SEDIMENT TRANSPORT ASSOCIATED WITH TIDAL ASYMMETRY IN STRATIFICATION, MIXING AND RESUSPENSION IN THE YORK RIVER ESTUARY

Observations collected along the microtidal, partially-mixed York River estuary suggest that landward transport of fine sediment often results from ebb-flood asymmetries in salinity stratification rather than classical gravitational circulation. Vertically sheared tidal currents interact with the along-channel salinity gradient such that ebb tides advect relatively fresher water over saltier water, enhancing salinity stratification, whereas flood tides advect relatively salty water over fresher water, reducing salinity stratification. Greater stratification on ebb lessens the intensity of near-bed turbulence which, in turn, reduces sediment resuspension during ebb relative to flood. Under this scenario, more sediment is transported in the lower water column on flood than on ebb, causing tidally-averaged sediment flux to be landward near the bed even though tidally-averaged near-bed velocity is minimal or seaward.