C. Hallin1,2*, P. van de Ven1, N.M. Elbers1, C. van IJzendoorn1, S. de Vries1
1 TU Delft
2 Lund University
In the last decade, there has been a tendency towards larger-scale nourishments to enhance coastal safety and reduce the environmental impacts of frequent nourishments. The Sand Motor and Hondsbossche Dunes are examples of so-called mega nourishments, where natural processes are used to redistribute the sediment both in the longshore and cross-shore direction.
One desired mechanism is that sand is transported by the wind over the beach into the dune area, reinforcing the existing dunes and stimulating new dune formation. However, sediment sorting processes due to grain-size-selective pick-up and deposition influence sand availability for aeolian transport (Hoonhout & de Vries, 2017). If the beach rarely is overwashed by waves, armour layers may develop with coarse material that impedes the aeolian transport (Strypsteen et al., 2021).
This study investigates how sediment sorting and armour layer development are influenced by the fill material, nourishment design, and sediment transport processes at the mega-nourishments Hondsbossche dunes and the Sandmotor.
About 100 sediment samples were collected along 25 cross-shore transects at each of the Sandmotor and Hondsbossche dunes mega nourishments. The samples were sieved to determine their grain size distribution. Local trends of the grain size characteristic parameters, such as median grain size and sorting, were investigated by correlation analysis.
The results revealed considerable grain size variability both in the longshore and cross-shore direction (Figure 1). In general, the coarsest samples and largest variability were found in the morphologically dynamic intertidal area. Aeolian transport processes have sorted the sediment across the nourishments so that the finest and most well-sorted sediments are found in the aeolian deposits in the dune areas. Eroding parts of the beaches generally display coarser sediments than accreting parts, which may locally influence the aeolian transport rates towards the dunes. Knowledge about sediment sorting processes at mega nourishments - how they relate to different management strategies and fill material - can be used to optimize the design of mega nourishments and promote dune build-up.
Figure 1: Median grain size of the surface sediments at the Sandmotor.
Hoonhout, B., & de Vries, S. (2017). Aeolian sediment supply at a mega nourishment. Coastal Engineering, 123, 11–20. https://doi.org/10.1016/j.coastaleng.2017.03.001
Strypsteen, G., van Rijn, L.C., Hoogland, M.D., Rauwoens, P., Fordeyn, J., Hijma, M., Lodder, Q.J. (2021) Reducing aeolian sand transport by using armour layer of coarse materials. Coastal Engineering, 166, 1-15. https://doi.org/10.1016/j.coastaleng.2021.103871