During hotter periods of the year, reservoir stratification results in the warm buoyant surface waters (epilimnion) floating on top of the colder, denser bottom layer (hypolimnion). Dams are commonly designed with deep offtakes which withdraw water from the hypolimnion, resulting in the release of colder-than-natural waters downstream which is referred to as cold water pollution. Due to the significant deleterious impacts associated with cold water pollution, we have investigated mitigation options via reservoir mixing with the aim of raising water temperatures near the deep water offtakes.
An extensive review of 138 destratification systems from 114 reservoirs were systematically assessed (Chaaya et al., 2025) for their ability to successfully mitigate cold water pollution, increase dissolved oxygen, reduce soluble metals, and suppress potentially toxic cyanobacteria growth. If correctly designed, bubble plumes were found to be the best overall solution for mitigating impacts associated with reservoir stratification. A significant finding of our assessment was that the success of bubble plumes is directly related to the provision of sufficient air flowrates to destratify the reservoir.
Commonwealth and NSW Government funding has been secured for a cold water pollution mitigation trial at Pindari Reservoir in northern NSW, Australia. When full, Pindari is 73 m deep with a storage capacity of 312 GL that can result in a large temperature stratification. A required airflow rate of 1000L/s was determined for destratifying established stratification and maintaining destratified conditions under peak summer conditions. This flowrate was determined through a combination of desktop methods and reservoir modelling with the AEM3D model.
Significant power savings could be made by varying the airflow rate depending on meteorological conditions, inflows and outflows, and the storage level. Solar renewable energy can be better designed for both peak demand, average demand, and potential sale of excess energy. An iterative approach was developed running the Pindari Reservoir AEM3D model to determine the minimum airflow required to maintain destratification on a weekly basis over a 10-year timeseries.
This paper presents that the required airflow rate andenergy usage is significantly reduced through a combination of (1) initiating system operation before stratification starts in early spring, and (2) using a variable speed drive (VSD) compressor with adaptive operational management that is guided by in-reservoir temperature measurements along with meteorological, hydrological and water levels observations. Statistical relationships between airflow and environmental variables are presented along with future works proposed during the operational trial to verify and refine these predictions.