Estimating effective longitudinal dispersion in the Chesapeake Bay

Abstract

An analysis of Environmental Protection Agency's Chesapeake Bay Program hydrographic dataset shows that the bay responds coherently to variability in freshwater flux. Mean salinity and salinity stratification both respond to variability in freshwater flux on time scales of roughly 90 days. Stratification is also influenced by local wind forcing but on much shorter (4–5 day) time scales. The volume of available data allows the effective longitudinal dispersion coefficient to be estimated as a function of either time or space. Values for this dispersion coefficient vary between 200 and 1000 m 2 s −1, with mean values around 650 m 2 s −1. The spatially dependent structure has a maximum roughly 75 km from the head of the estuary, and decreases gradually towards the mouth. The temporally varying effective dispersion varies spatially as the inverse of the estuarine cross-section, and temporally as the cube root of the freshwater flux, and is at least qualitatively consistent with models of estuarine circulation and results of previous field studies. Estimates of the numerical values of the dispersion are useful for better understanding distributions of other tracers within the bay, as well as providing another metric against which numerical models should be measured.