LANDSLIDE HAZARD MAPPING IN WATAUGA COUNTY, NORTH CAROLINA: METHODS AND RESULTS
Scanned and georegistered archival aerial photography and existing geologic maps were brought into a GIS environment, and used with orthophotography and a LiDAR (light detection and ranging) digital elevation model to map landslides and landslide (mainly debris fan) deposits. We used this preliminary mapping and debris flow susceptibility modeling to target field studies. Data were collected at 2,200 sites across the county. Detailed studies conducted at six debris flow initiation zones included triaxial shear strength and hydraulic conductivity testing on soil. Soil gradation and Atterberg limits testing were performed at 98 sites. These data were used in conjunction with the compiled geologic map to help establish relationships between soil properties and geologic units, which were then used to refine and calibrate the debris flow susceptibility model (stability index map).
Remote sensing, field studies and other sources identified 2,253 landslides, 2,100 of which are landslides (mainly debris flows) from the August 13-14, 1940 storm event. Debris flows account for 77% of the 2,253 landslides mapped. Debris flows occurred throughout the county in 1940 but were concentrated in the Elk Creek reentrant in the Blue Ridge Escarpment, and in highly dissected mountainous terrain near the Fork Ridge fault. Pre-existing landslide deposits cover approximately 68 km2 (8%) of the county. Rockslides occur in cut slopes, mainly in fine-grained metasedimentary rocks of the Grandfather Mountain formation where brittle deformation fabrics overprint mylonitic rocks along the Linville Falls shear zone.