Étude par télédétection de la dispersion en mer des eaux estuariennes issues de la Gironde et du Pertuis de Maumusson

The objective of this study, carried out in the Gironde estuary inlet, was to determine the nature of the seaward dispersion of estuarine water and suspended sediments during a spring tide and high river flow. On June 3, 19(7, three flights were made over the inlet, at mid ebb, low tide and mid flood. The planned flight at high tide was cancelled due to adverse meteorological conditions. During each flight, thermal infrared coverage was obtained, together with black and white photographs. These data were correlated with concomitant ground mesures of current velocity, salinity, temperature and suspended sediment concentration Four types of water masses were identified during the tidal cycle: estuarine "very warm and turbid" water; estuarine "cooler and turbid" water; coastal "cold and turbid" water; marine "very warm and non-turbiq': water. The tidal movement and dispersion of each of these water masses were analysed over the tidal cycle. The net tidal movement of the warm and turbid estuarine water is initially seaward to the West, then to the North, following the net coastal water dnft. To the North of the Gironde inlet, the Gironde waters mix with those from the Marennes-Oleron Bay. Further North, the turbid plumes dispersed by fragmentation are transported seaward, where they are gradually entrained to the south in the offshore shelf drift. A considerable quantity of the seaward-expelled Gironde suspended sediments is deposited in a mud field on the inner and mid shelf adjacent to the estuary. The rates of mixing and dispersion of the estuarine waters on the shelf are relatively low. Dispersion, which appears to attain a maximum during mid ebb, is reduced at the beginning of flood tide, and the turbid plumes are preserved for a time without mixing.