Open-File Report O-24-06, Multi-Hazard Risk Report for Douglas County, Oregon

DOGAMI OPEN-FILE REPORT SERIES

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Open-File Report O-24-06, Multi-Hazard Risk Report for Douglas County, Oregon, by Matt C. Williams and Jason D. McClaughry; 114 p. report, 9 tabloid size map plates, two Esri® geodatabases 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 Douglas 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 (114 p., 32.2 MB PDF)
Appendix E, Map Plates (9 p., 38.8 MB .zip file)
GIS metadata bundle (7 .xml files, 24 KB .zip file)
Full GIS data bundle, with .xml metadata (2.4 GB, .zip file; view .xml metadata links below)
Complete Publication Bundle (2.5 GB .zip file)

EXECUTIVE SUMMARY

This report was prepared for the communities of Douglas 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 2023 and 2024 by the Oregon Department of Geology and Mineral Industries (DOGAMI) within Douglas County (herein referred to as the study area). The purpose of this project is to provide communities with detailed risk assessment information to enable them to understand and compare hazards and act to reduce their risk. The risk assessment results quantify the consequences of natural hazards to each community and support the decision-making process in planning for disaster.

We arrived at our results 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 Douglas County by synthesizing assessor data, Federal Emergency Management Agency (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 (e.g., 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 Douglas County. A Cascadia Subduction Zone (CSZ) earthquake (Mw-9.0) and subsequent tsunami will cause extensive damage and losses for coastal portions of the county and moderate damage for areas farther from the coast. Moderate damage and losses would occur for some areas in Douglas County near the epicenter of a local crustal earthquake. 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. Flooding is identified as a threat for communities along the South Umpqua, North Umpqua, and Umpqua rivers and we quantify the number of elevated structures that are less vulnerable to flood hazard. The southern and eastern parts of the county are at significant risk from landslide hazard. There is extensive risk throughout most of the county from wildfire hazard. We also find that the highest potential for population displacement is associated with wildfire (40%) and landslide (11%) hazards.

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 Douglas County (rural)
City of Canyonville
City of Drain
City of Elkton
City of Glendale
City of Myrtle Creek
City of Oakland
City of Reedsport
City of Riddle
City of Roseburg
City of Sutherlin
City of Winston
City of Yoncalla
Cow Creek Band of Umpqua Tribe of Indians*
Community of Glide
Community of Green
Community of Tri-City
Community of Winchester
Community of Winchester Bay

*This federally recognized Indian Tribal Government is referred to as the Cow Creek Umpqua Tribe within this report.

Selected Countywide Results Total buildings : 73,438 Total estimated building value: $23.6 billion
Cascadia Subduction Zone Magnitude-9.0 Earthquake Scenario^a Red-tagged buildings^b: 2,583 Yellow-tagged buildings^c : 7,886 Loss estimate: $2 billion	Cascadia Subduction Zone Magnitude 9.0 Tsunami Inundation Number of buildings exposed: 861 Exposed building value: $204 million
Hypothetical Crustal Fault Magnitude 6.8 Scenario Red-tagged buildings^b : 2,307 Yellow-tagged buildings^c : 6,468 Loss estimate: $1.77 billion	100-year Flood Scenario Number of buildings damaged: 4,111 Loss estimate: $218 million
Landslide Exposure (High and Very High- Susceptibility) Number of buildings exposed: 9,024 Exposed building value: $2.4 billion	Wildfire Exposure (High and Moderate Risk): Number of buildings exposed: 31,046 Exposed building value: $10 billion
^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.