Use of numerical models to study land-based sedimentation and subsequent nearshore morphological evolution

Abstract

A beach sedimentation problem is presented. This problem involves assessing the impact and importance of land-based sediment sources (erosion from gullies during intense rainfall, and directly from cliffs by wave action) on morphodynamic changes at a beach. The site motivating the study is La Barrosa beach, Spain. The location is described and a discussion of possible approaches presented. A schematized representation of a gully and beach is used. A hybrid approach is adopted, in which sediment deposition by run-off (and to a lesser extent cliff erosion) is modelled using wave-resolving hydrodynamic models. It is found that significant amounts of sediment can be supplied by gully run-off during storms. For small run-offs the swash can be involved in determining the region of deposition within the gully area, with longer period motions favouring increased sediment deposition at gully sides. Simulations also show that swash motions can erode gully sides, and the presence of a cross-shore jet associated with the swash and gullies can also impact on sediment movement. Longer term, wave-averaged morphodynamic modelling (without a swash zone) indicates that run-off deposits subsequently diffuse laterally, whether under normal or oblique incidence. These simulations also reveal the possibility of underlying crescentic bar-shedding dynamics that may pertain during changes of wave conditions.