Recently completed studies of rainfall data gathered over the past 100 years have prompted dam safety revisions in Australia. Following Bureau of Meteorology revisions to methods for estimating extreme rainfall events, new dam safety regulations were established in 2007.
“While the possibility of an extreme rainfall event occurring is very low, it is possible,” says Phil Webber from Queensland-based water company SunWater. “As a responsible dam owner and operator it would be remiss of us not to take action.”
SunWater owns 19 major dams, 63 weirs and barrages and supplies over 40% of the water used commercially in Queensland. Following the Bureau of Meteorology’s announcements, SunWater embarked on a major dam spillway upgrade programme to proactively respond to the new projections.
A detailed assessment of all the company’s dams was carried out to determine a prioritised list of spillways requiring action. The criteria used to assess the structures included:
• Current rainfall event capability.
• Risk to affected populations should an extreme rainfall event occur.
• Integrity of the dam structure under increased water loads.
• Impact to provision of ongoing water supply during an extreme rainfall event.
• Economic impact of the dam’s failure during an extreme rainfall event.
Based on this, higher priority was given to dams with the lowest capacity to release excess water in a controlled manner, as well as those dams presenting an unacceptable level of risk to the community surrounding the dam. As a result the upgrade programme got underway in 2005 when four dams were selected as having spillways requiring prioritised action.
Fred Haigh dam
Fred Haigh dam was built in 1975. A 52m high rockfill dam with a crest length of 646m it has a reservoir capacity of 586m3x106. This A$4M (US$3.7M) upgrade was identified as being of the highest priority. The seven month project involved raising the top of the dam by 2m and providing a watertight gate to allow operational access to SunWater’s Monduran pump station. The first stage of the upgrade was completed in 2006 and the second stage will be undertaken in the near future.
Bjelke-Petersen dam
The 43m high Bjelke-Petersen rockfill dam was the second spillway to be upgraded, at a cost of A$5.7M (US$5M). Work on this 560m long dam with a reservoir capacity of 125m3x106 was started in February 2007 and completed by October 2007. The main dam and saddle dam wall were both raised by 1.2m and the saddle dam wall was extended. Again the second stage of upgrading this dam, which was originally built in 1988, will be undertaken in the near future.
Borumba dam
The first stage of upgrading the 53mhigh Borumba rockfill dam started in March 2008 and was completed by January 2009. The height of the dam wall was increased by 1.6m. The cost of the work at this 36-year-old dam was estimated at A$5.1M (US$4.7M).
Tinaroo Falls dam
Tinaroo Falls dam was built in 1959 and is a 53m high rockfill dam, 343m in length with a reservoir capacity of 42.6m3x106. Work started on the A$21M (US$19.5M) spillway upgrade in April 2009 and is due for completion by late 2010 – all of the work is to be completed in one stage.
Here the main dam wall will be reinforced with 32×10” wide steel cable anchors which will be inserted up to 80m through the dam wall and the foundation rock below. The cables will be tested to 2000 tonnes to ensure that in an extreme rainfall event the dam will not suffer from any undermining, erosion or lifting and will be able to pass flows safely through the spillway as designed. In addition, a 250m long, concrete protection slab will be placed along the downstream base of the dam wall. The height of the saddle dam will be increased by 30cm and filter zones will be put on the downstream face.
“We are very pleased with the progress we have made to date,” says Col Bendall, SunWater’s area operations manager. The project looks set to be finished on time and within budget.
Five of SunWater’s dams can already safely manage an extreme rainfall event, as identified by the Bureau of Meteorology’s new assessments, and do not require upgrading. These are: Beardmore; Cania; Coolmunda; Kinchant; and Peter Faust.
More photographs and information is available in the book ‘More than just the dam – the story of Tinaroo Falls’ which is available from Atherton Library in Australia
Email – athertonl@trc.qld.gov.au
