Theme for 2010:
ARkStorm: Examining a Potential California Flood Disaster

ARkStorm: A West Coast Storm Scenario

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Experts at federal, state, and regional levels have been assembled to design the large, but scientifically plausible, hypothetical storm scenario that would provide emergency responders, resource managers, and the public a realistic assessment of what is historically possible.

The ARkStorm is patterned after the 1861-1862 historical events but uses modern modeling methods and data from large storms in 1969 and 1986. The ARkStorm draws heat and moisture from the tropical Pacific, forming Atmospheric Rivers (ARs) that grow in size, gain speed, and, with a ferocity equal to hurricanes, slam into the U.S. West Coast for several weeks. Using sophisticated weather models and expert analysis, precipitation, snowlines, wind, and pressure data the modelers will characterize the resulting floods, landslides, and coastal erosion and inundation. These hazards will then be translated into the infrastructural, environmental, agricultural, social, and economic impacts. Consideration will be given to catastrophic disruptions to water supplies resulting from impacts on groundwater pumping, seawater intrusion, water supply degradation, and land subsidence. Possible climate-change forces that could exacerbate the problems will also be evaluated.

Flooding Emergency Response Exercise

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Formulating the ARkStorm Meteorology

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Recent storm episodes were "stitched" together to describe a rapid sequence of several major storms over the state, yielding precipitation totals and runoff rates beyond any that occurred during the individual (unstitched) historical events. This stitching approach is a new strategy that allowed the scenario-design team to avoid arbitrary scalings to achieve much greater-than-historical storm and flood totals, by instead allowing for the very real occasions when storms stall over parts of the state and when extreme storms have followed each other into the state over short periods of time. The scenario — called the ARkStorm — is quantified by a dynamical (regional weather-model) downscaling of historical observations of extreme winter storms of January 1969 and February 1986 to 6-km and 2-km grids over California. The weather model outputs were used to force a hydrologic model to estimate runoff, for comparison with historical runoff. The methods used to build this scenario, and key results, could also be applied to other, nonemergency or non-California applications.

Understanding and Forecasting Atmospheric Rivers

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Research, done by NOAA scientist in Boulder, CO, have shown that these strong and sustained land-falling AR's are associated with many if not a majority of floods in California. This presentation will review AR's and discuss how current NWS forecast offices can recognize and attempt to forecast these phenomena well in advance of landfall. The Columbus Day 2009 event will be used as an example. In addition, the idea of developing an AR threat index or intensity category will be presented. Finally the paper discusses how this knowledge of AR's could be used to prepare state and local officials for a hypothetical, but very believable catastrophic flood event being proposed as part of a statewide emergency response drill in 2011 called ARkStorm.

The Psychology of Uncertainty: Challenges to Communicating Risk

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Potential Flooding from ARkStorm

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As a part of the Map Modernization Program, FEMA created a seamless, digital National Flood Hazard Layer. Building on the success of Map Modernization, FEMA is transitioning to a new program entitled RiskMAP. RiskMAP's vision is to collaborate with State, local, and tribal entities to deliver quality flood hazard data that increases public awareness and leads to mitigation actions that reduce risks to life and property. An overview of how the seamless National Flood Hazard Layer was used to support the goals of both RiskMAP and ARkStorm, in this innovative pilot project, will be presented.

Panel Discussion: Flood Modeling Uncertainties

• Transcription of panel discussion (PDF*, 76 KB)
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Potential Landslide Impacts from ARkStorm

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Estimating the numbers and impact of landslides from the ARkStorm requires models relating rainfall to the numbers and distribution of landslides and detailed records of the impact of landslides in past storms. Neither of these basic data sets exists. Models that estimate the areas with the greatest susceptibility to landsliding do exist, however, and can be combined with the detailed maps of small areas showing the numbers of landslides triggered in individual storms. We have extrapolated the distribution of landslides mapped in past storms to areas where no such maps exist using the landslide susceptibility maps. Similarly, we have extrapolated the incomplete records of damage from landslides in past storms to the ARkStorm using the susceptibility maps. We anticipate that the ARkStorm would trigger thousands of landslides causing hundreds of millions of dollars in direct repair costs.

Infrastructure and Physical Damages Estimated for ARkStorm

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Between the three approaches, it is deemed realistic that an event like the ARkStorm would result in property damage costing $310 to $330 billion to repair, of which approximately $200 billion is from building damage from flooding, $100 billion from content damage from flooding, $5 billion from wind damage to buildings, and the balance from infrastructure damage. Demand surge could potentially increase this amount by 20%. Some highways in steep terrain could be heavily damaged by multiple landslides, in some cases taking months or more to repair. Electric power would be unavailable for up to 2 weeks in most places, but in some places with very high winds such as the southern Sierras, commercial electric power could take months to restore. Sewer systems in heavily flooded areas are subject to damage, with some counties having half their wastewater treatment plans flooded, damaging the electrical equipment, and requiring a month or more to restore. Water service is severely impacted in some places, especially where well pumps are flooded and contaminated with untreated wastewater, in some places taking several months to restore. Lifeline service providers are being given a second opportunity to review and offer revisions to these findings.

Economic Consequences of the ARkStorm Scenario

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The ARkStorm damages are transformed into sources of business interruption including evacuation, loss of building use and output, lifeline service (power, water, waste water) outages, reduced highway capacities, agricultural losses, and environmental damages. These business interruptions are described spatially and temporally to estimate effects on various economic sectors. The interdependencies among the sectors in the California economy are captured in a computable general equilibrium model to produce estimates of direct and indirect (ripple effects) economic impacts to sector activity over a two-year period.

Dr. Snell's Precipitation Measurements of December 1861-January 1862 are Valid

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  (PDF*, 624 KB)
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