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Open-File Report O-20-07, Local tsunami evacuation analysis of Nehalem Bay, Tillamook County, Oregon,
by Laura L. S. Gabel, Fletcher E. O’Brien, and Jonathan C. Allan; 53 p. report, one Esri® geodatabase with internal metadata, external metadata in .xml format.

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WHAT'S IN THIS REPORT?

This report shows modeled pedestrian evacuation routes to escape a local tsunami generated by an earthquake on the Cascadia Subduction Zone (CSZ) for the communities of Nehalem Bay, Tillamook County.

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ABSTRACT
Pedestrian evacuation routes were evaluated for a local tsunami generated by an earthquake on the Cascadia Subduction Zone (CSZ) in the Nehalem Bay region of Tillamook County, Oregon. Our analyses focused on a maximum-considered CSZ tsunami event covering 100 percent of potential variability, termed XXL and generated by a locally-generated magnitude 9.1 earthquake. Evacuation paths were limited to established roads, trails, and pedestrian pathways designated by local government reviewers as the most likely routes.

To assist in pedestrian tsunami evacuation, we produced maps and digital data that include the
following:

e BTW maps depict the minimum evacuation speed required to stay ahead of the tsunami wave in a given scenario. For planning purposes, we present a variety of scenarios that increase and decrease evacuation difficulty (due to additional complications and mitigation options, respectively). The base scenario uses the existing road and path network and includes a 10-minute delay from start of earthquake before beginning evacuation. Additional challenges to evacuation are discussed, including failure of nonretrofitted bridges and effects from liquefaction. In all cases, the identified minimum speeds must be maintained for the entire time it takes to evacuate from the inundation zone.

Given the model limitations defined in the Methods section, results show that evacuation for most of the Nehalem Bay communities examined is achievable at a moderate walking speed (4 fps, or 2.7 mph). Exceptions to this arise at Nehalem Bay State Park (especially on Nehalem Spit), the Nehalem Bay boat launch, and Tohl Ranch Road. Longer distances to high ground and reliance on non-retrofitted bridges make it difficult for evacuees from these locations to reach safety prior to the arrival of the tsunami. Liquefaction could present a significant challenge to evacuation across the region.

In this report, tsunami mitigation means actions used to improve the survivability of a local community population. The results presented in this study are about evaluating ways to help move people out of the tsunami zone in the shortest amount of time possible between the start of earthquake shaking and the arrival of the tsunami. Mitigation options may include adding new evacuation routes, constructing earthquake-hardened roads and trails (that is, built or remodeled to withstand shaking from a major earthquake and liquefaction), enhancing tsunami wayfinding signage along core routes, and/or installing a tsunami refuge, otherwise known as a vertical evacuation structure.


GEOGRAPHIC INFORMATION SYSTEM (GIS) DATA

Geodatabase is Esri® version 10.6 format.
Metadata is embedded in the geodatabase and is also provided as separate .xml and .html formatted files.

Nehalem_Bay_Tsunami_Evacuation_Modeling.gdb (GIS data bundle zip file)
   rasters
MaxTsunamiFlowDepth_XXL1 The maximum tsunami flow depth raster shows the maximum tsunami flow depths estimated for a maximum-considered XXL1 tsunami caused by a magnitude 9.1 CSZ earthquake. Data are from DOGAMI Open-File Report O-13-17. Flow depths were not directly incorporated into the evacuation difficulty analysis but are included in this report to assist in the evaluation of mitigation options.

.xml

.html

TsunamiWaveArrival_XXL1 The tsunami wave arrival time raster shows estimated wave arrival times for a maximum-considered XXL1 tsunami caused by a magnitude 9.1 CSZ earthquake. The tsunami starts toward shore at the moment the earthquake starts, but the shaking from the earthquake can last three to five minutes. Wave arrivals are shown as the time when wave depth reaches 6 inches or greater at any location, and arrivals are timed from the start of the earthquake.

.xml

.html

 
   feature dataset - XXL1_BridgesOut
Analyses focused on a maximum-considered Cascadia Subduction Zone tsunami event covering 100% of potential variability, termed XXL1 and generated by a magnitude 9.1 earthquake. Assume all non-retrofitted bridges within the inundation zone fail due to earthquake shaking.
XXL1_EvacuationFlowZones Polygons. Evacuation flow zones depict the nearest safety destination for every point in the inundation zone (on the road and trail network) assuming all non-retrofitted bridges within the inundation zone fail due to earthquake shaking.

.xml

.html

XXL1_EvacuationRoutes Polylines. Evacuation routes depict the most efficient routes to safety for every point in the inundation zone (on the road and trail network) assuming all non-retrofitted bridges within the inundation zone fail due to earthquake shaking.

.xml

.html

XXL1_WalkingSpeeds_Roads Polygons. Minimum evacuation speeds needed to stay ahead of the wave assuming all non-retrofitted bridges within the inundation zone fail due to earthquake shaking. This feature class contains BTW data on paved roads only.

.xml

.html

XXL1_WalkingSpeeds_Trails Polylines. Minimum evacuation speeds needed to stay ahead of the wave assuming all non-retrofitted bridges within the inundation zone fail due to earthquake shaking. This feature class contains BTW data on trails and beach networks only.

.xml

.html