Short-Term Morphodynamic Response of a Ridge and Runnel System on a Mesotidal Sandy Beach

Abstract

The sediment dynamics of a ridge and runnel system were observed on a mesotidal sand beach during fair weather conditions. Topographic surveys, grain-size-trend analysis and fluorescent-sand-tracer experiment show that sediment transport is clearly directed onshore on the ridge. However, neither growth nor cross-shore migration of the ridge was observed. The ridge is migrating towards the south at a speed of 25 m/day and a theoretical longshore transport rate of 16 m³/m/day: transport rates on the ridge are 8 times less important. Such a difference is attributed to the complex flow pattern associated with the ridge and runnel system. Longshore transport appears to be controlled mainly by strong currents concentrated within the runnel during the falling tide. During the rising tide, sediment exported by the rip channel are driven towards the next southwards ridge. The position of the rip currents remained constant during the five days of observation.