Hundreds of thousands of people in Los Angeles could experience at least a foot of flooding during a 100-year disaster, a new scientific study has found, highlighting the hazards of aging infrastructure in America’s second-largest city.
This is a much higher estimate of flood exposure in Los Angeles than the one produced by the federal government. That estimate classifies areas of the city containing about 23,000 residents as being at high risk in a 100-year event, or an event with a 1 percent chance of occurring in any year.
The discrepancy is explained, in part, because the new study takes a more realistic view of the city’s water infrastructure, said the report’s lead author, Brett F. Sanders, a professor of civil and environmental engineering at the University of California, Irvine.
Many of Los Angeles’s flood control channels have become clogged with sediment and vegetation, reducing the amount of water they can transport, Dr. Sanders said. Rather than assume these channels are good as new, he and his colleagues used survey data collected with lidar, a technology for creating detailed 3-D maps, to examine how well the city’s waterways would handle a severe storm in their actual state.
“Let’s not assume perfect performance from our infrastructure; let’s look at the most likely performance,” Dr. Sanders said. “When we do this in Los Angeles, the second largest city in the United States, the risk is actually more than an order of magnitude bigger than what FEMA said it was,” he said, referring to the Federal Emergency Management Agency.
Other big cities face similar hazards. Last year’s deadly flooding in New York from the remnants of Hurricane Ida highlighted the need to unclog drains and widen sewage pipes so downpours could be flushed away quickly.
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FEMA declined to comment directly on the study, which was published on Monday in the journal Nature Sustainability. In a written statement, David I. Maurstad, senior executive of the agency’s National Flood Insurance Program, said all homeowners should learn about flood exposure on their property, whether or not they live in areas FEMA considers high risk.
“When it comes to flood risk, we know: Where it can rain, it can flood,” he said.
Extreme droughts and wildfires may have been the climate threats at the top of Californians’ minds in recent years. But, because hotter air can carry more moisture, global warming is also causing the state to experience fiercer winter rainstorms — in particular, the ocean-born storms known as atmospheric rivers, so named for their long, sinuous shape and the prodigious amount of water they convey.
Moderate atmospheric rivers typically provide California with much of the precipitation it receives. But strong ones that come in quick succession can cause catastrophic flooding, as occurred in the winter of 1861-62, when relentless rain and snow across California and the Pacific Northwest wrecked homes and turned valleys into lakes.
Today, California has a roughly 1-in-50 chance each year of experiencing another weekslong megastorm of comparable intensity, scientists estimated recently. Global warming has roughly doubled those odds compared with a century ago, they found. And, as the planet warms further, the risk will continue to grow.
In the meantime, less ferocious storms will continue causing disruption. In January 2021, heavy atmospheric river-driven rainfall over Southern California washed away a section of highway near Big Sur and triggered dangerous flows of mud and debris on hillsides burned bare by wildfires. The estimated damage: $1.2 billion.
Dr. Sanders and his colleagues estimated flood risks in Los Angeles by subjecting the city, in a computer model, to 24 hours of rain at levels that, according to the National Oceanic and Atmospheric Administration, have a 1 percent chance of occurring in any year. They assumed this rain would not be able to soak into the ground, as might be the case if it came in the middle of a weekslong storm and the earth were already saturated.
The researchers studied how water might move through the city by building a high-resolution topographic map, one that could show details as small as three meters across, or about 10 feet. This allowed their analysis to account not just for levees and flood walls, but also for finer-scale infrastructure such as culverts and drain pipes. They performed their calculations on a supercomputer in Wyoming run by the National Center for Atmospheric Research.
The California Department of Water Resources and the Bureau of Engineering at the Los Angeles Department of Public Works both declined when The New York Times asked whether they would review the study ahead of its publication and comment on its findings.
Harrison Wollman, a spokesman for Los Angeles’s mayor, Eric Garcetti, said the city had held public information sessions on flood risk and worked with FEMA and the Army Corps of Engineers to update flood hazard maps. “This study will help better inform our efforts to prepare for an extreme weather event,” Mr. Wollman said.
The study also found that Black and Hispanic residents were disproportionately at risk, largely in neighborhoods south of downtown Los Angeles near the Los Angeles and San Gabriel Rivers.
These are not the Angelenos typically assumed to be most exposed to flooding, said Nicholas Pinter, an earth scientist at the University of California, Davis, who didn’t work on the study. “The iconic and widespread view of flood hazard in L.A. is of movie-star beach houses on the Malibu coast,” Dr. Pinter said.
Disadvantaged communities often struggle more than others to bounce back from floods, said Nícola Ulibarrí, an associate professor of urban planning and public policy at the University of California, Irvine, and an author of the study. People who rent their homes are less likely to be able to cover the costs of recovery; hourly workers are more likely to lose income because flooding prevents them from getting to work.
“If you look at the longer-term impacts of recovery, those are also not equally distributed,” Dr. Ulibarrí said.