Unable to wait for the outcomes of the Queensland floods royal commission all sorts of people have been quick out of the blocks to criticise the management of the Wivenhoe Dam. An early one suggested outright that the flood peak was man-made. That bloke has a bit of form in that he’s never wrong. A royal commission which disagreed would merely demonstrate their stupidity in his book.
Another line of argument suggests that the balance between water storage and flood mitigation should be altered in favour of flood mitigation. Quiggin suggested ( Kim’s Quick link post here) that the water storage compartment could be reduced to 75% of the present.
Legal eagles are circling, waiting to see whether the royal commission provides the basis for a class action.
I won’t link to all the other experts who have come out of the woodwork, giving opinions that a bit of work on the back of an envelope together with information publicly available, but not heretofore collected in one place, would have shown them to be foolish.
The situation is fairly simple, but first you need to understand some basic metrics about the dam.
The Wivenhoe Dam when full to the top of the wall has a storage capacity of 2.6 million megalitres (ML). (ML is by convention the basic unit of measurement in this discourse. Get used to it!) This is divided into two sections. There is a water storage compartment of 1.15 million ML (more on the adequacy of this later). On top of that sits a flood mitigation compartment of 1.45 million ML.
By convention the dam is deemed 100% full when the level is 1.15 million ML. At 2.6 million ML it is therefore 225% full. You will hear that these compartments are split 40:60. The simple application of a calculator will tell you that should be 44:56.
At 225% full the dam wall is threatened, so the managers try to keep the levels below 200%, at which time the sluice gates are ineffective and they have to open a series of valves which leads to an uncontrolled release. During the latest event the level peaked at 191%, I think, which is as close as they’d want to go for orderly operation. At that point they were pretty much obliged to release an amount equivalent to what was coming in. Hence we had a flood but we would have had a flood anyway, because the Wivenhoe only controls 40% of the Brisbane River catchment. There was plenty coming down the Lockyer Creek, which joins the Brisbane about 5km below the Wivenhoe wall, and the Bremer River, which enters further down via Ipswich.
There have been allegations that too much water was held back and then had to be released when the rain was pissing down. Any dolt could see it was going to rain.
Well I watch the weather pretty closely and I can tell you that any dolt can’t see that it’s going to rain. Often they say it’s going to rain and you wake to find that the weather influence is 50km out to sea, or has migrated north or south. There are plenty of instances where you get a significant downpour and then practically nothing for six months.
The Wivenhoe operates to strict rules, with rule number one that no politician should interfere. These rules have now been released. The highest priority is dam wall safety. The second is that you don’t release water that will flood urban areas.
The Brisbane CBD gauge is in a tidal area, and I understand zero is effectively sea level, which is an average midpoint between low and high tide. A fairly normal tide is 1.6m, a king tide is 2.6m. the flood peaked at 4.45m, so you can see there’s not a lot of leeway. The one riverside area I know well sees a highish tide with the river out of what a normal person would call its channel, spilling onto a low-lying grassy area. I’m not sure at what point the level becomes a minor flood.
Another rule is that the flood compartment must be emptied within seven days, all thing being equal.
From this site you can see the recent historic water levels of the dam:
From this you can see that the Wivenhoe probably knocked out three instances of perhaps minor flooding before the big one. The Oz, which is on a campaign about this, have latched on to some engineer’s emails and are making much of them. Just remember that when the levels hit 190% they had a dam wall to protect, a flood entering downstream from the dam wall, which in the event was close to 50% of the whole flood water, flooding already occurring in Brisbane, the prospect of king tides coming up, and water entering the dam at the rate of 2.6 million ML per day, with a forecast that the rain would break but not for some very long hours. The flood was being monitored with 200 gauges and constant reference being made to the BOM.
I’d be more than happy to cut them some slack, pending the inquiry.
Had they gone into the event with the dam at 75% capacity instead of 100% how much difference would it have made? They would have been 287,000ML better placed, that’s what. And that would have filled in about two and a half hours.
In a Courier Mail article of 18 January I can’t find on the net, we were told that the Brisbane River would have been flooding at the rate of 13,000 cubic metres per second, if Wivenhoe did not exist, compared to 9,500 in 1974. I came to the rough conclusion that Quiggin’s idea might have shaved centimetres off the flood.
Yesterday on local radio I heard an expert hydrologist say exactly the same thing. I thought his name was Professor Ackenasy, but google doesn’t call him into being. I listened to this guy, because as a younger man working for the relevant authority, he set up the operating rules for the Wivenhoe Dam. This is some of what he said.
The rules were set up on the basis of flood behaviour according to models. Now that we have an actual instance, of course they will be changed on review.
This flood, without the Wivenhoe, would have been about 1.5 metres higher, that is 6m instead of the actual 4.45 and 5.5 for 1974. The difference in using different release rates would have been trivial, a matter of a few centimetres.
The Wivehoe is useful for flood mitigation, but there are limits to what it can do. On the positive side it has completely knocked off two significant floods since it was built.
The Brisbane River according to the 160 years of records available, floods every 20 years on the average. This one was roughly one in 40 years, not the one in 100 typically quoted. Much of the rest of what he said I’ve already included in the above.
So settle down everyone, but that 1 in 40 figure is a bit alarming.
This post is already long, but I want to say a bit about storage, before we go cutting bits off it.
The idea is that we have a 1% probability of being reduced to the T1 trigger (40% capacity) in the next 10 years. Knocking off 25% of storage from the top would compromise this safety margin and make water much more expensive.
At T2 (30% capacity), you should be building climate-independent extra capacity and have another 30 months to complete it.
Under this planning regime, which takes into account population projections, we should have medium level restrictions no more than once in 25 years, those restrictions should not last more than six months and should amount to no more than a reduction of 15% of normal entitlement.
Sounds rational to me.
While we are at it here are the SEQ dam capacities. There are no more sites available, other than building the Wolfdene at huge cost of resumptions.
This map shows the rivers and dams in the area:
The Somerset is below Kilcoy and above the Wivenhoe at Esk. These together with the North Pine Dam to the east constitute the main Brisbane storage. The Locker Creek flows in from near Toowoomba. The next river system down is the Bremer which is shown as flowing to Bundamba, a suburb of Ipswich. The Lockyer Creek catchment is actually about 50% bigger than the Bremer and was one of the driest catchments in Qld during the drought. Neither has major dam sites.
The Woldene dam was to be on the Albert River just north of the Gold Coast.
This map shows the rivers more clearly:
A point of interest here is that the now-abandoned Traveston Dam was to be on the Mary River approximately west of Noosa, I think. It was a case of stuffing over one community to serve the needs of another, and about as popular with the locals as raiding the water resources of the Clarence to the south would have been.