutments were equal to that at bridge piers (Brice and Blodgett, 1978). Bridge scour has been identified as the most common cause o f highway bridge failures and it accounts for about 60% o f all bridge collapse in the United States (Deng and Cai, 2009). A study by Kandasamy and Melville (1998) showed that 6 o f 10 bridge failures which occurred in New Zealand during Cyclone Bola were related to abutment scour. In China, local scour damaged 49 railway bridges in 1994, resulting in an interruption o f railway traffic for 2319 hours (Zhu and Liu, 2012). Figure 1- 1 A typical local scour around a bridge abutment 1 It is noted that local scour around bridge foundations has negative impacts on the performance and stability o f bridges. In the past few decades, local scour around bridge abutments in open channels has received wide attention, and many scholars have conducted numerous studies on this topic (e.g. Laursen and Toch, 1956; Froehlich, 1989; Melville, 1997; Coleman et al, 2003; Dey 2005a, 2005b). To estimate the maximum scour depth, many relationships and formulae have been developed which can be grouped into four categories: regime approach, dimensional analysis, analytical or semi-analytical approach, and probabilistic approach (Zhang, et al, 2008). In Canada, ice cover can stay up to six months in some northern areas. The formation o f ice cover involves complex interactions between hydrodynamic, mechanical, and thermal process (Shen, 2010). Ice cover can result in many problems, such as ice jamming, flooding, restricting the generation o f hydro-power, block river navigation and affect the ecosystem balance. (Hicks, 2009). However, to my present knowledge, there is still very little research on the local scour under ice cover. Field observations indicate that ice cover significantly affects velocity distribution and sediment transport processes in rivers. An imposed ice cover can lead to an increased composite resistance and almost double the wetted perimeter. Understanding river ice process and ice effects on hydraulic structures is important for the design and operation o f hydraulic projects. To examine the influence o f ice cover on local scour around bridge abutments, the present research is conducted. In the present research, ice cover plays an important variable for the estimation o f scour depth around abutments. Compared to an open channel, ice cover changes the hydraulics by adding an extra boundary. Due to the limitations o f laboratory study and lack o f field data, the flow field in the scour hole is still not clear. The main objectives in this research are to investigate, the local scour development, equilibrium depth estimation, bed evolution, velocity distribution, sediment transport rate and numerical model verification under various ice cover conditions. 2 1.1 Literature review 1.1.1 Local Scour characteristics around bridge abutments in open channels The scour occurring around bridge abutments can be grouped into three categories: general scour, constriction scour, and local scour. General scour is the removal o f sediment from the bottom o f a river channel by the flow o f the river. While constriction scour is the removal o f sediment from the bottom and sides o f the river channel, due to the higher velocity caused by hydraulic structures such as a bridge. Local scour is caused by an acceleration o f flow in the vicinity o f structures, which may happen around bridge piers, abutments, or other objects that obstruct the flow in different ways (Chang, 2002). Local scour is a dynamic feedback process between turbulent flow and erodible boundaries. The vortex systems and the down flow have high turbulence which is the main cause o f local scour. Compared to general scour and constriction scour, local scour can cause serious damage to bridges. So in the following passage, the process o f local scour around bridge abutments will be the main interest. bridge abutment bow wave water surface '\7 r ^- — ........................................................................................V ................. wake vortices „ riverbed do wn flow i primary vortex | a Figure 1- 2 Flow and local scour around a bridge abutment The flow field around a bridge abutment in natural open rivers is very complex. Moreover, the complexity increases with the development o f a scour hole which leads to separation o f flow into three vortex flow systems around the abutment. Figure 1-2 shows a schematic diagram o f the flow field and scour hole around an abutment. Kwan and Melville (1994) suggested that the scour hole is mainly dominated by a large primary vortex and associated down flow. The primary vortex extends to the downstream o f the abutment and loses its identity after some 3 distance. Near the water surface a vertical pressure gradient is developed due to the stagnation o f the approaching flow. At the comer o f abutment downstream, the flow accelerates and leads to the development o f concentrated vortices, referred to as wake vortices. Wake vortices are created due to the separation o f flow upstream and downstream o f the abutment comers (Zhang, 2005). Under open channel condition, the flow patterns and maximum down-flow are relatively unaffected by changes in approach flow depth (Kwan and Melville, 1994). Under ice covered conditions, flow fields around the bridge abutments will be significantly changed. This hypothesis will be verified by the experimental study. Based on whether there is sediment transported by the approaching flow, local scour can be classified into two categories: clear-water scour and live-bed scour (Chabert and Engeldinger, 1956). Clear-water scour takes place in the absence o f sediment transport by approaching flow into the scour hole. Live-bed scour occurs when the scour hole is continuously fed with sediment from upstream. The time variation o f the clear-water scour and live-bed scour is shown schematically in Figure 1-3. Chabert and Engeldinger (1956) observed that the equilibrium clearwater scour depth is 10% greater than live bed scour depth. The clear-water scour and live-bed scour are determined by the critical velocity (Vc). The clearwater scour can occur when V/Vc < 1, while the live-bed scour will happen if V/Vc > 1, in which V is average flow velocity and Vc is the critical flow velocity for sediments. There are many formulae used to decide the value o f Vc. In this thesis, the equation from Laursen (1963) will be used for non-uniform sediments, Vc = Kuy}/6 D ^ (1-1) in which, y{is average flow depth in the main channel or overbank area at the approach section; Di0 is bed material particle size in a mixture in which 50% percent are smaller; K u equals to 6.19 (S.I. Units). 4 equilibrium clear-water scour depth equilibrium live-bed scour depth TJ -s o> T? 10% ( X clear-water scour live-bed scour Time, t Figure 1- 3 Time evolution o f clear-water scour and live-bed scour (After Chabert and Engeldinger, 1956) Studies on the local scour around bridge elements in open channels has been widely done in the past few decades and are still o f continuous interest for scholars. These studies can be grouped into two categories (Zhang, 2005). One is the prediction o f scour depth by using empirical or semi-empirical formulae based on field data or experimental data. The other is numerical simulation. There are basically three types o f scour depth estimation formulae from the literature (Lim, 1997): a. the regime approach, which relates the scour depth to the increased discharge or flow at the abutment; b. the dimensional analysis, where relevant dimensionless parameters describing the scour are correlated (most o f the past formulas are obtained from this way); c. analytical or semi-empirical approach, which are based sediment transport relationships between approach flow and shear stress around the abutment. A large amount o f scour formulae are available in the published literature. However, most o f these formulae were derived from limited variables related to the scour development (Barbhuiya and Dey, 2004): (a) Variables related to the approaching flow (flow depth, mean velocity, roughness, etc); (b) Variables related to bed sediment (grain size distribution, density, cohesiveness, etc); (c) Variables related to the flow (water density, dynamic viscosity, gravitational acceleration); (d) Variables related to the abutment and channel (abutment size and shape, channel width). 5 From the 1950s to 1980s, different forms o f empirical formulae were presented from earlier studies (e.g. Laursen and Toch, 1956; Laursen, 1963; Shen et al, 1969; Raudkivi and Ettema, 1983). From the 1990s to 2000s was the prosperous development period for scour research, during which many experimental studies were conducted and many formulae were derived. Some o f the representative studies include Melville, 1997; Lim and Cheng, 1998; Ettema et al, 1998; Kuhnle et al, 2002; Coleman et al, 2003. A comprehensive review o f the investigations on local scour formulae can be found in Melville (1997) and Barbhuiya and Dey (2004). Johnson (1995) compared 7 commonly used and cited formulae with a large set o f field data for both clear-water scour and live-bed scour. The results o f this study pointed out the necessity for further data collection and experimental research. For Hydraulic Engineering applications, the concept o f equilibrium scour depth in bridge hydraulics is essential for scour prediction. Three o f the commonly used formulae for predicting scour depth at abutments for open channels (Laursen, 1963; Melville 1992 and Lim, 1997) are briefly reviewed in the following passage. 1. The Laursen’s relationship (Laursen, 1963) was based on scour in a long contraction. For abutments that do not extend over the overbank region into the river channel, Laursen gave the following equation: ( 1-2) In which, L : the length o f abutment r. the ratio o f scour at the abutment to scour in a long contraction. y: the approach flow depth. W ith r =12 and using the binomial approximation, the equation can be simplified to: (1-3) 2. As defined by Melville and Coleman (2000), the functional relationship between scour depth and other dependent parameters is: ds = f [flow, bed sediment, bridge geometry, time] By using dimensional analysis method, Melville (1995, 1997) studied the development o f local scour at bridge abutments and developed an equation to estimate the maximum scour depth 6 under clear water conditions. By plotting many published data o f local scour depth d s at bridge abutment sites and using his own experimental data collected at the University o f Auckland, Melville (1997) proposed the following scour prediction equation: i = 2K , K JK ,K J C t K„, l y K^KXX^o, 1 < - < 2 5 4 = =2 i = 10 K, Kt K , K , K , K a , - > 2 5 VAv y ( 1- 4 ) y y In which, d x : equilibrium local scour depth; L : abutment length; y : approach flow depth; K ,; scour depth o f flow intensity; K d : sediment size; K a : sediment gradation; Ks : abutment shape (with values 1 for the vertical wall abutment, 0.75 for 45° wing wall abutment, and 0.5 for 1:1 sloping spill-through abutments); K e : abutment orientation; K c : channel geometry; K ] , A'*: adjusted values o f K s and K 0 ; Kx, K 0 and K, are all defined through experimental data in Melville’s study. For a vertical wall abutment, under the condition o f 1 < L / y < 25, the formula can be written as ds = 2(_yZ,)°5, which is close to Laursen’s equation. 3. Based on the continuity equation, scour geometry, and a generalized form o f the power law formula for flow resistance in an alluvial channel, Lim (1997) proposed an equation for estimation o f the maximum equilibrium scour depth. For vertical wall abutments, the scour depth can be simplified to: ds = 1.8(yL)0-5, which is in close agreement with the formulae derived by Melville (1997) and Laursen (1963). 7 Currently, even the open channel scour depth estimation is not a standard design because o f a lack o f reliable data (Hoffmans and Venhij, 1997). According to Melville (1997), “existing design m ethods...are adequate for prediction o f scour depth at abutments sited in channels that can be approximated by a rectangular shape”. For the scour in natural rivers, the formulae mentioned here involve strong empiricism and introduce many uncertainties. 1.1.2 Ice Covered issues on local scour In northern Canada, many rivers become ice covered in winter. The presence o f ice cover causes changes in the properties o f the flow such as: velocity profile, bed shear stress distribution, mixing properties, and sediment transport (Lau and Krishnappan, 1985). The riverbed evolution process will be significantly changed compared to that observed in open channels (Sui et al., 2010b). To my knowledge, the literature on the scour under ice cover is still limited (Krishnappan, 1984; Lau and Krishnappan, 1985; Tsai and Ettema, 1994; Beltaos, 1998; Ettema et al, 2000; Wang et al, 2008; Sui et al. 2010b). In the following passage, a brief literature review will be provided on the velocity distribution and sediment transport under ice cover. Lau and Krishnappan (1981) used the k-e model to calculate the velocity distribution by using different boundary roughness. Lau and Krishnappan (1985) proposed a method to calculate sediment transport by using k-e model in covered flows. Under ice covered flows, they found that the reduction in the bed shear stress had very significant effects on the sediment transport. From a series o f experiments, it was found that the top ice cover can cause an increase in depth, decrease in average velocity and diffusivity distributions (Figure 1-4). However, the bed shear stress and the eddy viscosity are both smaller than that corresponding free-surface flow values (Krishnappan, 1984). Ettema et al. (2000) reviewed methods o f estimating o f sediment transport in ice covered channels and proposed a method to estimate the sediment transport rate by using the parameters acquired from open channels. Wang et al. (2008) conducted an experimental study on the incipient motion o f sediment under ice cover and discussed the role o f flow velocity and critical shear Reynolds number in this process. Sui et al. (2010b) compared the velocity profile under different flow and boundary conditions. He found that lower critical dimensionless shear stress for incipient motion was needed if the sediment size is smaller. Moreover, the velocity profile 8 under ice cover is completely different compared to the velocity profile in open channels. The flow in the upper layer is primarily affected by ice cover resistance, whereas the lower portion o f the flow is influenced by the channel bed resistance (Sui et al, 2010b). Suspendend Sediment concentration \ water surface ! o velocity Figure 1-4 Comparison o f velocity and suspended sediment concentration distributions between covered flow and free surface flow (S: sediment transport rate; V: velocity profile. Adapted from Lau et al, 1985) As mentioned in section 1.1, the local scour can be separated to clear-water scour and live-bed scour. Since it is difficult to measure and track sediments transported from approaching flow, in this research, the clear water scour will be the focus. In natural rivers, when the flow condition satisfies or exceeds the criteria for incipient motion, sediment particles will start to move. Depending on the size o f the bed-material particles and flow conditions, if the motion o f the particle is rolling, sliding or sometimes jumping along the bed, it is called bed load transport. If the motion o f the particle is supported by the upward components o f turbulent currents and remained in suspension for a distance, it is called suspended load transport. Total load is the sum o f bed load and suspended load. In most natural 9 rivers, sediments are mainly transported as suspended load, while the bed load transport rate is about 5-25% o f that in suspension (Yang, 2003). In this research, both bed load and suspended load transport rates will be considered. According to Bagnold (1966), the motion o f the bed load particles is assumed to be dominated by gravitational forces, while the effect o f turbulence on the overall trajectory o f bed load is supposed to be o f minor importance. Based on this assumption, van Rijn (1984a) presented a method which enables the computation o f the bed load transport rate (qb) as the product o f the saltation height (jumping height, 8b), the particle velocity (ub) and bed load concentration (cb): q b = ub6bcb (1-5) For suspended load, van Rijn (1984b) computed it as the depth integration o f the local concentration and flow velocity. The particle fall velocity and sediment diffusion coefficient were studied in detail as the main controlling hydraulic parameters. The proposed relationships for the suspended load transport were also verified by using a large amount o f flume data. However, for sediment transport under ice cover, the quality and quantity o f data are still limited. Ettema and Daly (2004) reviewed the impacts o f river ice on sediment transport. Dimensional analysis o f variables associated with flow was used. Sediment transport under ice cover was described in terms o f key non-dimensional parameters characterizing the dynamics o f flow and sediment interaction. The ice cover can influence water drag on the bed, redistribute flow to generate turbulence, and reduce the rate o f flow energy expended along the bed. Currently, there are two main methods to estimate flow resistance under ice cover in alluvial channels. The first one is to assume that the bed resistance coefficients do not change with ice cover, for example, Manning, Chezy, or Darcy-Weisbach coefficients. The second one is the flow resistance behavior o f the bed can be determined by an ice cover flow as a composite o f two non-interacting flow layers, with the lower layer o f flow affecting the bed topography. Lau and Krishnappan (1985) simulated the sediment transport under ice cover by assuming that the lower layer in a covered flow can be treated as a free surface flow. The top ice cover causes an increase in depth and decrease in average velocity and diffusivity distributions. The shear stress is also used to characterize the channel scour, which is directly used to quantify resistance to motion. Hains (2004) used the shear stress analysis in experiments with smooth cover and rough ice cover. The results showed that increased shear stresses on the bed will increase bed erosion and scour depth. Hains and Zabilansky (2005) conducted a series o f 10 experiments to establish the sensitivity o f various parameters affecting sediment transport processes under ice cover. In their research, approaching flow velocities were selected primarily on clear water scour under both smooth and rough simulated ice covers. Open channel, floating cover and fixed cover experiments were presented. By revising the scour model o f Melville and Coleman (2000), two extra parameters were included in the equation: kCOver and Lc. The Melville’s equation was then modified as: ds = / [flow, bed sediment, abutment geometry, time, cover (kCOver, Lc)] In which, Lc= the length o f ice cover. kc0ver= the ice cover factor (roughness, wetted perimeter) For a floating smooth cover, the velocity profile is gradual, with the maximum velocity approximately at mid-depth. For a floating rough cover, the maximum velocity is also mid-depth but is greater in value than the smooth ice profile (Figure 1-5). ■ * • • tM ■ A ■ • • A A • ■ • t A A • op en water • ■ Floating Smooth 14 A ■ 0 5 • 10 A 15 20 25 30 36 Figure 1- 5 The velocity distribution o f open water, floating smooth and floating rough cover (After Hains and Zabilansky, 2005) There are also experiments on the local scour under ice cover by using different laboratorial flumes (Ettema, 2000; Wang et al. 2008; Sui et al. 2010b). While most o f these studies focus on the velocity distribution, the sediment transport under ice cover is not quantitatively analyzed, which restricts further study o f the local scour under ice cover. Only two studies on the 11 experimental study o f local scour around bridge foundations under ice cover can be found (Ackermann et al. 2002; Munteanu and Frenette, 2010). For a better understanding o f this phenomenon, more experiments need to be conducted for collecting data and also for the calibration o f numerical models in the future. 1.2 Research objectives Compared to the research o f local scour in open channels, the local scour study under covered conditions is very limited. Only a few papers can be found in the literature. For numerical simulation, there is still very limited mathematical model available that can be used in the present research. My study aims at contributing to the understanding o f local scour under ice cover and modeling o f flow and sediment movement around bridge abutments. The main objectives o f this study are listed as follows. 1.2.1 Objective One The impact o f velocity, flo w depth and sediment composition From previous studies, the velocity distribution under ice cover is different to that in open channels. The effect o f approaching velocity is incorporated in the scour predicting formulae in the form o f flow Froude number or shear velocity (Froehlich, 1989; Kandasamy, 1989). For the bridge abutment scour, Melville (1992) suggested flow depth has different impacts on short abutments (l/h>l) and long abutments (l/h>25). Characteristics o f sediment composition are commonly used in scour depth formulae. Derived from the particle size distribution curves, median sediment diameter dso and geometric standard deviation a g (ag=(d 84/di 6 )° 5) are the two most widely used sediment parameters in the study o f local scour. Dey and Barbhuiya (2004) indicated that for non-uniform sediments, due to the formation o f armor-layers in the scour hole, the scour depth is reduced significantly in open channels. Under ice cover, the impacts o f different approaching velocity, flow depth and sediment composition on scour hole development are still not clear. In the experimental study, by changing different approaching velocities, flow depths and sediment compositions, the real-time and maximum scour depth will be measured under ice cover. 1.2.2 Objective Two 12 Scour development around different types o f abutments Melville (1992, 1997) presented results o f laboratory investigations o f local scour at bridge abutments and piers in open channels. In the scour depth formulae, Melville used shape factors Ks to account the effect o f the shape o f abutments on equilibrium scour depth estimation. Semi­ circular can produce vortices o f feeble strength, while vertical abutment, which is similar to spurdikes, can produce strong turbulent vortices. A relatively large scour depth is observed around vertical abutments (Barbhuiya and Dey, 2004). In the study, a vertical wall abutment and a semi­ circular abutment model will be made to study the shape parameter on local scour. The dimension o f the abutment is shown in Figure 1-6. For the scour under ice cover, different shapes o f abutments are still not systematically studied. The value o f shape factor in the scour depth formulae has not yet been determined. 40 cm Vertical BA Semi-circular BA Figure 1- 6 Bridge abutment (BA) types used in experiments 1.2.3 Objective Three Dimensional analysis o f variables fo r the scour depth including ice cover Using the Buckingham n theorem, various formulae have been brought up by combining different parameters that affect the scour depth, such as abutment shape, approaching flow, fluid and sediment characteristics, channel geometry, and time. However, none o f these formulae has ever incorporated ice cover as a parameter. By using dimensional analysis and data collected from experiments, a relationship between ice cover and other variables will be derived. 13 1.3 Research innovations The experimental and numerical research are focused on the local scour around bridge abutment under ice cover conditions. The study has the following innovations: 1. The whole process o f local scour around bridge abutment under ice covered conditions will be simulated by a series o f large scale flume experiments; 2. The local scour process under different flow conditions, namely, open channel, smooth, rough will be compared; 3. Through Dimensional Analysis, empirical formula to estimate the scour depth under ice covers will be derived. 1.4 Outline of dissertation The dissertation focuses on the analysis o f data from experimental study. Chapter 2 is the methodology and experimental set up. Chapter 3 is the data analysis and discussion, which is separated into several parts. Each section discuss one aspect o f ice cover impacts on local scour around bridge abutment. Chapter 3.1 is the analysis o f ice cover impacts around the semi-circular abutment. Chapter 3.2 compares both square and semi-circular abutment under ice cover and open channel conditions. In this section, the shape factor o f abutment is introduced. By comparing a small scale flume experiments, Chapter 3.3 is introduced to show the impacts o f ice cover and non-uniform sediment. The large scale flume experiments shows interestingly different comparing to that from small scale flume experiments. Chapter 3.4 focuses on the analysis o f armor layer analysis around abutments. Since the sediments used in the present research are non-uniform, a clear armor layer is noticed around abutments. By including armor layer sediment size, the maximum scour depth is discussed. Empirical equations are also developed. Chapter 3.5 is used to show the analysis o f ADV measurements from the experiments. Finally, Chapter 3.6 shows the theory analysis o f incipient motion under ice cover. The dimensionless shear stress is calculated and compared. References 14 1. Acharya A, (2011). 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A three component bistatic coherent Doppler velocity profiler: error sensitivity and system accuracy. IEEE, Journal o f Oceanic Engineering, 27(3): 717-725. 19 2 METHODOLOGY The influence o f ice cover on local scour is a complex interaction among the ice cover, fluid flow, sediment, bridge abutment, bed geometry and channel geometry. An ice cover approximately doubles the wetted perimeter o f the river, which increases the flow resistance. In the present research, two main approaches will be used: experimental method and analytical study. Experimental study will provide original data o f the equilibrium scour depth and profile, which can be used for developing the empirical formulae o f scour depth under ice cover. The incipient motion is measured and monitored under ice cover. By conducting physical experiments, dimensional analysis can be employed to determine the effect o f ice cover on local scour. 2.1 Theoretical analysis To predict the location and geometry o f local scour in the vicinity o f hydraulic structures such as a bridge abutment and spur dike, theoretical analysis o f the bed shear stress and turbulence properties is necessary. In open channels, the measured velocity profiles can be used to calculate the following turbulent flow characteristics: mean velocities in three directions, Reynolds stresses and bed shear stress. Bed shear stresses can be calculated by using four methods (Acharya, 2011). In the present study, the turbulent kinetic energy (TKE) method will be used (Biron et al., 2004), which is as follows: x = Cx[0.5 p (u '2 + v '2 + w '2)] (2-1) Here, p is the water density, Ci=0.19 is a proportionality constant, u ’, v ’ and w ’ are flow velocity fluctuations in the longitudinal, transverse and vertical directions, respectively. For the local scour in open channel, a flow resistance calculation leads directly to the estimation o f the shear velocity associated with bed surface drag. To estimate the sediment transport rate under ice cover, it is first necessary to estimate flow resistance (or a relationship between flow depth and mean velocity o f the flow), and then the flow drag on the bed. To determine the shear velocity for the incipient motion o f sediments, the velocity profile under ice cover has to be measured. Ice cover alters mean flow distribution and flow turbulence characteristics. The flow velocity profile under ice cover can be categorized into an upper portion and lower portion. Divided by 20 the locus o f the point o f the maximum velocity, the upper portion o f flow is mainly affected by the ice cover and the lower portion o f flow is mainly affected by the river bed (Sui et al, 2010b). The forces acting on a sediment particle under ice cover include hydrodynamic drag, the hydrodynamic lift and the submerged weight, as shown in Figure 2-1. The drag force Fd is in the direction o f flow and the lift force F l is normal to the flow. The drag force F d is associated with the bed shear stress, while the lift force F l is also associated with Fd. Ice cover Flow ^ River bed / Velocity profile Figure 2-1 Schematic o f force on particle on a sloping bed under ice cover The shear velocity o f approaching flow will be calculated by using the log-law. The critical bed shear velocity can be determined by using the classical Shields Diagram. If the flow velocity profiles are available, the bed shear velocity u«c can be calculated by fitting a least squares regression to flow velocity and distance measurements from near the bed to 20% o f the depth using the following equation (Kuhnle et al. 1999; 2002): U*C = 5.75d(log h) in which u is time mean velocity at a distance o f h. The shear Reynolds number will be used here to study the incipient motion o f sediment particles. Re = (2-3) 21 in which u»c is the bed shear velocity, D 50 is the median grain size o f sediments and u is the kinetic viscosity o f water. 2 The dimensionless shear stress will be calculated by using the following equation: t . = pu‘c , gApD50 where Ap is the difference in mass density between sediment and water, g is the gravity. In this research, the velocity profile will be measured in the scour hole under simulated ice cover. Once the velocity profile is acquired, the flow resistance and the bed load sediment transport rate can be estimated. Thereby, the suspended sediment transport rate could be calculated based on the bed load transport rate. 2.2 Experimental study 2.2.1 Study site The experimental research has been conducted at Dr. Max Blouw Quesnel River Research Center (QRRC), Likely, BC. The QRRC is a University o f Northern British Columbia (UNBC) based research facility. There are six outdoor flow-through spawning channels in the research center. Each channel has dimensions o f 80 meters long, 2 meters wide and 1.3 meters deep. To conduct the experimental research, one channel was modified as an engineering flume during the summer o f 2011. 2.2.2 Experimental design and construction In reviewing the literature on experimental local scour research, only a few studies were conducted in large flumes (Sheppard et al., 2004; Morales et al., 2008). The experimental research, were conducted in a 2m wide flume, and can be treated as a large scale local scour experiment. To my knowledge, this is the first large scale experimental research on the local scour under ice over. A more detailed introduction o f the flume will be discussed below. In 2011, the flume was re-constructed to set up for experimental research. Prior to the modification, the flume had an upstream section and downstream section, which had a length o f 39.5m and 38.2m, respectively. The upstream 39.5m has been modified as a holding tank for the purpose o f keeping a constant discharge during the experiments. The experimental zone is located in the downstream 38.2m section o f the flume. Figure 2-2 shows the modification plan o f the flume at the QRRC. 22 Firstly, to directly observe and record the scour process, two 4m sections o f concrete flume wall were replaced with plexiglass. Since the flume has a width o f 2 m, it would be too much to cover all the flume bed with sand, so two sand boxes were made by elevating the flume bottom by 30 cm. The sand boxes are 0.3 m deep, 2 m wide and 5.6m and 5.8m long respectively. Other parts o f the flume bottom were covered by treated waterproof plywood. Different composition o f sands (dso) were put in the sand box to study the effect o f sediment composition on local scour. To create different velocities, three input valves were connected together which can adjust the amount o f water into the flume. It was measured that this method can produce at least six velocities for the scour simulation. Because o f the cold weather and heavy snow in Likely, a roof was also constructed to cover the experimental zone away from leaves, snow and wind. The modification o f the flume was finished in November, 2011. Figure 2-3 shows the modification process o f the flume at QRRC. 23 simulated ice cover simulated ice cover sand box 39.5m 11.3m 5.3m es A-A Holding tank ["a experimental zone 1 Sediment Tnfi / experimental zone 2 1 3ni 39.5m Sediment Trap 102m 3JUm Plan View Figure 2- 2 The modification plan for the flume at QRRC T T 25, the value o f Kd equals to 1 from Melville’s (1992) previous research, which is not practical for the present study. Since the abutment length remains constant, the non-uniform sediments were valued by including the Froude number as defined by the following equation: Fr = U 0 / y f e H ( 3 1 _5 ) where g is the gravitational acceleration, U0 is the approaching velocity, H is the approaching flow depth. 2.5 i .................... 0 0.1 0.15 r A 0.2 0.25 ............................. 0.3 Fr AD50=0.58mm *050=0.50111111 41 * D50=0.47nun 0.35 1.4 1.2 1 0.8 0.6 + „ •r 0.4 0.2 1 0 0.1 0.15 0.2 0.25 F 0.3 0.35 r >open channel + covered flow| Figure 3 . 1 - 7 (a) Variation o f maximum scour depth with the Froude number under different sediment composition (b) The comparison o f maximum scour depth in open channel and ice covered condition (Dso=0.50mm) The experimental data from Figure 3.1-7a indicates that under the same flow conditions, fine sediment composition can result in a deeper maximum scour depth. W ith the same sediment composition, the maximum scour depth increases with the Froude number. An imposed ice cover results in an increased composite resistance, so under ice covered conditions, the maximum scour is more than that in open channels (Figure 3.1-7b). To gain a better understanding o f the impact o f sediment grain size on the maximum depth, regression analysis was conducted. The maximum scour depth around the semi-circular abutment can be described by the following variables. ^tva x " „ V V i ^ 5 0 \b 1" ’ (3.,-6) In all, 27 experiments have been conducted to investigate the relationship between average scour depth and approaching flow depth, in which 9 experiments were in open channels, 9 experiments were under smooth ice cover and 9 experiments were under rough ice cover. By using the regression analysis, the following equations were derived from all the 27 experiments (Figure 3.1-8). 42 2.5 13 .0 x 10 - 3 X - 0.6490 H * dH R1 = 0 9 06: 8 .2 x 10 X - 0 4433 i.5 J /*/• 0.5 .V - 0.2734 100 300 200 400 500 -v = ( - = = ) ;1(-f-) • open channel *■ smooth c o m * rough c o m Figure 3 . 1 - 8 Dependence o f maximum scour depth on related variables For open channel: ^ = 4 . 2 x 1 0 3( - ¥ = f 1( ^ ) 34 - 0 .2 7 3 4 (3-1-7) For smooth cover: = 8 . 2 x i q -3( —^ L ) 5 >(J?50)-24 - 0 .4 4 3 3 H H (3.1-8) For rough cover: U S5 1, A o s )- 2 4 - 0 . 6 4 9 0 4gH H -= » - = 1 3 .0 x 10 J( - T = = r ( — H (3.1-9) As reported by Sui et al. (2010), with an increase in velocity and particle size, the maximum scour depth will increase. In the present research, regarding the local scour around the sem i­ circular abutment, the rough ice cover causes the largest average scour depth compared to those under both smooth ice cover and open channel. Hence, we compared the maximum scour depth under different flow conditions and with different composition o f bed sediments. It is interesting to note that the geometric characteristics o f the local scour depend mainly on the approaching flow velocity, bed sediment grain size as well as the cover condition. From Figure 3.1-8 and Equation 3.1-7 to 3.1-9, the impact o f sediment distribution is studied. To study the impact o f ice cover roughness on the local scour development around the semi-circular abutment, the following dimensional variables under covered flow were considered: 43 where ni is the ice cover roughness and nb is the channel bed roughness. According to the Hydraulic Design Handbook (1999), in an un-vegetated alluvial channel, the total roughness nb consists o f two parts. One is grain roughness ( n ) which is resulting from the size o f the particle and the other is skin roughness (n ) because o f the existence o f the bed forms. The total roughness can be expressed as: n = n'+n'' (3.1-11) However, there is no reliable method o f estimating n , so in the present research, the grain roughness was used as the channel bed roughness in the analysis. For mixtures o f bed material with significant portions o f coarse-grain sizes, the following equation from Hager (1999) was used. » = ° . ° 39 ^ ‘ (3]12) Ice cover presence alters the mean flow distribution and flow turbulence characteristics. For smooth ice cover, because the styrofoam panel has a relatively smooth concrete-like surface, by referring the Mays (1999), the value o f 0.013 was adapted. The roughness o f the ice cover was changed by attaching small cubes with dimensions o f 2.5cmx2.5cmx2.5cm 3.5cm from with a distance each other. By using the results o f discharge measurements through the ice and supporting field data related to the observed characteristics o f the underside o f the ice cover, Carey (1966) calculated Manning roughness coefficient was between 0.01-0.0281. From his calculation, a constant roughness o f 0.0251 was used for the winter period. Li (2012) reviewed several methods to calculate the M anning’s coefficient for ice cover, the following equation can be used depending on the size o f the small cubes. ^= 0 .0 3 9 ^ 6 (3.1-13) In which ks is the average roughness height o f the ice underside. So in the present research, the roughness coefficient was calculated as 0.021, which is also in the range o f Carey’s calculation. By using the regression analysis, the following empirical equation was developed: 2.5 - 0 0511 R 2 = 0 .9 3 4 8 1.5 0.5 • covered flow 0 10000 20000 30000 y/gH 40000 50000 H »/. 60000 70000 Figure 3 . 1 - 9 Dependence o f maximum scour depth on related variables under ice cover The correlation between maximum scour depth and above three variables is worth mentioning because as indicated in Figure 3.1-9 the regression relationship is strong. Meanwhile, it is also confirmed that the hypothesis for calculation o f the ice cover roughness is correct. One can also note that, under the condition o f same flow and bed material, the maximum scour depth under rough ice cover is deeper than that under smooth ice cover. However, under the same flow and cover condition, since the index for D 50 is negative (-4.85), with the decreasing in sediment grain size, the maximum scour depth will increase. W ith the same bed material and covered condition, the approaching velocity has a positive impact on the maximum scour depth. To apply this empirical equation in the hydraulic engineering field, the authors assume that during the winter the ice cover can be treated as smooth. While in early spring, with the ice breaking up and ice jamming processes, the ice cover can be treated as rough therefore increasing the flow velocities and increasing the local scour around bridge foundations. In this case, the sediment transport increases and the safety o f bridge infrastructures will be threatened. One can note from Equation 3.1-14 that, in the same river, with the increase in ice cover roughness, the maximum scour depth increases. During the ice break up period in spring time, due to the accumulation o f small ice chunks under side, local bridge scour should be monitored. Compared to the research o f armor layer development in the paper from Sui et al. (2010), the approaching water depth had a stronger impact on the maximum scour depth compared to 45 approaching velocity in open channels. However, for ice covered flow, the authors found that approaching velocity has a stronger impact compared to that o f the approaching water depth. One drawback regarding the proposed empirical equation is that roughness o f only two ice covers were tested. 3.1.3 Conclusion Experiments have been conducted in a large scale flume to study the impact o f ice cover roughness and non-uniform sediment on the local scour around semi-circular abutments. The location o f the maximum scour depth along the abutment is 60° from the flume wall. We noticed that the downstream slope in the scour hole is smaller compared to that in the upstream. In this research, the Froude number was also used to investigate the impacts o f non-uniform sediment composition on local scour. The scour volume and scour area were calculated and compared to open channel, smooth and rough cover conditions. Under ice cover, the average scour depth was always greater compared to that in open channels. The average scour depth under rough ice cover was 35% greater than that under smooth ice cover. By using dimensional analysis, an empirical equation o f the maximum scour depth was developed. The equation indicated that with an increase in sediment grain size, the maximum scour depth decreased correspondingly. In conclusion, ice cover roughness plays an important role for the maximum scour depth development. References 1. Ackermann N L, Shen H T, Olsson P, Local scour around circular piers under ice covers [C]. Proceeding o f the 16th IAHR International Symposium on Ice, International Association o f Hydraulic Engineering Research, Dunedin, 2002, New Zealand. 2. Andre R, Thang T, Mean and turbulent flow fields in a simulated ice-cover channel with a gravel bed: some laboratory observations [J]. Earth Surface Processes and Landforms, 2012,Vol. 37, pp: 951-956. 46 3. Carey K, Observed configuration and computed roughness o f the underside o f river ice St Croix river Wisconsin [J], Geological Survey Professional Paper, 1966, Vol. 550, Part 2, pp. B192-B198. 4. Deng L, Cai C S, Bridge scour: prediction, modeling, monitoring, and countermeasuresReview [J], Practice Periodical on Structural Design and Construction, 2009,15(2): 125-134. 5. Dey S, Barbhuiya A K, Turbulent flow field in a scour hole at a semicircular abutment [J], Canadian Journal o f Civil Engineering, 2005, Vol. 32, pp. 213-232. 6. Ettema R, Daly S, Sediment transport under ice. ERDC/CRREL TR-04-20. Cold regions research and Engineering Laboratory, 2004, US Army Corps o f Engineers. 7. Ettema R, Natako T, Muste M, Estimation o f scour depth at bridge abutments, NCHRP 24-20, 2010, The University o f Iowa, USA. 8. Hager W H, Wastewater Hydraulics [M]. Berlin: Springer-Verlag, 1999. 9. Hains D B, An experimental study o f ice effects on scour at bridge piers [C]. PhD Dissertation, 2004, Lehigh University, Bethlehem, PA. 10. Hicks F, An overview o f river ice problems [C] CRIPE 07 guest editorial Cold regions Science and Technology, 2009, 55: pp. 175-185. 11. Kandasamy J K, Melville B W, Maximum local scour depth at bridge piers and abutments [J]. J Hydraul. Res. 1998, 36:183-197. 12. Li S S, Estimates o f the Manning’s coefficient for ice covered rivers [J], Water Management, Proceedings o f the Institution o f Civil Engineers, 2012, Vol. 165, Issue WM9, pp. 495-505. 13. Mays L W, Hydraulic Design Handbook [M], MaGraw-Hill, 1999, pp. 3.12. 14. Melville B W, Local Scour at bridge abutments [J]. Journal o f Hydraulic Engineering, ASCE, 1992, Vol.l 18 (4), pp. 615-631. 15. Melville B W, Pier and Abutment scour: integrated approach [J], Journal o f Hydraulic Engineering, 1997. ASCE, Vol 123(2): 125-136. 16. Munteanu A, Frenette R, Scouring around a cylindrical bridge pier under ice covered flow condition-experimental analysis, R V Anderson Associates Limited and Oxand report, 2010. 17. NCHRP Web-only Document 175, Evaluation o f Bridge- Scour Research: Pier scour processes and predictions. 2011, NCHRP Project 24-27(01). 47 18. NCHRP Web-only Document 181, Evaluation o f Bridge-Scour Research: Abutment and Contraction Scour Processes and Prediction. 2011, NCHRP Project 24-27(02). 19. Sui J, Faruque M A A, Balanchandar R, Local scour caused by submerged square jets under model ice cover [J]. Journal o f Hydraulic Engineering, ASCE, 2009, Vol 135 (4), pp. 3 lb319. 20. Sui J, Wang J, He Y, Krol F, Velocity profile and incipient motion o f frazil particles under ice cover [J]. International Journal o f Sediment Research, 2010, Vol 25(1), pp. 39-51. 21. Sui J, Afzalimehr H, Samani A K, Maherani M, Clear-water scour around semi-elliptical abutments with armored beds [J]. International Journal o f Sediment Research, 2010, Vol. 25, No. 3, pp. 233-244. 22. Wang J, Sui J, Kamey B, Incipient motion o f non-cohesive sediment under ice cover - an experimental study [J]. Journal o f Hydrodynamics, 2008, Vol 20(1), pp. 177-124. 23. Zhang H, Nakagawa H, Kawaike K, Baba Y, Experimental and simulation o f turbulent flow in local scour around a spur dike [J]. International Journal o f Sediment Research, 2009, Vol. 24, No. 3, pp. 33-45. 48 3.2 Local scour around bridge abutments under ice covered condition: comparing of square abutment and semi-circular abutment Local scour is the engineering term used to describe sediment removal around hydraulic structures by running water. It may result in bridge failures as it can undermine piers and abutments that support bridges. The Federal Highway Administration has estimated that over 60% o f bridge collapses in the US was from local scour. Luigia et al. (2012) indicated that approximately 50 to 60 bridges fail on average each year in the US. A worldwide survey also indicated that, the main cause o f bridge collapse is natural hazards, among which flooding and scour is responsible for around 60% o f the failures (Imhof, 2004). An important consideration in bridge abutment design is to estimate the maximum scour to make sure the bridge foundation can be built deep enough to avoid the possibility o f undermining. In the past few decades, local scour around bridge abutments and piers in open channels has received wide attention and many scholars have conducted various studies on this topic (Laursen and Toch, 1956; Froehlich, 1989; Melville, 1997; Coleman et. al., 2003; Dey 2005; etc.). To estimate the maximum scour depth, several formulae have been developed. As reviewed by NCHRP in 2011, five major dimensionless parameter groups are classified for the scour depth estimation formulae. The first group is Flow and Sediment, which indicates flow interaction with sediment and can be used to classify clear-water or live-bed scour. The second group is Abutment and Sediment scale, which is related to the degree o f model scaling. The third group is Abutment and Flow geometry, which will measure abutment dimensions relative to the scale o f flow field. The fourth group is Abutment Flow distribution, which is the discharge per unit width in the approach and contracted sections. The fifth group is Scour and Geotechnical Failure, which is the scour that leads to the slope instability which is difficult to model in the laboratory. The five groups listed above include almost all the variables in the local scour estimation formulae around bridge abutments. However, in the northern areas, ice cover is an issue because it can stay as long as 5 months on some rivers. Ice cover can result in many problems such as ice jamming, flooding, restricting the generation o f hydro-power, block river navigation and affect the ecosystem balance (Hicks, 2009). Moreover, ice cover can also significantly change the flow field and change the flow properties around bridge foundations, such as velocity profile, bed shear stress distribution, mixing properties, and sediment transport (Lau and Krishnappan, 1985). 49 Ettema et al. (2000) developed a method to estimate the sediment transport in ice covered channels. Sui et al. (2000) derived interrelationships o f suspended sediment concentrations and riverbed deformation under ice cover in Hequ Reach o f Yellow River. Some other researches on the sediment transport and scour under ice including: Ettema and Daly, (2004); Hains and Zabilansky, (2004); Wang et al. (2008). However, to date there is limited research on the local scour around bridge structures under ice cover (Ackermann et al., 2002; Hains, 2004; Sui et al., 2009; Sui et al., 2010a; Sui et al., 2010b). To fill this gap, a series o f experiments were conducted to find the parameter that can describe the ice cover impact on the scour depth. 3.2.1 Experimental setup Based on the previous review o f local scour around bridge abutments, flume experiments were designed to evaluate the impacts o f ice cover on scour depth. The following three hypotheses were tested in this research. Hypothesis 1: Shape factor o f abutments Bridge abutments are designed in different sizes and shapes. The shape factor is important in abutment local scour estimation. According to Melville (1992), the effect o f shape can be expressed using a shape factor Ks. In open channels, the shape factor for square abutment is 1.0 and for semi-circular one is 0.75. The shape factor was examined under ice covered conditions. Hypothesis 2: Non-uniform sediment Most o f the existing work on local scour focuses on uniform sediments in small laboratory flumes with very few studies that look at non-uniform sediments. However, a more practical problem for engineers is that natural riverbeds are normally non-uniform. Three non-uniform different sediments were used in this study to see the scour contour and sediment deposition. The maximum scour depths from different non-uniform sediments were also compared. Hypothesis 3: Ice cover roughness impact Since the roughness o f the ice cover impacts the velocity distribution in the water regime. In this research, two different types o f ice cover were created, namely smooth cover and rough cover. The maximum scour depths under different covers were compared. Experimental design 50 Experiments were carried out in an outdoor flume in 2012. The flume is 40m long, 2m wide and 1.3m deep. The slope o f the concrete bottom is 0.2%. Figure 3.2-1(a) shows the flume geometry and design. Two abutment models were made from Plexiglass to permit observation o f the scour process during experiments. The dimensions o f the abutment can be found in Figure 3.2-1(b). A holding tank with a volume o f 90m3 was created upstream o f the flume. Two valves to adjust flow rate were connected to the holding tank. The flow depth can be adjusted by the tailgate at the end o f the holding tank. Two sand boxes were created to simulate riverbed, with a distance 10.2m from each other. Each sand box can have 30cm depth o f sediment (Figure 3.2-2). The velocity range in Sand box 1 is from 0.16 to 0.26m/s, while the velocity range in Sand box 2 is between 0.14 and 0 .2 1 m/s. (a) sim.iljlo,! icc cover \ simulated ice cover . sand box ? 11.3m iO ’ m 5 Km 5.3m V frtiv a l View Holding Tank !A Sand B oxffl 1 i .3m 39.5m A -A Sedim ent Tran / 5.ftm 10.2m 3ft .2m P lan View 51 S edim ent Tran Sand Box 5 .ftm 5.3m •- (b) i | , f2(1.3) and fi(3) > f2(3). It is also important to notice that under the same densimetric Froude number condition, the maximum scour depth in the vicinity o f square abutment is much larger than that around the semi-circular abutment. Another important consideration here is to include D so in the relationship between dmax and F0. By including the median sediment grain size, the connection between flow and sediment can be built under ice covered flow for both abutments. The data and fitting curve from Figure 4 has further strengthened the hypothesis that ice cover has a stronger impact on the maximum scour depth than shape factor o f the abutment. The maximum scour depth with three different bed sediments are compared in Figure 3.2-5. As predicted in the previous hypothesis, the maximum scour depth increases with the decrease o f Dso- For coarse sand (Dso=0.58), the maximum scour depth under smooth ice cover is similar to that in open channels, which indicates the smooth cover has less impact on the scour development. However, for fine sediments (Dso=0.50 and Dso=0.47), the ice cover has a stronger impact on the maximum scour depth than open channels. 59 200 1 180 160 140 5 120 J 10° | 80 60 40 20 0 0.58 0.5 0.47 D 50 (mm) ■ open channel msmooth cover « rough cover Figure 3 . 2 - 5 The variation o f D 50 with scour depth under different conditions Under the same flow condition in open channel, results also show that square abutment contributes a higher maximum scour depth compared to that from semi-circular one. With an ice cover as an extra boundary on top, as reported by Sui et al. (2010b), the location o f maximum velocity is closer to the channel bed than for the corresponding open channel flow. The increased gradient o f the near bed velocity leads to a higher bed shear stress, which contributes the deeper scour hole in covered flow. With the increase o f ice cover roughness, the locus o f maximum velocity moves closer to the bed compared to the smooth ice cover. This explains the reason o f a deeper scour depth under rough ice cover. Figure 3.2-6 indicates that the variation o f the maximum scour depth around the two abutments in open channels, smooth cover and rough cover in different bed sediments. 60 Square abutment Semi-circular abutment (a) (d) 1)50=0.58mm D50=0.58 mm 1.6 08 1.4 0.7 1.2 06 1 : 0.5 0.8 | 0.4 0.6 ' 0.3 0.4 0.2 0.2 0.1 0 23 1.3 0 28 1.3 1.8 13 Fo 1 open channel • smooth cover 18 Fo arough cover open channel • smooth cover * rough cover (e) (b) D50=0.50mm D50=0.50mm 24 1.4 2 1.2 12 1 1.8 1.6 ■§ 0.8 1.4 I 1.2 1 0.8 0.6 0.6 0.4 0.4 0.2 0 1.3 a 0.2 • 0 1.8 23 1.3 28 1.8 23 ' open channel • smooth cover 28 Fo Fo * rough cover 1open channel • smooth cover a rough cover (f) (c) D50=0.47mm D50=0.47mm 26 24 24 2 1.8 22 2 22 1 *I 16 | '1.24 1 0.81 0.4 0.4 0.2 1.8 1.6 1.4 1.2 0.8 0.6 0.6 0 0.2 0 1.3 1.8 1.3 23 1.8 28 23 28 Fo Fo • open channel 'o p e n channel • smooth cover • smooth cover a rough cover a rough cover Figure 3.2- 6 The variation o f maximum scour depth with different sediments and abutments 61 Figure 3.2-6 (a) - (c) show the variation o f maximum scour depth with different bed sediments around the square abutment, while Figure 3.2-6 (d) - (f) represent the variation o f maximum scour depth with the three sediments around the semi-circular abutment. It is clear that with an increase o f F0, the ratio o f maximum scour depth to approach flow depth increases correspondingly under all flow condition with or without ice cover. It can also be seen that under the same densimetric Froude number, with decreasing D 50, the scour depth difference between smooth ice cover and rough ice cover also decreases. For both abutments, with the decrease o f D 50, the impact o f ice cover roughness has a more clear impact on the scour depth. As mentioned above, the impact o f the shape factor for semi-circular abutment on scour depth is smaller than that in open channels. To find the impact o f shape factors, the derivative o f d m a x / h to Fo is possible by using Equation (3.2-2) and (3.2-3). (4=5.)’= v 1 / 2.0282Fo - 2.9937 h (3.2-6) (x 4 1bsl)'=0.5728F ' oo +0.1305 h (3.2-7) By making equation (3.2-6) equals to equation (3.2-7), it is found that when Fc =2.11, the difference between square abutment and semi-circular abutment is the smallest. At this point, the shape factor for semi-circular abutment is 0.66. From the calculation, the shape factor for semi­ circular abutment has a range from 0.66 -0.71. The following multi-relationship can be used to describe the impact o f shape factor on maximum scour depth: = m , K s) (3.2-8) h In which, F0 is the densimetric Froude number and Ks is the shape factor for different abutments. For square abutment, the shape factor has a value o f 1, which is same to that in open channels. However, for the semi-circular abutment under ice covered condition, the shape factor will be around 0.66-0.71, which is smaller than that in open channels. The presence o f an ice cover induces a redistribution o f the highest velocities compared with the open channel flow around bridge abutments, and thus leads to a higher available energy for the scouring phenomenon. The relationship in Figure 3.2-7 also shows that with an increase in D 50, the ratio o f maximum scour depth to flow depth decreases. While under the same flow and sediment condition, the ice cover can result in a deeper scour depth. By increasing the 62 densimetric Froude number, the ratio increases correspondingly under open channel, smooth cover and rough cover. But compared to the ratio in open channels, the rough cover has the largest value and smooth cover has the second largest. 26 14 2,2 2 18 1.6 I 14 I 12 I 0.8 0.6 0.4 02 0 1.3 1.3 1.7 19 IS 21 25 27 29 3.1 Fo • open channel « n ^ cover ......... *.roughcover........... open chamiel.................. ....................................................... rough cover Figure 3.2- 7 The variation o f maximum scour depth with different covered conditions The experimental research conducted by Munteanu and Frenette (2010) showed that, an increase up to 55% o f maximum scour depth can be reached around the bridge pier under ice covered conditions. From our study, for bridge abutments under rough ice cover, the increase on maximum scour depth is around 30% ~ 40% for all the sediments. W hile for the smooth cover, under the same flow condition, the increase o f maximum scour depth is less than 30%. From the authors’ understanding, the sediment transport under ice cover depends on the flow re­ distribution due to ice cover. A rough ice cover can cause more turbulence compared to the smooth ice cover and open channel. 3.2.3 Conclusions Ice cover plays an important role in the development o f local scour hole around bridge abutments. Experiments have been conducted to study the impact o f ice cover on bridge abutments with 63 solid foundations in the soil. Two types commonly used abutments were built, namely semi­ circular and square abutments. For three non-uniform sediments, the profiles o f local scour around abutments were plotted. By using Buckingham Pi theorem for dimensional analysis, the densimetric Froude number was used as one parameter to investigate the impact o f shape factor and ice cover roughness on maximum scour depth around abutments. Results indicate that the impact o f shape factor for semi-circular abutments on maximum scour depth is smaller in covered conditions than that in open channels. The range o f shape factor is between 0.66 and 0.71. Additionally, ice cover roughness also has a more pronounced impact on the maximum scour depth. However, due to the limitation o f experimental data, further experiments can lead to a higher degree o f certainty regarding the influence o f shape factor on scour for semi-circular abutment under ice covered conditions. Future work will include: flow velocity analysis in the vicinity o f bridge abutments under ice cover and analysis o f the armor layer. References 1. Ackermann N L, Shen H T, Olsson P, 2002, Local scour around circular piers under ice covers. Proceeding o f the 16th IAHR International Symposium on Ice, International Association o f Hydraulic Engineering Research, Dunedin, New Zealand. 2. Coleman S E, Lauchlan C S, Melville B W, 2003, Clear water scour development at bridge abutments, Journal o f Hydraulic Research, 41(5): 521-531. 3. Dey S, Barbhuiya A K, 2005, Time variation o f scour at abutments, Journal o f Hydraulic Engineering, ASCE, 131 (1): 11-23. 4. Ettema Robert, Daly Steven F, 2004, Sediment transport under ice. ERDC/CRREL TR04-20. Cold regions research and Engineering Laboratory, US Army Corps o f Engineers. 5. Ettema Robert, Braileanu, F, Muste M, 2000, Method for estimating sediment transport in ice covered channels, Journal o f Cold Regions Engineering, Vol. 14, No. 3, pp. 130-144. 6. Froehlich D C, 1989, Local scour at bridge abutments. Proc. Natl. Conf. Hydraulic Engineering, ASCE, 13-18. 7. Hains D B, 2004, An experimental study o f ice effects on scour at bridge piers. PhD Dissertation, Lehigh University, Bethlehem, PA. 64 8. Hains Decker, Zabilansky Leonard, 2004, Laboratory test o f scour under ice: Data and preliminary results. ERDC/CRREL TR-04-09. Cold regions research and Engineering Laboratory, US Army Corps o f Engineers. 9. Hicks F, 2009, An overview o f river ice problems: CRIPE 07 guest editorial Cold regions Science and Technology, 55: pp. 175-185. 10. Imhof D, 2004. Risk assessment o f existing bridge structures. PhD thesis, University o f Cambridge, UK. 11.Lau Y L, Krishnappan B G, 1985, Sediment transport under ice cover. Journal o f Hydraulic Engineering, ASCE, 111(6), pp. 934-950. 12. Laursen E M, Toch A, 1956, Scour around bridge piers and abutments. Iowa Highway Research Board Bulletin, No 4. 13. Luigia Brandimarte, Paolo Paron, Giuliano Di Baldassarre, 2012, Bridge pier scour: a review o f process, measurements and estimates. Environmental Engineering and Management Journal, Vol 11 (5). 14. Munteanu A, Frenette R, 2010, Scouring around a cylindrical bridge pier under ice covered flow condition-experimental analysis, R V Anderson Associates Limited and Oxand report. 15. Melville B W, 1992, Local scour at bridge abutments. Journal o f Hydraulic Engineering, ASCE, Vol 118 (4), pp. 615-631. 16. Melville B W, 1997, Pier and Abutment scour: integrated approach, Journal o f Hydraulic Engineering, ASCE, Vol 123(2): 125-136. 17. NCHRP Web-only Document 181, 2011, Evaluation o f Bridge-Scour Research: Abutment and Contraction Scour Processes and Prediction. NCHRP Project 24-27(02). 18. Sui Jueyi, Wang Desheng, Kamey B, 2000, Suspended sediment concentration and deformation o f riverbed an a frazil jammed reach, Canadian Journal o f Civil Engineering, Vol. 27,1120-1129. 19. Sui Jueyi, Faruque M A A, Balanchandar Ram, 2009, Local scour caused by submerged square jets under model ice cover. Journal o f Hydraulic Engineering, ASCE, Vol 135 (4), pp. 316-319. 65 20. Sui Jueyi, Afzalimehe Hossein, Samani A K, Meherani M, 2010a, Clear-water scour around semi-elliptical abutments with armored beds. International Journal o f Sediment Research, Vol 25(3), pp.233-244. 21. Sui Jueyi, Wang Jun, He Yun, Krol Faye, 2010b, Velocity profile and incipient motion o f frazil particles under ice cover. International Journal o f Sediment Research, Vol 25(1), pp. 39-51. 22. Wang J, Sui J, Kamey B, 2008, Incipient motion o f non-cohesive sediment under ice cover - an experimental study. Journal o f Hydrodynamics, Vol 20(1), pp. 177-124. 66 3.3 Scour morphology around bridge abutments with non-uniform sediment under ice cover Sediment transport, including the erosion o f river bed sediment, is a common problem in water resource management. Bridge abutments and piers in rivers are used to support the infrastructure of a bridge and are crucial for the safety o f bridges. Bridge abutments extend perpendicularly from the bank into the river flow. The erosion around bridge structures can weaken the structural stability o f a bridge and is a public safety concern. Local scour refers to the erosion o f sediment directly around infrastructure by running water. From an engineering perspective, determining the maximum scour depth is important so that provisions can be made in the design and construction (Chang, 2002). The local scour around bridge foundations is an important aspect o f river hydraulic engineering as studies have shown that local scour has caused huge economic loss around the world. For example, in 1987, 17 bridges were destroyed in New York and New England and in Georgia in 1994 over 500 bridges were damaged due to the scouring during flood events (Richardson and Davis, 2001). Furthermore, a nation-wide study conducted by the US Federal Highway Administration, found that 75% o f 383 bridge failures in 1973 involved abutment damage and 25% involved pier damage (Chang, 1973). Over the past few decades, local scour around bridge abutments in open channels has received wide attention, and many researchers have conducted numerous studies on this topic (e.g., Laursen and Toch, 1956; Froehlich, 1989; Melville, 1997; Coleman et al, 2003; Dey 2005). These studies can be broadly grouped into two categories (Zhang, 2005). The first is the prediction o f scour depth by using empirical or semi-empirical formulae based on field data or experimental data and the second method is based on numerical simulations. There are three main types o f scour depth estimation formulae (Lim, 1997): 1) the regime approach, which relates the scour depth to the increased discharge or flow at the abutment; 2) the dimensional analysis, where relevant dimensionless parameters describing the scour are correlated (most o f the past formulas are obtained from this way); and 3) analytical or semi-empirical approach, which is based on sediment transport relation between approach flow and increased shear stress at the abutment site. 67 Natural river beds are composed o f non-uniform sediment (i.e., a large range in grain-size), however, very few studies recognize the influence o f non-uniform bed sediment on the development and morphology o f local scour holes (e.g., Wu et al. 2000; Sui et al. 2010a, Zhang et al. 2012). The direct result o f non-uniform sediment transport in alluvial rivers is grain sorting. This can result in the formation o f armor layer around the bridge abutment and influence the development and the morphology o f scour holes. In the northern regions o f Canada, ice cover on rivers can present numerous engineering challenges as the ice cover can last for several months. River ice formation seasonally affects the water flow and sediment transport in alluvial channels. Field observations indicate that ice cover significantly affects velocity profiles and sediment transport processes in rivers (Figure 3.3-1). A solid ice cover can lead to an increased in the composite resistance and almost double the wetted perimeter. The armor layer and bed morphology o f the local scour coupled with non-uniform sediments under ice cover has not been studied extensively. There is little experimental research on the development and morphology o f local scour around bridge abutments under ice cover with non-uniform sediments (Ackermann et al, 2002; Sui et al, 2009, 2010b). A small-scale flume experimental study in open channel with artificial nonuniform sediments was conducted by Sui et al. (2010) and another large-scale flume experiment with simulated ice cover was conducted in 2012 by the authors. The scour morphology o f the local scour from these two studies is presented here. In addition, the grain size distribution was also measured around the bridge abutment in the second, large-scale, flume experiment. (a) (b) Figure 3.3-1 A comparison o f flow profiles with (a) and without (b) ice cover (Reproduced from Ettema and Daly, 2004) 68 3.3.1 M ethodology Small-scale flume experiment The first experiment was conducted in a 5.6m long, 0.31m wide, 0.4m deep recirculating flume (Sui et al., 2010a). The semi-circular abutment model was used (Figure 3.3-2) and the scour hole morphology was investigated under different flow conditions. As shown in Figure 3.3-2, the semi-circular abutment was placed in a sediment box which has a dimension o f lm long, 0.3m wide and 0.1m deep. To study the impact o f sediment grain size on the local scour development, two layers o f uniform sediment with different grain-sizes were put in the sediment box. The flow rate in the flume was controlled by an inlet valve and flow dissipaters were used in front o f the flume to reduce the turbulence. The semi- circular abutment model was leveled in the sand box and set against the flume wall. The flume was slowly filled to prevent a scour hole from developing prior to the initiation o f the experiment. A weir was installed at the rear o f the flume to control the depth o f water in the flume and once the appropriate water depth was reached, the valve was adjusted to obtain the desired discharge. In this study, a constant flow depth 0.06m was maintained for each run. A pump was used to circulate the water from the reservoir back to the head. |~ F" -f ~IF Wafer Surface regulator ScJtmnt) Trm Abutment D m ipoten Figure 3.3- 2 The experimental setup o f the small scale flume (left) and large scale flume (right) Observation indicated that the time for achieving maximum scour depth in uniform sediments was approximately 12 hours. Herein, the running time ranges between 12 and 15 hours at which 69 point the maximum scour depth was carefully measured by using a point gauge with a resolution o f ± 0.01mm. Table 3. 3-1 Experimental data o f small scale flume experiments H (m) d (mm) da (mm) 3da (mm) U (m/s) ds (m) 0.06 0.26 1.15 3.45 0.24 0.034 0.06 0.26 2.36 7.08 0.34 0.017 0.06 0.26 4.00 12.00 0.41 0.024 0.06 0.52 2.36 7.08 0.34 0.023 0.06 0.52 4.00 12.00 0.41 0.027 0.06 0.84 4.00 12.00 0.41 0.036 0.06 0.26 1.15 3.45 0.28 0.048 0.06 0.26 2.36 7.08 0.39 0.024 0.06 0.26 4.00 12.00 0.47 0.029 0.06 0.52 2.36 7.08 0.39 0.031 0.06 0.52 4.00 12.00 0.47 0.030 0.06 0.84 4.00 12.00 0.47 0.051 0.06 0.26 1.15 3.45 0.31 0.061 0.06 0.26 2.36 7.08 0.44 0.030 0.06 0.26 4.00 12.00 0.53 0.043 0.06 0.52 2.36 7.08 0.44 0.055 0.06 0.52 4.00 12.00 0.53 0.050 0.06 0.84 4.00 12.00 0.53 0.064 Uniform sediments have been used for the purpose o f comparison. Three bed materials were used with diameter o f 0.26mm, 0.52mm and 0.84mm. Coarse sediment with diameter (da) o f 1.15mm, 2.34mm and 4.0 mm were overlain on top o f the bed material to create an armor layer. From a previous study conducted by Froehlich (1995), the thickness o f the armor layer has been designated as 3da. The maximum scour depth o f the local scour around bridge abutment are found in Table 3.3-1, in which H is the flow depth, d is the diameter o f bed material, da is the 70 median diameter o f the armor layer particle, U is the mean flow velocity and ds is the scour depth near the abutment. Large-scale flume experiment The small-scale flume experiment has many advantages including ease o f use, greater control o f flow and bed conditions and lower power usage. However, compared to the natural rivers, smallscale flumes are not adequate to study the grain-size and ice cover impact on the bed morphology o f the local scour. To overcome the limitations o f a small-scale flume, a large-scale flume experiment was conducted in 2012. The setup o f the large-scale flume can be found in Figure 3.3-2. As shown in Figure 3.3-2, the flume has a dimension o f 40m long, 2.0m wide and 1.3m deep. Two sand boxes with a depth o f 0.3m were constructed 10.2m from each other. For the purpose o f observing scouring process from outside o f the flume, one side o f the flume wall in the sand box was replaced by Plexiglass. The semi-circular abutment model was also made from Plexiglass with a radius o f 20cm and 1.0m high. In this large scale experiment, the ice cover was simulated by using styrofoam panel which covered the whole sand box area. To study the bed morphology under different ice covers, a rough ice cover was created by attaching small cubes o f the Styrofoam to the underside o f the simulated ice cover. The small cubes have a dimension o f 2.5cm * 2.5cm x 2.5cm, with a spacing distance 3.5cm from each other. Three different non-uniform sediments were used in this flume. The Dso o f these three sediments were 0.58mm, 0.50cm, 0.47mm. It is important to note that the non-uniform sediment used here are natural sands which were purchased from a local aggregate mine. The velocity range in sand box 1 was 0.16 - 0.26m/s, while in sand box 2, the range is 0.14 - 0.21 m/s. At the beginning o f each experiment, the flume was slowly filled by adjusting the valves in the holding tank. To protect the scour from the initial filling o f the flume, a template was made to cover the bed material. After the required water depth was reached, the template was removed to start the scouring process. Observations indicated that the running time to reach maximum scour depth around the semi-circular abutment was approximately 24 hours. 71 To measure the approaching velocity and flow profile around the bridge abutment, a SonTek IQ velocity meter was installed in front o f the sand box. The velocity meter also provided flow depth, pressure and temperature. In the sand box, a staff gauge was installed for reading water depth directly. A 10MHz SonTek Acoustic Doppler Velocimeter (ADV) was also used at the end o f each experiment to measure the flow field in the scour hole. The ADV measures the phase change caused by the Doppler Shift occurs when the signal reflects o ff the particles in the flow. The running condition o f this large scale flume experiment can be found in Table 3.3-2. Table 3.3- 2 Experimental data o f small scale flume experiments Running D 50 Running condition (mm) time (h) Flume Water Approaching width depth velocity (m) (m) (m/s) 0.58 24 2 0.07 0.21 0.58 24 2 0.19 0.23 0.58 24 2 0.07 0.26 0.50 24 2 0.07 0.21 0.50 24 2 0.19 0.23 0.50 24 2 0.07 0.26 0.47 24 2 0.07 0.21 0.47 24 2 0.19 0.23 0.47 24 2 0.07 0.26 0.58 24 2 0.07 0.23 0.58 24 2 0.19 0.20 0.58 24 2 0.07 0.20 0.50 24 2 0.07 0.23 0.50 24 2 0.19 0.20 0.50 24 2 0.07 0.20 0.47 24 2 0.07 0.23 0.47 24 2 0.19 0.20 0.47 24 2 0.07 0.20 Open channel Smooth cover 72 0.58 24 2 0.07 0.20 0.58 24 2 0.19 0.20 0.58 24 2 0.07 0.22 0.50 24 2 0.07 0.20 0.50 24 2 0.19 0.20 0.50 24 2 0.07 0.22 0.47 24 2 0.07 0.20 0.47 24 2 0.19 0.20 0.47 24 2 0.07 0.22 Rough cover 3.3.2 Results and discussion Local scour pattern and profile At the end o f each experiment, digital photos were taken for the small-scale flume experiment and for the large -scale flume experiment, the local scour was manually measured and then mapped using Surfer 10, Golden Software. The local scour patterns o f the small-scale and largescale flume experiments are compared in Figure 3.3-3. As reported by Sui et al. (2010), in the small scale flume experiment, the coarse sediment tended to stay in the scour hole because due to the large mass compared to the fine-grained sediment and fine sediments were sheltered behind the coarse sediments. Because coarse particles need more energy to move to the downstream the development o f an armor layer depends not only on the approaching velocity, but also the sediment grain size and the thickness o f the armor layer. Depending on the conditions o f approaching flow and armor layer, the armoring process occurs in the upstream portion o f the scour hole. The sediment pile was highly compacted in the upstream portion o f the scour area (Sui et al. 2010). However, the downstream portion o f the scour was less compaction (Figure 3.3-3(a)). Figure 3.3-3(a) shows the sediment redistribution around in the small scale flume in open channels. It was also noted that the scour close to the upstream toe area around the abutment was generally deeper. In the downstream side o f the abutment, the local scour area is smaller compared to that from the upstream. The scouring process showed that the transport o f fine 73 particles on the bed surface left the large particles that formed a stable armor layer on top o f the scour hole. Figure 3.3-3(b) - (d) showed the grain-size distribution and scour pattern in the large scale flume under ice cover. It was also found that with artificial non-uniform sediments in the small flume there was a thick accumulation o f coarse sediment on top o f the fine- grained sediment. In contrast, the natural non-uniform sediments, relatively thin ribbons o f fine-grained sediment was observed in the scour hole, in-between the coarse sediments. As shown in Figure 3.3-3(c) and (d), the fine sediment ribbon is clearly marked. (a) (b) (c) (d) Figure 3.3- 3 (a) The local scour around the bridge abutment in the small-scale flume and (b) (c) and (d) The local scour around the bridge abutment in the large scale flume 74 Sediment transport is governed by flow condition, which is often characterized by the bed shear stress and grain-size. From the classical sediment transport theory, fine-grained sediment are more mobile than coarse sediment. However, this conclusion can only be drawn if the material is uniform and the grains are surrounded by identical ones (Hunziker and Jaeggi, 2002). From the small scale flume experiment, with the artificial non-uniform sediment, the medium and coarse particles were also trapped in the scour hole due to their large mass. In the large scale flume under the natural non-uniform sediment, the armor layer was smaller compare to that in the small scale flume. At the same time, two fine sediment ribbons were noticed in the scour hole, which was not detected in the small scale flume experiments. To study the sediment distribution under ice cover, the sediment was sieved and analyzed. Sediment size distribution around the abutment under ice cover Properties o f sediment include both individual particles and the sediment mixture as a whole. For particles, the size, shape and fall velocity are the main focus, while for sediment mixtures, the size distribution, specific weight, angle o f response are part o f interest. Due to the large variation in sands used in the large-scale flume experiment, this analysis will focus on samples using the sand with a D5o=0.50 mm. For easy interpretation, one local scour contour in the large scale flume under ice cover was used here to illustrate the locations o f sample collection. Figure 3.3-4 shows the 2D and 3D contour map o f the local scour hole, two sediment samples were collected from LI and L2. Samples from these two locations are shown in Figure 3.3-5. From Figures 3.3-4 and 3.3-5, the local scour along the abutment under ice cover with natural non-uniform sediments shows a similar trend o f the local scour compared to the small-scale flume with artificial non-non-uniform sediment. The upstream toe area has a deeper scour depth compared to other areas, however, a fine sediment pile was found at location L2. This feature was not present in the small-scale flume experiment, which can illustrate the fine sediment deposition o f the non-uniform sediment under ice cover. 75 Figure 3.3- 4 The scour contour in the large scale flume Figure 3.3- 5 The sediment samples LI (left) and L2 (right) Figure 3.3-5 shows the sediment sample collected from different locations along the semi­ circular abutment. As shown in Figure 3.3-5, the sample in LI was mainly coarse particles, and in contrast the sample from L2 was comprised mainly by fine sediments. Furthermore, it was observed from the cross section o f the scour hole along the abutment is that in the scour hole there is a sudden elevation increase (Figure 3.3-6). To investigate if there is any different o f sediment composition, samples were also collected at this location (L3). The cumulative size distribution from LI, L2, L3 are plotted in Figure 3.3-7. For the purpose o f comparison, the sand analysis o f the original natural non-uniform sediment was also plotted. It can be found that from the figure that, due to the sediment deposition effect, fine particles were 76 mainly collected at location L2, while at L2, the large particles o f armor layer accounts for most o f the sample. However, at L3, the particles were the coarsest and this is likely due to the interaction o f the abutment and ice cover. The primary vortex is decreasing before the point L3, while the wake vortex is the strongest at the point L3. Because o f the interaction o f these two vortices and ice cover, only large particles were trapped at location L3, most o f the fine particles were removed by the running water and turbulent vortices. Unfortunately, the relationship between the vortices and sediment movement is difficult to elucidate due to the limited experimental data. o- D5fMJ50mm — » cross section < V # • P A B C D F. F (j H I J K Q Figure 3.3- 6 The cross section o f the local scour along the abutment (left) and samples collected (right) l C 0.8 0.2 0 0 0.5 2 1.5 2.5 3 Grain size (mm) — •— LI m... L2 Figure 3.3- 7 The sand analysis o f samples 77 3.5 4 T he m axim um scour depth analysis Small scale flume To study the impact o f diameter o f bed material on the maximum scour depth (ds), the densimetric Froude number was used for the small-scale flume analysis, which can be defined in the following equation: F0 = U / y j g K p d (331) In which, g is the gravitational acceleration, U0 is the approaching velocity, Ap is mass difference between sediment and water, d is the diameter o f the bed material. 0.18 0.16 —, ^0,0025F: v -0.006IF„ +0.0404 0.14 R>= 0.8301 o.i2 ; a 01 0.08 0.06 ♦ ♦ 0.04 0.02 0 * 3 4 5 6 7 8 9 F o Figure 3.3- 8 The variation o f scour depth with densimetric Froude number in small-scale flume By plotting the maximum scour depth with the densimetric Froude number in the small scale flume, the following relationship can be found under open flow condition: ^ » = 0.0025F2 o - 0 .0 0 6 IFo + 0.0404 d (3.3-2) The maximum scour depth increases with an increase in densimetric Froude number. For natural non-uniform sediments in the large-scale flume, the following analysis was conducted. Large scale flume 78 Similarly, under smooth ice cover in the large-scale flume, the maximum scour depth varied with the densimetric Froude number and the data are found in Figure 3.3-9. However, for natural nonuniform sediment the diameter o f the bed material was replaced by the D 50. It can also noted from Figure 3.3-8 and Figure 3.3-9 that, for natural non-uniform sediments, a small increase in the densimetric Froude number can change the maximum scour depth significantly compare to that in the artificial non-uniform sediment. In addition, this research demonstrates that the turbulence in the large-scale flume has strong impact on the scour depth around semi-circular abutments. 3.5 3 * 2.5 § 2 ^50 = 2.7395F? -9.6867/-, +8.8126 5 R* m 0.5 *• ~ ' » 0 .4 2 J 2 ^ X*^ ^ „ ^ ' A A 0 2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 Fo Figure 3.3- 9 The variation o f scour depth under smooth ice cover in large scale flume 3.3.3 Conclusions By using experimental data gained from two flume experiments on the local scour around sem i­ circular abutments, the bed morphology o f the local scour with non-uniform sediment was examined. The small-scale flume experiment with the artificial non-uniform sediment showed the formation o f an armor layer and significant erosion around the abutment. The large-scale flume experiment with natural non-uniform sediment showed that under ice cover, at different locations o f the abutment, the sediment sorting process were more clear. The armor layer only forms in the scour hole while fine sediment deposition was located at the downstream o f the abutment. One relationship o f maximum scour depth with densimetric Froude number was also developed for the small scale flume experiment. However, due to the limitation o f experimental 79 data, the relationship between ice cover roughness and sediment movement is not clear and further experimental data are needed to clearly assess this. References 1. Ackermann N L, Shen H T, Olsson P, 2002, Local scour around circular piers under ice covers. Proceeding o f the 16th IAHR Intemnational Symposium on Ice, Intemnational Association o f Hydraulic Engineering Research, Dunedin, New Zealand. 2. Chang F F M, 1973, A statistical summary o f the cause and cost o f bridge failures. Office o f Research, Federal Highway Administration, Washington D C, US. 3. Chang H H, 2002, Fluvial processes in river engineering, Reissue 2002, Krieger Publishing Company, Krieger Drive, Malabar, Florida, pp. 80-104. 4. Coleman S E, Lauchlan C S, Melville B W, 2003, Clear water scour development at bridge abutments, Journal o f Hydraulic Research, 41(5), pp. 521-531. 5. Dey S, Barbhuiya A K, 2005, Time variation o f scour at abutments, Journal o f Hydraulic Engineering, ASCE, 131 (1), pp. 11-23. 6. Ettema R, Daly S F, 2004, Sediment transport under ice, Cold Regions Research and Engineering Laboratory, ERDC/CRREL TR-04-20, pp. 8. 7. Froehlich D C, 1989, Local scour at bridge abutments. Proc. Natl. Conf. Hydraulic Engineering, ASCE, pp. 13-18. 8. Laursen E M, Toch A, 1956, Scour around bridge piers and abutments. Iowa Highway Research Board Bulletin, No 4. 9. Lim S Y, 1997, Equilibrium clear-water scour around an abutment, Journal o f Hydraulic Engineering, 123(3), pp. 237-243. 10. Melville B W, 1997, Pier and Abutment scour: integrated approach, Journal o f Hydraulic Engineering, 123(2), pp. 125-136. 11. Richardson E V, Davis S R, 2001, Evaluating scour at bridges. HEC18 FHWA NHI-001, Federal Highway Administration, US Department o f Transportation, Washington, DC. 12. Sui J, Faruque M A A, Balanchandar R, 2009, Local scour caused by submerged square jets under model ice cover. Journal o f Hydraulic Engineering, ASCE, Vol 135 (4), pp. 316-319. 80 13. Sui J, Afzalimehr H, Samani A K, Maherani M, 2010a, Clear water scour around semi­ elliptical abutments with armored bed, International Journal o f Sediment Research, Vol. 25, No. 3, pp. 233-244. 14. Sui J, Wang J, He Y, Krol F, 2010b, Velocity profile and incipient motion o f frazil particles under ice cover. International Journal o f Sediment Research, Vol 25(1), pp. 39-51. 15. Wu W, Wang S, Jia Y, 2000b, Non-uniform sediment transport in alluvial rivers, Journal o f Hydraulic research, 38(6), pp. 427-434. 16. Zhang H, 2005, Study o f flow and bed evolution in channels with spur dykes, PhD Dissertation, Ujigawa Hydraulics Laboratory, Kyoto University, Japan. 17. Zhang H, Nakagawa H, Mizutani H, 2012, Bed morphology and grain size characteristics around a spur dike, International Journal o f Sediment Research, Vol. 27, No. 2, pp. 141-157. 81 3.4 A rm o r layer analysis of local scour aro u n d bridge abutm ents u n d e r ice cover Local scour is the erosion o f sediments in the vicinity o f bridge foundations. Depending on if there is a sediment supply from approaching flow, the scour can be categorized as either clear water scour or live-bed scour (Barbhuiya and Dey, 2004). Clear water scour occurs with the absence o f sediments transported from upstream while live-bed scour takes place when the scour hole is continuously fed with sediments by the approaching flow. Local scour is a challenging problem for hydraulic engineers. Most existing studies are conducted in small scale flumes with uniform sediment. Natural river beds are composed o f a mixture o f different sizes o f sand and gravel. Very few studies use natural sand due to its complexity. The finer materials will be transported faster than the coarser materials under the same flow conditions, and the remaining bed material becomes coarser. This coarsening process is stopped once a layer o f coarse material completely covers the river bed and protects the finer materials beneath it from being transported. After this process is completed, the river bed is armored and the coarser layer is called armor layer (Yang, 2003). The incipient velocity for non-uniform sediments varies more in comparison to that o f uniform sediment. Advances in the non-uniform sediment movement play a key role in theoretical analysis and engineering practice pertaining to channel and reservoir design, physical sediment model analysis and numerical simulation (Xu et al, 2008). Meyer-Peter and Mueller (1948) defined the formula describing armor layer sediment size by using one mean grain size o f the bed to calculate the sediment size in the armor layer. The following equation was developed. d- SD (3.4-1) where d is the sediment size in the armor layer, S is the channel slope, D is the mean flow depth, Ki is a constant equal to 0.058 when D is in meters; n is the channel bottom roughness or M anning’s roughness, and dw is the bed material size where 90% o f the material is finer. Yang (1973) developed his criteria by using the approach velocity to illustrate the incipient motion. For open channels, the logarithmic law for velocity distribution is applied. However, in his equation, the relative roughness effect was treated as constant due to insufficient data. Kuhnle 82 (1993) conducted flume experiments on the incipient motion o f gravel and sand mixtures with different ratios. By calculating critical shear stress for incipient motion, it was found that for gravel-sand mixtures, the gravel showed an increasing critical bed shear stress with increasing grain size. Dey and Barbhuiya (2004) examined clear water scour at abutments in armored beds. It was found that the scour depth with an armor layer in clear water scour is always greater than that without armor layer for the same bed sediment. Around bridge piers, Dey and Raika (2007) noticed that the scour depth with an armor layer is less than that without an armor layer for the same bed sediments when the scour hole is shielded by a compact secondary armor layer. Some recent relevant work on the non-uniform sediment transport can also be found from Khullar et al. (2010) and Jha et al. (2011). Guo (2012) gave a critical review o f pier scour in clear water for non-uniform sediments. The flow-structure-sediment factors were analyzed systematically and several empirical equations were reviewed. Zhang et al. (2012) found that the mean grain size and geometric standard deviation o f the bed sediments are two important and practical parameters in characterizing the changes in bed morphology and composition around spur dikes. Furthermore, river ice seasonally affects the flow distribution and results in a change in sediment transport in natural rivers around bridge foundations. The impact o f ice cover on sediment transport is important for cold regions in the northern hemisphere. The velocity field changes significantly under ice cover due to the presence o f an extra boundary layer. As identified by Melville (1992), the primary vortex, together with the down flow are the principal causes o f local scour around bridge abutments. With the presence o f ice cover, the down flow can be increased, which also increases the sediment transport around bridge abutments. Regarding the effects o f river ice on scour and sediment transport, studies such as Ackermann et al. (2002), Hains (2004), Hains and Zabilansky (2004), Munteanu (2004), Andre and Tran (2012) pointed out that combination o f increased ice cover roughness and pressure flow resulted in a larger scour depth. Smith and Ettema’s (1997) experiments showed that the two layer assumption was especially inadequate for characterizing flow resistance and sediment transport rates. Ettema et al. (2000) developed a new method for estimating sediment transport and identified the importance o f assessing flow resistance attributable to bed surface drag. Li (2012) obtained field estimates o f the composite Manning’s coefficient associated with ice cover. By using four different methods, winter measurements o f ice covered rivers in Canada were analyzed. The results show that the composite Manning’s coefficient ranges from 0.013 to 0.040. The results 83 are useful for modeling ice covered river flow and determining the sediment transport under ice cover. To date, there is still no research connecting the non-uniform sediment and ice cover. In this research, three non-uniform sediments and two types o f ice cover are applied to study the armor layer in the scour hole as well as the impacts o f ice cover on the maximum scour depth. 3.4.1 Methodology One large scale flume was used in this study. The set-up o f the flume is indicated in Figure 3.4-1. The flume was 2m wide, 1.3m deep and 38.2m long. Two 0.3m deep sand boxes were created to hold non-uniform sediment. The flume was covered with treated waterproof plywood acting as a false river bed. To compare the shape factor o f the abutment as recognized by Melville (1992), two abutments were made from Plexiglass. On the outside surface o f the abutment, different measuring lines have been drawn for the purpose o f comparing scour profiles at different locations (Figure 3.4-2). For the square abutment, the upstream surface and comer B are the locations o f the maximum scour depth from previous studies, so four equal distance lines were made along the upstream surface. While for the semi-circular abutment, 12 lines having an equal central angle o f 15° were drawn which are used when describing the scour depth along the abutment. Plexiglass window simulated iec cover 39.5m 11.3m 10.2m 5.8m Vertival View Holding lank 11.3m 5.6m 10.2m 38.2m Plan View 84 5.8m 5.3m Figure 3 . 4 - 1 The layout o f the experimental large scale flume < 1 I* z X' Y Measuring points along the Square abutment M easuring points along the Setni-C ireubr abutment F lo w A3 A2 Figure 3.4- 2 Dimensions and measuring points o f abutments Three natural non-uniform sediment mixtures were used in this study. The Dsos were 0.58mm, 0.50mm and 0.47mm respectively, with geometric standard deviation (ag) o f 2.61, 2.53 and 1.89. For all the three sediments, the value o f o g is larger than 1.4, which can be treated as nonuniform sediments (Dey and Barbhuiya, 2004). Two types o f ice covers were used, namely smooth cover and rough cover. Smooth ice cover was constructed from Styrofoam, while the rough ice cover is modified by attaching small cubes to underside o f the smooth cover. The small cubes have a dimension o f 2.5cm x 2.5cm * 2.5cm, with spacing o f 3.5cm from each other (Figure 3.4-3). 85 Figure 3.4- 3 Experimental flume set up and rough ice cover (up); Armor layer around the square abutment comer (bottom) A 10 MHz SonTek Acoustic Doppler Velocimeter (ADV) was used to measure the flow field at the end o f each experiment, which had the equilibrium scour depth. The sampling frequency for ADV was 25Hz. After each experiment, photos o f the local scour around the abutment were taken. After measurement was completed, sediment samples from different scour locations were collected. For the square abutment, samples from comer B were collected and for semi-circular abutment samples from location E to F were collected. Additionally, sediment samples from the downstream fine sediment ridge were collected for the purpose o f comparison with armor layer. Surface sampling was used in accordance with Ettema (1984). The thickness o f the natural armor layer varies from d to 3d, in which d is the particle size o f armor layer (Froehlich, 1995). For this study, surface samples were collected by a small scoop. At the end o f each experiment, sediments in the armor layer were sieved and analyzed. The bottom elevations were measured by using the measuring lines on the abutment. The scour contours were plotted using Surfer 10, Golden Software. In all, 54 experiments have been carried out. The experimental conditions are presented in Table 3.4-1. 86 Table 3.4-1 Test condition and non-uniform sediment composition o f each experiment Average Abutment Cover type condition D 50 (mm) Di6 (mm) Ds4(mm) D 9o(mm) depth (m) velocity (m/s) 0.58 0.28 1.91 2.57 0.07 0.26 0.58 0.28 1.91 2.57 0.07 0.21 0.58 0.28 1.91 2.57 0.19 0.21 0.50 0.26 1.66 2.09 0.07 0.26 0.50 0.26 1.66 2.09 0.07 0.21 0.50 0.26 1.66 2.09 0.19 0.21 0.47 0.23 0.82 1.19 0.07 0.26 0.47 0.23 0.82 1.19 0.07 0.21 0.47 0.23 0.82 1.19 0.19 0.21 0.58 0.28 1.91 2.57 0.07 0.20 0.58 0.28 1.91 2.57 0.19 0.16 0.58 0.28 1.91 2.57 0.07 0.23 0.50 0.26 1.66 2.09 0.07 0.20 0.50 0.26 1.66 2.09 0.19 0.16 0.50 0.26 1.66 2.09 0.07 0.23 0.47 0.23 0.82 1.19 0.07 0.20 0.47 0.23 0.82 1.19 0.19 0.16 0.47 0.23 0.82 1.19 0.07 0.23 0.58 0.28 1.91 2.57 0.07 0.22 0.58 0.28 1.91 2.57 0.07 0.20 0.58 0.28 1.91 2.57 0.19 0.14 Rough 0.50 0.26 1.66 2.09 0.07 0.22 cover 0.50 0.26 1.66 2.09 0.07 0.20 0.50 0.26 1.66 2.09 0.19 0.14 0.47 0.23 0.82 1.19 0.07 0.22 0.47 0.23 0.82 1.19 0.07 0.20 Open channel Square Smooth abutment cover 87 0.47 0.23 0.82 1.19 0.19 0.14 0.58 0.28 1.91 2.57 0.07 0.21 0.58 0.28 1.91 2.57 0.19 0.23 0.58 0.28 1.91 2.57 0.07 0.26 0.50 0.26 1.66 2.09 0.07 0.21 0.50 0.26 1.66 2.09 0.19 0.23 0.50 0.26 1.66 2.09 0.07 0.26 0.47 0.23 0.82 1.19 0.07 0.21 0.47 0.23 0.82 1.19 0.19 0.23 0.47 0.23 0.82 1.19 0.07 0.26 0.58 0.28 1.91 2.57 0.07 0.23 0.58 0.28 1.91 2.57 0.19 0.20 0.58 0.28 1.91 2.57 0.07 0.20 0.50 0.26 1.66 2.09 0.07 0.23 0.50 0.26 1.66 2.09 0.19 0.20 0.50 0.26 1.66 2.09 0.07 0.20 0.47 0.23 0.82 1.19 0.07 0.23 0.47 0.23 0.82 1.19 0.19 0.20 0.47 0.23 0.82 1.19 0.07 0.20 0.58 0.28 1.91 2.57 0.07 0.20 0.58 0.28 1.91 2.57 0.19 0.20 0.58 0.28 1.91 2.57 0.07 0.22 0.50 0.26 1.66 2.09 0.07 0.20 0.50 0.26 1.66 2.09 0.19 0.20 0.50 0.26 1.66 2.09 0.07 0.22 0.47 0.23 0.82 1.19 0.07 0.20 0.47 0.23 0.82 1.19 0.19 0.20 0.47 0.23 0.82 1.19 0.07 0.22 Open channel Semi­ Smooth circular cover Abutment Rough cover 3.4.2 Results and discussion Scour morphology and geometry Figure 3.4-3 shows the experimental setup, local scour morphology and armor layers around the abutments. Figure 3.4-4 shows the contour plotted by Surfer 10 around both square and semi­ circular abutment. 3*1 25* Ml- Figure 3.4- 4 Typical local scour contour around square abutment (left) and semi-circular abutment (right) The geometry o f the scour holes under both open channel and ice covered channel share some common features. For the square abutment, two scour holes were developed in the scouring process, one located in comer B, which is also where the maximum scour depth is located. The other smaller scour hole is located at comer C (Figure 3.4-2). For the semi-circular abutment, the maximum scour depth is located between E and F (Figure 3.4-2). For both square and semi­ circular abutments, the maximum scour depth is located between 45 to 60 degrees facing the approaching flow. Figure 3.4-3 shows one typical non-uniform scour hole around the square abutment. As shown in Figure 3.4-3, the scour hole is not completely covered by an armor layer. There are two fine sediment ribbons extending downstream from the main scour hole. For the armor layer development, an earlier formation o f the armor layer is detected in the upstream section. Due to the interaction between primary vortex and wake vortex behind the abutment, the geometry o f the scour area in the upstream differs substantially from that in the downstream. Noted by Sui et al. (201Ob), the point o f the maximum velocity is located at 60% o f water depth for smooth ice cover, while 70% under rough ice cover. Due to movement o f the maximum 89 velocity in the transverse direction, stronger turbulence can be generated around the abutment under ice covers. It is also interesting to note that, under the same flow conditions, the area o f the armor layer under ice cover is larger than that under open channels. Meanwhile, under rough ice cover, the armor layer area is the largest and extends the longest distance downstream comparing that o f smooth ice cover. While in open channels, the armor layer has the smallest area and shortest distance from the abutment. For non-uniform sediment transport under ice cover, Ettema (2002) mentioned that an imposed ice cover results in an increased composite resistance. The maximum velocity is located between the cover and channel bed, with its vertical location depended on the relative resistance coefficients o f the channel bed and cover. With rough ice cover, the location o f maximum velocity is lower than that with smooth ice cover and open channel because o f the relative large roughness coefficient. Our data o f maximum scour depth under rough cover and smooth cover supports the above conclusions. Grain size analysis of armor layer In the experimental research, three non-uniform sediments were used. Figure 6-5 shows the sieve analysis o f the three sediments. According to United States Standard Test Sieve procedure, the following sieves were selected for the analysis: 4.0mm, 2.0mm, 0.85mm, 0.5mm, 0.25mm, 0.15mm, 0.063mm. The distribution curves were plotted as the “percentage-finer-than’' curve; D 5 0 , Di6, Dg4 , D 9oare calculated from curves (Figure 3.4-5). 0.80 - au 0.40 a 0.20 - 0.00 0.000 1.000 2.000 3.000 4.000 sediment size (mm) D50=0.58mm — * — • D50=0.50mm 90 D50=0.47mm Figure 3 .4- 5 Distribution curves for the non-uniform sediment The armor layer initiated its development from the toe area and then extended to the downside o f the abutment. One can see from Figure 3.4-3 that the armor layer covers the outside o f the scour hole. However, because the maximum scour depth is located in the upstream o f the abutment, only the samples from the larger scour hole were analyzed for this paper. Figure 3.4-6 shows sediment samples o f the armor layer and fine sediment deposition o f the three uniform sediments. The armor layer generated in D5o=0.58mm sediment is covered by coarser particles. Meanwhile, coarse particles are also found in fine sediment ridge. With the decreasing o f D 50, more fine sediments can be found in armor layer while less coarse particles are found in the fine sediment ridge. As smaller D 50S have less coarse particles, the sediment size in the armor layer decreases. Smaller particles in the armor layer will provide less protection in the river bed around the bridge abutment. A smaller grain size in the armor layer can result in a deeper scour depth. Samples of armor layer in scour hole samples of deposition of the ridge distribution curve (a) D5o= 0.58mm — * - *rmor kyer » fine (edunrnt ndgft —*— oigiml_________ (b) D 5 0 - 0.50mm • re o ik y e r fine »d m * n i ridge ongiml 1 2 m dim tnt size (c) D 5o= 0.47mm 91 i 09 08 07 0* 03 0.4 —♦ - tm orkyftr 03 « S »%'» . , i $ j ♦ i I’r T T T p r r T T r i ^ 03 0.1 0 * 0 2 fiw «dtft»ntndg« original_________ 3 4 (mm) Figure 3.4- 6 Samples o f armor layer, fine sediment ridge and related distribution curves Effect of armor layer on maximum scour depth Zhang et al. (2012) pointed out that the extent o f the scour hole exhibits a strong relationship with the Dqo- The maximum scour depth and scour volume decrease with an increase o f the D% around a spur dike Dimensional analysis is used to study the relationship o f sediment size o f the armor layer and the maximum scour depth. Dimensional analysis provides a convenient way for building a framework for parameters on which the maximum scour depth depends. Given the complexity o f the interaction o f various parameters, NCHRP (2011) identified five major groups o f dimensionless parameters affecting the maximum scour depth: flow intensity, Froude number, sediment size, abutment and flow geometry, flow distribution and the abutment stability parameter. In this framework, the influence o f non-uniform sediment size is unclear, and the roughness o f ice cover is not considered. Herein, the maximum scour depth around bridge abutments depends on the following parameters in this study. = f ( U , p , p s , g , d , n h , n l , D 50, l , B , H ) (3.4-2) where dmax is the maximum scour depth around the abutment; U is the mean approach velocity; p and ps is the density o f the water and non-uniform sediment respectively; d is the armor layer grain size; is the Manning’s coefficient for the channel bed; n, is the Manning’s coefficient o f ice cover roughness; D 50 represents the median grain size; 1 is the width o f the abutment; B is the width o f the flume, and H is the approaching flow depth. For a flow-sediment mixture, the terms g, p, and p s should not appear as independent parameters. Additionally, abutment blockage ratio is also kept constant in this study. Equation 3.4-1 is used 92 to calculate the armor layer sediment size d. Since the armor layer sediment size is the main interest here, d is used in the calculation o f the densimetric Froude number. K=u/J(p,/p-\)gd (34 3) Equation 3.4-2 can be simplified as the following: = a (f y 50y (— )c(— y nb » (3 4 .4 ) The densimetric Froude number represents the interaction o f sediment and flow, D so /d represents the impact o f sediment composition on the armor layer particle size, nj/nb represents the ice cover roughness and channel bed roughness, and H/d represents the relationship between approaching flow depth to the armor layer particle size. In all, 54 experiments on the local scour around bridge abutments were conducted, while 18 o f which were under open channels for comparison. Under open channels, the ice cover roughness is treated as 0, and Equation 3.4-4 can be written as: ^ f - = A(Foy { ^ f f { ^ - y d d d (3.4-5) To study the impact o f the independent variables, namely F0, D so /d , H /d , and nj/nb, the following analysis was conducted. Figures 3.4-6 and 3.4-7 indicate the variation o f maximum scour depth to the densimetric Froude number. With the increase in F0, the value dmax/d increases correspondingly. Around both square and semi-circular abutments, under the same densimetric Froude number, the rough ice cover has the largest relative maximum scour depth. Due to the limitations o f experimental data in open channels, the data points around the semi-circular abutment are not as clear as that around square abutment. However, from Figure 3.4-7, under rough ice cover conditions, the scour depth still has the highest value compared to that with smooth ice cover and open channel conditions. 93 80 80 X ♦ 70 70 60 ♦ , 50 1 50 * x ♦ ♦ 40 * 20 60 40 ♦ 30 X 0 0.4 * 20 20 10 ♦ * X W X X ♦ x - • * x # 0 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 0.5 0.6 r# ♦ o p m d im u d > rough cover I | ♦cpnechnnd trough cover Figure 3.4- 7 Variation o f maximum scour depth with F0 at square abutment (left) and semi-circular abutment (right) Figures 3.4-8 and 3.4-9 compare the variation o f the maximum scour depth with different sediment composition and approaching flow depth. Even with the limited experimental data around the semi-circular abutment, an increasing trend is still present for both the square and semi-circular abutment. With the increase in relative flow depth, the maximum scour depth increase correspondingly. Moreover, the square abutment results in a larger scour depth than that o f semi-circular abutment under same flow conditions. 80 60 ♦ 70 X 60 70 ♦ 60 50 50 Xv 1 40 • * 30 ' ♦ E 20 ' 10 • ♦♦ * " .30 X * ♦ 20 0.0? 10 * f t 0• 0 X ♦ % m 1 40 E 0.1 0.15 o 0.2 0.25 27 D ?0 d ♦ open channel * 10 , i * 28 29 30 31 32 H/d * smooth cover ♦ open channel f smooth cover Figure 3.4- 8 Variation o f the maximum scour depth with related variable around square abutment 94 80 SO 7Q 70 60 I ! i 60 50 50 40 40 30 30 30 20 10 10 0 0.1 0 0.15 30 31 33 D?0 ;d ♦ open channel jr smooth cover ♦ open chanone! Figure 3.4- 9 Variation o f the maximum * smooth cover scour depth with related variable around semi-circular abutment Figures 3.4-10 and Figure 3.4-11 show the regression relationship o f the above variables around the square and semi-circular abutments respectively. For both types o f abutments, the rough ice cover leads to a greater dimensionless maximum scour depth compared to that under smooth ice cover and open channel. The slope in the figures 3.4-10 and 3.4-11 indicates that ice covered flow has a large slope compared to that o f the open channel. R2 - 0.8429 80 0 .0 0 0 2 70 X - 1 .4229 60 50 40 30 20 R2 = 0.8099 10 0 T ~T" r 100000 200000 300000 400000 500000 3 61 ( f ) 174 ( —S_) 003 ' ** d ♦ open channel A rough cover Figure 3 .4 -1 0 Dependence o f the maximum scour depth on related variables around square abutment 95 0 0 1E+10 2E+10 3E+10 4E+10 ♦ open channel 5E+10 6E+10 A rough cover Figure 3 . 4 - 11 Dependence o f the maximum scour depth on related variables around the semi­ circular abutment Additionally, under both rough and smooth ice covered conditions, the slope in the scour hole is same for both abutments, while the slope under open channel is smaller. However, under open channel conditions, the semi-circular abutment results in a small slope compared to the square abutment. The following regression relationships are derived. Around square abutment: Open channel: ^s*5- = 0 .0 0 0 2 ^ ) ' 74( ^ 0 ) 003( ^ ) 361 -4.9941 d " d d (3.4-6) Smooth cover: = 0.0002CFJ174( ^ o > ° 03( i L f 61 -5 .5 2 0 6 d d d (3.4-7) Rough cover: d = 0.0002(F„),74( ^ ) ao3(— )3'61 -1.4229 d Around semi-circular abutment: 96 d (3.4-8) Open channel: 4 = - = lx 10~9( F J 38( ^ ) 16(— )61-3 .4 7 6 8 d d d (3.4-9) Smooth cover: ^ = 2x 10-9(F0)38( ^ r ‘ 6(— )61-5.2705 d d d (3.4-10) Rough cover: d^* - 2 x l 0 ”9(Fo)38( ^ ) " l6(— )61 +0.0555 d ° d d (3.4-11) Equations 3.4-6 to 3.4-11 show that H/d has strongest impact on the maximum scour depth compared to other variables. From the derived relationships both the densimetric Froude number (variable index 1.74 and 3.8) and approaching flow depth (variable index 3.8 and 6.1) have stronger impacts on the maximum scour depth than Dso/d. Dso/d has the smallest impact for both square and semi-circular abutments. For non-uniform sediments, if the particle size o f the armor layer is larger, then the maximum scour depth around bridge abutments is smaller under the same flow conditions. In the practical engineering field, H/d has a relatively large value compared to F0 and Dso/d. However, the impact o f Dso/d is still not neglected for the consideration o f maximum scour depth estimation. Ice roughness and the armor layer Research on channel roughness has been conducted; however, for calculating ice cover roughness, there are still very few studies that can be referred. Carey (1966) calculated Manning’s roughness coefficient as 0.01-0.0281 by using supporting field data related to the observed characteristics o f the underside o f ice cover. From his calculation, a constant roughness o f 0.0251 was used for the winter period. For this study, for smooth ice cover, the Manning’s coefficient o f 0.013 was adapted by referring to Mays (1999). For this study, the rough ice cover was created by attaching small cubes with dimensions o f 2.5cmx2.5cmx2.5cm. Equation 12 is applied to calculate the Manning’s coefficient for rough ice cover by considering the roughness height o f the ice cube (Li, 2012). n, = 0 .0 3 9 &S !6 (3.4-12) 97 in which ks is the average roughness height o f the ice under side. By using the above equation, the rough ice cover has a Manning’s coefficient o f 0.021, which falls within the ranges mentioned by Carey (1966). For non-uniform sediment composition with significant portions o f coarse-grain sizes, the channel bed roughness is calculated by using the following equation from Hager (1999): n = 0 0 3 9 D ,/6 (3.4-13) The values o f dmax/d were compared for both smooth and rough cover under almost the same conditions in Figure 6-12. Rough cover causes a greater scour depth compared to open channel around both abutments. However, in some o f tests, higher velocity was applied under smooth cover, which caused a larger dimensionless maximum scour depth. s q u a re a b u tm e n t s e m i-c irc u la r a b u tm e n t Osmooth cover B smooth cover Brough c c . er Brough cover Figure 3 .4 -1 2 The impact o f ice cover roughness on the maximum scour depth Figure 3.4-13 shows the regression analysis around the square abutment under ice cover, as indicated by the following equation: d _ 0 0 0 0 1 (F ) 3 73( ^ 5 i ) _1 78(Z?L)0 77( ^ ) 3 01 d nh d b (3.4-14) Figure 3.4-14 shows the regression analysis for the semi-circular abutment, as indicated by the following equation: ^max _ ] x l O ^ F )860( ^ 50 )~430( H' )' °°(^ )300 d d nb 98 d (3.4-15) 80 70 0 .0001 60 50 40 30 20 10 ♦ square abutment 0 0 100000 200000 300000 x = ( F „ ) 3” 400000 ' d ( wi 500000 o rj f H 600000 700000 ^ U J nb Figure 3 .4 -1 3 Regression relationship under ice cover o f related variables around square abutment 70 60 • d 50 d = l x 10 ' e .V - 1 .0465 R1=0.9604 20 10 • semi-circular abutment 0 0 10000000 20000000 30000000 -V = tF, )!« d 40000000 ii. 50000000 60000000 d j Figure 3 .4 -1 4 Regression relationship under ice cover o f related variables around semi-circular abutment Indexes o f independent variables from above equations can be used to indicate the impact o f ice cover and armor layer sediment size. Compared to the ice cover roughness, particle size o f armor layer sediment has a stronger impact on the maximum scour depth. With the increase in particle diameter o f the armor layer, the maximum scour depth decreases correspondingly. Meanwhile, around the semi-circular abutment, the index for nj/nb equals to 1. From regression analysis, one can also notice the particle size o f the armor layer has a strong impact in reducing the 99 dimensionless maximum scour depth. This conclusion is in line with a previous study conducted by Sui et al. (2010a). In hydraulic engineering, a mixture o f coarse sediments in the vicinity o f bridge foundations can reduce maximum scour depth, which has a similar impact as riprap. 3.4.3 Conclusions By conducting experimental research on the maximum scour depth under ice cover around bridge abutments, the impact o f armor layer development and ice cover roughness is discussed. The armor layer grain size has a strong impact on the dimensionless maximum scour depth. With the increase in the particle size o f armor layer, the maximum scour depth decreases correspondingly. With the increases in ice cover roughness, the maximum scour depth increases. The relationships between maximum scour depth, water depth, densimetric Froude number, ice cover roughness, and armor layer grain size are derived by using dimensionless analysis. During the period o f ice cover formation and during the breakup o f ice jams, the roughness o f ice cover is beyond our present knowledge. The present study indicates the necessity for further ice scour research as it relates to hydraulic engineering. References 1. Ackermann N L, Shen H T, Olsson P, 2002, Local scour around circular piers under ice covers. Proceeding o f the 16th IAHR International Symposium on Ice, International Association o f Hydraulic Engineering Research, Dunedin, New Zealand. 2. 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Zhang H, Nakagawa H, Mizutani H, 2012, Bed morphology and grain size characteristics around a spur dyke, International Journal o f Sediment Research, Vol. 27, No. 2, pp. 141-157. 102 3.5 ADV m easurem ents of flow field along a ro u n d abutm ent u n d e r ice covers Local scour is a complex phenomenon resulting from the interaction o f the three dimensional turbulent flow around bridge foundations and sediment. Local scour around bridge abutments or piers has been an interesting topic for a long time. As mentioned by Melville (1992), 6 o f 10 bridge failures that occurred in New Zealand during Cyclone Bola were related to abutment or approach scour. Luigia et al. (2012) indicated that approximately 50 to 60 bridges fail on average each year in the US. The Federal Highway Administration has estimated that over 60% o f bridge collapses in the US is from local scour (NCHRP, 2011). Bridge foundations should be designed to withstand the effects o f scour resulting from designed floods. Estimation o f the scour depth at bridge foundations is a problem that has perplexed designers for many years. Improving the understanding o f local scour is therefore vital for the engineers responsible for the design o f bridge foundations. In cold regions o f northern hemisphere, ice cover is a big issue as it can stay as long as 6 months on some rivers. Ice cover can result in many problems, such as ice jamming, flooding, restricting the generation o f hydro-power, block river navigation and affect the ecosystem balance (Hicks, 2009). Numerous researches contributed lots work on the ice related hydrology and hydraulic research (Beltaos, 2000; Prowse, 2001; Ettema and Daly, 2004; Wang, 2008; Sui et al., 2009). Ice cover can significantly change the flow field and impact o f sediment transport in natural rivers. Lau and Krishnappan (1985), Ettema et al. (2000) developed their own methods for estimating the sediment transport under ice cover separately. Sui et al. (2000) derived interrelationships o f suspended sediment concentration and riverbed deformation under ice cover at Hequ Reach o f the Yellow River. Turcotte et al. (2011) reviewed the sediment transport process in ice affected rivers by documenting a range o f unique ice and sediment transport process. Considerable advances have been made concerning ice forces on structures, such as bridges (Brown, 2000) and dams as reported by Morse and Hicks (2005). However, very few researches have ever been conducted regarding the local scour around bridge foundations under different roughness o f ice cover. In addition, only a few experiments can be found on the local scour under ice cover (Ackermann et al. 2002, Hains and Zabilansky, 2004; Sui et al. 2010). To date, there is still no research measure the flow field in the scour hole around bridge abutment 103 under ice cover. To fill this gap, a series o f flume experiments were conducted in 2012. The objectives o f this research are as following. a. Compare the flow field measured by ADV in open channel and under ice cover around the semi-circular abutment. b. Compare the scour depth around the bridge abutment at different locations. c. Study the impact o f sediment composition on the maximum scour depth by introducing a dimensionless particle parameter. 3.5.1 Methodology A series of experiments were conducted in a large scale flume with the dimension o f 40m long, 2m wide and 1.3m deep. The flume is located at Quesnel River Research Center, Likely, BC, Canada. The slope o f the flume bottom is 0.2%. A holding tank was made in the upstream o f the flume with a volume o f 90m3 to keep a constant discharge rate in the experimental zone. Two valves were connected to adjust water into the holding tank for the purpose o f changing the flow velocity. From the holding tank, water overflowed from a rectangular weir to flow dissipaters in the experimental zone. One sand box was created in the flume with depth o f 30cm. Figure 3.5-1 shows the setup o f abutment and ice cover in the large scale flume. Figure 3 . 5 - 1 Experimental setup Since ice cover is the main interest here, we made two types o f ice cover for the research, namely smooth cover and rough cover. A smooth ice cover was created from the original Styrofoam. A 104 rough ice cover was created by attaching small cubes o f the Styrofoam to the underside o f the smooth ice cover. The small cubes have a dimension o f 2.5cm *2.5cm * 2.5cm, with a spacing distance o f 3.5cm from each other. Meanwhile, three different natural non-uniform sediments were used in the study. The D 50 o f the three sediments were 0.58cm, 0.50cm and 0.47cm. One round abutment with the diameter o f 20cm was made from plexiglass. The dimensions o f the abutment and coordinate system can be found in Figure 3.5-2. In front o f the sand box, a SonTek IQ was installed in the bottom o f the false floor to measure the approaching flow velocity, water depth, water temperature, etc. Meanwhile, a staff gauge was also installed in the sand box for depth measurement. A 10 MHz SonTek ADV was used to measure the flow field in the vicinity o f the abutment. Table 3.5-1 summarizes the experimental conditions and some preliminary results for each flume run. Table 3.5-1 The maximum scour depth under different conditions. Cover D 50 D l6 D 84 condition (mm) (mm) (mm) Water Average Maximum depth velocity scour depth (m) (m/s) (cm) 0.58 0.28 1.91 0.07 0 .2 1 0 .0 0.58 0.28 1.91 0.19 0.23 5.5 0.58 0.28 1.91 0.07 0.26 2.7 0.50 0.26 1 .6 6 0.07 0 .2 1 3.5 0.50 0.26 1 .6 6 0.19 0.23 7.0 0.50 0.26 1 .6 6 0.07 0.26 6 .0 0.47 0.23 0.82 0.07 0 .2 1 0 .0 0.47 0.23 0.82 0.19 0.23 15.0 0.47 0.23 0.82 0.07 0.26 15.0 0.58 0.28 1.91 0.07 0.23 2.3 0.58 0.28 1.91 0.19 0 .2 0 3.2 Smooth 0.58 0.28 1.91 0.07 0 .2 0 1 .0 cover 0.50 0.26 1 .6 6 0.07 0.23 6.5 0.50 0.26 1 .6 6 0.19 0 .2 0 6 .0 0.50 0.26 1 .6 6 0.07 0 .2 0 2.5 Open channel 105 0.47 0.23 0.82 0.07 0.23 13.5 0.47 0.23 0.82 0.19 0.20 12.0 0.47 0.23 0.82 0.07 0.20 3.0 0.58 0.28 1.91 0.07 0.20 2.2 0.58 0.28 1.91 0.19 0.20 3.5 0.58 0.28 1.91 0.07 0.22 4.7 0.50 0.26 1.66 0.07 0.20 4.0 0.50 0.26 1.66 0.19 0.20 7.5 0.50 0.26 1.66 0.07 0.22 9.0 0.47 0.23 0.82 0.07 0.20 4.0 0.47 0.23 0.82 0.19 0.20 17.0 0.47 0.23 0.82 0.07 0.22 13.7 Rough cover To make sure each experiment had the same conditions, the following steps were strictly followed in the experimental study. (1) Before each experiment, the abutment model was leveled and fixed in the sand box to make sure the abutment is upright to the flume bottom. On the outside surface o f the abutment, different measuring lines have been drawn for the purpose o f measuring scour depth. In all, 13 measuring lines (P ~ Q) were made for the round abutment (Figure 3.5-2). (2) At the beginning o f each experiment, the flume was slowly filled up by adjusting the valves in the holding tank. One template was made to cover the scour zone from initial scouring. After the required water depth was reached, the valves were adjusted to get a certain flow rate in the flume and the template was then removed to start the experiment. The duration o f scour experiments was 24 hours; enough for the maximum scour depth development in a large flume from the authors’ observation. Because the main interest o f the research here is the flow field in the scour hole. The downlooking ADV was used. The sampling interval is 0.04s. At each measuring point, the measurement last at least 20s. 106 2m Figure 3.5- 2 Abutment dimension and coordinate system. 3.5.2 Results and Discussion At the end o f each experiment, photos were taken for the scour profile around the bridge abutment. Meanwhile, by measuring the scour depth along the outside line o f the abutment, the profiles o f the scour hole were plotted by using Surfer 10, Golden Software. Figure 3.5-3 shows the typical scour profile under open channel, smooth cover and rough cover. 107 open channel, h=0.19m , v -0 21m s smooth cover, h=0.19m, v=O.20m/s -20 -10 0 10 108 20 30 -25 -20 -15 -10 -5 0 5 10 15 20 25 Figure 3.5- 3 Contours o f scour hole under open channel, smooth cover, and rough cover Scouring process and contours From our observation, the scour started form the toe area o f the abutment. In the first two hours, one large scour hole has already been formed. The maximum scour depth is located at the comer which is around 50° facing upstream. With the developing o f scour hole, more sediment moves out to the downstream, which can also be seen from the pile o f fine sediment ridge from Figure 3.5-3. The location o f the scour hole is independent on the covered condition. While for the dimensions o f the scour hole, from Figure 3.5-4, it can be seen that ice cover roughness has a more obvious impact on the profile o f the scour. Under rough ice cover, the scouring area is larger compared to that under smooth cover. During the scouring process, a primary vortex was observed in the upstream o f the abutment. As mentioned by Dey and Barbhuiya (2005), around the bridge pier, the horseshoe vortex is the primary reason for scouring. While under ice cover, several small horseshoe vortexes can be detected in the scour hole. The horseshoe vortex had the direction o f clock wise. Meanwhile, some horizontal vortexes were also noticeable. 109 In the downstream o f abutment, a fine sediment ridge can be seen from the outside o f Point J and K to downstream. From our observation, under rough ice cover, the ridge has a longer length compare to that under smooth ice cover. As shown in Figure 3.5-4, one can clearly notice the location o f the maximum scour depth around the bridge abutment. The profiles along the measuring points (A ~ K) are different under ice covers compare to that in open channels. Please also note that the approaching velocity in open channels is larger that those in covered channels. Following three observations are noted. (1) In open channel, the maximum scour occurs around C and D, while under ice covers, the maximum depth locates between B and C. Additionally, under rough ice cover, the maximum scour depth closes to B. From our understanding, with an increase in ice roughness, we can make the assumption that in the upstream o f the abutment, extra shear stress caused by the ice cover impacts location o f maximum scour depth. (2) In open channel, one fine sediment ridge can be noticed along from J to Q. However, there was no clearly fine sediment ridge under ice covers along the abutment. From our observation, fine sediment ridge locates at a distance downstream away from the abutment under ice covers. And with the increase in roughness o f ice cover, the start point o f fine sediment ridge further from the abutment. 110 -open channel 0.5 P,0 H, -3.5 A ,-4 E, -4.3 B, -5 G, -4.7 D ,-5.5 C ,-5.5 - -i- - smooth cover * P, G Q, 0 X ®> -0-2 * F ,- 0 $ * A, -2.2 ' x T , -0.3 * I, -0.7 i E, -2.2 / *'D, -2.8 X B, -3 ' ' - • * C, -3.2 rough cover 1,0 P,0 •-F, 0 \ \ \ ■ \ \ \ \ * B, -3.5 G, -0.8 / J- 0 Q’ ° / 4 E, -2.2 \ 4 A, -2.7 \ K, 0 0--- —• 0~ J D, -3.3 C, -3.5 Figure 3.5- 4 The scour profile along the round abutment under different conditions (3) Two scour holes can be noted along the abutment. One locates in the upstream surface o f the abutment, the other locates between measuring points F to I. Meanwhile, along the abutment, F 111 is the dividing point o f two scour holes. Under ice covers, the elevation at F is close to the original channel bed. (4) From our observation, under ice covers, the scouring process takes more time to reach the maximum scour depth. In open channels, in the large scale flume, the scour hole develops fast in the first 3 hours. While under ice cover, due to the increased opposing resistance, the scouring time is longer than that in open channels. Additionally, with the increase in roughness, the scouring process under rough cover is longer than that under smooth ice cover. Velocity distribution under ice cover As reported by Sui et al. (2010), due to the increased wetted perimeter o f flow caused by ice cover, the composite resistance increased correspondingly. The upper portion o f the flow is mainly affected by the ice cover, while the lower portion is mainly influenced by the channel bed. The maximum velocity locates between the cover and channel bed. However, in the vicinity o f the abutment, inside the scour hole, the flow field has never been studied. By using one down-looking ADV, 3D instantaneous velocity can be measured along the abutment. One should evaluate two parameters provided in the ADV file, which is signal-tonoise ratio (SNR) and the correlation (COR), to ensure the ADV measurements can provide an accurate representation o f the flow velocity (Wahl, 2000). According to the manufacturer, the SNR is a function o f turbidity, the amount o f particulate matter in the flow. COR is an indicator the relative consistency o f the behavior o f the scatters in the sampling volume during the sampling period. Here we used the following standard: SNR > 15 db, and COR >70%. The WinADV software program developed by the Bureau o f Reclamation’s Water Resources Research Laboratory was used to filter the ADV data from poor quality or erroneous data based on the two parameters mentioned above. The velocity field from the upstream surface o f the abutment to the downstream was measured. Figure 3.5-5 shows the Reynolds- averaged velocity measured along the abutment from A to K under open channels, smooth cover and rough cover. The following findings have been noticed. (1) From Point A to I, one can notice that in open channels, the velocity component in the Z direction is small compared to the X and Y components at the same elevation. Meanwhile, compare with the value in Z direction under smooth cover and rough cover, the velocities in the 112 Z direction in open channels are so small being close to zero. From our understanding, ice cover imposed extra force to the flow downwards which creates a higher velocity component in the Z direction. (2) From Point J to K, the velocity component is larger than that from A to I in open channels. J and K are the only two locations that have large Z direction velocity component under open channel condition. Additionally, the fine sediment ridge was located close to this zone. Due to the boundary layer o f the flume wall, small downward vortexes can be generated in the downstream o f the abutment. Our ADV measurement also proved this assumption. (3) Under ice covered condition from Point A to F, all the measured velocity components in Z direction have negative value. However, in the downstream, from G to H, the velocity component in Z direction has positive value. Based on this, we make the following assumption: in the upstream, the primary vortex moves downwards, while in the downstream, the vortex moves upwards from the bottom. (4) In the upstream o f the abutment, velocity component in the X direction is always positive, while the Y component has the trend from positive to negative. At Point C to E, the Y direction velocity is the least compared to that in other points. In other words, the Y direction velocity component decreases along the abutment in the upstream surface till the maximum scour depth. After that, the Y velocity component increases again to the downstream. Meanwhile, we make the conclusion that the Y direction velocity contributes less to the maximum scour depth compared to the velocity component in the X and Z direction. (5) As mentioned above, in open channels, the maximum scour depth locates between C and D. While under ice cover, the maximum scour depth locates between B and C. From our ADV measurement, we also notice that, in open channels, the X direction velocity contributes most to the scour hole development. While under ice covered condition, the Z direction velocity has the largest impact. (6) With the increase in ice cover roughness, the gradient o f velocity in all the three directions decreases, which can be found from the ADV measurements at all points. (7) No scour hole was observed in the downstream o f the abutment under ice cover. However, from I to K, velocity components are highly turbulent compared to that without ice covers. 113 Point A S t ?e « |13 -6 -4 4 Point B X Y -r -to -10 Point C * 10 ■12 -8 -10 -8 -ft 1-4 -2 ft 4 6 8 / n lM t;, r » i Point D >> -8 -4 114 15 Z Point E 1U vffctfty. m i •4 Point F ■*> \ 4 - Point G }. • 9 5 10 15 20 30 J5 9 vetKity. c w i Point H y to 20 2 115 4 6 8 |(| J2 Point I vHwM). m i Figure 3.5- 5 The velocity distribution along the abutment under different conditions: open channel (Left), smooth cover (Middle), rough cover (right) 3.5.3 Conclusion The local scour under ice covers were conducted in a large flume in 2012. Equal distance measuring lines were made along the round abutment to measure the 3D flow velocity in the scour hole. We found that in open channels, the maximum scour depth locates at the upstream surface o f the abutment with an angle about 50°, while under ice cover, the angle is around 60°. 116 By using a down-looking 3D ADV, the flow field at different locations and elevations were measured. Compared to the flow in open channel, the velocity component in Z direction contribute much to the development o f scour hole under ice covers. Based on the comparison o f flow distribution, the velocity field around the abutment was analyzed under open channel and ice covered conditions. References 1. Ackermann N L, Shen H T, Olsson P, 2002. Local scour around circular piers under ice covers. Proceeding of the 16th IAHR International Symposium on Ice, International Association o f Hydraulic Engineering Research, Dunedin, New Zealand. 2. Beltaos S, 2000. Advances in river ice hydrology. Hydrological Processes. Vol. 14, pp. 1613-1625. 3. Brown T G, 2000. Ice loads on the piers o f Confederation Bridge, Canada. Structural Engineer, Vol. 78, pp. 18-23. 4. Dey S, Barbhuiya A K, 2005. Turbulent flow field in a scour hole at a semicircular abutment. Canadian Journal o f Civil Engineering, Vol. 32, pp. 213-232. 5. Ettema R, Braileanu, F, Muste M, 2000. Method for estimating sediment transport in ice covered channels. Journal o f Cold Regions Engineering, Vol. 14, No. 3, pp. 130-144. 6. Ettema R, Daly S F, 2004. Sediment transport under ice. ERDC/CRREL TR-04-20. Cold regions research and Engineering Laboratory, US Army Corps o f Engineers. 7. Hains D, Zabilansky L, 2004. Laboratory test o f scour under ice: Data and preliminary results. ERDC/CRREL TR-04-09. Cold regions research and Engineering Laboratory, US Army Corps o f Engineers. 8. Hicks F, 2009. An overview o f river ice problems: CRIPE 07 guest editorial Cold regions Science and Technology, 55: pp. 175-185. 9. Lau Y L, Krishnappan B G, 1985. Sediment transport under ice cover. Journal o f Hydraulic Engineering, ASCE, 111(6), pp. 934-950. 10. Luigia B, Paolo P, Giuliano D B, 2012. Bridge pier scour: a review o f process, measurements and estimates. Environmental Engineering and Management Journal, Vol. 11 (5). 117 11. Melville B W, 1992. Local scour at bridge abutments, Journal o f Hydraulic Engineering. ASCE, Vol. 118, No. 4, pp. 615-631. 12. Morse B, Hicks F, 2005. Advances in river ice hydrology 1999-2003. Hydrological Processes, Vol. 19, pp. 247-263. 13.NCHRP Web-only Document 181, 2011. Evaluation o f Bridge-Scour Research: Abutment and Contraction Scour Processes and Prediction. NCHRP Project 24-27(02). 14. Prowse T D, 2001. River-Ice ecology: Part A. Hydrologic, geomorphic and water-quality aspects. Journal o f Cold Regions Engineering, Vol. 15, pp. 1-16. 15. Sui J, Wang D, Kamey B, 2000. Suspended sediment concentration and deformation o f riverbed an a frazil jammed reach. Canadian Journal o f Civil Engineering, Vol. 27,1120-1129. 16. Sui J, Faruque M A A, Balanchandar R, 2009. Local scour caused by submerged square jets under model ice cover. Journal o f Hydraulic Engineering, Vol. 135, No. 4, pp. 316-319. 17. Sui J, Wang J, He Y, Krol F, 2010. Velocity profile and incipient motion o f frazil particles under ice cover. International Journal o f Sediment Research, Vol. 25, No. 1, pp. 39-51. 18. SonTek, 2001, Acoustic Doppler Velocimeter (ADV) principles o f operation, SonTek ADV technical manual, SonTek, San Diego. 19. Turcotte B, Morse B, Bergeron N E, Roy A G, 2011. Sediment transport in ice affected rivers. Journal o f Hydrology, Vol. 409, pp. 561-577. 20. Wahl T L, 2000, Analyzing data using WinADV. Joint Conference on water resources engineering and water resources planning and management, Minneapolis, Minnesota, pp. 1-10. 21. Wang J, Sui J, Kamey B, 2008. Incipient motion o f non-cohesive sediment under ice cover - an experimental study. Journal o f Hydrodynamics, Vol. 20, No. 1, pp. 177-124. 118 3.6 T he incipient m otion of bed m aterial and sh ear stress analysis aro u n d bridge abutm ents un d er ice-cover Local scour refers to the scour caused by river obstructions such as bridge abutments, piers, and other objects that obstruct the flow (Chang, 2002). It has been identified as an important issue by civil engineers for a long time. Excessive scour can cause structural failure and even result in the loss o f life. According to Melville (1992), 29 o f 108 bridge failures in New Zealand between 1960 and 1984 were attributed to abutment scour. Over the past few decades, local scour around bridge abutments has received worldwide attention: Laursen and Toch, 1956; Froehlich, 1995; Melville, 1997; Coleman et al, 2003; Dey and Barbhuiya, 2005. However, almost all o f these studies were conducted in open channels. In the Northern Hemisphere, winter lasts up to six months, which is a big challenge for hydraulic engineers to estimate scour condition around bridges. To fill this gap, some researchers started to look at this problem from an experimental approach (Ackermann et al., 2002; Hains, 2004; Munteanu, 2004; Ettema and Daly, 2004; Sui et al., 2009, 2010; Munteanu and Frenette, 2010) and numerical approach (Beltaos, 2000; Wang et al., 2008). Ackermann et al. (2002) investigated the effects o f ice cover on local scour around bridge piers. By using uniform sediments, the author’s found that for equivalent averaged flow velocities, the existence o f an ice cover could increase the local scour depth by 25%~35%. For live bed scour, a rough cover gave a slightly larger scour depth than smooth cover. Munteanu (2004) conducted experiments on local scour around cylinders and found that under clear water conditions local scour increased up to 55 percent. Sui et al. (2010) mentioned that the flow velocity profiles under ice cover appear to be identical regardless o f the average flow velocity and flow depth. As reported by Wang et al. (2008), under ice covered conditions, flow velocity profiles can be divided into the upper portion which is from the ice cover bottom to the point o f the maximum velocity, and the lower portion, which is from the channel bed to the maximum velocity. When the channel bed and ice cover have different resistance coefficients, the maximum velocity will be closer to the surface with the smallest resistance coefficient. In practice, dimensionless shear stress is used to study the incipient motion. Dey and Barbhuiya (2005) investigated the three dimensional turbulent flow properties around a short vertical wall abutment both upstream and downstream o f the scour hole in open channels. By using the 119 Reynolds stresses, the bed shear stresses were also calculated. From their experiments, the maximum bed shear stresses were about 3.2 times that o f the incoming flow. Duan et al. (2009) examined the Reynolds stresses around a spur dike. It was found the Reynolds stress was 2-3 times that o f the incoming flow. Since the abutment and spur dike have similar contraction impact on the flow, all three studies showed the similar amplification factor o f bed shear stress in open channel flow. For non-uniform sediments, finer materials can be transported faster than coarser materials under constant flow conditions. The remaining coarser layer is called armor layer (Yang, 2003). With the development o f an armor layer, further sediment transport is inhibited. Non-uniform sediment makes up typical bed composition in natural rivers. To date, there are no known experimental studies on clear water scour around bridge abutments under ice covered conditions with non-uniform sediments. The effects o f ice cover and armor layer have to be considered in the analysis o f local scour. In this study, we started with a particle force analysis under ice cover by introducing armor layer particle size. Then the dimensionless shear stress was calculated. 3.6.1 Experimental setup and measurement Experiments were conducted in a 40m long, 2m wide and 1.3m deep flume located at Quesnel River Research Center, BC, Canada (Figure 3.6-la). The flume had a bottom slope o f 0.2% and a 90m3 volume holding tank was located in the upstream section o f the flume to keep a constant flow rate in the experimental zone. At the end o f the holding tank, water overflowed from a rectangular weir to flow dissipaters in the experimental zone. Two types o f ice cover were used in the research, namely smooth cover and rough cover. The ice cover was 6m long, which covered the experimental sand box area completely. Two abutment models were made from plexi-glass, semi-circular and square abutments (Figure 3.6-lb). The abutment model was located in the sand box to simulate a bridge abutment with a solid foundation in the floodplain. A smooth ice cover was created from Styrofoam panels, while a rough ice cover was created by attaching small cubes o f the Styrofoam to the underside o f the smooth ice cover. The small cubes had dimensions o f 2.5cm *2.5cm x 2.5cm, with a spacing distance o f 3.5cm from each other. In this study, three different natural non-uniform sediments were used in the flume. The D 50 o f the 120 three sediments was 0.58cm, 0.50cm and 0.47cm with geometric standard deviations (a g) larger than 1.4. To maintain clear water scour conditions, the approaching velocity was carefully chosen in this series o f experiments. A SonTek IQ was installed for flow velocity and water depth measurement. We also used a 10 MHz SonTek down looking ADV for scour hole velocity measurements. The sampling rate o f the ADV was 10Hz. ADV measurements were mainly located at four points around square abutment, A, B, C, D. Around the semi-circular abutment, the measurement points (from A to K) were along measuring lines marked on the abutment (Figure 3.6-lb). For the ADV measurement, two values were used to ensure the measurements can provide an accurate representation o f the flow velocity: signal-to-noise ratio (SNR) larger than 15db and the correlation (COR) between 70% and 100%. Then the data was analyzed by WinADV (Wahl, 2000). (a) sP irM *tfl* * window- \ siim iln Ja J ic e a>vtT r 1I I H o ld in g t'm k \ pr S « k) G o .\ r ! | 5M n tftm i 5.3m (b) Figure 3.6-1 Sketch o f experimental setup and abutment dimension 121 3.6.2 Results and discussion Incipient motion under ice cover The presence o f ice cover in the channel altered the flow characteristics to a great extent. From our observation, the incipient motion started from the toe area o f the abutment. Around the square abutment, the scour started at point B and extends to A and E. While around the semi­ circular abutment, the scour was firstly observed between Point D and E. The forces acting on a sediment particle at the bottom o f the scour hole under ice cover are shown in Figure 3.6-2. For most natural rivers, the river slopes are small enough that the component o f gravitational force acting on the particle in the direction o f flow can be neglected. As shown in Figure 3.6-2, the forces to be considered related to the incipient motion are the drag force F d, lift force F l, submerged weight W, and the resistance force F r. The angle o f the scour hole with vertical abutment is a. Abutment Icc cover Figure 3.6- 2 Incipient motion in the scour hole under ice cover The scour angle was calculated by measuring the upstream facing scour distance and maximum scour depth. For the square abutment, the distance from Point B to upstream was measured, while for the semi-circular abutment, since the maximum scour depth was located between Point D and E, the larger distance outwards from D and E was used. We found that around the square abutment, the average scour angle was 65°, while the average scour angle around the semi­ circular abutment was 74°. From the perspective o f preventing local scour, the larger the scour 122 angle, the better. Our study indicates the idea that streamline-like abutments cause less local scour depth under ice-covered condition. A sediment particle is at a state o f incipient motion when the following conditions have been satisfied: fFD = F * sin a [W = Fl +F r cos a By using Yang’s criteria (2003) for incipient motion, the drag force can be expressed as: FD ~- C D ^ .—— V2 ~ r d 4 2 (3.6-2) where C d is the drag coefficient at velocity Vd, p is the density o f water, and Vd is the local velocity at a distance d above the bed. In open channels, the shear velocity, shear stress or flow velocity profile can be calculated by using the logarithmic distribution law. The lift force acting on the particle can be obtained as: 2 4 2 (3.6-3) where C l is the lift coefficient at velocity V d . The submerged weight o f the particle can be given by: W = ^ ( p s-p )g 6 (3.6-4) By applying Equation 3.6-2 to 3.6-4 to Equation 3.6-1, the following relationship can be found: v3 F (3 6-5) CDctgcc + CL The lift coefficient and drag coefficient can be determined by experiments. Since the sediments used here are non-uniform sediment, in Equation 3.6-5, the diameter o f the sediment particle will be replaced by D 50, and the following can be derived. V = j— \3 -----------1------------ £ ) p CDctgoc + C, 50 (3 6 6) From Equation 3.6-6, one can note that, with the increase in scour angle, the velocity needed to move the particle in the scour hole will increase correspondingly. When the scour angle is equal to 90°, the velocity reaches maximum. However, when the scour angle is less than 90° the critical 123 velocity for incipient motion in the scour hole will be smaller compared to that with flat beds under the same flow conditions. Regarding the drag coefficient C d, since the Reynolds number in this research was larger than 105, the Stokes Law cannot be applied. By referring the relationship between drag coefficient and Reynolds number for a sphere, developed by G raf and Acaroglu (1966), the value o f C d can be determined. For the lift coefficient C l , the lift coefficient was a function o f shape and density o f the sediment particle. If C l=|3Cd, the trail and error method was used to get the value o f P in this research. Around the square abutment, the maximum scour depth was located around Point B, herein, the measurements at B were used to calculate the near bed velocity. While for the semi-circular abutment, the maximum scour depth was located between Point D and E, so the measurements at these two points were used. Under ice cover, if the flow velocity profile was available, as suggested by Kuhnle et al. (2008), the bed shear velocity can be calculated by fitting a least square regression to flow velocity and distance measurements from near the bed to 20% o f the depth using the following: U.c = du (3.6-7) 5J5d\gh in which, U*c is the critical bed shear velocity, “ is the mean flow velocity at a distance o f h. However, if the velocity profile was not available, the logarithmic velocity distribution assumption was one o f the generally accepted methods for calculating the shear velocity based on Prandtl and Einstein correction factor (Einstein, 1950). U = ________ - ________ 5.75 log10 12.27*4h Ao (3.6-8) Where Rh is the channel hydraulic radius, " is the average cross sectional velocity, D 50 is used to represent the particle size since the sediment used in this research is non-uniform sediment, x is the Einstein multiplication factor, here we used xr=l, and the ice cover can be included in the channel hydraulic radius. The critical shear velocities were calculated based on Equation 3.6-8. 124 At the end o f each experiment, an armor layer developed around the bridge abutment. To assess the impacts o f armor layers, Meyer-Peter and Mueller (1948) developed the following equation by using one mean grain size o f the bed to calculate the sediment size in the armor layer. SH d (3.6-9) Where d is the sediment size in the armor layer; S is the channel slope; H is the mean flow depth; Ki is the constant number equal to 0.058 when H is in meters; n is the channel bottom roughness or Manning’s roughness, and D 90 is the bed material size where 90% o f the material is finer. To further examine the relationship between near bed velocity and maximum scour depth, Figure 3.6-3 was plotted. From Figure 3.6-3, at least three observations can be noticed. 80 +A 10 11 12 13 14 15 16 17 18 19 V4flc •DSO^.SSmin *D50*0.50mm AD50=0.47inra Figure 3.6- 3 Incipient motion o f different sediments with the maximum scour depth (1) For the same sediment, with the increase in V d / U c , the value o f dmax/d decreases. From Figure 3.6-3, the slope o f the curve represents the changing rate o f scour depth. At the beginning, the scour depth increased quickly. Afterward, with the development o f the scour hole and formation o f the armor layer, the changing rate decreased correspondingly. The changing rate became 0 at the end, which means no variation in scour depth. In other words, the maximum scour depth was reached. (2 ) Under the same flow condition with the same value o f V d / U c , sediment with smaller D 50 had a larger maximum scour depth. In this case, sediment with Dso=0.47mm had the largest maximum scour depth. 125 (3) When the value o f V | 40 ♦ ■w 30 20 10 * 0 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02 0.022 0.024 0.026 0.028 Dimensionless shear stress Figure 3.6- 5 The maximum scour depth variation with dimensionless shear stress around square abutment 100 ! 1 | 60 40 0.004 0.008 0.012 0.016 0.02 0.024 0.028 D im e n s io n le s s s h e a r s tr e s s ♦ open channel Figure 3.6- 6 The maximum scour depth variation with dimensionless shear stress under ice cover and open channel (square abutment) 128 0 + 0.01 T- T.................. ..................... T............. 0 .0 1 2 0 .0 1 4 0 .0 1 6 0 .0 1 8 F...................r ...................r ................. ’1 0 .0 2 0 .0 2 2 0 .0 2 4 0 .0 2 6 D im e n s io n le s s s h e a r s tr e s s x smooth cover • rough cover Figure 3.6- 7 The maximum scour depth variation with dimensionless shear stress under smooth ice cover and rough ice cover (semi-circular abutment) 3.6.3 Conclusions The present study investigated the features o f incipient motion under ice cover with non-uniform sediments. Experiments have been conducted by using two abutment models and three nonuniform sediments, under open flow condition and two ice-covered conditions. The following are the main conclusions that can be drawn from this study: 1. The average scour angle around a semi-circular abutment is around 10 degrees larger than that around the square abutment under clear water conditions. The streamline-like abutment with a solid foundation in the floodplain causes less local scour depth than that caused by the square abutment under ice-covered condition. 2. Based on the scour angle around bridge abutment, it was found that for same non-uniform sediment, due to the formation o f an armor layer, the maximum scour depth remains constant. 3. W ith the increase in dimensionless shear stress, the maximum scour depth increases correspondingly. Additionally, the presence o f ice cover can result in a deeper maximum scour depth compared to that under open flow condition. In reality, when the ice cover forms in early winter and breaks up in early spring, the roughness coefficient o f ice cover (or ice jam ) is surprisingly larger than the stable covered period during winter. Therefore the scour depth around bridge abutment at this time may increase due to the enlarged roughness coefficient. 129 The present research deals with the incipient motion and dimensionless shear stress for nonuniform sediments under ice covered condition. Further work needs to be carried out to investigate the velocity profile under ice cover in the scour hole around abutments with solid foundations. Notation C d= drag coefficient at velocity Vd C l= lift coefficient at velocity Vd d= sediment size in the armor layer D 50 = Mean diameter o f sediment for which 50% o f the sample is finner (mm) F d= Drag force for incipient motion F l= Lift force F r= Resistance force g = gravity acceleration (ms‘2) H= mean flow depth n= M anning’s roughness value Re* = Reynolds number (-) S= channel slope U*c= critical bed shear velocity u= mean velocity at distance h from the bottom W= Particle submerged weight a = scour angle (-) p= density o f water ps= density o f sediment cyg= geometric standard deviation (-) x*—dimensionless shear stress (-) 130 References 1. Ackermann, N. L., Shen, H. T., Olsson, P. (2002) Local scour around circular piers under ice covers. Proc. Int. Conf. 16th IAHR International Symposium on Ice, IAHR, Dunedin, New Zealand. 2. Coleman, S. E., Lauchlan, C. S., Melville, B. W. (2003). Clear water scour development at bridge abutments, J. Hydraulic Res., 41(5), 521-531. 3. Dey, S., Barbhuiya, A. K. (2005). Turbulent flow field in a scour hole at a semicircular abutment, Can. J. Civ. Eng., 32,213-232. 4. Duan, J. G., He, L., Fu, X., Wang, Q. (2009). Mean flow and turbulence around experimental spur dike, Adv. Water Resour., 32, 1717-1725. 5. Ettema, R., Braileanu, F., Muste, M. (2000). Method for estimating sediment transport in ice covered channels. J. Cold Reg. Eng., ASCE, 14( 3), 130-144. 6. Ettema, R., Daly, S. (2004). Sediment transport under ice. ERDC/CRREL TR-04-20. Cold regions research and Engineering Laboratory, US Army Corps o f Engineers. 7. Froehlich, D. C., (1995). Armor limited clear water construction scour at bridge. J. Hydraulic Eng., 121, 490-493. 8. Graf, W. H., Acaroglu, E. R. (1966). Setting velocities o f natural grains. Bulletin o f the International Association o f Scientific Hydrology, 11(4). 9. Hains, D. B. (2004). An experimental study o f ice effects on scour at bridge piers. PhD Dissertation, Lehigh University, Bethlehem, PA. 10. Laursen, E. M., Toch, A. (1956). Scour around bridge piers and abutments. Iowa Highway Research Board Bulletin, No 4. 11. Melville, B. W. (1992). Local scour at bridge abutments. J. Hydraulic Eng., 118, 615631. 12. Melville, B. W. (1997). Pier and Abutment scour: integrated approach. J. Hydraulic Eng., 123(2), 125-136. 13. Munteanu, A. (2004). Scouring around a cylindrical bridge pier under partially icecovered flow condition. Master thesis, University o f Ottawa, Ottawa, Ontario, Canada. 131 14. Munteanu, A., Frenette, R. (2010). Scouring around a cylindrical bridge pier under ice covered flow condition-experimental analysis. R V Anderson Associates Limited and Oxand report. 15. Sui, J., Faruque, M. A. A., Balachandar, R. (2009). Local scour caused by submerged square jets under model ice cover. J. Hydraulic Eng., 135(4), 316-319. 16. Sui, J., Wang, J., He, Y., Krol, F. (2010). Velocity profile and incipient motion o f frazil particles under ice cover. International Journal o f Sediment Research, 25(1), 39-51. 17. Smith, B. T., Ettema, R. (1997). Flow resistance in ice covered alluvial channels. J. Hydraulic Eng., 123( 7), 592-599. 18. Wahl, T. L. (2000). Analyzing data using WinADV. 2000 Joint Conference on water resources engineering and water resources planning and management. Minneapolis, Minnesota, 1- 10 . 19. Wang, J., Sui, J., Kamey, B. (2008). Incipient motion o f non-cohesive sediment under ice c o v e r -a n experimental study. Journal o f Hydrodynamics, 20(1), 177-124. 20. Yang, C. T. (2003). Sediment transport, theory and practice. Krieger publishing company, Krieger Drive, Malabar, Florida. 132 4 GENERAL CONCLUSION Experiments have been conducted in a large scale flume to study the impact o f ice cover roughness and non-uniform sediments on the local scour around two types o f commonly used abutments. It is found that ice cover plays an important role in the development o f local scour hole around bridge abutments, including bed morphology, maximum scour depth, maximum scour depth location, armor layer etc. The general conclusions are as follows. The location o f the maximum scour depth along the abutment is around 60° from the flume wall for semi-circular abutment. While the maximum scour depth around square abutment locates in the upstream comer. The results indicate that the impact o f shape factor for semi-circular abutments on maximum scour depth is smaller in covered conditions than that in open channels. The range o f shape factor is between 0.66 and 0.71. The downstream slope in the scour hole is also smaller compared to that in the upstream. Under ice cover, the average scour depth is always greater compared to that in open channels. The average scour depth under rough ice cover is 35% greater than that under smooth ice cover. In this research, densimetirc Froude number is also used to investigate the impacts o f non-uniform sediment composition on local scour. The scour volume and scour area are calculated and compared to open channel, smooth and rough cover conditions. By using Buckingham Pi theorem for dimensional analysis, the impact o f shape factor and ice cover roughness on maximum scour depth around abutments is investigated. Empirical equations o f the maximum scour depth are developed, which indicates that with an increase in sediment grain size, the maximum scour depth decreased correspondingly. Furthermore, the impact o f armor layer development and ice cover roughness is discussed. The armor layer only forms in the scour hole while fine sediment deposition is located at the downstream o f the abutment. The armor layer grain size has a strong impact on the dimensionless maximum scour depth. With the increase in the particle size o f armor layer, the maximum scour depth decreases correspondingly. The relationships between maximum scour depth, water depth, densimetric Froude number, ice cover roughness, and armor layer grain size are derived by using dimensionless analysis. By using a down-looking 3D ADV, the flow field at different locations and elevations around two abutments was measured. Compared to the flow in open channel, the velocity component in 133 Z direction contributes much to the development o f scour hole under ice covers. Additionally, features o f incipient motion under ice cover with non-uniform sediments are studied at the end o f the research. It is interesting to find that the average scour angle around a semi-circular abutment is around 10 degrees larger than that around the square abutment under clear water conditions. The streamline-like abutment with a solid foundation in the floodplain causes less local scour depth than that caused by the square abutment under ice-covered condition. With the increase in dimensionless shear stress, the maximum scour depth increases correspondingly. In reality, ice cover is a big issue in the northern hemisphere. When the ice cover forms in early winter and breaks up in early spring, the roughness o f ice cover (or ice jam ) is completely different, compared to that with stable covered period during the winter. Up to date, the impact o f ice cover is beyond our knowledge. The present study indicates the necessity for further ice scour research as it relates to hydraulic engineering. Empirical equations developed from the present research can also be used for the estimation o f scour depth under ice cover in hydraulic engineering. 134 5 APPENDIX Table 5 - 1 Experimental data collected at non-uniform sand (Dso = 0.58 mm) date 0922 0923 0926 0927 0928 0929 0930 1001 1002 1003 1004 1005 1006 1007 1008 1009 1010 1011 abutment type flume cover maximum depth (cm) average velocity (m/s) approach depth scour volume(cm3) scour area (cm2) average scour depth (cm) square open 9.5 0.26 0.07 411.46 267.38 1.54 square open 4 0.21 0.07 288.18 355.35 0.81 square open 5.5 0.21 0.19 661.88 592.79 1.12 round open 0 0.21 0.07 0.00 0.00 0.00 round open 5.5 0.23 0.19 1433.09 1009.79 1.42 round open 2.7 0.26 0.07 570.46 782.08 0.73 round smooth 2.3 0.23 0.07 273.33 466.68 0.59 round smooth 3.2 0.2 0.19 696.74 907.55 0.77 round smooth 1 0.2 0.07 165.88 494.57 0.34 square smooth 3.1 0.2 0.07 167.72 264.03 0.64 square smooth 4 0.16 0.19 706.36 733.16 0.96 square smooth 8 0.23 0.07 1201.07 509.75 2.36 square rough 6.5 0.22 0.07 1400.24 904.46 1.55 square rough 5.7 0.21 0.07 993.01 766.47 1.30 square rough 5 0.14 0.19 785.24 518.72 1.51 round rough 2.2 0.21 0.07 238.63 540.96 0.44 round rough 3.5 0.2 0.19 459.50 376.34 1.22 round rough 4.7 0.22 0.07 1127.69 715.74 1.58 135 Table 5 - 2 Experimental data collected at non-uniform sand (D 5 0 = 0.50 mm) date abutment type flume cover maximum depth (cm) average velocity (m/s) approach depth (m) scour volume (cm3) scour area (cm2) average scour depth (cm) square open 16.4 0.26 0.07 24930.95 3977.91 6.27 square open 7 0.21 0.07 1131.11 1188.88 0.95 square open 6.5 0.21 0.19 1412.31 645.86 2.19 round open 0 0.21 0.07 0.00 0.00 0.00 round open 15 0.23 0.19 19095.94 3335.68 5.72 round open 15 0.26 0.07 16847.15 3401.70 4.95 round smooth 13.5 0.23 0.07 6520.80 1895.73 3.44 round smooth 12 0.2 0.19 5856.95 1758.72 3.33 round smooth 3 0.2 0.07 187.58 213.61 0.88 square smooth 8 0.2 0.07 1610.17 757.03 2.13 square smooth 6.5 0.16 0.19 1002.35 536.94 1.87 square smooth 15.5 0.23 0.07 12372.14 3101.20 3.99 square rough 16.5 0.22 0.07 14090.53 3885.92 3.63 square rough 8.3 0.21 0.07 3582.35 1421.59 2.52 square rough 8 0.14 0.19 2723.10 1151.93 2.36 round rough 4 0.21 0.07 565.81 469.97 1.20 round rough 17 0.2 0.19 13986.93 3020.83 4.63 round rough 13.7 0.22 0.07 3224.38 1150.13 2.80 1018 1019 1020 1021 1022 1023 1024 1025 1026 1027 1028 1029 1030 1031 1101 1102 1104 1103 136 Table 5 - 3 Experimental data collected at non-uniform sand ( D 5 0 = 0.47 mm) date 1107 1108 1109 1110 1111 1112 1113 1114 1117 1116 1115 1118 1119 1120 1121 1122 1123 1124 abutment type flume cover maximum depth (cm) average velocity (m/s) approach depth (m) scour volume (cm3) scour area (cm2) average scour depth (cm) square open 15 0.26 0.07 8277.42 2002.30 4.13 square open 7.5 0.21 0.07 1007.74 532.61 1.89 square open 6 0.21 0.19 1010.42 652.13 1.55 round open 3.5 0.21 0.07 778.15 717.05 1.09 round open 7 0.23 0.19 3278.91 1528.65 2.14 round open 6 0.26 0.07 3712.67 2240.62 1.66 round smooth 6.5 0.23 0.07 1674.01 1187.83 1.41 round smooth 6 0.2 0.19 2350.44 1549.43 1.52 round smooth 2.5 0.2 0.07 361.83 511.09 0.71 square smooth 4 0.2 0.07 540.29 547.31 0.99 square smooth 4.5 0.16 0.19 608.74 643.44 0.95 square smooth 10.5 0.23 0.07 4752.10 1631.03 2.91 square rough 11 0.22 0.07 4343.89 1657.09 2.62 square rough 6 0.21 0.07 1537.33 832.45 1.85 square rough 4.5 0.14 0.19 999.56 808.63 1.24 round rough 4 0.21 0.07 481.45 507.46 0.95 round rough 7.5 0.2 0.19 2190.03 881.93 2.48 round rough 9 0.22 0.07 5892.06 2264.36 2.60 137 Table 5- 4 Scour contours a t D 5 0 = 0 . 5 8 m m Date Contour 0922 0923 •25 -20 -15 -10 -5 0 -5 0 5 10 15 20 0926 •25 -2G 138 -15 -10 5 10 15 20 139 140 -25 -20 -15 -10 -5 •30 -20 -10 -30 -20 -10 141 0 5 0 0 10 15 20 25 142 pa 0.6 04 02 U to ie I-0 2 '04 •06 •oa 143 Table 5- 5 Scour contours at D 50 = 0.50 mm Contour 144 145 146 147 485859^98286 -50 -40 -30 -20 -10 0 10 20 1101 -35 -30 75 -20 -15 -»0 -5 10 «5 70 7 Wt: 3 25 2 15 1 05 0 1102 -0 5 -1 •1 5 -2 •2 5 •3 -3 5 ■4 -20 -15 148 -10 -5 0 5 10 15 20 1103 20‘^ * I f 1i^^H ~ I -40 -30 -20 -10 0 10 20 1104 -50 149 -40 -30 -20 -10 0 10 20 30 40 Table 5- 6 Scour contours a t D 5 0 = 0.47 mm Date Contour I 1107 -to -SO -20 -30 -10 C 1° 1-0 5 -! r5 1108 1-2 5 1-4 5 1-6 1 -6 5 1-6 -30 -20 -10 0 10 20 1109 201915^3094114 -30 -20 -10 150 0 10 1 -6 5 08 06 04 02 -4£-0l6 -02 -04 •0 6 -0 6 •1 151 152 oa 06 04 02 L e - o ie -0 2 -0 4 1-0 6 -0 6 ie*ooi 1E*001 iE * 0 0 t 153 •35 -30 *25 -20 154 -15 -10 -3 0 5 10 15 20 -9 •1E-K301 155