Relentless rain increases chance of landslides

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MERCER ISLAND - After 14 days of relentless rain in Western Washington, NOAA is putting out a warning about the elevated threat of landslides.

Every time it rains, Bankim Shah holds his breath.

“We’re living on the edge, if you will,” he says. “Literally and mentally too.”

Last spring, a landslide caused major damage across the street from his Mercer Island home.

“One night just like this, the rain was pouring and the road just gave away.”

Shah hired a geotechnical engineer to check the soil when he first moved to Mercer Island, so he didn’t think slides would be a problem. But there have been a few in his neighborhood, and in other parts of King County, over the last week. UW geomorphologist David Montgomery says that’s because of all the rain we’ve had this fall.

“It’s the combination of duration and intensity of rain. You can think of it as how much water is getting into the ground. If it's already really wet, it's going to take that much less to trigger slides.”

He says not all slopes are prone to slides. If homeowners aren’t sure what kind of soil is in their neighborhood, they can consult a geologist or look at a government hazard map to see where there have been issues in the past. Since more rain is in our forecast, he says anyone living or working near a hillside should be aware of the potential for problems.

“Keep your eyes open,” he says. “Are things changing on a hillside? Are cracks developing and growing?”

Shah and his neighbors have put up a temporary berm to divert the water on their street, until a permanent retaining wall can be built.

“We are keeping our fingers crossed that we can make our way through this winter,” he says. “We don't want trees to fall down and damage the road, and we don't want too much rain to take the rest of the hillside with it.”

Montgomery shared these tips for homeowners who are concerned about the possibility of landslides:

LANDSLIDE PREPAREDNESS

  • Get a ground assessment of your property. Your county or city geologist or planning department may have specific information on areas vulnerable to land sliding.
  • Seek advice of geotechnical experts for evaluating landslide hazards or designing corrective techniques to reduce landslide risk.
  • Plant ground cover on slopes to stabilize the land, and build retaining walls.
  • Plan at least two evacuation routes since roads may become blocked or closed.
  • Make arrangements for housing in the event you need to evacuate your home.
  • Plan for "earthquakes" and "severe storms" that can cause a landslide.

LANDSLIDE WARNING SIGNS

  • Doors or windows stick or jam for the first time.
  • New cracks appear in plaster, tile, brick or foundation.
  • Outside walls, walks or stairs begin pulling away from the building.
  • Slowly developing, widening cracks appear on the ground or on paved areas such as streets or driveways.
  • Underground utility lines break.
  • Bulging ground appears at the base of a slope.
  • Water breaks through the ground surface in new locations.
  • Fences, retaining walls, utility poles or trees tilt or move.
  • You hear a faint rumbling sound that increases in volume as the landslide nears. The ground slopes downward in one specific direction and may begin shifting that direction under your feet.

There are several government maps, that mark landslide hazard areas:

Seattle: http://www.seattle.gov/dpd/cs/groups/pan/@pan/documents/web_informational/dpdd017622.pdf

King County: https://tribkcpq.files.wordpress.com/2015/12/landslide-hazard-map.pdf

USGS: https://tribkcpq.files.wordpress.com/2015/12/of06-1139_508.pdf

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