COMMUNITY-BASED GEOLOGIC HAZARD MAPPING IN NORTHWESTERN NORTH CAROLINA

Geologic hazards in the northwestern mountain counties of North Carolina and adjacent parts of Tennessee are primarily slope failure and flood hazards. Slope failures are initiated on steep slopes and move downhill to form landslide deposits in run-out zones. Using these various settings, we developed and use a six-unit map that includes three slope failure zones and three zones where previous landslide deposits or flood hazards are present. The three slope failure zones include a Green zone of gentle slopes (<20%) outside the new 100 year floodplain; an Orange zone of moderate slopes (20% - 30%), where slope aspect does not parallel failure planes; the Red zone —steep slopes > 30% in which there is a higher probability of slope failure due to joint/foliation orientations. The other three hazard zones include an Orange landslide zone — a zone between 20% and 30% slope that is underlain by landslide deposits; a Red landslide zone with slopes > 30% underlain by landslide deposits; and a flood zone. The distinction between red and orange zones is modified based on fracture orientation relative to the slope and rock type, i.e. slopes with fractures oriented to facilitate wedge failures and S-surface failures and underlain by weak rock may be upgraded from orange to red based on these factors. Slopes lacking such conditions may be down graded from red to orange. Floodplains may be divided into floodways and floodway fringe zones where available data make such a subdivision possible.

The mapping process includes outlining slope steepness zones and floodplains using DEM (where available) and site evaluations, field mapping of fracture orientations, and a fracture analyses using stereonets. Hazard zones are outlined and assigned a hazard designation based on slope steepness and aspect, fracture orientations and flood potential. We have applied these techniques to developing geologic hazard maps for the communities of Seven Devils, NC and Banner Elk, NC.