Extreme floods occurring more frequently than federal estimates – report

New analysis has revealed that current federal data on extreme rainfall severely underestimates the likelihood of flood events.

According to findings released by the non-profit organization First Street Foundation, the US government’s precipitation frequency estimates fail to adequately capture the frequency and severity of extreme precipitation in the face of climate change.

As such, events classified as a “1-in-100-year flood” occur more frequently than predicted.

In fact, First Street’s peer-reviewed model found that some 51% of Americans reside in areas that are twice as likely to experience a 1-in-100-year flood compared to the predictions of Atlas 14, the widely used precipitation frequency estimates by the National Oceanic and Atmospheric Administration (NOAA).

Jeremy Porter, head of climate implications for First Street, said the discrepancy is due to the infrequent updates to Atlas 14, which has not kept pace with intensifying rainfall events caused by the climate crisis.

Extreme floods and the impact of climate change

First Street’s study revealed that approximately 21% of the country can expect a 1-in-100-year flood to occur every 25 years.

Meanwhile, more than 1.3 million people across 20 counties, including parts of Indiana, Kentucky, Pennsylvania, and North Carolina, may experience these extreme flood events at least once every eight to 10 years.

“The magnitude of the changes in expected rainfall intensity are startling for many areas in the United States,” said Jungho Kim, First Street’s senior hydrologist and a lead author on the study. “And it is important that Americans are fully aware of this consequence of climate change that can impact their lives and homes.”

The research also shed light on regions like the Northeast, the Ohio River Basin, Northwestern California, the Texas Gulf Coast, and the Mountain West, where rainfall for a 1-in-100-year event might occur at least every five to 10 years.

Furthermore, it highlighted the impact of climate change on densely populated cities. One example is Houston, Texas, where the likelihood of a 1-in-100-year flood event went up 335% from Atlas 14, making it a 1-in-23-year event.

Another concern raised by First Street is the recent allocation of $1.2 trillion through the Infrastructure Investment and Jobs Act (IIJA) for capital investment and infrastructure spending until 2027.

Given that many of these projects will require engineering expertise to withstand climate-related risks, including accurate flood design standards, the group said estimates in NOAA’s Atlas 14 could lead to billions of dollars being spent on projects that may not stand the test of time.

“The fact that the nation will not have the most accurate estimates of extreme precipitation likelihoods available at the time of the design of these projects means that many of them will be out of date on the day they are opened to the public,” said Matthew Eby, founder and executive director of First Street Foundation.

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