ARkstorm Warning for California: Experts Fear Rare Mega Storm Three Times As Damaging As San Andreas Earthquake

Experts have spoken out after officials declared a California dam unprepared for a rare mega-storm that could flood vast areas of the state.

At recent public meetings, the U.S. Army Corps of Engineers told Los Angeles County locals that the 60-year-old Whittier Narrows Dam on the San Gabriel River could fail in an extreme storm scenario, The Los Angeles Times reported.

Scientists fear California isn't prepared for a hypothetical storm known as "Atmospheric River 1,000," or ARkstorm for short. The scenario imagines ferocious rain pummeling the West Coast—overwhelming existing flood-protection systems and submerging swathes of California.

The United States Geological Survey (USGS) has said that such a storm "is estimated to produce precipitation that in many places exceeds levels only experienced on average once every 500 to 1,000 years."

The agency likens an ARkstorm to the devastating floods that swept through California's central valley in the winter of 1861-2. Battered by rain in winter storms that raged for weeks, the region became impassable without a boat.

In 2011, the USGS estimated an ARkstorm could cost California upwards of $725 billion in damage and economic losses. That's well over three times the agency's 2008 Big One cost estimate, which put a $200 billion price on a magnitude 7.8 earthquake on the southern San Andreas fault. In 2017, data analytics firm CoreLogic raised that figure to $289 billion for an even bigger hypothetical quake.

The engineering Corps wants up to $600 million in federal funding to fix up the Whittier Narrows Dam but needs congressional funding. Corps officials fear the dam's failure could cause "very significant loss of life and economic impacts," The Times reported.

At present, the 56 foot high dam, which can hold back approximately 12.2 trillion gallons of water according to Corps engineers, could fail in several ways. Rising waters could spill over its crest, liquid could erode the sandy soil below and high rates of precipitation could prompt the spillway on the river to open, overwhelming the channel with more than 20 times the liquid it could safely hold, The Times reported according to federal engineers.

A government study modeled extreme but rare hypothetical storm scenarios that have a small chance—0.1 percent and even lower—of occurring each year, the newspaper reported. Even the most likely of these storms—which has a one in 900 chance of striking each year—would see floodwaters sweep through cities from Pico Rivera to Long Beach as the dam fails.

University of California Los Angeles climate scientist Daniel Swain said the Corps are "categorically underestimating" the potential impact of a mega-storm. "We only have about a century of records to refer to in California. So, they are extrapolating in the dark," he told The Times.

Local officials are setting up emergency measures to help protect the area until Whittier Narrows Dam repairs are finished in 2026. But as a warming climate makes dam failures potentially more likely, as per a recent Geophysical Research Letters study, scientists argue California's water infrastructure system needs a wider revamp.

As water historian and Whittier Narrows area expert David Reid told The Times: "The false sense of security included in the phrase '900-year flood' combined with the promises of 20th century water infrastructure have put us in a bind. That's because a mega-flood is impossible to predict. And if the water infrastructure fails, we're in big trouble."

California Flood
A road is blocked by floodwaters on February 15, 2019 in Forestville, California. Justin Sullivan/Getty Images