LANDSLIDE INVENTORY, SUSCEPTIBILITY AND RISK ANALYSIS OF THE EUGENE-SPRINGFIELD REGION, LANE COUNTY, OREGON

The Oregon Department of Geology and Mineral Industries (DOGAMI) partnered with the cities of Eugene and Springfield, and Lane County, to conduct a Regional Landslide Susceptibility and Risk Analysis project, funded by the Federal Emergency Management Agency’s Risk MAP program. The study area covers over 230 mi² and straddles the Coast Range and the Western Cascades physiographic provinces where they meet at the southern end of the Willamette Valley, the center of much of the population and agricultural production of Oregon. City planners and emergency managers, in particular, sought aid to better understand the landslide risk in their jurisdictions. Using DOGAMI’s Protocol for Inventory mapping of Landslide Deposits from Light Detection and Ranging (lidar) Imagery: Special Paper 42 method, along with both shallow and deep landslide susceptibility mapping methods (following protocol in Special Papers 45 and 48, respectively), we produced inventory maps and two types of susceptibility maps. We distinguish shallow and deep landslides at 15 foot depth. Shallow susceptibility maps highlight areas of low, moderate and high risk, based on slope angle, landslide inventory of <15 foot deep landslides, and an engineering geologic map, used to create a Factor of Safety map. Deep susceptibility is based on geology, slope, slope aspect, and mapped deep landslide deposits from the inventory, divided into high, moderate and low susceptibility. Preliminary results include a total of 630 mapped landslide, rock fall and debris flow fan deposits, with 22 attributes describing type of movement and size for each. We used these landslide hazard maps to conduct a risk analysis, with building footprints, tax lot values, land use, distributed population, roads and critical facilities. Here we present the exposure of these asset classes to the nine combinations of hazard types and risk, in monetary value, population, and building counts. We used Hazus-MH to simulate various earthquake scenarios, tallying damage and losses due to earthquake-triggered landslides, specifically. We are working with local, county, and other governmental entities to incorporate the maps into planning tools, such as their Natural Hazard Mitigation Plan and Comprehensive Plan.