Time Scale for Contraction Scour at Bridges

Several methods exist for predicting rates of pier and abutment scour. Contraction scour equations are developed using sediment transport equations for live-bed conditions and initiation of motion concepts for clear-water conditions. This paper extends these concepts to predict the rate of contraction scour primarily at tidal bridges under storm surge conditions. The method is incorporated into a spreadsheet and can be used for either clear-water or live-bed conditions. The ultimate scour predicted with this approach compares very well with the standard contraction scour equations and, depending on the amount of upstream sediment supply, can occur within a few hours to several months.