Role of flocculation and settling processes in development of the mangrove-colonized, Amazon-influenced mud-bank coast of South America

• Flocculation is an important process in the dynamics of Amazon-derived mud banks.
• Hindered settling favoured by waves results from high suspended concentrations.
• Enhanced settling occurs in low-energy leading and inner edges of mud banks.
• Flocculation, hindered and enhanced settling characterize mud bank migration.
• Migration is driven by modulation of these processes by wave energy gradients.

The Guianas coast downdrift (northwestward) of the Amazon River mouth is characterized by the periodic migration of mud banks that originate from the Amazon. The characteristic sizes of these banks as well as their rates of alongshore migration have been estimated from remote sensing. However, the physical mechanisms leading to their displacement are not yet fully understood. The present work is aimed at investigating a number of micro-scale processes involved in the coastal and estuarine dynamics of fine sediments, and expected to occur during the migration of mud banks. The relative magnitudes of flocculation, hindered settling and consolidation have been determined.Sampling of coastal mud was carried out during a field survey in 2001 that focussed on the fluid mud layer in the leading edge of a mud bank in French Guiana. Settling column experiments were conducted under quiescent conditions for various mean sediment concentrations in the range of 2.5–110 g.l− 1, which is typical of mud bank concentrations. The time dependent vertical profiles of suspended sediment concentration were monitored using an optical settling tank equipped with 32 pre-calibrated optical sensors. The corresponding settling velocities were deduced from the equation of the conservation of mass.The results show that the timescales of hindered settling and consolidation processes are much larger than the timescales of mixing mechanisms such as tides or propagating waves. The individual floc settling velocities are too small to counterbalance the turbulent mixing induced by breaking waves. Hindered settling, favoured by wave action and by the high background suspended sediment concentrations, is thus an overarching process in wave-driven mud bank migration. This pervasive hindered settling regime is characteristic of the wave-exposed outer and leading edges of mud banks where active mobilization of mud assures mud bank migration. As a result, consolidation is theoretically precluded. The experiments pointed out, however, that additional flocculation and differential settling should enhance sedimentation during slack water and under low wave conditions. We deduce from this that enhanced settling in the inner, subtidal–intertidal, parts of leading edges of banks during such conditions is important in the temporary sedimentation that generates gel-like fluid mud patches and mud bars. These low-energy inner, leading parts of banks form an accreted substrate colonized by mangroves, which further contribute to enhanced settling and rapid sedimentation.