Mission Valley flooding
Mission Valley flooding in 2016. File photo via Twitter

Researchers and engineers warn that California may be due for rain of biblical proportions — what experts call an ARkStorm, it was reported Tuesday.

Scientists call it California’s “other big one” and say it could cause three times as much damage as a major earthquake ripping along the San Andreas Fault, the Los Angeles Times reported.

This rare mega-storm, which some say is rendered all the more inevitable by 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, according to the newspaper.

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 cited by The Times.

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.

In the event of a rare storm, the Whittier Narrows Dam could overflow and erode. The U.S. Army Corps of Engineers has a proposal to protect it in such an event.

The corps is seeking up to $600 million in federal funding to upgrade the 3-mile-long dam and say the project has been classified as the agency’s highest priority nationally, due to the risk of “very significant loss of life and economic impacts,” The Times reported.

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

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.

A government study, however, used computer models to estimate the effects of 900-year, 7,500-year and 18,000-year storm events. Officials say as many as 1 million people could be affected.

— City News Service

Chris Jennewein

Chris Jennewein is Editor & Publisher of Times of San Diego.