Modellingand Design of Reservoir Aeration Destratification Systems

ReportNo WSAA 23

April 1991

SYNOPSIS

Thisreport describes the results of a study programme directed at improving designmethods for air bubbler destratification systems for reservoirs. The studyconsisted of laboratory and field investigations; further development of thebubble plume algorithm installed in the reservoir simulation model DYRESM;validation of the model against specific laboratory data and against field datafrom the Harding Dam in the northwest of W.A.; formulation of a designprocedure; design of an aeration system for Harding Dam; monitoring of theaerator performance and validation of the simulation algorithm against thisfield data; and recommendations regarding future operation of the system inHarding Dam in the light of the observed and simulated bubbler performance.

Thelaboratory investigations identified a parameter, the plume number, for whichthe efficiency of destratification was a maximum. This plume number includesthe 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 numberis larger than the optimum value, there is insufficient air flow for the waterfrom the deep part of the reservoir to be carried to the surface; if the numberis 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, thereis therefore an optimum air flow rate. Plume theory also indicates that thetotal entrainment from a number of sources is greater than that for a singlesource with the same total air flow rate.

Thebubble plume simulation algorithm included in the reservoir simulation modelDYRESM was validated against the laboratory data and limited field dataobtained in Wungong and Harding Dams in W.A. The model was used to design anaeration system for Harding Dam, based on principles that are applicable forall reservoirs. Most designs will depend, however, on the operating criteriaspecified by the operating authority. These could include maintenance of adestratified condition over seasonal time scales, rapid destratification of aninitially stratified reservoir, or maximising the efficiency of energyconversion to achieve these or other goals.

TheHarding Dam aerator was installed and monitored during 1989. The design waspartially successful in destratifying the reservoir, and after evaluation andmodification of the model using the intensive data set gathered during theperiod of operation, modification of the design was recommended. The modifiedaeration model was shown to be a reliable tool for designing reservoirdestratification systems. The model is a significant improvement over currentdesign methodologies as it allows the designer to evaluate alternative designstrategies under temporally varying conditions. The model makes it possible forthe operator to adjust the aeration strategy to suit the changing conditions sothat increased destratification efficiencies may be achieved.

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