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Rare mega storm could overwhelm LA-area dam and flood dozens of cities, experts say

Louis Sahagun, Los Angeles Times on

Published in Weather News

LOS ANGELES -- Scientists call it California's "other big one," and they say it could cause three times as much damage as a major earthquake ripping along the San Andreas fault.

Although it might sound absurd to those who still recall five years of withering drought and mandatory water restrictions, researchers and engineers warn that California may be due for rain of biblical proportions -- or what experts call an ARkStorm.

This rare mega storm -- which some say is rendered all the more inevitable because of climate change -- would last for weeks and send more than 1.5 million people fleeing as floodwaters inundated cities and formed lakes in the Central Valley and Mojave Desert, according to the U.S. Geological Survey. Officials estimate the structural and economic damage from an ARkStorm (for Atmospheric River 1,000) would amount to more than $725 billion statewide.

In heavily populated areas of the Los Angeles Basin, epic runoff from the San Gabriel Mountains could rapidly overwhelm a flood control dam on the San Gabriel River and unleash floodwaters from Pico Rivera to Long Beach, says a recent analysis by the U.S. Army Corps of Engineers.

In a series of recent public hearings, corps officials told residents that the 60-year-old Whittier Narrows Dam no longer met the agency's tolerable-risk guidelines and could fail in the event of a very large, very rare storm, such as the one that devastated California more than 150 years ago.

Specifically, federal engineers found that the Whittier Narrows structure could fail if water were to flow over its crest or if seepage eroded the sandy soil underneath. In addition, unusually heavy rains could trigger a premature opening of the dam's massive spillway on the San Gabriel River, releasing more than 20 times what the downstream channel could safely contain within its levees.

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The corps is seeking up to $600 million in federal funding to upgrade the 3-mile-long dam, and says the project has been classified as the agency's highest priority nationally because of the risk of "very significant loss of life and economic impacts."

The funding will require congressional approval, said Doug Chitwood, lead engineer on the project.

Standing atop the 56-foot-tall dam recently, Chitwood surveyed the sprawl of working-class homes, schools and commercial centers about 13 miles south of Los Angeles and said, "All you see could be underwater."

The dam -- which stretches from Montebello to Pico Rivera and crosses both the San Gabriel and Rio Hondo rivers -- is one of a number of flood control facilities overseen by the corps. Throughout much of the year, it contains little water.

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