Scour of the streambed in the vicinity of bridge piers and abutments during floods has resulted in more bridge failures nationally than all other causes in recent history. Sudden bridge failure due to scour may result in tragic loss of life as well as large economic loss.

To address the problem of bridge scour on a national basis, the Federal Highway Administration (FHWA) established in 1991 a program to (1) conduct scour-related research and data collection, (2) improve methods for evaluation of scour, and (3) identify potentially scour-critical bridges on federal-aid highways. In Montana, the USGS initiated a program in cooperation with the Montana Department of Transportation (DOT) to address all three goals of the national program.

The Montana USGS began a program to assess scour using detailed hydraulic analyses at about 80 key bridges. In addition, the USGS began collecting on-site scour data at about 6 bridges. At several of these, the collected data will be used to calibrate and investigate use of a mobile-bed hydraulic model (BRI-STARS) for detailed scour analyses. The USGS also developed a method for the rapid-estimation of scour that could be used to quickly assess scour potential at a large number of sites by a deadline date set by the FHWA. This rapid-estimation method was intended for use as a screening tool that would quickly identify which bridge sites might require more detailed future hydraulic analyses.

Holnbeck, S.R., and Parrett, Charles, 1997, Method for estimation of scour at highway bridges based on limited site data: U.S. Geological Survey Water-Resources Investigations Report 96-4310, 79 p.

Holnbeck, S.R., and Parrett, Charles, 1996, Rapid-estimation method for assessing scour at highway bridges based on limited site data: U.S. Geological Survey Fact Sheet FS-244-96, 4 p.

The rapid-estimation method was used to estimate scour at over 1,500 bridge sites in Montana. Work is ongoing on completion of detailed hydraulic scour assessments at about 15 sites. On-site scour data collection is continuing.  Montana bridge scour data were entered into the Bridge Scour Data Management System (BSDMS), as part of a national effort to report bridge scour data. Assembly of existing data pertaining to scour measurements at piers in coarse bed streams was begun. Pier scour data were collected at a site having coarse streambed material and a site having contraction scour under clear-water conditions.