SEDIMENT TRAPPING AND LOCALIZED MUD ACCUMULATION IN TWO ESTUARIES, SOUTHEASTERN US

A region of elevated suspended sediment concentrations in an estuary, or estuarine turbidity maximum (ETM), has been identified in systems with a wide range of river discharge, sediment input, size, and tidal range. A number of mechanisms have been suggested for the occurrence of the turbidity maximum, but typically it occurs at the landward extent of the salt intrusion and results from a combination of processes involving enhanced settling from river input due to flocculation and landward advection in bottom waters to the point of convergence at the null zone. The ACE Basin and Winyah Bay, SC, are two estuaries that exhibit elevated concentrations of suspended sediment well downstream of the primary ETM at the head of the salt intrusion, and these regions coincide with localized areas of fine bed sediments bounded by sands. In the ACE Basin, the mud reach is found in the lower Ashepoo, a river with minimal sediment input, but is downstream of Fenwick Cut, a manmade connection to the larger, Edisto River which has a greater sediment load. The Cut experiences rapid accumulation of fine-grained sediments and serves as a source to the Ashepoo where sediments are rapidly trapped in an area of persistent salt stratification, forming fluid muds during periods of high discharge. Winyah Bay, a partially-mixed estuary with a tidal range of 1 m and an average fresh water discharge of 500 m³ s^-1 exhibits a traditional ETM, as described above, with fluid muds and rapid accumulation rates. However, areas of high concentrations of suspended sediments in the water column also occur downstream and are not typically associated with tidal resuspension. These areas are associated with recurring fronts during short periods of both the flood and ebb phases of the tide. During ebb, sediment trapping is enhanced due to increased stratification, convergence of bottom flows, and a reduction in boundary shear stress. During flood, a tidal intrusion front can form, concentrating sediments in the near bottom where they are deposited during decelerating currents. Both instances act to trap sediments in lower regions of the estuary, creating areas of increased deposition of fine sediments.