ModellingOptimum Conditions for Reservoir Destratification

usingMechanical Mixers

ReportNo WSAA 24

April 1991

SYNOPSIS

Forthe last two years three submersible mixers have been used at Myponga Reservoirin South Australia for destratification. The mixers lift cold water from nearthe bottom of the reservoir to the surface and offer a number of advantagesover a conventional air curtain including less power; lower noise levels; andgreater flexibility of operation.

Thisreport presents the results of using a mathematical model to simulate mixerperformance. The optimum position and orientation for the mixers have beendetermined, a number of operating scenarios examined in detail, and theperformance of the mixers compared with a conventional aeration destratifier.The results are generally applicable to any reservoir.

Themodel has also been used to assess the effectiveness of operating a singlemixer fitted with a shroud. The purpose of the shroud is to stop entrainment ofwarmer water as the cold water is lifted to the surface. A mixer with shroudwas field tested and the data collected compared with model simulations.

Theeffect of the mechanical mixers on water quality was assessed by acomprehensive monitoring program. Data for nutrients, heavy metals,temperature, dissolved oxygen, chlorophyll and phytoplankton are discussed indetail. Changes in the phytoplankton community structure since 1963 have alsobeen examined to determine if the mixers have had a major impact.

Thereport concludes that the model can be used to simulate the performance of themixers but that insufficient field data was available to validate the mixerwith shroud. Although the mixers have improved water quality in the reservoirthey have not been an unqualified success.

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