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ARkStorm stands for an atmospheric river (“AR”) that carries precipitation levels expected to occur once every 1,000 years (“k”). The concept was presented in a 2011 report by the U.S. Geological Survey (USGS) intended to elevate the visibility of the very real threats to human life, property and ecosystems posed by extreme storms on the West Coast.

In an ARkStorm scenario, a month’s worth of rain dumps as much as 10 feet of water on California, fed by a conveyor belt of tropical moisture. In addition to the expected damages (which include interruption of water conveyance), the storm has several public policy implications, most notably emergency preparedness and whether to pay now to mitigate expected damage or pay a lot more later for recovery.

“An ARkStorm is plausible, perhaps inevitable,” the USGS report said. “There may be no pattern that forces the storms to occur with clockwork regularity, so such an event could occur in any year.”

ARkStorm rainfall totals in some places would overwhelm the flood protection system in many areas. The Central Valley would experience a hypothetical flood 300 miles long and 20 miles wide, while Orange County, Los Angeles County, San Diego County and the Bay Area would all be hit with serious flooding. “Both because of its large geographic size and the state’s economic interdependencies, an ARkStorm would affect all California counties and all economic sectors,” according to USGS.

The deluge of water would cause hundreds of landslides and property damage would run in the hundreds of billions of dollars. Coupled with factors such as agricultural losses, the costs of restoring pipelines and business interruptions costs, an ARkStorm “could cost on the order of $725 billion, which is nearly three times the loss deemed to be realistic … for a severe Southern California earthquake, an event with roughly the same annual occurrence probability,” according to USGS.

California flooding references often point to the winter of 1861 when the city of Sacramento was 10 feet underwater. USGS says a repeat of a storm that size is certainly capable of happening. What’s surprising, experts say, is that California isn’t walloped by such super soakers more often. Until the next big storm arrives, the authors of the ARkStorm report believe there is time for planners to consider the level of readiness and whether further investment is necessary.

“Although enhancing state flood protection is very costly; not doing so may be even more so,” the report said. “Enhancing urban sections of the state flood protection system to 500-year levels could realistically cost $10s of billions. Not doing so could realistically cost $100s of billions when such a storm occurs.”

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