Spatial distribution of bed level changes and responsible sediment exchanges across the Dutch shoreface

Abstract

The lower shoreface is an important part of the coast because it is the foundation of the beach and dunes that prevent the hinterland from flooding. The lower shoreface has been under reviewed and sediment transport and the morphodynamics have been poorly quantified. The study objectives are therefore to quantify the large-scale bed level changes, find the responsible sediment exchanges and validate the transport model results of Grasmeijer et al. (2022). Bathymetric changes based on Vaklodingen between 2019-2017 and transport data between 2013-2017 from Grasmeijer et al. (2022) are combined to study the shoreface of the Dutch Holland coast in different zones (both longshore and depth-dependant cross-shore). The results show flattening of the lower shoreface due to larger net erosion in the shallow part compared to the deeper part. Three sections were identified: an eroding northern section, an accumulating central section and a stable southern section. The modelled transport data shows an increasing total and longshore transport towards the north due to increasing tidal flood dominance (Grasmeijer et al., 2022). The overall onshore-directed cross-shore transport increased towards smaller water depths. The modelled transports were shown to result in longshore and cross-shore exchanges. These are dependent on the transport magnitude, the shoreface width and the vicinity of mega nourishment. The shoreface width modulates the length of the lateral boundaries and seaward boundary is curving so that transport is oblique to this boundary along the majority of the shoreface This results in alongshore sediment exchanges for the different zones. Additionally, bathymetry- and transport-based sediment volume changes are determined and compared. The volumes show almost no resemblance. Possible explanations are inaccuracies of the bathymetric data, the excluded sediment exchange with the upper shoreface and the excluded effect of undertow in the transport model results. Vaklodingen result seem to result in uncertainties so the results must be considered carefully. The combination of bathymetric data and modelled transport data however is a useful method to study the lower shoreface.