Spillway at Wivenhoe

An article in The Australian today highlights the role of the Wivenhoe Dam in the flooding that affected Brisbane:

The official records from SEQWater show that, at 6am on Friday, January 7, Wivenhoe Dam, Brisbane’s insurance policy to protect the city and surrounding suburbs from a massive rainfall and flood event, was at about 106 per cent capacity. This means that Wivenhoe had filled to 100 per cent of its capacity for water supply with a total 1.15 million megalitres, and it was 6 per cent into its additional 1.45ML of storage for flood mitigation.

On Saturday, January 8, it is understood to have let about 100,000ML go; on Sunday, when Mr Goodwin’s family was there, a further 116,000ML were released.

By 9am on Monday, the levels in the dam had soared to just over 148 per cent, and it was reported that managers at the dam had “scrambled”.

That afternoon, the extreme rainfall over Toowoomba and the Lockyer Valley unleashed a maelstrom that the Bureau of Meteorology had not predicted. While the run-off from this event did not fall into Wivenhoe’s catchment of about 7000 sq km, the dam’s operators were caught by surprise and released 172,000ML as the capacity went past 150 per cent.

Through Tuesday, as Wivenhoe continued to rise past 175 per cent and then 190 per cent, the situation was becoming critical as the available buffer for more rain had been almost fully taken up. Nobody wanted the dam to go to 200 per cent, and the theoretical maximum of 225 per cent needed to be avoided at all costs.

One of the crucial questions that will be asked in a commission of inquiry, called late yesterday by Premier Anna Bligh, is whether the releases from the flood storage compartment of a little over 200,000ML on the weekend were too little, too late, necessitating a huge outpouring to get levels down quickly.

The operators of the dam gave the order on Tuesday, cranking up the release to a staggering peak rate of 645,000ML a day. At that point the Brisbane River flood was not a case of if but when: the computer modelling showed major flooding from this Wivenhoe discharge was inevitable and would peak in the 36 hours the water would take to reach the city gauge.

The release from Wivenhoe at a peak rate of 645,000ML a day represented up to 30 per cent of the dam’s total capacity. Nobody was under any misapprehension about the consequences. It was this release from Wivenhoe that represented about 80 per cent and perhaps more of the volume in the Brisbane River.

A rainfall event that could have been comfortably managed by the dam if its flood compartment had been lower had turned into a major flood that would devastate thousands of homes. (source)

So the question to be asked is, was it an oversight not to release water earlier, and at a more controlled rate, in anticipation of the floods, or was it to preserve water in the dam due to some diktat from the climate change department? Hopefully the inquiry will answer that question.

Regular ACM contributor Bruce (brc) wrote a lengthy comment on an earlier item which I believe rightly deserves a post of its own. I would like to thank Bruce for his thorough and down to earth assessment of the current Queensland floods which commentators on all sides would do well to imitate:

Look, as a 3rd generation lifelong South East Queensland (“SEQ”) resident, all this
 world wide attention and theories are starting to irk me as talking
 heads start to spout off things of which they appear to have little
 understanding. SEQ (and Queensland in general) is periodically subject to 
intense widespread rain events: 1893, 1974, 2011. They happen. Even
 smaller intense rain events occur at least once a decade. The 1893 
floods were larger, and peaked on 3 separate occasions. Does anyone
 care to tell me how AGW could have done that, given that horseback 
was the primary transportation method at the time, and electricity
 was something played with in laboratories?

If anything, the lower 
levels in 1974 and 2011 is proof that AGW makes the flooding less 
worse (I say with tongue in cheek). There’s also a popular meme 
going around (James Delingpole and Andrew Bolt) that somehow green 
interfering caused the death and destruction. Nothing could be 
further from the truth. And believe me, I love a good outing of
 ridiculous green policy as much as the next person.

Here are the facts:

• The 
majority of loss of life was caused by flash-flooding in and around
 Toowoomba (700m above sea level) and the Lockyer valley below the 
Toowoomba range. The streams that caused the devastation in
 Toowoomba are normally babbling brooks one can leap with a vigorous 
jump. While some warning may have helped, many deaths were caused 
by people undertaking risky actions like trying to drive across 
flooded bridges.
• The scrapped Traveston Crossing dam project on 
the Mary river would not have saved Gympie from flooding. It would
 have been 100% full (like every other dam in the region) prior to 
the large rain events – it has been raining steadily for two 
months. In any case, it was the residents of Gympie that campaigned 
the most against the dam. Not because of lungfish (the figleaf that
 the environment minister used) but because it was a bad idea. A 
flat alluvial sandy plain is not the ideal location for a dam. It
 would have been wide and shallow on porous soil. And it would have 
subsumed a huge area of productive farmland. It was correct for the
 dam to be scrapped, and many engineers publicly stated this. That 
it was scrapped under environmental reasons was just the out for an 
embarrassed Federal government saddled with the plans after the 
former premier announced it to save his political hide (Brisbane
 was under severe water restrictions at the time) but then scarpered
 anyway. It was chosen because the area had never, and would never, 
vote for Labor anyway, so it was the best place to put it, safely
away from Brisbane voters. It would have been full, and would not
 have saved Gympie from flooding. And the townspeople in Gympie are 
used to flooding anyway, and go about moving out of the way with a 
cheery disposition.
• Wivenhoe dam – conceived and built after the 
1974 floods – has done a very good job in extremely difficult
 circumstances. It has managed to keep the peak level of floods 
1m lower than predicted, by delicately balancing the inflows and
 outflows and timing with the low tide in the Brisbane river. It was 
already at 150% (and releasing continually, as it has been for
 months) when this large rainfall event hit. SEQ Water are to be 
commended with the way they handled this, with the Dam balanced 
within 1m of the peak level allowable before dam-protection levees
 give way to protect the wall (with devastating consequences for 
those downstream).

It’s difficult for people who don’t live in 
Queensland to understand the volumes of water we’re talking about
 here. This is not some drizzling Victorian rain or misty English 
weather. This is a proper, tropical summer monsoon rainfall a bit
 further south than it normally is. The written history of Queensland is
 only about 200 years long, but it is peppered with tales of huge
 floods that astound new observers. People see the 1974 markers on
 buildings around Brisbane and think it can’t possibly have 
happened. The puny infrastructure put in the way of these periodic 
deluges is nothing compared with the water volumes. It will happen 
again, at least once per lifetime of the average person. There’s 
nothing that can be done. After all, it’s just weather.