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Open-File Report O-20-06, Natural hazard risk report for the Lower Columbia–Sandy watershed, Oregon, including the Cities of Gresham, Sandy, and Troutdale and Unincorporated Communities of Government Camp and The Villages at Mt. Hood,
by Lowell H. Anthony, Matt C. Williams, and John M. Bauer; 81 p. report, 9 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 communities in the Lower Columbia-Sandy watershed. The risk assessment can help communities better plan for disaster.

Report downloads:

EXECUTIVE SUMMARY

This report was prepared for the communities of the Lower Columbia–Sandy watershed in 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 2017 by the Oregon Department of Geology and Mineral Industries (DOGAMI) within the study area. The purpose of this study is to provide communities within the study area a detailed risk assessment of the natural hazards that affect them to enable them to compare hazards and act to reduce their risk. The risk assessments contained in this study quantify the impacts of natural hazards to these communities 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 best available hazard data, and performing natural hazard risk assessment.

In 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. With these data 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 were the best available at the time of writing.

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 FEMA Hazus®-MH methodology, and (2) calculated number of buildings, their value, and associated populations exposed to earthquake and flood, or susceptible to varying levels of hazard from landslides, wildfire, channel migration, and volcanic lahars.

The findings and conclusions of this report show the potential impacts of hazards in communities within the Lower Columbia–Sandy watershed. A Cascadia Subduction Zone (CSZ) earthquake will cause low to moderate damage and losses throughout the study area. Higher building losses are expected from a Mount Hood Fault Zone magnitude (M) 6.9 earthquake relative to a CSZ earthquake. We ran Hazus-MH earthquake simulations to illustrate the potential reduction in earthquake damage through seismic retrofits. Flooding is a minor threat for many communities in the study area and we quantify the number of elevated structures that are less vulnerable to flood hazard. Our analysis shows that landslide is a widespread hazard and is present for some communities within the study area. Exposure analysis shows that communities in the Villages at Mt. Hood are particularly vulnerable to channel migration hazard. The best data available at preparation of this report show that wildfire risk is moderate for the overall study area. Exposure analysis shows that buildings in the riverine valleys of the study area are vulnerable to volcanic lahar hazard. Our findings also indicate that some of the study area’s critical facilities are at high risk from earthquake hazard. We also found that the two biggest causes of population displacement are a CSZ earthquake and a volcanic lahar hazard. Lastly, we demonstrate that this risk assessment can be a valuable tool to local decision-makers.

Results were broken out for the following geographic areas:

Selected Study Area Results
Total buildings: 25,659
Total estimated building value: $8.1 billion
Cascadia Subduction Zone (CSZ)
Magnitude 9.0 Earthquake

Red-tagged buildingsa: 1,467
Yellow-tagged buildingsb: 553
Loss estimate: $558 million
Mount Hood Fault Zone (MHFZ) Magnitude 6.9 Earthquake
Red-tagged buildingsa: 1,106
Yellow-tagged buildingsb: 406
Loss Estimate: $462 million
100-year Flood Scenario
Number of buildings damaged: 295
Loss estimate: $6.8 million
Landslide Exposure (High and Very High Susceptibility)
Number of buildings exposed: 1,205
Exposed building value: $295 million
Channel Migration Zone (High Risk)
Number of buildings exposed: 1,632
Exposed building value: $323 million
Wildfire Exposure (High Risk)
Number of buildings exposed: 340
Exposed building value: $116 million
Lahar (Medium Scenario)
Number of buildings exposed: 953
Exposed building value: $144 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

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.

Lower_ColumbiaSandy_Watershed Risk_Report_Data.gdb:
Feature dataset: Asset_Data:    
Building_footprints polygons .xml
Communities polygons .xml
UDF_points points .xml
Raster data: Hazard Data:    
FL_Depth_10yr.xml grid .xml
FL_Depth_50yr.xml grid .xml
FL_Depth_100yr.xml grid .xml
FL_Depth_500yr.xml grid .xml

 

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

Plate 1, Building Distribution Map of Lower Columbia-Sandy Watershed, Oregon (3 MB PDF)


Plate 2. Population Density Map of Lower Columbia-Sandy Watershed, Oregon (3 MB PDF)


Plate 3. M9.0 CSZ Peak Ground Acceleration Map of Lower Columbia-Sandy Watershed, Oregon (3 MB PDF)


Plate 4. M6.9 MHFZ Peak Ground Acceleration Map of Lower Columbia-Sandy Watershed, Oregon (3 MB PDF)


Plate 5. Flood Hazard Map of the Lower Columbia-Sandy Watershed, Oregon (3 MB PDF)


Plate 6. Landslide Susceptibility Map of the Lower Columbia-Sandy Watershed, Oregon (3 MB PDF)



Plate 7. Wildfire Risk Map of the Lower Columbia-Sandy Watershed, Oregon (3 MB PDF)



Plate 8. Channel Migration Hazard Map of Lower Columbia-Sandy Watershed, Oregon (3 MB PDF)



Plate 9. Lahar Hazard Map of Lower Columbia-Sandy Watershed, Oregon (3 MB PDF)