Alongshore variability of nourished and natural beaches

Abstract

Alongshore variability in topography (i.e. height differences in bed level along the coast) can exist on both natural and nourished beaches. An important question prior to implementation of a nourishment project is how alongshore variability is going to evolve and, related to this variability, the effect of the project upon swimmer safety. In this thesis observations and analysis of the alongshore variability in nearshore topography of a nourished Dutch beach are discussed. Three and a half years of nearly monthly measurements with a jetski survey system and GPS backpack show in detail how the topography of the beach changed after installation of a large nourishment. The results show that the first period just after implementation of the nourishment showed similar magnitudes of variability than the total period investigated, and no apparent effect of the nourishment shape on the variability could be discerned. Moreover, changes in topographic variability were found to be much slower than previous observations, f.i. at the US east coast or Australian east coast. The formation process of alongshore variability is examined in more detail by examining field measurements of an Australian natural beach and using conceptual numerical modelling. Special attention herein is paid to the effect that different sea states (swell vs. wind sea) have upon flow circulation and the formation of topographic variability in the nearshore, to put the results in the thesis into the context of the Dutch wind sea wave climate. This analysis has shown that the typical Dutch wind sea wave climate is likely to result in slower formation of alongshore variability due to the short wave period (the time between consecutive waves) and high wave angles (angle at which it waves approach shore) in this wave field. Very low frequency flow velocity oscillations can be forced by variations in wave height and are related to transient rip currents. Analyses in the thesis show that these oscillations are typically small in the Dutch wave climate. However, the steep profile of a nourished beach could enhance the magnitude of these motions, such that substantial fluctuations in current can be present at a Dutch nourished beach.