Evaluation of Bridge Scour Research: Pier Scour Processes and Predictions (2011)

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ACKNOWLEDGMENT 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. 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, FTA, Transit Development Corporation, or AOC 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 Research Council, 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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i ACKNOWLEDGMENTS NCHRP Project 24-27(01) was conducted by a Research Team comprising Robert Ettema, George Constantinescu, and Bruce Melville. They were assisted by the following people who formed a consultative Expert Team for the project: • Mr. David Andres, Northwest Hydraulics Consultants, Edmonton, Alberta • Mr. Sterling Jones, formerly with the Federal Highway Administration’s Hydraulics Laboratory, Mclean, Virginia • Dr. Max Sheppard, Ocean Engineering Associates, Gainesville, Florida • Dr. Mutlu Sumer, Technical University of Denmark, Lyngby, Denmark • Dr. Lyle Zevenbergen, Ayres Associates Inc., Fort Collins, Colorado Additionally, Terry Sturm from Georgia Tech University, as the lead principal investigator for the companion project NCHRP 24-27(01), gave useful insight assisting the present project. Michael Kundert, of IIHR Hydroscience and Engineering, prepared the majority of the report’s figures. Suggestions, technical and editorial, were provided by the Panel appointed for this NCHRP Project. The Panel members are listed in the table below. The research team gratefully acknowledges the suggestions provided by the Panel.

ii CONTENTS ACKNOWLEDGMENTS ................................................................................................... i CONTENTS ........................................................................................................................ ii LIST OF TABLES ...............................................................................................................v LIST OF FIGURES ........................................................................................................... vi LIST OF SYMBOLS ...........................................................................................................x EXECUTIVE SUMMARY .................................................................................................1 CHAPTER 1: INTRODUCTION ........................................................................................9 1.1. Introduction .......................................................................................................9 1.2. Motivation .......................................................................................................10 1.3. Objectives .......................................................................................................11 1.4. Key Considerations .........................................................................................11 1.5. Complexities ...................................................................................................13 1.6. Report Organization ........................................................................................14 1.7.Relationship to Other NCHRP Projects ...........................................................15 CHAPTER 2: SCOUR AS DESIGN CONCERN .............................................................19 2.1. Introduction .....................................................................................................19 2.2. Pier Function and Structure ............................................................................19 2.3. Design Depth for Pier Foundation ..................................................................21 2.4. Current U.S. Design Methods for Pier Scour .................................................24 2.5. Need for Structured Design Approach ............................................................25 2.6. Synopsis of Post 1990s Research ....................................................................26 CHAPTER 3: PIER-SCOUR PROCESSES ......................................................................29 3.1. Introduction .....................................................................................................29 3.2. Pier Foundation Material ................................................................................30 3.3. Pier Flow Field ................................................................................................32 3.3.1. Narrow Piers ..................................................................................33 3.3.2. Transition Piers ..............................................................................36 3.3.3. Wide Piers ......................................................................................36 3.4. Erosion of Foundation Material ......................................................................41 CHAPTER 4: PARAMETER FRAMEWORK .................................................................44 4.1. Introduction .....................................................................................................44 4.2. Variables at a Cylindrical Pier in a Single Foundation Stratum .....................45 4.3. Primary and Secondary Parameters ................................................................47

iii 4.4. Parameter Influences .......................................................................................49 4.4.1. Flow-field Scale, y/a ......................................................................49 4.4.2. Relative Coarseness, a/D ...............................................................52 4.4.3. Pier Face Shape, 55 4.4.4. Pier Aspect Ratio, b/a ....................................................................56 4.4.5. Pier Alignment, 57 4.4.6. Flow Intensity, V/Vc .......................................................................58 4.4.7. Sediment Non-uniformity, g ........................................................61 4.4.8. Power of Turbulence Structures, Eu and Re ..................................63 4.4.9. Time Rate of Scour, tV/a ...............................................................66 4.5. Data Quality and Gaps ....................................................................................71 CHAPTER 5: PIER SITE COMPLICATIONS .................................................................72 5.1. Introduction .....................................................................................................72 5.2. Pier Structure ..................................................................................................73 5.3. Abutment Proximity........................................................................................78 5.4. Bridge-Deck Submergence .............................................................................82 5.5. Woody Debris, or Ice, Accumulation .............................................................84 5.6. Channel Morphology ......................................................................................90 5.7. Layered Sediments ..........................................................................................92 5.8. Scour of Cohesive Soil....................................................................................94 5.9. Scour of Weak Rock .......................................................................................95 5.10. Suspended Sediment (Silt and Clay) in Flow ...............................................96 5.10.1. Flow Field Modification ................................................................96 5.10.2. Bed Erosion Resistance ..................................................................98 5.10.3. Conclusion .....................................................................................99 CHAPTER 6: LEADING PREDICTION FORMULAS .................................................100 6.1. Introduction ...................................................................................................100 6.2. Richardson and Davis (2001) Method ..........................................................101 6.3. Sheppard-Melville Method (NCHRP Project 24-32) ...................................103 6.4. Discussion .....................................................................................................104 6.4.1. Reflection of Proven Parameter Relationships ............................105 6.4.2. Capacity to Include Recently Identified Parameter Influences ....107 6.4.3. Limits of Methods ........................................................................108 CHAPTER 7: PROPOSED DESIGN METHODOLOGY ..............................................110 7.1. Introduction ...................................................................................................110 7.2. Structured Design Methodology ...................................................................110 7.3. Uncertainty and Conservatism in Design Estimation ...................................114 7.4. Single-column Piers in the Narrow- and Transition-Pier Categories ...........116 7.5. Common Piers Forms ...................................................................................118 7.6. Common Pier Forms in Complex Situations ................................................118 7.6.1. Abutment proximity .....................................................................119 7.6.2. Woody Debris or Ice ....................................................................119 7.6.3. Bridge Deck Over-topping ..........................................................120

iv 7.6.4. Channel Morphology ...................................................................120 7.6.5. Armoured Boundary Surface, Layered Bed Sediment ................120 7.6.6. Weak Rock ...................................................................................120 7.7. Wide, Complex, or Uncommon Pier Forms .................................................120 7.7.1. Hydraulic Modeling .....................................................................121 7.7.2. Numerical Modeling ....................................................................122 CHAPTER 8: RESEARCH AND EDUCATION ISSUES .............................................126 8.1. Introduction ...................................................................................................126 8.2. Research Needs for Single-Column Piers .....................................................126 8.2.1. Design Issues ...............................................................................127 8.2.2. Scour Processes ............................................................................128 8.3. Research Needs for Complex Pier Forms and Complicating Site Factors ...130 8.3.1. Design Issues ...............................................................................130 8.3.2. Scour Processes ............................................................................131 CHAPTER 9: CONCLUSIONS ......................................................................................138 9.1. Introduction ...................................................................................................138 9.2. Conclusions ...................................................................................................139 REFERENCES ................................................................................................................145 APPENDIX: EVALUATION OF PIER SCOUR METHODS .......................................160 A-1. Introduction ..............................................................................................160 A-2. Evaluation Criteria ...................................................................................160 A-3. Expression of Parameter Influences .........................................................160 A-4. Comparison of Scour Depth Predictions..................................................166 A-5. Discussion ................................................................................................180

v LIST OF TABLES Table 4-1. Classification of local scour processes at bridge piers in terms of y/a (Melville and Coleman 2000); the limits are approximate values beyond which different trends occur ...............................................................................................................50 Table 4-2. Shape factors for uniform piers (Richardson et al. 2001) ................................56 Table 4-3. Comparison of local scour depths for the pier shapes shown in Figure 4-8 (Mostafa 1994) ..........................................................................................................57 Table 7-1. Structured design approach ............................................................................113 Table 8-1. Priority range for research needs (Adapted from NCHRP 24-8 (Parola et al. 1996)) ......................................................................................................................134 Table 8-2. Research topics and priorities for Single-Column Piers Table 8-3. Research topics and priorities for .................................135 considerations complicating scour-depth estimation Table 9-1. Summary of proposed structured design methodology ..................................143 ................................................................................................................136 Table A-1. A chronological listing of pier scour equations .............................................160 Table A-2. Dimensionless parameters included in the selected pier scour equations (The table is divided to indicate methods proposed since 1990) .....................................165 Table A-3. Range of parameter values for Figures A-1 through A-12 ............................167 Table A-4. Some characteristics of the remaining 5 equations .......................................181

vi LIST OF FIGURES Figure 1-1. Three length scales (structure, flow depth, and sediment size (or shear strength when considering laboratory hydraulic models)) prescribe the flow field at a pier. The inherent difficulty of equally scaling the three lengths makes hydraulic modeling intrinsically approximate ..........................................................................16 Figure 1-2. Sketches showing a “long” multi-span bridge with multiple piers and abutments; (a) oblique perspective, and (b) cross-sectional view. In some cases pier-pier and/or pier-abutment interactions may be significant ................................17 Figure 1-3. A sketch showing the foundations of a “short” three-span bridge. Depending on the water level and scour around the foundation, the flow fields in the vicinity of the pier may be significantly different ......................................................................18 Figure 2-1. A simple pier form comprising two circular cylinders ...................................20 Figure 2-2. A common pier structure used for two-lane bridges. The pier comprises a column supported by a pile group with a pile cap ....................................................20 Figure 2-3. A common pier form in a flow situation complicated by debris or ice accumulation .............................................................................................................21 Figure 2-4. Some bridges, such unusually large bridges, or bridges in unusual circumstances, require large piers of uncommon design ..........................................21 Figure 2-5. Scour reduces the effective length of friction-bearing piles ...........................22 Figure 2-6. Scour reduces pier support, causing pier settlement (a)  (b), bottom rotation of pier (a)  (c), or top rotation of pier (a)  (d) ....................................................23 Figure 2-7. Bridge pier settled vertically owing to scour reduction of pier support ..........24 Figure 2-8. Pier tipping owing to scour; (a) forward tipping, (b), backward tipping. These photos raise interesting questions: How does scour develop when a pier rotates as it loses support or gets pushed back by flow pressure? Does pier tipping deepen scour? Also, (a) illustrates pier propensity to collect woody debris ....................................24 Figure 2-9. Overview of structured design approach .........................................................28 Figure 3-1. Sketch showing flow through a bridge site involving complex interactions between the floodplain, the main channel and the piers situated close to the floodplain and main channel, especially during high flow conditions ....................30 Figure 3-2. Differences in scour form at a cylinder; (a) sand bed, (b) clay bed (Briaud et al. 2004), and (c) rock bed (Hopkins and Beckham, 1999). The maximum depth of scour is approximately similar for each material, but the location of deepest scour differs .......32 Figure 3-3. Scour in a weak cohesive material (snow) ......................................................32 Figure 3-4. The main flow features forming the flow field at a narrow pier of circular cylindrical form. Early research focused on flow immediately upstream of the pier (dashed area) .............................................................................................................34 Figure 3-5. Variation of flow field with reducing approach flow depth; narrow to transitional pier of constant pier width. The sketches contain the horseshoe vortex, the bow vortex and the lee-wake vortices. The downflow is represented by the vertical arrow close to the upstream face of the pier ................................................37 Figure 3-6. Main features of the flow field at a wide pier (y/a < 0.2) ...............................38

vii Figure 3-7. Visualization of the main vortices forming the horseshoe vortex system, (HV) system in the mean flow field around a circular pier in a scoured bed. HV1 is the main necklace vortex; HV2 and BAV are secondary necklace vortices; JV is a junction corner vortex (Kirkil et al., 2008) ...............................................................40 Figure 3-8. Numerical simulation showing example flow paths (and fine-sediment paths) around a pier during scour; (a) top view, (b) side view (Kirkil et al., 2008) ............41 Figure 3-9. Distribution of instantaneous bed shear stress around a narrow circular pier with scour hole. Note the high value beneath the leg of the main necklace vortex on the right side of the pier. This streak of vorticity is detaching from the horseshoe vortex and is convected behind the pier parallel to the deformed bed (Kirkil et al., 2008) .........................................................................................................................43 Figure 3-10. Numerical simulation showing distribution of instantaneous bed friction velocity in the flow past a high aspect ratio rectangular cylinder at the start of the scour process (flat bed) .............................................................................................43 Figure 4-1. Variables influencing pier scour at a cylindrical pier ....................................46 Figure 4-2. Influence of y/a on local scour depth expressed as ys Figure 4-3. Influence of sediment coarseness on local scour depth at piers for clear-water scour conditions (Melville and Coleman 2000) ........................................................53 /a (Melville and Coleman 2000) .........................................................................................................................50 Figure 4-4. Influence of sediment coarseness on local scour depth at piers at different flow intensities for live-bed scour conditions (Melville and Coleman 2000) ..........53 Figure 4-5. Local scour depth variation with sediment coarseness (Melville and Coleman 2000) .........................................................................................................................54 Figure 4-6. Influence of sediment size a/D50 on local scour depth ys Figure 4-7. Basic pier shapes .............................................................................................55 /a (Lee and Sturm 2008) .........................................................................................................................55 Figure 4-8. Cylinders differing in cross-sectional shape, but having the same projected width to the flow (Mostafa 1994) .............................................................................56 Figure 4-9. Local scour depth variation with pier alignment (Laursen and Toch 1956) ...58 Figure 4-10. Local scour depth variation with flow intensity, V/Vc Figure 4-11. Influence of flow intensity on local scour depth in uniform sediment (Melville and Coleman 2000) ...................................................................................61 (Melville and Coleman 2000) .........................................................................................................................59 Figure 4-12. Influence of flow intensity on local scour depth in non-uniform sediment (Melville and Coleman 2000) ...................................................................................61 Figure 4-13. Influence of sediment non-uniformity on local scour depth at piers subject to clear water scour (Melville and Coleman 2000) .......................................................62 Figure 4-14. Local scour depth variation with sediment non-uniformity (Melville and Coleman 2000) ..........................................................................................................63 Figure 4-15. Variation of ys Figure 4-16. Time-development of scour at a cylindrical pier subject to clear-water or live-bed conditions ....................................................................................................67 /a with Euler number (Ettema et al. 2006) ............................65 Figure 4-17. Temporal development of local scour at piers, clear-water conditions (Melville and Chiew 1997) .......................................................................................69 Figure 4-18. Equilibrium time-scale variation with flow shallowness, flow intensity and sediment coarseness ..................................................................................................70

viii Figure 5-1. Common pier structures comprise a column on a slab footing, or column on pile cap with underpinning piles (Melville and Coleman 2000) ...............................74 Figure 5-2. Scour depth variation for four cases of non-uniform pier shape (Melville and Coleman, 2000). A fifth case (not shown) is when the pile cap is fully above the water surface ............................................................................................................76 Figure 5-3. Influence of non-uniform shape on local scour depth at piers (Melville and Raudkivi 1996) ..........................................................................................................77 Figure 5-4. The disassembly approach proposed by Jones and Sheppard (2000) for estimating scour depth at a pile-supported pier ........................................................78 Figure 5-5. Abutment proximity close to a pier may substantially alter the flow field and scour at a pier ............................................................................................................79 Figure 5-6. A pier close to the abutment is within the flow field generated by the abutment ...................................................................................................................80 Figure 5-7. Normalized scour depth at pier relative to scour depth at a spill-through abutment, and wing-wall abutment, on an erosion resistant or fixed flood plain (F) or an erodible flood plain (E). The smallest value of Lp /yf Figure 5-8. Bridge beams submerges by flood water ........................................................83 coincides with the toe of the spill-through abutment, at the edge of the fixed floodplain. The error bars indicate relative dune height .....................................................................................82 Figure 5-9. Scour at piers beneath a partially submerged bridge deck (adapted from Guo et al. 2009) ................................................................................................................84 Figure 5-10. Accumulation of woody debris (a), and ice rubble (b) at bridge waterways, affects flow locally at a pier and through the entire bridge waterway ......................86 Figure 5-11. Woody debris accumulation at a single pier (Lagasse et al. 2010) ...............87 Figure 5-12. Rectangular accumulation of debris at a pier (Lagasse et al. 2010) ..............89 Figure 5-13. Local scour-depth variation with quantity of floating debris (Melville and Coleman 2000) ..........................................................................................................90 Figure 5-14. Pier scour and abutment/contraction scour on flood plain. Channel geometry and vegetation substantially affect the approach flow to the pier .............92 Figure 5-15. Local scour in layered sediments (Breusers and Raudkivi, 1991); yc = depth to top of coarse layer, ys Figure 6-1. Normalized equilibrium scour depth, y = depth to top of fine layer ................................................94 s/a*, versus flow intensity ratio, V/Vc, for constant values of y/a* and a* Figure 7-1. Envelope of potential maximum scour depth for clear-water and live-bed scour conditions at piers ..........................................................................................117 /D (Sheppard and Melville) ................................104 Figure A-1. Comparison of normalized local scour depth predictions using 22 different equations/methods for transition from clear-water to live-bed scour conditions. Pier width large compared to the water depth, fine sand ...............................................168 Figure A-2. Comparison of normalized local scour depth predictions using 22 different equations/methods for a particular live-bed scour condition. Pier width large compared to the water depth, fine sand ..................................................................169 Figure A-3. Comparison of normalized local scour depth predictions using 22 different equations/methods for transition from clear-water to live-bed scour conditions. Pier width equal to water depth, fine sand .....................................................................170

ix Figure A-4. Comparison of normalized local scour depth predictions using 22 different equations/methods for a particular live-bed scour condition. Pier width equal to water depth, fine sand .............................................................................................171 Figure A-5. Comparison of normalized local scour depth predictions using 22 different equations/methods for transition from clear-water to live-bed scour conditions. Deep water relative to pier width, fine sand ...........................................................172 Figure A-6. Comparison of normalized local scour depth predictions using 22 different equations/methods for a particular live-bed scour condition. Deep water relative to pier width, fine sand ................................................................................................173 Figure A-7. Comparison of normalized local scour depth predictions using 22 different equations/methods for transition from clear-water to live-bed scour conditions. Pier width large compared to water depth, very coarse sand. ........................................174 Figure A-8. Comparison of normalized local scour depth predictions using 22 different equations/methods for a particular live-bed scour condition. Pier width large relative to water depth, fine sand, very coarse sand ...............................................175 Figure A-9. Comparison of normalized local scour depth predictions using 22 different equations/methods for transition from clear-water to live-bed scour conditions. Pier width equal to water depth, very coarse sand .........................................................176 Figure A-10. Comparison of normalized local scour depth predictions using 22 different equations/methods for a particular live-bed scour condition. Pier width equal to water depth, very coarse sand .................................................................................177 Figure A-5. Comparison of normalized local scour depth predictions using 22 different equations/methods for transition from clear-water to live-bed scour conditions. Deep water relative to pier width, very coarse sand ...............................................178 Figure A-6. Comparison of normalized local scour depth predictions using 22 different equations/methods for a particular live-bed scour condition. Deep water relative to pier width, very coarse sand ....................................................................................179