Agriculture is the dominant land use in the Werribee River catchment. Flow regime of the river has been dramatically altered to provide water for drinking and irrigation in this area, resulting in a significant depletion of flows in the lower reaches of the river in recent years. As a result of depleted flows and high levels of nutrients, this area is affected by large-scale accumulation of floating weeds and blue-green algal blooms. 

Due to high nutrient levels, it is unrealistic to reduce weeds and algal blooms through water quality management. Therefore, the best approach is to prevent their build up by supplying a large enough flow. This study assessed the performance of a set of environmental flow releases as the mitigation measure for this ongoing issue.

A 3D hydrodynamic (HD) and advection-dispersion (AD) model of the lower Werribee river, was developed using MIKE by DHI 3D FM model. Hydraulic control points along the river were identified during a field survey. A series of ADCP velocity profiles were collected across the river by Water Technology during 2 environmental flow release events. Modelled velocities were successfully calibrated and verified against the velocity measurements. The AD model assessed the performance of the flow release for residence time improvement and the prevention of accumulation of blue-green algae blooms.

It was concluded that lower environmental flow releases in the range of 6 to 80 ML/d are not sufficient to achieve satisfactory flushing of the lower Werribee River.  An environmental flow of 100-200 ML/d performed best to mitigate algal bloom accumulation.