Shallow landslides are significant natural hazards in Oregon, and identification of areas susceptible to future landslides is a critical step in reducing risk. Recent advances in identification of areas susceptible to shallow landslides are mostly based on geographic information system (GIS) calculations of the slope stability using the infinite slope equation. This technique was further improved with high-resolution light detection and ranging (LiDAR)–based digital elevation models (DEM) converted to very accurate slope data as input into the GIS models. However, these models still underestimate and overestimate the susceptibility in certain areas compared to past landslide events and field observations. One significant overestimation we noted occurs in regionally flat areas with isolated steep slopes that have very little relief. We developed a method to remove these isolated overestimated areas using a neighborhood analysis with a maximum relief of 1.22 m (4 ft). Because landslides that originate on the steep slope may extend back into the flat area above the slope, or out onto the flat area at the toe of the slope, we applied a 9 m (30 ft) buffer (twice our defined depth to failure for shallow landslides) for all of the areas with a calculated factor of safety (FOS) less than 1.5. We tested the methods on three landslide inventory databases examining two main criteria: (1) capture rate (overall and individual landslides) and (2) reduction in total map area susceptibility coverage while maintaining a high capture rate. We found the two methods maintained a capture rate between 90% and 99% while at the same time reducing the total map area susceptibility zones from 64% to 42%.

Abstract Most of Oregon has been mapped as having low radon potential. Overall, elevated levels of indoor radon are found in only 4% of Oregon homes compared to 8% of homes nationally. Most maps show Portland, Oregon’s largest city, as having low to moderate potential for indoor radon. Using data collected by the Oregon Health Division’s Radiation Protection Services Section, we found Portland to have elevated radon values in 22% of homes. A radon potential map has been produced for 39 zip code regions of Portland based on the rank sums of maximum radon values, average indoor radon values, and the percentage of homes with radon values >4 pCi/1. Eight zip codes have high, 15 moderate, and 16 low radon potential. The maps were constructed by taking the indoor radon values from 1,135 homes, categorizing the data in terms of radon values, and plotting the points geographically by zip codes. Trends became obvious when geologic maps were compared to the areas of radon potential. Most of the high potential sites lie on highly permeable Missoula flood sediments. The data and map of radon potential were presented to the public through television and radio interviews and newspaper articles in June of 1994. The public, already familiar with the zip codes, could understand how to use the map. People in high to moderate categories were strongly encouraged to test their homes. A telephone number was given to receive free information on radon testing and mitigation.