FRONTAL CONVERGENCE CAUSES SEDIMENT TRAPPING IN THE HUDSON RIVER ESTUARY

Sediment trapping in the turbidity maximum zone of the Hudson River estuary exhibits large variations at seasonal, fortnightly and tidal timescales. Seasonal variations result from changes in position of the salt intrusion due to variations in freshwater discharge. During high flow, the salt wedge is pushed near the mouth, and the large input of terrigenous sediment is trapped in the most seaward reaches of the estuary. When the discharge decreases, the salt intrusion advances, and the sediment trapping zone moves landward. The landward movement of sediment is not continuous, but rather it occurs as fortnightly pulses during spring tides, when a large volume of sediment is resuspended.

The trapping of sediment involves the interplay of resuspension, tidal advection and frontogenesis. During flood tides, resuspension tends to be maximal, and the sediment is transported rapidly landward. During the early ebb, the sediment flux reverses, but as the ebb progresses, the near-bottom salinity gradient becomes concentrated in certain zones due to topographic effects. The gradient increases to form bottom fronts, behind which the ebbing flow becomes weak or may even reverse. The flow convergence at these fronts causes rapid convergence of sediment transport. A stunning example of this convergence process was observed in late May, 2001, based on timeseries data from a bottom tripod. Frontal convergence resulted in high concentrations of sediment during the late ebb, which settled during slack water to produce 20 cm of sediment deposition over approximately 1 hour. The following flood remobilized most of the sediment, leaving several cm of net deposition. This intense trapping process continued over several spring-tide periods between late May and late June, yielding approximately 20 cm of net deposition.