Sediment Transport in a Coastal Plain Estuary

Observations of net flow and suspended sediment were made in the Rappahannock Estuary, Virginia, to delineate patterns of transport that lead to partial entrapment of sediment in the middle estuary. The estuary is characteristic of the partly mixed type; its main channel is bordered by extensive submerged shoals. River-borne sediments are flushed downstream through the upper estuary mainly over the shoals. In the middle estuary, sediment is partly entrapped on the north shoals or deposited on the basin floor. Another part is redistributed headward in the channel along with estuarine produced organic constituents. Upstream transport by density currents is augmented by alternating tidal flow that favors differential settling and selective transport of coarse-grained particles. Near the head, rapid advective mixing and moderate tidal turbulence act to deter deposition and recirculate sediment downstream over the shoals. Engineering measures in similar systems should be planned to take advantage of natural self-flushing features.