On December 4th 2007, a 3-Mm3 landslide occurred along the northwestern
shore of Chehalis Lake. The initiation zone is located at the intersection
of the main valley slope and the northern sidewall of a prominent
gully. The slope failure caused a displacement wave that ran up to
38 m on the opposite shore of the lake. The landslide is temporally
associated with a rain-on-snow meteorological event which is thought
to have triggered it. This paper describes the Chehalis Lake landslide
and presents a comparison of discontinuity orientation datasets obtained
using three techniques: field measurements, terrestrial photogrammetric
3D models and an airborne LiDAR digital elevation model to describe
the orientation and characteristics of the five discontinuity sets
present. The discontinuity orientation data are used to perform kinematic,
surface wedge limit equilibrium and three-dimensional distinct element
analyses. The kinematic and surface wedge analyses suggest that the
location of the slope failure (intersection of the valley slope and
a gully wall) has facilitated the development of the unstable rock
mass which initiated as a planar sliding failure. Results from the
three-dimensional distinct element analyses suggest that the presence,
orientation and high persistence of a discontinuity set dipping obliquely
to the slope were critical to the development of the landslide and
led to a failure mechanism dominated by planar sliding. The three-dimensional
distinct element modelling also suggests that the presence of a steeply
dipping discontinuity set striking perpendicular to the slope and
associated with a fault exerted a significant control on the volume
and extent of the failed rock mass but not on the overall stability
of the slope.