Evaluation of Potential Bridge Scour in Montana

Problem:

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.

Objective:

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 (MDT) to address all three goals of the national program.

Approach:

The USGS Montana Water Science Center 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 selected bridges. At several of these, the collected data were used to calibrate and investigate use of a mobile-bed hydraulic model (BRI-STARS) for detailed scour analyses. Bridge scour data continue to be collected, with recent efforts including data collection associated with pier scour on coarse-bed streams. In addition to more conventional techniques that use sounding weights deployed from the bridge deck, acoustic transducers are also being used to collect scour data and to monitor scour at selected bridges. 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. 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.

Publications:

Holnbeck, S.R., 2005, Sediment-transport investigations of the upper Yellowstone River, Montana, 1999 through 2001: Data collection, analysis, and simulation of sediment transport: U.S. Geological Survey Scientific Investigations Report 2005-5234, 69 p.

Chase, K. J., and Holnbeck, S. R., 2004, Evaluation of pier-scour equations for coarse-bed streams: U.S. Geological Survey Scientific Investigations Report 2004-5111, 24 p.

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.

Status:

The rapid-estimation method was used to estimate scour at over 1,600 bridge sites on interstate and primary highways in Montana. Work continues in applying the method to over 1,600 bridges on secondary roads in the state. Fieldwork at the sites is also being conducted to investigate remnant scour from historic floods. On-site scour data collection and scour monitoring is continuing.  Montana bridge scour data have been 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 completed and a report published. Collection of pier scour data for coarse bed streams continues, and results will be summarized in a planned report.

Cooperator: Montana Department of Transportation

Project Chief: Stephen R. Holnbeck