National Academies Press: OpenBook

Effects of Debris on Bridge Pier Scour (2010)

Chapter: Chapter 1 - Introduction and Research Approach

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Suggested Citation:"Chapter 1 - Introduction and Research Approach." National Academies of Sciences, Engineering, and Medicine. 2010. Effects of Debris on Bridge Pier Scour. Washington, DC: The National Academies Press. doi: 10.17226/22955.
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Suggested Citation:"Chapter 1 - Introduction and Research Approach." National Academies of Sciences, Engineering, and Medicine. 2010. Effects of Debris on Bridge Pier Scour. Washington, DC: The National Academies Press. doi: 10.17226/22955.
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Suggested Citation:"Chapter 1 - Introduction and Research Approach." National Academies of Sciences, Engineering, and Medicine. 2010. Effects of Debris on Bridge Pier Scour. Washington, DC: The National Academies Press. doi: 10.17226/22955.
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Suggested Citation:"Chapter 1 - Introduction and Research Approach." National Academies of Sciences, Engineering, and Medicine. 2010. Effects of Debris on Bridge Pier Scour. Washington, DC: The National Academies Press. doi: 10.17226/22955.
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Suggested Citation:"Chapter 1 - Introduction and Research Approach." National Academies of Sciences, Engineering, and Medicine. 2010. Effects of Debris on Bridge Pier Scour. Washington, DC: The National Academies Press. doi: 10.17226/22955.
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41.1 Scope and Research Objectives 1.1.1 Background On Sunday, March 16, 1980, a portion of the Perkins Road Bridge over Nonconnah Creek in Memphis, Tennessee, col- lapsed during high flow (Figure 1.1). The bridge failure resulted in one death and litigation was brought against numerous defendants. Investigation showed that at about 11:00 p.m. during a rainstorm, the two northbound lanes of spans 3 and 4 of the Perkins Road Bridge had collapsed. Inspection of the bridge after the collapse was conducted by a Shelby County bridge inspection crew. Initial inspection found (1) large drifts of debris on the upstream side of the bridge, (2) at least one pile that was broken but still in place, and (3) other piles completely missing. To address the charge of negligence and other issues and to provide a basis for litigation support, detailed hydrologic, hydraulic, and scour analyses were completed. The objectives were to establish the hydraulic, erosion, and sedimentation characteristics of the Nonconnah Creek system. The analyses involved application of basic hydrologic, hydraulic, geo- morphic, and sediment transport principles and yielded esti- mates of the contribution of degradation, contraction scour, local scour, and the impact of debris (Lagasse and Schall 1980). The investigation focused on the impacts of debris on pier scour (Figure 1.2). Calculation procedures for scour predated but paralleled the current recommendations in FHWA’s Hydraulic Engineering Circular No. 18, known as HEC-18 (Richardson and Davis 2001), and resulted in the estimates shown in Table 1.1 for local and contraction scour. To estimate the impact of debris on local pier scour, the pier width was increased incrementally to represent blockages that would reduce conveyance through the bridge opening by 20% up to 45%. It was concluded that increasing the “blockage” by assuming that the debris would increase pier width to reduce conveyance through the bridge by more than 40% to 50% would radically alter the flow conditions implicit in the pier scour equation. The results clearly showed that a 20% block- age would undermine the estimated 12 ft of embedment on the pile foundation of the pier that failed. Surprisingly, this crude model of debris scour has not been changed substantially in the 28 years since this bridge failure. The approach of increasing the width of the pier using engi- neering judgment to account for debris is still suggested in the fourth edition of HEC-18 (Richardson and Davis 2001), which represents the current state of the practice in the United States. 1.1.2 Objectives Waterborne debris (or drift), composed primarily of tree trunks and limbs, often accumulates on bridges during flood events. Debris accumulations can obstruct, constrict, or re- direct flow through bridge openings resulting in flooding, dam- aging loads, or excessive scour at bridge foundations. The size and shape of debris accumulations vary widely, ranging from a small cluster of debris on a bridge pier to a near complete blockage of a bridge waterway opening. Debris accumulation geometry is dependent on the characteristics and supply of debris transported to bridges, on flow conditions, and on bridge and channel geometry. The effects of debris accumulation can vary from minor flow constrictions to severe flow contrac- tion resulting in significant bridge foundation scour. Qualitatively, the impacts of debris have been well docu- mented (see for example, Chang and Shen 1979, Diehl 1997, Parola et al. 2000). However, a pressing need remains for state DOTs and other bridge owners to have improved prediction methods for the geometry (size and shape) of typical debris accumulations, the conditions under which debris can be expected to develop, and the resulting depth and extent of scour at bridge piers. Currently, only limited guidance is avail- able on which to base critical public safety decisions during flooding on debris-prone rivers. There is a need for accurate methods of quantifying the effects of debris on scour at bridge C H A P T E R 1 Introduction and Research Approach

pier foundations for use by DOTs and other agencies in the design, operation, and maintenance of highway bridges. The objectives of this research were to develop (1) guide- lines for predicting the size and geometry of debris accumu- lations at bridge piers and (2) methods for quantifying scour at bridge piers resulting from debris accumulations. 1.2 Research Approach 1.2.1 Overview This research project produced results on two related prob- lems: (1) predicting the accumulation characteristics of debris from potentially widely varying source areas, in rivers with different geomorphic characteristics, and on bridges with a variety of substructure geometries and (2) developing improved methods for quantifying the depth and extent of scour at bridge piers considering both the accumulation variables and the range of hydraulic factors involved. One might infer from previous studies, as well as the geo- morphic characteristics of rivers in different physiographic regions and the different characteristics of woody vegetation and river bank erosion processes, that there might be some regional bias in debris characteristics and in debris impact on bridges. These regional characteristics were investigated dur- ing Phase 1 in the Task 1 literature search and in the Task 2 site reconnaissance and survey. The evidence available does not support the hypothesis that there is a regional bias in debris characteristics. In fact, a photographic archive of debris at bridges across the United States assembled to assess typical debris geometry relationships demonstrates that debris accu- mulations can be grouped into a finite number of common shapes that can be found in most physiographic regions. For Task 3, Diehl’s extensive study “Potential Drift Accumulation at Bridges” (1997) for FHWA was the obvious starting point. The preliminary guidelines for Task 3 expanded on the three major phases suggested by Diehl for assessing the potential for debris production and accumulation at a bridge site: (1) estimate the potential for debris production and deliv- ery, (2) estimate the potential for debris accumulation on individual bridge elements, and (3) calculate the hypothetical accumulations for the entire bridge. The Task 4 laboratory plan considered different accumu- lation configurations (e.g., floating raft, submerged wedge, etc.) to develop a matrix of alternatives for laboratory testing. Thus, the Task 1 and Task 2 results and the debris photo- graphic archive were used to determine the number of alter- natives that must be included in the Task 4 laboratory plan to investigate a full range of debris characteristics. 5 Figure 1.1. Perkins Road Bridge, March 17, 1980 (flow from right to left). Figure 1.2. Debris at Perkins Road Bridge, March 16, 1980. Total Erosion Debris Blockage (%) Local Scour Depth (ft) Contraction Scour Depth (ft) Degradation Depth (ft) (ft) (m) 0 6.2 1.5 0.5 8.2 2.5 20 12.5 1.5 0.5 14.5 4.4 45 18.8 1.5 0.5 20.8 6.3 Table 1.1. Total erosion potential at Perkins Bridge for 1980 flood.

1.2.2 Research Plan Modifications The Interim Report (Task 5) provided all findings and rec- ommendations from Phase 1, including the suggested labora- tory test plan for Phase 2. At the Interim Report panel meeting, the following topics were presented and discussed: • Survey and field pilot study results • Photographic archive—sources, distribution, and applica- tions • Preliminary guidelines for debris production and prediction • Laboratory test plan • Conceptual framework for predicting debris scour at piers Panel guidance was requested on the following topics: • Requirements for additional field work • Guidelines for predicting size and geometry of debris • Laboratory testing matrix and priorities (including scaling factors) • Debris scour prediction—conceptual framework As a result of the Interim Report meeting with the panel, several significant changes were made to the research approach based on findings of the field pilot study during Phase 1: • Plans for extensive field work at debris sites were eliminated. • The photographic archive of debris sites should be expanded and examined for all relevant information on trends or pat- terns that could be used as a guide by the practitioner. • A case study should be developed to illustrate the applica- tion of the final guidelines (Task 6) for production of debris and accumulation at bridges. • Considering the testing budget, laboratory tests should be conducted at a single scale. • Guidelines for inspection, monitoring, and maintenance related to debris at bridges should be developed. 1.3 Research Tasks Considering the research approach discussed and outlined previously, the following specific tasks were completed to accomplish project objectives. These tasks incorporate panel guidance and parallel, with some modifications, those sug- gested in the original research project statement. 1.3.1 Phase 1 Tasks Task 1, Review the Technical Literature The research team conducted a thorough review of the tech- nical literature from foreign and domestic sources to assess the adequacy and extent of existing information on debris accu- mulations and the effect of these accumulations on bridge scour. The literature review identified research in progress as well as completed work. As part of the literature review, a photographic archive was compiled to assess typical debris geometry relationships. Photographs of debris at bridges were acquired from a num- ber of contributors. The primary contributor was Debris Free, Inc. of Ojai, California, who provided almost 50% of the pho- tos. Numerous photographs were provided by 22 state DOT personnel in response to the project survey. The remaining photographs were acquired from in-house sources, Internet sites, and referenced publications. A total of 1,079 photographs were acquired from the various sources. Task 2, Conduct Survey and Site Reconnaissance The research team determined typical debris accumula- tions by surveying state DOTs and other agencies and by site reconnaissance of debris accumulations at bridge piers. The site reconnaissance (pilot study) included field measurements, photographs, geomorphologic information, channel types and flow patterns, and associated bridge and pier geometrics. Task 2 required determination of typical debris accumu- lations by surveying state DOTs and other agencies. The results of the survey are analyzed in Chapter 2 and included in Appendix B. The field pilot study to bridge sites in southeastern Kansas was completed during the period April 25–28, 2005. Overall coordination was provided by Kansas DOT. A detailed field trip report is included as Appendix C. Results and recommen- dations are summarized in Chapter 2. The panel concurred with the recommendation that no additional field work should be conducted to obtain additional measurements of the geom- etry of debris accumulations. The panel directed that the remaining field work budget be allocated to developing a case study for application of the final guidelines (Task 6). Task 3, Develop Preliminary Guidelines Based on the Task 1 literature search and the empirical data collected in Task 2, the research team developed preliminary guidelines for predicting the size and geometry of debris accumulations at bridge piers. Here, Diehl’s extensive study “Potential Drift Accumulation at Bridges” (1997) for FHWA was the starting point. Task 4, Develop Phase 2 Laboratory Plan The research team developed a detailed description of the Phase 2 laboratory experiments proposed for assessing the effects of debris accumulations on scour at bridge piers. Typical configurations of debris accumulations identified in Tasks 1 6

and 2 were categorized, and selected debris configurations were prioritized for use in the Phase 2 laboratory experiments. A testing program was proposed to address three main objectives: 1. Effect of debris accumulation on local hydraulic condi- tions at bridge piers 2. Quantification of scour resulting from predicted debris accumulations 3. Sensitivity of hydraulic conditions, and resulting scour patterns, to variations in debris accumulation geometries All laboratory testing was completed at the Colorado State University Engineering Research Center Hydraulics Laboratory. Task 5, Submit Interim Report The research team prepared and submitted an Interim Report documenting the information developed in Tasks 1 through 5. The Interim Report contained a detailed discus- sion of the typical debris configurations to be used in the Phase 2 laboratory experiments and the preliminary guide- lines for predicting the size and geometry of debris accumu- lations. The research team met with the NCHRP 24-26 panel to discuss the Interim Report and the revised work plan. 1.3.2 Phase 2 Tasks Task 6, Finalize Task 3 Guidelines Based on panel comments and guidance during the Interim Report meeting, the research team finalized the Task 3 pre- liminary guidelines for predicting the size and geometry of debris accumulations. Case Study. A case study was developed to provide an example of how the practitioner should apply the guidelines for assessing debris production and predicting debris accumu- lation at a bridge site. Developing the case study also assisted in refining and finalizing the guidelines. The intent of the case study is to provide practitioners with an example that documents the step-by-step process used to apply the guidelines for a specific bridge site. The final guide- lines include procedures for documenting: • Geomorphic factors that affect stream stability • Debris production from the watershed and tributary net- work • Transport and delivery to the bridge. The guidelines also provide guidance on how to use the information collected at a bridge site to estimate potential accumulation sizes and geometry, and from that, apply the appropriate scour prediction relationship. For the case study, field data sheets specific to the debris problem were developed. The sheets can be used to document site characteristics such as channel type and size, channel insta- bility, bank erosion and retreat, and bank vegetation character- istics in detail. The sheets were designed to document potential or existing debris size, production, transport, and storage char- acteristics. The field data sheets were incorporated into the final guidelines. Guidance is also provided on obtaining sup- plemental information such as watershed size, channel plan- form, land use conditions, and peak discharges, which can be documented from aerial photography and gage data that are readily available (e.g., TerraServer and U.S. Geological Survey Water Resources websites). Debris Photographic Archive. The panel instructed the research team to extract as much additional information from the debris photographic archive as possible with the resources remaining in Task 6. The database was developed on Microsoft® Excel spreadsheets, which were expanded in Task 6. The existing photographs in the database were reviewed and additional fields of the database were populated with National Bridge Inventory (NBI) data, aerial photography, bridge plans, and inspection reports for a selected subset of bridges. The photographic archive also contains case studies to illustrate the use of these data sources to expand the archive for a typical bridge. Task 7, Laboratory Studies Using the typical debris configurations agreed on during the Interim Report meeting, the research team conducted the laboratory experiments according to the approved work plan. The goal of the laboratory testing was to provide sufficient data for a range of debris accumulations to develop adjust- ment factors to the HEC-18 pier scour equation (see Task 8). The adjustment factors considered included a correction factor to the overall equation (such as the Kw factor for wide piers) and an adjustment to the pier width used as an input variable to the equation (similar to the HEC-18 complex pier approach). The goal of the laboratory plan was to develop a series of tests for a wide range of debris configurations that could be run quickly and efficiently. The tests were performed for single debris clusters at individual piers, which is the primary type of debris accumulation identified by all regions in the (Task 2) survey. The majority of the testing was performed for clear- water sediment transport conditions. The testing encompassed a range of debris characteristics including debris accumulation shape, thickness, width, length, porosity, and roughness. The range of debris accumulation size that was tested in the labo- ratory is related to actual debris accumulations observed by the research team in the field or from the survey sources and the photographic archive. 7

Task 8, Develop Scour Prediction Methods Based on the results of Task 7, the research team developed methods for predicting the depth, shape, and lateral extent of scour at bridge piers resulting from debris accumulations. The methods are suitable for incorporation into HEC-18. Task 9, Submit Final Report The research team submitted a final report that documents the entire research effort. They also provided a companion executive summary that outlines the research results. 1.4 Research Results As a result of this research, bridge owners now have docu- mentation, guidelines, and analytical procedures to quantify the effects of debris-induced scour on bridges: • A fully documented database on debris and case studies, photographs, and data related to debris generation, move- ment, accumulation, and scour at bridges that can be used to inform and train design and maintenance personnel on debris-related hazards • Necessary guidelines for predicting the size and geometry of debris accumulations at bridge piers • Methods for quantifying scour at bridge piers resulting from debris accumulations • Guidance for incorporating debris effects in one- and two- dimensional hydraulic modeling • Worked example problems and a case study illustrating the application of the guidelines and analytical methods The end results are practical, implementable guidelines for bridge owners that enhance their ability to predict debris- related hazards at bridges and to design, operate, inspect, and maintain bridges considering those hazards. 1.5 Documentation Organization Findings from this research are available in three documents: • NCHRP Report 653 – Findings from the review of current practice and site reconnaissance – Overview of laboratory testing results – Interpretation and appraisal of findings and results – Conclusions and recommendations – Suggested research – Guidelines for predicting size and geometry of debris accumulations at bridge piers – Methodology for predicting scour at bridge piers with debris loading • NCHRP Web-Only Document 148 [available on the TRB website (www.trb.org) by searching for “NCHRP Web-Only Document 148”] – Photographic archive of debris at bridge piers – Survey of practitioners – Field pilot study report • Reference Document [available from the NCHRP Proj- ect 24-26 web page (http://144.171.11.40/cmsfeed/TRB NetProjectDisplay.asp?ProjectID=725)], which contains detailed laboratory testing results from Colorado State University 8

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 653: Effects of Debris on Bridge Pier Scour explores guidelines to help estimate the quantity of accumulated, flow event debris, based on the density and type of woody vegetation and river bank condition upstream and analytical procedures to quantify the effects of resulting debris-induced scour on bridge piers.

The debris photographic archive, the survey questionnaire and list of respondents, and the report on the field pilot study related to development of NCHRP 653 was published as NCHRP Web-Only Document 148: Debris Photographic Archive and Supplemental Materials for NCHRP Report 653.

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