Effect of grain size sorting on the formation of shoreface-connected sand ridges

Abstract

Field data of shoreface-connected ridges show persistent spatial variations of mean grain size over the bed forms. In the shore-normal direction, the profiles of bottom topography and mean grain size are approximately 90° out of phase. To investigate the mechanisms responsible for the observed grain size distribution and the influence of sediment sorting on the temporal and spatial characteristics of shoreface-connected ridges, a model is developed and analyzed. A linear stability analysis of an alongshore uniform basic state (describing a storm-driven flow on a microtidal inner shelf) with respect to small bottom perturbations is carried out. The transport of nonuniform sediment is described by formulations for both bed load and suspended load, both of which account for dynamic hiding effects. A one-layer model for the bed evolution and a bottom friction term, which depends on the grain size, are used. The initial formation of the ridges is studied for a bimodal sediment mixture. The results of the model indicate that the phase shift between bed topography and mean grain size for shoreface-connected ridges is due to the selective transport via suspended load of grains with different sizes. A net stabilizing effect on the growth of bed forms and enhanced migration are predicted, caused by the bimodal character of the sediment. The wavelengths of the bed forms are only slightly affected. Including a tidal current or a grain size dependent formulation for the bottom friction has no effect on the results. A physical explanation for the model results is also given.