Intensified rains pose hidden flood risks in the US

As climate change intensifies severe downpours, the infrastructure protecting millions of Americans from flooding is at growing risk of failure, according to new calculations of expected precipitation in every county and locality in the contiguous United States.

The calculations suggest that one in nine residents of the lower 48 states, largely in densely populated regions including the Mid-Atlantic and Texas Gulf Coast, are at significant risk from downpours that bring at least 50 percent more rain per hour than local pipes. , channels and culverts can be designed to drain.

“The data is shocking and it should be a wake-up call,” said Chad Berginnis, executive director of the Association of State Floodplain Managers, a nonprofit organization focused on flood risk.

The new rain estimates, released Monday by the First Street Foundationa New York-based nonprofit research group also has worrying implications for homeowners: They indicate that 12.6 million properties across the country are at significant flood risk, despite the federal government not being required to carry flood insurance.

The country will invest hundreds of billions of dollars in new and improved roads, bridges and ports in the coming years under the bipartisan infrastructure plan that President Biden signed into law in 2021. First Street’s calculations suggest that many of these projects are built to standards that are already outdated.

Matthew Eby, First Street’s executive director, said he hoped the new data could be used to future-proof these investments, “so we don’t spend $1.2 trillion knowing it’s wrong.”

The National Oceanic and Atmospheric Administration, the agency under the Commerce Department that produces the precipitation estimates used by planners and engineers around the country, declined to comment.

NOAA’s estimates are “the floor, not the ceiling,” said Abdullah Hasan, a White House spokesman. “States and localities often consider additional factors best suited to their local geography when making project decisions.”

Any additional increase in global warming increases the likelihood of intense rain in many places for one simple reason: Hotter air can hold more moisture. But NOAA’s estimates of expected rainfall are updated only occasionally. And, as NOAA scientists described in a recent report prepared in conjunction with university researchers, the agency’s estimates assume that the intensity and frequency of extreme rainfall has not increased in recent decades, despite ample evidence to the contrary.

The result, according to First Street, is that NOAA significantly underestimates the risk of heavy rain in some of the nation’s largest cities: Baltimore, Chicago, Dallas, Detroit, Houston, New York, Philadelphia and Washington. Other places where there are major discrepancies between First Street and the NOAA’s precipitation estimates include the Ohio River Basin, northwestern California, and parts of the Mountain West.

In other areas, including those east of the Sierra Nevada and Cascade Range, First Street believes NOAA overestimates the likelihood of heavy rain, implying resources there may not be best spent on upgrading flood infrastructure.

NOAA and its predecessor agencies have been publishing data on forecast rain and snow for decades. The most recent estimates, covering nearly every part of the country, are contained in a multi-volume publication called Atlas 14. (Another set of estimates, called Atlas 2, covers the northwestern states.)

Pick any point on the map and the NOAA Atlases tell you the probabilities there of different precipitation events — that is, a certain number of inches falling over a certain time span, from five minutes to 24 hours to 60 days.

But the Atlas’s estimates are based on rainfall measurements collected over the past several decades, or in some places since the 1800s, “in a climate that just doesn’t exist anymore,” says First Street chief Jeremy R. Porter. research into climate implications.

First Street, on the other hand peer-reviewed methods to estimate precipitation, use only precipitation data from this century, and only data collected by state-of-the-art government weather stations. (First Street plans to release additional documentation on July 31 about how it calculated its new estimates.)

NOAA is working on updating the atlas estimates to take better account of the warming climate. But the agency says initial data for Atlas 15 may not be ready until 2026.

First Street’s rain estimates also raise questions about federal government guidelines regarding flood risk for homes.

The Federal Emergency Management Agency maps areas of the country that it calculates are at significant risk from a 100-year flood, or an area with a 1 percent chance of occurring in any given year. FEMA’s maps guide decisions by builders, insurers and banks and determine whether homeowners should buy flood insurance.

But First Street’s data suggests that 17.7 million properties across the country are at risk in a 100-year event. Of those, only about 5 million properties also fall within a FEMA flood risk area. That means millions of other homeowners may be making decisions with an incomplete understanding of the actual physical and financial risks they face.

In Houston, 145,000 properties are in First Street’s 100-year flood zone, but not in FEMA’s. New York has 124,000 such properties; Philadelphia, 108,000; and Chicago, 78,000.

In an emailed statement, FEMA said it welcomed outside efforts to improve the country’s understanding of flood risks, but cautioned that First Street’s assessments were based on data and methods different from its own.

“FEMA’s process is careful not to underestimate or overestimate current flood risk,” the statement said. “The accuracy of the flood data required to maintain the nation’s largest flood insurance program and the nation’s largest regulatory land use program is fundamentally different from the level of accuracy required to support the First Street Foundation.”

NOAA began publishing Atlas 14 in 2004, meaning all sewers, culverts, and stormwater basins built since then may have been sized to standards that no longer reflect Earth’s current climate. But much of America’s infrastructure was built before, meaning it was designed to specifications that are likely even more outdated, said Daniel B. Wright, an associate professor of civil and environmental engineering at the University of Wisconsin-Madison.

“Certainly, updating Atlas 14 is something that needs to be done,” said Dr. Wright. “But the problem is huge, in that right now there are trillions and trillions of dollars worth of things that are based on horribly outdated information.”