COMMUNITY RISK REDUCTION THROUGH LIDAR-BASED LANDSLIDE INVENTORY MAPPING IN EASTERN OREGON

Landslides are common in Oregon, representing a serious natural hazard. A first step to reduce landslide-related damage and losses is by knowing where previous landslide activity has occurred and then take appropriate risk reduction actions. The Oregon Department of Geology and Mineral Industries (DOGAMI) utilizes high-resolution lidar to map the extent and distribution of landslide deposits and provides information such as failure type and age. Inventory maps have been developed for many communities in densely vegetated western Oregon, but few have been created for sparsely vegetated arid regions east of the Cascade Range. DOGAMI received Federal Emergency Management Agency Cooperating Technical Partners (FEMA CTP) funding in 2020 to support lidar-based landslide inventory mapping in Wasco and Grant counties, both located in eastern Oregon. Previous studies cataloged 135 landslides in Wasco and 81 in Grant. New lidar-based mapping delineated 95% more landslides, with 2,693 in Wasco and 1,507 in Grant. New mapping also provides a significantly greater level of confidence and detail, both in terms of spatial extent and landslide characteristics. The activation, reactivation, and distribution of landslides in Wasco and Grant counties is related to geologic conditions such as rock strength, structure, and contacts, but also to steep slopes, accumulations of colluvium, deep valley incision, precipitation, infrequent but very intense or long duration rain or rain-on-snow events, earthquake shaking, certain human activities, or some combination of these factors. During the projects, several community brainstorming meetings were held and landslide risk reduction actions were developed. High priority actions include: 1) use of new landslide inventory maps to determine where future site-specific geotechnical analysis must be performed prior to development, 2) public education and awareness, and 3) updating land use codes. We conclude lidar-based landslide inventory mapping results in significantly improved outcomes over other methods, even in areas of very sparse vegetation, such as eastern Oregon. The substantial increase in landslides identified in Wasco and Grant counties and the improved understanding of the hazard, underscores the critical need to expand lidar-based landslide mapping and risk reduction activities to other communities in eastern Oregon.