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Open-File Report O-20-16, Natural Hazard Risk Report for Clatsop County, Oregon, Including the Cities of Astoria, Cannon Beach, Gearhart, Seaside, and Warrenton and the Unincorporated Communities of Arch Cape, Svensen-Knappa, and Westport,
by Matt C. Williams, Lowell H. Anthony, and Fletcher E. O'Brien; 93 p. report, 7 tabloid size map plates, one Esri® geodatabase with internal metadata, external metadata in .xml format.

What's in this report?
This report describes the methods and results of a natural hazard risk assessment for Clatsop County communities. The risk assessment can help communities better plan for disaster.

Report downloads:

EXECUTIVE SUMMARY

This report was prepared for the communities of Clatsop County, Oregon, with funding provided by the Federal Emergency Management Agency (FEMA). It describes the methods and results of the natural hazard risk assessment performed in 2018 by the Oregon Department of Geology and Mineral Industries (DOGAMI) within the study area. The purpose of this project was to provide communities with a detailed understanding of their risk from natural hazards, to give communities the ability to compare their risk across multiple hazards, and to prioritize and take actions that will reduce risk. The results of this study can also inform the natural hazard mitigation planning process.

We arrived at our findings and conclusions by completing three main tasks: compiling an asset database, identifying and using the best available hazard data, and performing a natural hazard risk assessment.

To complete the first task, we created a comprehensive asset database for the entire study area by synthesizing assessor data, U.S. Census information, Hazus-MH general building stock information, and building footprint data. This work resulted in a single dataset of building points and their associated building characteristics. Using this dataset, we were able to represent accurate spatial location and vulnerability on a building-by-building basis.

The second task was to identify and use the most current and appropriate hazard datasets for the study area. Most of the hazard datasets used in this report were created by DOGAMI and some were produced using high-resolution lidar topographic data. While not all the data sources used in the report are countywide, each hazard dataset was the best available at the time the analysis was performed.

In the third task, we performed risk assessments using Esri® ArcGIS Desktop® software. We used two risk assessment approaches: (1) estimated loss (in dollars) to buildings from flood (recurrence intervals) and earthquake scenarios using FEMA Hazus®-MH methodology, and (2) calculated number of buildings, their value, and associated populations exposed to earthquake, tsunami, flood, landslides, coastal erosion, and wildfire hazards.

The findings and conclusions in this report show the potential impacts of hazards in communities within Clatsop County. A Cascadia Subduction Zone (CSZ) event (earthquake and tsunami) will cause extensive damage and losses throughout the county. Our findings indicate that most of the study area’s critical facilities are at high risk during a CSZ event. We also found that the two biggest causes of population displacement are a CSZ event (earthquake and tsunami) and landslide hazard. We demonstrate the potential for the reduction in damages and losses from seismic retrofits through building code simulations in the Hazus-MH earthquake model. Flooding is a threat for some communities in the study area, and we quantify the number of elevated structures that are less vulnerable to flood hazard. Our analysis shows that new landslide mapping based on improved methods and lidar information will increase the accuracy of future risk assessments. The risk from coastal erosion is high for most communities along the open coast. During the time of writing, the best available data show that wildfire risk is high in areas along Youngs Bay and the Columbia River. Lastly, we demonstrate that this risk assessment can be a valuable tool for local decision-makers.

Results were broken out for the following geographic areas:

Selected Study Area Results
Total buildings :25,829
Total estimated building value: $5 billion
Cascadia Subduction Zone (CSZ)
Magnitude 9.0 Earthquake
a
Red-tagged buildingsb: 5,045
Yellow-tagged buildingsc: 1,314
Loss estimate: $1.2 billion
Cascadia Subduction Zone
Tsunami Inundation

Number of buildings exposed: 8,810
Exposed building value: $1.7 billion
100-year Flood Scenario
Number of buildings damaged: 2,529
Loss estimate: $41 million
Landslide Exposure (High and Very High Susceptibility)
Number of buildings exposed: 7,335
Exposed building value: $1.2 billion
Coastal Erosion Exposure (Moderate Hazard)
Number of buildings exposed: 349
Exposed building value: $135 million
Wildfire Exposure (High Hazard)
Number of buildings exposed: 2,467
Exposed building value: $340 million
aResults reflect damages caused by earthquake to buildings outside of the tsunami zone. Earthquake and tsunami results combined estimate the total damages from a CSZ M9.0 event.
bRed-tagged buildings are considered to be uninhabitable due to complete damage.
cYellow-tagged buildings are considered to be of limited habitability due to extensive damage.

 


GEOGRAPHIC INFORMATION SYSTEM (GIS) DATA

Geodatabase is Esri® version 10.2 format.
Metadata is embedded in the geodatabase and is also provided as separate .xml formatted files.

Each dataset listed below has an associated, standalone .xml file containing metadata in the Federal Geographic Data Committee Content Standard for Digital Geospatial Metadata format.

Clatsop_County_Risk_Report_Data.gdb:
Feature dataset: Asset_Data:    
Building_footprints polygons .xml
Communities polygons .xml
UDF_points points .xml

 

APPENDIX E: MAP PLATES (PDFs, 11 x 17 inches each)

Plate 1, Building Distribution Map of Clatsop County, Oregon (5 MB PDF)


Plate 2. Population Density Map of Clatsop County, Oregon (5 MB PDF)


Plate 3. CSZ M9.0 Peak Ground Acceleration Map of Clatsop County, Oregon (3 MB PDF)


Plate 4. Tsunami Inundation Map of Clatsop County, Oregon (5 MB PDF)


Plate 5. Flood Hazard Map of Clatsop County, Oregon (5 MB PDF)


Plate 6. Landslide Susceptibility Map of Clatsop County, Oregon (8 MB PDF)



Plate 7. Wildfire Hazard Map of Clatsop County, Oregon (3 MB PDF)