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Lakes and reservoirs provide us with important stores of freshwater, and with popular sites for recreation.  Many lakes and reservoirs are also polluted, and play an important role in global biogeochemical cycles.  Through an NSF-funded interdisciplinary project, combining expertise in hydrodynamics (Environmental Hydrodynamics Laboratory) and biogeochemistry (Global Change and Watershed Biogeochemistry Lab), we are studying the processing of pollutants in lakes, with particular emphasis on Nitrogen and methane pollution.

The lakes we have studied display a fascinating range of hydrodynamic behaviour, from daily-period development and destruction of stratification, to newly-discovered vertically propagating lakewide internal waves.  The internal waves cause dramatic fluctuations in near-bed stratification and mixing rates, which are likely important to lakebed removal of nitrate pollution.  Internal waves sometimes form undular bores or break as they propagate up the sloping lakebed (measurements of an undular bore are shown below).  You can read more about these waves, and nearbed stratification/mixing, in two powerpoint presentations, one for a general audience, and another (two files) aimed at physical oceanographers.  A more detailed description of progress to-date is provided in these four papers:

1. Vertical propagation of lakewide internal waves, published in the journal `Geophysical Research Letters’.

2. Upslope internal wave Stokes drift, and compensating downslope Eulerian mean currents, observed above a lakebed, published in the Journal of Physical Oceanography (examines how waves transport water laterally through the lake, in a manner reminiscent of the Stokes drift that is balanced by undertow on a beach).  

3. Turbulent production in an internal wave bottom boundary layer maintained by a vertically propagating seiche, published in the Journal of Geophysical Research (Oceans) [examines the energy balance relating supply of internal wave energy to the lakebed and loss of wave energy to turbulence].  

4. Chemical mixing in the bottom boundary layer of a eutrophic reservoir: The effects of internal seiching on Nitrgen dynamics (examines the interactions between waves and near-bed biogeochemistry, in press for the journal `Limnology and Oceanography’).

Ph.D. student Jeff Nielsen is now finding that simple models, based on Richardson-number closures, predict the observed thickening of near-bed mixed layers, allowing us to predict rates of irreversible boundary layer mixing.  This work is continuing in collaboration with the Global Change and Watershed Biogeochemistry Lab.

UndularBore