A numerical model of the James River estuary, Virginia, U.S.A.

Abstract

A numerical model of a partially mixed estuary is postulated, in which temporal changes in density current and vertical salinity stratification at a given point depend only on the longstream gradient of cross-sectional average salinity, S̄, and the tidal speed, ‖U‖, averaged over a tidal period. The salt conservation requirement leads to a partial differential equation on S̄: under steady state conditions this becomes an ordinary differential equation, that can easily be solved analytically for an estuary bed of any shape. The qualitative features of the solution are similar to those of real and laboratory model partially-mixed estuaries.

The time-dependent equation on S̄ is soved numerically, for the James River, U.S.A., in the 2-month period following Hurricane Agnes (June 1972). Agreement with observation is good, considering the extreme simplicity of the model. In particular, it is found in both observation and model, (i) that salt penetration up the James River appears to respond strongly and rapidly to changes in salinity at the mouth, overshadowing the responses to changing river flow and the spring neap cycle; (ii) that stratification depends primarily on the spring-neap tidal cycle, and very little on river flow.