Rare mega-storm could wash away portions of California

Scientists say the impending earthquake is not the only "Big One" Californians should be worried about

For years we have been hearing that California is due for the "Big One", in reference to the mega earthquake the state is expecting. Now, scientists say that is not the only "Big One" Californians should be worried about, because Southern California could also experience an ARkStorm (for Atmospheric River, 1 in 1000 year event).

This hypothetical, but scientifically realistic, "mega-storm" scenario was developed and published by the United States Geological Survey (USGS) and if it were to occur, it would cause major flooding from the coast to the desert, impacting over a million-and-a-half people.

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Image: Atmospheric river impacting California from cover of ARkStorm Magazine (Source: USGS)

A team of researchers and engineers warned that this other ¨Big One¨ could cause three times more damage than a major earthquake along the San Andreas fault.

ARkStorm addresses massive U.S. West Coast storms analogous to those that devastated California in 1861–62. Storms of this magnitude are projected to become more frequent and more intense as a result of climate change.

The purpose of this study was to design the large, but scientifically plausible, hypothetical storm scenario hitting both northern and southern California. The wind, precipitation and flooding, and coastal hazards were translated into physical, environmental, social, and economic damages to provide emergency responders, resource managers, and the public a realistic assessment of what is historically possible.

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Map of areas that would potentially flood from the hypothetical ARkStorm (USGS)

The hypothetical storm would last for weeks, and floodwaters would inundate large areas from the coast to the Mojave Desert. In the Los Angeles Basin, runoff from the San Gabriel Mountains would cause the San Gabriel River to overflow from Pico Rivera to Long Beach, and that would definitely test current control measures.

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Simulations of these potential scenarios indicate that cities like Downey could end up under 15 feet of floodwater. The government study also contemplated simulations for 900, 7,500 and 18,000 year storm events. In all cases, much of the region ended-up under catastrophic flooding, with a quarter of California's homes affected, and a statewide bill of $725 billion (USD).

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Map of potential agricultural losses from the hypothetical ARkStorm (USGS)

SEE ALSO: Billion-dollar weather and climate disaster bill for 2018

Some of main effects caused by the hypothetical ARkStorm would include:

  • The evacuation of 1.5 million residents.

  • Damage to power, phone, water and sewer would take weeks or months to restore.

  • The Central California Valley would experience hypothetical flooding 300 miles long and 20 or more miles wide.

  • Serious flooding would also affect urban areas around Orange County, Los Angeles County, San Diego, San Francisco Bay area and other coastal communities.

  • Hurricane force winds of up to 200 km/h could affect some areas, especially in interior mountain ranges.

  • Hundreds of landslides would damage roads, highways and homes.

  • Property damage would exceed $US 300 billion mainly 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, of which $20 to $30 billion would be recoverable through public and commercial insurance.

Landslide Susceptability

Map illustrates landslide susceptibility in California (USGS)

“We only have about a century of records to refer to in California. So, they are extrapolating in the dark,” says Daniel Swain, a scientist at the UCLA Institute of The Environment.

It should be noted that there is still a very small chance of a storm of this size striking this year or the next, but science suggests that it will happen eventually.

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And California wouldn’t be the only area impacted by the ARkStorm. The entire West Coast would feel the impact.

The Great Flood of 1862 inundated Oregon, Idaho, New Mexico, Nevada, Arizona, and Utah, and California has potential to be next on the list.