Setting the Stage: The 2014 water year (October 1, 2013 to September 30, 2014) began with an unseasonably dry spell. Then, the pattern abruptly changed and precipitation arrived with a vengeance. From mid February to late March 2014, rain and snow were so heavy they nearly made up for what hadn’t arrived in the fall. The result was an anomalously wet late winter.
That’s the stage. Enter the loaded weapon: A mass of soil—16 billion kilograms (18 million tons), or roughly three times the mass of the Great Pyramid of Giza—made of glacial till and sand precariously perched atop the 190-meter-tall (623 foot) bluff overlooking the Washington community.
The heavy rains in the late winter super-saturated the soil. Somewhat like a sponge, the more water the soil soaked up, the heavier it got, until the entire mass was sent sliding. (The exact trigger—if there was a single trigger—of the slide is still largely unknown.)
Henn and colleagues’ analysis looked at soil moisture and precipitation accumulations as the possible contributors to the slide. Using the Variable Infiltration Capacity Macroscale Hydrologic Model to reconstruct conditions at the time of the landslide, the researchers calculated that the local soil moisture was unseasonably high due to the high amounts of precipitation in the weeks leading up to the landslide. In fact, six days before the slide the bluff’s soil moisture was calculated as having a 43-year return period (meaning the soil moisture conditions were so high they had a 1 in 43, or 2.32%, chance of occurring in any given water year). Precipitation accumulations ending on the day of the landslide were also—not surprisingly—really high: racking up return periods as high as 88 years for 3-week accumulations.
Note On Research Methods: The researchers’ conclusions were reached by gathering data from several precipitation gauges located near the site of the landslide—two primary gauges located 3 km (about 2 miles) and 18 km (11 miles) away and nine other stations located within 80 km (50 miles). The data were broken down into 28 different time frames ranging from 1 day to 10 years with time frames all ending on the date of the landslide. Results were fitted with a probability distribution. The distributions were then compared with the 2014 accumulations to estimate their probabilities leading up to the landslide. While the previous few days were wet, they weren’t exceptionally so; it was the precipitation totals in the 1 to 6 weeks before the slide that were exceptionally high compared to similar time periods. Soil moisture reconstructions were taken from University of Washington’s Drought Monitoring System for the Pacific Northwest, an effort supported by CIRC.