Large Eddy Simulation is used to investigate the main coherent structures playing a role in the erosion process (e.g., horseshoe vortex system forming around the base of the groynes, eddies shed in the separated shear layer) and the associated bed shear stress distributions around isolated and multiple groynes placed in straight channels. For isolated groynes we investigate the flow at conditions corresponding to the start of the scouring process (flat bed) and to its end (equilibrium scour bathymetry). Also, we consider the flow past two vertical groynes and we study the effect of groyne submergence (fully emerged vs. 40% relative submergence depth) on the horseshoe vortex system forming at the base of the upstream groyne and bed shear stress distribution in the groynes region. Large amplifications of the turbulence (e.g., resolved kinetic energy) inside the horseshoe vortex system and of the bed shear stress below are observed in all the cases. © 2008 Taylor & Francis Group, London.