Open-File Report O-23-06, Multi-Hazard Risk Report for Benton County, Oregon

DOGAMI OPEN-FILE REPORT SERIES

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Open-File Report O-23-06, Multi-Hazard Risk Report for Benton County, Oregon, by Matt C. Williams and Nancy C. Calhoun; 96 p. report, 10 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 Benton County communities. The results quantify the impacts of natural hazards to each community and enhance the decision-making process in planning for disaster.

PUBLICATION DOWNLOADS

Text report, including all appendices (96 p., 47.0 MB PDF)
Appendix E, Map Plates (10 p., 47.2 MB .zip file)
GIS metadata bundle (3 .xml files, 11 KB .zip file)
Full GIS data bundle, with .xml metadata (14.8 MB, .zip file; view .xml metadata links below)
Complete Publication Bundle (106.8 MB .zip file)

EXECUTIVE SUMMARY

This report was prepared for the communities of Benton County, Oregon, with funding provided by the Federal Emergency Management Agency (FEMA). It describes the methods and results of a natural hazard risk assessment performed in 2022 by the Oregon Department of Geology and Mineral Industries (DOGAMI) within the study area. The purpose of this project is to provide communities with detailed risk assessment information to enable them to compare hazards and act to reduce their risk. The risk assessment results quantify the impact of natural hazards to each community and enhance the decision-making process in planning for disaster.

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.

In the first task, we created a comprehensive asset database for Benton County by synthesizing assessor data, U.S. Census information, FEMA 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 (i.e., construction materials, number of floors, usage, etc). Using these data, we were able to represent accurate spatial locations and vulnerabilities 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 produced using peer-reviewed methods and with high-resolution, lidar topographic data. Although not all the data sources used in the report provide complete, countywide information, each hazard dataset used was the best available at the time of the analysis. Data sources and coverage are discussed in detail for each hazard in Assessment Overview and Results.
In the third task, we analyzed risk using Esri® ArcGIS Desktop® software. We took two risk assessment approaches: (1) estimated loss (in dollars) to buildings from floods and earthquakes using the Hazus-MH methodology, and (2) calculated the number of buildings, their value, and associated populations exposed to earthquake, and flood scenarios, or susceptible to varying levels of hazard from landslides, channel migration, and wildfire. Details on recurrence intervals, susceptibility, hazard levels and other particulars are discussed in detail for each hazard in Assessment Overview and Results.

The findings and conclusions of this report show the wide range of potential impacts hazards could have on the communities of Benton County. A Cascadia Subduction Zone (CSZ) earthquake (Mw-9.0) will cause extensive damage and losses throughout the county, with most of the critical facilities at high risk. The Turner and Mill Creek Fault Mw-6.6 earthquake showed localized high damages for areas in the northeastern portion of Benton County. We demonstrate the potential for reduction in earthquake damages and losses through seismic retrofits using the building code simulations in the Hazus-MH earthquake model. We also find that the highest potential for population displacement is associated with earthquake, flood, and landslide hazards. Flooding is identified as a threat for some communities in the county (Alsea, Corvallis, Philomath, and Albany) and we quantify the number of elevated structures that are less vulnerable to flood hazard. Our analysis shows that areas with moderate to steep slopes or at the base of steep hillsides are at the greatest risk from landslide hazards, which are present throughout the communities and rural county. Over 400 buildings along Marys River and North Fork Alsea River were exposed to channel migration hazard. Wildfire exposure analysis shows a higher risk for buildings within the wildland-urban interface (WUI) in the western and northern parts of the county.

The information presented in this report is designed to increase awareness of natural hazard risk, to support public outreach efforts, and to aid local decision-makers in developing comprehensive plans and natural hazard mitigation plans. This study can help emergency managers identify vulnerable critical facilities and develop contingencies in their response plans. The results of this study are designed to be used to help communities identify and prioritize mitigation actions that will improve community resilience.

Results were broken out for the following geographic areas:

Unincorporated Benton County (rural)	City of Adair Villate
City of Albany*	City of Corvallis
City of Millersburg	City of Monroe
City of Philomath	Community of Alpine
Community of Alsea	Community of Bellfountain
Community of Blodgett	Community of Kings Valley
Community of Summit

*The portion of the city of Albany within Linn County is included in this report.

Selected Countywide Results Total buildings : 61,091 Total estimated building value: $19 billion
Cascadia Subduction Zone Magnitude 9.0 Earthquake Scenario Red-tagged buildings^a : 2,552 Yellow-tagged buildings^b : 8,936 Loss estimate: $2.9 billion	Turner and Mill Creek Fault Magnitude-6.6 Earthquake Scenario Red-tagged buildings^a : 1,898 Yellow-tagged buildings^b : 5,956 Loss estimate: $2 billion
100-year Flood Scenario Number of buildings damaged: 2,067 Loss estimate: $88 million	Landslide Exposure (High and Very High- Susceptibility Number of buildings exposed: 2,078 Exposed building value: $497 million
Channel Migration Zone* (Erosion Hazard Area - 30-year): Number of buildings exposed: 402 Exposed building value: $96 million	Wildfire Exposure (High and Moderate Risk): Number of buildings exposed: 1,777 Exposed building value: $481 million
^aRed-tagged buildings are considered uninhabitable due to complete damage ^bYellow-tagged buildings are considered limited habitability due to extensive damage *Results are limited to the study are of Appleby and others (2021), which covers the North Fork Alsea River and Marys River.

GEOGRAPHIC INFORMATION SYSTEM (GIS) DATA

Geodatabase is ESRI® version 10.7 format. Metadata are embedded in the geodatabase and are also provided as separate .xml format 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.