Clear water abutment scour in a compound channel for extreme hydrologic events

Peak discharges during large floods can often result in submerged orifice flow (also called “pressure flow”), or embankment and bridge overtopping flow, in which the embankment and abutment foundation of a bridge are subjected to severe scour and possible failure. In this study, abutment scour experiments were carried out in a wide, laboratory compound channel to investigate the characteristics of abutment scour for the three flow types of free, submerged orifice, and weir (overtopping) for an erodible but riprap-protected embankment and abutment. Detailed bed contours and three-dimensional (3D) velocities and turbulence quantities were measured with acoustic Doppler velocimeters. The results show that the contracted flow around an abutment (due to lateral or vertical flow contraction) and the local turbulent structures near the downstream face of the bridge are the main features of the flow field responsible for the maximum scour depth near the abutment. Experimental results for combined abutment and contraction scour depth are presented in terms of the theoretical long contraction scour for all three types of flow. The experimental results for maximum scour depth are compared with those of other investigators, and it is shown that the erosional strength of the embankment defines an upper and lower bound for maximum scour depth for a solid abutment versus a riprap-protected embankment and abutment