Size, shape and density sorting around two barrier islands along the north coast of Holland

Abstract

The sand movement on and around two barrier islands flanking the tidal flat area north of The Netherlands is studied by means of size, shape and density sorting. Since there is mainly fine sand in the area, the competency of the transporting media is seldom reached and selection mainly occurs on shape and density. In the deeper off-shore, a coarse grain-population shows inherited characteristics from a higher energy early transgressive phase, whereas a finer population is consistent with the present-day hydrodynamic situation. Tidal channels in between the islands form outer tidal deltas. These deltas protect the islands against the waves produced by N-W gales and they form a "shadow" for the W-E longshore currents. The coastline of the islands is still retreating. The removal of sand mainly occurs during storm surges. The eroded material is redeposited in sorted zones parallel to the coast. Only the susceptible material, that is deposited farthest from the beach, can be transported by the longshore currents. The coarsest, most spherical and densest grains remain closest to the beach and can re-enter the beach as prograding bars during more quiet conditions. This process gives rise to a stepwise diminishing grain-size from island to island towards the east. The eastern island is situated on top of older river and/or tidal channel deposits, which are easy to erode. This gives the tidal inlet, and hence the tidal delta, a great lateral mobility. The eastern island is therefore very mobile. The western island is bordered by a deep tidal channel, which cuts into more resistent glacial deposits. This keeps the channel and hence its outer tidal delta in the same site. The western island therefore did not change longitude and constantly retired over its own deposits. This resulted into completely different sand properties for both islands, which mainly shows in the garnet percentages and the high rollability values for the western island.