Aeolian Sand Transport on a Ridge and Runnel Beach: Preliminary Results from Leffrinckoucke Beach, Northern France

The aeolian transport conditions on a macro-tidal ridge and runnel beach in northern France were monitored over two 30-minute experiments, representing respectively conditions of onshore and offshore to shore-parallel wind flow. Several anemometers, a portable weather station, acoustic grain impact counters (saltiphones), temperature and ground moisture probes, and aeolian sand traps were deployed in order to determine the effects of topographic, moisture and bedform variations on aeolian sand transport. The instruments were deployed across a ridge/runnel/ridge system on the mid- to upper beach, and on the upper beach terrace linking this ridge and runnel system to the dune front. The results show that the near-ground wind velocities are slowed by the pronounced ridge and runnel topography of the upper beach. The ridge and runnel system segments the fetch, whatever the wind direction. The experiment involving an onshore wind was associated with negligible rates of sand trapping in spite of suitable wind conditions. Significant trapping occurred during the offshore to shore-parallel wind experiment, but this was limited to the upper beach terrace and upper ridge and runnel, and there was little downwind transport below this upper ridge/runnel/ridge set. These preliminary experiments and the field observations suggest that the important degree of wave-tidal bedform development over the beach surface and the high moisture levels in the runnels and sometimes on the ridges, both common characteristics of ridge and runnel beaches, tend to limit sand mobilisation. It is tentatively suggested from this data set that this ridge and runnel beach is characterised by a moderate and balanced exchange of sand with the dune front.