Damned if they do, damned if they don't

BACK last September and October the warning bells began to toll across Brisbane and elsewhere in Queensland that the Sunshine State should brace for flooding rains and an unusually active cyclone season.

The warnings were not speculative musings from the Bureau of Meteorology, whose severe weather scientists briefed the Bligh government's cabinet and disaster management committees behind closed doors.

Nor were the warnings withheld from anyone: the bureau's public relations arm ensured that media releases were widely distributed and prominently published, ensuring maximum possible coverage.

There was no sugar-coating of the risks or the messages. Many mainstream media outlets headlined the bureau's severe weather forecasters stressing that confirmation of a fundamental change in the weather - to the La Nina phase, after years of El Nino-related drought - would likely bring flooding rains to Queensland. The bureau went as far as raising the potential for six tropical cyclones to hit Queensland. As Cyclone Wanda proved in 1974 when it unleashed a maelstrom on Brisbane and caused the last major flood, cyclones dump enormous amounts of rain in just hours.

Few people - and particularly the operators of Brisbane's Wivenhoe Dam, which was at full supply level at the time, meaning it was at 100 per cent capacity for water supply (not including its additional significant capacity to deal with flood mitigation) - could claim to have been oblivious to warnings of the grave risks.

The Queensland government-owned and operated Wivenhoe Dam was built to mitigate a major flood and provide a safe water supply for a growing population in Australia's third largest city.

In the wettest years since the dam's commissioning after the 1974 floods, it has worked like a giant sponge to store the catchment's rainfall run-off that would otherwise flow straight to the floodplains of Brisbane.

But to ensure Brisbane is given optimum protection from the extreme rainfall events which have occurred in southeast Queensland over thousands of years, Wivenhoe Dam must have enough capacity to store the run-off from such events.

Like most large dams, its potential to mitigate a huge flood can be stretched or contracted. If the dam is already storing a large volume of water before an extreme rainfall event occurs, its capacity to also absorb the sudden run-off is limited. To prevent the risk of the levels getting dangerously high and triggering an emergency release to stave off a catastrophic collapse, the dam's operators must let water go, and this water then becomes part of the flood in the Brisbane River.

Now, as the city faces years of rebuilding after a devastating flood caused to a significant degree by the sudden releases this week of massive volumes of water from Wivenhoe Dam, there are critical issues demanding scrutiny by an independent inquiry.

These issues go to the heart of the objective of safe and conservative management of the Wivenhoe Dam, and the extent to which the sudden releases of the volumes of water (necessary because of the diminishing capacity of the dam to store in-flow from the catchment) this week caused much of the flood hours later that would devastate thousands of properties and cause incalculable financial and other losses.

On October 4, the bureau's weather services manager, Ann Farrell, was unequivocal: "This is a well-established La Nina. Normally there may be one or two indicators present and it's a borderline La Nina, but this year all the indicators are present."

Her colleague, severe weather forecaster Tony Auden, said: "Historically, in similar years with similar ocean patterns, we've seen six to seven [cyclones] out to sea and at least one severe coastal crossing in each of those years."

Two weeks earlier, on September 22, long-range weather forecaster Hayden Walker warned that southeast Queensland would be "very likely" to get a severe cyclone with catastrophic results, adding "it could be one of the worst natural disasters in [Australian history].

Campbell Newman, Brisbane's mayor and an engineer who has made it part of his administration's mission since 2004 to repeatedly warn a blissfully complacent city of its flood risks, went out on a limb on October 11 by declaring that a repeat of the 1974 floods could be brewing.

He said on that day: "Back in '73 there was an extended period of wet weather leading up to January '74 when we had a cyclonic rain event which turned into a rain depression and then dumped a whole lot of rain in the catchment areas. That's something that will one day happen again and given that this is the first time we have had so much rain, plus with full dams, again it is of concern. We have got a very wet period at the moment - we have got full dams and all we would need is another big rain event in the next few months in the catchment areas and we could have flooding."

On December 23, the weather bureau, which was still briefing officials and disaster committees about the heightened risk, made another public statement with its regional head Jim Davidson stressing Queenslanders must "prepare for heavy rain and flooding during the holiday period".

A common refrain after a natural disaster is that it is easy to be wise in hindsight and that Wivenhoe Dam's operators performed an exceptional job under extraordinary circumstances to ensure the dam minimised the flooding. This might be correct.

The alternative and gravely serious scenario is that Wivenhoe Dam's operators - conditioned by years of El Nino-linked drought that had reduced dam volumes to perilously low levels, enveloped political masters in crisis and ushered in severe water restrictions - were determined in the latter months of 2010 to store more water in the dam than necessary.

Under this scenario the operators of the dam did not err on the side of caution; did not adequately heed the serious and repeated warnings of extreme weather; did not have sufficient buffer for the major rainfall event that occurred; and were compelled in a crisis situation to release thousands of megalitres quickly, turning what would have been a moderate flood in Brisbane into a natural disaster.

Hydrologist Aron Gingis, a Melbourne-based rainfall expert formerly of Monash University, contacted The Weekend Australian to urge a public debate about the dam's influence on the flood. According to Gingis, the dam's operators bear a heavy responsibility for their significant contribution to the flooding.

"They had no right prior to the start of the wet season - when the forecasts were all pointing strongly to exceptional rainfall - to keep so much water in the dam," said Gingis. "I tried to warn them about the coming disaster and to urge them before it was too late . . . to release much more water to give themselves more storage room for a big one.

"When they finally did release, it was because they had received so much inflow this week that they were afraid the whole system would collapse. There is no doubt in my professional opinion that most of the flooding in Brisbane should have been avoided. It is extraordinary to me that people are not asking more questions about this. Brisbane should have been protected by Wivenhoe Dam. Instead, the dam is a large part of the reason the city has flooded."

It is important to understand that the dam's total storage capacity is 2.6 million megalitres, of which 1.15 million megalitres is used for water storage. During a flood situation, the dam is designed to hold back a further 1.45 million megalitres. One of the questions for a public inquiry will be whether maintaining 100 per cent of the capacity for water storage was prudent given the warnings from September 2010.

The Weekend Australian asked the operators of the dam for a briefing, but they declined and sought questions in writing.

The first question was: "What do the operators of Wivenhoe Dam say to the suggestion that the warnings by the Bureau of Meteorology's severe weather forecasters in September and October of a confirmed La Nina, and the heightened possibility of flooding rains and cyclone activity, should have necessitated a much more conservative approach to Wivenhoe Dam; specifically, that it should have led to the volume of water in Wivenhoe being significantly reduced over the ensuing weeks to give the dam a much larger buffer?" The questions were not answered.

What is clear, however, is that the sudden controlled releases from the dam before the flood in Brisbane were urgent because, as Premier Anna Bligh acknowledged late yesterday in her media briefing, "we were very close to an uncontrolled release".

As an uncontrolled release could have had extremely serious repercussions - and potentially been catastrophic in a worst-case scenario with an over-topping of the wall - experts in meteorology, hydrology and engineering will turn their minds in coming months to why Wivenhoe Dam was forced to cut it so fine.

The extreme rain and run-off from the freak event over Toowoomba on Monday did not fall into the catchment for Wivenhoe. But the storm's intensity, with almost 200mm falling in less than an hour, caused serious alarm among the dam operators because they realised that such a weather system - if it were to form and drench Wivenhoe's catchment for several hours - would go close to a Probable Maximum Precipitation event, a feared monster that would create more in-flow than the dam could release.

The consequences of a PMP are so severe that it will keep the dam's operators awake at night for a long time to come, according to a senior source close to the dam. It could have driven what is known as the Probable Maximum Flood, an event with an average frequency of just one in every 100,000 years.

The strategies for operating the dam are meant to be strictly regulated. They are also supposed to be above political interference.

"It has been operated around an approved strategy calculated by experts who are concerned to minimise risk," said the source.

Retired engineer Ian Chalmers, a key project supervisor in the construction of Wivenhoe Dam between 1977-85, defended the decisions of the operators in the past week, adding they will do a better job next time.

"These questions are all valid, but put it this way - you would have to have very large balls to [significantly reduce the dam's volumes in the months after the weather warnings] after 10 years of drought, because if you had got it wrong you would be accused of wasting the water," Chalmers said.