Caractérisation et dynamique des turbidités en zone côtière : L'exemple de la région marine Bretagne Sud

The space and time turbidity variability in Southern Brittany was studied, from the combination of results of hydrological campaigns, long-term instrumented moorings, satellite observations and 3D process-based numerical modelling. Previous observations exhibit a cross-shore concentration gradient with values hardly exceeding 30 mg/l beyond the littoral zone. Acoustic profilers (ADCP) were moored to measure currents, waves and the associated dynamics of fine particles. Time series of mass concentration in the whole water column were estimated from the analysis of the backscattered signal. During wave episodes, values can reach 100 mg/l at 1.50 m above bottom, just outside the Bay of Vilaine. The empirical calibration of the signal with independent measurements of turbidity is very satisfactory and more efficient than the inversion of a backscattering model, due to instrumental and environmental uncertainties. These data were used to calibrate a hydrodynamic and sediment transport numerical model (MARS-3D/SiAM) in a realistic configuration (tide, wind, river flows, sediment cover) with a simple parameterization of sedimentary processes (erosion, deposition). Wave forcing is computed with SWAN model. Results of a 5 months simulation show the important role of wind circulations and hydrological structures on the spatial distribution of turbidity, which is mainly generated by waves. Mean turbidity levels and fluxes in specific areas are also estimated.