Dam safety at risk following study into flood events

Industry-funded research says existing models for potential maximum rainfall are outdated. It suggests that existing dams are at greater risk due to spillway inadequacy, risking dam safety and the safety of local populations.

The rainfall model that engineers use to help them design critical infrastructure, such as large dams and nuclear power plants, needs to be updated to take into account climate change.

That is the conclusion of a paper published in Water Resources Research, which calculates that the ‘Probable Maximum Precipitation’ (PMP) estimates for 546 large dams across Australia are expected to increase between 14 and 38 per cent 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 existing models of PMP 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 frequent.

Improved modelling vital for dam safety

“There are a lot of risks involved with dams, given the amount of water they are holding back. 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 lead author Johan Visser from UNSW Sydney’s Water Research Centre.

“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, you first need to calculate what is the greatest depth of rainfall meteorologically possible over that area in a certain amount of time. That is called the Probable Maximum Precipitation – PMP.

“The problem is that PMP calculation is based solely on historical data with no consideration for future climate conditions. 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. We also wanted to know how these estimates might change in the future when considering a potential increase in atmospheric moisture due to known climate change,” said Visser.

Incorporation of climate models

The current PMP guidelines for various timeframes and locations across Australia are collated and published by the Bureau of Meteorology. The study was funded by 10 of Australia’s leading water providers and dam owners from across Australia.

The new research reanalysed existing meteorological records. It added in more up-to-date data from the last 20 years that was not previously included. The researchers then calculated potential changes in the future by incorporating the latest climate scenario modelling. This modelling came from the highly respected 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 of the paper show that PMP estimates have increased across Australia over the past 60 years. It is 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 analysed.

For the worst-case future scenario, PMP estimates over large dam locations in Australia could increase by 38 per cent on average.

The modelling suggests an average increase in PMP of 13 per cent across Australia under the most conservative modelling.

The researchers say they have shown the current method of calculating PMP is likely outdated. It does not take into consideration the potential consequences of current changes in atmospheric conditions. The models also do not account for the changes predicted into the future.

Dam safety a critical problem

“For each 1-degree Celsius rise in temperature, the atmosphere can hold approximately 7 per cent more water,” said Mr Visser.

“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. 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. That includes 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 PMF estimates will also increase. Dam owners regularly review the safety performance of their dams under a wide range of failure scenarios. This research reinforces the importance of ensuring that in the future these reviews consider the implications of a warming climate on extreme floods.

“One of the issues with the methodology presently used to estimate probable maximum rainfalls is that the last time it was updated in Australia was around the late 1990s or early 2000s. Therefore, 20 years’ worth of atmospheric moisture and rainfall data is already not included.

Professor Rory Nathan of the University of Melbourne, one of the lead collaborators involved in the research, said: “No country in the world has yet updated the operational procedures used to estimate PMPs to account for climate change, and this research provides the first evidence that these procedures need to be reviewed.

“It will take some time to work out the best way of using these findings to update industry practice. We are facing a climate crisis. This research adds to the urgency with which we need to curb our carbon emissions.”

Global importance of PMP modelling

The researchers say it is hard to make a generalised recommendation with regards to potential upgrades to dam infrastructure.

That’s because the location, climate and design of each dam varies significantly. 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. The increase in extreme rainfalls associated with a warming climate is just one factor that dam owners need to consider when regularly reviewing the safety performance of their dams.

But they highlight the fact that previous amendments to PMP estimates have resulted in large-scale dam enhancements.

The Warragamba Dam supplies water to Sydney. It had its wall raised by 5.1 metres and an auxiliary spillway (18,000 m³/s capacity) constructed. This followed 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.

Professor Ashish Sharma, co-author of the paper from UNSW’s School of Civil and Environmental Engineering, said: “This is an important piece of research, not just for Australia but also across the world. The approach to PMP is developed and recommended by the World Meteorological Organization. It 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. 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.”