Marine and aeolian sediment transport at the Hondsbossche Dunes

Abstract

The Hondsbossche Dunes is a seven-kilometre-long dune, beach and shoreface nourishment that serves as a primary flood defence since 2015. In addition to its protective function, the Hondsbossche Dunes stimulate ecological habitat development in the coastal area, in line with Ecoshape's 'Building with Nature'-approach. Its morphologic development is monitored to simulate the morphological response to marine processes and aeolian sediment transport, and to determine the optimal profile for dune growth and nature development.The monitoring program of Ecoshape studied the volume change of the beach and the dunes. The decrease of sediment volume on the beach is not equal to the increase of sediment volume on the dunes. The hypothesis is that the remaining sediment must stem from the intertidal zone and the shoreface. The research objective of this study is to investigate the contribution of sediment supply and marine processes in the intertidal zone to changes in morphology and bed composition in the beach and dune. The results of this data analysis show that sediment is transported from the intertidal zone to the shoreface in the north and south of the Hondsbossche Dunes. This behaviour is different from the middle of the domain, where sediment is transported from the shoreface to the intertidal zone. The direction of cross-shore sediment transport between the shoreface and the intertidal zone may depend on the crest level of sandbars in the shoreface: The sandbars in the middle of the Hondsbossche Dunes break more short waves than those in the north and south, which causes 'freeing' of long waves and net landward sediment transport. The modelling study suggests that wave runup and the presence of soil moisture reduce aeolian sediment transport from the intertidal zone to the beach, because grains in the intertidal zone are immobile when they are moist. This reduction results in increased erosion at the dry part of the beach, because the wind picks up sediment from the beach instead of the intertidal zone. The soil moisture content also significantly coarsens the bed composition in the intertidal zone, because the difference in threshold for aeolian transport between fine and coarse grains becomes larger. Hydraulic mixing does not contribute to significant morphological changes in the intertidal zone, but redistributes grains on the bed of the intertidal zone toward its initial distribution by stirring up the bed of the intertidal zone. These marine processes only affect dune growth if aeolian sediment transport is supply limited, which is mainly the case for Profile 2 north and Profile 4. Aeolian sediment transport in Profile 3 south is mainly transport limited, because the dune growth volume in most simulations is equal to the dune growth capacity following the formulation of Bagnold (1941). It seems that dune growth in this profile is less affected by marine processes. These results are likely to be related to the finer bed composition and wider beach at Profile 3 south.