Nonlinear modelling of shoreface-connected ridges; Impact of grain sorting and interventions

Abstract

The evolution of shoreface-connected sand ridges on micro-tidal inner shelves and the variations in the mean grain size over these ridges are investigated with process-based models. A review of previous studies is presented, as well as new results that concern the influence of grain sorting on the finite-amplitude behaviour of the ridges, the application of the model to La Barrosa beach and the role of wave-topography feedbacks. The ridges initially form due to morphodynamic self-organisation, in which the presence of waves and a storm-driven current are crucial. Predicted growth time scales, migration speeds, topography and spatial pattern of the mean grain size agree with field data collected on micro-tidal shelves in the case that both bedload and suspended load sediment transport are accounted for, together with spatially non-uniform wave orbital motion. The model can not successfully explain the presence of large-scale ridges observed on La Barrosa inner shelf, because strong and complex behaving tidal currents occur in that area. Nonlinear model simulations show that on the long term the height of the ridges evolves towards a finite, constant value, whilst their migration speed hardly changes during the evolution. In the saturated stage the ridges have asymmetrical profiles, with steep slopes on the downstream sides. The maximum variation in mean grain size also tends to a constant value and during the evolution the spatial lag between the patterns in the mean grain size and topography decreases. The processes that cause these changes are identified and explained. Model results can be obtained for transverse bottom slopes up to 50% of their observed values on micro-tidal shelves. Extrapolation of results to realistic values of the inner shelf slope yields, in case of Long Island shelf, a final height that agrees with observed ridge heights, but the modelled variation in mean grain size is small compared to field data. Finally, the response of ridges to large-scale interventions is considered. Experiments reveal that extraction of sand on the inner shelf causes a decrease of the sand volume stored in the surf zone.