A series of storms will continue to roll in from the Pacific Ocean adding rain to the soggy, steep slopes of the Cascades and foothills, as hopes for finding survivors from the Oso, Wash., landslide dwindle.
The landslide in Oso occurred on Saturday, March 22, 2014, along Route 530 and the North Fork of the Stillaguamish River. Essentially, the entire side of a steep mountain gave way due to saturation.
The lives of more than two dozen people were claimed by the Oso landslide, with dozens more feared dead now that search efforts have continued for nearly a week. According to the Associated Press, those individuals involved in recovery efforts are now using shovels and other hand tools to search for the missing. Hope for finding survivors is diminishing.
Meanwhile, up to several additional inches of rain are forecast to fall from the Washington and Oregon coasts to the western slopes of the Cascades into the first part of April.
Besides complicating search and rescue efforts, additional rainfall, thaws in the high country and saturated, unstable ground can lead to additional debris flow incidents such as mudslides and landslides, as well as flash, urban and small stream flooding.
How Landslides Occur
Water collects in the soil and tends to build up above a less permeable layer such as rock or clay. Under the right conditions, the weight of the top layers of saturated soil can rapidly give way causing a large mass of mud and other debris to slide downhill suddenly. The risk can be elevated in areas of sparse vegetation or where wildfires recently destroyed vegetation. A lack of solid bedrock on the slopes can make the area more prone to mudslides.
The resulting debris flow can act like quicksand, block streams, damage roadways, and in some cases slide through homes, businesses and farms, destroying everything in its path and placing lives in peril. Often rocks, trees and other structures can be carried along in the flow.
Flooding problems can be made worse, where debris flows have blocked the channel of streams and rivers.
Very Wet Conditions of Late
March has been a wet month across the Northwest.
"Storms have had a free ride moving in from the Pacific Ocean this month, rather than taking an indirect route through British Columbia," AccuWeather Meteorologist Eric Leister said. "Earlier in the winter, storms were before forced northward, away from the region."
The metro area of Seattle-Tacoma, Wash., lies 20 miles to the southwest of Oso, where near-record rainfall has fallen during March. Seattle-Tacoma International Airport has received just over 8 inches of rain for the month as of Friday morning, March 28. The normal rainfall for the entire month is 3.72 inches.
"Already Friday morning, March 28, another 0.25 of an inch of rain has fallen on the area," Leister said.