Advection and diffusion of shore-attached sand nourishments

Abstract

Understanding of the behaviour of coastline perturbations at soft-coastlines is essential for modelling coastal evolution at decadal time scales. Many coastline models do, for example, implicitly assume dominant diffusive behaviour of coastline features. The validity of this assumption is investigated for the Dutch coast on the basis of data on coastline perturbations. Bathymetrical data for a number of nourishments were used to assess the relative importance of diffusion with respect to advection of the sediment along the coast. For this purpose, the volume of sediment is computed for cross-shore transects along the coast. The alongshore distribution of this sediment over time (as a result of dispersion by waves and currents) is then analysed by means of simple shape parameters: a mean alongshore position and standard deviation of the nourishment sand from the centre. Next, the nourishments were also characterized with an advective and diffusive parameter by fitting of an advection-diffusion equation. These parameters then give a classification of the nourishment behaviour. It was found that the behaviour of nourishments at the Dutch coast is dominated by diffusive processes, while advective processes have some influence on the alongshore transport in the shallow part of the surfzone for smaller nourishments.