America’s dams were not built for today’s climate-charged rains and floods

As flooding battered Appalachia in the wake of Hurricane Helene, residents became familiar with a new norm in the US post-storm scenario: dams at imminent risk of failure.

Officials said last week that several dams are on the brink, including Tennessee’s Nolichucky Dam and North Carolina’s Walters and Lake Lure dams. People in nearby communities were ordered to evacuate.

In the end, the dams held. But the close calls highlighted the stress on the nation’s dams, many of which are more than half a century old and none of which were designed for the higher levels of precipitation brought on by climate change.

Many of the dams “absolutely serve a useful function for communities, whether it’s helping to hold water for irrigation or hydropower,” said Tom Kiernan, president and chief executive of American Rivers, an environmental nonprofit. But many others, he said, “are outdated, unsafe, abandoned.”

The average dam in the U.S. is 60 years old, according to the American Society of Civil Engineers, even though dams “are typically designed and built to last 50 years,” Kiernan said. There are more than 92,000 dams nationwide, according to a US Army Corps of Engineers inventory. Of these, about 16,000, or nearly 20 percent, are classified as having “high hazard potential.” This does not mean that they are at an increased risk of failure, but that their failure would involve a significant loss of life or property. This group includes the 98-year-old Lake Lure Hydroelectric Dam. The number of high-risk potential dams in the US has increased as development encroaches on once-rural areas.

Over the past decade, extreme rainfall has caused partial or complete dam failures in Connecticut, Minnesota and Michigan. In 2017, heavy rains brought California’s Oroville Dam into crisis mode and forced the evacuation of more than 180,000 people. Between 2015 and 2018, North and South Carolina saw more than 100 dams break due to record flooding, according to a Congressional Research Services report.

Only 3% of US dams are under federal control; some are run by states, but most are privately owned. In 2022, the Association of State Dam Safety Officials, a nonprofit organization, estimated that $75.7 billion is needed to rehabilitate non-federal dams. The Inflation Relief Act allocated $2 billion for dam decommissioning and upgrades.

“We need tens of billions more,” Kiernan said.

As greenhouse gases in the atmosphere increase temperatures, storms become more intense. This increases the danger to dams, especially the earthen dams that comprise the majority of those in the US. Excess rainwater poses a risk in at least two ways. The first is overtopping – when water spills over the edge of the dam, eroding it and leading to structural failure. The second is sediment and debris carried by floodwaters, which can choke systems like spillways that are supposed to help dams compensate for too much rain.

For more than 75 years, infrastructure whose failure could cause loss of life – including dams and nuclear power plants – has been built or upgraded to withstand flooding using a standard called Probable Maximum Precipitation (PMP). The PMP was developed by the federal government beginning in the 1940s, a process led by what is now the National Oceanic and Atmospheric Administration.

The standard is based on the most extreme rainfall events of the past, with the idea that infrastructure built to withstand these extremes will be future-proof. But the data used to maintain the standard hasn’t been updated nationally since 1999, and in some regions hasn’t been updated for 60 years.

Some states have acted on their own initiative. “There are probably 20 states now that have updated their PMPs in the absence of federal guidance,” often turning to the private sector for help, said Bill McCormick, who serves as an extreme precipitation expert for the State Dam Safety Association . The officials. North Carolina began its own upgrade effort before Helene hit, he said.

But many states have not started. And in the absence of federal oversight, it’s unclear whether state updates go far enough.

Not only is the historical data that informed the PMP based on yesterday’s climate, it is not entirely reliable. “There are numerous flaws in the way we’ve collected the data for over 100 years,” said James Smith, a professor of civil and environmental engineering at Princeton University. “But the biggest and hardest to deal with is climate change,” he said.

Smith pointed to the catastrophic flooding in western North Carolina as a vivid example of the problem. There, the PMP is based in part on the great flood of 1916, when more than 8 inches of rain fell in 24 hours. Helene’s rainfall — which was estimated to be 50 percent higher than it would have been without climate change — exceeded it in places by 7 inches.

For a long time, NOAA “had no legislative mandate” to update the federal PMP, McCormick said. This changed in 2022 with the passage of the Precipitation Change Estimates and Research Provision Act (PRECIP). The law required the agency to conduct a rainfall estimation study, including the PMP, within two years. In June, the National Academies of Sciences, Engineering and Medicine issued that report, written by a committee that Smith chaired. It urges the US to update how it calculates the standard to account for climate change.

PMP does not just affect infrastructure as it is designed and built, but throughout its life. Dams and nuclear power plants are regulated and monitored by state and federal agencies and maintained over time. “Even if it’s 100 years old, if the PMP changes, a dam has to be able to pass the PMP without failing,” Smith said.

So far, only two countries — Norway and Sweden — have rules requiring dam owners to incorporate future climate forecasts into their operations, said Javier Fluixá-Sanmartín, a dam safety engineer at Swiss firm HYDRO Exploitation SA. But more such rules are likely to come in other countries.

In the US, the National Academies report is a key first step. But the biggest question will be funding. Many dam owners cannot necessarily afford to upgrade.

“Not every dam is a hydroelectric dam. So not every dam generates revenue by producing power,” McCormick said.

Photo: The Broad River after Hurricane Helene in Bat Cave, North Carolina on October 1. Photographer: Sean Rayford/Getty Images

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Flood