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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 REPORT 761 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction Peter F. Lagasse Ayres AssociAtes Fort Collins, CO Michel Ghosn ccNy/cUNy New York, NY Peggy A. Johnson PeNNsylvANiA stAte UNiversity University Park, PA Lyle W. Zevenbergen Ayres AssociAtes Fort Collins, CO Paul E. Clopper Ayres AssociAtes Fort Collins, CO Subscriber Categories Bridges and Other Structures â¢ Hydraulics and Hydrology TRANSPORTAT ION RESEARCH BOARD WASHINGTON, D.C. 2013 www.TRB.org Research sponsored by the American Association of State Highway and Transportation Officials in cooperation with the Federal Highway Administration
NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM Systematic, well-designed research provides the most effective approach to the solution of many problems facing highway administrators and engineers. Often, highway problems are of local interest and can best be studied by highway departments individually or in cooperation with their state universities and others. However, the accelerating growth of highway transportation develops increasingly complex problems of wide interest to highway authorities. These problems are best studied through a coordinated program of cooperative research. In recognition of these needs, the highway administrators of the American Association of State Highway and Transportation Officials initiated in 1962 an objective national highway research program employing modern scientific techniques. This program is supported on a continuing basis by funds from participating member states of the Association and it receives the full cooperation and support of the Federal Highway Administration, United States Department of Transportation. The Transportation Research Board of the National Academies was requested by the Association to administer the research program because of the Boardâs recognized objectivity and understanding of modern research practices. The Board is uniquely suited for this purpose as it maintains an extensive committee structure from which authorities on any highway transportation subject may be drawn; it possesses avenues of communications and cooperation with federal, state and local governmental agencies, universities, and industry; its relationship to the National Research Council is an insurance of objectivity; it maintains a full-time research correlation staff of specialists in highway transportation matters to bring the findings of research directly to those who are in a position to use them. The program is developed on the basis of research needs identified by chief administrators of the highway and transportation departments and by committees of AASHTO. Each year, specific areas of research needs to be included in the program are proposed to the National Research Council and the Board by the American Association of State Highway and Transportation Officials. Research projects to fulfill these needs are defined by the Board, and qualified research agencies are selected from those that have submitted proposals. Administration and surveillance of research contracts are the responsibilities of the National Research Council and the Transportation Research Board. The needs for highway research are many, and the National Cooperative Highway Research Program can make significant contributions to the solution of highway transportation problems of mutual concern to many responsible groups. The program, however, is intended to complement rather than to substitute for or duplicate other highway research programs. Published reports of the NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM are available from: Transportation Research Board Business Office 500 Fifth Street, NW Washington, DC 20001 and can be ordered through the Internet at: http://www.national-academies.org/trb/bookstore Printed in the United States of America NCHRP REPORT 761 Project 24-34 ISSN 0077-5614 ISBN 978-0-309-28356-4 Library of Congress Control Number 2013950551 Â© 2013 National Academy of Sciences. All rights reserved. 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, or Transit Development Corporation 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. NOTICE The project that is the subject of this report was a part of the National Cooperative Highway Research Program, conducted by the Transportation Research Board with the approval of the Governing Board of the National Research Council. The members of the technical panel selected to monitor this project and to review this report were chosen for their special competencies and with regard for appropriate balance. The report was reviewed by the technical panel and accepted for publication according to procedures established and overseen by the Transportation Research Board and approved by the Governing Board of the National Research Council. The opinions and conclusions expressed or implied in this report are those of the researchers who performed the research and are not necessarily those of the Transportation Research Board, the National Research Council, or the program sponsors. The Transportation Research Board of the National Academies, the National Research Council, and the sponsors of the National Cooperative Highway Research Program do not endorse products or manufacturers. Trade or manufacturersâ names appear herein solely because they are considered essential to the object of the report.
The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distinguished scholars engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their use for the general welfare. On the authority of the charter granted to it by the Congress in 1863, the Academy has a mandate that requires it to advise the federal government on scientific and technical matters. Dr. Ralph J. Cicerone is president of the National Academy of Sciences. The National Academy of Engineering was established in 1964, under the charter of the National Academy of Sciences, as a parallel organization of outstanding engineers. It is autonomous in its administration and in the selection of its members, sharing with the National Academy of Sciences the responsibility for advising the federal government. The National Academy of Engineering also sponsors engineering programs aimed at meeting national needs, encourages education and research, and recognizes the superior achievements of engineers. Dr. C. D. Mote, Jr., is president of the National Academy of Engineering. The Institute of Medicine was established in 1970 by the National Academy of Sciences to secure the services of eminent members of appropriate professions in the examination of policy matters pertaining to the health of the public. The Institute acts under the responsibility given to the National Academy of Sciences by its congressional charter to be an adviser to the federal government and, on its own initiative, to identify issues of medical care, research, and education. Dr. Harvey V. Fineberg is president of the Institute of Medicine. The National Research Council was organized by the National Academy of Sciences in 1916 to associate the broad community of science and technology with the Academyâs purposes of furthering knowledge and advising the federal government. Functioning in accordance with general policies determined by the Academy, the Council has become the principal operating agency of both the National Academy of Sciences and the National Academy of Engineering in providing services to the government, the public, and the scientific and engineering communities. The Council is administered jointly by both Academies and the Institute of Medicine. Dr. Ralph J. Cicerone and Dr. C. D. Mote, Jr., are chair and vice chair, respectively, of the National Research Council. The Transportation Research Board is one of six major divisions of the National Research Council. The mission of the Transporta- tion Research Board is to provide leadership in transportation innovation and progress through research and information exchange, conducted within a setting that is objective, interdisciplinary, and multimodal. The Boardâs varied activities annually engage about 7,000 engineers, scientists, and other transportation researchers and practitioners from the public and private sectors and academia, all of whom contribute their expertise in the public interest. The program is supported by state transportation departments, federal agencies including the component administrations of the U.S. Department of Transportation, and other organizations and individu- als interested in the development of transportation. www.TRB.org www.national-academies.org
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 24-34 by Ayres Associates, Fort Collins, Colorado. Peter F. Lagasse, senior vice president, served as principal investigator (PI). Michel Ghosn of the City College of New York/City University of New York and Peggy A. Johnson of the Pennsyl- vania State University served as co-principal investigators (Co-PIs). They were assisted by Lyle W. Zeven- bergen, manager of river engineering at Ayres Associates (currently program manager/hydraulic engineer at Tetra Tech), and Paul E. Clopper, senior water resources engineer at Ayres Associates. The authors wish to acknowledge the contributions of Will deRosset of Ayres Associates and David Zachmann, consultant, in developing the rasToolÂ© software that links the U.S. Army Corps of Engineers 1-dimensional hydraulic model with Monte Carlo simulation techniques. This linkage was integral to achieving project objectives. A technical advisory team consisting of David Williams, Robert Ettema, Arun Shirole, and Harry Capers provided input at the outset of the project and/or periodic review of draft project documents. The authors also gratefully acknowledge the advice and support of NCHRP panel members throughout this project. Panel member Denis D. Stuhff, who passed away suddenly on June 27, 2012, is respectfully acknowl- edged for his contributions to this project. Mr. Stuhff was known for his depth of knowledge of bridge structures, hydraulics, and environmental engineering; for his commitment to improving highway safety for the people of Utah; and for contributing to advancing the state of practice in his chosen disciplines through activities such as serving on the panel for NCHRP Project 24-34. The research team would like to dedicate this report to Mr. Stuhff. CRP STAFF FOR NCHRP REPORT 761 Christopher W. Jenks, Director, Cooperative Research Programs Crawford F. Jencks, Deputy Director, Cooperative Research Programs Waseem Dekelbab, Senior Program Officer Danna Powell, Senior Program Assistant Eileen P. Delaney, Director of Publications Sharon Lamberton, Assistant Editor NCHRP PROJECT 24-34 PANEL Field of Soils and GeologyâArea of Mechanics and Foundations Steve Ng, California DOT, Sacramento, CA (Chair) Larry A. Arneson, Federal Highway Administration, Lakewood, CO Bilal Ayyub, University of Maryland, College Park, MD Andrzej J. Kosicki, Maryland State Highway Administration, Baltimore, MD Michael J. Orth, Kansas DOT, Topeka, KS Rick Renna, Florida DOT, Tallahassee, FL Denis D. Stuhff, Utah DOT, Salt Lake City, UT (deceased) Xiong âBillâ Yu, Case Western Reserve University, Cleveland, OH Kornel Kerenyi, FHWA Liaison G. P. Jayaprakash, TRB Liaison
This report presents a reference guide to identify and evaluate the uncertainties associ- ated with bridge scour prediction including hydrologic, hydraulic, and model/equation uncertainty. Tables of probability values to estimate scour depth with a conditional prob- ability of exceedance when a bridge meets certain criteria for hydrologic uncertainty, bridge size, and pier size are included in the reference guide. For complex foundation systems and channel conditions, a step-by-step procedure is presented to provide scour factors for site- specific conditions. The reference guide also includes a set of detailed illustrative examples to demonstrate the full range of applicability of the procedures. The report will be of imme- diate interest to hydraulic and bridge engineers. Current practice for determining the total scour prism at a bridge crossing involves the calculation of the various individual scour components (e.g., pier scour, abutment scour, contraction scour, and long-term channel changes). Then, using the principle of superposi- tion, these individual components are considered to be purely additive and the total scour prism is then drawn as a single cumulative line for various frequency flood events (e.g., 50-year, 100-year, and 500-year flood events). The scour equations are generally under- stood to be conservative in nature and, with the exception of the contraction scour equa- tions, have been developed as envelope curves for use in design. This approach does not provide an indication of the uncertainty involved in the computation of any of the indi- vidual components. Uncertainties in hydrologic and hydraulic models and the resulting uncertainty of relevant inputs (e.g., design discharge, velocity, depth, and flow distribution between the main channel and the floodplain) to the scour calculations will all have a sig- nificant influence when evaluating the risk associated with scour prediction. To develop an overall estimate of confidence in the calculated scour depths, one must use engineering judgment and examine the level of confidence associated with the results of the hydrologic analysis, the level of confidence associated with the hydraulic analysis, and the level of confidence associated with each of the scour components. Scour reliability analysis involves quantification of the uncertainties in each of these steps and then combines them in such a way that the overall estimate of confidence is known for the final prediction of scour. Research was performed under NCHRP Project 24-34 by Ayres Associates with the assis- tance of the City College of the City University of New York and the Pennsylvania State Uni- versity. The objective of NCHRP Project 24-34 was to develop a risk-based methodology that can be used in calculating bridge pier, abutment, and contraction scour at waterway crossings so that scour estimates can be linked to a probability consistent with Load and Resistance Factor Design (LRFD) approaches used by structural and geotechnical engineers. This Reference Guide is oriented toward the practitioner. The research agencyâs final report documenting the complete results of the research is not published but is available online at www.trb.org by searching âNCHRP Project 24-34.â F O R E W O R D By Waseem Dekelbab Staff Officer Transportation Research Board
1 Summary 4 Chapter 1 Introduction and Applications 4 1.1 Introduction 5 1.2 Applications 5 1.2.1 Transportation Facilities 6 1.2.2 Floodplain Risk and Flood Control Facilities 7 1.2.3 Channel Restoration and Rehabilitation Works 7 1.3 Organization of the Reference Guide 11 Chapter 2 Uncertainty in Hydraulic Design 11 2.1 Introduction 11 2.2 Hydrologic Uncertainty 11 2.2.1 Overview 13 2.2.2 Evaluating Hydrologic Uncertainty 15 2.3 Hydraulic Uncertainty 15 2.3.1 Overview 17 2.3.2 Evaluating Hydraulic Uncertainty 18 2.4 Uncertainty in Bridge Scour Estimates 18 2.4.1 Background 20 2.4.2 FHWA GuidanceâIncorporating Risk in Bridge Scour Analyses 22 2.5 LRFDâA Hydraulic Engineering Perspective 22 2.5.1 Introduction 22 2.5.2 Reliability 24 2.5.3 LRFD Code Calibration 27 Chapter 3 Evaluating Uncertainty Associated with Scour Prediction 27 3.1 Introduction 27 3.2 Determining Individual Scour Component Uncertainty 28 3.3 Parameter and Model Uncertainty 28 3.3.1 Parameter Uncertainty 28 3.3.2 Model Uncertainty 30 3.4 Development of Supporting Software 30 3.4.1 HEC-RAS 30 3.4.2 Integration of HEC-RAS and Monte Carlo 32 3.4.3 Discharge 33 3.4.4 Manning Roughness Coefficient 33 3.4.5 Downstream Boundary Friction Slope 34 3.4.6 Summary 34 3.5 Implementing the Software 34 3.5.1 Approach 34 3.5.2 Hydraulic Parameter Uncertainty 38 3.5.3 Testing and Adjusting the Software 39 3.6 Summary and Preview of Applications C O N T E N T S
41 Chapter 4 Bridge Scour Equations and Data Screening 41 4.1 Introduction 41 4.2 Pier Scour Data 41 4.2.1 Pier Scour Laboratory DataâCompilation, Screening, and Analysis 47 4.2.2 Pier Scour Field DataâCompilation and Analysis 47 4.3 Contraction Scour 47 4.3.1 Clear-Water Contraction Scour Laboratory DataâCompilation and Screening 49 4.3.2 Clear-Water Contraction Scour Laboratory DataâAnalysis 51 4.4 Abutment Scour Data 51 4.4.1 Abutment Scour Laboratory DataâCompilation 52 4.4.2 NCHRP Project 24-20 Abutment Scour Approach 57 4.4.3 Abutment Scour Data Screening and Analysis 59 Chapter 5 Probability-Based Scour Estimates 59 5.1 Introduction 59 5.2 Approach 59 5.2.1 Background 60 5.2.2 Calibration of Level I Statistical Parameters 60 5.2.3 Level I Applications for Typical Site Conditions 61 5.3 Level I Analysis and Results 68 5.4 Level II Analysis and Results 70 5.4.1 Step-by-Step Procedure for Level II Analysis 72 5.4.2 HEC-RAS/Monte Carlo Simulation Results for Pier Scour 75 5.4.3 HEC-RAS/Monte Carlo Simulation Results for Contraction Scour 78 5.4.4 HEC-RAS/Monte Carlo Simulation Results for Abutment Scour 80 Chapter 6 Calibration of Scour Factors for a Target Reliability 80 6.1 Approach 80 6.1.1 Background 80 6.1.2 Reliability Analysis 81 6.1.3 Reliability Calculation Process 81 6.1.4 Calibration of Design Equations 81 6.1.5 Simplified Example 82 6.2 Validation of the Simplified Procedure 82 6.2.1 Overview of the Procedure 84 6.2.2 Case Studies for Validation 84 6.3 Implementation of Reliability Analysis for Sacramento River Bridge Data 84 6.3.1 Pier Scour Designed Using HEC-18 Method 87 6.3.2 Pier Scour Designed Using Florida DOT Method 87 6.3.3 Contraction Scour Designed Using HEC-18 Method 88 6.3.4 Total Pier and Contraction Scour Using HEC-18 Methods 88 6.3.5 Total Pier and Contraction Scour Using Florida DOT Method 89 6.3.6 Total Scour at an Abutment Using NCHRP Project 24-20 Method 89 6.3.7 Summary 90 6.4 Calibration of Scour Factors 92 Chapter 7 Illustrative Examples 92 7.1 Overview 92 7.2 Example Bridge No. 1: Maryland Piedmont Region 95 7.3 Example Bridge No. 2: Nevada Great Basin Subregion
99 7.4 Example Bridge No. 3: California Pacific Mountains Subregion 102 7.5 Example Bridge No. 4: Missouri Interior Lowlands Subregion 106 7.6 Example Bridge No. 5: South Carolina Atlantic Coastal Plain Subregion 112 Chapter 8 Conclusions and Observations 112 8.1 Conclusions 113 8.2 Observations 113 8.2.1 Data Analysis Issues 114 8.2.2 Importance of Hydrologic and Hydraulic Uncertainty 114 8.2.3 Roadway Overtopping 115 8.2.4 Total Scour 116 References A-1 Appendix A Glossary B-1 Appendix B Summary of Scour Factors in Tabular and Graphical Form Note: Many of the photographs, figures, and tables in this report have been converted from color to grayscale for printing. The electronic version of the report (posted on the Web at www.trb.org) retains the color versions.