FORENSIC ANALYSES OF TWO LANDSLIDES USING LIDAR TOPOGRAPHIC DATA

We present two case studies where lidar topographic data were utilized to evaluate conditions preceding or following two significant landslides in the Portland, Oregon metropolitan area. In the first case study we discuss the use of lidar data in evaluating a large landslide that occurred in 1969 during construction of Interstate 205. This landslide was a translational failure where Columbia River basalt flows slid on an underlying sedimentary interbed that was exposed during rock excavation. We have compared the 10-meter USGS DEM, which represents the pre-failure topography, with a recent vintage lidar DEM in our analysis of the cause and approach to stabilization of this landslide failure. In the second case study we discuss the use of lidar topography in evaluating a landslide that destroyed an expensive residence in rural Clackamas County. The lidar data were used to confirm the presence of a pre-existing landslide on the property and in reconstructing the filling and grading during site development. In both cases utilizing lidar topographic data was crucial in the forensic analyses of these landslides.