Author(s): Jorg Imberger
Linked Author(s): Jörg Imberger
Keywords: No Keywords
Abstract: Presently there are many one dimensional and three dimensional water quality models available on the market. These models are readily available and are used extensively throughout the world to assess the impact of management strategies on ecological processes in these natural systems However, in the literature there is very little information available as to the accuracy of the hydrodynamics, the suitability of the process description of the ecological processes or the predictive capability overall of such models. The presentation will begin with an example of how one such model, the one dimensional model DYRESM, can be used to design destratification systems for lakes and reservoirs. The example chosen is from Lake Samsonvale, a major water supply reservoir for the city of Brisbane in Queensland, Australia. The model was used to design the destratifier and to assess how well the bubbles mixed the water when imbedded in the seasonal cycle of the natural mixing processes. The aim of the installation was to reduce the levels of manganese and iron in the deeper part of the water by inducing an oxygenation through the water surface transfers of the lake as a whole. A secondary aim was to see whether the increased oxygen at the bottom would reduce the sediment nutrient supply sufficiently to reduce the likelihood of blue green algae blooms which had been documented over the previous years. The bubbler was designed and installed and two years ecological data now exists and has been analysed. The results show that the bubbler was successful at increasing oxygen levels, decreasing nutrient fluxes from the sediments, almost completely reducing the concentrations of manganese and iron, and in the first year of operation, also decreasing the total biomass of cylindrospermopsis to a very considerable degree. However, monitoring during the second summer showed that in areas away from the bubbler, the concentrations of cylindrospermopsis had returmed to their pre-bubbler installations levels. From the hydrodynamics point of view, the results also showed that the algorithms used in the model did not accurately predict the rate of destratification and showed that it is necessary to understand much better the entrainment processes into a bubble plume. This example therefore highlights the need for careful post simulation comparisons to continually improve our numerical codes. It also revealed the need for one dimensional, two dimensional and three dimensional water quality models, each being required for a different purpose.