Comparing calculated and observed vertical suspended-sediment distributions from a Hudson River Estuary turbidity maximum

Suspended-sediment concentrations calculated using a vertical suspended-sed iment distribution equation were compared to observations from a field stud y of the lower turbidity maximum of the Hudson River Estuary. At four stati ons, an instrumented tripod measured vertical profiles of suspended-sedimen t 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 (ba sed on the gradient Richardson number). Suspended-sediment profiles were ca lculated with a stratification-modified Rouse equation, using (1) reference concentrations measured at 20 cm above the bed, (2) estimates of shear vel ocity based on the quadratic stress law, and (3) a constant sediment settli ng velocity of 0.22 cm s(-1). Differences between mean calculated and obser ved total suspended load for each station were - 17, 7, 14 and 58%, respect ively. An uncertainty analysis revealed that the two parameterizations most likely to account for differences of this magnitude were those used for se ttling velocity and stratification. Best results were found when substituti ng a power law relationship for settling velocity based on suspended-sedime nt concentration. This demonstrates the improvement which a power law formu lation can provide over the commonly used constant w(s) parameterization in fine sediment environments. (C) 2001 Academic Press.