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September 12, 2016

Tsunami evacuation planning in coastal communities is typically based on maximum evacuation zones that reflect a combination of all potential extreme tsunamis. However, in the case of a smaller tsunami, this approach may result in more people being evacuated than need to be, and in doing so, may overly disrupt the local economy, and strain resources needed during emergency response.

Evacuations are intended to keep a population safe and reduce losses, but what are the costs of lost work and wages, or accidental injuries that may occur during an evacuation?

To reduce the potential for future over-evacuations, multiple evacuation zones are being mapped and included in emergency planning documents called “tsunami evacuation playbooks.” The California Office of Emergency Services and the California Geological Survey are developing these playbooks for California coastal communities. The various zones are designed to provide evacuation options that better match the size and arrival time of an impending tsunami. U.S. Geological Survey scientists and these state partners used the low-lying coastal city of Alameda, California, in San Francisco Bay, as a case study to explore community implications and benefits associated with four different tsunami-evacuation zones. The city of Alameda has a population of more than 79,000 people spread across Alameda Island, Coast Guard Island, and Bay Farm Island, and approximately half of them live or work in areas susceptible to tsunamis generated by both local and distant earthquakes.

“Community leaders and emergency managers struggle to maintain the balance between keeping as many people safe from a tsunami as possible without evacuating more people than necessary,” said Jeff Peters, USGS geographer and lead author of a new study to estimate the potential community benefits of planning for multiple evacuation zones.

Evacuation modeling summarized in the new study indicates that the tens of thousands of individuals living in tsunami evacuation zones are likely to have sufficient time to evacuate any one of the zones before tsunami waves reach Alameda shores. Therefore, the benefit of using smaller evacuation zones for certain tsunamis instead of the maximum zone is intended to help reduce community disruptions in Alameda, without compromising the safety of its residents and visitors.

The new study estimates the number of residents and employees who would not need to evacuate zones smaller than the current maximum zone. This also demonstrates benefits in reducing potential impacts to the quality of life by not requiring evacuations at certain banks, government offices, libraries, and markets during smaller tsunami events. The multi-zone approach may also reduce the need to evacuate people who may need assistance, such as those located at K-12 schools, child day-care centers, medical offices, and adult-residential-care centers.

Finally, the study showed that adequate refuge exists at the center of Alameda Island and on Bay Farm Island during evacuations of zones smaller than the maximum zone. For an evacuation to the current maximum zone, people on Bay Farm Island will likely need to evacuate to nearby Oakland which highlights opportunities for cross-jurisdictional coordination and planning.

The full report by Jeff Peters and others, “Intra-community implications of implementing multiple tsunami-evacuation zones in Alameda, California,” was published in the journal Natural Hazards, and is online.

map of Alameda, Calif. with various shades of red indicating tsunami evacuation zones
Map of the city of Alameda, California indicating tsunami evacuation zones. Light pink zones are the first to be evacuated for small events. Dark red areas would be evacuated only for an expected large tsunami. Jeff Peters, U.S. Geological Survey. 

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