LANDSLIDE SUSCEPTIBILITY MAP FOR SHALLOW LANDSLIDES FOR THE WEST HILLS OF PORTLAND, OREGON USING GIS AND LIDAR

The West Hills of Portland, Oregon are prone to landslides on the steep slopes covered by loess (ML soils) overlying basalt. Using the approaches used by Harp et al. (2006) in Seattle, a comprehensive landslide database from the 1996 landslide event in Portland was combined with an accurate LiDAR basemap to produce a susceptibility map. The whole map was based on the infinite slope model. Sixteen maps were produced using the different variables for the slopes: phi ranged between 28 and 32 degrees (with an average of 27.8 +/- 3.8 degrees); cohesions of 270 and 320 psf (with an average of 270 +/- 250 psf), average thicknesses of 4 and 7 based on an average scarp height of 5 +/- 4 ft. (with a range of 1-20 ft). The model is especially sensitive to thicknes of the slides. The factor of safety (FOS) maps were broken into three categories (0-1 for high susceptibility, 1-1.5 for moderate susceptibility, and >1.5 for low susceptibility. The map that had the best correlation with the data had the following data: phi of 28 degrees, cohesion of 270 PSF, and a thickness of 7 ft. The mapping was done at the 1:6000 scale with north shadows.