KNOWN SLIDE AREAS: UPDATING THE CITY OF SEATTLE'S CENTURY-OLD LANDSLIDE DATABASE

Landslides pose a substantial risk to the built environment in the City of Seattle, and the City has been keeping records of landslide activity for over 100 years. To mitigate landslide risk, the Seattle Municipal Code regulates development on properties with landslide-prone areas, which include “Known Slide Areas.” Properties with known slides on the site are required to undergo additional geotechnical review before obtaining development permits.

The City of Seattle has extensive paper records documenting landslides over the past century. These records were used to create a Known Landslide map in 1990 as part of the Interim Critical Areas Ordinance. Much of the information was then evaluated, field checked, and compiled in the Seattle Landslide Study (Shannon & Wilson, 2000) and adopted as an updated Known Landslide Areas layer in GIS in 2001. This GIS layer has also been periodically updated to reflect landslides reported to the City since 2001.

While the Known Landslide Areas layer in GIS is extensively used by City departments, consultants, and the public, there are issues with the current layer. Landslides from different source documentations were mapped using different protocols. Some landslides are mapped as points and some are mapped as extensive polygons encompassing numerous slides. In addition, landslide records digitized from old, low-resolution maps appear in the GIS layer as large circles that do not accurately represent the extent of sliding.

For this project, we are reviewing all landslide files maintained by the Seattle Department of Construction and Inspections and the Seattle Department of Transportation, making evidence-based changes to the Known Slide Areas GIS layer. In places where we have information that indicates mis-mapped points, additional slide events, or multiple properties affected, we are adding this information to the Known Slide Areas layer. We are also adding recent landslides reported to the City since 2012 and correcting artifacts of digitization from older maps. In addition, we are scanning the paper documents and attaching them to the GIS landslide features. The result will be a Known Slide Areas layer with increased spatial resolution, higher accuracy of affected properties, and electronic documents accessible to the public for download from private computers.