STORM INFLUENCES ON A TIDAL SANDBANKS SURFACE (MIDDELKERKE BANK, SOUTHERN NORTH-SEA)

The results are presented of two types of ''pre-storm'' and ''post-sto rm'' surveys, carried out on the Middelkerke Bank (southern North Sea) . The first type of survey was planned to detect detailed morphologica l changes in two selected areas of the bank totalling 3 km(2); for the survey in these shallow waters to be completed in a fast and accurate way, use was made of a special hovercraft-based hydrographic system. The other type of survey concerned grab samplings covering the sandban k in order to study spatial variations in surficial grain size. The pr e-storm and post-storm morphological surveys showed that small and med ium dunes disappear under stormy conditions. A 5 m westerly displaceme nt of most of the large and very large dunes has been interpreted as t he result of bedform migration under normal (fair-weather), locally do minant ebb flow. The lowering of the large dunes' crests by up to 1.2 m is considered an effect of the storms. The accretion of the lower pa rt of the northwestern flank is attributed to the deposition of sand, derived from shallow parts of the sandbank during storms. The grain-si ze surveys showed that especially the central and northern parts of th e sandbank were affected by grain-size changes, the northwestern flank showing a clear coarsening and the landward southeastern flank a fini ng with respect to the fair-weather situation. It is put forward that the waves approaching from the north cause an extra winnowing on the e xposed flank of the bank. On the southeastern flank of the bank, which is better protected against the wave action, fine particles are then deposited.