2009 – Stirling Dam – Conservation of Water during Remedial Works

Bob Wark, Alex Gower. Graeme Mann

Stirling Dam is a 53 m high extreme hazard zoned earthfill dam located in south west WA. Construction was completed in two phases between 1939 and 1947. Recent safety reviews confirmed that the societal risk exceeded the ANCOLD guideline tolerable limit due to inadequate spillway capacity and the lack of embankment filters. Remedial work would involve: widening the spillway; removing the downstream shoulder of the dam; adding downstream filters; and reconstructing the downstream shoulder fill. Rock from the spillway excavation would be used to provide the fill for the downstream shoulder. The works optimisation involved a 3 m raising of the embankment to provide the required spillway capacity.

The design criteria included: ensuring the risks of failure during construction were to be no higher than the risks prior to remedial works; maintaining reservoir operation during construction; and no river releases based on median monthly inflows. This required the spillway crest to be temporarily lowered during construction to provide adequate flood capacity while the embankment height was reduced. A key feature of the design had also been the scheduling of the storage drawdown and remedial works to manage the failure risk and probability of river releases during construction. Higher than average inflows after contract award resulted in water levels above the scheduled drawdown curve. This lead to river releases to prevent spillway flows and rescheduling the  onstruction over two seasons.

Keywords: Stirling Dam, water conservation, embankment filters, spillway capacity, construction scheduling