2001 – The Augmentation of Spillway Capacity and Storage Capacity of Candowie Dam

Mike Taylor and Doug Halloran

Candowie Dam is a 15m high embankment dam with a storage capacity of 2182 ML. It is the primary source of water for the Westernport Region Water Authority which includes Phillip Island and the town of Cowes south- east of Melbourne.

The existing spillway, comprising a 21m long concrete ogee profile crest discharging into a concrete chute which converges to a width of 7m, has a capacity to only accommodate the I in 6 000 annual exceedance probability (AEP) flood, well short of the required capacity of the 1 in 40 000 AEP flood.
In addition, Westernport Water would like to increase the yield of Candowie Dam as far as economically possible, within the scope of the spillway works.

A solution has been developed whereby the spillway capacity could be increased to accommodate the 1 in 40 000 AEP flood and at the same time the full supply level could be raised by 900mm resulting in an increase in storage of 573 ML and an increase in yield of 580 ML per year.
The solution comprises the following:

1. Lowering the existing spillway crest by 850 mm
2. The installation of 1.75m high precast concrete ‘Hydroplus’ fusegates on the lowered crest.

The fusegates are designed to tip off incrementally with the initial tip off occurring when the flood exceeds the 1 in 200 AEP flood. The tip offs are actuated purely by hydrostatic pressure developed by the rising flood level and programmed so that at no stage does the outflow flood peak exceed the inflow flood peak.

Westernport Water can accommodate the risk (0.5% per year) of the occasional loss of the existing top 830mm of storage resulting from a tip-off.
The total cost of the augmentation is estimated to be in the order of $ 700 000.