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The other Big One: California’s pending megaflood

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155 years ago, California turned into a sea, and it could happen again

Large waves blocking the view of homes in Pacifica.
Pacifica, 2005
Brocken Inaglory

Northern California is still drying out from storms that flooded town from Gilroy all the way up to Sacramento.

All things considered, the state weathered the weather fairly well. But there’s always another storm brewing, including the very real possibility of one that weather scientists dub “the other Big One,” a California tempest so big that it would rival or surpass a mega-quake.

In 2011, the United States Geological Survey (USGS) released their report on the ARKstorm Scenario. The “AR” in “ARK” stand for “atmospheric river,” a weather phenomena in which a band of moisture floats through the air.

“Think of an atmospheric river as a fire hose that funnels moisture from the tropical Pacific towards California,” as Megan Barber, writing for Curbed SF, put it.

And the “K” stands for 1K. As in, the kind of atmospheric river that visits California only once every 1,000 years or so, although some estimates pegs its frequency as low as 500 years.

The results are similar to a hurricane, except they can last for weeks. USGS tapped 117 “scientists, engineers, public-policy experts, [and] insurance experts” to suss out what it would look like:

The Central Valley experiences flooding 300 miles long and 20 or more miles wide.

Serious flooding also occurs in Orange County, Los Angeles County, San Diego, and the San Francisco Bay Area. Wind speeds reach 125 miles per hour. [...] Hundreds of landslides damage roads, highways, and homes. Property damage exceeds $300 billion, most from flooding.

[...] Agricultural losses and other costs to repair lifelines, dewater (drain) flooded islands, and repair damage from landslides, brings the total direct property loss to nearly $400 billion. Flooding evacuation could involve 1.5 million residents.

In all, 25 percent of the state could end up submerged. And the total cost could surge as high as $725 billion—more than three times the estimate of a mega-earthquake. $1.17 billion of that would be in San Francisco alone.

A plausible flooding scenario in an ARK-level storm.

"If we had a catastrophic disaster that takes down the California economy, that is a problem of national significance," former USGS Director Macia McNutt told the New York Times, emphasizing the financial cost to the entire country.

It’s sometimes seems easy to scoff at such warnings. But when compiling the storm profiles, USGS didn’t have to guess what might happen. They had precedent to consider.

From December 1861 to January 1862, it rained for 45 straight days in California. The Great Flood of 1862 wrought so much destruction that some parts of the state remained underwater six months later. The center of California turned into a 300-mile sea.

Finance-wise, the storms “bankrupted the state, destroyed the ranching industry, drowned 200,000 head of cattle [and] changed California from a ranching economy to a farming economy” in essentially one stroke, seismologist Lucy Jones told NPR in 2013.

The state capital temporarily moved to San Francisco, as Sacramento was navigable only by boat.

Sacramento, utterly flooded in 1862.
A newspaper illustration of the same scene.

But the waters didn’t spare the Bay Area: The entirety of Napa, for example, sloshed around in four feet of deluge by the time rains stopped. “The water reached 30 feet, submerging telegraph poles that had just been installed between San Francisco and New York,” notes Scientific American.

And San Francisco preacher SC Thrall declared it heaven’s judgment on California, our own personal Noah-style smiting.

“He who visited the nation with war, has smitten us with flood,” said an overzealous Thrall, pointing out that since the West Coast was sitting out the Civil War this evened the scales.

The 1862 flood served as a model for the ARKstorm, but of course, 1862 was not 500 to 1,000 years ago. The ARKstorm, if it ever comes, could be even bigger. “1862 is not a freak event,” write the ARKstorm authors. “And not the worst case.”

The good news, if any, is that unlike earthquakes, meteorologists can see big storms coming. In fact, a lot of the same tools used to predict climate change issues, like the Coastal Storm Modeling System, are constantly employed to anticipate storms brewing.

Chances are, most modern Californians will go their entire lives without a storm on this level. But Mother Nature always cashes in her marker sooner or later.