Major dams in California are five times more likely to flood this century than the last one due to global warming, a new study finds, possibly leading to overtopping and catastrophic failures that threaten costly repairs and evacuations.
That means Californians can expect more disasters like the Oroville Dam, which failed in 2017 after days of flooding had filled state reservoirs to 85% of their capacity, leading to the evacuation of more than 180,000 people and losses of around $300 million.
In June, an analysis led by UCLA researchers concluded the Oroville Dam spillway overflow was worsened by climate change. The results of the new Geophysical Research Letters study suggest that there are at least six other major dams in California that have an even higher potential flood risk than the Oroville Dam did, study author Amir AghaKouchak of the University of California, Irvine, told Angle News. “The Oroville Dam shows what the chances of these failures look like, and might become the new normal.”
Right now, heavy snows build California’s mountain snowpack. But higher temperatures in the coming decades will mean that much of the state’s mountain snow will instead fall as rain, leading to floods, the new analysis found.
“In a warmer world, we expect more precipitation, which influences flooding,” AghaKouchak said. “We tried to look into the future to ask about the probability of that affecting dams.”
California’s New Don Pedro, Shasta, Lewiston, and Trinity Dams face the highest risk of flooding in this century, according to the new report, which combined the results of 10 climate models assuming “business as usual” for the global climate — the roughly 7 degree Fahrenheit rise path that the world is now on — with a hydrological model of 13 major dams and rivers in California. The study also analyzed a less severe future where humanity decides to lower its greenhouse gas emissions.
The state of California acknowledged that climate change poses a risk, but also said its dams have been designed to withstand storms so massive that they happen only once every 1,000 years, according to Chris Orrock of the California Department of Water Resources. (That means, roughly speaking, that these storms have a 0.1% chance of happening every year).
In September, however, California’s legislature released a report noting that the state has 678 “high hazard” dams with many needing repairs or upgrades. It also raised questions about flood and earthquake risks to the state’s levees, and noted the American Society for Civil Engineering estimated around $18.6 billion in necessary repairs and upgrades.
According to a 2014 Intergovernmental Panel on Climate Change report, climate change will increase over this century, largely driven by the burning of fossil fuels such as coal and oil that release heat-trapping greenhouse gasses into the atmosphere. That warmer air will hold more moisture, which will lead to heavier rain and snowfall. Essentially, the heaviest storms will become heavier in a warmer world, a pattern already seen today in North America, particularly in the increased number of heavy downpours in Northeastern and Midwestern states.
“It is clear that climate change has already changed California’s climate in ways that the study outlines,” climate scientist Noah Diffenbaugh of Stanford University told Angle News. “The state needs to address questions about an aging water infrastructure built for snow … instead of the rain we will see in the future.”
For California, the study finds these warmer temperatures will mean more rain and less snow will fall in the mountains that serves as the snowpack reservoir for the state’s rivers and dams. Snowmelt will happen earlier, and floods will result. What used to be 1-in-100 year floods will now happen every 20 years to the four most threatened dams, essentially turning a yearly 1% flood risk into a 5% one.
By the middle of the century, the risk of these floods will peak in the fall or winter, when what was once heavy snowfall will instead fall as rain. The climate projections are fairly standard, climate scientist Charles Curry of the Pacific Climate Impacts Consortium (PCIC) in Victoria, Canada, and jibe with another recent study by Curry’s team of increased future flood risks on Canada’s Fraser River, which empties into the Pacific Ocean at Vancouver.
AghaKouchak, the study author, stressed that they broadly defined failure in the study not only as a catastrophic collapse, but also less severe problems, such as overtopping of dams or damage to foundations and spillways that exceed the limits of their design. Estimating the risk of catastrophic collapses would require a structural engineering analysis of each dam, which was not party of his study.
The key uncertainty in the study is future lifetime of the dams, many of which will be rebuilt to higher flood standards in the coming decades. But for all the dams in the study, whether new or old, the risk of hydrological failure increases in the coming decades — even with the more conservative climate models. “This strikes me as the most important result of the paper,” said Curry.
Weather reports suggest that snow falling on California this week may turn to rain in the days ahead, possibly leading to flooding, said Diffenbaugh. “There is some timeliness to reporting on this now.”