Marion County, Oregon

Oregon has a variety of natural hazards, including earthquakes, landslides, flooding, wildfires, volcanoes, channel migration, tsunamis, and coastal erosion. The keys to managing the risks are, one, having enough information about the hazard and, two, taking the proper steps to reduce the risk from that hazard.

This story map, which is a summary of the natural hazard risk assessment report, was prepared for the communities of Marion County, Oregon, with funding provided by the Federal Emergency Management Agency (FEMA). It describes the methods and results of the natural hazard risk assessments performed in 2021 and 2022 by the Oregon Department of Geology and Mineral Industries (DOGAMI) within the study area. The risk assessment quantifies the impacts of natural hazards to these communities and enhances the decision-making process in planning for disaster. 

Click on this  LINK  to access the full risk assessment report at the DOGAMI website.

For this risk assessment, DOGAMI applied a quantitative approach to buildings and population. We limited the project scope to buildings and population because of data availability, the strengths and limitations of the risk assessment methodology, and funding availability. We did not analyze impacts to the local economy, land values, or the environment.

Depending on the natural hazard, we used one of two methodologies: loss estimation or exposure. Loss estimation was modeled using methodology from Hazus®-MH (Hazards U.S., Multi-Hazard), a tool developed by FEMA for calculating damage to buildings from floods and earthquakes. Exposure involves categorizing buildings based on their location relative to various hazard zones. To account for impacts on population (permanent residents only), estimated population for each community in 2020 were associated with residential buildings.

Data Sources and Methods

DOGAMI used a deterministic scenario method in Hazus-MH for this study to estimate losses on a building-by-building basis. The deterministic scenario is based on a specific seismic event, such as a Cascadia Subduction Zone (CSZ) magnitude (Mw)-9.0 event. The Mt. Angel Fault Mw-6.8 was selected as the most appropriate for communicating earthquake risk for Marion County. The site-specific information used in the scenario (coseismic landslide, liquefaction, and soil class amplification) was developed by Madin and others (2021) and the ground motion data was calculated within Hazus.  

Data Sources and Methods

A typical method for determining flood risk is to identify the probability of flooding and the impacts of flooding. The flood depth grids provided by FEMA were used in this risk assessment to determine the level to which buildings may be impacted by flooding. Flood depth grids for two flooding scenarios (100- and 500-year) were used in Hazus-MH for loss estimations and, for comparative purposes, exposure analyses.

Data Sources and Methods

The data from the combined Statewide Landslide Susceptibility Map (Burns and others, 2016) and new landslide mapping (Calhoun and others, 2020) were used in this report to identify the general level of susceptibility of a given area to landslide hazards, primarily shallow and deep landslides. Building and critical facilities data were overlaid on landslide susceptibility zones to assess the exposure for each community. The total dollar value of exposed buildings was summed for the study area and the number of people threatened by landslides was also estimated.    

Data Sources and Methods

Channel migration is a dynamic process by which a stream’s course may change over time due to changes in climate and precipitation patterns, land use, and how we manage our waterways.  To assess the exposure for each community, we overlaid buildings and critical facilities on the channel migration zones (CMZ) developed by Appleby and others (2021) for the Pudding River and the Santiam and North Santiam Rivers. To assess the exposure within each community, we overlaid buildings and critical facilities on the 30-year erosion hazard area (EHA) within the CMZ. While there is risk throughout the CMZ, we chose to examine the structures within the 30-year EHA, because it represents the area of greatest probability of being at risk from channel migration during the next 30 years. The total dollar value of exposed buildings was totaled for the study area, and the number of people potentially displaced from the 30-year EHA was estimated. 

Data Sources and Methods

For this assessment, the Overall Wildfire Risk dataset — a dataset included in The Pacific Northwest Quantitative Wildfire Risk Assessment (PNRA) (Gilbertson-Day and others, 2018) — was used to measure the level of risk to communities in Marion County. The PNRA is a comprehensive report that includes a database of spatial information related to wildfire hazard developed by the United States Forest Service (USFS) for the states of Oregon and Washington. The building locations were compared to the geographic extent of the wildfire risk categories. This assessment determined the total dollar value of exposed buildings in the study area and estimated the number of people threatened by wildfire.

Data Sources and Methods

The lahar zones (areas of volcanic debris flow) used in this report were created by Walder and others (1999) and were based on previous volcanic eruptions to estimate the extent of potential lahars on Mount Jefferson. Three nested lahar zones were computed based on an estimated volume of debris that could suddenly flow from Mount Jefferson. For this risk assessment, the locations of buildings and critical facilities were compared to the geographic extent of the lahar hazard area zones to assess the exposure for each community. The total dollar value of exposed buildings was totaled for the study area; the number of people at risk from lahar hazards was also estimated.

The following areas of research are needed to better understand hazards and to reduce community risk to natural hazards through mitigation planning. These research areas, while not comprehensive, touch on all phases of risk management and focus on awareness, planning, emergency response, mitigation funding opportunities, and hazard-specific risk reduction activities.