Changing Vulnerability of Coastal Communities to Tsunamis

Tsunamis have the potential to cause considerable damage to communities along the U.S. Pacific Northwest coastline. There have been an estimated 565,000 tsunami-related deaths throughout recorded history, and tsunami hazards continue to threaten lives and livelihoods in coastal communities throughout the world. Studies to better understand potential losses from future tsunamis and the range of possible risk-reducing strategies have included the distribution of developed land within hazard zones, population exposure, building integrity, demographic sensitivity, evacuation potential, economic loss estimation, vertical-evacuation siting, and institutional resilience. Each of these approaches provides practitioners and policymakers with information for developing risk-reduction strategies tailored to local conditions and needs, instead of more generic, regional strategies.

As coastal communities expand over time, the potential societal impact of tsunami inundation changes. Vulnerability assessments and related risk-reduction efforts are typically based on current landscape conditions, demographic distributions, and development patterns in hazard zones. Extreme events that create significant tsunamis, however, may not occur for many years into the future, while coastal at-risk communities continue to grow. Vulnerability assessments based on current conditions therefore are only static snapshots of existing conditions and fail to capture likely changes in hazard exposure over time. Although these efforts have value for outreach, preparedness, and evacuation planning to help protect lives today, they may be insufficient for long-term mitigation projects (e.g. capital improvement plans, vertical-evacuation refuges) that should anticipate future exposure and related societal risk-reduction needs based on projected development trends.

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To understand how community exposure to tsunami hazards may change in coming decades, we projected future development (i.e. urban, residential, and rural), households, and residents over a 50-year period (2011-2061) along the Washington, Oregon, and northern California coasts. These projections relied upon the U.S. Geological Survey’s LUCAS model, which was used to projects future developed land use based on historical development trends. We then compared our development projection results to tsunami-hazard zones associated with a Cascadia subduction zone (CSZ) earthquake. Changes in tsunami-hazard exposure through 2061 were estimated for 50 incorporated cities, 7 tribal reservations, and 17 counties relative to current (2011) estimates.

The study found that, across the region, the 2016 population exposure in tsunami-hazard zones was projected to increase by 3,880 households and 6,940 residents. The top ten communities with highest population exposure to CSZ-related tsunamis in 2011 are projected to remain the areas with the highest population exposure by 2061. The largest net population increases in tsunami-hazard zones were projected in the unincorporated portions of several counties, including Skagit, Coos, and Humboldt.

This study of projected changes in community exposure to tsunami hazards focuses on the use of spatially explicit LULC state-and-transition modeling to project changes in developed land, households, and residents. Based on our analysis, we reach several conclusions that bear on future land change modeling and community-exposure studies related to sudden-onset hazards, such as tsunamis.

• Land-change simulation modeling serves as an exploratory tool aimed at helping local governments characterize and visualize the hazard-exposure implications of business-as-usual development and to recognize future growth in current hazard-mitigation planning.
• Projected new development in tsunami-hazard zones is not equally distributed along the study area, where 15% of jurisdictions are projected to have no additional development, 46% are projected to have less than 10 ha, and 39% are projected to have more than 10 ha.
• Tsunami-hazard zones associated with a Cascadia subduction zone earthquake in the Pacific Northwest are projected to increase by 3,880 households and 6,940 residents by 2061, over 2011 estimates. Oregon had the largest increase in exposure (51% of new households and 45% of new residents), followed by Washington (41% and 40%, respectively) and California (8% and 15%, respectively).
• The top ten communities in 2011 in terms of population exposure to CSZ-related, tsunamis are projected to remain the same areas with the highest population exposure by 2061.
• The largest net increases in residential exposure to tsunami-hazard zones are projected in the unincorporated portions of several counties, including Skagit, Coos, and Humboldt. These increases may represent greater life-safety issues in Humboldt and Coos counties than in Skagit due to substantial differences in tsunami wave-arrival times across the region.
• Land-simulation projections provide insight on implications of future land-use decisions but increases may be tempered by changes in risk tolerance and the ability to convert certain land cover classes (e.g., wetlands) to development.

This paper, “Projecting community changes in hazard exposure to support long-term risk reduction: a case study of tsunami hazards in the U.S. Pacific Northwest” was published in International Journal of Disaster Risk Reduction.

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