By Erick M. Boehmler

What is Scour?
Scour is a general colloquial term used to identify the process of streambed erosion under and near bridges. Scour occurs when moving water lifts, and rolls or carries sand and rocks (streambed material) in the streamflow direction. During certain flow conditions, material is carried away by the moving water and the streambed erodes. This process of streambed erosion or degradation is called general scour.

Under most natural conditions, the general scour process occurs gradually. However, scour is enhanced, in some cases, at bridges over waterways because of the constant interaction of water with the structure. In addition to general scour, there are two other types of scour that occur at bridge sites, contraction scour and local scour.

Contraction scour is a process of streambed degradation which results from the acceleration of water approaching and flowing under a bridge. This process occurs when the width of the bridge opening is narrower than the width of the waterway approaching the bridge. An example of the flow acceleration may be observed by putting a finger over the end of a running garden hose. Similarly, water approching a narrower bridge accelerates. Water that flows more quickly can lift, and carry or roll more sand and rocks out from under a bridge.

Local scour also is a process of streambed degradation. However, it results from turbulence in the water, which occurs mostly at the walls supporting a bridge. An example of the turbulence is similar to that of a tornado. Just as tornadoes are formed in air by the interaction of wind blowing in different directions at different altitudes, eddies or whirlpools are formed in water by the interaction of water flowing in different directions at different depths. In the same way that objects may be lifted from the ground and thrown a mile away by a tornado, eddies lift sand and rocks from the streambed near the walls that support a bridge and carry them out from underneath the bridge.

Our Mission, This Project, and You
The mission of the U.S. Geological Survey is to provide sound scientific information to Federal, State, and local officials, and the public regarding scour processes that pose a potential threat to public safety. The results of this project increase the awareness and understanding of the interaction between human activities and the environment of rivers and streams. Ultimately, such information helps State and local authorities minimize and reduce the adverse effects of scour and stream instability on bridges and improves the safety of Vermont residents.

One of the more extreme examples of scour and its effects was the collapse of the Interstate Route 90 bridge over Schoharie Creek in New York State during a flood in 1987. Ten lives were lost and traffic was routed around the site during a lengthy reconstruction period, costing millions of dollars. The Vermont scour project is being done in response to such costly tragedies so that hazardous scour situations might be avoided at bridge sites throughout the state.

Our Assessment Process
Bridge scour potential is being evaluated at bridge sites in Vermont by use of a two-leveled approach. The first level is an assessment of the stream channel. The assessment consists of an evaluation of stream channel features that are indicators of the stability or instability of the channel. In the stream reach near a bridge, changes in the channel shape, changes in channel depth, the degree of channel bends, the accumulation or absense of excess streambed material at channel bends, the extent of densely rooted vegetation on the banks, and changes in channel width and bank slope are evaluated. These indicators provide evidence of channel movement or instability. The channel under a bridge also is examined for remnant scour holes along the walls supporting a bridge and exposure of the foundation of the bridge.

Information from the first-level evaluation is considered in part with the second level of the assessment process. In the second level of the assessment, the degree of streambed erosion that might be expected under a bridge during a rare flood is determined. This is done by applying a computer program designed to model the flow of water under a bridge. The model results are used to evaluate the depth of streambed material that may be removed from underneath the bridge.

In Vermont, approximately 400 of the 2,400 bridges over waterways were selected by the Vermont Agency of Transportation for evaluation of scour potential. The bridge sites are examined by a team consisting of personnel from the fields of engineering, hydrology and geology from our New Hampshire / Vermont District office in Pembroke, N.H. Reports of the stream stability and scour assessments are on file with the Vermont Agency of Transportation in Montpelier. All reports have been completed.

Conclusion of Project
Collection of historical data, Level I and Level II field data for all 400 scour susceptible bridges in Vermont is complete. Flood frequency estimates and analyses of grain size distributions at each of the 400 sites is also complete. Quality assurance checks and processing of Level I data into report format has also been completed for the 400 sites. Hydraulic models and scour analysis have also been done for 400 sites. Fourhundred reports (one report per bridge) have been approved and released as Open-File Data Reports.

Vermont Bridge Scour Open-File Reports listing is available in .pdf format (162 KB), published 1996-1998.

More Information
For questions regarding this project, contact

Kenneth Toppin: Supervisory Hydrologist
New Hampshire / Vermont District
U.S. Geological Survey, Water Resources Division
361 Commerce Way
Pembroke, NH 03275-3718
Phone: (603) 226-7800

Related USGS links:
Geology -- Water -- Geography