Tsunami-evacuation planning is challenging for California communities due to the many potential tsunamigenic sources. One maximum-inundation zone based on a composite of all estimated tsunami sources is currently used for all tsunami evacuation planning in California. An unintended consequence of using one composite evacuation zone is the potential for over-estimate the need for evacuation during smaller tsunami events. In order to help avoid this problem, the California Geological Survey and the California Office of Emergency Management are developing the Evacuation Playbook of phases or zones that relate to specific modeled tsunami events to further assist emergency managers and planners. We investigate the community-level implications of this evacuation playbook approach for Alameda Island and Bay Farm Island, the two main regions of the island community of Alameda, CA.
In the past, U.S. Geological Survey reports estimated a significant difference between numbers of Alameda residents in the maximum-inundation zone and those exposed if an event-based inundation zone is used. A population exposure analysis was conducted for the four Alameda evacuation phases in order to identify variations in the types of residents and businesses within each phase. A pedestrian-evacuation analysis using an anisotropic, path distance model was also conducted to estimate clearance times for each evacuation phase zone. Results suggest that for phases 1 and 2, Alameda Island could face challenges evacuating many boat docks and a large beach, while Bay Farm Island is unaffected for these phases but might be challenged with evacuating for phase 3 and the maximum phase due to long distances to safety. Initial results also illustrate potential challenges for communicating the need for evacuations due to expected short arrival times for tsunamis generated by local earthquake sources. A better understanding of the population exposure within each tsunami-evacuation phase and the time it would take to evacuate out of each phase by foot will help emergency managers decide which phase to evacuate during an actual tsunami event.
