EVALUATING ROOT STRENGTH INDEX AS AN INDICATOR OF LANDSLIDE-PRONE SLOPES IN EASTERN KENTUCKY

Slope angle is one of the primary metrics by which landslide likelihood is assessed even though slopes of equivalent steepness do not experience failure at the same rate. Landslides also occur in areas where low slope thresholds would not predict it, a phenomenon which may be attributable to degraded tree coverage, or low root strength. Until now, regional maps of landslide susceptibility in Kentucky have not included root strength as a variable because it is a highly complex and area-specific factor. Lidar-derived root strength indices may be a viable proxy for true root strength, but these maps reflect vegetation conditions at the time of lidar acquisition so relating low root strength index to slope failure is difficult when landslide age is unknown. A July 2022 storm event triggered over 1000 new landslides in eastern Kentucky over a five-day period, providing an ideally time-constrained dataset to assess root strength indices prior to the landslides. After generating maps of root strength index using 2014 lidar coverage, we examined the distribution of values contributing to the 2022 landslides. Overall, we find that the median root strength index values of the area contributing to these new landslides were ~1/6 that of the maximum values observed and appreciably lower than values observed on comparative intact slopes and that there exists a statistical threshold of root strength index below which even low-gradient slopes are susceptible to landslides. We propose that lidar-derived root strength indices and resulting threshold maps are a valuable GIS layer to incorporate into future assessments of landslide susceptibility.