Risk of dam failures rises as climate shifts
Heavy rain amplifies threat from crumbling structures
Oklahoma City’s Overholser Dam suffers from floodgates that won’t open, concrete buttresses that are cracked and support beams that are crumbling. If the dam failed during a record rainstorm, the resulting flood would endanger thousands of lives and property worth millions in the heart of the city.
The Lake View Terrace mobile home park and $800,000 houses in the new Wheeler District. A Dell computer service center, a wastewater plant and a convention center. Dozens of hotels at two Interstate 40 interchanges. I-40 itself.
All of it, according to the worst-case scenario calculated by the U.S. Geological Survey, stands in the path of the floodwaters. And at least some version of this flood scenario has grown more likely, USA TODAY found, as the dam’s problems mount and powerful rainstorms descend more often on most of the United States east of the Rockies.
Thousands of U.S. dams could pose a growing threat to nearby communities as the climate changes, a USA TODAY analysis of climate and dam data found. They are in places that make them likely to harm people or property if they fail, and they face increased risk
of failing because river-swelling rainfalls have become more common, testing the limits of these structures.
Nearly 3,000 dams are already flagged as being in poor or unsatisfactory condition and, like Overholser, they need to be repaired or replaced. For 7,000 others that are in areas experiencing increasingly frequent downpours and whose potential failure would put people or property in harm’s way, there is no public information about their condition.
Some dam safety officials say structures designed to handle massive downpours will shrug them off no matter how often storms darken the skies.
But Upmanu Lall, a Columbia University engineer who studies the effect of climate change on dams, said this confident outlook assumes the dam hasn’t deteriorated and that it was built correctly in the first place. “The thing they’re not thinking about is that there is a difference between what was designed and what exists today,” Lall said.
Congress recognized the nation’s aging dams by including $3 billion for repairs in the giant infrastructure bill it passed last year. But in March, the Association of State Dam Safety Officials estimated the cost to repair every dam not owned by the federal government at $75 billion.
Engineering experts who assess failed dams say many problems go unnoticed because rules require only visual inspections, not deep exams of a dam’s current structure and historical problems.
The failures are already happening. In 2020, a week of soaking rain followed by a rainstorm that occurs about once in 25 years destroyed the Edenville Dam in central Michigan, causing 11,000 people to evacuate. The earthen embankment was constructed in part with loose sands, and engineers overestimated its stability. The dam’s owner knew for years that the floodgates couldn’t open all the way, and the dam failed before the problem could be fixed.
In 2019, a bomb cyclone slammed a torrent of water and river ice into Nebraska’s Spencer Dam, which breached, sweeping a man to his death. The postmortem noted the dam had problems with ice jamming its gates in the past.
In 2015, an offshore hurricane fueled a 500-year rainstorm in South Carolina that caused the failure of 36 dams. The National Oceanic and Atmospheric Administration said those failures contributed to floods that killed 19 people and damaged at least $1.5 billion worth of property.
More than 200 U.S. dams failed in downpours, floods and other wet weather since 2000, unleashing the water in their reservoirs to surge downstream, according to data kept by Stanford University.
Many were smaller structures that presented less downstream danger than Oklahoma City’s Overholser Dam. But recent dam disasters have killed people, wiped out roads and bridges, toppled power lines and devastated buildings.
At Overholser, the city has hired engineers to design repairs, the city’s utilities director said. But the gates aren’t scheduled to be fixed until 2027, and the crumbling concrete will wait until 2030 for completion. During repairs, a temporary dam will be built to provide additional protection.
“We must ensure that our nation’s infrastructure and dams are capable of withstanding more frequent and severe storms and floods fueled by the climate crisis,” wrote U.S. Rep. Annie Kuster, DN.H., who has sponsored legislation to fix dams, in response to questions from USA TODAY.
John France, an engineer who worked on the Edenville Dam failure investigation and others, said climate change has challenged what engineers thought they knew about the floods dams would have to endure.
“The methods that we’ve been using have been based on past performance, assuming that they are going to continue to happen,” he said. “Now the science is telling us, with the climate changing, that is probably not true.”
It doesn’t take a 1,000-year storm
The climate data is clear: Intense storms are occurring with increasing frequency.
Just in the space of four weeks this summer, six storms classified as having a 1 in 1,000 chance of occurring in any given year – climatologists call them 1,000-year storms – occurred in the United States. In Death Valley, California, a storm produced 1.5 inches of rain – a normal year’s worth – in just hours. In Texas, 15 inches in 24 hours flooded Dallas’ eastern suburbs. There were two such millennial storms in 2021.
Officials evacuated portions of Jackson, Kentucky, below the Panbowl Lake Dam when record flooding from another 1-in-1,000 storm hit the eastern part of the state in July, raising fears the dam would break. That dam was already on the state’s radar after flooding in March 2021 caused water to seep through an embankment, threatening its stability.
But as the Edenville and Spencer dams showed, it doesn’t take a 1,000year storm to break a dam with problems. Both of those dams fell in rainfall that had about a 1 in 25 chance of occurring in any given year, engineers said.
Jeongwoo Hwang, a postdoctoral research scholar at North Carolina State University’s civil and environmental engineering program, found dam failures since 2000 tended to follow a pattern: long, saturating rain or snow followed by the kind of storm that would not be considered extreme.
“Basically, many dams that have been failing recently have been failing from nonextreme events,” said Lall, the Columbia University engineer.
At Spencer, water and ice spilled over the top of the dam. That’s especially dangerous for the earthen portions of a dam because the rapidly flowing water can quickly erode the downstream face, causing the dam to fail, Lall said.
Even a concrete spillway designed to handle overflowing water can fall apart, Lall said, if it is in poor condition. That’s what happened in 2017 at the Oroville Dam in California. The concrete spillway cracked and slid, and dam operators faced a quickly eroding hillside despite a flood that was a small fraction of the size the dam was designed to handle. That incident led to the evacuation of 188,000 people and a $1.1 billion repair bill, according to the Association of State Dam Safety Officials.
More frequent storms in the 25- to 100-year range could mean more erosion and structural deterioration, said France, the engineer. A dam failure, if it comes, might not be the result of one deluge but the cumulative effect of many.
“The flood handling structures on these dams are going to be operated and tested more frequently than they have in the past,” he said. “We may see more spillways have problems just because they’re going to have more water flowing through them more often than they used to.”
USA TODAY worked with rainfall projections calculated by climate researchers at the University of Wisconsin-Madison, which show increases in heavy rainstorms east of the Rocky Mountains. Their models predict that as temperatures rise, that trend will continue. On average, USA TODAY found, for dams east of the Rockies, what was once considered a 25-year storm in 1995 is now expected to occur about every 18 years. That means such a storm is about 40% more likely today than it was in 1995.
For more than 5,600 high- and significant-hazard dams, what was once a 25-year storm in 1995 is expected to become a 15-year storm by 2055, making it about 65% more likely. Inspectors have classified the conditions of 940 of those dams as poor or unsatisfactory.
Although storms at every level are increasing in frequency, USA TODAY examined the recurrence of 25-year storms because storms of that size were enough recently to break the Edenville and Spencer dams, each of which had deficiencies.
At Edenville, the storm arrived after days of soaking rain. The already saturated ground sent runoff into rivers and ultimately into Wixom Lake, behind the dam.
The combined effect was a 100- to 200-year flood that pushed water levels in the lake to a record high. Investigators concluded that the saturated, loose sands in the embankment of the earthen dam lost strength and began to move, causing the dam to fail. A second dam gave way downstream. Between those two failures and flooding unrelated to the dams, rising water affected 3,000 homes and caused massive infrastructure damage for a combined $245 million in losses, according to a postmortem published this year.
On its own, the 2 inches of rain a bomb cyclone dumped upstream of the Spencer Damin March 2019 shouldn’t have been a problem. But thick river ice, melting snow and frozen ground converged all at once, raising the Niobara River and hurling water and sheets of ice over the dam, engineers later found. The dam breached in two places.
Had inspectors looked into the dam’s history, said Rob Ettema, a Colorado State engineering professor who investigated the failure for the state, they’d have known ice jams had caused problems there at least three times since the dam was constructed, and they likely could have addressed the issue.
Irfan Alvi, a Maryland-based engineer who examined the failures at Oroville and Edenville, said storms can reveal the effects of damage that might accumulate in a dam “gradually, over time,” like a hidden cancer.
“And then, you may not detect them until pretty late in the process,” Alvi said, “or maybe when it’s too late.”
The Overholser Dam
Like both the Edenville and Spencer dams, the Overholser Dam has gates that don’t work properly to release rising water. The dam has been rated in poor condition, or in need of repairs, for the past three years.
In 2015, the the USGS modeled what would happen if Overholser failed during a historic storm.
Water would spill over the top. Over the next three and a half hours, the flow would scour the downstream face of the dam’s earthen berm until a 291-footwide section collapsed and Lake Overholser gushed toward the city’s center.
This wouldn’t be a tsunami but a swelling flood that would take 16 hours to inundate neighborhoods, highways, schools and businesses in a strip along the Oklahoma River about 2 miles wide and more than 30 miles long. A USA TODAY analysis of census data estimates that about 27,000 people live in the path drawn by flood modelers.
In 1923, this was not a test. The earthen parts of the dam gave way in a heavy rain, and the resulting flood killed five people and caused $3.5 million in damage, about $60 million in today’s dollars. The city rebuilt the dam in 1924.
Nearly a century after it last broke, the dam faces serious deficiencies and a rainier climate.
What was once a 25-year storm near Oklahoma City – 6 inches in a day – has become nearly 20% more likely in any given year, according to the University of Wisconsin data.
The USGS simulation of an Overholser collapse didn’t examine how big a storm it would take to cause the dam to fail. Researchers focused instead on where floodwaters would end up.
But inspectors in 2021 said Overholser’s malfunctioning floodgates couldn’t reliably prevent water from pouring over the top if the dam encountered the biggest upstream flood current standards require it to handle, the extreme event that the 2015 federal study modeled.
That’s the dam’s most dangerous problem, said Zachary Hollandsworth, who leads the dam safety program at the Oklahoma Water Resources Board. Next is the cracked and worn-down concrete buttresses under the gates, he said. Those do not meet safety requirements for stability, according to a detailed study engineers presented to the city in September. For the past several years, inspectors’ reports noted widening cracks.
But it wasn’t until 2021, two years after inspectors first identified serious problems, that the city commissioned the deeper study. It took yet another year to complete.
The September report, commissioned by city utilities director Chris Browning, recommends spending $48 million to replace gates and shore up the concrete portions of the dam. Design is expected to be complete by 2025, with repairs unfolding over the subsequent five years. The gates will come first.
Browning said he believes Overholser can withstand the region’s weather but doesn’t know what would happen if a rare storm like the one near Dallas this summer dumped 15 inches in 24 hours.
“I do know that we’ve had a number of events since I’ve been here that were pretty good rain events, and we were able to open those gates just fine and get the water downstream,” he said.
But, he said, “I am concerned about the dam. That’s why we’re doing this study, and that’s why we’re going to have a corrective action plan.”
More money, better inspections
Like other owners of problem dams around the U.S., Oklahoma City found that just figuring out what needs to be done regarding repairs can be expensive. Two years passed from the time inspectors flagged serious problems at Overholser until the Oklahoma City Water Utility Trust paid for a $500,000 engineering study to identify fixes.
More dam owners should do such indepth studies of their dams, said Hollandsworth, Oklahoma’s chief dam safety official. But as in most states, Oklahoma law only requires a visual inspection, he said, unless specifically ordered by the board that oversees dams. Owners often can’t afford more. “It’s harder to deal with smaller cities that do not have those resources,” Hollandsworth said. “It’s hard to convince a dam owner when no other deficiencies have ever been noted.”
Another challenge: Upgrading dams that are a known problem is hard because so many are privately owned.
“If it’s just one person or one family that owns a dam, it can be very difficult for them to come up with that money,” said David Griffin, program manager for safe dams at the Georgia Department of Natural Resources.
The nation also has an incomplete picture of which dams could do the most damage if they failed, experts fear. Dams considered low-hazard because they were built in rural areas may now pose an underappreciated threat because agencies responsible for rating hazards haven’t caught up to changes that have occurred since construction.
Some counties haven’t fully mapped out the areas that would be flooded if their dams broke, Griffin said, so over the years they have approved building permits for subdivisions without knowing they were at risk.
France and others who have seen hidden deficiencies bring about a dam’s demise, advocate less frequent but more intensive dam inspections than are being done in most cases now.
“If we don’t,” France said, “then the Orovilles and the Edenvilles will keep happening.”