Application of the Observation Method for Scour to Two Texas Bridges

Abstract

Bridge scour is a major cause of damage to bridge foundations and abutments. Approximately 17,000 scour-critical bridges exist in the United States. Scour-critical bridges are bridges with foundations that are unstable for calculated and/or observed scour conditions. This designation comes in part from the use of overconservative methods that predict excessive scour depths in erosion-resistant materials. Other available methods capable of overcoming this overconservatism are relatively uneconomical because they require site-specific erosion testing. This paper presents the assessment of two bridge case histories using the observation method for scour (OMS). OMS is a relatively new quantitative bridge-scour assessment method that accounts for time-dependent scour depth using field measurements. This method, which does not require site-specific erosion testing, was developed as a first-order assessment method for use in combination with routine bridge inspections. 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 vulnerability of the bridge to scour depends on the comparison between the predicted and allowable (threshold) scour depths. The case histories presented in this paper consist of two Texas bridges, one designated as scour critical and the other as stable by the Texas Department of Transportation (TxDOT). Both stable and scour-critical bridges were selected to test OMS and also compare it with TxDOT's scour designation. These case histories serve to demonstrate the validity and applicability of OMS to full-scale bridges and to provide practitioners with two potentially useful real-life case histories that could serve as examples for engineering practice. A validation process was performed on the two case histories using historical scour measurements and flow data. The validation exercise showed that there was good agreement between predicted scour depths using OMS and field measurements. OMS was then applied as a bridge-scour assessment tool to both bridges using the 100-year flood as the future flood and the outcome of OMS compared with the original TxDOT designation. As a result of this, the originally scour-critical bridge was found to be stable according to OMS. The bridge originally designated as stable was also found to be stable according to OMS.