RICHARD H. JAHNS DISTINGUISHED LECTURE: THE LANDSLIDE THAT ATE LAPRAK

Located in a remote region of west-central Nepal and two to three days’ walk from the nearest roadhead, the Himalayan village of Laprak (28°13’4.8”N, 84°48’12.7”E) lies along a steep river valley at an elevation of about 2150 m. It also sits atop a large landslide, part of which was reactivated during an exceptionally heavy rainstorm in July 1999 and has continued to move intermittently during summer monsoon seasons. The active part of the landslide is a structurally constrained mass of relatively thin sandy colluvium derived from jointed quartzite and schist bedrock of the High Himalayan Crystalline Sequence. Analysis of a field-generated digital terrain model of the active landslide suggests that it consists of three major segments and makes clear the extent of bedrock structural control of the slope morphology. Limit equilibrium stability modeling—constrained by field observations, a limited program of geotechnical testing, and finite element simulation of seepage—support the geomorphic interpretation of three distinct sections. The remote location of Laprak, well beyond the nearest road, and lack of money rule out capital intensive remedial options such as drain installation, retaining or pier wall construction, or wholesale regrading. Likewise, wholesale relocation is a socially unrealistic solution. Gradual replacement of existing dry stone masonry buildings with lightweight, flexible, and easily repaired wood and metal structures, however, is a possibility.