Brisbane floods: did the dams work?

By Carolyn Barry 15 January 2011
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As Brisbane cleans up its murky waters, the community wonders how well the dams worked.

NOW THAT BRISBANE’S FLOODWATERS are receding and the long clean up begins, considerations are turning to just how well the city’s infrastructure and systems handled the disaster.

The Wivenhoe Dam has played a central role in managing the massive water flow into the Brisbane River catchment area, and therefore, to urban areas.

From lessons learned after the 1974 flood, the dam – built in 1984 and located some 80 km upstream from Brisbane – was constructed to mitigate future severe floods and prevent smaller ones.

“If were not for Wivenhoe, we would have faced flooding the likes of which we cannot comprehend,” Brisbane Mayor Campbell Newman says.

Managing the dam water

According the managing authority, SEQ Water Grid Manager, the Wivenhoe dam was designed to reduce the level of a 1974-sized flood by 2 m. Given this flood’s peak of 4.46 m (1 m less than 1974), without the dam, the river would have risen much higher, inundating many more areas.

“The flood storage and gates and spillways are designed to deal with floods much more extreme than what we’ve seen,” says Dan Spiller, director of operations at SEQ Water Grid Manager. “What this [latest flood] has shown is just how the dam is supposed to operate.”

The full capacity of the dam is 225 per cent, with 100 per cent referring to storage of the region’s drinking water supply and the excess above that meant for flood storage. The additional water is released in amounts and times that are manageable for the river system and its downstream towns. This is done closely in conjunction with information from the Bureau of Meterology.

At 200 per cent, the dam’s five gates start to become overwhelmed and the water is forced out through overflow valves. If the reservoir tops 225 per cent, the water simply spills over the top of the dam into the valley below.

Just three years ago, the dam’s reservoir bottomed out at 17 per cent at the height of a prolonged drought. With months of intense rains, brought about by a strong La Niña, and recent heavy rains, the water deluged up to 1 million megalitres (ML), twice that of Sydney Harbour, each day over a few days prior to the flood, forcing the dam up to a peak of 190 per cent, Dan says. “And we were able to catch that and release it in a much more controlled way.” 

Preparing the dams for more floods

The inundation to the catchment area was such that nothing was going to prevent a severe flood. But the dam provided the ability to slow the flow down, Dan says.

In a blessing from Mother Nature, clear skies have spared time for the dam to play catch-up, allowing the dam authority to steadily decrease its reserve in controlled releases.

“What we’re trying to do is empty [the dams] now in seven days,” says Dan, to shore up rooom for any more floods that may be on the way. And with experts predicting the La Niña to hang around until March, it’s a possibility that Brisbane is taking seriously.