A Few More Inches and Heaven Help Brisbane

Posted on Sat 02/05/2011 by

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Andrew BoltBy Andrew Bolt

Look very closely at this image. It is taken from a position on the lake side of Wivenhoe Dam looking towards and across the Dam’s wall, that thin black line you can see. Between that area of green in the middle distance you can see the frothing of the water as it is frantically released from the 5 wide open gates of the spillway. Again, note how close the water is to the top of the dam wall. That’s about a foot or so, and it got even higher than that again, in fact as close as four inches to the top of the wall. During the crisis, and after those releases, I made a necessarily long Post about those levels at this link…..TonyfromOz.

Here’s how close to utter disaster Brisbane came during last month’s floods. This is the Wivenhoe dam, built to save the city from flooding, at the moment its operators were desperately trying to lower the levels before it overtopped and parts of the dam gave way.

Reader Bruce explains:

Not many people have seen this picture yet. This is Wivenhoe Dam at its peak, i.e. at 191+% capacity.

Note that the main 5 floodgates are fully open. Note also the spray / white water just visible above the trees beyond the right hand end of the dam wall. This is from the spillway which is cut into the base rock and is out of sight to the right (west) of the wall you can see.

Brisbane dodged a very big wet bullet in those 24 hours, no thanks to Anna Blight and her minions in Queensland water.

This is why those floodgates were turned on full:

These calculations, yet to be tested by SEQWater, show that the urgent release from the dam of huge volumes at unprecedented rates of flow of up to 7500 cubic metres per second, when the operators were gravely concerned late on January 11 that the dam’s rising levels could trigger a collapse of the system, produced most of the flood in the Brisbane River….

Wivenhoe Dam engineering officer Graham Keegan’s … 20-odd emails – from January 5 until the crisis at the dam had passed late the following week – become urgent in tone early on Tuesday, January 11, with notification that “we are entering conditions where dam safety overrides other concerns – although minimisation of urban flooding remains very important”.

A few hours later at 9.50am he reported “the flood situation has moved into a critical phase”.

Communications with the FOC were difficult, river levels were rising rapidly, and the dam’s flood storage capacity was diminishing.

By that evening, Keegan …warned that the dam was expected to reach “a maximum level of 75.5m provided no further significant rainfall occurs”.

“This is 0.1m below the trigger level for (an uncontrolled discharge) – this is the major focus of the current release strategy,” Keegan said….

A collapse of Wivenhoe, which would occur from over-topping because of the inflow from the catchment exceeding outflow from the dam’s gates, would be catastrophic. An engineering paper by the dam’s operators a decade ago found that “the population at risk within a distance that would result in less than three hours’ warnings of a dam failure is between 57,000 and 244,000, depending on the time of day and nature of the breach”.

Since a safety upgrade a few years ago, keeping water below three collapsible engineered levee banks, known as “fuse plugs”, is the key to maintaining control of the dam. Should the levels rise to 75.7m and trigger a fuse plug, a very large release occurs to ease pressure, but the outpouring is not controlled by any gate – only by the speed at which the levee or bank is eroded by the water.

If a mistake was made, it was by not emptying the dam enough beforehand to accommodate the flood water that could have been predicted in a La Nina season. The frantic turning on of the floodgates that resulted – just at the wrong time – may have caused much of Brisbane’s flooding:

LEAKED email communications from a Wivenhoe Dam engineering officer underline concerns that the Brisbane River flood was mostly caused by massive releases from the dam after it had held on to water too long over a crucial 72 hours before the severe rainfall that hit the region last week.

The emails, which become increasingly urgent in tone as the situation became critical as the dam’s levels rise rapidly, were provided to The Australian by a source who said the stream of data had convinced him the river flood of Brisbane could have been largely avoided if the dam’s operators had taken action much earlier.

A commission of inquiry will examine whether the dam’s operators erred in permitting the dam’s flood compartment to be severely limited for a major rainfall event because of their strategy to let the dam’s levels rise over the weekend of January 8-9.

According to figures from Wivenhoe’s operator, SEQWater, the dam’s capacity went from 106 per cent full on the morning of Friday, January 7, to 148 per cent full on the morning of Monday, January 10, due to the limited weekend releases. Experts have said this severely compromised the dam’s ability to store additional runoff.

But how much worse it could have been. Check the photo again.

Andrew Bolt is a journalist and columnist writing for The Herald Sun in Melbourne Victoria Australia.

Andrew Bolt’s columns appear in Melbourne’s Herald Sun, Sydney’s Daily Telegraph and Adelaide’s Advertiser. He runs the most-read political blog in Australia and is a regular commentator on Channel 9′s Today show and ABC TV’s Insiders. He will be heard from Monday to Friday at 8am on the breakfast show of new radio station MTR 1377, and his book Still Not Sorry remains very widely read.

Read more excellent articles from Andrew Bolt’s Blog

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