PRELIMINARY RESULTS OF RECENT LANDSLIDE SUSCEPTIBILITY MAPPING IN ALBEMARLE AND NELSON COUNTIES, VIRGINIA

In 2018 and 2020, the Virginia Department of Energy was awarded Pre-Disaster Mitigation grant funds from the Federal Emergency Management Agency, through the Virginia Department of Emergency Management (VDEM), to complete a landslide inventory and susceptibility mapping in Albemarle and Nelson Counties. The eastern Blue Ridge of these counties were affected by Hurricane Camille in 1969 and a strong series of storms in 1995 which generated thousands of debris flows. Landslides from both storm events have been studied and mapped by the U.S. Geological Survey (1973, 1996, 1999), however recently available 1-meter resolution LIDAR has greatly improved the accuracy of mapping efforts. Inventory mapping of recent (1969-present) and ancient landslide features is complete and > 7900 recent landslides have been identified. Approximately 89% of these landslides occurred during Hurricane Camille and were primarily debris flows and slides. Initial susceptibility mapping for the western half of the study area was completed and delivered to VDEM in 2022. These maps show potential initiation areas for shallow, translational slope movements (i.e., debris flows) during a high-rainfall event as a color-coded map. This map was produced using a series of ESRI geoprocessing tools to calculate a weighted overlay raster. Parameter values for the raster were derived from topographic LIDAR data (curvature, slope), digital soil data (cohesion, soil depth), and geologic data. The final output raster was then assigned into three susceptibility zones (high, moderate, low) based on calculated parameter value ranges and known landslide occurrences. Parameter ranges were adjusted to maximize the number of landslide locations per unit area captured in high-hazard zones. High and moderate hazard zones from the susceptibility map, average debris flow width and length, and LIDAR topographic data were then used to produce areas of predicted debris flow pathways using hydrologic flow path tools in ESRI ArcPro. Additional feature classes indicate where public infrastructure (roads, railroads, etc.) intersect these pathways and may be at risk during a future rainfall event. Susceptibility mapping for the western study area has provided a “proof of concept” for the remaining eastern study area and will be delivered to VDEM in September 2024.