Modelling and Design of Reservoir Aeration Destratification Systems

This report was produced for the Urban Water Research Association of Australia, a now discontinued research program.

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Modelling and Design of Reservoir Aeration Destratification Systems

Report no. UWRAA 23

April 1991

Synopsis

This report describes the results of a study programme directed at improving design methods for air bubbler destratification systems for reservoirs. The study consisted of laboratory and field investigations; further development of the bubble plume algorithm installed in the reservoir simulation model DYRESM; validation of the model against specific laboratory data and against field data from the Harding Dam in the northwest of W.A.; formulation of a design procedure; design of an aeration system for Harding Dam; monitoring of the aerator performance and validation of the simulation algorithm against this field data; and recommendations regarding future operation of the system in Harding Dam in the light of the observed and simulated bubbler performance.

The laboratory investigations identified a parameter, the plume number, for which the efficiency of destratification was a maximum. This plume number includes the effects of the stratification, the air flow rate and the reservoir depth, based on the performance of a single source bubble plume. If the plume number is larger than the optimum value, there is insufficient air flow for the water from the deep part of the reservoir to be carried to the surface; if the number is smaller than the optimum value, there is more air supplied than is required. In either case, the efficiency of energy conversion from that input by the airflow to mixing is not optimal. For each stratification and water depth, there is therefore an optimum air flow rate. Plume theory also indicates that the total entrainment from a number of sources is greater than that for a single source with the same total air flow rate.

The bubble plume simulation algorithm included in the reservoir simulation model DYRESM was validated against the laboratory data and limited field data obtained in Wungong and Harding Dams in W.A. The model was used to design an aeration system for Harding Dam, based on principles that are applicable for all reservoirs. Most designs will depend, however, on the operating criteria specified by the operating authority. These could include maintenance of a destratified condition over seasonal time scales, rapid destratification of an initially stratified reservoir, or maximising the efficiency of energy conversion to achieve these or other goals.

The Harding Dam aerator was installed and monitored during 1989. The design was partially successful in destratifying the reservoir, and after evaluation and modification of the model using the intensive data set gathered during the period of operation, modification of the design was recommended. The modified aeration model was shown to be a reliable tool for designing reservoir destratification systems. The model is a significant improvement over current design methodologies as it allows the designer to evaluate alternative design strategies under temporally varying conditions. The model makes it possible for the operator to adjust the aeration strategy to suit the changing conditions so that increased destratification efficiencies may be achieved.

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