USING A GEOGRAPHIC INFORMATION SYSTEM (GIS) TO MODEL SLOPE INSTABILITY AND DEBRIS FLOW HAZARDS IN THE FRENCH BROAD RIVER WATERSHED, NORTH CAROLINA Anne Carter Witt Dept. of Marine, Earth and Atmospheric Sciences, Box 8208, North Carolina State

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USING A GEOGRAPHIC INFORMATION SYSTEM (GIS) TO
MODEL SLOPE INSTABILITY AND DEBRIS FLOW HAZARDS
IN THE FRENCH BROAD RIVER WATERSHED, NORTH
CAROLINAAnne Carter WittDept. of Marine, Earth
and Atmospheric Sciences, Box 8208, North
Carolina State University, Raleigh, N. C. 27695,
awitt48662_at_aol.com
Results SINMAP model runs have been completed for
the watershed using a 30-meter digital elevation
model (DEM) and 119 landslide point locations.
Results using the programs default parameters
were compared with those for four recharge events
(50mm/d, 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 tended to
underestimated areas of instability. By adjusting
soil parameters, 93 (111/119) of the mapped
landslide locations occurred in the higher
thresholds for instability. Unstable areas
accounted for 63 of the watershed, or about
4,421km².
Recently, 10-meter DEMs have become available for
Western North Carolina, distributed through The
National Map Seamless Data Distribution System
(http//seamless.usgs.gov/). As SINMAP is highly
sensitive to the accuracy of the DEM, 10-meter
DEMs are an improvement from the coarser 30-meter
DEMs and should provide a finer resolution when
predicting instability. Currently, SINMAP model
runs have been completed for Haywood County using
the default settings and a moderate recharge
event (125mm/d). As with the 30-meter DEM, the
default parameters underestimated areas of
instability. For a recharge rate of 125mm/d, 21
of the 22 mapped landslide locations occurred in
the higher thresholds for instability. Unstable
areas accounted for 73 of the county, or about
1,034km².

• Overall Evaluation and Conclusions
• SINMAP (and the infinite slope equation) seems to
  be most sensitive to adjustments to two basic
  parameters transmissivity and cohesion (root and
  soil). Adjustments to other variables (recharge,
  soil friction angle, soil density) showed little
  change in the predicted areas of unstable land.
  So even as recharge increased there was little to
  no change in predicted instability.
• SINMAP seems to overpredict the amount of
  instability in the watershed.
• In the field, it was noted that bedrock
  foliation, fracturing, and jointing tend to
  concentrate groundwater flow in the watershed and
  failure tends to occur along these planes of
  weakness. SINMAP cannot take this type of
  groundwater movement into account.
• SINMAP is adequately suited for preliminary slope
  stability studies on large areas (i.e.
  county-wide).

References Cruden, D.M. and Varnes, D.J., 1996,
Landslide Types and Processes, in Turner A.K.,
and Schuster, R. J., eds., Landslides
Investigation and Mitigation - Special Report
247 Washington, D.C., Transportation Research
Board, National Research Council, p. 36-75. Pack,
R.T., Tarboton, D.G., and Goodwin, C.N., 1998,
Terrain stability mapping with SINMAP, technical
description and users guide for version 1.00
Terratech Consulting Ltd. Salmon Arm, B.C.,
Canada, Report Number 4114-0, 68 p.