DEFINING AND DELINEATING ANCIENT LANDSLIDE DEPOSITS DURING HAZARD MAPPING IN VIRGINIA

In 2017, the Virginia Department of Energy, Geology and Mineral Resources Program received funding through a FEMA Pre-Disaster Mitigation Grant to create landslide hazard maps for western Albemarle and Nelson Counties in the Virginia Blue Ridge. The area experienced significant storm events in 1969 and 1995 that generated thousands of debris flows and has numerous areas of unconsolidated colluvial deposits. Accurately delineating modern landslides (those from 1969 to the present) was straightforward as headscarps and tracks are sharply defined in 1-meter LIDAR-derived hillshades and slopeshades. Landslide types were identified using the Cruden and Varnes (1996) classification system for material and movement. However, identifying and defining ancient landslide deposits was more complicated. Traditional geologic mapping often captures all surficial geologic material in an area, including those formed by alluvial and gradual colluvial processes. As this project only focused on potential hazards from various types of landslides, deposits resulting from these processes were omitted from the mapping. Older, extremely weathered deposits, generally mapped as high elevation debris fans on 1:24,000-scale geologic maps, were also excluded. Both deposits and modern landslides were digitized using ESRI ArcPro at 1:6,000. Deposit mapping captured areas defined as “debris fan” (n=69) and “colluvium undifferentiated” (n=726), collectively referred to as “debris deposits”, “relict landslides” (n=96), and “talus” (n=3). Debris deposits are low-relief accumulations of unconsolidated bouldery material representing many landslide events over geologic time. They form at the base of steep mountain slopes and have an obvious uphill source. LIDAR derived 10-foot (3-meter) topographic contours are straight and broad along the base of the deposits. Deposit polygons were ended uphill in colluvial hollows were material accumulation ends (or is unmappable at the map scale) and where contours become tightly curved. Deposits are considered a moderate hazard for development as oversteepened cuts and embankments in unconsolidated sediment may be more prone to failure. First and second order streams along and within the margins of these deposits are also likely locations for future debris flows.