Contrasting sand beach morphodynamics in a mud-dominated setting: Cayenne, French

The 15 km-long bedrock promontory of Cayenne, French Guiana, comprises three headland-bound fringing sandy beaches (>2< 5 km-long and referred to as type 1 beaches), and numerous, more or less highly embayed, short pocket beaches (0.1-1 km-long, type 2 beaches) exhibiting variable morphodynamic changes in response to mud banks migrating from the Amazon. Aerial photographs, satellite images and field studies of these mudaffected beaches show alternations between 'mud-bank' phases, when mud banks migrating northwestwards from the mouth of the Amazon River attain the Cayenne area, leading to extensive wave energy dissipation, 'inter-bank' phases, characterised by relative scarcity of mud and enhanced wave activity, and transitions between these two phases. During inter-bank phases, higher incident waves lead to the cleanout of mud from the intertidal beach and induce 'normal' beach morphodynamics. Transitional phases are associated with the variable presence of mud on these beaches, and result in the largest morphodynamic variability on the longer type 1 beaches. These beaches undergo alternations in longshore drift direction that are associated with a rare form of beach rotation induced by wave-mud interaction. These phases are associated with short-lived sand bed collapse features affecting the intertidal beach profile. Profile changes on type 2 beaches are much weaker, and are basically cross-shore, with little or no evidence of longshore sand drift, because of the poor propensity for development of longshore gradients in wave energy in these short embayed beaches. The presence of mud alters the behavioural patterns of beaches on the Amazon-influenced coast of South America, which is normally influenced by seasonal changes in wave energy due to trade-wind activity. ADDITIONAL INDEX WORDS: Amazon mud supply, mud banks, longshore drift, muddy beach, beach rotation