Landslide volumes span many orders of magnitude,but large-volume slides tend to travel
farther and consequently can pose a greater hazard. In northwest Washington State, USA, and a
landscape abounding with landslides ample and small,the recent occurrence of the large-volume
and tragically deadly State Route 530 (Oso) landslide is a stark reminder of the hazards
associated with glacial terraces lining valleys of the western Cascade Range. What controls
the differences in location and size of these slope failures? Here, we examine the control on
landslide volume and failure style by terrace sedimentary architecture. We analyze lidar
topographic data in three nearby valleys and find significant variation in landslide deposit
volumes, and morphology,and relative mobility in each valley. Geologic data show that each site
differs in the thickness and position of outwash, tills, and glaciolacustrine clays. Combining
a three-dimensional limit-equilibrium slope-stability analysis (Scoops3D) with simulations
of variably saturated groundwater flow (VS2Dt),we show that landslide volumes are highly
sensitive both to the distribution of fabric strength as well as the location of perched water
tables. Modeled landslides match observed failure sizes and depths in all valleys when the
effects of variably saturated groundwater flow are included. The position and thickness of
low-strength strata act as first-order controls on landslide volume, with peak volumes for
stratigraphic geometries similar to that of the valley containing the Oso landslide. Knowledge
of feedbacks between lithology and hydrology is therefore critical to assess the landslide
hazard and evolution of landscapes composed of stratigraphically layered units.
Source: usgs.gov