Heavy Rainfall Increases Landslide Chances; New Weather Statement Issued

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Several potent weather systems have produced heavy rainfall and some flooding over the Burien area over the last few days, and the National Weather Service issued a “Special Weather Statement” warning of the increased likelihood of landslides.

Sea-Tac Airport has received around 2.5 inches of rain over the past five days, and the ground is surely getting saturated. The good news is that, starting Monday, things will dry up a bit.

Also, a record high was set at the airport today, with a temp of 55 degrees, breaking the old record of 54 set in 1981. A record high of 56 was set at the Olympia Airport, and Seattle hit 56.

Here’s the statement in its entirety:

Statement as of 4:09 PM PST on January 16, 2011

A series of weather systems has produced locally heavy rainfall across western Washington over the past 5 days. SeaTac Airport has received around 2.5 inches of rain while locations near the Cascades… along the coast… and in the south interior have received much more. Soil moisture levels around western Washington are high. Another round of precipitation ranging from 0.25 to 0.5 inches in The Lowlands and over an inch in the mountains is expected tonight as a cold front moves across the area. The rainfall may put continued pressure on soil instability… maintaining a risk of landslides through Monday morning.

Rainfall in the forecast… on top of the cumulative rainfall over the last three weeks… will keep western Washington soils to the point where they are near the USGS landslide index. A diminishing risk of landslides is expected Monday through the remainder of the week as conditions dry out somewhat.

While the USGS rainfall thresholds were designed for the Seattle area of the Puget Sound… most of The Lowlands of western Washington are similarly susceptible to landslides caused by wet soils. Areas most susceptible to landslides under these conditions are steep coastal Bluffs and other steep hillsides.

For more information about current conditions… visit www.Weather.Gov/Seattle… select hydrology… and then scroll down for the link to the USGS landslide information Page.

Here’s more info on landslide warning signs from the USGS:

Landslide Warning Signs:

  • Springs, seeps, or saturated ground in areas that have not typically been wet before.
  • New cracks or unusual bulges in the ground, street pavements or sidewalks.
  • Soil moving away from foundations.
  • Ancillary structures such as decks and patios tilting and/or moving relative to the main house.
  • Tilting or cracking of concrete floors and foundations.
  • Broken water lines and other underground utilities.
  • Leaning telephone poles, trees, retaining walls or fences.
  • Offset fence lines.
  • Sunken or down-dropped road beds.
  • Rapid increase in creek water levels, possibly accompanied by increased turbidity (soil content).
  • Sudden decrease in creek water levels though rain is still falling or just recently stopped.
  • Sticking doors and windows, and visible open spaces indicating jambs and frames out of plumb.
  • A faint rumbling sound that increases in volume is noticeable as the landslide nears.
  • Unusual sounds, such as trees cracking or boulders knocking together, might indicate moving debris.

Areas that are generally prone to landslide hazards

  • On existing old landslides.
  • On or at the base of slopes.
  • In or at the base of minor drainage hollows.
  • At the base or top of an old fill slope.
  • At the base or top of a steep cut slope.
  • Developed hillsides where leach field septic systems are used.

Areas that are typically considered safe from landslides

  • On hard, non-jointed bedrock that has not moved in the past.
  • On relatively flat-lying areas away from sudden changes in slope angle.
  • At the top or along the nose of ridges, set back from the tops of slopes.
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