Catastrophic flooding a century ago prompted civilian leaders and engineers to tame the raging rivers of the Los Angeles Basin with dams, gullies and concrete.
Now scientists warn that in a warming world, the region must expect an increase in epic downpours that could quickly overwhelm its aging flood defense system and unleash floodwaters over low-lying working-class communities.
On Tuesday, the Los Angeles County Board of Supervisors approved a motion asking the Department of Public Works to prepare a report on the viability of existing flood control infrastructure, as well as plans to reduce flood risk and build resilience in disadvantaged communities.
The application, written by supervisor Janice Hahn, was inspired by a recent study led by researchers at UC Irvine. The report aims to provide county officials with data needed to determine how severe and where flooding is hitting hardest, and to introduce tools and apps that would help speed recovery.
“The latest available scientific data warns that the potential for a catastrophic flood is real,” Hahn said, “and that it would be most devastating for black communities in low-lying neighborhoods.”
“Now the county needs to take a close look at our infrastructure and see what we can do to protect these communities if and when the storm we’re all worried about hits,” she added. “I’m sure some of the challenges this report will present will be daunting, but we will not let these communities down.”
The study, published in the journal Nature Sustainability, is one of the first to examine how extreme weather events due to climate change may affect the region, whose development has been guided by social and racial divisions that favored white residents.
The study’s unprecedented combination of high-resolution flood modeling and socioeconomic data suggests major flooding would occur between the Dominguez Channel to the west and the Los Angeles River to the east.
Communities most at risk include Carson, Paramount, Compton, Bell Gardens, South Gate, North Long Beach and parts of downtown Long Beach, including the south end of Pine Street near the Long Beach Convention Center.
Analysis of data from 1,767,588 parcels of land within approximately 2,700 square miles shows that approximately 874,000 people and up to $108 billion in property stretching south from the Santa Monica Mountains to Long Beach are at risk of flooding .
That’s about 30 times more people at risk than suggested by the Federal Emergency Management Agency, according to Brett Sanders, a professor of civil and environmental engineering at UC Irvine.
Until recently, it was assumed that a flood event of this magnitude would occur every 1,000 to 10,000 years. But new research suggests there’s about a 50/50 chance of seeing another one of this magnitude in the next 40 years.
Now, among the top priorities of the U.S. Army Corps of Engineers in Southern California is spending an estimated $600 million to modernize the 63-year-old Whittier Narrows Dam, located in a natural gap in the hills about 11 miles east of downtown Los Angeles was built.
The earth dam was placed in the agency’s highest risk category when it was determined that three potential failure modes threatened more than 1 million people downstream from Pico Rivera to Long Beach.
These include the premature opening of the San Gabriel River spillway gates, erosion resulting from water pipes passing through the dam’s foundation, and flooding during an extreme flood event.
“Our infrastructure was built at a time when the county only cared about two things: economic development and safety from damaging storms,” Sanders said. “Now we have the opportunity to rebuild it with new goals.”
“Today we are dealing with drought, heat waves and water conservation,” he said. “We want rivers with trees and ecological connections to the land around them.
“We also want to make sure all communities are equally served by the infrastructure,” he said.
https://www.latimes.com/environment/story/2022-12-06/l-a-county-seeks-flooding-fix-in-face-of-climate-change L.A. County seeks flooding fix in face of climate change