MAPPING LANDSLIDE THRESHOLDS, USING LIDAR IN THE WEST HILLS OF PORTLAND, OREGON

Mapping landslides in western Oregon is problematic due to the heavy vegetation. The West Hills of Portland, Oregon are prone to landslides as there is loess (ML soils) overlying basalt on steep slopes. The geology coupled with high levels of precipitation during the winter and parts of spring lead to high numbers of slope failures. Landslide indicators are hard to distinguish, on traditional aerial photographs or satellite imagery which would be used for inventory maps in areas that aren't populated. LIDAR (light detection and ranging) penetrates the vegetation for a view of the ground surface. The LIDAR data were acquired by Portland State University from the Puget Sound LIDAR Consortium (PSLC) which flew the West Hills area in winter of 2004. In this project we attempted to define large-medium-small landslides in the West Hills, Portland using LIDAR. We created contour lines with different intervals (5, 15, 50 feet) and compared the number of landslides identified in each. Within the different contour intervals we also measured the area of landslide that we were able to delineate. The intervals set at 5 and 15 feet allowed us to see “small” landslides (~5,000 sq ft), as well as the large ones. The 50 foot interval depicted smaller landslides with an area of 20,000 sq ft. While the 5 and 15 feet contour lines were the best, they also took 5- 10 seconds to load every time we panned the screen. We found hill shading wasn't as accurate as contour lines, and it sometimes presented a false image of the area. To maximize the image, we draped contour lines over the hill shade, which seemed to give a better view of what the ground morphology was really like. The 5 foot contour at a scale of 1: 3000 was the best, but the longest to load. We could see some of the same landslides in the 15 foot contour with a scale of 1: 6000, especially with the hill shade image below. We recommend using the 50 foot contour at a scale of 1: 10000and when locating larger slides in areas that are heavily populated, it is difficult to discern the housing complexes with disturbed slopes.