RECENT LANDSLIDE HAZARD AND RISK STUDIES IN OREGON

Landslides are a significant hazard in Oregon. DOGAMI has completed several recent landslide and risk projects with communities in Oregon including the City of Astoria on the coast, the communities around Mt. Hood, and a portion of the Portland region. The primary triggers of new and reactivated landslides include: volcanic activity, prolonged intense rainfall, earthquakes, and man. Multiple methods of risk analysis have been used including exposure and FEMA’s HAZUS. All of the dataset creation and analysis are performed in a GIS primarily using lidar-based topographic data. The combination of the GIS and lidar allows for 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 on risk reduction actions.

In the City of Astoria project, we found 120 landslides, of which 83 were estimated or known to have moved historically. We also ran a HAZUS earthquake scenario with and without co-seismic landslides to see the effect of the landslides triggered by an earthquake. The loss ratio increased from 12% to 21%, indicating the landslide triggered in the earthquake would result in nearly the same amount of damage as the earthquake shaking alone. In the Mt. Hood project, over 47,000 buildings were accurately located and attributed with generalized zoning/land use classes including: 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. In the Portland region study (NW Clackamas County), we located 2,885 landslides which cover 7% of the study area (1/3 of the Portland region). We found more than 7,000 residents and more than 3,000 buildings with a combined land and building value of almost $900M are located on existing large, deep landslides. We have found that communities in Oregon seem to respond better when presented with hazard and risk, rather than with hazard alone.

In our continued lidar-based landslide inventory mapping, we have mapped more landslides in the past 5 years than in the previous 60. Our current database (SLIDO 3.1) has 38,755 landslide polygons and 12,095 historic landslide points.