It is difficult to make high quality planning decisions ahead of time especially when they involve spending a lot of money and also when mitigating against one type of catastrophe can conflict with the mitigation needed for a different type of catastrophe. (In the case of Wivenhoe 1 in 2000 year floods versus 1 in 100 year floods versus drought)

Planning for Wivenhoe shows this in the information coming out after the floods.

SEQWater shed massive volumes of water on January 11 resulting in devastating flooding in Brisbane but if they hadn’t the structural integrity of the dam could have been compromised.

Engineers and other experts are expected to put submissions to the commission of enquiry on the Wivenhoe Dam’s role in the Brisbane flood that identify serious shortcomings in public policy and operational management of the dam.

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

The upgrade included building 3 ‘fuse plugs’ which are designed to automatically collapse and release a large volume of water to quickly lower the level of Wivenhoe during extreme rainfall.

The first 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 1450000ML. This removed about 250000 ML of capacity from the storage compartment.(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.)

The cut in capacity for flood mitigation by at least 250000 megalitres to about 1.2 million megalitres was one of the cheapest options to meet new safety rules (tens of millions of dollars cheaper than raising the dam’s wall). This went ahead in 2006 after the dam’s operators were warned that the dam would over-top with catastrophic results in very rare (once in several thousand years on average) precipitation events.

However the upgrade removed a large part of its vital storage capacity to manage more frequent rainfall events such as the 2011 Brisbane flood.