Morphodynamic evolution of an intertidal mudflat under the influence of Amazon sediment supply – Kourou mud bank, French Guiana, South America

• We analyse sedimentary processes in a mudflat along an open coast.
• We show the important role of biofilm on mudflat structure.
• We link the constant accretion of the mudflat to tidal supply and biofilm migration.
• We evaluate the sedimentation rates in the mudflats of French Guiana.
• We show the influence of mud cracking on mudflat erosion and mud pebble formation.

The coastal environment between the Amazon and the Orinoco Rivers is characterised by the migration of large mud banks formed by accretion of the muddy discharge from the Amazon. The migration of mud in the shallow coastal waters is associated with the creation of mudflats that form a surface for the development of coastal mangroves after consolidation. This study focuses on the fine-scale sedimentary processes involved in the morphodynamic evolution of a consolidated mudflat and its erosion. Mudflats can be divided into two areas: the seafront and the inner part between the seafront and the land. This study highlights a link between tidal mud supply, biofilm migration and increasing elevation in the latter area. The migration of a biofilm through each cycle of tidal supply prevents erosion and permits the continuous accretion of the entire mudflat over several years. This increase in topography is also modulated by fortnightly tidal cycles. Desiccation greatly impacts the mudflat's structure at a yearly scale. This process plays an important role in the erosion of the seafront area under wave action by allowing the formation of mud pebbles, which are progressively abraded into fluid mud supplied to the inner part of the mudflat by over-wash processes. This study provides a better understanding of the behaviour of mudflats on the wave-exposed coast downdrift of the mouth of the Amazon by describing: (1) the processes involved in sediment exchanges between mudflats and mud banks, (2) the mechanisms associated with the persistence of mudflats along the French Guiana coast downdrift of the mouth of the Amazon, and (3) the processes involved in the erosion and recycling of these mudflats.

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