Fine sediment trapping, deposition, and remobilization at in tidal salt wedge front

The Connecticut River estuary is an energetic salt wedge estuary characterized by strong stratification, strong salinity fronts, and strong tidal and river velocities. Surficial sediment cores show that the bed is predominantly sandy in the estuary, and yet several distinct regions have significant fine sediment on the bed. We use a hydrodynamic and sediment transport model to examine the processes leading to trapping and accumulation of fine sediment in the estuary. The model reproduces key aspects of the salinity and sediment distributions observed from moorings and surveys as well as the fine sediment distribution found in bed samples. In the model, trapping is associated with bottom salinity fronts during ebbs at multiple bathymetric constrictions along the estuary. A zone of flow convergence and enhanced sediment concentration is created at an oblique angle to the channel where the front intersects the bed. The spatial gradient in stratification and bottom stress promotes deposition downstream of the front, as suspended sediment advects along the pycnocline and settles into the region of decreased turbulence. The trapping can be ephemeral and the sediment resuspended depending on the extent to which mixing destroys the front later in the ebb. Fine sediment remaining in the frontal zone following ebbs is primarily found in deeper channels, but it is remobilized during flood tides and transported laterally to lower stress regions on the shoals. Early in the flood, increased tidal velocities suspend fine sediment throughout the estuary, but at relatively low concentrations. The highest concentrations are due to convergence and enhanced bed stress at the salt wedge front that advects up-estuary with the flood tide. The coupling of ebb tide trapping and flood tide redistribution seems to be a persistent feature, but the location of the fine sediment deposition varies seasonally with the discharge and extent of the salinity intrusion.