Scour around Bankline and Setback Abutments in Compound Channels

Prediction of bridge abutment scour is particularly difficult in compound channels because of the effects of the flow interaction between the floodplain and main channel superimposed on the flow obstruction, and contraction caused by the bridge abutment and highway embankment. Previous laboratory studies have focused on rectangular channels, although it is generally recognized that compound channels can have an additional geometric influence on the maximum clear-water scour depth. It is the purpose of this paper to integrate experimental results for clear-water abutment scour for different compound channel geometries by taking into account the flow redistribution that occurs between floodplain and main channel in the contracted section created by the bridge opening. In addition, sediment size, bridge backwater, abutment shape, and approach velocity and depth are included in a proposed scour depth relationship. Both setback and bankline abutments are considered. Experimental evidence is presented for the characteristic nonuniform lateral velocity distributions associated with compound channels in the bridge approach section as affected by backwater, and for the entrainment of floodplain flow into the main channel between the approach and bridge opening cross sections. Scour depth patterns are shown as remnants of the floodplain flow acceleration and entrainment process. It is demonstrated that a common relationship for scour depth can be developed for both setback and bankline abutments provided that a consistent definition of scour depth is utilized. The experimental results are applied successfully to a field case of measured abutment scour, but the need for additional high-quality, real-time field data is emphasized.