MORPHODYNAMICS OF BARS ON THE EBB DELTA OF A MEGATIDAL INLET (NORMANDY, FRANCE )

摘要

The purpose of this paper is to describ the morphology and attempt to understand the morphodynamics of bars located on an ebb delta close to an inlet mouth in a megatidal context. Although there have been numerous studies on the morphology and physical processes affecting such bars, these concern only microtidal and mesotidal settings. The west Cotentin coast (Normandy, France) is a fine example of a megatidal coast with one of the largest tidal ranges in the world. The mean spring tidal range is up to 10 m. Fieldwork was undertaken during one month and involved the deployment of an electromagnetic current meter, a pressure sensor and DGPS surveys to measure topographic changes every 2 days. Along the western coast of Cotentin, in spite of the very large tidal inlet, bars are generally small (2 m high and 250 m long) compared with other tidal environments. During the campaign, the bar migrated exclusively landward over a distance of 4 m. Its volume did not change significantly. This slow migration rate is attributed to the large tidal range. In the megatidal setting, the tidal water level fluctuations control the duration of the emergence and flooding periods, as well as the translation rate of different hydrodynamic zones across the ebb delta surface. Field observations showed that bar migration and the shoal volume variation patterns during the experiment may be divided as a function of hydrodynamical conditions. These results clearly indicate a morphodynamic behaviour pattern strongly hinged on the specific local megatidal hydrodynamic conditions.