Using a GIS (Geographic Information System) to Model Slope Instability and Debris Flow Hazards in the French Broad River Watershed, North Carolina
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Date
2005-04-28
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Abstract
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, slope movement and failure. Since the late 1800's, several intense storms and hurricanes have tracked through the French Broad watershed initiating thousands of debris flows and causing severe flooding. Studying the history of debris flows has identified triggering mechanisms that are particular to North Carolina and the recurrence interval of these events.
This study was initiated to investigate and predict the spatial distribution of regional slope instability within the French Broad watershed by comparing the results of two GIS-based modeling applications: SINMAP (Stability Index Mapping) and SHALSTAB (Shallow Landsliding Stability Model). As extensions to ArcView® 3.x, SINMAP and SHALSTAB use a modified form of the infinite slope equation to compute and map slope-instability by calculating either a factor of safety (SINMAP) or the critical steady-state rainfall intensity necessary to trigger slope instability (SHALSTAB). In both models, topographic slope is derived from digital-elevation data while parameters for soil and climate are considered more variable and can be adjusted to better match existing conditions. An inventory of actual debris flow locations, collected from aerial photography, field reconnaissance, and a literature review of historic mass wasting events, are used to verify model results.
SINMAP model runs have been completed for the watershed using a 30-meter and 10-meter digital elevation model (DEM) and 142 landslide point locations. Results using the program's default parameters were compared with those for four recharge events (50, 125, 250, and 375 mm/d). In the latter, parameters for soil density, cohesion, internal soil friction angle, and transmissivity were adjusted to better match existing watershed conditions.
As with the SINMAP model, SHALSTAB was used to model instability using both a 10-meter and 30-meter DEM. Limitations in the SHALSTAB program only allow smaller (county-size) DEMs to be processed. Because of these limitations Haywood County was chosen for several model runs in SHALSTAB for comparison to the SINMAP results. Parameters for soil density, soil depth, cohesion, and soil friction angle were adjusted and results were compared to 23 mapped debris flow locations.
The modeled results for the default SINMAP and SHALSTAB parameter values underestimate the extent of instability in the study area. By adjusting soil parameters, SINMAP calculated 88% -to- 94% of the inventoried landslides would fall into the "lower threshold", "upper threshold", and "defended" stability classes. Generally, predicted areas of unstable land did not change, even as recharge increased.
SHALSTAB calculated 91% to 100% of the mapped debris flows would occur in the three most unstable stability classes for low values of soil friction angle (26°) and cohesion (0). Overall, SHALSTAB seems to over-predict areas of instability for these values. An increase in cohesion or soil friction angle decreases the amount of land predicted as unstable (88%) but increases the landslide density. When the results of the two programs are compared directly, the results are very similar. Visually the SHALSTAB results seem to cluster while the SINMAP results are more dispersed.
In the field, it was noted that bedrock foliation and fracturing concentrate groundwater flow in the watershed and failure tends to occur along these planes of weakness. Neither model takes into account either antecedent moisture or the effect that geologic structure can have on concentrating groundwater flow.
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French Broad, North Carolina, SHALSTAB, SINMAP, debris flow, landslide
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Degree
MS
Discipline
Marine, Earth and Atmospheric Sciences
