Simulation of turbidity maximums in the York River, Virginia

Two of the most important processes in cohesive sediment transport, erosion rate and settling velocity, were the focus of this study. Settling velocities were estimated by the Owen tube method and the acoustic Doppler velocimeter (ADV) method. A novel erosion model, namely a constant erosion rate model, was implemented in a three-dimensional hydrodynamic eutrophication model (HEM-3D) to simulate the turbidity maximums in the York River system, Virginia. Two one-month periods of model simulations were conducted to mimic typical dry (November--December, 2001) and wet (March--April, 2002) seasons. In order to have enough data to verify the model, four slack water surveys were carried out during each period to measure salinity and total suspended solid (TSS) profiles. Because of the unexpected extremely low freshwater discharge during both those periods, all survey results showed similar salinity and TSS distributions. The estuarine turbidity maximums were abnormally located about 30 km upstream from West Point, with TSS concentrations on the order of 102 mg/L. Laboratory Owen tube experiments showed that the settling velocity was related to the TSS concentration, highlighting the importance of sediment availability on settling velocity and the less important salinity effect. The estimated settling velocities from four sets of ADV field measurements were much higher than that from the Owen tube laboratory experiments and better reproduced the turbidity maxima for slackwater simulations. These suggested that turbulence may have a dominant effect on settling velocity, and the ADV method seems to be an effective and suitable way to estimate the settling velocity in turbulence dominated environments. Based on a newly found erosion behavior, a constant erosion rate model was implemented in a three-dimensional numerical model such that erosion occurs only during accelerating phases of the tide. Specifically, the Four Factor Model was suggested that consists of (i) a reference constant erosion rate, (ii) hydrodynamic effects, (iii) spatial variability of the bed condition, and (iv) temporal variability of the bed condition. The Four Factor Model successfully simulated the turbidity maximums in the York River system