February 6, 2011
Close to the brink
Brisbane's flooding came very close to being a lot worse -- from the Melbourne Herald Sun:A few more inches and heaven help BrisbaneThe link above: the operators were gravely concerned late on January 11 that the dam�s rising levels could trigger a collapse of the system goes to a damning article at The Australian:
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:This is why those floodgates were turned on full: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.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.
The great avoidable flood: an inquiry's challengeIt will be interesting to see if the people responsible for the waffling will be sacked. A tragic story... Posted by DaveH at February 6, 2011 2:43 PM
At 12.26pm on Wednesday, January 5, those in the loop for receiving advice on the operations at Wivenhoe Dam received a timely alert.
It was headed "Bureau of Meteorology (BOM) Severe Weather Warning - Dam Flood Operations". Its author, Wivenhoe Dam engineering officer Graham Keegan, wanted to ensure that those authorised to receive his emails understood that significant rainfall of 100mm to 200mm "may occur during the next few days".
Relaying information from his colleagues at the dam's Flood Operations Centre (FOC), he added: "Somerset and Wivenhoe Dams are still above (full supply level) and rising slowly due to continuing base-flows from their catchments. As the catchments are still wet it is likely that we will be releasing floodwaters in the near future if BOM's forecasts are accurate. Please be prepared. We will keep you up to date with our plans as this event develops."
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