Processus de sédimentation des vases dans l'estuaire de la Gironde Contribution d'un traceur radioactif pour l'étude du déplacement des vases

Sedimentary processes of the mud in the Gironde estuary Contribution of a radioactive tracer to the study of mud displacements

¹ Docteur ès-Sciences, Université de Bordeaux I
² Docteur ès-Sciences, Laboratoire National d'Hydraulique
³ Commissariat à l'Energie Atomique
⁴ Laboratoire Central d'Hydraulique de France

Résumé

The hydraulic and sedimentological processes operating in the Gironde estuary (SW France) are linked to the interaction between the river discharge (mean values between 800-1,000 m3/s), tides (ranges from 2 to 6 m), the suspended sediment influx from the rivers (2 to 3 million tons per year), and the general morphology of the estuary, consisting in two channel systems separated by a series of islets, shoals and bars. The fluvial discharge undergoes important seasonal variations, and the length of the salinity intrusion varies correspondingly : during periods of low river flow (summer-fall) it extends above Bec d'Ambes (73 km from the inlet), and during high river flow, below Pauillac (45 km from the inlet). The residual circulation pattern is linked to the intensity of the density gradients, and also varies seasonally, following changes in the ratio of river flow to tidal prism. Generally speaking, in the zone of the saline intrusion the residual currents are stratified : upstream on the bottom, and downstream toward the surface. The channel and shoal topography can locally disrupt the residual currents by amplifying the flood or the ebb flow. The principal null point, or zone of convergence of the bottom residual currents upstream of which the bottom residuals are always oriented downstream, generaly coincides with the upstream extremity of the salinity intrusion, and follows the same seasonal migrations. There is a marked lateral salinity gradient in the lower estuary, with less saline water flowing along the right bank (looking downstream). A well marked "turbidity maximum" forms at the upstream limit of the salinity intrusion, in the zone of bottom residual current convergence. This turbidity maximum migrates seasonally, in response to the migration of the salinity intrusion and null point. The concentration of suspended sediment in the turbidity maximum is very high (0.1 to 10 g/l). The deposition of suspended sediment in the main estuarine channel occurs through the formation of pools of extremely turbid bottom water (100-300 g/l). Practically no movement is discernible in these pools of "fluid mud", and intense bottom deposition by settling occurs. These pools are formed in the turbid core of the turbidity maximum, in zones of "motionless lenses", on the bottom occuring at flood slack by the amplification of the velocity time assymetry in the salinity intrusion. This fluid mud follows a cycle of erosion and deposition linked to the monthly cycle in tidal ranges. During neap tides the mud accumulates, and during spring tides, it is eroded and resuspended into the overlying turbidity maximum. It appears, from suspended sediment flux measurements in the lower estuary, and analysis of the plume of turbid estuarian water escaping out on to the continental shelf during river floods and spring tides, that a significant amount of alluvial suspended sediment bypasses the estuary and is deposited in a silt and olay zone on the inner continental shelf. Possibly part of this estuarine water is recirculated in an estuarine and lagoonal system further north. In order to study and assess the diffusion of the fluid mud into the turbidity maximum during spring tides, and also to follow the upstream migration of these fluid mud pools during a period of decreasing river discharge, a radioactive tracer study was made of the fluid mud. This study showed that a very slow movement, on the order of 0.1-0.2 cm/s occurs in the fluid mud, and appears to be directed downstream. Also it was shown that at each cycle of erosion and deposition of the fluid mud, which reformed further upstream at each cycle, the same sediment appears to be recycled.