The main water reservoir for Brisbane, Wivenhoe Dam, can hold approximately 3 trillion litres of water. Ideally, when at operational volume, it holds about 1 trillion (that’s a million million or tn) litres for the city’s water supply. So that leaves about 2 tn litres of empty space, which is used for flood mitigation.
Before the weather event of February 2022, the dam was just over half full of its operational volume, at 0.56 tn litres. So the dam had even more reserved space (2.44 tn litres) for flood mitigation.
Over the weather event (named as a “rain bomb” where 1100mm of rain fell at nearby Mt. Glorious), approximately 1.5 tn litres (net) was added (there were significant releases from the dam wall, so it could have been more). But it is important to note: in just 5 days, about 60% of the reserved space was used up! Fortunately, there was still about 1 tn litres in the flood mitigation reserve.
Now this seems a lot, but such a limited reserve starts to narrow options if other unlikely events come into play. What if the weather pattern persisted just another 3 days? What if there was equipment failure or a terrorist attack, etc? There are heaps of fail-safe systems at the dam (eg. controlled gates, an emergency spillway, fuse plugs) that should work to protect the dam. But it was built to a budget with an earth/rock wall, not concrete. It was also engineered before the effects of climate change were apparent. This type of dam construction is not supposed to have water overtopping the wall and would likely fail in such a situation. At which point all 3 tn litres has to go somewhere. Fast.