Effects of time and channel geometry on scour at bridge abutments

Experiments are described to investigate local scour at bridge abutments. The experiments were performed in a two-stage channel using abutments that extended different distances onto the floodplain including right up to the edge of the main channel (Melville, Type III). To ensure the largest scour depths the conditions on the floodplain upstream of the abutment were close to critical conditions for the bed material. The time evolution of the scour and the ultimate scour depth were measured. The time development of the local scour corresponded well with the theories of Ettema and Franzetti and the theory of Whitehouse for scour at horizontal cylinders in the marine environment. Melville has suggested that scour at abutments on floodplains can be approximated by scour in rectangular channels if an imaginary boundary is assumed, separating the flow in the main channel from that on the floodplain. The experimental results confirm the validity of Melville's suggestion for the configurations tested in the experiments.