An Example from the Landslide Hazard Zonation Project: The Garrard Creek Watershed Administrative Unit (WAU), Lewis, Grays Harbor, and Pacific Counties, Washington

Landslides have been studied using several approaches – inventory, heuristic, statistical, and deterministic. Each point of view has advantages and limitations for assigning landslide potential. This poster presents a comprehensive study of the landslide hazard zonation of the Garrard Creek Watershed Administrative Unit (WAU) by using the Landslide Hazard Zonation Protocol of the Washington State Department of Natural Resources (DNR), Forest Practices Division. The Garrard Creek watershed is located on the northern extent of the Coast Ranges physiographic province and covers 20795.8 acres. The study area is prone to landsliding due to a combination of several factors such as high rain fall during the wet season, rock types, high degree of weathering, and steep slopes. With the goal of identifying areas within the Garrard Creek watershed that have a low, moderate, or high risk of landslides, a representative sample of 539 mass-wasting features were inventoried from aerial photography, LiDAR imagery, and field investigations. This analysis divided the watershed into 11 mass wasting landforms that were assigned slope stability hazard ratings from low to very high. The overall hazard rating for Garrard Creek WAU was found to be moderate. The data developed during this project consist of a multi-temporal landslide inventory map, a landform hazard zonation maps, and a report detailing the landslide hazard findings. The data for this watershed and other completed WAUs are available to the public via DNR, Forest Practices Division website: http://www.dnr.wa.gov/BusinessPermits/Topics/LandslideHazardZonation/Pages/fp_lhz_completed.aspx