Modeling tidal sand wave formation in a numerical shallow water model: the role of turbulence formulation

Tidal sand waves are prominent dynamic bottom features in shallow sandy seas. Up to now, the processes controlling the formation of these bedforms have only been studied in stability sand wave models, in which geometry, boundary conditions and turbulence models are schematized. In this paper we present simulations of sand wave formation and migration with a numerical shallow water model (Delft3D), in which we restrict us to bedload transport and analyse the initial formation stage only. First, it is shown that the reproduction of the basic sand wave formation mechanisms in a numerical shallow water model requires careful treatment of model geometry, initial profile, vertical resolution and lateral boundary conditions. Second, an intercomparison between the Delft3D model and a nonlinear stability sand wave model is performed. Next, we compare the results for two of the built-in turbulence models: constant vertical eddy viscosity model (commonly used in stability sand wave models) and a more advanced spatially and temporally variable vertical eddy viscosity model (k–ε turbulence model). Finally, the model results are compared with field data on sand wave length. The k–ε turbulence model shows good agreement with the field data, whereas the constant vertical eddy viscosity model overestimates the wave length of the sand waves considerably