Even so, the authors noted, their assessment likely underestimated the impact on highways due to a number of factors. Furthermore, restoration may take longer than anticipated.

Three important limitations of this scenario were recognized during the initial panel discussions. First, these restoration times may underestimate competition for limited resources. Second, these restoration timelines were laid out without full consideration of the needs for evacuation. Third, the landslide assessment for northern California was not available at the time of the panel discussions. New landslide information might affect the assessment of both damage and restoration. Other considerations that were overlooked during the panel discussions also may affect restoration.

—Overview of the ARkStorm Scenario

ARkStorm Meteorology
The historical storms that were the basis for ARkStorm—and most severe precipitation in California—were probably the result of a phenomenon termed “atmospheric rivers”, jets of warm moist air that originate over the mid-latitude north Pacific Ocean and transport that moisture to California where much of the moisture turns to rain and snow that falls on the state.
These atmospheric rivers, the meteorological conditions that produce them, and the resulting precipitation and winds that affect California, can be simulated by using computer models.
An Atmospheric River originating over the central Pacific on 16 February 2004. Source: USGS. Click to enlarge.

The ARkStorm team designed a large, scientifically realistic meteorological event followed by an examination of the secondary hazards (for example, landslides and flooding), physical damages to the built environment, and social and economic consequences. The hypothetical storm depicted in the ARkstorm scenario would strike the US West Coast and be similar to the intense California winter storms of 1861 and 1862 that left the central valley of California impassible.

The storm is estimated to produce precipitation that in many places exceeds levels only experienced on average once every 500 to 1,000 years.

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.

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.

—Lucy Jones, chief scientist of the USGS Multi-Hazards Demonstration Project and architect of ARkStorm

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.

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.

Key findings of the ARkstorm report are:

1. Megastorms are California’s other “big one.” A severe California winter storm could realistically flood thousands of square miles of urban and agricultural land, result in thousands of landslides, disrupt lifelines throughout the state for days or weeks, and cost on the order of $725 billion. An event like the ARkStorm could require the evacuation of 1,500,000 people. Because the flood depths in some areas could realistically be on the order of 10-20 feet, without effective evacuation there could be substantial loss of life. These impacts, the authors noted, are not exhaustive.

2. An ARkStorm would be a statewide disaster. Extensive flooding is deemed realistic in the California Central Valley, San Francisco Bayshore, Los Angeles and Orange Counties, several coastal communities, and various riverine communities around the state.

3. An ARkStorm could produce an economic catastrophe. Perhaps 25% of buildings in the state could experience some degree of flooding in a single severe storm. That degree of damage would threaten California with a long-term reduction in economic activity, and raise insurance rates statewide—perhaps nationwide or more—afterwards.

4. An ARkStorm is plausible, perhaps inevitable. Such storms have happened in the California historic record (1861-1862), but 1861-1862 is not a freak event, not the last time the state will experience such a severe storm, and not the worst case. An ARkStorm would be unlike any storm that has occurred in living memory: 6 megastorms that were more severe than 1861-1862 have occurred in California during the last 1800 years, and there is no reason to believe similar storms won’t occur again. There may be no pattern that forces the storms to occur with clockwork regularity, so such an event could occur in any year.

5. The ARkStorm is to some extent predictable. Unlike earthquakes, for the ARkStorm there exists a capability to partially predict key aspects of the geophysical phenomena that would create damages in the days before the storm strikes. While these predictive systems already have some important capabilities, there could be great benefit in enhancing their accuracy, lead time, and the particular measures they can estimate. This represents a great challenge scientifically and practically. A game-changing attention to this problem is needed, likely of a scope similar to what is currently done for hurricanes and tornadoes.

6. California flood protection is not designed for an ARkStorm-like event. Much has been done to protect the state from future flooding, but the state flood-protection system is not perfect. The existing systems are designed, among other things, to protect major urban areas from fairly rare, extreme flooding. The level of protection varies: some places are protected from flooding that only occurs on average once every 75 years; others, on average every 200 years. But the levees are not intended to prevent all flooding, such as the 500-year streamflows that are deemed realistic throughout much of the state in ARkStorm.

7. Planning for ARkStorm would complement planning for earthquakes. The ShakeOut exercise has become an annual activity in California, with more than 6 million people participating each year. Many of the same emergency preparations are useful for a severe winter storm: laying in emergency food and water, shelter preparations, exercising emergency corporate communications, testing mutual aid agreements, and so on.

8. Those considering flood mitigation should consider ARkStorm. Governments, businesses, public and private utilities, and individuals have the opportunity now to explore the costs and benefits of physical improvements to their infrastructure to reduce future damage.

9. Hurricane Katrina is a relevant, cautionary experience. Just under 1 year before Katrina, the USACE requested $4 million from Congress for a study on how to protect New Orleans from a category-4 hurricane, which, according to one recent estimate, would have cost on the order of $30 billion. Congress deemed the cost of the study to be too high at the time. The actual storm ultimately cost the federal government in excess of $100 billion, resulted in perhaps $150 billion in total economic loss, and killed 1,800 people. The alarm over the Californian flood-protection systems has already been raised; this study echoes prior ones.

10. There are many ways in which scientific improvements could help to manage risk from severe winter weather. Several research issues are raised by ARkStorm, such as the need for a statewide—or even nationwide—end-to-end stochastic model of severe weather, physical impacts, and socioeconomic consequences. Researchers identified the need for a convenient way to talk about the size of such a California winter storm; better elevation data and historic landslide maps to improve coastal inundation and landslide models; better asset location data in HSIP Gold to improve the understanding of essential facilities exposed to risk; and various reforms to NFIP.

Resources

• Overview of the ARkStorm Scenario