Indiana Potential Scour Index

When debris rafts accumulate against the face of a bridge, scouring effects may be accelerated by flow directed to the streambed. (Click image to enlarge)

In 1991, the USGS, in cooperation with the Indiana Department of Transportation (INDOT), began collecting data at 5,587 bridges in the State of Indiana that were built or maintained with federal aid. Initially, the data were collected to enable INDOT to identify bridges with existing channel instability or streambed-scour problems using the Simon method. Subsequently, however, it became necessary to develop an alternative method which, although similar to the Simon method, was more appropriate for the geographic provinces of Indiana.

This method is referred to as the “INDOT potential-streambed-scour index,” and is designed to help identify bridges that may be susceptible to the effects of scour. This index gives weighting points in each of four categories that may in part control the amount of streambed-scour that occurs at a given site: bed material, attack angle, debris accumulation, and contraction ratio.

Potential Scour Criteria

  •          Bed material characteristics
  •          Attack angle of high flow to bridge elements
  •          Debris accumulation potential at bridge opening
  •          Bridge opening contracting high flows


The total potential-scour score for a given site is calculated as the summation of the points from each category. The only exceptions are for sites where the bridge has been built on bedrock or is well protected by riprap. For these sites, the potential-scour score is set to 0. With this potential-scour weighting scheme, potential-scour scores can range from 0 (best case) to 100 (worst case). This index is limited by the fact that it does not take into consideration the overall stream geometry, bank materials, streamflow velocity, flood-channel width, and propensity for debris to accumulate at the bridge because of bridge design.

For more information specific to the Indiana streambed-scour index, see:

USGS publication WRIR 95-4264; Robinson, Bret A. and Thompson R.E., 1995, An observed-streambed-scour index for selected bridges in southwestern Indiana, 1991.

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