WHAT IS SCOUR?
Scour is the removal of sediment (soil and rocks) from streambeds and streambanks caused by moving water.
Although scour may occur at any time, it is usually more significant during high flows, when the water is
swift and deep. Swiftly moving water has more energy (turbulence and velocity) to lift and transport
sediment than slowly moving water.
Three areas at bridges are affected by scour:
- At pier and abutment foundations. (Piers are pillars supporting a bridge.
Abutments are the supports at each end of a bridge.) Commonly, the water flows
faster around piers and abutments making them susceptible to local scour.
- Across a bridge opening. Contraction scour occurs when water accelerates as
it flows through a bridge opening that is narrower than the channel and floodplain
upstream from the bridge.
- Throughout a large section of stream, both upstream and downstream from a bridge.
This degradation in a stream is usually a long-term process that results in lowering
the streambed.
WHY BE CONCERNED ABOUT SCOUR?
If too much sediment supporting bridge piers and abutments is scoured by a stream, then the bridge
could fail or become unsafe for travel. Many bridge failures have been caused by scour, some of which
resulted in deaths. In 1987, the Interstate 90 bridge over Schoharie Creek in New York failed because
of scour, and 10 people lost their lives. On March 10, 1995, the Interstate 5 bridge over Arroyo Pasajero
in California collapsed, and seven people died. After the Schoharie Creek failure, the Federal Highway
Administration mandated every state to assess bridges for existing scour and to identify sites where scour
may become a problem. Iden- tification of existing and potential scour-susceptible bridges will assist the
Pennsylvania Department of Transportation (PennDOT) in monitoring and correcting problems before bridges
fail or become unsafe.
WHAT'S BEING DONE ABOUT SCOUR AT BRIDGES?
In response to that mandate, the PennDOT and the U.S. Geological Survey (USGS) initiated a cooperative
study to examine approximately 16,000 bridges throughout the Commonwealth of Pennsylvania, which are
greater than 20 feet in length and over water.
The PennDOT manages the bridge-scour evaluation program for all State- and locally owned bridges over
water. Through review of their Bridge Management System database and consultation with their District
Offices, the PennDOT identified approximately 2,200 bridges scattered around the state as having potential
for scour. These bridges are the first group of sites that are being evaluated by the USGS.
A scour assessment by the USGS begins in the PennDOT District Office responsible for maintaining the
bridge being assessed. Bridge plans, historical inspections, and available maintenance records are
reviewed.
A field team then visits the site and collects the following information:
- Site characteristics including the location and appearance of stream bends (meanders), amounts of
woody vegetation stabilizing the streambanks, type of sediment in the streambed and streambanks, and
any evidence of scour.
- Survey of the bridge features and the channel upstream and downstream of the site.
- Precise bridge location using Global Positioning Satellite (GPS) data.
- Video of the bridge and channel to document existing site conditions.
The information gathered during the scour assessments is transferred to a computerized database and
used to identify a bridges potential for scour. The PennDOT then uses these USGS results, in concert
with information gathered by the PennDOT personnel, to prioritize remediation as needed.
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