Influence of time- and depth-dependent eddy viscosity on the formation of tidal sandwaves

Existing model studies on sandwave dynamics point to the importance of turbulence, and particularly its temporal and spatial structure. We present an idealized model of sandwave formation, in which turbulence is accounted for by adopting a time- and depth-dependent eddy viscosity. Following linear stability analysis, the basic and perturbed flow solutions are obtained using a spectral method involving eigenfunctions that simplify the vertical stress term. First model results show favourable convergence properties of the basic state, expressing the generation of higher harmonics due to the time-dependent eddy viscosity. The perturbed state, which requires further investigation in detail, reveals large flow gradients close to the bed.