APPLICATION OF NEAR-REAL-TIME MONITORING TO STUDY OF COASTAL BLUFF INSTABILITY, SNOHOMISH COUNTY, WASHINGTON

Recently the U.S. Geological Survey (USGS), in cooperation with the Burlington Northern Santa Fe Railway (BNSF) and its consultant, Shannon and Wilson, Inc., instrumented two sites on unstable coastal bluffs near Edmonds and Everett, Washington. We designed instrument arrays to identify the mechanisms by which precipitation triggers shallow landslides based on stratigraphy and topography of the sites and observations of recent slides. The coastal bluffs at these sites are underlain by subhorizontally bedded glacial and interglacial sediments, which include sandy outwash overlying glaciolacustrine silt deposits. Shallow landslides commonly occur in weathered glacial deposits and slope deposits (colluvium) on the bluffs after periods of relatively heavy rainfall or snowmelt. Observations indicate that water enters the slopes by direct infiltration and by lateral flow through sandy layers that rest on less permeable layers of silt or clay. The instrumentation consists of rain gauges, soil-moisture reflectometers, soil-temperature probes, and pore-pressure transducers in shallow (1-2 m) holes. Data are transmitted regularly over a commercial line-of-site radio network to a facility in Kent, Washington where they are posted to an FTP server. The data are processed and served (http://landslides.usgs.gov/pugetrt/index.html) at USGS offices in Golden, Colorado. Measurements made during the winter rainy season of 2001-2002 indicated wetting fronts propagating downward through the soil. Moisture response of the soil appears to vary with storm duration and intensity as well as antecedent moisture conditions. Limited measurements indicate that the near-surface soil moisture increased abruptly following several days of rain at the end of October 2001. The soil remained relatively wet throughout the winter, with volumetric water content varying by a few percent. About 50 mm of rain that fell on December 16 and 17, 2001 produced a sharp wetting front that propagated rapidly downward. Soil moisture gradually declined over the next two weeks during which we recorded only a few millimeters of precipitation. In contrast, 32 mm of rain that fell over three days, (January 6-8, 2002) produced a diffuse wetting front, followed by a gradual decline in soil-moisture content during the next two weeks when relatively little rain fell.