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Flow and turbulence structure around abutments with sloped sidewalls

Köken, Mete
Results of eddy-resolving numerical simulations are used to investigate flow and turbulence structure around an isolated abutment with sloped sidewalls at conditions corresponding to the start (flatbed) and the end (equilibrium bathymetry) of the scour process. Besides cases where the abutment is not protected against scour using riprap, the paper considers cases where a riprap apron of constant width is present around the base of the abutment at the start of the scour process. The paper also discusses the effect of large-scale turbulence (e.g., horseshoe vortex system, eddies shed in the separated shear layer, vortices forming in the recirculation regions) on sediment entrainment and transport. Using information about the position and coherence of the large-scale turbulent structures present in the vicinity of the bed surface, the paper discusses some of the reasons for the very different bed scour patterns observed at equilibrium scour conditions for cases without and with a riprap apron. The effect of the degree of bluntness of the upstream side of the abutment on flow and turbulence structure is discussed based on comparison of results obtained for abutments with sloped sidewalls with those obtained for a vertical-wall abutment of similar length. The later type of abutment is characterized by a stronger downflow, the formation of a more coherent horseshoe vortex system, and the formation of a deeper scour hole.