Observation Method for Estimating Future Scour Depth at Existing Bridges

Abstract

Bridge scour can cause damage to bridge foundations and abutments. Bridges with foundations that are unstable for calculated and/or observed scour conditions are termed scour-critical bridges. There are approximately 17,000 scour-critical bridges in the United States. This designation comes in part from the use of overly conservative methods that predict excessive scour depths in erosion-resistant materials. Other methods capable of overcoming this overconservatism are relatively uneconomical because they require site-specific erosion testing. This paper proposes a new bridge scour assessment method. The new method, termed the observation method for scour (OMS), was developed for the Texas Department of Transportation's statewide bridge scour assessment program. The proposed method does not require site-specific erosion testing and accounts for time-dependent scour in erosion-resistant materials. OMS was developed for use as a first-order assessment in combination with a routine bridge inspection program. OMS uses charts that extrapolate or interpolate measured scour depths at the bridge to obtain the scour depth corresponding to a specified future flood event. The scour vulnerability depends on the comparison between the predicted and allowable scour depths. This paper also includes a new hydraulic-hydrologic analysis procedure for the determination of flow parameters required in OMS. This procedure was developed specifically for the state of Texas. The new hydraulic-hydrologic analysis procedure could possibly be applied to other regions that have sufficient flow gauges. The nine case histories used to validate OMS showed good agreement between predicted and measured values. OMS was then applied to 16 bridges, 10 of which were scour-critical bridges that had sufficient information for OMS to be carried out. Six out of these 10 bridges were found to be stable according to OMS.