The effect of a storm surge on near-shore sediments in Ameland-Schiermonnikoog area (N. Netherlands)

Abstract

After a severe storm part of the sample localities have been resampled in the shallow off-shore (to about 20 m) and the beach of an area, which was previously surveyed under quiet conditions. It appeared that in the shallow off-shore: (1) the depth had not changed much, (2) half of the samples were finer after the storm, some were coarser and some had not changed much, (3) the garnet percentages as well as the mica concentrations had both decreased after the storm, (4) the post-storm sands were considerably less rollable than before and the Shape Disribution Character had changed into an upsloping type, which is characteristic for receiving deposits. The observed features are explained by erosion of the shallow sea floor with transport by undertow towards deeper water. Subsequently, deposition took place of finer material that had been eroded in the tidal flat area and taken out in front of the coast by the first post-storm ebb tide. This material was reworked and spread out later on, loosing its mica and part of the fines by winnowing. Since most of the depositional processes were due to capacitive overloading rather than to competency, the grain 'size' largely failed as a diagnostic criterion and the conclusions reached in this paper are mainly based on grain-shape and density differences.