Effects of tides on mixing and suspended sediment transport in macrotidal estuaries

Abstract Studies in estuaries on the French Atlantic coast, with tides ranging from 4 to 7 m, have shown that in macrotidal estuaries, semidiurnal and fortnightly tidal cycles play a significant role in controlling hydrological and sedimentological processes. During the fortnightly neap-spring cycle of tidal amplitudes, the tidal prism varies considerably, bringing about significant changes in the ratio of river flow to tidal volume. This in turn causes large variations in mixing, and the estuaries can change from a relatively well mixed state during spring tides, to partially mixed, or even well stratified, during neap tides. The changes in the tidal averaged water volume of the estuaries during the neap-spring sequence induces a residual accumulation of water in the estuary during increasing tide range and a residual seaward discharge during decreasing tide range. This purely tidal effect can probably influence the flow of fresh water and suspended sediment seaward from the estuary. During every semidiurnal tidal cycle, large amounts of fine sediment are alternatingly eroded, resuspended and deposited, and the turbidity maximum in these estuaries appears as essentially a tide-induced phenomenon. Upstream transport of suspended sediment caused by the ebb-flood asymmetry of the shoaling tidal wave appears to create a tidal sediment trap in the upper estuary, even in the absence of density circulation. Neap–spring tide cycles control sedimentation and seaward escape of suspended sediment. Large-scale erosion and resuspension occurs during spring tides, increasing the residence time of fine sediment in the aquatic environment, and providing a mechanism for enhancing seaward escape of suspended sediment. In hypersynchronous estuaries, where tidal damping by friction is less than the effect of the upstream convergence, tidal amplitude and power dissipation can attain a maximum within the estuary. Numerical simulation of suspended sediment in the Gironde estuary shows that this process also contributes to the formation of the turbidity maximum.

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