Abutment scour at small, severely contracted bridges

Abutment scour at small, severely contracted bridges is not specifically addressed in current scour guidelines. Many of the abutments at these bridges are vertical and set along the main channel banks. The long roadway approach section and narrow bridge opening force floodplain waters to re-enter the main channel at the bridge, causing a severe contraction in flow area that results in both contraction and local scour. Current scour guidelines assume that contraction and local scour processes are independent and are determined separately and summed for a total scour depth. Because of the severe contraction in flow area, independent scour processes cannot be assumed. Thus, the practice of assuming independence may result in significant over-estimations of scour depth at severely contracted bridges. In this study, a relatively new scour model, ABSCOUR, is tested to determine its ability to provide more realistic scour estimates, at severely contracted bridges by accounting for flow non-uniformity in a single total scour equation. The program format is also examined to determine its applicability to a wide range of environments. The results showed that scour depth predictions at prototype bridges were excellent, and that program applicability to a variety of environmental conditions was promising.