The United States Geological Survey (USGS) developed a model for landslides and rainfall events for the Seattle area a few years ago linking rainfall thresholds with landslides. Most periods of landslides require multiple days of wet weather. It is rare that a single storm will trigger landslides, but a series of moderate storms will push the saturation level of soils to the point of failure. We have reached that threshold for the second time this winter due to a series of moderate storms.
It was certainly noticeable while walking to work. Overland flow was developing within my own neighborhood as the ability of soils to soak up anymore water has been exceeded. Since the development of this statistical model, it has proven to pretty accurate. It seems that every time a warning is issued the rail line south of Everett is blocked by slides coming off of the steep slopes above.
The National Weather Service Seattle office includes the USGS warning in its special weather statements. For those living on hills with any history of shallow landslides or along small streams susceptible to slide induced debris flows or flooding, precaution appropriate for the risk should be taken.
SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE SEATTLE WA
447 AM PST FRI JAN 21 2011
WAZ001-503>519-220200-SAN JUAN COUNTY-WESTERN WHATCOM COUNTY-SOUTHWEST INTERIOR-EAST PUGET SOUND LOWLANDS-WESTERN SKAGIT COUNTY-
EVERETT AND VICINITY-SEATTLE/BREMERTON AREA-TACOMA AREA-
ADMIRALTY INLET AREA-HOOD CANAL AREA-LOWER CHEHALIS VALLEY AREA-
OLYMPICS-EASTERN STRAIT OF JUAN DE FUCA-WESTERN STRAIT OF JUAN DE FUCA-NORTH COAST-CENTRAL COAST-WEST SLOPES NORTHERN CASCADES AND PASSES-
WEST SLOPES CENTRAL CASCADES AND PASSES-447 AM PST FRI JAN 21 2011
RAINFALL AMOUNTS OF ABOUT A QUARTER TO AN INCH FELL DURING THE PAST 24 HOURS...ENDING AT 4 AM PST THIS MORNING...ON TOP OF ALREADY SATURATED SOILS. ANOTHER TENTH TO A HALF INCH OF RAIN IS EXPECTED TODAY...WITH LOCALLY HIGHER AMOUNTS IN THE MOUNTAINS.
THIS AMOUNT OF RAIN WILL ONLY SERVE TO PUT EXTRA PRESSURE ON SOIL INSTABILITY...LEADING TO AN INCREASED RISK OF LANDSLIDES. THERE HAVE BEEN LANDSLIDES IN WESTERN WASHINGTON DURING THE PAST WEEK.
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.
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