Sediment Retention by a Mediterranean Posidonia oceanica Meadow: The Balance between Deposition and Resuspension

Suspended-sediment concentrations calculated using a vertical suspended-sediment distribution equation were compared to observations from a field study of the lower turbidity maximum of the Hudson River Estuary. At four stations, an instrumented tripod measured vertical profiles of suspended-sediment concentration, current velocity, salinity and temperature through a tidal cycle. Bed and suspended-sediment samples were also analysed to determine inorganic sediment size distributions. Velocities were as high as 1·3 m s-1, with suspended-sediment concentrations up to 2000 mg l-1. When a well-defined pycnocline existed, cross-isopycnal mixing was strongly damped (based on the gradient Richardson number). Suspendedsediment profiles were calculated with a stratification-modified Rouse equation, using (1) reference concentrations measured at 20 cm above the bed, (2) estimates of shear velocity based on the quadratic stress law, and (3) a constant sediment settling velocity of 0·22 cm s-1. Differences between mean calculated and observed total suspended load for each station were -17, 7, 14 and 58%, respectively. An uncertainty analysis revealed that the two parameterizations most likely to account for differences of this magnitude were those used for settling velocity and stratification. Best results were found when substituting a power law relationship for settling velocity based on suspended-sediment concentration. This demonstrates the improvement which a power law formulation can provide over the commonly used constant wsparameterization in fine sediment environments.