Tide-Induced Sediment Resuspension and the Bottom Boundary Layer in an Idealized Estuary with a Muddy Bed

Abstract

Sediment transport and bottom boundary layer (BBL) in an idealized estuary with a muddy bed were studied by numerical simulations. The focus was placed on description and prediction of the dynamics of nepheloid layer (a fluid?mud layer) developed in the estuary because of the coupling effect of the seawater and resuspended sediment concentration. The Princeton Ocean Model was coupled to a sediment transport model to conduct the numerical experiments. A semidiurnal tide with a spring?neap cycle was used to force the model at the estuary entrance. A stability function was introduced to the bottom drag coefficient Cd for a slip bottom boundary condition in order to consider the effects of sediment-induced stratification. When the seawater density is not affected by the resuspended sediments, spring tides resuspend sediments to the sea surface near the estuary entrance where the bottom stress is larger than the critical stress value. The sediment distribution in the BBL near the entrance is dominantly affected by the vertical eddy diffusion, and the time series of the sediment concentration presents two high value peaks within a tidal cycle. Above the BBL the sediment concentration is primarily controlled by the horizontal tidal advection; thus a semidiurnal oscillation in sediment concentration is predicted. When the seawater density and the sediment concentration are coupled, the sediments resuspended by the spring tides are only distributed in the bottom layer with a thickness of a few meters. A lutocline is developed above a nepheloid layer where the vertical sediment concentration gradient is of maximum. The settlement of the nepheloid layer gives rise to the resuspension events that are characterized with an abnormally high value in sediment concentration within a thin wall layer that is overlaid by a thicker layer with much smaller concentration. This two-layer sediment distribution structure was observed on the continental shelf off the mouth of the Amazon River. These resuspension events may be referred to as ?resuspension hysteresis? with respect to the tidal forcing frequency. The frequency of the resuspension hysteresis is controlled by both the sediment settling velocity and the turbulence intensity, and is lower than that of the tidal forcing. A hyperpycnal plume is also established near the entrance, generating a cross-estuary tidal mean flow on the order of 1 cm s?1 there. Variability in Cd between the spring and neap tides is predicted because of the sediment-induced stratification, and the prediction agrees, in general term, with observations in south San Francisco Bay.