USING A GEOGRAPHIC INFORMATION SYSTEM GIS TO MODEL SLOPE INSTABILITY AND DEBRIS FLOW HAZARDS IN THE

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USING A GEOGRAPHIC INFORMATION SYSTEM (GIS) TO
MODEL SLOPEINSTABILITY 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
Introduction Catastrophic, storm-generated mass
wasting is a destructive erosional process in the
portion of the Southern Appalachians that extends
through North Carolina. Steep slopes, a thin soil
mantle, and extreme precipitation events all
increase the risk of slope instability and
failure. Since the early 1900s, several intense
storms and hurricanes have tracked through the
French Broad watershed initiating hundreds of
debris flows and causing severe flooding. In the
Appalachian Mountain chain, it has been estimated
that as many as 1,700 debris flows occurred in
the 20th century, killing at least 200 people and
destroying thousands of acres of farm and
forested land.

• Watershed Area 7,000 km²
• Counties Transylvania, Henderson, Buncombe,
  Madison, and Haywood and portions of Avery,
  Mitchell, and Yancey
• Major Rivers French Broad, Nolichucky, Toe, and
  Pigeon Rivers
• Population 426,000

• Research Objectives
• To investigate and predict the spatial
  distribution of regional slope instability within
  the French Broad watershed by analyzing the
  results of the GIS-based modeling application
  SINMAP (Stability INdex MAPping) (Pack, et al.,
  1998)
• To identify triggering mechanisms particular to
  the area that influence instability and failure
• To determine which soil and geologic units are
  the most prone to instability and those that are
  the most stable, in general and
• To study the effect of parameter variation on the
  modeling program.

History The mountains of North Carolina have a
long history of producing destructive debris
flows through the 20th century. The prior record
is incomplete, as the evidence and scarring of
previous debris flows has been quickly removed by
vegetation and weathering. By reviewing both
historical rainfall events that have triggered
debris flows, and the locations of these debris
flows, predictions can be made about future
debris-flow locations and the triggering
mechanisms that are common to the region.
SINMAP 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
(30-meter and 10-meter DEMs) 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.
The final output of a SINMAP session is a series
of maps that define areas of potential terrain
instability shaded green areas are considered
stable while dark red areas have a high
probability of failure based on parameter inputs.
Debris flows are swift-moving mass-wasting events
that occur predominantly in shallow,
coarse-grained soil on steep slopes during
periods of exceptionally heavy precipitation.
They are particularly dangerous to life and
property because they travel quickly, often
strike without warning, and may be of sufficient
density to move large boulders, trees, and cars
(Cruden and Varnes, 1996).
SINMAP Stability Index map for 125mm recharge and
using a 30-meter DEM