Determining Scour Depth Around Structures in Gravel-Bed Rivers (2023)

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2023 N A T I O N A L C O O P E R A T I V E H I G H W A Y R E S E A R C H P R O G R A M NCHRP RESEARCH REPORT 1031 Determining Scour Depth Around Structures in Gravel-Bed Rivers Elowyn Yager Angel Monsalve Jaeho Shim Daniele Tonina Caitlin Keady University of Idaho Boise, ID Carter Borden Sarah Borden Centered Consulting International, LLC Boise, ID Subscriber Categories Bridges and Other Structures • Hydraulics and Hydrology • Maintenance and Preservation Research sponsored by the American Association of State Highway and Transportation Ofcials in cooperation with the Federal Highway Administration

NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM Systematic, well-designed, and implementable research is the most effective way to solve many problems facing state departments of transportation (DOTs) administrators and engineers. Often, highway problems are of local or regional interest and can best be studied by state DOTs individually or in cooperation with their state universities and others. However, the accelerating growth of highway transporta- tion results in increasingly complex problems of wide interest to high- way authorities. These problems are best studied through a coordinated program of cooperative research. Recognizing this need, the leadership of the American Association of State Highway and Transportation Officials (AASHTO) in 1962 ini- tiated an objective national highway research program using modern scientific techniques—the National Cooperative Highway Research Program (NCHRP). 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C O O P E R A T I V E R E S E A R C H P R O G R A M S AUTHOR ACKNOWLEDGMENTS The research reported herein was performed under NCHRP Project 24-48 in the Civil and Environmental Engineering Department and the Center for Ecohydraulics Research at the University of Idaho (UI). Authors of the report are Dr. Elowyn Yager (P.G., Professor at UI), Dr. Angel Monsalve (Postdoctoral Scholar at UI), Dr. Jaeho Shim (Postdoctoral Scholar at UI), Dr. Daniele Tonina (P.E., Professor at UI), Caitlin Keady (Research Assistant and M.S. student at UI), Dr. Carter Borden (Centered Consulting International, LLC), and Sarah Borden (Centered Consulting International). The work was done under the general super- vision of Professor Yager, with Professors Yager and Tonina being Principal Investigators on the original proposed work. CRP STAFF FOR NCHRP RESEARCH REPORT 1031 Christopher J. Hedges, Director, Cooperative Research Programs Waseem Dekelbab, Deputy Director, Cooperative Research Programs, and Manager, National Cooperative Highway Research Program Camille Crichton-Sumners, Senior Program Officer Mazen Alsharif, Senior Program Assistant Natalie Barnes, Director of Publications Heather DiAngelis, Associate Director of Publications Doug English, Senior Editor NCHRP PROJECT 24-48 PANEL Field of Soil and Geology—Area of Foundations and Scour Charles S. Hebson, Maine Department of Transportation, Augusta, ME (Chair) Jon E. Bischoff, Utah Department of Transportation, Salt Lake City, UT Celso Castro-Bollinaga, North Carolina State University, Raleigh, NC Shunyi Christopher Chen, North Carolina Department of Transportation, Raleigh, NC Joanna Crowe Curran, Herrera Environmental Consultants, Seattle, WA Donald F. Hayes, U.S. Army Engineer Research and Development Center, Meridian, MS Terrence McCleary, McCleary Engineering (deceased) Michael J. Orth, Kansas Department of Transportation, Topeka, KS Kornel D. Kerenyi, FHWA Liaison Christophe Chevalier, French Institute of Science and Technology for Transport, Development, and Networks (IFSTTAR) Liaison Franziska Schmidt, French Institute of Science and Technology for Transport, Development, and Networks (IFSTTAR) Liaison

NCHRP Research Report 1031: Determining Scour Depth Around Structures in Gravel-Bed Rivers provides state departments of transportation (DOTs) with new equations for pre- dicting bridge pier scour depths for live bed and clear water scour in coarse-bedded rivers. This report, the Excel Scour Prediction Tool, and laboratory examples provide practical tools for predicting scour depths and prioritizing scour mitigation measures. The erosive action of flowing water that removes boundary material from channel beds or banks around bridge foundations is known as scour. In gravel-bed rivers, the inter- action of bridge piers and large heterogeneous gravel particles with the approaching flow can generate coherent turbulent structures in the flow. In addition to increasing the shear stress applied by the flow onto the bed, these structures create a highly variable bed shear stress field that affects gravel-bed mobility, which potentially compromises bridge foun- dation integrity. Formulas used to predict scour depths around bridge foundations have been developed for sand-bed rivers with near uniform sand-sized sediment. Heterogeneity of bed material and variable near-pier shear stresses have not been adequately considered in existing bridge scour equations. This has led to the overestimation of scour depths in gravel-bed rivers. Under NCHRP Project 24-48, “Develop a Formula for Determining Scour Depth Around Structures in Gravel-Bed Rivers,” the University of Idaho was tasked with developing a rigorously tested and rapidly deployable methodology for bridge owners that will accurately predict scour at bridge foundations in gravel-bed rivers by (1) providing reliable data for fluid-induced forces, required to generate incipient motion in the surface and subsurface layers of gravel-bed rivers; and (2) combining fluid-induced erosion forces and incipient motion data to predict scour depths at bridge foundations with gravel-bed rivers. In addition to the conduct of research published as NCHRP Research Report 1031, several deliverables, not included in the published report, are available on the National Academies Press website (nap.nationalacademies.org) by searching for NCHRP Research Report 1031: Determining Scour Depth Around Structures in Gravel-Bed Rivers. The deliverables include a PowerPoint presentation, an Excel Scour Prediction Tool, Excel Laboratory Examples in Standard International and United States customary units, and Field Example Predictions in SI units. An implementation plan is also on the website. The Excel Scour Prediction Tool provides updated scour prediction equations intended for practitioners within state DOTs involved in bridge asset preservation and scour mitigation. F O R E W O R D By Camille Crichton-Sumners Staff Officer Transportation Research Board

Note: Photographs, figures, and tables in this report may have been converted from color to grayscale for printing. The electronic version of the report (posted on the web at nap.nationalacademies.org) retains the color versions. 1 Summary 3 Chapter 1 Background 3 1.1 Introduction to Bridge-Pier Scour in Coarse-Bedded Channels 5 1.2 Framework for New Scour Equation 7 Chapter 2 Research Approach 7 2.1 Large-Scale Laboratory Experiments on Bridge-Pier Scour and Critical Shear Stresses 21 2.2 Numerical Modeling of Near-Bed Shear Stresses in the Scour Hole 24 2.3 Small-Scale Laboratory Flume Experiments and Numerical Model Validation 29 2.4 Field Measurements of Bridge-Pier Scour 39 Chapter 3 Findings and Applications 39 3.1 Measured Controls on Bridge-Pier Scour in Laboratory Experiments 44 3.2 Development of New Bridge-Pier Scour Equations Using Laboratory Experiments 58 3.3 Testing and Modification of Scour Equations Using Field Data 68 3.4 Example Steps of Application of New Scour Equations for Practitioners 79 Chapter 4 Conclusions and Suggested Research 79 4.1 General Conclusions 79 4.2 Implementation Concerns and Implementation and Future Research Recommendations 82 References 86 Appendix A Summary of Engagement with State DOTs 90 Appendix B Sediment Sampling Recommendations 92 Appendix C Measured and Predicted Scour Depths in Database C O N T E N T S