Short-term profile changes of sandy pocket beaches affected by Amazon-derived mud, Cayenne, French Guiana

The 15 km-long bedrock promontory of Cayenne, French Guiana, comprises several headland-bound fringing sandy beaches and numerous, more or less highly embayed, short pocket beaches that are affected by mud banks migrating alongshore from the Amazon to Venezuela. The influence of mud banks on this coast may be considered within a four-phased cycle characterised by a mud bank phase that leads to extensive wave energy dissipation, an inter-bank phase of relative mud scarcity and enhanced wave activity, and the two transitional phases between these two phases. Three profiles from two short ( < 400 m long) pocket beaches were variably surveyed between November 2001 and November 2004, a period characterised by the last stages of a bank phase, a short transition, and an inter-bank phase. The profiles comprise an upper sandy segment linked to a relatively flat muddy lower beach that extends offshore, the sand-mud contact and elevation of this muddy lower profile depending on exposure to wave energy. Profile changes were basically cross-shore, with little or no evidence of longshore sand transfers, because of the poor propensity for development of longshore gradients in wave energy in these short embayed beaches. Differences in the intensity of profile change were due to variations in the degree of exposure to waves which strongly depended on the presence, inshore, of wave-dissipating mud. These pocket beaches differ from the more open and longer beaches of Cayenne, which show periodic alternations in longshore drift that lead to rare forms of beach rotation induced by wave interactions with the mud banks.