LANDSLIDE HAZARD AND RISK ANALYSIS AT THE OREGON DEPARTMENT OF GEOLOGY AND MINERAL INDUSTRIES

Landslides are a significant hazard in Oregon causing millions to 100s of millions of dollars in damage annually. In order to improve our understanding of the hazard and risk, and thus a better ability to compare and prioritize, four main tasks are critical to reach this goal and include: creation of a landslide inventory, creation of landslide susceptibility maps, creation or acquisition of asset data, and risk analysis. DOGAMI has completed several projects with communities in Oregon including the City of Astoria, the communities around Mt. Hood, and a portion of a highway corridor connecting Portland with the coast. Multiple methods of risk analysis have been used including exposure and FEMA’s HAZUS-MH. All of the dataset creation and analysis are performed in a Geographic Information System (GIS) primarily using lidar-based topographic data. The combination of the GIS and lidar allows for accurate and fine-scale resolution of the hazards and the assets, which make the results of the risk analysis more credible and thus more likely to be used. Some datasets, such as the landslide inventory, buildings, and primary electric transmission system components are significantly improved when delineated using the lidar. For example, the mostly pre-lidar based landslide inventory for the Portland metropolitan area has roughly 1,000 landslide polygons and our new lidar-based inventory has over 7,000. In the Mt. Hood project, over 47,000 buildings were accurately located and attributed with generalized zoning/land use such as residential, commercial, or public and the dollar value of the building and the surrounding land. In the communities around Mt. Hood, we found 1,250 buildings and combined land and buildings worth $300M reside on debris flow fans, compared to 235 and $93M respectively located within the 500-year flood hazard zone. HAZUS-MH has been used primarily to examine the potential damage from earthquake triggered landslides during major crustal and subduction earthquakes. In Astoria, HAZUS analysis results showed 50% of the estimated losses in a major earthquake will be caused by the landslides compared to the earthquake shaking alone. We have found that communities in Oregon seem to respond better when presented with hazard and risk than with hazard alone.