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The Other 'Big One': Is California Due For A Devastating Megaflood?

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Caution: flooding (Photo by Sam72 via Shutterstock)
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When Californians talk about the Big One, we're usually talking about a massive, disastrous earthquake. But some scientists believe we need to dedicate just as much attention—if not more—to the possibility that we're also due for massive, disastrous megafloods that could deluge both Northern and Southern California.The last time it happened was in 1861, and the Central Valley in particular was deluged by an intense rainstorm that lasted for 43 days, killed thousands and caused horrible flooding that temporarily shut down Sacramento. A new article by Scientific American surveys geological studies suggesting that the 19th century megaflood wasn't a freak event but a regular occurrence that hits California every other century or so:

[...]the unsettling bottom line is that megafloods as large or larger than the 1861-62 flood are a normal occurrence every two centuries or so. It has now been 150 years since that calamity, so it appears that California may be due for another episode soon.

That's not even the worst of it, and if you've been paying attention, you can guess what's coming next: climate change could make these sorts of storms more frequent.The Scientific American article explains the data that alarms scientists. Most of the huge storms that batter California and prompt flooding are caused by "atmospheric rivers," which are long streams of water vapor a mile in the sky. They can carry water from the tropics in epic vapor streams that stretch across the ocean and condense into water when they hit the Sierra Nevadas. (You might know them as a "Pineapple Express"). Most climate change models show that the amount of rain and show delivered by these "atmospheric rivers" could increase 10 percent by the end of the century.

So what would a megaflood look like? The U.S. Geological Survey, the same agency behind ShakeOut, ran a scenario that is about half as bad as the 1861 flood. Instead of envisioning a rainstorm that lasted 43 days, it considered the possibility of a rainstorm lasting 23 days.

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The scenario nicknamed ARkStorm was still pretty grim—and three times as costly as the Big One:


When project leaders ran the events of ARkStorm through a variety of weather, runoff, engineering and economic models, the results suggested that sustained flooding could occur over most lowland areas of northern and southern California. Such flooding could lead to the evacuation of 1.5 million people. Damages and disruptions from high water, hundreds of landslides and hurricane-force winds in certain spots could cause $400 billion in property damages and agricultural losses. Long-term business and employment interruptions could bring the eventual total costs to more than $700 billion. Based on disasters elsewhere in recent years, we believe a calamity this extensive could kill thousands of people (the ARkStorm simulation did not predict deaths). The costs are about three times those estimated by many of the same USGS project members who had worked on another disaster scenario known as ShakeOut: a hypothetical magnitude 7.8 earthquake in southern California.


The one possible upside Californians of the 21st century have over 19th century Californians is that scientists are able to do a better job predicting these sorts of storms days in advance. But the article suggests scientists study the issue and that city planners and state leaders take the possibility of these megafloods as seriously as we do earthquakes.