Circulation patterns and implication for fine sediment transport in a preserved tropical estuary: The case of the Maroni (French Guiana)

Abstract Compared to temperate estuaries, tropical ones are less documented. Among them, the numerous estuaries along the Guiana shoreline present at least two original features: they are still preserved from anthropogenic modifications, and their hydro-morphodynamics are influenced by longshore migration of outer coastal mud banks. This study deals with the Maroni estuary, a mesotidal preserved tropical estuary, currently spotting more attention to better anticipate changes related to the development of harbour activities. Field data collected over semi-diurnal tidal cycles in the lower estuary revealed the presence of a turbidity maximum and seasonally contrasted hydro-sedimentary features. A one-year 3D hydrodynamic simulation covering a wide-range of tidal and fluvial conditions was performed to further analyse the seasonal variability of stratification and residual circulation patterns. The estuary is well-mixed for a very low river discharge (lower than 200 m3/s). It becomes partly mixed in both spring and neap tide in case of a moderate river discharge of 700 m3/s. The strongest stratification, which could be associated to a salt wedge near the mouth, appears during neap tides combined with higher discharge. Residual currents along the navigation channel are usually seaward except during neap tides when the discharge is large enough ( ∼ 700 m3/s) to generate stratification and landward residual current near the bottom to mid depth until the first km 15 from the mouth. Residual inflow starts from the location of the mud bank and extends along the navigation channel near the eastern bank. The inflow extension matches with observed highly turbid areas and fresh mud deposits, suggesting an outer supply of mud. In summary, this exploratory work allows a first description of current dynamics of the Maroni, and is a first step before a comprehensive study of changes in estuarine circulation and fine sediment transport under different configurations of coastal mud banks.