USING A GEOGRAPHIC INFORMATION SYSTEM (GIS) TO MODEL SLOPE INSTABILITY AND DEBRIS FLOW HAZARDS IN THE FRENCH BROAD RIVER WATERSHED, NORTH CAROLINA

Catastrophic, storm-generated mass wasting is a destructive erosional process in the portion of the Southern Appalachians that extends through western North Carolina. Steep slopes, a thin soil mantle, and extreme precipitation events all increase the risk of slope instability and failure. Since the early 1900’s, several intense storms and hurricanes have tracked through the French Broad watershed initiating hundreds of debris flows and causing severe flooding.

This study was initiated to investigate and predict the spatial distribution of regional slope instability in the French Broad watershed using the GIS-based modeling application SINMAP (Slope Instability MAPping). As an extension to ArcView® 3.x, SINMAP computes and maps a slope-stability index by calculating factors of safety using a modified form of the infinite slope equation. Topographic slope is derived from digital-elevation data while parameters for soil and climate are considered more variable and are entered as upper and lower bounded values. Actual debris flow and slide locations are used to verify the model results.

SINMAP model runs have been completed for the watershed using a 30-meter digital elevation model (DEM) and 56 landslide point locations collected from aerial photography, field reconnaissance, and a literature review of historical mass wasting events. Results using the program’s default parameters were compared with those for three recharge events (125mm/d, 250mm/d, and 375mm/d). In the latter, parameters for soil density, internal soil friction angle, and transmissivity were adjusted to better match existing watershed conditions. Default parameters underestimated areas of instability: 17% of the watershed (1196 km²) was predicted to be unstable and 50% of known landslides (28/56) occurred within these areas. By adjusting soil parameters, 86% (48/56) of the mapped landslide locations occurred in the higher thresholds for instability. Unstable areas accounted for 60% of the watershed, or about 4160 km². Generally, predicted areas of unstable land did not change, even as recharge increased. In the field, it was noted that bedrock foliation and fracturing influence groundwater flow in the watershed and failure tends to occur along these planes of weakness. Further adjustments to the model are needed to take this into account.