Stratification,Mixing and Water Quality in

DarwinWater Supply Reservoirs

ReportNo WSAA 34

November 1991





Theobjectives of the project were to demonstrate the applicability of thereservoir model DYRESM for Manton River Reservoir and Darwin River Reservoir,and to develop causal relationships between the physical behaviour of thereservoirs and water quality. This report details the validation of DYRESM forthese reservoirs, examines the relationships between the frequencies of periodsof stratification and destratification and changes in water quality; anddiscusses management implications.


Comparisonof simulated temperature profiles with field data showed that the modelsimulated the timing of seasonal stratification and destratification fairlyaccurately throughout the validation period and can be confidently used toexamine the impacts of different hydrological and management scenarios on thephysical structure of the water column in Darwin River Reservoir and MantonRiver Reservoir if onsite wind data is available.


Inthese tropical reservoirs oxygen demand is increased due to the high watertemperatures. Iron and manganese concentrations in the hypolimnion increasedramatically following the depletion of oxygen. Water quality problems associatedwith these conditions are largely avoided at the present time by using a systemof variable offtakes and by drawing water from above the thermocline. However,wind induced vertical mixing may result in poor water quality throughout thewater column. This is particularly a problem during the wet season in MantonRiver Reservoir, where topographic sheltering from the prevailing winds resultsin a shallow surface mixed layer.


Incontrast, Darwin River Reservoir is exposed to the prevailing winds, and issubject to more wind induced vertical mixing. Throughout most of the year thethermocline is much deeper in Darwin River Reservoir compared to Manton, andwhen mixing occurs oxygen levels in the surface waters are only slightlyreduced due to the large volume of oxygenated water that mixes withdeoxygenated water.


Poorsurface water quality in both reservoirs is likely to result from the passageof cyclones, which occur either side of the monsoon period when the reservoirsare stratified. Winds associated with cyclones may be strong enough to causecomplete vertical mixing in both reservoirs.


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


Witha fairly simple system consisting of a chain of thermistors in the reservoirand an anemometer and the data transmitted back to a base computer, LakeNumbers could be calculated in real time and provide a valuable management toolfor optimising water quality.


Copiesof the Report are available from WSAA, price $A30. Orders may be placed throughthe Bookshop at or by email to