GIS Data Catalog

King County retained Shannon & Wilson to map deep-seated landslides within the study limits using the procedure in the Oregon State Department of Geology and Mineral Industries (OR DOGAMI) Special Paper 42 (Burns and Madin 2009).

Shannon and Wilson mapped 930 deep-seated landslides in the study area during this work.

A report prepared by Shannon & Wilson to accompany their mapping is included as Appendix A of the report.

Many features of deep-seated landslides share similar topographic characteristics including steep, often arcuate head scarps defining the upslope limit of the slide and a distinct deposit of displaced debris downslope.

The debris was often identifiable by its lobate form and hummocky upper surface.

These features varied in their topographic distinctness from features that appeared sharp and unambiguous to features that were muted and subtle.

It is likely that some features mapped as slides based on their topographic character may have formed in other ways and it is also likely that some prehistoric deep-seated landslides present in the study limits were not identified because they were not distinguishable with the available imaging.

A numerical confidence rating method set by the DOGAMI method (e.g., headscarp and slide morphology) was used to assign ratings to each mapped landslide.

Twenty attributes, i.e. relative age of landslide, LiDAR source and date, and whether the slide appeared natural or human modified, of each landslide, were documented.