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Bridge Scour

Study Area: Statewide
Period of Project: Ongoing
Project Number: CO251
Project Chief: Mike Stevens
Cooperator: Colorado Department of Transportation (CDOT)

BACKGROUND:

The most common cause of bridge failure is structural instability caused by the scouring of streambed material around bridge foundations during floods. Annually, the cost is estimated to be about $50 million for repair of flood damage to bridges on the Federal-Aid system for major highways. Empirical scour evaluation methods can be unreliable and do not always provide acceptable predictions for local scour around piers and abutments at existing bridges.

All bridges on spread footings or with unknown foundations have been evaluated by the Colorado Department of Transportation (CDOT) for scour susceptibility. A number of the bridges are indicated to be scour critical and require some type of countermeasure. One acceptable countermeasure for bridges listed as scour critical is to monitor a cross section at a bridge during and/or after a flood to detect any significant scour that might lead to bridge failure. If cross-section monitoring is selected as a countermeasure, an action plan could be implemented that includes an alert notification process, flood watch process, and inspection or assessment procedures.

Development of a reliable scour monitoring system would greatly enhance the transportation communities capability to ensure public safety and determine the need and timing of road closures or for installing more costly permanent countermeasures.
The extent (depth) of scour can vary depending on the magnitude and duration of a flood event. The scour that occurs during a 50- to 100-year flood event during a 2-3 day period (rainfall runoff) can be substantially less than that of a 25-year event that occurs over a time duration of several weeks (snowmelt runoff). To document the observed maximum scour at a site, it is necessary to include scour observations and data collection for a range of streamflows and durations.

OBJECTIVES:

The objectives of this study for a multi-year period are:

1. To determine the magnitude of local scour at selected scour critical bridges over a range of streamflows
2. To provide scour information that can be used by CDOT to evaluate bridge safety and possible road closures during high streamflows
3. To provide additional data to the statewide and national scour database for comparison of predicted scour depths versus observed depths. Information contained in the national scour database can be used to determine the applicability of recommended scour equations to Colorado.

APPROACH

The approach is to choose five bridges per year from the CDOT scour-critical list for monitoring during the snowmelt period. Low-water surveys of bridge geometry, bed-material composition, and channel configuration will be completed prior to snowmelt if possible. Cross-sections profiles and streamflow will be measured at the upstream side of each bridge six times during the peak snowmelt period.