PROGRESS IN DEVELOPING A METHOD FOR SEMI-QUANTITATIVE SLOPE STABILITY ASSESSMENT: THE WASHINGTON STATE LANDSLIDE HAZARD ZONATION PROJECT 2001-2009

For the past ~8 years the Washington State Landslide Hazard Zonation Project (LHZ) has created a GIS database of landslides and landforms that are used as a screening tool to assess the potential for landslides in areas subject to forest management. State and local agencies, forest industry, and the public use the database to apply the Forest Practice Rules. To date 59 reports have been completed [http://www.dnr.wa.gov/BusinessPermits/Topics/LandslideHazardZonation/Pages/fp_lhz_completed.aspx].

LHZ has utilized previous mass wasting modules of Watershed Analysis (WSA) methodology and enhanced the process by: 1) using a quasi-statistical approach in delineating landforms, 2) establishing a consistent approach to mapping and describing landslides and landforms, and 3) involving diverse stakeholders through an external review process. The overall result is a more consistent approach to mapping landslides and creating forest practice sensitivity hazard zones. Process details are described in LHZ Protocol Version 2.1 [http://www.dnr.wa.gov/Search/Results.aspx?k=landslide%20hazard%20zonation].

The LHZ mapped 20,175 (117,059 ac) landslides and delineated landforms on ~2.5 million acres within ~70 watersheds throughout the state. These data can be utilized in other types of investigations to assess impacts of storm events and differentiate contributing factors when delineating unstable slopes. For example, Sarikhan, (GSA Abstract, 2009) has included these data into a statewide database of all landslides mapped through LHZ and other studies to develop a statewide landslide rate. The above normal storm events in December 2007 and January 2009 produced large numbers of landslides in some watersheds without WSA mass wasting prescriptions or hazard delineations. This demonstrates the ongoing need for LHZ products. These recent storms also provide an opportunity to evaluate and refine the LHZ protocol by studying the distribution of new landslides within designated landforms mapped through the LHZ protocol. Landslides are dynamic and continue to shape the landscape; we should continue the process of mapping landslides and creating hazard zones to assist forest management.