Predicting the effects of mechanical destratifiers on water quality in Toowoomba’s reservoirs

The study of the effect of mechanical destratifiers on water quality in Toowoomba’s reservoirs is urgently needed to understand the behaviour of the reservoirs for management purposes. In this study, a 1-D hydrodynamic model (DYRESM) coupled with an aquatic ecosystem model (CAEDYM) is adopted for water quality prediction in the water column for the next 50 years. The AWBM hydrological model and the ClimGen weather generator model are used to support the data preparation for water quality prediction. The simulation results are separated into two periods which are the period November – April represented by the warm period and the period May – October represented by the cold period. The results are used to assess the sustainability aspect and risk factors in the vertical profiles of the reservoirs. A new water quality index is introduced to assess the water quality level in the storages without and with the use of the mechanical destratifiers. The main conclusions of this study are summarized below: (1) A strong thermal stratification occurs in the storages during the warm period. In this period, Cyanobacteria have a high concentration. (2) The water quality index (WQI) in Cooby storage will be a good or an excellent level. The WQIs of Cooby tend to decrease from an excellent level to a good level without the use of the mixers. The continuous operation of the artificial mixers is able to increase the WQI values by an average of 15 grade points and produces excellent water quality for the next 50 years. (3) The WQIs of Cressbrook storage will be a good or an excellent level. The WQIs of Cressbrook reservoir have a tendency to increase from a good to an excellent level. The WQIs in the surface layer remain the same without and with the use of the mechanical mixers. The artificial mixers are able to slightly increase the WQI values at the pumping elevation and the average of all layers by an average of four grade points. (4) Without artificial mixers, safe levels of raw water from the Cooby storage can be attained at 8 – 10 m depth. The best/optimal water quality can be achieved with multi-level withdrawals. The use of the artificial mixers can extend the withdrawal range to 9 – 20 m depth. The optimal water quality can be achieved with a fixed pumping elevation at 15 m depth. (5) Without surface mixers, the safe levels can be attained at the layer between 14 m and 30 m depth all the time. The nitrate and total phosphorus levels have a high probability of being unsafe. The use of the mechanical surface mixers is able to widen the range of the safe level between 16 m and 37 m depth. This can give more alternative layers for withdrawals.