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UBC Theses and Dissertations

Impact of an artifical circulation device on the heat budget of an ice-covered mid-latitude lake Rogers, Christopher K.

Abstract

Two lakes located in the Southern Interior Plateau of British Columbia were selected for a field investigation in order to assess the impact of artificial circulation on the heat budget of a small mid-latitude ice-covered lake. The heat transfer algorithm developed by Patterson & Hamblin (1988) for high latitude lakes was extended to include the impacts of snowmelt due to rain, sediment heat transfer, snow-ice formation, and day today variations in snow density, snow conductivity and albedo. Since the lakes considered are nearly isothermal in winter, the new model ignores the internal hydrodynamics of the problem. This model was tested on Harmon Lake, which was selected as a control for the study. The model was further modified to include the effects of artificial circulation at nearby Menzies Lake. These effects include polynya development, and a substantial reduction in average temperature. Heat losses due to free convective evaporation and direct snowfall on open water were added to the set of standard aerodynamic formulae used to determine the net meteorological heat flux across a water surface. Turbulent heat transfer from the circulated water to the ice cover was estimated based on an empirical surface velocity relationship derived from field measurements. The size of the polynya is estimated by means of a simple heat balance which also involves the surface velocity function. The Harmon Lake predictions agree well with the field data. All discrepancies could be accounted for by parameter uncertainties and expected observation error. It was found that sediment heat transfer may be important in early winter in preventing a net loss of heat from the lake water. Significant heat gains in the latter part of winter, however, are attributed to the penetration of solar radiation. Once calibrated, the Menzies Lake predictions are also good. It was found that, over the period in which lake temperature dropped, the average heat loss due to turbulent heat transfer between water and ice was three times that across the polynya surface. The former heat flux continued to increase as the lake warmed up again, while the latter fell, on average, over a short period until increased solar heating resulted in a reversal in the direction of heat transfer across the polynya. Discrepancies between early winter ice thickness predictions and observations, could not be accounted for. It is suspected, however, that these discrepancies are a result of the impact which the heat flux across the polynya may have on the heat flux through the ice and snow cover. In this thesis, it is assumed that these two fluxes are independent of one another.

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