Cheap Wivenhoe Dam upgrade reduced its capacity

The actual capacity of Brisbane's Wivenhoe Dam for flood storage is significantly less than claimed by the Queensland government

The actual capacity of Brisbane's Wivenhoe Dam for flood storage is significantly less than claimed by the Queensland government -- and about 20 per cent below the dam's capacity when it was built to reduce the serious flooding of Australia's third largest city.

The compromise in flood storage is one of the reasons the dam operator, SEQWater, shed massive volumes of water at the worst possible time on January 11, resulting in most of the devastating flooding in Brisbane, The Weekend Australian can reveal.

The dam's capacity to store floodwater was significantly compromised four years ago when engineering works occurred.

But the reduction has not been previously communicated to the general public by the Queensland government or SEQWater, which asserts that the flood storage capacity has not changed in the 25 years since the dam's opening.

Engineers and other experts, who are studying the dam's key role in the Brisbane flood, told The Weekend Australian the official claims that the dam's storage compartment still has a total capacity of 1.45 million  megalitres are demonstrably false.

The capacity for flood mitigation was cut by at least 250,000 megalitres to about 1.2 million ML as a result of a dam upgrade four years ago that was one of the cheapest options to meet new safety rules.

The reduction is equivalent to almost half the volume of Sydney Harbour.

A commission of inquiry headed by Supreme Court justice Cate Holmes, whose terms of reference include investigating the role of Wivenhoe Dam in Brisbane's devastating flood, will receive submissions from engineers and other experts who have identified what they describe as serious shortcomings in public policy and operational management for the dam.

The Weekend Australian has sought briefings and data from Natural Resources Minister Stephen Robertson and from SEQWater -- which has refused to release information since stating it performed well in last month's flood and reduced the inundation in Brisbane.

The cheaper upgrade went ahead in 2006 after the dam's operators were warned that the dam would over-top with catastrophic results in a probable maximum precipitation event, measured by the Bureau of Meteorology as likely once in several thousand years on average.

As a result of the upgrade, which was tens of millions of dollars cheaper than raising the dam's wall, the dam was compliant for an event with an average return of about 5000 years. However, the upgrade removed a large part of its vital storage capacity to manage more frequent rainfall events such as those last month.

The upgrade included building three so-called "fuse plugs", which were designed to automatically collapse and release a large volume of water to quickly lower the level of the lake at Wivenhoe Dam during extreme rainfall.

The trigger-point of 75.7m for the first fuse plug is 1.3m below the 77m level at which the dam's capacity was calculated when built to be 1,450,000ML.

About 250,000ML of capacity was removed from the storage compartment withe the upgrade. In addition to the loss of 250,000ML of capacity between the trigger point of 75.7m and 77m, many tens of thousands of megalitres more potential capacity is removed by SEQWater's priority to keep the level at least 70cm below the trigger point.

Mr Robertson did not respond to detailed questions yesterday, instead issuing the following statement: "I am advised by my department that the fuse plugs are designed to protect the structural integrity of the dam.

"The dam's ability to manage extreme flood events was increased during the upgrade program which included the fuse plug installation. Therefore the reality is, the flood mitigation benefit of the dam has not been altered. The flood management of Wivenhoe Dam is now subject to investigations of the independent commission of inquiry."