PROGRAMS AND PLANS--Guidelines for the Transmittal of Bridge-Scour Information to Cooperators In Reply Refer To: November 18, 1991 Mail Stop 415 OFFICE OF SURFACE WATER TECHNICAL MEMORANDUM NO. 92.03 Subject: PROGRAMS AND PLANS--Guidelines for the Transmittal of Bridge-Scour Information to Cooperators This memorandum provides guidelines for the transmittal of bridge- scour data and information to cooperators. Many districts are conducting investigations of channel-bed scour at bridges to provide information and understanding to cooperators to promote public transportation safety. The specific objectives of these investigations are diverse; however, they may generally be grouped into those that improve scour-prediction techniques and those that evaluate scour potential for existing structures. Investigations to improve scour-prediction techniques and to improve definitions of scour processes include collection of channel-geometry data during normal and flood-flow conditions, as well as hydraulic, bed-material, and bridge-geometry data. After appropriate review, data from these investigations may be transmitted under a cover letter to cooperators. Of course, cooperators may be notified immediately of potentially hazardous bridge-scour conditions. Interpretive results from these studies will be transmitted to cooperators in Director-approved reports. Investigations regarding scour potential and channel instability at selected bridges provide that information at three levels of detail. The first level of detail consists of qualitative and quantitative data collected during a site visit and summarized to provide an index to assess scour potential. The data are fluvial- geomorphic information and bridge characteristics related to bridge-scour potential. The data may be summarized into an observed scour index (for observed scour conditions) and/or a potential scour index. It is important to note that the potential scour index is not for a boundless range of flow conditions, but is typically limited to bank-full flood conditions, which may be envisioned by the assessor without the benefit of hydraulic analysis. Over 15,000 of these channel instability assessments are being done in ongoing studies. Many existing studies have used the assessment form and index developed for West Tennessee and described in a Director-approved 1989 proceedings article by Simon, Outlaw, and Thomas. Accuracy, transferability, and other issues related to scour assessment procedures are being investigated by a work group composed of scour assessment project chiefs and organized by the Office of Surface Water. Basic data from the level-one assessments may be transmitted under a cover letter to cooperators provided that the data are reviewed for adequacy and accuracy. Developing a procedure to summarize these data into an index requires judgment and interpretation. Thus, the procedure must be documented in a Director-approved report before transmittal of scour indices for selected bridges. Modifications to an existing procedure must also be documented in a Director-approved report. The second level of detail includes information collected in the level-one analysis, estimation of flood magnitude for specified recurrence intervals, a step-backwater computation to obtain hydraulic characteristics for these floods, and use of specific empirical equations to estimate local scour. Cooperators use this information and their bridge foundation data to analyze the susceptibility of bridges to scour and to determine where scour countermeasures are needed. About 600 of these scour evaluations will be done in ongoing investigations. The information from this level scour evaluation is rather analogous to the results of indirect measurements of peak discharge. These scour evaluations follow a specified procedure which usually does not require hydrologic interpretation. Following regional review, information from these evaluations may be transmitted under a cover letter. If undocumented procedures are used in a level-two evaluation, it should receive further review and be transmitted in a Director- approved report. The level-three evaluations are based upon sediment transport modeling of the stream through the bridge site. About 30 of these analyses are planned in ongoing investigations. Results from these analyses will be transmitted to cooperators in Director- approved reports. Ernest F. Hubbard Acting Chief Office of Surface Water DISTRIBUTION: A, B, S, FO, PO