 |
| Wivenhoe Dam flood mitigation |
THE Queensland Government is re-assessing the safety of Wivenhoe Dam after a report suggested the risk of having a large volume of water stored above the state capital had been underestimated.
Population growth during the past decade has increased the potential for disaster if Wivenhoe or Somerset dams were to fail.
The report, prepared by the Department of the Interior and the US Army Corp of Engineers, stressed there were "no dam structural safety issues" during the 2011 floods.
Wivenhoe and Somerset dams are classified as having the highest "population at risk" categories in Queensland law.
The report, which backed the actions of the four dam engineers during the 2011 floods, has called for a re-assessment of the risk posed by a possible collapse of Wivenhoe or Somerset, or both.
The State Government says it is already working on a reviewed risk assessment of the two dams.
The report said earlier risk assessments might have overlooked the southeast's rapidly expanding population.
It said previous estimates of a failure at both Somerset and Wivenhoe dams during a flood had been for a potential loss of 89 lives.
However, those estimates did not take into account the recent population explosion in the southeast.
With more residents now living in the area, escape to a safe area in the event of an emergency would be made more difficult. This could potentially result in more lives being lost.
"It was assumed that there would be at least two hours of warning time for a night time failure, and at least three hours of warning for a daytime failure," the report said.
"These warning times lead to very low fatality rates. The methodology does not account for flooding of large populations where the ability of people to safely evacuate may be restricted."
The report suggested there was always significant uncertainty in any loss of life estimates where large populations were at risk.
"Loss of life due to dam failure may be underestimated," it said.
Seqwater said the most recent risk assessment for Wivenhoe Dam was completed by the Wivenhoe Alliance, as part of the spillway upgrade project in 2005.
"A further specific risk assessment of both Somerset Dam and Wivenhoe Dam is currently in progress as part of the portfolio risk assessment being undertaken by Seqwater for its 26 large referable dams," Seqwater said in a statement.
That risk assessment will involve reassessment of the population at-risk estimates for both dams. A world-recognised expert in population risk assessments, Wayne Graham, is part of a team working on the assessment.
"Seqwater has, and always will, operate and maintain Wivenhoe Dam to the highest possible standard to ensure the structural safety of the dam," Seqwater said.
Worst case scenario
ONE of the worst-case scenarios for Wivenhoe is for the dam to overflow and compromise the integrity of the dam wall.
The dam can store about five times the amount of water stored in Sydney Harbour but in normal conditions uses only about 40 per cent of this space for drinking water, leaving the remaining greater area purely for flood mitigation.
When wet weather hits, the dam can fill rapidly, with the manual providing guidance on levels of water releases moving from W1 (low release) to W4 (emergency release).
The dam manual instructs the dam engineers to begin dumping water at the W4 rate when the dam level reaches the critical stage of 74m.
But the dam is also fitted with several fuse plugs, the first of which will automatically activate when water levels reach just under 76m.
The fuse plugs allow part of the wall to give way automatically, spilling thousands of megalitres into the Brisbane River but - hopefully - maintaining the integrity of the wall.
In the January 2011 flood, it is believed the water rose to within almost 90cm of the fuse plugs.
www.CourierMail.com.au
26.9.12
