The down-wind change of aeolian sand transport over a beach, Egmond aan Zee, the Netherlands

Abstract

High coastal dunes are vital to protect the hinterland from marine flooding during storms. To preserve these dune systems, insight into their erosion and growth is important. In previous years, research mainly focused on dune erosion. In recent years, there has been a substantial interest in the growth of the dune areas. At sandy beaches, aeolian wind-blown sand transport ensures the return of sand towards the dunes at a slow pace, i.e. days to months. Aeolian sand transport fluxes arise over a sandy beach in down-wind distance, from the top of the swash zone towards the dune area. At sandy beaches, aeolian sand transport rate depends on the wind climate and the surface parameters. This research report is based on field data with short timescales, i.e. minutes to hours, at a sandy beach located south of Egmond aan Zee, the Netherlands, and focuses on how aeolian sand transport fluxes vary in down-wind direction over a bar-trough beach towards the dune area. The main aim of this research is to give more insight in how aeolian mass fluxes vary over the beach and to detect which factor has the most influence on aeolian mass fluxes in down-wind distance. During the field campaigns, aeolian sand fluxes were measured using Modified Wilson and Cooke (MWAC) sediment catchers to quantify the total mass flux. To measure the wind speed and direction, cup anemometers and sonic anemometers were used during the field campaign. This research shows that a beach area can be divided into three sub-areas, where on each part of the beach unique factors increase or limit aeolian sand transport in down-wind direction. At the intertidal area it is observed that morphology limits aeolian sand transport, while the wind speed is the most important factor to start aeolian sand transport. At the upper beach, it is shown that the conditions to start or to continue aeolian sand transport are most favourable. The morphology ensures for limiting influence, while the combination of fetch length and wind speed ensures for most increase in aeolian mass flux. At the dune foot, it is noticed that the morphology is slightly more unfavourable than at the upper beach area. It is also indicated that during oblique wind conditions the wind speed is relatively lower at the dune foot than at the intertidal beach and the upper beach area. The wind direction is most important at the dune foot, because the wind direction ensures that the fetch is long enough to increase to its maximum. In order to give better predictions for aeolian sand transport towards the dunes during model studies, there must be considered that there are several factors that increase or limit aeolian sand transport which alternate one another over the beach. Furthermore, in the results is shown that the wind speed decreases near the dunes during oblique wind directions, which might influence the amount of aeolian sand transport towards the dunes. Therefore all these different increasing or limiting parameters should be taken into account when predicting aeolian sand transport over the beach towards the dunes.