Googong Dam Spillway Upgrade

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Googong Dam was completed in 1978 and forms one of the main water supply storage reservoirs for the Australian Capital Territory (ACT) and surrounding region. It is a 66m high zoned earth and rockfill embankment on the Queanbeyan River and has been classified as an extreme consequence category dam in accordance with the guidelines in DSC (2002). Successive flood events in 1978 and through the 1980s resulted in extensive erosion in the unlined section of the spillway chute, including a large erosion hole, up to 19 m deep and 25 m wide, in the upper part of the unlined spillway chute. Staged remedial works were undertaken in the 1980s to protect the eroded structure.

GHD was engaged in 2006 to undertake a review of the historical performance and prediction of future performance of the structure. This engagement included a review and assessment of previous work, the development and comparison of options and ultimately the detailed design of the preferred arrangement. A scale physical model was constructed as a validation tool during the detailed design phase. The project also included an increase in the capacity of the spillway to discharge the required spillway design flood which had significantly increased due to changes in hydrological methods in recent years. 

The scope of the upgrade works included:

  • Filling of the erosion hole with backfill concrete
  • Construction of an extension to the existing concrete-lined chute with an anchored reinforced concrete slab
  • Placement of anchored reinforced concrete protection over the downstream cut slope
  • Construction of walls in the spillway chute extension up to 17m high
  • Excavation of a plunge pool below the end of the spillway
  • Anchorage of the existing concrete chute slab to resist uplift pressures
  • Retrofitting of drains in the existing concrete chute slab
  • Raising of the walls in existing chute to contain the PMF

The construction of the remedial works was undertaken as a construction-only package by the Bulk Water Alliance involving ACTEW Corporation (the Owner Partners), GHD (the Design Partner) and Abigroup and John Holland (the Construction Partners). Construction of the remedial works was completed in late 2010. Other packages included in the Alliance were the design and construction of the Enlarged Cotter Dam, an inter-basin transfer pipeline from the Murrumbidgee River to Googong Dam and other bulk water assets.

The project was awarded a high commendation in the 2011 Engineers Australia Excellence Awards.

Photos: Completed spillway discharging the flood of record in December 2010 (left) and aerial view of project towards the end of construction in October 2010 (right).  Courtesy of Bulk Water Alliance.

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