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NCHRP Web-Only Document 249: Combining Individual Scour Components to Determine Total Scour Terry Sturm Irfan Abid Georgia Institute of Technology Atlanta, GA Bruce Melville Xiaozhou Xiong University of Auckland Auckland, New Zealand Thorsten Stoesser Bruno Fraga Bugallo Ken Vui Chua Cardiff University Cardiff, Wales Steven Abt Colorado State University Ft. Collins, CO Seungho Hong West Virginia University Morgantown, WV Contractorâs Final Report for NCHRP Project 24-37 Submitted September 2017 ACKNOWEDGMENT This work was sponsored by the American Association of State Highway and Transportation Officials (AASHTO), in cooperation with the Federal Highway Administration, and was conducted in the National Cooperative Highway Research Program (NCHRP), which is administered by the Transportation Research Board (TRB) of the National Academies of Sciences, Engineering, and Medicine. COPYRIGHT INFORMATION Authors herein are responsible for the authenticity of their materials and for obtaining written permissions from publishers or persons who own the copyright to any previously published or copyrighted material used herein. Cooperative Research Programs (CRP) grants permission to reproduce material in this publication for classroom and not-for-profit purposes. Permission is given with the understanding that none of the material will be used to imply TRB, AASHTO, FAA, FHWA, FMCSA, FRA, FTA, Office of the Assistant Secretary for Research and Technology, PHMSA, or TDC endorsement of a particular product, method, or practice. It is expected that those reproducing the material in this document for educational and not-for-profit uses will give appropriate acknowledgment of the source of any reprinted or reproduced material. For other uses of the material, request permission from CRP. DISCLAIMER The opinions and conclusions expressed or implied in this report are those of the researchers who performed the research. They are not necessarily those of the Transportation Research Board; the National Academies of Sciences, Engineering, and Medicine; or the program sponsors. The information contained in this document was taken directly from the submission of the author(s). This material has not been edited by TRB.
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iv CONTENTS SUMMARY 1 1. INTRODUCTION 8 1.1 Motivation 8 1.2 Problem Statement 9 1.3 Research Objective 10 1.4 Research Approach 11 1.5 Embankment Failure Conditions 13 1.6 Organization of the Report 14 2. BACKGROUND RESEARCH 15 2.1 Introduction 15 2.2 Abutment Scour 15 2.2.1 Abutment Scour Around Solid Abutment 16 2.2.2 Abutment Scour Around Erodible Abutment 22 2.3 Lateral Contraction Scour 26 2.4 Vertical Contraction Scour (Pressure Scour) 29 2.5 Pier Scour 33 2.6 Field Studies 36 2.7 Physical Model Studies 42 2.8 Scour Component Interaction 47 2.9 Computational Studies 48 2.10 Summary 52 3. RESEARCH METHODOLOGY 54 3.1 Introduction 54 3.2 Experiments at Georgia Tech 55 3.2.1 Experimental Setup 55 3.2.2 Experimental Procedure 58 3.2.3 Scour Interaction Categories 61 3.3 Experiments at University of Auckland 64 3.3.1 Experimental Setup 64
v 3.3.2 Experimental Procedure 67 3.4 Computer Modeling at Cardiff University 71 3.4.1 Large-Eddy Simulation 71 3.4.2 Three-Dimensional Reynolds-Averaged Navier-Stokes Equations 74 3.4.3 Two-Dimensional Reynolds-Averaged Navier-Stokes Equations 75 3.4.4 Numerical Setups and Boundary Conditions 75 3.4.5 Pre-Cursor Simulations 78 4. RESEARCH FINDINGS 81 4.1 Introduction 81 4.2 Clear-Water Scour Experimental Results 81 4.2.1 Summary of Results 81 4.2.2 Water Surface Profiles 86 4.2.3 Flow Fields and Clear-Water Scour 87 4.2.4 Time Development of Scour 96 4.2.5 Organization of Scour Prediction Methodology 99 4.2.6 Prediction of Pier Scour Alone 101 4.2.7 Prediction of Vertical Contraction Scour Alone 103 4.2.8 Category I Scour Interactions 104 4.2.9 Category II Scour Interactions 113 4.2.10 Category III Scour Interactions 120 4.2.10.1 Effect of Pier on Abutment/Contraction Scour (Category III) 123 4.2.10.2 Effect of Abutment/Contraction Scour on Pier Scour (Category III) 125 4.2.11 Category IV Scour Interactions 130 4.2.10 Summary of Clear-Water Scour Analysis 132 4.3 Live-Bed Scour Experimental Results 135 4.3.1 Summary of Results 135 4.3.2 Water Surface Profiles 136 4.3.3 Flow Fields and Live-Bed Scour 137 4.3.4 Time Development of Scour 142
vi 4.3.5 LBS Prediction for SSA/BLA (Category II) 143 4.4 CFD Model Results 149 4.4.1 Validation of the Large-Eddy Simulations 149 4.4.2 Time-Averaged Flow 160 4.4.3 Instantaneous Flow and Turbulence 167 4.4.4 Effect of Scoured Bathymetry on Flow and Turbulence 184 4.4.5 Three-Dimensional RANS Simulations 187 4.4.5.1 Validation of the Three-Dimensional RANS Simulations 187 4.4.5.2 Effect of Abutment Length on Hydrodynamics and Potential Scour 188 4.4.6 Two-Dimensional or Depth-Averaged RANS 192 4.4.6.1 Validation of the RANS Simulations 192 4.4.6.2 Effect of Bridge Opening on Hydrodynamics and Scour Predictions 194 4.4.6.3 Application of the Two-Dimensional RANS Model to Prototype Geometry 197 4.4.7 CFD Summary 202 5. METHODOLOGY FOR SCOUR DEPTH PREDICTION 203 5.1 Introduction 203 5.2 Methodology 203 5.3 Flow Chart 206 5.4 Combined Scour Example Problem 211 5.4.1 Problem Statement 211 5.4.2 Water Surface Profiles from HEC-RAS 214 5.4.3 Scour Calculations for Free Flow 215 5.4.3.1 Scour Calculations for Left Abutment, Pier #1 and Pier #2 218 5.4.3.2 Scour Calculations for Right Abutment, Pier #3 and Pier #4 220 5.4.3.3 Comparison with HEC-RAS Calculations Based on HEC-18 222 5.4.4 Scour Calculations for Submerged Orifice and Overtopping Flow 226 5.4.4.1 Scour Calculations for Left and Right Abutments (Q2) 227 5.4.4.2 Scour Calculations for Left and Right Abutments (Q3) 228
vii 5.4.5 Application Considerations 230 6. CONCLUSIONS AND RECOMMENDATIONS 232 6.1 Overview 232 6.2 Conclusions 233 6.3 Recommendations 236 7. REFERENCES 238 8. LIST OF SYMBOLS AND ABBREVIATIONS 249 9. APPENDIX A. Bed Elevation Contours and Photographs at Equilibrium for Clear-Water Scour Experiments at Georgia Tech A-1 10. APPENDIX B. Bed Elevation Contours and Photographs at Equilibrium for Clear-Water Scour Experiments at University of Auckland B-1
viii LIST OF FIGURES Figure 1-1. Photos of different types of bridge scour. 8 Figure 1-2. Classes of bridge flow. 10 Figure 1-3. Bridge scour components and their interactions. 11 Figure 2-1. Definition sketch for abutment scour in a compound channel 16 Figure 2-2. Typical cases of abutment positions in compound channels (reproduced from Melville and Coleman (2000)). 20 Figure 2-3. Abutment scour conditions: Scour Condition A - bank failure and failure of the abutment face, Scour Condition B - failure of the abutment face, and Scour Condition C - breaching of the approach embankment (reproduced from Ettema et al. 2010) 23 Figure 2-4. Definition sketch for idealized long contraction scour ( 1Q = cQ = main channel flow rate at approach flow section; 2Q = tQ = total flowrate in channel at contracted section; B1 = Bm1 = approach-flow main channel width; B2 = Bm2 = contraction main channel width; scd = contraction scour depth). Reproduced from Sturm et al. (2011). 27 Figure 2-5. Definition diagram for pressure-flow scour at bridges 30 Figure 2-6. Effect of flow intensity on pressure-flow scour at bridges 32 Figure 2-7. Available data for pressure-flow scour at bridges 33 Figure 2-8. Aerial photograph at Houfeng Bridge in 2007; flow right to left (Hong et al. 2012) 37 Figure 2-9. Aerial photograph of Big Sioux River Bridge at Flandreau, South Dakota (Larsen et al. 2011) 39 Figure 2-10. Aerial photograph of James River bridge near Mitchell, South Dakota (Rossell and Ting 2013) 40 Figure 2-11. Minnesota River near Belle Plaine. (Wagner et al. 2006) 42 Figure 2-12. Prototype Ocmulgee R. Bridge 43 Figure 2-13. Ocmulgee R. Bridge model (1:45) 43 Figure 2-14. Physical model scour results compared with field data for Ocmulgee R. model. 44
ix Figure 2-15. Contraction scour depths for Ocmulgee R. model (ym2a = YMAX = flow depth at location of maximum scour depth in main channel; ym1 = Ym1 = initial flow depth in the approach flow main channel; Vf1 and Vm1 = approach flow floodplain and main channel flow velocities, respectively; Vm1 = approach flow velocity in main channel; Vmc1 = critical velocity in the approach flow main channel. 45 Figure 2-16. Laboratory model of Towaliga River Bridge (Hong and Sturm, 2010) 46 Figure 2-17. Comparison of measured field and laboratory scour cross sections (C.S.) from submerged orifice flow (Q = 1048 m3/s) for Tropical Storm Alberto in July 1994. Initial C.S. and bridge C.S. after experiment are laboratory measurements (Hong & Sturm 2009). 46 Figure 2-18. Dependence of width-averaged TKE ( bK ) across scour hole on 12 / qq . (Hong et al. 2015). 48 Figure 2-19. Simulated water surface (top), measurement locations (bottom left) and close-up photograph of the laboratory experiment (bottom right). 51 Figure 2-20. Longitudinal water surface profiles along two locations, which are at the channel centerline (Profile A) and at one-third of the channel width (Profile B) at the abutment face. 51 Figure 2-21. Streamlines of the time-averaged flow over a submerged bridge. a) oblique view from behind and b) in a horizontal plane near the bed 52 Figure 3-1. Modified Towaliga River model 55 Figure 3-2. (a) Model bridge cross-section including embankment and bridge deck; (b) Model cross-section (Sec. A-A) of embankment and bridge deck; (c) Plan view of flume test section with LSA on left floodplain and BLA on right floodplain. All 1:45 scale model dimensions in ft. 56 Figure 3-3. Top and side views of model bridge deck (dimensions in inches at 1:45 model scale). 57 Figure 3-4. Schematic of 5-ft wide Auckland LBS flume: (a) profile and dimensions; and (b) compound channel cross section 65 Figure 3-5. Schematic of compound channel cross section in 8-ft wide Auckland CWS flume. 66 Figure 3-6. (a) Roughening of floodplain in 5-ft wide LBS flume; (b) overhead view of upstream bank protection, embankment riprap layer and riprap apron (flow from left to right). 66 Figure 3-7 Computational domain of the numerical simulations 76
x Figure 3-8: Domain of the compound channel of the pre-cursor simulations in plan view 79 Figure 3-9: Contours of the instantaneous (top) and time-averaged streamwise velocity in the cross-section of the compound channel 80 Figure 3-10: Measured (dots) and simulated (lines) profiles of the streamwise velocity of the approach flow 80 Figure 4-1. Water surface profiles for CWS in Georgia Tech flume for free, submerged orifice, and overtopping flows: (a) La/Bf = 0.41; (b) La/Bf = 0.77. 87 Figure 4-2. Flow acceleration from approach flow section to downstream face of bridge with La/Bf =0.41 and 0.77. 88 Figure 4-3. Distribution of discharge per unit width between floodplain and main channel as affected by relative approach flow depth for two different compound channel geometries. 89 Figure 4-4. Distribution of flow rate per unit width in approach flow and bridge sections. 91 Figure 4-5. Velocity vectors located 0.20 in. (5 mm) above the bed for La/Bf = 0.41. 92 Figure 4-6. Cross-section distributions of flow properties and scour for La/Bf = 0.41 and 0.77. 94 Figure 4-7. Bed elevation contours after equilibrium scour for La/Bf = 0.41 and 0.77. 96 Figure 4-8. Time development of maximum depth of scour in linear and log coordinates for La/Bf = 0.41 and 0.77 in F, SO, and OT flows. 98 Figure 4-9. Path of maximum scour depth with time for La/Bf = 0.41 and 0.77 in F, SO, and OT flows. 98 Figure 4-10. Distinguishing LSA from SSA for scour prediction. (a) Definition sketch; (b) Comparison of data for maximum scour location with HEC-18 criterion for LSA. (Scour depths for circled data points were plotted as SSA in determining scour depth prediction formulas.) 100 Figure 4-11. Proposed criterion for classifying abutment lengths as SSA or LSA for purposes of scour depth prediction (CWS). 101 Figure 4-12. Comparison of measured and calculated dimensionless pier scour depth for CWS results at Georgia Tech. (ds = pier scour depth; a = pier width). 102 Figure 4-13. Comparison of vertical contraction scour data from this study with data from other sources. 103 Figure 4-14. Amplification factor for maximum depth of CWS around LSA in the form suggested by Ettema et al. (2010). 106
xi Figure 4-15. Predicted maximum flow depth of scour hole for LSA, Category I scour 109 Figure 4-16. Predicted maximum CWS flow depth of scour hole for LSA, wingwall abutment, Category I scour (Theoretical contraction scour (C.S.) shown for reference). 110 Figure 4-17. Comparison of field data with predicted maximum flow depth of scour hole for LSA, Category I scour. 111 Figure 4-18. Comparison of measured and predicted clear water, interactive abutment and contraction scour by HEC-18 procedure and suggested Category I Model (dashed lines represent ±10% of perfect agreement). 113 Figure 4-19. Predicted maximum CWS flow depth of scour hole for BLA, Category II scour. (Theoretical contraction scour shown for reference). 115 Figure 4-20. Comparison of BLA scour prediction equation with SSA data, Category II scour. (Theoretical contraction scour shown for reference). 116 Figure 4-21. Comparison of BLA scour prediction equation with wingwall (WW) data, Category II scour. (Theoretical contraction scour shown for reference). 116 Figure 4-22. Comparison of Eq. (4-10) with field data for Category II scour. 118 Figure 4-23. Comparison of measured and predicted maximum CWS depth for Category II scour with predictions from HEC-18 additive methodology. 119 Figure 4-24. Comparison of measured and predicted maximum CWS depths for Category II scour with predictions from proposed Eq. (4-10). 119 Figure 4-25. Illustration of pier interaction with abutment/lateral contraction scour for free flow, La/Bf = 0.41. 121 Figure 4-26. Effect of pier location on abutment/contraction scour and riprap movement for SO flow, La/Bf = 0.41, Vf1/Vfc =0.589, Vm1/Vmc =0.725 122 Figure 4-27. Category III abutment/contraction scour: Ratio of maximum abutment/contraction scour depth with pier to that without pier for rectangular column piers and wall piers (Yf2maxo from Eq. (4-8) for Category I scour). 124 Figure 4-28. Performance of Eq. (4-8) developed for Category I scour in predicting Category III scour at an abutment in the presence of a pier. 125 Figure 4-29 Ratio of maximum pier scour depth near an abutment to that for an isolated pier (dsmaxo) as a function of distance of pier from abutment. 126 Figure 4-30. Excess Category III pier scour for rectangular column piers in the presence of an abutment. 128 Figure 4-31. Excess Category III pier scour for wall piers in the presence of an abutment. 128
xii Figure 4-32. Measured vs. predicted Category III pier scour in the presence of an abutment using HEC-18 procedure (abutment scour depth + contraction scour depth + pier scour depth). 129 Figure 4-33. Measured vs. predicted Category III pier scour in the presence of an abutment using excess pier scour equations (Eq. 4-12 and 4-13). 130 Figure 4-34. Bed elevation contours for (a) vertical contraction scour alone, and (b) combined vertical contraction and pier scour (Category IV). 131 Figure 4-35. Comparison of Category IV measured and predicted interactive pier and vertical contraction scour 132 Figure 4-36 Water surface profiles for LBS in UOA 5-ft flume for free, submerged orifice, and overtopping flows: (a) La/Bf = 0.80; (b) La/Bf = 0.50. 137 Figure 4-37. Flow acceleration from approach flow section to downstream face of bridge with La/Bf =0.80 and 0.50. 138 Figure 4-38. Distribution of flow rate per unit width in approach flow and bridge sections. 139 Figure 4-39. Cross-section distributions of flow properties and scour for La/Bf = 0.80 and 0.50. Note: Some Kb/u*c2 values are not available for Run 5 LBS UOA due to shallow flows on FP. 140 Figure 4-40. Bed elevation contours after equilibrium scour for La/Bf = 0.80 and 0.50. 142 Figure 4-41. Time development of maximum depth of scour in linear and log coordinates for La/Bf = 0.80 in SO and OT flows under two different approach flow velocities. 143 Figure 4-42. Envelope curves of Ettema et al. (2010) compared with LBS data. 144 Figure 4-43. Illustration of Qot/Q>Qobst/Q as a criterion for determining the condition qm2/qm1<1. 145 Figure 4-44. Comparison of LBS scour data for SSA with CWS relationship (Eq. (4-10)) for BLA/SSA (Category II). 146 Figure 4-45. Comparison of both LBS scour data and CWS data with CWS relationship (Eq. (4- 10)) for BLA/SSA (Category II). 147 Figure 4-46. Comparison of measured Category II LBS data (SSA) with predictions using HEC- 18 additive procedure (Error limits of ±20% relative to line of perfect agreement line are shown). 148 Figure 4-47. Comparison of measured Category II LBS data (SSA) with predictions using Eq. (4- 10). (Error limits of ±20% relative to line of perfect agreement are shown). 148
xiii Figure 4-48. Schematic top view of the simulation domain, with cross sections highlighted where experimental measurements were taken. 150 Figure 4-49. Schematic cross-section view of the domain with y coordinate values across the top 150 Figure 4-50. Profiles of the normalized streamwise time-averaged velocity for Run 1 (free flow long setback abutment) at all five cross sections. 151 Figure 4-51. Profiles of the normalized water surface elevation for Run 1 at all five cross sections. 152 Figure 4-52. Profiles of the normalized streamwise time-averaged velocity for Run 10 (free flow short setback abutment) at three cross sections. 153 Figure 4-53. Profiles of the normalized water surface elevation for Run 10 at all five cross sections 154 Figure 4-54. Profiles of the normalized streamwise time-averaged velocity for Run 2 (submerged orifice flow and long setback abutment) at all five cross sections. 155 Figure 4-55. Profiles of the normalized water surface elevations for Run 2 at all five cross sections. 156 Figure 4-56. Profiles of the normalized streamwise time-averaged velocity for Run 11 (submerged orifice flow and short setback abutment) at two cross sections. 157 Figure 4-57. Profiles of the normalized water surface elevation for Run 11 at two cross sections 157 Figure 4-58. Profiles of the normalized streamwise time-averaged velocity for Run 3 (overtopping flow and long setback abutment) at three cross sections 158 Figure 4-59. Profiles of the normalized water surface elevation for Run 3 at three cross sections. 159 Figure 4-60. Profiles of the normalized streamwise time-averaged velocity for Run 12 (overtopping flow, short setback abutment) at three cross sections. 159 Figure 4-61. Profiles of the normalized water surface elevation for Run 12 at three cross sections. 160 Figure 4-62. Streamlines of the mean flow of Run 1 (free flow, long setback abutment) color- coded with the normalized time-averaged mean streamwise velocity (U/Ubulk). 161 Figure 4-63. Streamlines of the mean flow of Run 10 (free flow, short setback abutment) color- coded with the normalized time-averaged mean streamwise velocity (U/Ubulk). 162
xiv Figure 4-64. Streamlines of the mean flow of Run 2 (submerged orifice, long setback abutment) color-coded with the normalized time-averaged mean streamwise velocity (U/Ubulk). 163 Figure 4-65. Streamlines of the mean flow of Run 11 (submerged orifice, short setback abutment) color-coded with normalized time-averaged mean streamwise velocity (U/Ubulk). 164 Figure 4-66. Streamlines of the mean flow of Run 2 (overtopping, long setback abutment) color- coded with the normalized time-averaged mean streamwise velocity (U/Ubulk). 165 Figure 4-67. Streamlines of the mean flow of Run 12 (overtopping, short setback abutment) color-coded with the normalized time-averaged mean streamwise velocity (U/Ubulk). 166 Figure 4-68. Free surface of the instantaneous flow of Run 1 (free flow, long setback abutment) color-coded with the water surface elevation. Dark blue represents areas of lower water surface. 167 Figure 4-69. Coherent turbulence visualised by isosurfaces of the Q-criterion color-coded with the normalized time-averaged streamwise velocity (U/Ubulk). 168 Figure 4-70: Contour lines of normalized, near-bed turbulent kinetic (TKE) overlaid onto contours of the scoured experimentally observed bathymetry (a) and vice versa (b) for the free flow, long setback abutment run. 170 Figure 4-71. Free surface of the instantaneous flow of Run 10 (free flow, short setback abutment) color-coded with the water surface elevation. Dark blue represents areas of lower water surface. 171 Figure 4-72. Coherent turbulence visualised by isosurfaces of the Q-criterion color-coded with the normalized time-averaged streamwise velocity (U/Ubulk) 172 Figure 4-73: Contour lines of normalized, near-bed turbulent kinetic (TKE) overlaid onto contours of the scoured experimentally observed bathymetry (a) and vice versa (b) for the free flow, short setback abutment run. 173 Figure 4-74. Free surface of the instantaneous flow of Run 11 (submerged orifice flow, short setback abutment) color-coded with the water surface elevation. Dark blue represents areas of lower water surface. 174 Figure 4-75. Coherent turbulence visualised by isosurfaces of the Q-criterion color-coded with the normalized time-averaged streamwise velocity (U/Ubulk) for the submerged orifice flow with a long setback abutment. 175
xv Figure 4-76. Contour lines of normalized, near-bed turbulent kinetic (TKE) overlaid onto contours of the scoured experimentally observed bathymetry (a) and vice versa (b) for the submerged orifice flow, long setback abutment run. 176 Figure 4-77. Coherent turbulence visualised by isosurfaces of the Q-criterion color-coded with the normalized time-averaged streamwise velocity (U/Ubulk) for the submerged orifice flow with a short setback abutment. 177 Figure 4-78. Contour lines of normalized, near-bed turbulent kinetic (TKE) overlaid onto contours of the scoured experimentally observed bathymetry (a) and vice versa (b) for the submerged orifice flow, short setback abutment run. 178 Figure 4-79. Free surface of the instantaneous flow of Run 12 (overtopping flow, short setback abutment) color-coded with the water surface elevation. Dark blue represents areas of lower water surface. 179 Figure 4-80. Coherent turbulence visualised by isosurfaces of the Q-criterion color-coded with the normalized time-averaged streamwise velocity (U/Ubulk) for the overtopping flow with a long setback abutment. 180 Figure 4-81. Contour lines of normalized, near-bed turbulent kinetic (TKE) overlaid onto contours of the scoured experimentally observed bathymetry (a) and vice versa (b) for the overtopping flow long setback abutment run. 182 Figure 4-82. Coherent turbulence visualised by isosurfaces of the Q-criterion color-coded with the normalized time-averaged streamwise velocity (U/Ubulk) for the overtopping flow with a long setback abutment. 183 Figure 4-83. Contour lines of normalized, near-bed turbulent kinetic (TKE) overlaid onto contours of the scoured experimentally observed bathymetry (a) and vice versa (b) for the overtopping flow short setback abutment run. 184 Figure 4-84. Contour lines of near-bed turbulent kinetic (TKE) at various elevations overlaid onto the experimentally observed bathymetry. 185 Figure 4-85. Streamlines of the near-bed flow through the scour hole. 186 Figure 4-86. Contours of the streamwise velocity together with streamlines (top row) and contours of the normalized TKE in a near bed plane without (left column) and with (right column) scour hole on the floodplain (Runs 1 and 18). 187 Figure 4-87. RANS-simulated and measured profiles of the normalized streamwise time- averaged velocity for Run 1 at three cross sections. 188
xvi Figure 4-88. (left) Depth-averaged streamwise velocity field and streamlines predicted by the 3D RANS simulations for the long (LSA), medium (MSA), short (SSA) and bank setbacks (BLA); (right) normalized scour predictors (q2/q1, V2(R)/u*c, and Kb/u*c2) at the down_toe. 190 Figure 4-89. Profiles of measured and 3D RANS -predicted scour predictors (q2/q1, V2(R)/u*c, and Kb/u*c2) at the down_toe location. 191 Figure 4-90. Profiles of the depth-averaged normalized streamwise velocity as measured (open circles) and predicted by LES (solid line) and 2D RANS (dashed line) for the free flow, short setback abutment at all five cross sections. 192 Figure 4-91. Profiles of the depth-averaged normalized lateral velocity as measured (open circles) and predicted by LES (solid line) and 2D RANS (dashed line) for the free flow, short setback abutment at all five cross sections. 192 Figure 4-92. (left) Depth-averaged streamwise velocity field and streamlines predicted by the 2D RANS simulations for the long (LSA), medium (MSA), short (SSA) and bankline (BLA); (right) normalized scour predictors (q2/q1, V2(R)/u*c, and Kb/u*c2) at the down_toe. 195 Figure 4-93. Profiles of measured and 2D RANS -predicted scour predictors (q2/q1, V2(R)/u*c, and Kb/u*c2) at the down_toe location. 196 Figure 4-94. Predicted maximum normalized depth of scour hole for Category I scour as measured in the laboratory (squares) and as predicted by the 2D RANS obtained predictor variables (grey symbols) together with proposed Category I Model curve. 197 Figure 4-95. Predicted maximum normalized depth of scour hole for Category II scour as measured in the laboratory (squares) and as predicted by the 2D RANS obtained predictor variables (grey symbols) together with proposed Category II model curve. 197 Figure 4-96. Bathymetry of the Ocmulgee River Bridge at Macon, GA 198 Figure 4-97. Mesh and Location of the cross sections of interest and the unstructured mesh used during the 2D RANS simulations. 199 Figure 4-98 Velocity vectors around the bridge piers. 200 Figure 4-99. Comparison of SRH-2D velocity distribution at C.S.-4 with physical model and prototype data 201 Figure 4-100. Comparison of HEC-RAS velocity distribution at C.S.-4 with physical model and prototype data 201 Figure 5-1. Definition sketch for abutment scour in a compound channel. 206
xvii Figure 5-2. Flow chart demonstrating the methodology. 209 Figure 5-3. The upper abutment is Category I (abutment/lateral contraction scour for long setback abutment); the lower abutment is Category II (abutment/lateral contraction scour for short setback abutment), Run 1. 210 Figure 5-4. (a) The pier scour is Category IIIb (pier scour is affected by the abutment/ contraction scour with Lp/Yf1 = 1.8, Run 7 (OT flow)); (b) Category IIIa (pier scour is affected by the abutment/contraction scour with Lp/Yf1 = 3.7, Run 9 (OT flow)). 210 Figure 5-5. (a) Category IVa (pier scour alone, Run 43); (b) Category IVb (pier scour plus vertical contraction scour, Run 37). 211 Figure 5-6. Upstream and downstream bridge cross sections from HEC-RAS. 212 Figure 5-7. Cross-section layout along main stem of Flat Creek for HEC-RAS analysis. 213 Figure 5-8. Water surface profiles for design flows of Q1, Q2, and Q3. 214 Figure 5-9. Comparison of minimum scour elevations for NCHRP 24-37 and HEC-18 methods 223 Figure 5-10. HEC-RAS scour output with depiction of predicted scour holes using HEC-18 methods. 224
xviii LIST OF TABLES Table 2-1. Abutment shape factors for Melville scour formula 19 Table 2-2. Summary of published field data 37 Table 3-1. List of CWS experimental runs at Georgia Tech. (F = free flow, SO = submerged orifice flow, OT =overtopping flow, Qot = overtopping discharge, Q =total discharge, T.W. = tailwater, Vfc1 = critical approach flow velocity on the floodplain, Vmc1 = critical approach velocity in the main channel, A = local abutment scour; P = pier scour; L = lateral contraction scour and V = vertical contraction scour). See Figure 2-1 for definition of all other variables. 60 Table 3-2. Experimental runs organized by combinations of scour components 63 Table 3-3. Experimental conditions for LBS experiments in the UOA 5-ft wide flume (Bf/Bm = 2.3) 69 Table 3-4. Experimental conditions for CWS experiments in the UOA 8-ft wide flume (Bf/Bm = 2.0) 70 Table 3-5: Details of the numerical grids used for the various models 77 Table 3-6: Resolution of the Immersed Boundaries 78 Table 3-7: Grid resolution details of the pre-cursor simulations 79 Table 4-1. Experimental maximum abutment scour depth results for CWS and LSA. (GT = Georgia Tech, UOA = University of Auckland, F = free flow, SO = submerged orifice flow, OT = overtopping flow, LSA = long setback abutment, WW = wingwall abutment,) 82 Table 4-2. Experimental maximum abutment scour depth results for CWS around BLA. (GT = Georgia Tech, BLA = bankline abutment, WW = wingwall abutment). 84 Table 4-3. Experimental maximum scour depth results for CWS around SSA. (GT = Georgia Tech, UOA = University of Auckland, F = free flow, SO = submerged orifice flow, OT = overtopping flow, SSA = short setback abutment, WW = wingwall abutment). 85 Table 4-4. Experimental maximum scour depth results for CWS with piers in place. (GT = Georgia Tech, F = free flow, SO = submerged orifice flow, OT = overtopping flow, Lp = distance from abutment toe to pier, W = wall pier.) 86 Table 4-5. Succession of regression models applied to the full CWS, LSA data set (Category I). 108
xix Table 4-6. Succession of regression models applied to the full CWS, BLA data set (Category II). 114 Table 4-7. Summary of proposed combined scour equations and prediction errors. 133 Table 4-8. Experimental conditions for LBS experiments in UoA 5-ft flume (Bf/Bm = 2.35) 135 Table 4-9. Predicted scour depths for Category I and Category II scour and 2D RANS model calculated predictor variables for various abutment lengths. 196 Table 5-1. Summary of proposed combined scour equations. 204 Table 5-2. Water surface profile output data for three discharges (Profile Summary Table). 215 Table 5-3. Detailed cross-section output at approach flow section (RS 1500) for PF1 (Q1) 216 Table 5-4. Detailed cross-section output at bridge (BR D) for PF1 (Q1). 216 Table 5-5. Detailed cross-section output at RS 1185 just downstreamof bridge for PF1 (Q1). 217 Table 5-6. Variables for scour calculations for Q1, free flow. (LOB = left overbank, MC = main channel). 217 Table 5-7. Bridge Scour; River=Flat Creek; Reach= Main Stem; RS = 1225 BR (HEC-RAS) 225 Table 5-8. Variables for scour calculations for Q2, submerged orifice flow, and Q3, overtopping flow, in left overbank. 227 Table 5-9. Ranges of variables in laboratory experiments. 230
xx AUTHOR ACKNOWLEDGMENTS The research reported herein was performed under NCHRP Project 24-37 by the School of Civil and Environmental Engineering at the Georgia Institute of Technology (GT). GT was the primary contractor for this study and subcontracts were issued to University of Auckland, Cardiff University, and Colorado State University. The project was administered fiscally by Grants and Contracts and managed by the Contracting Officer in the Office of Sponsored Programs at GT. Dr. Terry W. Sturm, P.E., Professor of Civil and Environmental Engineering at GT was the Project director and co-Principal Investigator. Other co-principal investigators were Dr. Bruce Melville of the University of Auckland, Dr. Thorsten Stoesser of Cardiff University, and Dr. Steven Abt of Colorado State University who are co-authors of the report. The other authors of the report are Dr. Seungho Hong, formerly post-doctoral research associate at GT and now Assistant Professor at West Virginia University, Irfan Abid, Ph.D. candidate at GT, Xiaozhou Xiong, Ph.D. candidate at the University of Auckland, Bruno Fraga Bugallo, post-doctoral research associate at Cardiff University, and Ken Vui Chua, Ph.D. candidate at Cardiff University. The work was done under the general supervision of Professor Sturm at GT.
xxi ABSTRACT This report presents and documents the results of a study of how different components of bridge scour should be combined to obtain an improved estimate of total scour depth for bridge foundation design. The scour components are abutment scour, lateral contraction scour, pressure scour (vertical contraction scour), and pier scour. In current design practice, separate estimates of individual scour components are added as a conservative approach to estimating maximum scour depths. This method discounts the concurrent interactions among scour components that may result in more realistic scour depth estimates that are smaller than for the conservative approach of simple addition of components. The research approach consisted of experiments conducted at the Georgia Institute of Technology and the University of Auckland, and numerical simulations of the flow field executed at Cardiff University. Both live-bed and clear-water scour experiments were performed in large compound channels with movable sediment beds. Embankment/abutment lengths were varied from approximately 40% of the floodplain width to full blockage of the floodplain. Flow types of free flow through the bridge, submerged orifice flow, and bridge overtopping flow were investigated. Four categories of scour component interaction were identified, and a formula was proposed for estimating the maximum scour depth in each case based on the experimental results and validation with a few available field measurements. Numerical model results validated the physics of the scour component interaction and guided the development of the equations as well as the method for calculating scour prediction parameters needed in the scour prediction formulas. The findings indicate that current methods overestimate maximum scour depth from 20% to 45% at the 95% level of confidence, while approximately 80% of maximum scour depth estimates using equations developed in this study fell within ±10% of the line of perfect agreement.
