Modeling the formation and migration of sand waves: The role of tidal forcing, sediment size and bed slope effects

Abstract

Tidal sand waves are rhythmic bedforms existing widely in shallow shelf seas and are formed by the interaction of tidal currents and topography. Using a process-based numerical model, Delft3D, the wave lengths and migration rates of sand waves were simulated and verified with field measurements. The physical mechanism that controls the evolution of sand waves was mainly the balance between bedload transport, suspended load transport and the slope effect. It was found that the bedload transport multiplier, which reflects the bed slope effect, was a key parameter to reproduce the observed sand wave dynamics accurately. If the bedload transport multiplier is tuned with the actual grain size, it fits the observations on wavelength of sand wave much better. Both the migration rates and wavelengths were better predicted by the process-based numerical Delft3D model compared to a linear stability analysis sand wave model, since the former adopted sophisticated process formulations necessary for accurate field predictions. Next, sand wave formation and evolution under different environment settings, including tidal forcing and sediment sizes, were examined systematically. It was found that the preferred wavelength (LFGM, fastest growing mode) of the sand wave increased with increasing tidal current magnitudes and decreasing sand diameters. Sand waves were only formed within a certain range and combination of tidal current magnitude and sand diameters. Downstream- and upstream-migration of sand waves were predicted by considering residual currents or tidal constituent of higher harmonics.