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Climate change made rain estimates outdated. Now they put US at risk for more flooding.

When 16.5 inches of rain fell in 24 hours at Cypress Creek near Houston in April 2016, officials called it a 1-in-100-year rain event.

Seventeen months later, Hurricane Harvey delivered even more rain — 19.9 inches.

The 100-year label was never intended to mean two such storms couldn't happen within the same century. But a warming climate is increasing the frequency of intense storms and playing havoc with outdated federal benchmarks — exposing the U.S. to a greater risk of flooding at a time when the country is looking to build toward the future.

Congress and the National Oceanic and Atmospheric Administration are looking at measures to bring about change later this year, but experts say time is of the essence.

Engineers, architects and others use the rain benchmarks to design highways, dams and retention ponds to be sure the structures can handle heavy rains. The 100-year label, for example, tells them a storm of a certain size has a 1% chance of occurring in any given year.

Intense rain from the remnants of Hurricane Ida caused widespread flooding in the Northeast last fall. The overflowing Rahway River sent flood waters pouring into this business, Casa Mia, on Essex Street in Millburn, New Jersey, where cleanup was underway on Sept. 2, 2021.
Intense rain from the remnants of Hurricane Ida caused widespread flooding in the Northeast last fall. The overflowing Rahway River sent flood waters pouring into this business, Casa Mia, on Essex Street in Millburn, New Jersey, where cleanup was underway on Sept. 2, 2021.

But the numbers — called precipitation frequency estimates — are outdated in at least 18 states, some by as much as 50 years. And the estimates, done on a state or regional basis, have never taken into account future projections for more intense rain.

Experts say this antiquated approach leaves not only existing structures at increased risk of flooding, but also anything built now using the old standards.

“We have a broken national process for understanding and predicting the relationship between rainfall and frequency,” said Chad Berginnis, executive director of the Association of Floodplain Managers.

Chad Berginnis, executive director of the Association of Floodplain Managers
Chad Berginnis, executive director of the Association of Floodplain Managers

When Texas officials sat down with the National Weather Service in 2018 to update their decades-old benchmarks, they found that a 100-year storm in Houston had become a 25-year storm.

Studies in several other states found that 100-year rainstorms now fall two or three times more often than they used to.

With the nation poised to start spending the billions of dollars approved in November's federal infrastructure plan, it’s more important than ever for the rainfall benchmarks to be accurate, said Rob Moore, a senior policy analyst with the Natural Resources Defense Council.

The nation's aging stormwater and wastewater systems already can't keep up with the rainfall increases, Moore said. And the country's flood losses topped $1.6 billion last year.

Building under the old standards — especially without taking into account projected increases — could add to those flood losses, Moore said. “If we build wastewater or drinking water systems that get flooded, that’s not money well spent."

MORE: Chicago's treasured shores are being swamped by rising waters. The city is again trying to turn the tide.

'We can do better'

The federal infrastructure plan approved in November includes nearly $500 million to modernize the rain benchmarks and flood mapping.

And Congress is considering legislation this spring that would require the National Weather Service to work out a more comprehensive way of doing the rainfall estimates and updating them more often.

Under the current method, estimates are produced on a rotating basis among states, but only when states agree and commit to reimburse the federal agency for the work. The latest round of updates began in 2002, but remains incomplete, with no new estimates for Montana, Idaho, Oregon, Washington and Wyoming.

While individual states might have their own independently produced estimates, the federal benchmarks for the five states were released almost 50 years ago. One set of documents still available to help planning in those states was published in 1961 and 1964.

NOAA tried to solicit interest last year among the five states to update the estimates, but only Idaho and Montana agreed to pledge funds. The update is proceeding for those two states and Wyoming.

The benchmarks in Texas were similarly outdated when the state and weather service began the 2018 update.

After adding decades of new rain data, they found the amount of rain required to meet the 100-year threshold in Austin increased by as much as 3 inches.

For the 13 states in the Ohio River valley, including some of the states seeing the biggest increases in intense rainfall, frequency estimates haven’t been updated in 16 years. Since then, the heaviest rainfall events — the ones that used to occur three days a year in 1990 — are up 139% in Akron and 129% in Dayton.

Similar increases in the most intense rains have occurred in Madison, Wisconsin; Fort Wayne, Indiana; and Sioux City, Iowa, to name a few.

Massive flooding occurred along the Ohio River in 2018, including this flooding in downtown Aurora, Indiana.
Massive flooding occurred along the Ohio River in 2018, including this flooding in downtown Aurora, Indiana.

Meanwhile, flood losses continue to escalate, claiming lives and costing billions. The annual average losses paid by the National Flood Insurance Program — $1.6 billion last year — is nearly 10 times higher than in 1999.

While the federal rain benchmarks haven't taken into account the changing climate, NOAA plans to unveil a proposed strategy for doing so within weeks, said Mark Glaudemans, who oversees the program as director of the National Water Center’s geo-intelligence division.

That's timely because the legislation pending before Congress would require the agency to take future rainfall projections into account as part of its modernization effort.

A similar bill would update flood standards and create a national flood information system to improve communication of flood risks. Flood estimates are different than rainfall estimates because they rely on topography, pavement and development to figure out how much runoff could occur in any given area. But experts told USA TODAY those estimates are similarly outdated and also use the outdated rain benchmarks.

Both bills were sponsored in the House by Rep. Mikie Sherrill, D-New Jersey, and have seen bipartisan support in the House and Senate, Sherrill said. “We're working to get this done as quickly as possible,” she said.

The new rain reality hit hard when the remnants of Hurricane Ida flooded much of her district. Rainfall rates surprised forecasters with their intensity.

Photos: See Ida's damage in New Jersey

The rain fell so hard and fast that one of Sherrill's constituents had to be rescued by boat. She said the current was so strong the woman's rescuers had a hard time steering. Sherrill visited the home afterwards and said the homeowners were “just devastated.”

“We can do better,” she said.

That's crucial, said Moore, the policy analyst at the the Natural Resources Defense Council.

“We are not incorporating what we know about the future into our decisions about what we build, where we build and how we build as a nation,” he said. “We are digging ourselves a bigger hole, that’s just going to keep filling up with water."

More bad news for climate change: Pandemic dip in carbon emissions was temporary, report says

This article originally appeared on USA TODAY: Outdated by climate change, rain estimates expose US to more flooding