The influence of Amazon-derived mud banks on the morphology of sandy headland-bound beaches in Cayenne, French Guiana: a short- to long-term perspective

The 1600-km-long South American coast between the Amazon and the Orinoco Deltas has a prograded muddy shoreline comprised of sandy cheniers. The 15-km-long bedrock coast of Cayenne, French Guiana, differs from the rest of this fluctuating alluvial coast in that it comprises several headland-bound fringing sandy barriers that have experienced limited progradation and do not behave as cheniers that migrate landward over a muddy base. The limited progradation of these barriers is probably due to blanketing of fluvial sand by the cover of Amazon mud on the inner shelf, which precludes shoreward reworking to supply the beach and barrier systems. Aerial photographs, satellite images and recent field observations show that alternations between,mud bank' phases, when Amazon-derived mud banks migrating northwestwards attain the Cayenne area, and 'interbank' phases, characterised by relative scarcity of mud, result in marked spatial and temporal variations in beach dynamics and morphology. These include periodic alternations in longshore drift that lead to a rare form of total or partial beach rotation that does not result, as is generally reported for nonmud bank-affected beaches, from seasonal variations in deepwater wave approach directions. These beach morphological changes have been defined in terms of a simple, four-stage conceptual model comprising 'bank', 'interbank' and transitional phases. In extreme 'bank' phases, the mud banks responsible for muddy coastal progradation on the Amazon-Orinoco coast may directly weld onto the beaches for periods ranging from months to years, leading to a rare example at the world scale wherein the dynamics of ocean-facing beaches are completely muted under the protective cover of mud that may become rapidly colonised by mangroves. The observations also suggest that these changes do not affect the medium-term (order of tens of years) beach sand budgets. Mud welding onto the beaches may, however, sequester at short time scales (order of months to years) sand eroded from the beaches. Subsequent mud erosion leads to release of this sand and its restitution to the beach sand budget. (C) 2004 Elsevier B.V. All rights reserved.