Landslide Susceptibility Map for Shallow Landslides for the West Hills of Portland, Oregon Using GIS and Lidar

The West Hills of Portland, Oregon (45 degrees 33' North, 122 degrees, 45' West) are prone to landslides on the steep slopes covered in loess ( ML soils) overlying basalt bedrock. The loess is the problematic unit. The relatively simple geology, the propensity for landslides, an accurate LiDAR base map and a landslide database make the West Hills an ideal place to produce a landslide susceptibility map for shallow (25 ft. or 7.6 m depth) landslides using the approach of Harp et al. (2006). The landslide database was used to correlate the susceptibility map with the actual area of the landslides to ascertain the accuracy of the output susceptibility maps. The output maps were calculated using an algorithm based on the Infinite Slope Model modified by Harp et al. (2006)using the Portland Hills silt data. The soil values used for the shear strength of the loess were 28 and 32 degrees for the friction angle phi (27.8 +/- 3.8 degrees with a range of 18-32 degrees), cohesions of 270 and 320 psf (cohesion = 270 +/- 250 psf with a range of 0-698 psf), and landslide thickness of 4 and 7 ft. (t=4 +/- 3 ft based on the scarp height of 5 +/- 4 ft). The model was especially sensitive to changes in thickness, and to a lesser extent phi and cohesion.