VADOSE-ZONE RESPONSE TO RAINFALL LEADING TO SHALLOW LANDSLIDE INITIATION ON THE PUGET SOUND BLUFFS, WASHINGTON

On January 14, 2006, a shallow translational landslide occurred at a USGS-instrumented site on an unstable coastal bluff near Edmonds, Washington. The 24 m long x 10 m wide x 1-2 m deep landslide formed in colluvium on a 45-degree slope. This landslide was one of several that occurred during the winter season of 2005-2006 along the rail corridor between Seattle and Everett, Washington. Shallow landslides are common in the colluvial deposits that mantle many of the steep bluffs along the Puget Sound and are often triggered by heavy rainfall or rapidly melting snow. The bluffs at the instrumented site are underlain by subhorizontally bedded glacial and interglacial sediments, which include glacial outwash sand and glaciolacustrine silt deposits. The well-consolidated glacial deposits readily weather mechanically to form a loose, highly permeable colluvium. The instrument array at the site consisted of rain gauges, water content profilers (8 depths to 2 m below ground surface), soil temperature sensors, and 2 tensiometer nests (6 instruments each to 1.5 m below ground surface).

Instrumental observations show that infiltration of rainwater was dominantly vertical and the pore-water response to rainfall at depths below about 1 m was highly dependent on initial soil-moisture conditions. For example, record 24-hour rainfall on initially dry soils (~12% soil-water content at 0.8 m depth) in the fall of 2003 led to a small increase in pressure head and water content that peaked nearly 6 days after the storm. However, once initial water contents reached about 20% (typically between late October and early December), the response at depth was much greater and occurred within hours of the beginning of rainfall. Between late December 2005 and the middle of January 2006 about 200 mm of rain was recorded at the Edmonds field site. By the early January, soil-water contents at depths below 1 m exceeded 30%. In the following days, rainfall infiltration produced peaks in measured pore-water pressure and water content at depths of 1-2 m below the ground surface. Several shallow landslides along the rail corridor, including the one at the monitoring site occurred concurrently with measured peaks in the pore-water response at depth.