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 Open-File Report O-22-04, Natural Hazard Risk Report for Washington County, Oregon, including the cities of Banks, Beaverton, Cornelius, Durham, Forest Grove, Gaston, Hillsboro, King City, North Plains, Sherwood, Tigard, and Tualatin, by Matt C. Williams and William J. Burns; 98 p. report, 10 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 natural hazard risk assessments for Washington County communities. The risk assessments can help communities better plan for disaster.

Report downloads:

EXECUTIVE SUMMARY

This report was prepared for the communities of Washington County, Oregon, with funding provided by the Federal Emergency Management Agency (FEMA). It describes the methods and results of the natural hazard risk assessments performed in 2021 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 assessments contained in this project quantify the impacts of natural hazards to these communities and enhances the decision-making process in planning for disasters.

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

In the first task, we created a comprehensive asset database for the entire study area 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. With 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 were produced using 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.

In the third task, we performed risk assessments using Esri® ArcGIS Desktop® software. We took two risk assessment approaches: (1) estimated loss (in dollars) to buildings from flood (recurrence intervals) and earthquake scenarios 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, wildfire, and channel migration.

The findings and conclusions of this report show the potential impacts of hazards in communities within Washington County. Although earthquake damage will occur throughout the entire county, extensive damage and losses are more probable in the area near the Gales Creek Fault, such as the city of Forest Grove, and areas with liquefaction-prone soils. Our findings indicate that most of the critical facilities in the study area are at high risk from an earthquake. We used multiple Hazus-MH earthquake simulations to illustrate the potential reduction in earthquake damage through seismic retrofits. Some communities in the study area have moderate risk from flooding and we found a small percentage (<1%) of flood exposed buildings were elevated above the 100-year flood elevation. Our analysis shows that areas with moderate to steep slopes or at the base of steep hillsides are at greatest risk to landslide hazards, such as the west side of the Portland Hills and the southwestern portions of Beaverton and Tigard. Nearly 300 buildings in the unincorporated county were exposed to channel migration hazard located along the streams within the Tualatin River Watershed. Wildfire exposure analysis show a higher risk for buildings within the wildland-urban interface (WUI) portions of the county. We found that population is most at risk of displacement from landslide 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:

* Small portions of the cities of Lake Oswego, Portland, Rivergrove, and Wilsonville that were within Washington County were not individually examined in this report. However, building data within these portions were included within the “Unincorporated Washington County (rural)” jurisdiction.

Selected Countywide Results
Total buildings: 213,901
Total estimated building value: $75 billion

Gales Creek Fault
Magnitude 6.7 Earthquake Scenario  

Red-tagged buildingsa: 1,807
Yellow-tagged buildingsb: 6,049
Loss estimate: $2 billion

100-year Flood
Number of buildings damaged: 1,323
Loss estimate: $60 million

Landslide Exposure (High and Very High-Susceptibility)
Number of buildings exposed: 8,997
Exposed building value: $2.7 billion

Channel Migration Zone (Erosion Hazard Area - 30-year) 
Number of buildings exposed: 886
Exposed building value: $271 million

Wildfire Exposure (High and Moderate Risk)

Number of buildings exposed: 2,297

Exposed building value: $590 million 

aRed-tagged buildings are considered to be uninhabitable due to complete damage.
bYellow-tagged buildings are considered to be of limited habitability due to extensive damage.  

 


GEOGRAPHIC INFORMATION SYSTEM (GIS) DATA

Geodatabases are Esri® version 10.7 format.
Metadata is embedded in the geodatabases 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.

Washington_County_Risk_Report_Data.gdb:

Feature dataset: Asset_Data

 

 

Building_footprints

polygons

.xml

Communities

polygons

.xml

UDF_points

points

.xml


Washington_County_Depth_Grids .gdb:

Raster datasets:  

 

 

FL_Depth_10

grid

.xml

FL_Depth_50

grid

.xml

FL_Depth_100

grid

.xml

FL_Depth_500

grid

.xml


 

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

Plate 1, Building Distribution Map of Washington County, Oregon (4.6 MB PDF)

MapDescription automatically generated

 

 

Plate 2, Population Density Map of Washington County, Oregon (4.3 MB PDF)

 

 

Plate 3, Gales Creek Magnitude 6.7 Earthquake Shaking Map of Washington County, Oregon (2.5 MB PDF)

 

 

Plate 4, Liquification Susceptibility Map of Washington County, Oregon    (3.3 MB PDF)

 

 

Plate 5, Site Amplification Class Map of Washington County, Oregon (2.0 MB PDF)

 

 

Plate 6, Coseismic Landslide Susceptibility (Wet) Map of Washington County, Oregon (3.8 MB PDF)



Plate 7, Flood Hazard Map of Washington County, Oregon  (8.5 MB PDF)

 



Plate 8, Landslide Susceptibility Map of Washington County, Oregon  (5.7 MB PDF)

 



Plate 9, Wildfire Risk Map of Washington County, Oregon  (4.6 MB PDF)

 



Plate 10, Channel Migration Hazard Map of Washington County, Oregon  (3.6 MB PDF)