Joint Workshop on Abutment Scour: Present Knowledge and Future Needs - June 2008 (2009)

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Research Results Digest 334 Subject Areas: IA Planning and Administration Responsible Senior Program Officers: E. T. Harrigan and D. A. Reynaud March 2009 JOINT WORKSHOP ON ABUTMENT SCOUR: PRESENT KNOWLEDGE AND FUTURE NEEDS—JUNE 2008 This digest summarizes key findings from a workshop sponsored by NCHRP Project Panels 24-15, 24-20, and 24-27 on the state of knowledge of bridge abutment scour. The workshop was held 23–24 June 2008 at the National Academies’ Keck Center, Washington, DC. The workshop notes on which this digest is based were prepared by Larry Arneson, Federal Highway Administration, and Peter Lagasse, Ayres Associates, Inc. INTRODUCTION The United States Geological Survey (USGS) defines scour as the hole left behind when sediment (sand and rock) is washed away from the bottom of a river.1 Although scour may occur at any time, scour action is especially strong during floods. Swiftly flowing water has more energy than calm water to lift and carry sediment down river. Scour affecting bridges may be classi- fied as follows: 1. Local scour is the removal of sedi- ment from around bridge piers or abutments. 2. Contraction scour is the removal of sediment from the bottom and sides of a river channel at the bridge opening; it is caused by the increase in the speed of water as it moves through a bridge opening that is narrower than the river channel. 1Warren, Linda P. “Scour at Bridges—What’s It All About?, Stream Stability and Scour Assessment at Bridges in Massachusetts,” U.S. Geological Survey Open-File Report 93-480, Online Only, http://ma. water.usgs.gov/publications/ofr/scour.htm. 3. Degradational scour is the general, more global removal of sediment from the river bottom by the flow of the river that, while a natural process, may cause removal of large amounts of sediment over time at the bridge site. The obvious danger of scour at or near a bridge is that the scour will undermine the piers and abutments that support the bridge and cause its catastrophic failure. In 1987, the 35-year-old bridge carrying Interstate 90 over Schoharie Creek in New York State failed with loss of life during a spring flood. This flood was classified as a 50-year event; the bridge had experienced a 100-year event soon after its construc- tion. The failure was initiated by extensive scour under one of the bridge piers; the absence of adequate countermeasures to scour at the pier as well as unnoted damage from the 100-year and later events con- tributed to the pier’s failure. In the aftermath of this failure and the comprehensive investigations into its causes, the Federal Highway Administra- tion (FHWA) instituted requirements for the states to identify overwater bridges vulnerable to scour and determine those where scour was severe. These inventories NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM

of bridge-site examinations, often undertaken with the assistance of the USGS, have allowed the states to plan and conduct maintenance and rehabilitation to remedy present scour and slow or prevent its future development. Besides these practical measures, the states— individually and collectively through the Nat- ional Cooperative Highway Research Program (NCHRP)—FHWA, and USGS embarked in the early 1990s on coordinated research programs to quantify and model the mechanisms of bridge scour and develop effective, efficient countermeasures to its occurrence. Table 1, which presents active, com- pleted, and planned NCHRP projects related to all aspects of bridge scour, illustrates the scale of these programs. Similar compilations are shown for FHWA at http://www.fhwa.dot.gov/engineering/hydraulics/ index.cfm and for USGS at http://water.usgs.gov/nrp/ currenttopics.html#sed. Of particular interest in Table 1, the common objectives of NCHRP Projects 24-27(01), 24-27(02), and 24-27(03) are to (1) critically evaluate the bridge- scour research completed since the early 1990s and (2) recommend the adoption of specific research results to AASHTO in its development of new edi- tions of two key highway hydraulic engineering guidance documents: Policy for Design of Highway Drainage Facilities and Recommended Procedures for Design of Highway Drainage Facilities. With these projects underway, the members of NCHRP Project Panels 24-15, 24-20, and 24-27 met with invited technical experts on 23–24 June 2008 in a facilitated workshop to: 1. Discuss possible changes to the FHWA docu- ment Evaluating Scour at Bridges, Hydraulic Engineering Circular No. 18 (HEC-18) aris- ing from the results of recently completed research on abutment scour, and 2. Forecast the next 5 years of research needs in bridge scour. Table 2 lists the participants in the 2-day work- shop and their affiliations. Participants were drawn from the project panels for NCHRP Projects 24-15, 24-20, and 24-27; the research teams involved in the three projects; and NCHRP staff. An experienced team from the Center for Leadership and Organiza- tional Change at the University of Maryland–College Park facilitated the workshop. 2 Table 1 NCHRP bridge scour projects Project Period of Number Project Title* Performance 24-06 Expert System for Stream Stability and Scour Evaluation 1993–1999 24-07 Countermeasures to Protect Bridge Piers from Scour 1995–1998 24-07(02) Countermeasures to Protect Bridge Piers from Scour 2001–2006 24-14 Scour at Contracted Bridge Sites 1998–2004 24-15 Complex Pier Scour and Contraction Scour in Cohesive Soils 1999–2002 24-15(02) Abutment Scour in Cohesive Soils 2004–2008 24-16 Methodology for Predicting Channel Migration 1999–2003 24-18 Countermeasures to Protect Bridge Abutments from Scour 2001–2003 24-18A Countermeasures to Protect Bridge Abutments from Scour 2003–2006 24-20 Prediction of Scour at Bridge Abutments 2002–2007 24-24 Criteria for Selecting Numeric Hydraulic Modeling Software 2004–2006 24-25 Guidelines for Risk-Based Management of Bridges with Unknown Foundations 2004–2006 24-26 Effects of Debris on Bridge-Pier Scour 2004–2007 24-27(01) Evaluation of Bridge Scour Research: Pier Scour Processes and Predictions 2008– 24-27(02) Evaluation of Bridge Scour Research: Abutment and Contraction Scour Processes 2008– and Predictions 24-27(03) Evaluation of Bridge Scour Research: Geomorphic Processes and Predictions FY 2009 24-29 Scour at Bridge Foundations on Rock FY 2009 24-32 Scour at Wide Piers and Long Skewed Piers 2007— 24-33 Development of Design Methods for In-Stream Flow Control Structures FY 2009 *For detailed information, go to http://www.trb.org/CRP/NCHRP/NCHRPProjects.asp?AreaID=24.

The following section of this digest summa- rizes the changes recommended for HEC-18 and the 5-year research needs identified by the work- shop participants. FINDINGS The fourth and latest edition of HEC-18 was pub- lished in 2001 and is available online from the FHWA at http://isddc.dot.gov/OLPFiles/FHWA/010590.pdf. HEC-18 provides guidelines for (1) designing new 3 Table 2 Workshop participants Name Title Affiliation Kenneth Akoh-Arrey Larry Arneson Stephen Benedict Bart Bergendahl Jean-Louis Briaud Kuang-An Chang Hamn-Ching Chen Stanley R. Davis Robert Ettema Daryl J. Greer Larry Harrison Robert W. Henthorne J. Sterling Jones Kornel Kerenyi Andrzej Kosicki Peter Lagasse William L. Moore III Tatsuaki Nakato Steve Ng Jorge Pagan-Ortiz Richard A. Phillips Rick Renna Bradford M. Rognlie Amy Ronnfeldt Terry Sturm Larry J. Tolfa Mehmet T. Tumay Facilitators and NCHRP Staff Judy Tso Amy Ginther Edward Harrigan David Reynaud Bridge Hydraulics Engineer Senior Hydraulic Engineer Hydrologist Senior Hydraulic Engineer Professor of Civil Engineering Professor of Civil Engineering Hydraulic Engineer Professor of Civil Engineer Director, Division of Planning Consultant Regional Geologist Consultant Hydraulics Laboratory Manager Assistant Division Chief, Bridge Design Division Senior Vice President Consultant Professor of Civil Engineering Senior Bridge Engineer Principal Bridge Engineer-Hydraulics Bridge Hydraulics Engineer State Hydraulics Engineer Senior Bridge and Hydraulics Engineer Assistant Hydraulics Engineer Professor of Civil Engineering Engineer Associate Dean for Research Organizational Development Specialist Organizational Development Specialist Senior Program Officer Senior Program Officer Arizona DOT Federal Highway Administration U.S. Geological Survey Federal Highway Administration Texas A&M University Texas A&M University Texas A&M University Jacobs Civil, Inc. University of Wyoming Kentucky Transportation Cabinet Kansas DOT Federal Highway Administration Maryland State Highway Administration Ayres Associates, Inc. Iowa State University CalTrans Federal Highway Administration South Dakota DOT Florida DOT Kansas DOT Texas DOT Georgia Tech New York State DOT Louisiana State University Center for Leadership and Organizational Change, University of Maryland– College Park Center for Leadership and Organizational Change, University of Maryland– College Park NCHRP NCHRP and replacement bridges to resist scour, (2) evaluating existing bridges for vulnerability to scour, (3) inspect- ing bridges for scour, and (4) improving the state of practice of estimating scour at bridges. HEC-18 repre- sents the current state of knowledge and practice on bridge scour, and the document is subject to a contin- uous, cooperative program of review and revision by the state DOTs, the FHWA, and their consultants. The potential changes to HEC-18 considered by the workshop participants are presented in Table 3. These are based, to a large extent, on the workshop

participants’ discussion of the findings and conclu- sions of several active and completed NCHRP and USGS research projects. Results of NCHRP Projects 24-15(02) and 24- 20, were presented by the two principal investiga- tors, Professors Jean-Louis Briaud of Texas A&M University and Robert Ettema of the University of Wyoming, respectively. Professor Briaud stressed the need for HEC-18 to provide engineers with a better understanding of how the behavior of both cohesive and cohesionless materials affects development of scour in streambeds and embankments. Professor Ettema discussed his research team’s work on incor- porating embankment failure into the abutment scour process, including laboratory testing to support mod- eling of different abutment failure mechanisms. In his presentation, Professor Terry Sturm of the Georgia Institute of Technology discussed possible changes to HEC-18 and the two AASHTO documents based on his team’s analysis of past research on abut- ment scour in NCHRP Project 24-27(02). Professor Sturm pointed out the difficulty of estimating abut- ment scour compared to pier scour as well as the prac- tical inseparability of abutment scour from contraction scour, even though each depends on different physical processes. Dr. Stephen Benedict of the USGS and Mr. Stanley Davis of Jacobs Civil, Inc. discussed their work on the validation of scour mechanisms and predictive models with measured field data. Obvi- ously difficult, such validation studies are nonethe- less invaluable in reconciling different scour models and refining and calibrating them to achieve reliable predictions. Finally, the workshop participants developed seven recommended research problem statements for conduct in the next 5 years as NCHRP or state pooled- funds projects; these are summarized in Table 4. 4 Table 3 Potential changes to Evaluating Scour at Bridges, Hydraulic Engineering Circular No. 18 (HEC-18) Technical and Editorial Improvements 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. Changes to Policies and Procedures 1. 2. 3. 4. 5. 6. 7. 8. 9. Include section on measurement and classification of soil properties. Include section on field data sets available to verify and validate scour prediction models. Note strengths and weaknesses of the data sets. Organize and categorize data with graphs and tables to assist the engineer in devel- oping an understanding of field performance trends. Add a discussion of the geomorphology and scour processes found in gravel bed rivers. Incorporate in both HEC-18 and HEC-20. Separate abutment design into two steps: (1) embankment stability design and (2) abutment foundation design, to account for hydraulic relief due to potential embankment failure. Include a section addressing incorporation of scour prevention in bridge construction practice. Include practical recommendations on abutment scour geotechnical failures. Consider addressing relief bridge scour. Discuss the physics of scour and how it influences the methodologies for scour reduction and prevention. Actively edit the document to achieve uniformity throughout of voice, language, and format, as well as smooth flow from section to section. Critique all design methodologies to improve their clarity and ease of use. Include a policy on channel degradation. Include a policy on riprap protection of abutments. Provide a list of forecasted changes in future editions. Develop a procedure to validate abutment scour equations with field data. Provide more quantitative abutment scour guidance derived from the physics of the underlying processes. Include a discussion of how the use of estimated or default versus measured values in design calculations affects the precision of calculated scour depths. Improve the procedure for calculating contraction scour to take account of soil type. Provide a procedure for calculating lateral channel movement. Provide a venue for discussing the impact and implementation of results from non-federal, non-NCHRP research.

Table 4 Recommended near-term (5-year) NCHRP or state pooled-fund research problem statements Recommended # Title Objective(s) Funding 1. 2. 3. 4. 5. 6. 7. Statistical Prediction of Unknown Foundations Risk Approach to Bridge Scour Predictions Probabilistic Approach to Prediction of Scour at Bridges Influence of Soil Properties on Scour Processes Interaction of Abutment and Contraction Scour Validation of Abutment Scour Equations Abutment Scour in Cohesive and Non-Cohesive Soils Develop practical tools to determine the type and penetration of unknown foundations. Develop a methodology for calculating the probability that scour will not exceed a given depth under the impact of a 50-, 100-, or 500-year flood event. (1) Develop a methodology to accurately estimate the variation present in physical and design factors that most significantly affect scour at bridge piers and abutments and approach embankments. (2) Develop stochastic prediction models that can account for the variation in significant factors and yield probabilistic estimates of scour depth. (3) Integrate the probabilistic models into a single comprehensive model for conducting a complete assessment of overall bridge reliability. (1) Characterize the effects of erodibility and related properties of natural and manufactured soils on scour processes. (2) Develop guidance for selection of proper scour methodology based on soil properties. Identify location, magnitude, and mechanisms of simultaneous abutment and contraction scour in laboratory and field. Verify and validate abutment-scour equations developed in NCHRP Projects 24-15 and 24-20 using USGS field data sets. Develop a unified design approach for abutment scour in cohesive and non-cohesive soils based on the results of NCHRP Projects 24-15(02) and 24-20. $750,000 $500,000 $350,000 $500,000 $600,000 $600,000 $500,000

Transportation Research Board 500 Fifth Street, NW Washington, DC 20001 These digests are issued in order to increase awareness of research results emanating from projects in the Cooperative Research Programs (CRP). Persons wanting to pursue the project subject matter in greater depth should contact the CRP Staff, Transportation Research Board of the National Academies, 500 Fifth Street, NW, Washington, DC 20001. COPYRIGHT PERMISSION 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.