Following the tragic Oso landslide, King County Executive Dow Constantine and the King County Flood Control District called for modernizing 1990’s-era landslide hazard maps.

LiDAR imaging – a surveying technology that measures distance by illuminating a target with laser light – produced updated high-resolution geologic maps. The new landslide mapping also relied on recent geologic mapping prepared by State and Federal agencies.

King County said the maps support earlier conclusions by geologists that there are no communities with the same unique combination of conditions present at the site of the 2014 landslide in Snohomish County near Oso. However, landslide hazards are a reality in Western Washington, and the maps show areas that may be at some risk.

The new maps are available at www.kingcounty.gov/landslides. King County and the Flood Control District will sponsor a series of landslide workshops in October and November at four locations across the county to help residents interpret the new maps and learn about landslide geology, how to reduce risk, landslide response services and more.

“When a tragedy of such magnitude strikes a neighboring community, your first thoughts are for the families of those involved and helping however you can. The next step is to learn and prepare. The multi-departmental initiative to better understand potential hazardous areas helps us protect public safety,” Executive Constantine said.

“Maps and technology alone will not make our residents safer. We must continue to take a thoughtful approach to permitting, land use, emergency response, and communications.”

“Updating these maps with advanced technology makes us better informed and in better position to protect people, property and critical public infrastructure,” said Flood Control District Chair Reagan Dunn. “It will also help us be more strategic in how we work to reduce flood risks.”

The four landslide hazard workshops will occur in late October and early November, with an additional open house on Vashon-Maury Island still to be scheduled. Start time for the workshops is 6 p.m. They are set:

• Tuesday, Oct. 25 – Lake Wilderness Lodge, Maple Valley.
• Thursday, Oct. 27 – Meadowbrook Farm Interpretive Center, North Bend.
• Tuesday, Nov. 1 – Green River Community College, Auburn.
• Thursday, Nov. 3 – Tolt Middle School, Carnation.

To stay informed about the workshops, sign up for email announcements at www.kingcounty.gov/landslides.

While these maps are more accurate than those produced in 1990’s, the County said landowners should always consult a geologist for an on-site assessment of potential geologic hazards.

While code revisions could be considered at some point, no changes in land-use regulatory codes are proposed at this time as a result of the landslide mapping updates.

To better understand landslide risks along rivers, the King County Flood Control District funded approximately $700,000 of the mapping effort, which was carried out by the King County Department of Natural Resources and Parks.

The Flood Control District focused on identifying potential hazards within river corridors, and included portions of some cities as well as unincorporated King County within those corridors. Landslide hazard information will be used in River Corridor Planning to address flood-related hazards and reduce flood risks.

At the same time, the King County Department of Permitting and Environmental Review completed a $70,000 companion effort to update landslide hazard mapping in the unincorporated areas beyond the river corridor areas.  This mapping, which includes Vashon-Maury Island but does not include the forest production areas, resulted in an improved screening tool that allows people to view an area or zoom to a specific parcel to check if there are any indications of potential hazard.  If there are, additional geotechnical evaluation likely will be required as part of any future development or redevelopment proposal.

In total, the County and Flood Control District spent about $976,000 to improve King County landslide hazard maps, including just over $206,000 for the LiDAR imaging technology.

“This was a vital and valuable investment in being better prepared for the future and in making sure our residents and businesses are more fully informed about landslide risks here in King County,” said Flood Control District Supervisor Kathy Lambert, whose district includes part of the Snoqualmie River basin.

The workshops are the best place to get comprehensive information about the landslide hazard map update. People who cannot attend may contact the Water and Land Resources Division (206-477-4800) if they have questions about potential hazards within river corridors. People seeking information about unincorporated areas that are not in the river corridors, or who have development-related questions, should call the Department of Permitting and Environmental Review at (206-477-0342).