Current application of scour-prediction equations overestimates scour depths around abutments and in contracted openings at many locations. Such excessive scour-depth prediction results in construction of unnecessarily deep foundations or installation of unnecessary countermeasures. Current methodologies available for predicting scour around abutments are based on laboratory-derived equations and cover only a narrow range of the wide variation of field conditions that commonly occur. Development of realistic scour-prediction equations must be based, at least in part, on field data. The use of field data is vital to understanding scour processes, to developing physical and numerical model studies that represent field conditions, to developing sound guidelines for implementing scour-prediction methodology, and for improving scour-prediction equations.
Without an accurate field description of the dominant scour processes, the processes and associated parameters that may substantially influence scour during flood events cannot be accurately modeled in laboratory studies. Examples of processes identified in the field but not adequately represented in laboratory studies are the failure of spill-slope embankments into scour holes around bridge abutments, scour at piers affected by flow around abutments, pressure scour, and scour caused by flow redirected and contracted by ice or debris accumulations. The inclusion of these and other processes is likely to substantially change the methodology used to predict scour.
The objectives of this research are as follows: (1) based on field data, describe and quantify the degree of influence of processes affecting scour magnitude in contracted openings, (2) provide field data for use in verification of physical and numerical model studies, and (3) develop quantitative guidelines for applying scour-prediction methodology at contracted bridge sites for a wide range of common field situations. These objectives should be accomplished using real-time and post-flood data collected at sites where floods occurred prior to the initiation of the project and at sites where flood events occur during the project.
To accomplish these objectives, the following tasks are recommended: (1) Conduct a critical review of published and unpublished literature to identify problems in applying scour-prediction methodology to common field conditions around abutments and in contracted openings. (2) Contact state and federal agencies to develop a list of well-documented cases of scour-damaged bridges and bridges that have not sustained scour damage during floods. Select sites from this list to be used in Task 9. The selected sites shall represent a wide range of common field conditions. (3) Prepare and submit for review a technical memorandum describing the field equipment, field procedures, and data to be collected for evaluating total scour in bridge openings. The conditions considered in this document shall include but not be limited to the following: abutment scour, contraction scour, debris-influenced scour, scour in erodible and non-erodible material, scour in cobble and boulder bed streams, scour at complex or skewed piers, and pressure scour. Prepare generic field forms with detailed descriptions of the spatial and temporal distribution of the data to be collected. Estimate the accuracy of the measurements proposed; measurements and their associated accuracy must be consistent with what is needed to verify physical and numerical models, as well as to conduct field-based research. (4) Develop criteria for selecting and prioritizing bridges for real-time flood data collection. Prepare a plan for the following: (a) monitoring the United States for flood conditions, (b) identifying favorable measurement conditions, and (c) deploying equipment and personnel to sites throughout the contiguous United States and Alaska. Include the methods to be used for monitoring, names and phone numbers of all points of contact, criteria to trigger deployment of personnel, response times to various parts of the United States, and names, telephone numbers, and daily deadlines for all common carriers proposed to ship equipment and/or personnel. (5) Within 6 months of contract award, submit an interim report that discusses the information developed in Tasks 1 through 4. The interim report shall also contain a detailed work plan for completing the remaining project tasks as a stand-alone document. (6) Monitor the United States for flood conditions, deploy equipment and personnel to sites matching approved criteria, and collect real-time data as safety allows. Coordinate this task with personnel from Federal Highway Administration, United States Geological Survey, state departments of transportation, and other governmental agencies. (7) Collect field data at significantly damaged or heavily scoured bridges immediately after flood waters recede and at nearby sites where little or no scour damage has occurred. Include the data necessary to complete hydrologic, geomorphic, geotechnical, hydraulic, and sediment transport analyses. Obtain scour-depth information from highway agencies at sites where scour damage has been repaired. (8) After review and approval by the NCHRP, provide a report via the Internet within 6 months of each flood-event deployment. The report will include a summary of events and results, a description of the dominant scour processes observed, and a list of findings to be used in ongoing and planned model studies. Specific data analyses with these reports are not necessary. (9) Prepare a database containing real-time and post-flood data collected in Tasks 6 and 7. In addition, collect and include historical data for these sites and for the sites selected in Task 2. Information in this database shall be referenced to a common horizontal and vertical datum for each site. Prepare a summary of individual bridge data with available supporting historical, real-time, and post-flood data. This summary and the database shall be made available via the Internet. (10) Apply, to the Task 9 sites, current methodology (e.g., Hydraulic Engineering Circular 18
) for predicting scour at contracted bridge sites. Compare the computed scour values to the measured scour. Discuss and propose modifications to current scour methodology and guidelines where computed and measured scour values differ. Illustrate the applicability of the proposed scour-prediction methodology using at least 10 case studies that cover a wide range of common field conditions, including at least 3 highly contracted openings (more than 50% contraction). (11) Submit a final report that documents the entire research effort and includes, as a stand-alone document, recommended guidelines for applying the modified scour-prediction methodology at contracted bridge sites. Include high-quality graphics and photographs to assist highway engineers in recognizing field conditions.
Research is complete.
The research agency's final report, "Scour at Contracted Bridges," is available online as NCHRP Web Document 83