Long Oral Presentation

Sam Davidson ¹ , Kathryn Russell ² , Christine Lauchlan Arrowsmith ¹

Why did you do it?

Water supply reservoirs disrupt sediment transport, accumulating sediment which must be removed to maintain normal operations. Traditional desilting methods adversely impact ecosystems and involve costly transportation/disposal of sediment.

Conducted two desilting events at McMahons Weir located on McMahons Creek, a small tributary of Yarra River using a gravity drawdown flushing method. A scour valve was opened fully, and the reservoir water level allowed to draw down, taking sediment with it. Dissolved oxygen was monitored and used to indicate when the desilt should be halted. The effectiveness of the desilting approach was assessed using pre and post bathymetric surveys, drone photography during draw down and the monitoring of suspended sediment concentrations in reservoir outflow. The effects of the desilting on the geomorphology and water quality of McMahons Creek were also monitored.

We learned that with the current method balancing effectiveness and environmental risk is difficult. The method showed potential, with some sediment redistribution in the reservoir during the trial indicating that with full drawdown over a long enough period significant sediment transport through the low flow channel was possible. However, the events were halted early due to a rapid decline in dissolved oxygen from mobilised anoxic mud which presented a risk to fish. A revised method will be tested at a similar nearby site, Armstrong Weir, using an extended drawdown which is aimed at slowly mobilising accumulated anoxic material to allow a full drawdown to be undertaken.

Sediment deposition in medium to small water supply reservoirs impacts reservoir operations and increases maintenance. This method shows the potential to be able to desilt these reservoirs cost effectively and with minimal environmental impact if dissolved oxygen concentrations can be maintained at safe levels. The proposed method also enables sediment connectivity, critical for maintaining channel form and supporting aquatic and riparian ecosystems.