Sediment sorting at a large scale nourishment

Abstract

A new approach in coastal maintenance is the application of a large scale nourishment as natural feeder of the coast. The Sand Motor constructed near Kijkduin on the Holland Coast is a pilot project of this new nourishment strategy. The objective of this research is to contribute to a better understanding of the sediment transport processes at nourishments by analysing the sediment sorting processes. Field data of the sediment characteristics in the Sand Motor area is used to determine the sediment sorting in the Sand Motor area. A numerical model (Delft3D) is used to simulate the sediment sorting processes and time scales in the cross-shore direction at a large scale nourishment. The measurement data showed an increase of the sediment sorting in the longshore direction. The sediment at the Sand Motor was coarser than the sediment in the area North of the Sand Motor. It is hypothesised that the fining North of the Sand Motor is caused by the deposition of relatively fine material that is eroded at the Sand Motor. A measured decrease of the uniformity coefficient at the Sand Motor is in accordance with this hypothesis. In the cross-shore direction the minimum median grain size in the transects was found to move seaward after the construction of the Sand Motor. It is assumed that the wave conditions preceding these measurements influenced the location of the minimum median grain size. Higher wave conditions are assumed to lead to a minimum median grain size located further offshore due to the stronger undertow. The model results show a strong influence of the sedimentation and erosion in the profile on the sediment sorting pattern. This pattern is found to be independent of the initial bathymetry and initial sediment composition of the nourishment. It is shown that extreme wave heights have a large influence on the sediment distribution in the profile as wave conditions determine the area where sediment sorting takes place. Furthermore, the sediment distribution also responds more quickly to higher waves. Finally, the hypothesis is confirmed that the location of the minimum median grain size in the profile depends on the wave conditions. Model results show that the location of the minimum median grain size is located at the edge of the morphologically active profile and is thus related to the wave height via the closure depth formulation. To conclude, this research consists of new measurement information on the sediment composition in the Sand Motor area and a comparison of this data set with existing measurement campaigns. This information shows the influence of the Sand Motor on the alongshore and cross-shore sediment sorting and states the influence of the wave conditions on the location of the minimum median grain size in the profile. The model study shows the utility of a relatively simple Delft3D model to study the influence of different parameters on the sediment sorting processes and the timescale of these processes.