Mechanisms controlling seaward escape of suspended sediment from the Gironde: A macrotidal estuary in France

Abstract

The main parameters controlling suspended-sediment transport in and out of the Gironde estuary are river flow and tides. These combine to control water mixing, density circulation and the trapping of suspended sediment in the turbidity maximum. Seaward escape of sediment is related to the position of the maximum in the estuary, which is a function of river flow; and the amount of sediment in suspension as controlled by tide range. Seaward escape of sediment appears to follow two cycles: (1) a 14-day neap—spring tidal cycle, and (2) a seasonal river flow cycle. Maximum escape occurs during spring tides and high river flow. During these periods, the turbidity maximum is located downstream in the estuary and the increased seaward surface density flow supplies to the shelf large quantities of suspended sediment. At this time, net seaward flux is more than 100 times that measured during conditions of low river flow. Thermal imagery and aerial photography in the inlet, repeated during a spring tide cycle, show an initial seaward drift of the surface estuarine water and suspensions, followed by a northward transport and mixing with water from the adjacent inlet. In succeeding tides, the turbid plume is dispersed, fragmented and transported seaward and to the north in the coastal drift system. Most of the suspended sediment expelled from the estuary is deposited on the shelf between 30 and 70 m depth, where wave resuspension is low and where suspended-sediment concentrations are high enough to supply and maintain a permanent mud deposit.