Effects of cross-channel bathymetry and wind direction on destratification and hypoxia reduction in the Chesapeake Bay

A coupled estuarine hydrodynamic model and water quality model were used to analyze differences in destratification and anoxia/hypoxia reduction by wind directions in the north-south oriented Chesapeake estuary, USA. The predominant cross-channel bathymetry in the Bay's anoxic center is asymmetric with a steeper and narrower shoal on the eastern shore than on the western shore, which modifies wind-induced circulation differently for two opposite wind directions. Model experiments of winds for 2-day at 8 m/s indicated that, for a stratified water over the aforementioned asymmetric bottom topography, the easterly wind caused greater destratification and hypoxia reduction than the westerly wind. This is a result of differential modulations on the two wind-induced cross-channel circulations by the asymmetric cross channel bathymetry. The downwelling along the gentle slope in the easterly wind was characterized with stronger baroclinicity than the downwelling along the steep slope (nearly perpendicular to surfaces of constant density) in the westerly wind. On the broad slope, there undergo greater contrasting density readjustments to the vorticity changes around the bottom boundary layer (BBL) during upslope and downslope motions. During the upslope condition, the flow in BBL tends to decelerate under adverse pressure gradient which leads to a stable condition in the outer layer; whereas, during the downslope condition, the BBL tends to accelerate under favourable pressure gradient, which leads to unstable condition in the outer layer of the large scale flow. Overall, the easterly wind caused greater anoxia reduction than the westerly wind during the entire wind period. A similar case was found for northerly versus southerly winds in the early stages of the wind period; modulated by the aforementioned bathymetry on the wind-induced cross-channel circulation, the northerly wind caused greater anoxia reduction than the southerly wind. However, as wind continues, the wind-induced along-channel circulation influences a larger area of greater hypoxia in the mid-Bay, by which the southerly wind causes a greater destratification and anoxia reduction than the northerly wind. This can supersede the greater destratification and anoxia reduction by the northerly wind under the bathymetry- affected cross-channel circulation.

Keywords: Chesapeake, hydrodynamic, water quality, hypoxia, destratification, bathymetry, wind