Effect of wave–bedform feedbacks on the formation of, and grain sorting over shoreface-connected sand ridges

Abstract

The influence of wave-bedform feedbacks on both the initial formation of shoreface-connected sand ridges (sfcr) and on grain size sorting over these ridges on micro-tidal inner shelves is studied. Also, the effect of sediment sorting on the growth and the migration of sfcr is investigated. This is done by applying a linear stability analysis to an idealized process-based morphodynamic model, which simulates the initial growth of sfcr due to the positive coupling between waves, currents, and an erodible bed. The sediment consists of sand grains with two different sizes. New elements with respect to earlier studies on grain sorting over sfcr are that wave-topography interactions are explicitly accounted for, entrainment of sediment depends on bottom roughness, and transport of suspended sediment involves settling lag effects. The results of the model indicate that sediment sorting causes a reduction of the growth rate and migration speed of sfcr, whereas the wavelength is only slightly affected. In the case where the entrainment of suspended sediment depends on bottom roughness, the coarsest sediment is found in the troughs; otherwise, the finest sediment occurs in the troughs. Compared to previous work, modeled maximum variations in the mean grain size over the topography are in better agreement with field observations. Settling lag effects are important for the damping of high-wavenumber mode instabilities such that a preferred wavelength of the bedforms is obtained.

A Morphodynamic Modeling Study on the Formation of the Large‐Scale Radial Sand Ridges in the Southern Yellow Sea

“…The modeled sand ridges are more irregular than the simulated ones, because of the homogeneous bed sediment configurations used in the present model. On the other hand, processes such as sediment sorting and sand‐mud interactions should be addressed in future research, which can play an important role on the morphodynamic evolution of sand ridges (De Swart et al, ; Vis‐Star et al, ). Winds and wind‐driven waves are not considered. Previous studies have shown that wind‐driven waves affect the morphology of sand ridges (Calvete et al, ), particularly during low tidal levels (Reynaud et al, ).…”

Formation of rhythmic sorted bed forms on the continental shelf: an idealised model

“…We include hiding, because previous studies on grain-sorting phenomena (Seminara et al 1996;Walgreen et al 2003;Vis-Star et al 2009;Van Oyen & Blondeaux 2009) show that it is an important factor controlling the transport of a sediment mixture and the grain size distribution. The sediment transport due to bedslope effects is quantified following Kovacs & Parker (1994),…”

“…On the inner shelf, sediment sorting patterns along shoreface-connected ridges have been studied by Walgreen, de Swart & Calvete (2003), de Swart et al (2008 and Vis-Star, de Swart & Calvete (2009). These bed forms differ from sorted bed forms both in length and orientation.…”