Large-scale field campaign for improving nourishment strategies in the Netherlands - Exploring the effects of wind

Climate change challenges us to come up with effective and efficient solutions to protect the Dutch coast. Sand nourishments are part of these solutions. The fate of the nourished sediment under the action of waves and tidal currents is still hard to predict. This is especially the case for ebb-tidal deltas, where the highly varied bathymetry introduces complex interactions between the different forcings.

Within the projects KUSTGENESE2.0/SEAWAD, a large-scale field campaign was carried out in September 2017 at the ebb-tidal delta and basin of the Ameland Inlet (the Netherlands). 5 landers were installed; each equipped with 2 ADVs, 2 ADCPs, 4 OBS, a LISST, a sonar ripple scanner and a CTD. The frames were placed in the main channel, on the lower foreshore, and on the shallower ebb shield. Additionally, 6 ADCPs were placed on the tidal watersheds of the basin.

The measurement period included calm conditions and two stormy periods (Sebastian and Xavier). During the storm Sebastian wave heights of over 6 m were measured, leading to significant stirring up of sediment. Wind affected the flow at shallow locations, modifying the tidal asymmetry. In particular, the combination of stirring up by waves and wind-driven flow led to additional net fluxes of sediment. The effect of wind on the tidal watersheds was so strong that the tidal flow reversed. A drop in salinity was found, suggesting that fresh water from the Afsluitdijk sluices was transported over the watershed towards the inlet. This indicates increased connectivity between distant parts of the Wadden Sea system, leading to large-scale sediment transport pathways.

The results show that wind has substantial influence on sediment transport via: (i) increase in wave height; (ii) change in local tidal asymmetry; (iii) change in large scale flow patterns (between basins and between basin and ebb-tidal delta). Not only the wind speed, but also timing of the storms and wind direction are found to be of crucial importance for sediment transport.