LANDSLIDES IN PORTLAND, OREGON 2008-2009: BROKEN WATER PIPES AND HEAVY RAINFALL AS TRIGGERS

Over 20 major landslides were studied in the area of Portland, Oregon during the fall and winter of 2008-009. The first, and the most spectacular, was a house that came crashing down the hill on Burlingame Place in the West Hills on October 8, 2008 during a time of the year when the soils are dry. It hit three houses at the bottom of the slope after traveling over 80 meters before completely collapsing. The value of the house was over $1.5 million. The fact that insurance did not cover the catastrophe and the owner escaped without injury made the headlines. A broken water pipe was determined to be the cause. Water was noted passing under the house the day before the slide. Water meter readings noted large increases in usage since July 2008. On January 1, 2009 the city had a rainfall of 8.1 cm in 24 hours causing the following slides. In Lake Oswego, a suburb south of Portland, a shallow landslide on a road diverted storm water onto a second slope below the road causing another slide that crashed into a house valued at over $1 million destroying it. No one was injured. In nearby Oregon City, a 500 cubic meter earth flow formed in an ML soil cover of an old landfill; no damages occurred. In Estacada to the east of Portland, reactivation of an ancient landslide that destroyed a house on the lower slope of the old slide blew up the house’s propane tank causing the house to burn down. The four occupants escaped safely. Four additional earth flows were mapped on the adjacent slope nearby. In nearby Gresham, a large mudflow was formed from the liquefaction of a small earth flow in loess at the top of an ancient volcano. The mudflow traveled over 2 km in length. In the West Hills of Portland, runoff from a small road caused a small earth flow (250 cubic yards) to form.