Formation and structure of the turbidity maximum in the macrotidal Charente estuary (France): Influence of fluvial and tidal forcing

International audienceUnderstanding estuarine sediment dynamics, and particularly turbidity maximum dynamics, is crucial for the management of these coastal systems. Various processes impact the formation, movement and structure of the turbidity maximum. Several studies have shown that tidal asymmetry and density gradients are responsible for the presence of this suspended sedimentary mass. The Charente estuary is a highly turbid system (with suspended sediment concentrations mostly in excess of 5 g/L) that remains poorly understood, despite its strong impact on local activities. In this study, a three-dimensional hydrosedimentary model is developed to represent the sediment dynamics of this estuary. Model validation demonstrates good accuracy, especially on reproducing semi-diurnal and spring-neap variability. Several simulations are performed to evaluate the influence of tides and river discharge on the turbidity maximum. Mean and maximum suspended sediment concentrations (S S C) and sediment stratification, are calculated. S S C transects are also used to visualise the suspended sediment distribution along the estuary. The turbidity maximum generally oscillates between the river mouth and the Rochefort area (20-30 km upstream). The model shows strong variations at different time scales, and demonstrates that S S C is mainly driven by deposi-tion/resuspension processes. Spring-neap comparisons show that the turbidity maximum is not well-defined during neap tides, for low and mean runoff conditions. Simulations of spring tides and/or high runoff conditions all result in a compact suspended sedimentary mass. Performing simulations without taking density gradients into account demonstrates that tidal asymmetry is the main mechanism leading to the formation of the turbidity maximum. However, density gradients contribute to maintaining the stability of the turbidity maximum. Vertical stratification traps sediments at the bottom. Longitudinal stratification ensures a sharper edge at the downstream limit of the suspended sedimentary mass, preventing a massive export of sediments

Morphology, hydrography and sediment dynamics in a mangrove estuary : the Konkoure Estuary, Guinea

The Konkoure Estuary in the Republic of Guinea is a poorly understood atypical mangrove system. Sediment dynamics in tropical estuaries are controlled by a combination of processes including river discharge, morphology, salinity, erosion and deposition processes, the settling of mud, physico-chemical processes and mangrove swamps. Here we present a consistent set of data aimed at characterising the estuary and thus, increasing our understanding of tropical systems, as well as studying the impact of human intervention in the region. Water elevations, current measurements, salinity, suspended sediment concentrations, bathymetry and sediment cover are presented following a 3 year survey of the Konkoure Estuary. Here we provide conclusive evidence that the Lower Konkoure is a shallow, funnel shaped, mesotidal, mangrove-fringed, tide dominated estuary, well mixed during low river discharge. The estuary becomes stratified during high river flows and spring tides whereas a salt wedge appears during neap tides. The Konkoure Estuary has been described as hypersynchronous, and has three terminal outlets, two of which are landward-directed, attesting to a tidal pumping effect, while the third one is seaward-directed, and is controlled by the mangrove. The suspended matter is transported by the tidal effect within the middle estuary and is therefore trapped in the Turbidity Maximum zone (TMZ). The location of the TMZ is river-controlled and is correlated with residual currents but not with salinity front. A dam, constructed 130 km upstream, impacts on the hydrodynamics, and reduces the salinity intrusion by about 25%. It causes an increased low river discharge whereas its efficiency over high river flows is unclear. (c) 2006 Published by Elsevier B.V