Landslide Risk is High

posted Dec 12, 2010, 1:56 PM by smike@guemesfire.org
...130 AM PST SUN DEC 12 2010

...HEAVY RAINFALL OVERNIGHT AND TODAY WILL LEAD TO INCREASED RISK OF LANDSLIDES IN WESTERN WASHINGTON..

HEAVY RAINS OF UP TO 2 INCHES OVER THE WESTERN WASHINGTON LOWLANDS SINCE NOON SATURDAY...AND UP TO 4 INCHES IN THE MOUNTAINS...HAVE INCREASED SOIL WETNESS TO HIGH LEVELS AROUND WESTERN WASHINGTON. HEAVY RAIN WILL CONTINUE TODAY AND TONIGHT...WITH ANOTHER 1 TO 2 INCHES OVER THE LOWLANDS AND ANOTHER 2 TO 6 INCHES OVER THE MOUNTAINS.

THIS AMOUNT OF RAIN WILL RESULT IN SOIL INSTABILITY. CUMULATIVE RAINFALL OVER THE LAST THREE WEEKS HAVE SOAKED SOILS TO THE POINT WHERE THEY ARE ABOVE THE U.S.G.S. LANDSLIDE INDEX.  WHEN CUMULATIVE RAINFALL OVER THE PREVIOUS 3 DAYS AND PREVIOUS 18 DAYS EXCEED THE U.S.G.S. CUMULATIVE THRESHOLD INDEX...RAINFALL INDUCED LANDSLIDES ARE POSSIBLE DURING INTENSE RAINFALL OR WHEN RAINFALL AMOUNTS EXCEED TWO INCHES IN 24 HOURS.  A DIMINISHING RISK OF LANDSLIDES WILL CONTINUE FOR SEVERAL DAYS AFTER TODAY.

AT LEAST TWO LANDSLIDES HAVE ALREADY BEEN REPORTED DURING THIS HEAVY RAIN EPISODE...ONE ALONG CHUCKANUT DRIVE IN THE NORTH INTERIOR AND ANOTHER NEAR VANCOUVER WASHINGTON.

WHILE THE U.S.G.S. RAINFALL THRESHOLDS APPLY TO THE SEATTLE AREA...MOST OF THE LOWLANDS OF WESTERN WASHINGTON ARE SUSCEPTIBLE TO LANDSLIDES DUE TO WET SOILS. AREAS MOST SUSCEPTIBLE TO LANDSLIDES UNDER THESE CONDITIONS ARE STEEP COASTAL BLUFFS AND OTHER STEEP HILLSIDES. FOR MORE INFORMATION ABOUT CURRENT CONDITIONS...SEE THE FOLLOWING INTERNET WEB PAGES:

HTTP://WWW.WRH.NOAA.GOV/SEW  (FOR WEATHER FORECASTS; ALL LOWER CASE)

HTTP://WWW.WRH.NOAA.GOV/SEW  (ALL LOWER CASE; LINK TO THE LANDSLIDE SECTION OF THE HYDROLOGY PAGE )

HTTP://LANDSLIDES.USGS.GOV/MONITORING/SEATTLE/RTD/PLOT.PHP (ALL LOWER CASE)

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