After the Oso slide, Snohomish County took criticism for allowing homes beneath a deadly landslide hazard. Would any of Washington's 38 other counties have done it differently? Each defines landslide hazards and sets rules its own way. Fourteen specify no buffer between hazards and homes. Pick a county below for a hypothetical illustration of how its landslide rules would have restricted development had they been applied to Oso. How did we make this?
Reporters from KUOW and EarthFix examined the Critical Areas Ordinance for each of Washington’s 39 counties. Each ordinance was analyzed to determine how close development is allowed to “geologically hazardous areas.”
Determining minimum buffers
Some counties' ordinances specify vegetation buffers or building setbacks at certain distances from hazards. Others mandate stricter development rules and geotechnical reports within a certain distance of hazards. Several required both. This analysis considered the larger of the two distances as the county’s minimum buffer for restricted development near landslide hazards.
Defining the hazard area
Defined landslide hazards as shown on the are derived from geospatial data obtained from Snohomish County Information Services. The data encompass “geologically hazardous areas” as defined by the county, which include past landslides and slopes steeper than a 30 percent incline.
Creating the map
Using geospatial analysis software, boundaries were drawn around the defined Oso landslide hazards for each of the minimum buffers determined in the analysis.
Data for the 2014 Oso slide deposit come from the Geotechnical Extreme Events Reconnaissance. Aerial images from before and after the slide, as well as data of homes damaged in the Oso slide, come from Esri.
A summary of each county’s rules was sent to county officials for review and comment. Several counties provided feedback, which was considered in the final production of the map. The analysis and visualization were reviewed by several experts in land use, engineering and geology.
* Shaded area shows Oso landslide slope and adjacent areas classified as geologic hazards by Snohomish County.
** Most jurisdictions can adjust their standard buffers up or down, case by case. Some buffers shown are required no-build zones; some are zones with stricter rules for development.