Stratification, Mixing and Water Quality in Darwin Water Supply Reservoirs

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

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Stratification, Mixing and Water Quality in Darwin Water Supply Reservoirs

Report no. UWRAA 34

November 1991


The objectives of the project were to demonstrate the applicability of the reservoir model DYRESM for Manton River Reservoir and Darwin River Reservoir, and to develop causal relationships between the physical behaviour of the reservoirs and water quality. This report details the validation of DYRESM for these reservoirs, examines the relationships between the frequencies of periods of stratification and destratification and changes in water quality; and discusses management implications.

Comparison of simulated temperature profiles with field data showed that the model simulated the timing of seasonal stratification and destratification fairly accurately throughout the validation period and can be confidently used to examine the impacts of different hydrological and management scenarios on the physical structure of the water column in Darwin River Reservoir and Manton River Reservoir if onsite wind data is available.

In these tropical reservoirs oxygen demand is increased due to the high water temperatures. Iron and manganese concentrations in the hypolimnion increase dramatically following the depletion of oxygen. Water quality problems associated with these conditions are largely avoided at the present time by using a system of variable offtakes and by drawing water from above the thermocline. However, wind induced vertical mixing may result in poor water quality throughout the water column. This is particularly a problem during the wet season in Manton River Reservoir, where topographic sheltering from the prevailing winds results in a shallow surface mixed layer.

In contrast, Darwin River Reservoir is exposed to the prevailing winds, and is subject to more wind induced vertical mixing. Throughout most of the year the thermocline is much deeper in Darwin River Reservoir compared to Manton, and when mixing occurs oxygen levels in the surface waters are only slightly reduced due to the large volume of oxygenated water that mixes with deoxygenated water.

Poor surface water quality in both reservoirs is likely to result from the passage of cyclones, which occur either side of the monsoon period when the reservoirs are stratified. Winds associated with cyclones may be strong enough to cause complete vertical mixing in both reservoirs.

The relationship between Lake Number, which describes the susceptibility of a waterbody to wind induced vertical mixing, and water quality in the hypolimnion of the reservoirs was examined. A strong inverse relationship between Lake Number and dissolved oxygen concentration was found in both reservoirs. During periods of high Lake Number the dissolved oxygen in the hypolimnion is rapidly depleted and iron and manganese concentrations increase; when Lake Number falls, turbulent vertical mixing transports oxygen to the hypolimnion and iron and manganese concentrations decrease.

With a fairly simple system consisting of a chain of thermistors in the reservoir and an anemometer and the data transmitted back to a base computer, Lake Numbers could be calculated in real time and provide a valuable management tool for optimising water quality.


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