Time evolution of the horseshoe vortex system forming around a bridge abutment

Scour forming around an isolated bridge abutment with curved end is investigated in this study. Clear water scour experiments are conducted for an abutment in a 25 m long sediment flume at a Reynolds number of 45000. 3D scour patterns forming around the abutments are obtained using an array of acoustic transducers along a grid refined up to 1-2 cm spacing at different stages of the scour. Detached Eddy Simulation (DES) is performed at the same channel Reynolds number for flat bed case (initiation of the scour) and for two deformed bed cases (intermediate stages of scour). Incoming flow in the simulations were fully turbulent containing unsteady velocity fluctuations. Variations in the structure and intensity of the horseshoe vortex system are investigated. At the initial stage of the scour the main and secondary horseshoe vortices undergo aperiodic bimodal oscillations. Those oscillations cause the horseshoe vortices to induce large bed shear stress values beneath them. As the scour hole starts forming secondary necklace vortex, HV2, gets closer to the primary necklace vortex, HV1, and merges with it at a location close to the abutment tip. © 2010 American Society of Civil Engineers