The rainfall model that engineers use to design critical infrastructure such as large dams needs to be updated to take into account climate change, according to research from the University of New South Wales Sydney.
A paper published in Water Resources Research calculates that the probable maximum precipitation (PMP) estimates for 546 large dams across Australia are expected to increase 14% to 38% on average due to increasing atmospheric moisture. The research suggests that existing dams will be at greater risk under climate change than what is currently assumed.
The researchers say PMP models have not been updated for at least 20 years. More recent meteorological events show that the climate is warming and making storms more intense and more frequent.
“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,” said Johan Visser from UNSW Sydney’s Water Research Centre, lead author on the paper. Engineers design dams to accommodate the largest flood event that could reasonably be expected to occur at a particular location, known as the probable maximum flood – PMF. Visser said a first step is to calculate the greatest depth of rainfall meteorologically possible over that area in a certain amount of time, referred to as PMP.
“The problem is that PMP calculation is based solely on historical data with no consideration for future climate conditions,” he said. 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 analyze 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.”
The PMP guidelines across Australia are collated and published by the Bureau of Meteorology. The new research – which was funded by 10 water providers and dam owners – reanalyzed existing meteorological records and added in more up-to-date data from the past 20 years. The researchers then calculated potential future changes by incorporating the latest climate scenario modelling from the Coupled Model Intercomparison Project Phase 6 (CMIP6). These climate models are used to explore how a range of global socioeconomic choices over the next century will affect greenhouse gas emissions.
The results show that 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 there would be a systematic increase in the PMP. This was confirmed using climate model simulations, indicating further increases for every climate scenario analyzed.
For the worst-case future scenario, where green policies are not implemented and carbon emissions remain highest through to 2100, PMP estimates over large dam locations in Australia could increase by 38% on average. Even using the most conservative (low) estimates regarding emissions and subsequent climate change, the modelling suggests an average increase in PMP of 13% across Australia.
Study results show that maximum persisting dewpoints (input in PMP calculations) over Australia has increased over the historical record and is expected to increase further over the next 80 years for all modelled climate scenarios known as Shared Socioeconomic Pathways (SSPs), resulting in increased PMP estimates.
“For each 1-degree Celsius rise in temperature, the atmosphere can hold approximately 7% more water,” said Visser. “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 analyzed. So, under any future scenario, our estimate is that the PMP will increase. That is concerning because if PMPs are increasing, then our PMF 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 researchers say it is hard to make a generalized recommendation with regards to potential upgrades to dam infrastructure based on the implications of the results of their new study. Individual risk and cost assessments would need to be done. In addition, climate change is continually evolving, and the increase in extreme rainfalls associated with a warming climate is just one of the factors dam owners need to consider when reviewing safety performance.
But they highlight the fact that previous amendments to PMP estimates have resulted in large-scale dam enhancements. For example, Warragamba Dam that supplies water to Sydney had its wall raised by 5.1 m and an auxiliary spillway (18,000 m3/s capacity) constructed after hydrological studies in the 1980s showed the original spillway (10,000 m3/s capacity) was undersized and the risk of dam break was unacceptably high.
This research was supported by Hydro Tasmania, Melbourne Water, Murray-Darling Basin Authority, Queensland Department of Natural Resources Mines and Energy, Seqwater, Snowy Hydro, Sunwater, West Australian Water Corporation, WaterNSW and the Australian Research Council.
