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You are here : eLibrary : IAHR World Congress Proceedings : 36th Congress - The Hague (2015) ALL CONTENT : Hydro-environment : Currents and heat fluxes induce stratification leading to hypoxia in green bay, lake michigan
Currents and heat fluxes induce stratification leading to hypoxia in green bay, lake michigan
Summer bottom water hypoxia has been a persisting water quality issue for decades in Green Bay, Lake Michigan¡¯s
largest embayment. The nutrient loading of the bay has been fairly constant, yet the magnitude and duration of hypoxia is
highly variable from year to year. The bay¡¯s morphology (~ 22km x 190 km) with high riverine inflow and restricted mixing
at the southern end of the bay, combined with large water mass exchange with Lake Michigan at the northern end of the
bay, causes changes in the hydrodynamic structure play and sets up water column stratification that leads to hypoxic
conditions. The onset of hypoxia is related to thermal stratification, which results from both direct meteorological forcing,
i.e. low winds, high air temperatures, and increased solar radiation, and from indirect meteorological forcing that drives
circulation patterns resulting in the southerly incursion of cooler bottom waters onto highly reducing organic rich
sediments. This circulation pattern can stratify a well mixed water column within hours, and can set up stable stratification
that persist for days to weeks during which time sediment oxygen demand rates are sufficient to deplete hypolimnetic
oxygen. Therefore the morphometry, circulation patterns, and the thermal input-output balance of the bay interact to
produce conditions that lead to variations in the degree, extent and duration of hypoxia. Previous field measurements
showed that it is common for two layers to flow through the mouth of the bay in opposite directions during the stratified
season. A 3D hydrodynamic model developed in this study, supported by existing and new field measurements,
demonstrates the onset of stratification by the combined effect of surface heat flux and near bottom cold water transported
southerly from Lake Michigan. Model and field data reveal layered flow, with near-bottom cold water transported into the
bay and near surface warmer water transported out of the bay. Model visualizations and calculation of net transport for a
cross section of the bay help to understand the time scales involved in the stratification process.
File Size : 2,375,010 bytes
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Chapter : IAHR World Congress Proceedings
Category : 36th Congress - The Hague (2015) ALL CONTENT
Article : Hydro-environment
Date Published : 18/08/2015
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