by John Bwarie

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Imagine California being bombarded for 45 days with one strong winter storm after another. That's what happened in 1861-62, causing severe flooding up and down the state. Geologic evidence suggests that earlier, prehistoric floods were even bigger. Although no living person remembers such a catastrophe, there's no reason it couldn't happen again.

For emergency-planning purposes, scientists unveiled in mid-January a hypothetical California scenario that describes a storm that could produce as much as 10 ft of rain, cause extensive flooding (in many cases overwhelming the state's flood-protection system), and damage nearly one-quarter of all the houses in California.

The "ARkStorm scenario," prepared by the U.S. Geological Survey (USGS) and released at the ARkStorm Summit in Sacramento, California, on January 13-14, combines prehistoric geologic flood history in California with modern flood mapping and climate-change projections to produce a hypothetical, but plausible, scenario aimed at preparing the emergency-response community for this type of hazard.

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The USGS, the Federal Emergency Management Agency (FEMA), and the California Emergency Management Agency (Cal EMA) convened the 2-day summit to engage stakeholders from across California to take action as a result of the scenario's findings, which were developed over the past 2 years by more than 100 scientists and experts.

"The ARkStorm scenario is a complete picture of what that storm would do to the social and economic systems of California," said Lucy Jones, chief scientist of the USGS Multi-Hazards Demonstration Project and architect of ARkStorm. "We think this event happens once every 100 or 200 years or so, which puts it in the same category as our big San Andreas earthquakes. The ARkStorm is essentially two historic storms (January 1969 and February 1986) put back to back in a scientifically plausible way. The model is not an extremely extreme event."

Jones noted that the largest damages would come from flooding—the models estimate that almost one-fourth of the houses in California would experience some flood damage from this storm.

"The time to begin taking action is now, before a devastating natural-hazard event occurs," said USGS Director Marcia McNutt. "This scenario demonstrates firsthand how science can be the foundation to help build safer communities. The ARkStorm scenario is a scientifically vetted tool that emergency responders, elected officials, and the general public can use to plan for a major catastrophic event to help prevent a hazard from becoming a disaster."

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To define impacts of the ARkStorm, the USGS, in partnership with the California Geological Survey, produced the first statewide landslide-susceptibility maps for California—the most detailed landslide-susceptibility maps ever created. The project also resulted in the first physics-based modeling system for analyzing severe-storm impacts (beach erosion, coastal flooding, and cliff failures) under present-day scenarios and under various climate-change and sea-level-rise scenarios. This modeling system, designed by USGS coastal geologist Patrick Barnard and collaborators at the Netherlands-based research institute Deltares, is also capable of incorporating real-time atmospheric data inputs for potential use in real-time warning systems along the U.S. west coast.

Because ARkStorm research raised serious questions about existing national, state, and local disaster policy and emergency-management systems, the ARkStorm scenario, while still in preparation, became the theme of the 2010 California Extreme Precipitation Symposium, held June 23 at the University of California, Davis, and attended by more than 200 leaders in meteorology and flood management. ARkStorm is part of efforts to create a National Real-Time Flood Mapping initiative to improve flood management nationwide. ARkStorm also provided a platform for emergency managers, meteorologists, and hydrologists to work together to develop a scaling system for west coast storms.

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"Cal EMA is proud to partner with the USGS in this important work to protect California from disasters," said Cal EMA Acting Secretary Mike Dayton. "In order to have the most efficient and effective plans and response capabilities, we have to have the proper science to base them on. Californians are better protected because of the scientific efforts of the United States Geological Survey."

According to FEMA Region IX Director Nancy Ward, "The ARkStorm report will prove to be another invaluable tool in engaging the whole of our community in addressing flood emergencies in California. It is entirely possible that flood-control infrastructure and mitigation efforts could be overwhelmed by the USGS ARkStorm scenario, and the report suggests ways forward to limit the damage that is sure to result."

The 2-day January summit included professional flood managers, emergency managers, first responders, business continuity managers, forecasters, hydrologists, and decision makers. Many of the scientists responsible for coordinating the ARkStorm scenario presented the science behind the scenario, including meteorology, forecasting, flood modeling, landslides, and physical and economic impacts.

The ARkStorm scenario is the second scenario from the USGS Multi-Hazards Demonstration Project led by Jones, which earlier created the ShakeOut earthquake scenario. Listen to a podcast interview with Jones about the ARkStorm scenario.

Read USGS Open-File Report 2010-1312, Overview of the ARkStorm Scenario.