Effect of Increasing Depth on Salinity in the James River Estuary

Nichols, Maynard M.

Item

Effect of Increasing Depth on Salinity in the James River Estuary

Nichols, Maynard M.

Abstract

The effects of channel deepening on the salinity and density flow in the James River estuary, Virginia, were studied to predict changes that might affect oyster production. A hydraulic model with 1: 1,000 horizontal and l: 100 vertical scales was employed to integrate three-dimensional changes in salinity and velocity through reaches of variable bottom geometry. After natural characteristics of the tide, current, and salinity were reproduced in the model, tests were run at three levels of steady river inflow, before and after a 3-meter channel deepening. Results were combined with corollary field observations to evaluate changes in present-day ecological conditions. Deepening produced the greatest salinity change in the middle estuary where the major cut was performed. The lower water layer located mainly in the channel became saltier by about 0.5 part per thousand, whereas the upper layer over the oyster shoals became fresher by about 0.2 part per thousand. Changes in bottom water salinity were greatest at intermediate inflow and least at very low inflow. High fresh-water inflow created the greatest change in vertical salinity gradient. With greater stratification, tidal velocities were less effective in promoting vertical mixing between lower and upper estuarine water layers, and the net volume transport in each layer was reduced. Since the changes in salinity and flow pattern due to channel deepening were small, no effects inimical to the oyster fishery were predicted. Similarly the prospective changes in sedimentary regime will not offset the beneficial effects of the proposed deepening project.