Dams at risk: ‘Mother of all floods’ to get bigger as climate warms

By Glenn Morrison

Dams around the world are designed to cope with some truly humungous floods, but a paper recently published in Water Resources Research, estimates that the biggest bouts of rain ever likely to fall across Australia are expected to rise by between 14% and 38% on average, due to increasing atmospheric moisture from climate change.

There are 546 large dams in Australia, and they might not cope with that level of rainfall.

“There are a lot of risks involved with dams given the amount of water they are holding back,” says Johan Visser from UNSW’s Water Research Centre, lead author on the paper in collaboration with academics from the University of Melbourne.

“Some of the worst floods around the world were due to extreme storms overwhelming a dam, causing it to fail and release a wall of water downstream.

“Hydrologists are therefore revising their estimates of the Probable Maximum Flood (or PMF) the dams were designed to withstand, which could mean some are now at risk of overtopping.”

The PMF is defined as the flood that might be expected from “the most severe combination of critical meteorological and hydrologic conditions”.

Also in Cosmos: Why can’t the weirs stop the Murray from flooding?

Hydrologists say estimates of the Probable Maximum Precipitation (PMP) likely to fall across a water catchment – used to predict floods and set by the Bureau of Meteorology – are 20 years out of date.

“The problem is that PMP calculation is based solely on historical data with no consideration for future climate conditions,” says Visser.

“This means that many large dams constructed decades ago were designed using information representative of a cooler climate.

“The purpose of this research was to analyse whether PMP estimates have changed over the last six decades and how these estimates might change in the future if we take into consideration a potential increase in atmospheric moisture due to known climate change.”

Vissey points out that for each 1°C rise in temperature, the atmosphere could hold about 7% more water.

As a result, and as the researchers discovered, extreme rainfall events and floods are already becoming more frequent and more intense.

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