Transport of suspended sand and mud over a mud-sand bed

This study is focused on the experimental and numerical modelling of sand and mud transport over mud-sand beds with percentages of fines (< 63 μm) up to 50% in conditions with currents, waves and combined currents and waves. Both field and laboratory experiments with mud-sand beds have been performed. Detailed measurements of near-bed hydrodynamic and sediment transport processes have been made in the muddy tidal ferry channel between Holwerd and Ameland in the Dutch part of Wadden Sea. Laboratory flume experiments with currents and waves over a pure fine sand bed show the generation of small-scale sand ripples and strong ripple-induced vortex motions resulting in relatively high sand concentrations close to the bed. The near-bed sediment dynamics of a fine sand bed change drastically when a small amount of cohesive sediments (mud 10%–15%) is added to the sand bed. Bed properties which are changed are the percentage of fines, the dry bulk density (packing) and the cohesivity. The results of exploratory long-bed experiments with various mud-sand mixtures show that the mud particles at the mud-sand surface are washed out and small-scale isolated barchan-type sand ripples develop at the bed surface. The bed ripple heights are suppressed resulting in flatter ripples with less vorticity and as a consequence lower sand concentrations and transport. The critical bed-shear stress (cbs) is not much influenced by cohesive effects if the percentage of fines (< 63 μm) is smaller than about 15%, while for pfines > 15%, the critical bed-shear stress increases for increasing values of pfines. Laboratory results also indicate that the bed ripple development and near-bed sand transport may already be affected for a lower percentage of fines (10%–15%). Various modelling methods are used and discussed, both for the flume and field data.