Supported by ten leading Australian water organisations, the study from the University of New South Wales and the University of Melbourne found that the current rainfall model upon which engineers base their designs of critical infrastructure such as the 546 large dams which are currently in operation around the country is around twenty years old.
Instead, the paper argues that increasing atmospheric moisture means that dams will need to handle between 14 percent and 38 percent greater maximum rainfall compared with what they are being designed for under current modelling.
Johan Visser, a senior water resources engineer from the UNSW Sydney Water Research Centre and the lead author of the paper in collaboration with academics from the University of Melbourne, said the potential consequences are serious.
“There are a lot of risks involved with dams given the amount of water they are holding back,” Visser said.
“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.”
According to Visser, engineers design dams to accommodate the largest flood event that could reasonably be expected to occur at a particular location.
This is known as the Probable Maximum Flood (PMF).
To work that out, engineers need to calculate the greatest depth of rainfall which is meteorologically possible over that area in a certain amount of time. This is known as the Probable Maximum Precipitation (PMP).
Problems arise, however, in two areas.
First, the methodology which is currently used to estimate likely maximum rainfall was last updated in the late 1990s or early 2000s. (The current probable maximum precipitation guidelines for various timeframes and locations across Australia is collated and published by the Bureau of Meteorology.)
This means that it is 20 years out of date and does not include the past two decades of atmospheric and rainfall data.
Moreover, the models do not account for the likely future consequences of climate change and consequential increasing atmospheric moisture.
In their new research, the academics aimed to determine whether PMP estimates have changed over the last six decades and how these estimates might change in the future if we consider a potential increase in atmospheric moisture due to known climate change
To do this, the academics reanalysed existing meteorological records and added in more uptodate data which had not been previously used.
They then incorporated the latest climate scenario modelling from the respected Coupled Model Intercomparison Project Phase 6 (CMIP6).
The results of the paper published in Water Resources Research show that in PMP estimates have increased across Australia over the past 60 years and are expected to increase further due to predicted increases in atmospheric moisture.
Based on the trajectory of the observed data, it was evident that there would be a systematic increase in the PMP.
This was confirmed using climate model simulations, indicating further increases for every climate scenario analysed.
Even using the lowest possible and most conservative estimates regarding emissions and subsequent climate change, the modelling suggests an average increase of 13 percent probable maximum rainfall across large dam locations throughout Australia through to the year 2100.
In the worst-case scenario, this could rise to 38 percent.
Should Dams be Upgraded?
The researchers say it is hard to make a generalised recommendation with regards to potential upgrades to dam infrastructure across Australia based on the implications of the results of their new study.
That’s because the location, climate and design of each specific dam varies significantly, and individual risk and cost assessments would need to be done on a case-by-case basis.
It also needs to be stressed that climate change is continually evolving, and the increase in extreme rainfalls associated with a warming climate is just one of the factors that dam owners need to consider when regularly reviewing the safety performance of their dams.
Nevertheless, the researchers highlight the fact that previous amendments to PMP estimates have resulted in large-scale dam enhancements.
For example, the Warragamba Dam that supplies water to Sydney had its wall raised by 5.1 metres and an auxiliary spillway (18,000 m3/s capacity) constructed after hydrological studies in the 1980’s showed the original spillway (10,000 m3/s capacity) was undersized and the and risk of dam break was unacceptably high.
Visser says dam owners will need to consider the effect of a warming climate when undertaking reviews about the safety of their assets.
“For each 1-degree Celsius rise in temperature, the atmosphere can hold approximately 7 per cent more water,” he said.
“We can already see in the observational record that extreme rainfall events and floods are becoming more frequent and more intense.
“The observational data shows that atmospheric moisture has been increasing over the historical record, and when you also factor in future climate projections from climate models, then you see that it’s projected to increase even further into the future.
“That is true for all the scenarios we analysed, even the most conservatively low estimates where there’s an increasing shift toward sustainable practices. So, under any future scenario our estimate is that the PMP will increase.
“That is concerning because if PMPs are increasing then our probable maximum flood estimates will also increase.
“Dam owners regularly review the safety performance of their dams under a wide range of failure scenarios, and this research reinforces the importance of ensuring that in the future these reviews consider the implications of a warming climate on extreme floods.”
The research was supported by leading Australian water organisations including Hydro Tasmania, Melbourne Water, Murray-Darling Basin Authority, Queensland Department of Natural Resources Mines and Energy, Seqwater, Snowy Hydro, Sunwater, West Australian Water Corporation, and WaterNSW, and also by the Australian Research Council.
Professor Ashish Sharma, corresponding author of the paper from UNSW’s School of Civil and Environmental Engineering, said the research was important not just for Australia but across the world.
This is because the approach to PMP is developed and recommended by the World Meteorological Organization and is used more or less consistently by countries around the globe.
“Anybody who is planning to build a large dam needs to think 50 to 100 years ahead, and this research makes clear that future planning now needs to consider the future impacts of climate change on the plausible upper limit of rainfall extremes,” Sharma said.