ASSESSING THE LANDSLIDE PROBABILITY ON COASTAL BLUFFS NEAR MUKILTEO, WASHINGTON THROUGH HYDROLOGIC MONITORING

Shallow, precipitation-induced landslides on coastal bluffs along Puget Sound, WA, pose a safety threat to the adjacent Burlington Northern Santa Fe (BNSF) railway corridor and often lead to service interruptions. Previous research in the Puget Sound region has identified rainfall thresholds and antecedent moisture conditions that are linked to periods of heightened probability of landsliding on hillsides. Guided by this research, we present the details of two new hydrologic monitoring sites that we installed along the coastal bluffs in Mukilteo, WA. We designed the system to observe hydrologic conditions leading to shallow slope failures along the railway corridor. We selected the two monitoring sites to facilitate a paired study of an active landslide and a currently stable slope that may be prone to future sliding. The two sites are 0.5 km apart and share similar topography and geology. Instrumentation at both sites includes rain gauges, tensiometers, soil moisture content sensors, and piezometers installed at various depths and locations within the colluvium. Additionally, we dug five test pits at each site to characterize the partially consolidated glacial deposits and overlying colluvium, and collected samples of these unstable materials for laboratory analyses. Subsurface water movement is difficult to predict in glacial deposits, but understanding patterns in subsurface hydrology is key to prediction of landslides. Because the two monitoring sites are located on glacial deposits that vary between coarse gravel- and cobble-rich layers and low permeability silt and clay deposits, in-situ monitoring is an important step toward predicting hydrologic movement and subsequently, landslide potential. One of the primary goals of our study is to understand how these material and density contrasts might influence water movement and slope stability.