Evolution of Three-Dimensional Flow Field and Vortex Structures around a Single Spur Dike during the Scouring Process

Large eddy simulation (LES) was conducted to better understand the evolution of the flow field and vortex structures near a single spur dike. The fixed-bed boundaries used in the simulations were reconstructed from the topography measured during the experiments at several representative times. The numerical results showed that the presence of a scour hole increases the passage area for the flow near the bed such that more fluid in the upper layer is transformed into the downflow, resulting in a stronger near-wall jet. This enhanced jet weakens the rotational strength of the primary horseshoe vortex (HV1), causing it to be dominated by the backflow mode. With further development of scouring, HV1 is able to regain its strength and stability. In addition, a spiral flow with an approximately vertical axis of rotation is gradually formed owing to topographic changes. The low-pressure core near the bed inside this spiral flow results in the development of excess pore water pressure in the sand layer, which could decrease the effective shear stress in the sand layer and cause upward seepage.