Large Eddy Simulation (LES) is used to numerically investigate the horseshoe vortex (HV) system around a vertical bridge abutment located on a flat bed in a straight channel with vertical lateral walls corresponding to conditions at the initiation of the scour process. The simulation is performed with upstream fully turbulent flow including the turbulent fluctuations obtained from a precalculated LES simulation. The dynamics of the instantaneous coherent structures associated with the HV system around the base of the abutment and the spectral content of the flow in this region are analyzed. Due to the flat bed conditions, for which the HV system is not stabilized by the presence of the scour hole, very large random variations in the structure, position, size and overall intensity of the turbulent HV system are observed. In particular, the HV system appears to oscillate between a zero-flow mode in which the main HV is situated closer to the abutment and the near-bed jet flow beneath it is weak and separates early, and a back-flow mode in which the near-bed jet flow is stronger and separates at a larger distance from the abutment, and the main HV eddy is larger. It is observed that the legs of the horseshoe vortices can interact, at times, with the eddies shed inside the detached shear layer (DSL). The distribution of the bed shear stress shows that the largest values are present in the strong acceleration region near the tip of the abutment, but high bed shear stress values are observed beneath the HV system. It is found that the bed shear stress fluctuations around the local mean values can be very high, especially in the region beneath the separated shear layer. The pressure fluctuations and resolved kinetic energy levels are found to be very high inside the HV region compared to the surrounding flow. These high values are produced primarily by the low-frequency chaotic switching of the HV system between the zero-flow mode and the back-flow mode. © 2007 ASCE.