Development Of The Turbidity Maximum In A Coastal Plain Estuary : Final Report

A study of the turbidity maximum in the Rappahannock Estuary; Virginia was conducted to determine how high concentrations of suspended sediment accumulate to form a maximum. Time-series observations of current velocity, salinity and suspended sediment over 8 to 18 tidal cycles reveal that the maximum forms in a convergence of bottom residual currents near the transition between fresh and salty water. Sediment supplied mainly by the river is transported into the convergence by density currents and accumulates since velocity is nearly zero and settling exceeds upward mixing. The maximum forms in the middle estuary after freshet or flooding and shifts upstream with a landward shift of the salt intrusion head and diminished river inflow. At the same time, its intensity is reduced by settling out, reduced strength of the convergence and increased mixing. Prime prerequisites for development are a strong convergence and high river inflow. The maximum modulates transport through estuaries to the sea by trapping materials and deposition. High turbidity can be alleviated by increased haline mixing and reduced inflow.