Turbulent coherent structure under dam-break driven swash – A 3D numerical study

It is well-known that in the surf zone, wave-breaking-induced turbulent coherent structures referred to as obliquely descending eddies (ODEs) can impinge the bed enhancing sediment suspension processes. However, the flow characteristics in swash zone are different from those in the surf zone due to shallow depth and inundation intermittency. Sediment transport in the swash zone is dominated by bore turbulence (swash front) during the uprush, and by the growing boundary layer during the backwash. The evolution and impact of wave-breaking-induced turbulent coherent structures in the swash zone, particularly their characteristics during uprush and backwash, are investigated in this study through 3D large-eddy simulation. As a first step, the numerical model is validated using the measured data of O’Donoghue et al. (2010, Coastal Eng., 57(5), 513–530) for swash flow generated by dam-break waves. The model is validated with measured evolution of free surface, flow velocity profiles, statistics of turbulent velocity fluctuation and shoreline position. Simulation results demonstrate that turbulent coherent structures during uprush are different than the ODEs observed in the surf zone. However, these structures can still impinge the bed frequently and induce large bottom stress and near bed turbulence. On the other hand, turbulent coherent structures during backwash interact less frequently with the bed. However, the contour of bottom shear stress show interesting finger pattern aligned with onshore/offshore direction.