Multi-Hazard Mitigation Plan: 09. Tsunamis

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Chapter 9:Hazard Analysis—Tsunamis 9.3.2.2 Evacuation Zone Maps Hawaii was the first state to develop tsunami evacuation maps. The tsunami evacuation maps were created in 1989-91 using one-dimensional numerical models.47 This approach used historical inundation records to infer the nearshore tsunami heights, which in turn provide the inundation limits through repeated calculations at transects perpendicular to the adjacent coastlines. However, the original inundation maps for Hawaii data on which these were based are no longer available. Technical advances enable a more rigorous approach using a two-dimensional model. The results of the 2-D model indicate that the existing evacuation lines may need to be moved further inland in certain areas. Additionally, the 2-D model could better define inland waterway embankment inundation which was not addressed by the 1-D model. However, because of the extensive cost to remap the entire evacuation zone, remapping is prioritized to recent coastal developments and densely populated areas. A new generation of tsunami evacuation zone maps is being prepared in 2009-2010, based on 2-D inundation modeling by the University of Hawaii, School of Ocean and Earth Science and Technology(UH SOSET). The analysis uses 2-dimensional modeling with updated bathymetry and topographical data utilizing recent LIDAR topographical surveys, which provide a higher resolution of the shallow reef forms below sea level and shoreline above sea-level than previously available. The National Tsunami Hazard Mitigation Program requires 2D numerical modeling and credible worse-case scenarios when mapping tsunamis affecting Alaska, California, Washington, Oregon, and Hawaii. The tsunami inundation and runup analysis performed by UH SOSET uses scenarios from tsunami-genic source regions along the Pacific Rim, see Figure 9-9. The methodology to meet this standard essentially uses the same 5 historical events (4 from Aleutian and one from Chile) used by the original tsunami map, but uses 2D methodology; also added hypothetical events assumed to be 2X the Great Sumatra Tsunami (beyond credible magnitudes), from other directions (west, e.g., Japan, and south Pacific, e.g., Marianas, and East Pacific, e.g., Cascadia), to test the model results. The hypothetical events do not appear to generate greater inundation than the historical suite of events due to directionality effects in the wave propagation. The 2-D modeling has been validated as producing good results when hindcasting historical inundation and runup data. The analysis has provided updated inundation zones around the island at locations indicated in Figure 9-10. Evacuation maps are soon to be updated in 2010 based on the results of this analysis. The inundation and run-up analysis is based on scenarios not on statistical probabilistic methods. An improved analysis would develop "Maximum credible inundation" maps based on probabilistic estimation. At some point, building code maps will need to incorporate tsunami inundation. Building designers need the tsunami inundation data to properly site and design structures against tsunami forces. 9.4 Risk Assessment Similar to the coastal flooding hazard an annualized loss can be estimated based on the exposure and the probabilistic tsunami inundation. 47 Curtis,George,D.,Hawaii Inundation/Evacuation Map Project,Final Report,Joint Institute for Marine and Atmospheric Research,University of Hawaii,April 1991. 9-11 Hawaii County Multi-Hazard Mitigation Plan