DNR Publishes Inventory of King County Landslides
News Date: 
January 24, 2019

State geologists cataloged 2,838 landslides in the western two-thirds of the county using new lidar mapping

The Washington Geological Survey, a division of the Washington State Department of Natural Resources, published a new inventory of known landslides in the western two-thirds of King County today.
The Geological Survey inventoried 2,838 landslides throughout King County using new lidar data, raising awareness of a relatively common geologic hazard. By knowing where landslides are, local officials and residents can take proper care to reduce the likelihood of reactivating the slides.
“Our abundant rain and glacially carved geology put much of Western Washington at a high risk for landslides,” said Commissioner of Public Lands Hilary Franz, the elected official who oversees DNR. “By releasing this data and making people aware of landslide danger, we hope to lower the risk to hundreds of thousands of King County residents.”
Lidar uses airplane-mounted lasers to create detailed three-dimensional maps of the Earth’s surface. The lidar maps produced for King County allow geologists to see past landslides that are typically hidden by vegetation and development. In addition to lidar, the geologists used aerial imagery, field observations, and previous mapping to identify landslides.
The detailed landslide inventory will help city and county planners, homeowners, emergency managers, and others identify areas that may need further geologic investigation before development or other uses are approved.
A high density of landslides were found along Puget Sound bluffs, river corridors, and upland areas of the Cascades.
For existing communities built on or near landslides, mitigation measures – retaining trees and native vegetation on slopes, or keeping downspouts and driveways from draining onto slopes, for instance – can reduce the potential of landslides reactivating.
The landslide inventory for King County also includes an inventory of alluvial fans, fan-shaped features created by streams, floods or debris flows that leave sediment behind. Alluvial fans are typically located on valley floors at the base of steep slopes where water transports sediment onto the fan surface. Geologists found 1,251 fans in the inventory area.
Small landslides, such as those that may affect roads during rain events, were not part of this inventory because they are not typically visible in lidar maps. A landslide susceptibility map is planned to be released later this year to identify areas where small landslides may happen in the future.
The Landslide Inventory for Western King County, Washington can be accessed at bit.ly/KingInventory. A brief paper discusses the Geologic Survey’s major findings in further detail, and there is a GIS database of the inventory. Due to the size of the study area and the detailed nature of the data, no electronic or paper maps are available, though maps can be printed from the Washington Geologic Information Portal at dnr.wa.gov/geologyportal.
The Legislature directed the Department of Natural Resources to identify and map landslides throughout Washington and disseminate the information to the public. This year, DNR has made a $1.4 million capital budget request and a $1.1 million operating budget request to the Legislature to do further work to better understand landslide hazards and glacial geology across the state.
Further information on the Geological Survey’s work on landslides in Washington can be found at dnr.wa.gov/landslides.
Kenny Ocker
Communications Manager
Department of Natural Resources