USING LIDAR TO OBJECTIVELY MAP BEDROCK LANDSLIDES AND INFER THEIR MECHANICS AND MATERIAL PROPERTIES

A map of extant slope failures is the most basic element of any landslide assessment. Landslide inventory maps are tedious to compile, difficult to make in vegetated terrain, often subjective, and normally simply outline slide boundaries without offering information about landslide mechanics. The surface of most slides is rougher, on a local scale of a few meters, than adjacent unfailed slopes. We exploited this characteristic to automatically detect and map landslides in landscapes represented by high-resolution DTMs produced from lidar data. At a large landslide complex and surrounding terrain, 1D, circular (2D) and spherical (3D) statistics were used to map the local surface roughness in the DTMs over a spatial scale of 1.5 to 10 m. Any of the statistics can be employed to automatically detect and map the overall slide complex. In general, results were superior using the 3D methods. Interior features with a minimum size of surface folds that have a wavelength of about 16 m and amplitude of about 1 m were readily mapped. The Laplacian operator also accurately maps kinematic units within the slide and the folds and levees within and at the margins of the units. 2D power spectra analyses were used to begin to explore how roughness varies with length scale. Initial results indicate that no dominant length scale of roughness exists for smooth, unfailed terrain. In contrast, zones with different styles of landslide deformation exhibit distinctive spectral peaks that correspond to the scale of deformation features, such as compression folds.

Physical models and analytical models based on simple elastic deformation theory are being used to investigate the relationships between the geometry of small folds and thrust faults seen in the high-resolution DTMs and slide mechanics and material properties. In a bench top physical model, both fold wavelength and amplitude vary systematically with material plasticity and depth of the basal shear surface. Field and model fold amplitudes and wavelengths are accurately predicted by elastic deformation theory if the material is near its plastic limit. At higher water contents, the fold amplitudes are lower than predicted. At the test landslide complex, it appears possible in one inset earthflow to solve inversely for the depth of the failure surface from lidar measurements of detailed surface topography.