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King County looks to update landslide threat strategies


Detailed laser data shows flood and slide threats all over King County. It’s critical information that you might not get if you buy a property; knowledge you might not have even if you’ve owned a property for years.

The county uses lidar, a sophisticated laser technology to create a 3D map of geologic risks from Shoreline to Enumclaw and east to the foothills of North Bend. Lidar images, gathered during airplane flights, can reveal historic landslides and old river channels to help predict slides and floods.

“If we’re able to predict where those things are going to happen, we’re able to take a variety of actions to
protect the public from those and those actions might include land use regulation, buying out houses that are at risk from flooding or channel migration,” said John Bethel, a geologist with the county’s Department of Natural Resources and Parks. That’s been done in the Snoqualmie Valley and along the Tolt River, for example.

Bethel focuses on rivers and flood planes. But from the lidar data, he knows there are landslide threats similar to what happened in Oso, in Snohomish County.

“Whether or not they exist on that scale, perhaps not, but there are certainly areas in neighborhoods that face some level or risk,” said Bethel.

If you’re buying property, you should know that a seller has some limited obligation to disclose potential hazards to a buyer. KIRO Real Estate expert Tom Kelly says the seller must reveal information that is considered to be detrimental, such as a history of slides or floods, if known. If you expect the county to notify you, understand that the county is not involved in real estate transactions in that sense, unless the buyer wants a permit to remodel or is building a new house.

“But if you’re just buying a property from somebody else, the county has no role in that at all and there are certainly properties available that are scary,” warned Bethel.

King County has a webpage with extensive lidar and other hazard data available to the public for free but it’s complicated and the site requires some extra software. If you’d hire an electrician or a plumber to work on your house, Bethel suggests you might consider hiring a geologist to assess those risks.

The county works to ensure that new construction is done in safe areas but that’s been limited to the last 20-30 years.

“There’s a great many both subdivided lots and buildings that were built before that where none of those kinds of controls were in place and so I think probably the most potentially hazardous properties that are available in King County are old ones,” Bethel explained.

After the deadly slide in Oso, some people complained that the community was not told of the landslide risk. King County will alert a community from time to time about a specific geologic risk, such as flooding, but it doesn’t happen in every case.

Bethel recently briefed King County Executive Dow Constantine.

“Just in the preliminary discussions, that’s a point of discussion that’s come up as to whether or not we should be moving in that direction,” Bethel said.

Landslide hazard mapping in King County began in the early 1990s. “It’s time to update that,” said Bethel. He expects that new policies on dealing with geologic hazards and notifying the public are possible in the weeks and months ahead.

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