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Open-File Report O-20-13, Landslide hazard and risk study of Tillamook County, Oregon
by Nancy C. Calhoun, William J. Burns, and Jon J. Franczyk; Esri geodatabase with internal metadata, external metadata in .xml format; text report.

Download complete publication including GIS data and appendices (81 MB .zip file)

Main report only (44 p. PDF, 7 MB)

Appendices only (450 KB .zip file):
Appendix A. Exposure Analysis Results (Microsoft® Excel® spreadsheet and Adobe® PDF formats)
Appendix B. Hazus-MH Analysis Results (Microsoft® Excel® spreadsheet and Adobe® PDF formats)

WHAT'S IN THIS REPORT?
This report updates a landslide inventory, shallow and deep landslide susceptibility, and landslide risk for a portion of Tillamook County, Oregon. This information can help communities better reduce risk from landslides.

EXECUTIVE SUMMARY
This study provides an updated geospatial inventory of landslide hazard data (including risk to properties and infrastructure) throughout Tillamook County, Oregon. These data are critically important for community planners, emergency managers, and the public at large.

Tillamook County has experienced hundreds of landslides in the past 50 years, many of which have been recorded in the Statewide Landslide Information Database for Oregon (SLIDO). However, no regional-scale landslide hazard study exists. Over 25,000 people reside in the study area portion of Tillamook County, with critical lifeline highway routes, including Highway 6 (Wilson River Highway) and U.S. Highway 101 (Oregon coast Highway) potentially at risk from landslides.

For this study we used existing protocols established by the Oregon Department of Geology and Mineral Industries (DOGAMI) for 1) making a landslide inventory; that is, mapping existing landslide deposits, 2) modeling deep and shallow landslide susceptibility in order to demonstrate where landslides may occur in the future, and 3) assessing landslide risk through exposure analysis and using the FEMA Hazus Multi-Hazard model. These established methods allow for a consistent scientific framework in which to perform the mapping, while also allowing for direct comparison to other areas in Oregon in order to understand the relative risk.

The study area is ~325 square miles (841 square kilometers), spans the length of the county, and includes numerous incorporated and unincorporated communities and transportation corridors. Results from our mapping and risk assessment indicate the following:

Our analyses indicate that the study area experiences moderate to high landslide hazard, which tends to be concentrated in several discrete communities and along certain key road corridors, notably in the northern portion of the study area, Neahkahnie, Nehalem, and Wheeler, and along U.S. Highway 101. The primary landslide hazard in the study area is exposure of existing structures and roadways to deep landslides and debris flow fans. Substantive risk reduction activities for this type of landslide hazard include controlling the input of water onto slopes within the moderate and deep landslide susceptibility zones and on existing deep landslides, and avoiding adding material (weight) to the tops of susceptible slopes or, conversely, removing material from the bottoms of slopes (by excavation or grading).

To assist the local communities, we recommend the following:

STUDY AREA

DOGAMI 
Open-File Report O-20-13 study area

 

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.

O-13-20_TillamookLandslideandRiskStudy.gdb:

Datasets:

Coseismic_Hazard_Data (feature dataset)

Landslide_Inventory (feature dataset)

Tillamook_Deep_Suscept (feature class, polygons)

Tillamook_Shallow_Suscept (raster dataset)

Tillamook_Study_Area (feature class, polygon)