Bar Morphodynamic Behaviour on the Ebb Delta of a Macrotidal Inlet (Normandy, France)

Abstract

The purpose of this paper is to describe the morphodynamics of bars located on an ebb delta of a macrotidal inlet system. 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 macrotidal coast with one of the largest tidal ranges in the world. The mean spring tidal range is up to 11 m. Fieldwork was undertaken during one month and involved the deployment of an electromagnetic current meter, a pressure sensor, and differential global positioning system surveys to measure topographic changes. Along the western coast of Cotentin, in spite of the very large tidal inlets, 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 a macrotidal setting, the tidal water level fluctuations control wave height during storms, the duration of emergence and flooding periods, and the intensity and direction of longshore tidal currents. Field observations showed that bar migration rate is correlated with wave activity, whereas volume variations are controlled both by mean tidal currents and wave conditions. These results indicate a morphodynamic behaviour strongly related to specific local macrotidal hydrodynamic conditions, the tidal prism influence being less important.