IDENTIFYING LOCATIONS THAT ARE PRONE TO LANDSLIDES FOR THE NATIONAL RISK INDEX, BOTH THE SOURCE LOCALITIES AND THE DOWNSLOPE IMPACTED AREAS

The National Risk Index (NRI) is an interactive mapping and data-based interface that enables users to identify and understand natural hazard risks with minimal technical knowledge. Started in 2016, and available at hazards.fema.gov/nri/, the NRI includes analyses of 18 different natural hazards from flooding to earthquakes. One of the factors, landslides, identifies areas of the country that are prone to landslides and the infrastructure potentially impacted. Currently, the database identifies landslide risk in the contiguous US and Hawaii (not Alaska) based on 16,000 historical landslides occurrences combined with the landscape slope and relief at a 90-meter scale. For the upcoming release of version 1.20 of the NRI, an improved and more detailed landslide susceptibility identification methodology was developed. Landslides can be characterized using two separate but related components: (1) the source area where the land initially gives way and (2) the downslope impacted areas. The methodology for determining the source localities was adapted from the 2020 study identifying landslide prone localities in Puerto Rico by Hughes and Schulz (2020). The Puerto Rico study combined not only slope, but eight other risk factors including land cover, soil properties, and precipitation amounts. The Puerto Rico approach was analyzed and adjusted for use on a national scale, including limiting the analysis to four primary risk factors: slope, land cover, precipitation, and soil properties at a 30-m resolution. A wider inventory of historical landslide occurrences was also identified from the USGS, containing over 80,000 events. The historical locality information was combined with each of the risk factors to determine landslide prone source areas based on a scale of five categories: Very High, High, Medium, Low, and Very Low. These landslide prone source areas were then used to identify the potential downslope impacted localities from each source locality, based on the landscape slope and direction of landslide flow. Overall, the updated and improved methodology to identify landslide prone localities for NRI v1.20 has greatly improved the resolution and identification of the population and infrastructure potentially impacted by future landslides.