Author(s): Xiaozhou Xiong; Brucew. Melville; Heide Friedrich
Keywords: Abutment; Compound channel; Overtopping; Live-bed; Sand dunes
Abstract: A laboratory study of total scour around spill-through abutments situated on the floodplain of compound channels is presented, for situations where both local scour and contraction scour occur together. The experiments include abutments near the main channel bank and abutments set-back from the main channel. Free surface and overtopping flow conditions were investigated in a 45-m long recirculating laboratory flume. The tests were characterized by a livebed scour regime in the main channel (0. 46-m wide) and clear-water scour regime on the floodplain (1. 08-m wide) .The test models were designed to represent a two-lane bridge at a 1: 45 geometric scale. Roughness elements on the floodplain, both upstream and downstream of the mobile section, were used to adjust the flow distribution. Riprap rocks were placed around the abutment, based on the FHWA-NHI-09-112 (US) guidelines. Five ultrasonic depth-sounders were used to obtain scour development in the vicinity of the abutment toe, the dynamic-equilibrium scour depth, detailed bed-scour contour plots and sand dune heights around the abutment. A 3D down-looking velocimeter and a side-looking acoustic Doppler velocimeter were used to measure the flow distribution upstream of the abutment. For tests with a short set-back abutment, the development and propagation of the sand dunes significantly affect scour development and the final scour bathymetry around the abutment, while for tests with a long set-back abutment, the effect of sand dunes is negligible and the effect of local turbulence on scour is minimal. For overtopping flow conditions, the countermeasures selected using the FHWA-NHI-09-112 (US) guidelines, were found to be inadequate. The need for additional experiments, quantifying the effect of overtopping and sand dunes propagation, is emphasized.