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Open-File Report O-12-05, Regional Landslide Hazard Maps of the City of Silverton, Marion County, Oregon, by William J. Burns and Katherine A. Mickelson. 21 p. PDF report, 3 PDF plates., scale 1:8,000, 36 x 43 inches, shp files.

Order publication CD-ROM, $15, from Nature of the Northwest.

Deliverables of this study include the following:

Executive Summary

On December 30, 2008, the Oregon Department of Geology and Mineral Industries (DOGAMI) entered an intergovernmental agreement with the City of Silverton, Oregon (IA no. 41460-11242008) to perform regional landslide hazard evaluation of the City of Silverton.

DOGAMI Special Paper 42, Protocol for Inventory Mapping of Landslide Deposits from Light Detection and Ranging (Lidar) Imagery (Burns and Madin, 2009) was used to create a landslide inventory of the City of Silverton area; 110 landslide deposits were located during this project. Of these, 25 are within or directly adjacent to the city. Of these 25, nine were classified as shallow, seven as deep, and two as debris flow deposits. The other seven (of the 25) are areas of rock fall and/or debris slide deposits.

The Protocol for Shallow-Landslide Susceptibility Mapping (Burns and others, 2012) was used to create a shallow-landslide susceptibility map of the City of Silverton area. Approximately 5% of the City of Silverton is classified as highly susceptible to shallow landslides, 19% as moderately susceptible to shallow landslides, and 76% as less susceptible to shallow landslides.

We followed the deep-landslide susceptibility mapping method outlined by Burns (DOGAMI Open-File Report O-08-09 [2008]) to create a deep-landslide susceptibility map of the City of Silverton. Approximately 0.2% of the City of Silverton is classified as highly susceptible, 1% as moderately susceptible, and 99% as less susceptible to deep landslides.

We developed the landslide inventory and landslide susceptibility maps with the best available data and documented methods, but several limitations underscore that these maps are designed for regional applications and should not be used as an alternative to site-specific studies in critical areas. These limitations are described in detail on Plates 1–3.

These maps are intended to provide users with basic information regarding landslides and the susceptibility to landslides within the mapped area. The data are particularly suitable for incorporation and into regional GIS databases for a multitude of purposes. These include but are not limited to city and county hillside development ordinances, issuance of building permit conditions, public works planning and operations, and environmental and sustainability issues. We reiterate that these data are not appropriate for site-specific evaluations.

The City of Silverton has a relatively low to moderate landslide hazard, when compared to other communities in Oregon.

Plate 1, Landslide inventory map:

City of Silverton landslide inventory map

 

Plate 2, Shallow-landslide susceptibility map:

City of Silverton shallow-landslide susceptibility map

 

Plate 3, Deep-landslide susceptibility map:

City of Silverton deep-landslide susceptibility map