The State Government is refusing to allow the massive emergency capacity of Wivenhoe Dam for the long-term storage of drinking water until more is known about the effect on flood risk.
The dam has a huge “buffer” capacity – designed only to soak up water that would otherwise flood Brisbane.
It could take as little as 72 hours for Wivenhoe Dam’s massive flood compartment to fill if southeast Queensland sees similar conditions this summer as occurred during Brisbane’s disastrous 1974 floods.
Under cautious operating rules set by Parliament managers must empty the entire 1.45 million megalitres within seven days of filling to prevent a second flood event over-running the dam.
Wivenhoe is now considered “full” after recent rains took it to 1.17 megalitres and water was last week being spilled into the Brisbane River to maintain the waterline.
Opposition water spokesman Jeff Seeney said raising the waterline by just 2m would add 228000 megalitres and provide an extra “no-fail” yield of 31000 megalitres a year. This was equivalent to the total annual yield from the Tugun desalination plant.
SEQwater and the Queensland Water Commission are investigating implications for flooding if part of the flood component is held back to be used as drinking water storage.
Water grid spokesman Dan Spiller said issues under investigation included the dam itself roads and bridges private infrastructure and impact scenarios of different size floods.
“There are hydrological and engineering issues and departments like Emergency Services and the Brisbane City Council have an intense interest” he said. “We need overwhelming evidence that benefits will exceed risks.
“… We have to be absolutely conservative in this and given the water around it’s not the time to do it.”
Another issue is that about half the total Brisbane River catchment of 15000sq km is below Wivenhoe – an area big enough to flood Brisbane Ipswich and surrounds.
Natural Resources Minister Stephen Robertson is adamant he is not prepared to risk summer flood protection.