I understand your point Duckman, but the Wivenhoe principle is to minimise or negate many low level flood events. The problem is that how big is big enough to be safe.
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PS: Is the premier worried about a possible dam burst? She looks grim saying (on the TV news) these releases are not optional, they are necessary to operate the dam as it was designed and to protect the people down stream.
It is a fact that the water stored in Wivenhoe dam is water that would otherwise have flowed downstream.
It could be expected that capturing some water upstream, would reduce the frequency of flooding downstream although it may be of little benefit in an extreme event. That is, the presence of a flood control dam will avoid a lot of smaller floods but will not avoid a catastrophic flood event.
That scenario is very common to virtually all safety devices and systems. For example, A seatbelt and air bag will reduce the risk of injury in most motor vehicle accidents but is of no real benefit in a head-on collision between a car and a large truck at highway speed.
Aircraft safety systems and the life jackets we are all told can be found under the seats may well help under a moderate emergency scenario. But if a wing falls off the plane then no amount of oxygen masks and life jackets fitted with whistles and lights is going to save you.
It's basically the same with most safety-related systems. Your chances of avoiding a moderate incident, or surviving it, are greatly improved. But in a worst case scenario the safety systems are basically useless. Same with flood control dams.
Looking at another flood where I have some figures, the Hobart floods of 1960, it is a fact that Hydro dams retained about 30% of the water upstream in the Derwent catchment. Likewise Hobart City Council dams retained significant amounts of water upstream in the Hobart Rivulet (Mt Wellington) catchment. Had this water not been retained, flooding in the CBD and at towns in the Derwent Valley would have been far worse than it was.
Likewise the frequency of flooding in the Huonville area has decreased since Lake Pedder was dammed in 1972. Prior to that, floods were more frequent in the Huon.
But all that said, if a truly major rainfall event occurs then Brisbane, Hobart, Huonville and everywhere else near a river are going to be flooded no matter how many dams (within practical limits...) have been built upstream. Flood control dams will avoid 95% of floods, but they won't stop the really big one.
As for a dam burst, rockfill dams in general are not designed to cope with water flowing directly over the top of the dam structure. There are exceptions which do have spillways on the downstream face of a rockfill dam, but Wivenhoe is not such an exception. If the dam were to overflow as such, as distinct from simply being on spill, then serious erosion and consequent collapse would be a real possibility.
In reality however I would not be overly concerned about the risk of a flood resulting in structural failure of Wivenhoe dam. It is not totally impossible, but if there is to be a flood of that magnitude then there isn't going to be much left of Brisbane with or without Wivenhoe bursting. Wivenhoe was built for flood control and has huge spillways, indeed the gates are amongs the largest in the world. It's not as though it's doesn't have a spillway or anything like that (actually, there are some pretty large dams around that don't have spillways but there is sound engineering behind that - it comes down to catchment area versus storage capacity).
As of 9am this morning, there were 2050 GL in Wivenhoe versus full supply level (flood control) of 2615 GL and normal FSL (for water supply purposes) of 1165 GL. Information from SEQ Water.
In layman's terms, that means the dam is normally allowed to fill up to 1165 GL, with any surplus water being released via the spillway. However, the actual capacity of the dam is 2615 GL. The reason for normally keeping the level lower is to enable the additional storage of flood waters in an emergency such as the one we have now.
So why are they letting water out if the dam is not likely to burst? The basic aim is to hold back as much as possible when it matters most given that capacity of the dam is limited. That is, someone has done the maths (hopefully) and worked out that flooding will be less if some water is let out now, such that more can be held back around the time when river levels are expected to peak. Obviously there's a bit of estimation being done there, but that is the theory. Trying to lessen the overall severity of flooding through manipulation of water release rates.
If I were in Brisbane right now then Wivenhoe dam bursting would be the least of my worries. I would be more concerned about whether or not the water coming out of it (Brisbane's water supply) will remain safe to drink. Likewise I would be more concerned about all the other consequences of the flood.
A dam could burst yes. And a bridge could collapse and you could be struck by lightning. Assuming that nobody does anything silly, like trying to operate outside the dam's safe operating criteria (
let the engineers make the decisions on water release, not politicians), then I wouldn't be worried about the dam itself.