2002 – Lake Daylesford Dam: Safety Review Following Overtopping

Mike Taylor, Paul Maisano and Rod Conway

Daylesford Dam forms an ornamental lake, known locally as Lake Daylesford, situated on Wombat Creek within the heart of Daylesford in Victoria. It is a focus of the local tourism industry and is vitally important to the Daylesford community as a recreational, social and environmental asset, with important heritage value.
On 24 October 2000, the 12m high embankment was overtopped following heavy rainfall and was in danger of breaching. This could have resulted in loss of the dam and lake, downstream damage to roads and the environment and possible loss of life. The overtopping of the dam prompted the Hepburn Shire Council, land manager for the dam, to initiate a safety review of the dam as well as the commissioning of a Dam Surveillance Program and a Dam Safety Emergency Plan.

The spillway is of the side-channel type with a 30m long concrete sill at the entrance discharging into a 5m wide unlined trough and chute. The existing spillway can only accommodate a peak flow of 24m3/s, which represents an AEP of less than 1 in 20. The required flood capacity in terms of the latest ANCOLD guidelines on spillway adequacy is for an AEP of 1 in 1 000 which equates to 120m3/s.
Following discussions with Hepburn Shire Council, and an evaluation of public usage of the Lake Daylesford area, it was assessed that the following constraints apply when considering options for increasing spillway capacity:

• A lowering of the full supply level is not an option due to existing shoreside development.
• Any raising of the embankment should be kept to a minimum as the crest forms a cross-roads for a large number of pathways and hiking trails.
• Due to the heritage value of the dam, changes to the appearance of the structure should be minimised.
• Due to the high level of recreational use of the site, the remedial works should be compatible with minimising any disruption to the use of the area by the public during construction.

The proposed solution includes the following:

• Widening the unlined spillway trough and lowering the invert.
• Utilising rock from the spillway excavation for a stabilising fill on the downstream face with a filter zone to reduce the risk of piping failure, with a modest ( <1 m) raising of the embankment crest.
• Reconstructing the spillway crest and access bridge.