Paul F. Hamblin

Meteorological Forcing and Water Level Fluctuations on Lake Erie

Abstract Analyses of both overtake wind and water level data in Lake Erie are presented for the summer periods of 1979 and 1980. The new data sets are compared to the more conventional land-based data and, where appropriate, to the results of several numerical models. Differences between land-based and open-water data are particularly significant on the shorter time scales of a few hours to several days in length (for winds and less than a few hours in the case of water levels). In practical applications of Lake Erie meteorological forcing functions these differences are shown to be important to such problems as storm surge forecasting. Thus, further research into the relation between land-based and lake-based forcing is recommended.

Modeling Nutrient Upwelling in Lake Malawi/Nyasa

Abstract In response to a need for improved understanding of the nutrient budget for an African Great Lake, a field investigation of the physical and chemical limnology in Lake Malawi/Nyasa was undertaken. Continuous water temperature profiles at two stations along the longitudinal axis in the southern portion of the lake, shore-based winds, and frequent nutrient profiles at one of the temperature measurement sites provide the basis for a simple time dependent mathematical model of the vertical fluxes of six key nutrient species. Two components of nutrient flux are simulated, namely, the turbulent and the upwelling flux. Due to persistent southerly winds, vertically directed currents known as upwelling flows dominate the transport except near the surface where turbulent transport plays a role. On four occasions from May to August, 1997, AVHRR thermal imagery, nearshore winds, and longitudinal temperature transects demonstrated upwelling in the southern extremity of the lake. Thus, upwelling is responsible for the bulk of nutrient transport into the photic zone in the area studied. These results represent the first quantification of nutrient upwelling for a large African lake.

Response of the Thermal Structure of Lake Ontario to Deep Cooling Water Withdrawals and to Global Warming

It has been proposed to cool buildings in downtown Toronto using cold, deep water withdrawn from Lake Ontario. 50 m3 s−1 of sub-thermocline water would be withdrawn from a depth of 80 m at a temperature close to 4°C, distributed to chillers throughout the city, and discharged on the surface in the nearshore zone at a temperature of 12°C. Compared with electrically-powered chillers, the Deep Lake Water Cooling (DLWC) scheme is thermodynamically elegant and environmentally beneficial in many ways. Using a one-dimensional thermodynamic model of Lake Ontario, this paper assesses the physical impact of the DLWC scheme on the whole lake under present and future conditions. We conclude that the lake could presently absorb the heat from the proposed Toronto installation and 20 others like it without major lake-wide physical changes. At a conservative estimate of 1000 m3 s−1 with an 8°C temperature differential, the DLWC scheme would reject approximately 33.5 GW of “waste heat” to Lake Ontario. Under our best estimate of a 2 x CO2 climate scenario, the deep waters could be 2–3°C warmer than they are now. The DLWC system would be less efficient, but the lake would retain a similar overall cooling capacity. Neither biological consequences nor the local physical impact of the discharge of heated subsurface water from the DLWC system are addressed in this study.

Wind forcing of internal waves in a long narrow stratified lake

Abstract A vertically integrated approach to the analysis of thermistor chain and wind data from a long, narrow, stratified lake indicates that forcing events are short relative to the response time. Correlations between forcing and isotherm tilts clearly indicate stratification response times dependent on internal wave speed. In addition, it is possible to infer basin-scale internal wave damping where over 80% of the potential energy in the internal wave setup is lost in one internal wave period.

Water Quality Modeling of Caged Aquaculture Impacts in Lake Wolsey, North Channel of Lake Huron

Abstract Nutrient release from fish farming could have an impact on water quality in restricted areas of the Great Lakes. Unprecedented hypolimnetic anoxia, discovered in a small bay of the LaCloche Channel in the vicinity of a caged aquaculture operation (Ontario Ministry of Environment, September 1997), stimulated an environmental review by provincial regulators of the increasing number of caged aquaculture operations in the North Channel area of Lake Huron. They found that water exchange rates between the restricted zones around the cages and surrounding waters are a key component in determining assimilative capacity and evaluating potential impacts on water quality. A caged aquaculture operation exists in Lake Wolsey (area 2,315 ha), a restricted embayment along the northwestern shoreline of Manitoulin Island and which ultimately drains into the North Channel of Lake Huron. The morphometry of Lake Wolsey lends itself to the formulation of a simple box model for the change in total phosphorus concentration due to loading from the caged operation and a specified exchange across the entrance of the embayment. Using water level readings at the entrance of the bay, the geometry of the entrance, and a one-dimensional exchange flow model, the exchange was found to be in the order of 14.3 m 3 /s on average. This relatively vigorous exchange is due to the presence of large wind and lunar tides in the North Channel. Such an exchange flow would result in a residence time of 215 days for the 25 m deep basin. Based on an estimated open water season (May to November) loading of 1,352 kg phosphorus for the fish culture operation, the resulting elevation of the total phosphorus concentration would be 5.1 μg/L above the measured background level of 8 μg/L, a total close to that observed, 12 μg/L. Similarly, in the ice-covered season with lower feeding rates the model predicted 7.9 μg/L comparing favorably to the observed spring turnover value of 8.0 μg/L. In Lake Wolsey , the long-term effects of caged aquaculture at its current level of production are probably minimal but in the short-term could result in algal blooms and increased oxygen demand.

Surface Meteorological Observations over Lake Malawi/Nyasa

Abstract Over-lake surface meteorology is essential for the estimation of lake evaporation and for the forcing of lake water quality and circulation models. A roving meteorological station was mounted aboard the research vessel, R/V Usipa, on Lake Malawi/Nyasa. Ship velocity and position were recorded, thus permitting winds to be corrected aboard the moving platform. This type of data is particularly sparse for tropical lakes. Of the eight full-lake cruises over the period from February, 1997 to May 1999, four yielded usable data comprising 3,316 30-min averages but restricted mainly to the wet season. In addition to winds, air temperatures, relative humidities, and water temperatures were recorded. Similar parameters were measured concurrently at up to four fixed locations on the shoreline providing a basis for comparison. An examination of the longest running series of winds and air temperatures showed no obvious interannual differences in wind speed and air temperature. Based on wind spectra, winds were divided into a diurnal component and a longer term smoothed component by digital filtering. Analysis of the shipboard winds shows an unexpectedly strong diurnal wind field in the open lake which appears to be dominated by the diurnal atmospheric circulation along the eastern shoreline. This effect is likely due to the interaction between southeast trade winds and the diurnal wind field. The long-term smoothed winds vary along the longitudinal axis of the lake, being weakest at the extremities and strongest in the middle. Calculations of an average evaporation rate based on observed meteorological data from all temporal scales using three methods resulted in a mean of 6.4 ± 1 mm/d. Diurnal meteorological fluctuations accounted for 36% of the total evaporation.

A Comparison of Drogues, Current Meters, Winds, and a Vertical Profiler in Lake Erie

Abstract As part of an extensive survey of the temperature and currents of the central basin, Lake Erie, a vertical automatic profiling system (EVAPS) was deployed at an offshore location for 3 days in August 1980. This system, which is considered to be new to Great Lakes studies, consisted of acoustic current meters, temperature sensors, and a pressure gauge which by the use of a bottom mounted winch could be made to ascend and descend through the water column. The profiler permitted the collection of continuous vertical temperature and velocity profiles. From these profiles, temperatures and the horizontal components of the flow were extracted at several depths and compared to standard measurements from current meters, drogues, and a meterological buoy. Such a comparison demonstrated satisfactory agreement of the conventional data with the profile measurements and thus establishes the validity of this novel approach to measurement of flow in a large lake.

Observations, Evaporation and Preliminary Modelling of Over-Lake Meteorology on Large African Lakes

Water quality models of lakes require accurate specification of the advective and turbulent transport fields. These are usually obtained from lake hydrodynamic models. In turn, hydrodynamic models require accurate specification of meteorological forcing. Uncertain specification of meteorological forcing over large lakes is one of the main reasons for the lack of correspondence between three-dimensional hydrodynamic models and observations of lake currents, temperatures and water levels. This is especially the case for intermontane lakes where sheltering effects of the surrounding topography disturb the air flow and generate such other mesoscale meteorological features as slope winds which can reinforce lake breezes.

Evaluation of Sediment Traps in Lake St. Clair, Lake Ontario, and Hamilton Harbour

Abstract Sediment traps are simple, inexpensive devices that yield time-integrated samples of material suspended in the water column. Although many different designs have been proposed, there seems to be general agreement that the cylindrical settling tube is a design capable of yielding quantitative results in relatively calm waters. The reliability of these or any other trap in shallow water with significant wave orbital motions is unknown. This paper describes our attempts to assess the field performance of conventional settling tubes and two versions of horizontally-ported chambers in shallow, wave-dominated water. We find that catch rates of the horizontally-ported chambers correlate strongly and positively with the root mean square horizontal flow velocity over the trapping interval. We suspect a similar dependence exists for the settling tubes. With present levels of understanding, results from sediment traps of these designs deployed in shallow, wave-dominated water should be given qualitative status only.

Modeling the thermal stratification of water filled mine pits

Meromixis, where a stratified body of water rarely mixes completely throughout its depth, is considered beneficial for containment of undesirable chemical species in water filled abandoned mine pits. Here we discuss a modeling effort examining the development of the thermal stratification in such a body of water. The model is a simple algorithm based on a one dimensional diffusion equation. The heat fluxes and boundary conditions are discussed as well as the mechanisms affecting turbulent diffusion and penetration of the mixed layer. This is especially important in the prediction of possible fall and spring overturns. The model is described in the context of the Brenda Mines Pit near Peachland. The temperature data does not strongly indicate meromixis, however, dissolved oxygen measurements show there might be a barrier to vertical penetration of heat and momentum.