From the USGS... For emergency planning purposes, scientists unveiled a hypothetical California scenario that describes a storm that could produce up to 10 feet of rain, cause extensive flooding (in many cases overwhelming the state’s flood-protection system) and result in more than $300 billion in damage.

The "ARkStorm Scenario," prepared by the U.S. Geological Survey and released at the ARkStorm Summit in Sacramento on Jan. 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.

The USGS, the Federal Emergency Management Agency and the California Emergency Management Agency convened the two-day summit to engage stakeholders from across California to take action as a result of the scenario's findings, which were developed over the last two 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."

To define impacts of the ARkStorm, the USGS, in partnership with the California Geological Survey, created the first statewide landslide susceptibility maps for California that are the most detailed landslide susceptibility maps ever created. The project also resulted in the first physics-based coastal storm modeling system for analyzing severe storm impacts (predicting wave height and coastal erosion) under present-day scenarios and under various climate-change and sea-level-rise scenarios.

Because the scenario raised serious questions about existing national, state and local disaster policy and emergency management systems, ARkStorm became the theme of the 2010 Extreme Precipitation Symposium at U.C. Davis John Muir Institute of the Environment, attracting over 200 leaders in meteorology and flood management. ARkStorm is part of the 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.

"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 it 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 two-day summit included professional flood managers, emergency mangers, 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. More information about the ARkStorm Summit is online. The ARkStorm Scenario, USGS Open-File Report 2010-1312, is also online.