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ABSTRACT
Witt, Anne Carter. Using a GIS (Geographic Information System) to model slopeinstability and debris flow hazards in the French Broad River watershed, North Carolina.
(Under the direction of Dr. Michael M. Kimberley.)
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
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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
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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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USING A GIS (GEOGRAPHIC INFORMATION SYSTEM) TO MODEL SLOPE
INSTABILITY AND DEBRIS FLOW HAZARDS IN THE FRENCH BROAD
RIVER WATERSHED, NORTH CAROLINA
by
ANNE CARTER WITT
A thesis submitted to the Graduate Faculty of North Carolina State University
in partial fulfillment of the
requirements for the Degree of
Master of Science
MARINE, EARTH AND ATMOSPHERIC SCIENCES
Raleigh
2005
APPROVED BY:
Chair of Advisory Committee ______________________________________ Dr. Michael M. Kimberley
Committee Member ______________________________________
Dr. Jeffery C. Reid
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BIOGRAPHY
Anne Carter Witt was born in Lynchburg, VA on August 10, 1977. She grew
up in Forest, VA and graduated from Brookville High School in 1995. In 1999, she
graduated from Mary Washington College in Fredericksburg, VA with an undergraduate
degree in Geology. Before being accepted into the Masters degree program at North
Carolina State University, she worked for three years as a GIS analyst with Dewberry and
Davis, LLC in Fairfax, VA.
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ACKNOWLEDGEMENTS
I would like to express sincere thanks and appreciation to the following
individuals: Rick Wooten with the North Carolina Geological Survey who provided me
with landslide data and an open discussion on the complexities of SINMAP. His
guidance, patience and knowledge of slope movements and the geomorphology of
western North Carolina are greatly appreciated. Thanks are also extended to Jody Kuhne
from the North Carolina Department of Transportation whose tattered, long-forgotten
landslide map was really the foundation for a digital landslide inventory in North
Carolina.
In addition, I would like to thank Drs. Helena Mitasova and Lonnie Liethold for
their advice and counsel is serving as members on my committee. I would also like to
thank Dr. Mary Schweitzer for being a last minute substitution during my thesis defense
on one of the busiest days of her life. Many thanks are also extended to Dr. Jeff Reid for
his time, guidance, critical comments, and dogged determination to see me finish. Thanks
are also extended to Dr. Michael Kimberley for his editorial expertise, assistance, and
support. He generously donated both a computer and his own office space to help with
my research.
I would also like to express my sincere gratitude to all of my family and friends.
Thanks go out to my fellow graduate students for their sometimes backwards but well-
meaning encouragement and enlightened conversations. I also would like to acknowledge
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best friend, Jamie Gibson. Finally, I would like to thank my parents and brother who
provided the foundation for me to be the best that I could be and instilled a lifelong love
of learning. Thank you for your love and support.
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TABLE OF CONTENTS
Page
LIST OF TABLES .......................................................................................................... vii
LIST OF FIGURES ....................................................................................................... viii
CHAPTER 1: INTRODUCTION.................................................................................... 1
1.1 R ESEARCH AIM AND OBJECTIVES ........................................................................ 3
1.2 SIGNIFICANCE AND SCIENTIFIC APPLICATIONS .................................................... 41.3 LANDSLIDE CLASSIFICATION ............................................................................... 6
1.3.1 Classification of Sharpe (1938) ...................................................................... 7 1.3.2 Classification of Varnes (1978) and of Cruden and Varnes (1996) ............... 8
1.4 COMMON CAUSES OF SLOPE MOVEMENTS .......................................................... 91.4.1 Precipitation ................................................................................................. 10 1.4.2 Human Interference ...................................................................................... 10
1.4.3 Tectonic Activity............................................................................................ 11 1.4.4 Geologic Material and Structures ................................................................ 11
1.5 DEBRIS FLOWS................................................................................................... 12
1.6 DEBRIS FLOWS WITHIN THE BLUE R IDGE AND WESTERN NORTH CAROLINA ..... 14
1.7 SINMAP AND SHALSTAB.............................................................................. 161.8 LIMITATIONS OF R ESEARCH............................................................................... 18
CHAPTER 2: PROJECT SETTING ............................................................................ 21
2.1 I NTRODUCTION .................................................................................................. 21
2.2 GEOLOGY........................................................................................................... 212.3 SOILS ................................................................................................................. 23
2.4 CLIMATE ............................................................................................................ 252.5 VEGETATION...................................................................................................... 26
CHAPTER 3: PRE-HISTORIC AND HISTORIC DEBRIS FLOWS IN WESTERN
NORTH CAROLINA..................................................................................................... 33
3.1 QUATERNARY DEBRIS FLOWS ........................................................................... 33
3.2 MODERN FLOODING AND DEBRIS FLOWS .......................................................... 373.2.1 June, 1876..................................................................................................... 38 3.2.2 May, 1901 ..................................................................................................... 39
3.2.3 July, 1916...................................................................................................... 40 3.2.4 August, 1940 ................................................................................................. 42 3 2 5 N b 1977 44
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4.2.2 Debris Flow Inventory.................................................................................. 68
4.2.3 Soil Data ....................................................................................................... 68
4.2.4 Soil Density ................................................................................................... 69 4.3 SINMAP PARAMETERS ..................................................................................... 69
4.3.1 T/R (Ratio of Transmissivity to Effective Recharge)..................................... 71
4.3.2 Dimensionless Cohesion ............................................................................... 72 4.3.3 Internal Soil Friction Angle.......................................................................... 72
4.4 SHALSTAB PARAMETERS ............................................................................... 73
CHAPTER 5: RESULTS AND DISCUSSION ............................................................ 80
5.1 I NTRODUCTION .................................................................................................. 805.2 DEBRIS FLOW I NVENTORY................................................................................. 80
5.3 SINMAP R ESULTS ............................................................................................ 81
5.3.1 Results – 30-meter DEM............................................................................... 83
5.3.2 Results – 10-meter DEM............................................................................... 84 5.3.3 SINMAP Interpretation................................................................................. 85
5.4 SHALSTAB R ESULTS....................................................................................... 87
5.4.1 Results – 30-meter DEM............................................................................... 89
5.4.2 Results – 10-meter DEM............................................................................... 90 5.4.3 SHALSTAB Interpretation ............................................................................ 92
5.5 SINMAP VS. SHALSTAB................................................................................ 935.6 GEOLOGY AND SOILS ......................................................................................... 94
5.7 JOINTING, FRACTURING AND FOLIATION ........................................................... 97
CHAPTER 6: CONCLUSIONS .................................................................................. 116
REFERENCES.............................................................................................................. 122
APPENDICES............................................................................................................... 133
APPENDIX A: GEOLOGIC U NITS ................................................................................... 134APPENDIX B: GENERAL SOIL DATA............................................................................. 139
APPENDIX C: SLOPE MOVEMENT DATA FOR THE 2004 HURRICANES FRANCES AND IVAN
..................................................................................................................................... 148
APPENDIX D: DEBRIS FLOW I NVENTORY..................................................................... 152
APPENDIX E: SINMAP R ESULTS................................................................................. 157APPENDIX F: SHALSTAB R ESULTS ........................................................................... 161
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LIST OF TABLES
Page
Table 2.1: Table of the average, median, minimum, and maximum precipitation totals
(mm/month) from 1895 to 2001 for the mountains of North Carolina
(NCDC Climate Data Online, 2003)................................................................31Table 3.1: Prehistoric debris flow studies in the southern Blue Ridge and the age-dating
techniques utilized. ..........................................................................................53
Table 3.2: Major storms within the French Broad Watershed and their minimum,average, and maximum precipitation amounts. ...............................................63
Table 4.1: Table of the hydraulic conductivity (K, m/hr), transmissivity (T, m²/hr),and T/R (m) values used for each precipitation threshold (50 mm/d, 125mm/d, 250 mm/d, and 375 mm/d) in the SINMAP analysis. The numbers
in blue are the lower bound values while the numbers in red are the upper
bound values. ...................................................................................................78Table 5.1: The parameters used in all of the SINMAP model runs...................................99
Table 5.2: SINMAP stability index definitions (Pack et al., 1998b). ..............................100
Table 5.3: Mapped instability classes used in the SHALSTAB model analysis. ............104
Table 5.4: Table comparing q/T and log (q/T) values and the precipitation raterequired to initiate instability for soils with a transmissivity of 65 m²d and
17 m²/d (after Dietrich and Asua, 1998). .......................................................105
Table 5.5: Parameters used in the SHALSTAB model runs............................................105Table 6.1: The advantages and disadvantages of SINMAP and SHALSTAB. ...............121
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LIST OF FIGURES
Page
Figure 1.1: The morphology of a typical debris flow found in the Southern Appalachians
(courtesy of the North Carolina Geological Survey, 2003). ............................19Figure 1.2: Threshold precipitation values necessary for producing debris flows in the
southern Appalachian Mountains. Storms likely to start debris flows occur
above the 125 mm/d threshold. Storms with precipitation values higher than250 mm/d are deemed “rare” but do occur in North Carolina (after Eschner
and Patric, 1982). .............................................................................................20Figure 2.1: Location Map of the French Broad Watershed in western North Carolina. ...27Figure 2.2: General geologic map for the French Broad Watershed. Individual geologic
unit descriptions can be found in Appendix A (adapted from North Carolina
Geological Survey, 1985). ...............................................................................28Figure 2.3: General soil map for the French Broad Watershed. Individual soil descriptions
can be found in Appendix B (adapted from U.S. Department of Agriculture,
1998). ...............................................................................................................29
Figure 2.4: The U.S. Department of Agriculture guide for the textural classification of soils. This guide is only for soils with a particle size of less than 2 mm in
diameter. A rock fragment modifier (gravelly, cobbly, stony, bouldery)
prefaces the textural name if particles larger than 2mm compose more than15% of the soil (Buol et al., 2003)...................................................................30
Figure 2.5: Graph based on the data from Table 2.1 (NCDC Climate Data Online,
2003). ..............................................................................................................31Figure 2.6: Average annual precipitation in inches within the French Broad Watershed.
(Adapted from data provided by North Carolina Center for GeographicInformation and Analysis map server (http://204.211.135.111)). ...................32
Figure 3.1: Old slope movement deposit along a private drive in Maggie Valley, North
Carolina (age not determined). There is significant soil development in this poorly sorted colluvial deposit. It is located along a chute where a modern
debris flow occurred. .......................................................................................52
Figure 3.2: Areas of major debris flows and landslides in western North Carolina (after
Scott, 1972)......................................................................................................54Figure 3.3: A sketch map of debris flows that occurred along Gouges Creek in Mitchell
County, North Carolina in May, 1901 (Myers, 1902). ....................................55
Figure 3.4: Map showing some of the hurricane and storm paths that have affectedwestern North Carolina as reported by the U.S. National Hurricane Center and
the U.S. Geological Survey – Water Resources Branch (1949). .....................56
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moving northwestward over the watershed. This is the greatest recorded
streamflow for the gauge at Asheville. ............................................................58
Figure 3.7: Total storm precipitation for August 14-15, 1940 adapted from U. S.Geological Survey, 1949). ...............................................................................59
Figure 3.8: Total storm precipitation for August 28-31, 1940 (adapted from U. S.
Geological Survey, 1949). ...............................................................................59Figure 3.9: Total storm precipitation for November 2-5, 1977 (adapted from Neary and
Swift, 1987)......................................................................................................60
Figure 3.10: Total storm precipitation (inches) for the remnants of Hurricane Frances(National Weather Service, 2004b)..................................................................61
Figure 3.11: Total storm precipitation (inches) for the remnants of Hurricane Ivan(National Weather Service, 2004c)..................................................................61
Figure 3.12: A debris flow that blocked the westbound lanes of Interstate-40 near Old
Fort Mountain in McDowell County (North Carolina Geological Survey,
2004a). .............................................................................................................62Figure 3.13: The initiation zone of the debris flow that occurred on Fishhawk Mountain
and devastated the Peeks Creek area of Macon County on September 17, 2004
(Wilett, 2004)...................................................................................................62
Figure 4.1: The infinite slope equation as defined by Hammond et al., (1992) and Pack etal., (1998b) where C r is root cohesion, C s is soil cohesion, θ is slope angle, ρ s
is soil density, ρ w is the density of water, g is acceleration due to gravity, D is
the vertical soil depth, Dw is the vertical height of the water table, and Φ is the
internal soil friction angle. In the SINMAP model, the ratio of the vertical soildepth to the vertical soil height is simplified so that depth is measured
perpendicular to the slope (h). (Diagram after Hammond et al., 1992 and
Otteman, 2001) ................................................................................................76Figure 4.2: Default parameters used in the SINMAP model analysis. The values for the
gravitational constant and the density of water were not adjusted in thisstudy.................................................................................................................77
Figure 4.3: Default values used for SHALSTAB..............................................................79
Figure 5.1: Landslide inventory map for the French Broad Watershed. The cluster of location points in southern Buncombe County is due to the extensive mapping
of 1977 debris flows done by Pomeroy (1991) and Otteman (2001). .............98
Figure 5.2: A comparison of 30-meter and 10-meter DEMs in Haywood County, NorthCarolina. In the coarser 30-meter DEM, there is a great deal of pixelation.Lower resolution can lead to an underestimation of both the slope and
instability in the study area. .............................................................................99
Figure 5.3: SINMAP results for a 30-meter DEM and using default parameters............101Figure 5.4: SINMAP results for 125 mm/d recharge and using a 30-meter DEM. .........102
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In the legend, the first number is the degree if soil friction angle and the
second number is the amount of cohesion (N/m³). ........................................107
Figure 5.9: Cumulative percent of debris flows for each log (q/T ) instability category for a variety of soil parameters for a 30-meter DEM (after Dietrich et al.,
2001). .............................................................................................................107
Figure 5.10: Cumulative percent of the area if Haywood County for each log ( q/T )instability category for a variety of soil parameters for the 10-meter DEM
(after Dietrich et al., 2001).............................................................................108
Figure 5.11: Cumulative percent of debris flows for each log (q/T ) instability category for a variety of soil parameters for a 10-meter DEM (after Dietrich et al.,
2001). .............................................................................................................108Figure 5.12: Mapped log (q/T ) results for a 10-meter DEM of Haywood County. The
parameters used for this model run are essentially that same as those used in
the SINMAP lower bounds: 26° soil friction angle, and zero cohesion. .......109
Figure 5.13: SHALSTAB results for a soil friction angle of 35° and cohesion of 2000 N/m³. ..............................................................................................................110
Figure 5.14: Comparison of the output of SINMAP and SHALSTAB for 125 mm of
recharge, 35° soil friction angle, 1922 kg/m³ soil density, and zero soil
cohesion for a location in Haywood County. The red areas are calculated asunstable by both programs whereas the grey areas are calculated to be stable.
Visually the SHALSTAB results seem to cluster better while the SINMAP
results are more scattered. Overall, the results are very similar. ..................111Figure 5.15: Results for SINMAP and SHALSTAB. Red areas are the areas calculated by
both models as being unstable. The purple areas were only calculated by
SHALSTAB and the green areas were only calculated by SINMAP............112Figure 5.16: The mean stability index for each geologic unit in the French Broad
Watershed. The most unstable unit, Zchs, is located in the northwestern portion of Haywood County. The most stable units, bz and Ctzp, are located in
the southeastern portion of the study area......................................................113
Figure 5.17: The mean stability index for each soil unit in the French Broad Watershed.The most unstable soil unit in the watershed is NC104, which is located in
western Haywood County. The most stable soils are located around the
floodplains of the Pigeon and French Broad Rivers. .....................................114
Figure 5.18: Picture taken in October, 2003 along the Blue Ridge Parkway, near Mt.Mitchell. Even during this light rain event, water is pouring out from fractures
in the rock. .....................................................................................................115
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CHAPTER 1: INTRODUCTION
Catastrophic, storm-generated mass wasting is a destructive erosional process in
the portion of the Southern Appalachians that extends through western North Carolina.
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) (Pack et al., 1998b)
and SHALSTAB (Shallow Landsliding Stability Model); (Dietrich and Montgomery,
1998).
Mass wasting is a general term used to describe the “dislodgement and downslope
transport of soil and rock material under the direct application of gravitational body
stresses” (Jackson, 1997). Mass wasting can range from very slow creep to a rock
avalanche. The term is often synonymous with the term “mass movement” (Crozier,
1986). Both Crozier (1986) and Varnes (1978) advocate the use of the term “slope
movements” for mass movement restricted to slopes, as “slope movement” appears to be
a suitably neutral, all-encompassing term. Throughout this paper the term “slope
movement” will be used interchangeable with the terms “mass wasting” and “landslide”
to describe the movement of a mass of rock, soil and debris downslope. Of the several
types of slope movements that occur in the Appalachians, rapid mass movement,
particularly debris flows, are considered the most dangerous and will be the focus of this
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early 1900’s, several intense storms and hurricanes have tracked through western North
Carolina, 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 (Scott, 1972).
As extensions to ArcView®
3.x GIS software, both SINMAP and SHALSTAB
compute and map areas of potential slope instability based upon digital-elevation data
and observed landslide locations. These models combine steady-state hydrologic
concepts and an infinite-slope-stability analysis with a digital elevation model (DEM) to
calculate either a factor of safety (SINMAP) or the critical steady-state rainfall intensity
necessary to trigger slope instability (SHALSTAB).
As in any landslide investigation using modeling software, model results should
be compared with mapped landslide locations whenever possible. In this study,
preliminary field investigations were completed in three key locations in Haywood,
Yancey, and Transylvania Counties, initially modeled as having a high probability for
slope instability and failure. Aerial photography was used to search for debris-flow scars,
in the development of a landslide database for the region, and to complete more-detailed
SINMAP and SHALSTAB model runs for those locations. During field work in these
areas, soil and geologic data was collected for each location; debris-flow locations were
more precisely located with a hand-held global positioning system, and the overall
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1.1 Research Aim and Objectives
Few comprehensive research studies comparing slope-stability-modeling software
have been completed within a specific drainage basin in North Carolina. Most of these
studies have tended to focus on identifying landslide hazard areas on the U.S. West Coast
or in other countries (Appt et al., 2002; Dietrich et al., 2001; Montgomery et al., 2001;
Zaitchick et al., 2003). Other studies have focused on identifying relict landforms and
debris aprons in the Appalachians formed during periglacial conditions (Gryta and
Bartholomew, 1987; Clark, 1987; Kochel, 1987; Jacobson et al., 1989; Liebens and
Schaetzel, 1997). This study focuses on identifying debris-flow hazard areas and the
factors that affect this instability. The objectives of this research are as follows:
1. To identify areas within the French Broad watershed prone to debris flows.
2. To identify triggering mechanisms particular to the watershed that promoteinstability and failure.
3. To determine which soil and geologic units are the most prone to instability andthose that are the most stable, in general.
4. To compare the results, reliability, and effectiveness of two widely used, andtheoretically similar, GIS-based programs that model shallow landslide potentialin a watershed in North Carolina.
5. To study the effect of parameter variation on each of the modeling programs.
As part of this study, preliminary debris-flow hazard maps have been produced,
for comparison, using SINMAP and SHALSTAB. Results from both computer programs
were compared to an inventory of recent debris-flow and landslide locations determined
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1.2 Significance and Scientific Applications
The French Broad river basin is located in western North Carolina, in the central
portion of the Appalachian Mountain chain. The French Broad River itself flows through
the City of Asheville, a major commercial and manufacturing center, and a popular
mountain resort area. According to data collected by the U.S. Census Bureau (2000),
approximately 426,000 people live within the French Broad watershed and this
population is predicted to increase, particularly in and around the City of Asheville.
Debris flows hazards are a major concern in mountainous areas as debris fans are favored
areas of development due to their flat building surface and location above the floodplain
(Ritter et al., 1995). With continued development and tourism in the forested areas of the
Blue Ridge, the risk to people and property will increase because of debris flows,
especially during periods of high precipitation.
The major hazard to human life and property from debris flows is from burial or
impact by boulders and other debris. Usually starting on steep hillsides, debris flows can
accelerate to speeds between 15-55 kph (10-35 mph) (Highland et al., 2004) and often
strike without warning. Because of their relatively high density and viscosity, debris
flows can move and even carry away vehicles, bridges and other large objects (Cruden
and Varnes, 1996).
The economic costs of slope movements are can be both “event-related” and
“preventative” (Crozier, 1986). Event-related costs may involve the removal of debris
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mapping, and laboratory testing. After an initial analysis has been completed, policies for
the establishment, management, and inspection of preventative measures should be
completed (Crozier, 1986). This may also include the restriction of development in areas
considered landslide-prone or the removal or conversion of existing developments.
Control measures in prone areas may also be designed and implemented including
controlled drainage, planting, slope-geometry modification, and structures such as rock
fences or other barriers (Schuster and Kockelman, 1996).
Continued poor land-management practices and deforestation add to the risk of
soil mass movement by increasing runoff, erosion, and flooding. Human activity also
disturbs large volumes of geologic materials with the construction of housing
developments, commercial buildings, mines and quarries, dams and reservoirs, and
particularly the emplacement of transportation systems along steep slopes (Schuster,
1996). Roadcuts and other altered or excavated areas along slopes are particularly
susceptible to debris flows. Repeated landslides and rock falls have plagued the Interstate
40 corridor as it winds through the mountains of Western North Carolina and the Pigeon
River Gorge (Leith et al., 1965; Glass, 1981; Kaya, 1991; Wieczorek, 1996).
By combining a theoretical model of slope instability with actual debris-flow
locations, landslide hazard areas may be efficiently identified and mapped for regulatory
purposes. Identifying areas already at risk for slope failure could prevent loss of property
and lives. This would be a valuable resource to government agencies, as well as city and
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download from the World Wide Web at no cost, making them economical for both
government and businesses.
Debris flows are not just an agent of destruction, but also play a critical role in the
processes of erosion and sediment transport in the Blue Ridge Mountains. They provide a
major means of removing weathered material from steep areas that normally experience
little concentrated surface drainage (Scott, 1972). By studying the causes, characteristics,
and effects of debris flows in the Southern Appalachians, we can better understand their
erosional importance, as well as their destructive influences. Improved knowledge of the
Blue Ridge may also be extended to comparable sub-tropical mountainous areas in other
parts of the world.
1.3 Landslide Classification
There have been several attempts to create a suitable landslide classification
system that is useful for both scientific and engineering purposes. Early attempts
occurred in Europe during the late 1800’s and early 1900’s (Sharpe, 1938). These
schemes tended to be incomplete; in that slow movements such as creep were ignored,
they were developed for a single specific purpose or were developed based on only
regional observations. The more successful classification systems are those that provide
clear and unambiguous terminology that can be applied in a number of different
situations (Crozier, 1986). Generally, these systems classify landslides on the basis of the
kind and quality of material moved, its water content, and the type, size, and rate of
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in Denver, CO. But this classification system, like many before it, was influenced by the
classification system proposed by C.F.S. Sharpe (1938).
1.3.1 Classification of Sharpe (1938)
In the late 1930’s, C. F. S. Sharpe became one of the first American scientists to
create a widely accepted landslide classification system. Sharpe created his classification
based primarily on the kind and rate of movement (making a distinction between slides
and flows) and the forms of the resulting deposits. Other important factors included the
moisture content in the moving mass and the type of material involved. Based on these
factors, mass movement was separated into four groups, i.e. slow flowage, rapid flowage,
sliding, and subsidence.
Sharpe (1938) defined the mass-wasting process that typically occurs in the
Southern Appalachians as debris-avalanche, a type of rapid flowage. A debris avalanche
is a rapidly-moving, sliding flow with a long, narrow track that occurs on steep terrain in
humid mountainous areas with significant vegetative cover. This type of slope movement
tends to have higher water content than does a slow flow. They are usually preceded by a
period of heavy precipitation “which increases the weight of the unadjusted material and
aids in its lubrication” (Sharpe, 1938, p. 61). The material of a debris avalanche is
variable and can consist of soil, rock, vegetation and ice. Initial failure typically occurs
between the soil-rock interface or within loose debris on slopes between 20 and 40
degrees. When there is less water present in the same type of material, the movement
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1.3.2 Classification of Varnes (1978) and of Cruden and Varnes (1996)
The landslide classification system of D. J. Varnes (1978) was well-received and
has been repeatedly updated, the latest update having been published in Cruden and
Varnes (1996). The goal of the current version of this classification was to provide
definitions and vocabulary that allow an investigator to observe and describe a landslide
in the field succinctly and unambiguously.
The Cruden and Varnes (1996) classification scheme emphasizes the type of
material and the type of movement in a slide. Two terms are needed to describe any
landslide, i.e. one that describes the material (rock, debris, earth) and one that describes
the movement (fall, topple, slide, spread, flow). Other descriptors can then be added in
front of the two-term classification as more information about the movement becomes
available. These include the water content, rate of movement, the current activity of the
slide (reactivated, inactive, etc.), distribution, and overall style (complex, composite,
etc.).
Like the classification scheme of Sharpe (1938), Cruden and Varnes (1996) uses
the terms debris avalanche, debris slide, and debris flow to describe rapidly-flowing
mass-wasting events. The terms “debris avalanche” and “debris slide” in this
classification are varieties of the more general debris flow. “Debris avalanche” is an older
term used to in the literature to describe extremely large, rapidly-moving mass
movements that involve large amounts of ice, snow, and soil. A “debris slide” generally
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Driving forces cause material to move downslope and can increase depending on the
mass of the material involved in the movement and the slope angle (Easterbrook, 1999).
Frictional resistance opposes deformation or motion and is caused by the friction between
grains within the material or by the material at the base of the slope. The resistance of a
material to shear along a slip surface due to an applied external force can be referred to as
the materials shear strength. Shear strength is determined by analyzing the amount of
cohesion, effective normal stress, and internal friction between material particles (Ritter
et al., 1995). When the gravitational driving force is greater than the frictional resistance
the slope will fail (Easterbrook, 1999).
While a single trigger may actually cause a slope to fail, a number of other
destabilizing factors are often needed to contribute to a reduction in a material’s shear
strength in order to ultimately initiate movement (Crozier, 1986). First, though, the
topography must have a sufficient slope. Usually slopes of 35° or greater are prone to
instability simply because of the effects of gravity (Sidle et al., 1985). But often other
factors, such as soil and root cohesion, increase the shear strength of materials on a slope.
1.4.1 Precipitation
A number of slope movements are generated by an excessive amount of water,
usually due to heavy precipitation or a moderate rainfall lasting several days. Slope
movements in soil and weathered rock are usually produced on steep slopes during the
most intense part of a storm (Wieczorek, 1996). Intense rainfall results in a higher
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material or along an existing plane of weakness. Thin, loose soils on hillslopes with
sparse vegetation are particularly prone to failure during an intense rainfall.
Soils with a high rate of hydraulic conductivity (the amount of water that will move
through a porous medium in unit time) have the ability to transmit water more quickly
downslope. This decreases soil pore pressure and may help to increase shear strength.
Typically, an unconsolidated, coarse grained, well- rounded, and well-sorted soil will
have a higher value of hydraulic conductivity (Fetter, 1994).
1.4.2 Human Interference
Road construction is a major contributor to slope failure and their mitigation can
often incur enormous public cost. Excavation of the toe of a hillslope by emplacing a
road, quarry, canal, or other type of cut, removes support and may induce anthropogenic
slope moment (Cruden and Varnes, 1996). Road fill and traffic also increases weight on a
hillslope, increasing shear stress on materials (Sidle et al., 1985). In developed areas,
slope saturation may occur, even during moderate recharge events, because of
concentrated run-off from rerouting of drainage systems during road construction and
from man-made structures such as drainpipes, buildings, and paved impervious surfaces.
Vegetation often acts to stabilize slopes and increases shear strength and root
cohesion. Foliage intercepts rainfall whereas roots and stems extract moisture from the
soil (lowering pore-water pressure), increase surface roughness (Greenway, 1987), and
provide an interlocking network that strengthens unconsolidated sediment (Easterbrook,
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1.4.3 Tectonic Activity
In tectonically-active areas, volcanic eruptions and earthquakes have often caused
slope instability and failure by causing uplift or tilting (Cruden and Varnes, 1996).
Volcanic ash deposited on steep volcanic peaks, combined with the rapid melting of snow
or heavy rainfall, can initiate deadly lahars, debris flows, and mudflows. Slope
movements involving loose, saturated, low-cohesion soils commonly occur as a result of
earthquake-induced liquefaction, a process in which shaking temporarily raises pore-
water pressure and reduces shear strength. Fault zones may also contain fractured,
crushed, or low-metamorphic-grade rock that contain inherent weakness and may be
susceptible to failure (Sidle et al., 1985).
1.4.4 Geologic Material and Structures
Some particular rock and soil types may be inherently weak and can influence
slope instability. Organic soils and clays naturally have low shear strength and are
particularly prone to weathering processes (Cruden and Varnes, 1996). A predominantly
clay soil layer can prevent vertical infiltration of water and cause a buildup of pore-water
pressure, while also providing a smooth, low-friction surface for failure. In contrast,
sandy soils commonly erode quickly despite rapid infiltration and sandy slopes may be
undermined by rivers and run-off. A permeable shallow soil overlying a hard,
impermeable rock layer may also be susceptible to mass movement. As water builds up
and travels along the rock-regolith interface, cohesion between the layers diminishes and
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for groundwater during rain events and reducing cohesion between layers or bedding
(Sidle et al., 1985). Structures parallel to the ground surface and downslope-dipping beds,
particularly those that separate two distinct lithologic units, may also act as a viable
failure plane.
1.5 Debris Flows
Although several types of slope movements have been described in the high-relief
portions of the French Broad watershed, this study focuses on debris flows, i.e., rapid
downslope movement of regolith. It was noted during field work that a number of the
regularly-occurring mass-wasting events were actually simple rock falls or slides along
over-steepened roadcuts. Debris flows seem to be a less common mass-wasting event
than those mentioned above, but may be devastating when they occur in populated areas.
Unfortunately, there is a considerable amount of inconsistency in terminology for
modern rapid channelized downslope movement of poorly sorted sediment. The terms,
debris torrent, debris avalanche, debris flow, debris slide, mudflow, and mud flood are
occasionally used interchangeably (Pierson and Costa, 1987). These terms all have
similar descriptive characteristics for grain-size and the rate of downslope movement, but
individually are often difficult to distinguish from one another in the field (Ritter et al.,
1995). Other authors also use the terms, alluvial fan, debris fan, hillslope, toe-slope or
foot-slope deposit to refer to relict fan-shaped features found throughout the
Appalachians produced by repeated depositional episodes by debris flow processes
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The term “debris flow” is used herein to describe swift-moving mass-wasting
events that occur predominantly in shallow, silty-to-gravelly soil on steep slopes during
periods of exceptionally heavy precipitation (Cruden and Varnes, 1996) (Figure 1.1).
“Debris” defines a material that contains 20-80 percent coarse-grained particles larger
than 2mm. These materials may include boulders to clay with varying amounts of water
(Ritter et al., 1995). The “flows” begin in depressions or hollows on steep slopes and
tend to move downslope following preexisting drainage channels. The most common
movement interface is between the bedrock-soil contact, but slippage may also occur
within deep soils (Clark, 1987). Debris flows from several different sources often
converge into one main drainage channel, increasing the flow’s overall volume of water
and material (Highland et al., 2004). Debris flows can travel for several kilometers before
releasing their suspended load and coming to rest upon reaching an area of low gradient
(Ritter et al., 1995).
Debris flows may be triggered in a variety of ways. The most common trigger is
an abundant amount of moisture, either from intense rainfall or rapid snowmelt, or a
combination of heavy precipitation and antecedent soil moisture. Topography also
influences debris-flow initiation by concentrating subsurface flow and determining slope.
Soil thickness, conductivity, soil strength, bedrock-fracture flow, and root strength also
influence the spatial distribution of debris flows and other types of shallow landslides
(Montgomery and Dietrich, 1994). During a heavy rain event, piezometric head in the
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laden slurry that gains material as it travels downslope or as a shallow-slope movement
that is mobilized into a flow (Ritter et al., 1995).
1.6 Debris Flows within the Blue Ridge and western North Carolina
Debris flows have been reported to be the most common form of rapid slope
movement in the Blue Ridge (Mills et al., 1987). Debris flows have occurred throughout
the Blue Ridge province and have been specifically documented in Virginia (Woodruff,
1971; Williams and Guy, 1973; Gryta and Bartholomew, 1987; Gryta and Bartholomew,
1989; Mazza and Wieczorek, 1997), North Carolina (Holmes, 1917; Gryta and
Bartholomew, 1983; Pomeroy, 1991; Wooten et al., 2001), West Virginia (Jacobson et
al., 1989) and Tennessee (Clark et al., 1987). Mills et al. (1987) suggest that the
abundance of this type of slope movement may be due to the amount of thick,
unconsolidated colluvium derived from crystalline rock. This colluvium is highly
permeable and susceptible to weathering, both of which contribute to the generation of
slope movements.
In western North Carolina, debris flows are activated primarily by either localized
severe storms that produce intense rainfall for several hours or by more regional
moderate storms that may last for several days (Wieczorek, 1996). Most debris-flow-
producing storms can be linked to the incursion of warm, tropical air masses over the
mountains between May and November, or the remnants of hurricanes and tropical
storms (Kochel, 1990).
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Precipitation rates that readily induce debris flows in western North Carolina range from
125 mm/day (Neary and Swift, 1987) to the upper end of observed precipitation (560
mm/day). These thresholds may vary due to lithology, vegetation, and topography but
generally catastrophic rainfall is required to initiate debris flows in heavily vegetated
areas (Kochel, 1990). Under these conditions, rapid infiltration and a corresponding
increase in soil saturation brings the soil mantle to field capacity. This tends to occur in
shallow (< 1 m thick) mountain soils on slopes averaging 25-40 degrees, overlying an
impermeable horizon of metamorphic rock or saprolite (Eschner and Patric, 1982). A
temporary rise in piezometric pressure within slope sediment causes an increase in shear
stress while decreasing shear strength. This, combined with a decrease in soil cohesion,
reduces the shear resistance force enough to lessen the stability of the soil and eventually
induce failure (Neary and Swift, 1987).
Typically, debris flows have a characteristic long narrow shape. In the Southern
Appalachians, the width of a debris flow, or the “chute,” may range from only a few
meters to 60 m wide, although most tend to be narrow (Mills et al., 1987). Runout lengths
rarely exceed 600 m, but a few have been known to extend as far downslope as 1600 m
(Scott, 1972). Although single linear chutes are common, larger tracks are often produced
by coalescing debris flows that form a dendritic pattern upslope (Scott, 1981). This shape
is due to the formation of a majority of debris flows in topographic hollows (Scott, 1972).
At their bases, debris accumulates as a washout fan of colluvium or may merge with
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1.7 SINMAP and SHALSTAB
There are many approaches to the problem of predicting shallow slope movements,
and almost as many predictive models. Simple models, based on identifying and
classifying high-hazard areas on the basis of critical slope angle, do not take into account
the effects of topographic form or position and lithology. More complex approaches to
prediction should consider a wide range of variables such as drainage area, bedrock
geology, soil thickness and cohesion, precipitation, vegetation and land use. Two such
models, which take many of these variables into account, are SINMAP and SHALSTAB.
SINMAP was designed as an extension to ArcView®
GIS, a product of
Environmental Systems Research Institute, Inc. SINMAP is applied to shallow
transitional landsliding phenomena controlled by shallow groundwater convergence
(Pack et al., 2001), as is generally found in western North Carolina. The SINMAP
methodology is based on the infinite-slope equation, an equation that has been found to
be adequately accurate in the analysis of debris flows for planning purposes in the U.S.
Western Cordillera (Hammond et al., 1992) as well as in western North Carolina
(Otteman, 2001).
A digital elevation model (DEM) is used to determine inputs of topographic slope
and catchment area. Landslide point data is added to compare locations of predicted
instability with areas of actual instability and to evaluate the accuracy of the model
results. Other parameters for both natural material and climatological properties are
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ranges are reached and yet stability is still retained, the stability index (defined as the
factor of safety) is calculated as greater than one (Pack et al., 2001). The default output of
a SINMAP session is a series of map grids that define areas of potential terrain
instability; shaded green areas are considered stable whereas dark red areas have a high
probability of failure, based on parameter inputs (Pack et al., 1998b).
Like SINMAP, SHALSTAB is an extension of ArcView
®
and is theoretically
similar to SINMAP. The SHALSTAB model is also based on the infinite-slope equation
and uses a DEM for the input of topographic grid information. SHALSTAB assumes
steady-state, saturated flow parallel to the slide surface and uses Darcy's law to estimate
the spatial distribution of pore pressures (Dietrich and Montgomery, 1998).
The authors of SHALSTAB intended the model to be as simple as possible and
nearly “parameter free” (Dietrich et al., 2001). In other words, most of the input variables
are derived directly from the slope and area grids created from the input DEM and do not
require the user to calculate specific parameters based on known soil properties which,
without direct sampling and laboratory investigations, are often difficult to derive
(Dietrich et al., 2001). Although SHALSTAB does allow for adjustments to certain soil
parameters to better match existing conditions, the model does not provide upper and
lower boundaries for parameters like SINMAP. Moreover, the model does not allow for
an adjustment to recharge, but rather calculates the effective recharge (precipitation
minus evapotranspiration) needed to induce failure for each cell within the DEM grid.
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should be compared with mapped landslide features whenever possible (Dietrich and
Montgomery, 1998).
1.8 Limitations of Research
The prime limitation to both SINMAP and SHALSTAB is the availability of
high-quality digital-elevation data i.e. a 10-meter scale or smaller. Surface topography,
determined from digital-elevation data, has great bearing on the location and frequency of
shallow landsliding predicted by both models. Currently, only 30-meter and 10-meter
DEMs are available for the western portion of North Carolina. Both SINMAP and
SHALSTAB require the input of high-quality digital-elevation data to identify areas of
steep slope. As more accurate DEMs and LIDAR data are released for this portion of the
state, more accurate slope-instability and landslide-hazard maps can be developed.
Limitations to SINMAP and SHALSTAB include the lack of high-quality soil
data necessary to parameterize the models, the size of the study area, and the need to
generalize soil and climate parameters over such a large area. Better quality results can be
derived from larger-scale studies covering towns or individual neighborhoods. Such
studies should collect and analyze soil samples as well as create a detailed inventory of
all present and past debris flows for the study area.
The authors of SINMAP and SHALSTAB did not intend for these models to
provide a complete forecast of all types of landslide potential. These models were
designed to predict only the potential for shallow landsliding, not deep-seated slides, rock
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Figure 1.1 - The morphology of a typical debris flow found in the Southern Appalachians (courtesy of
the North Carolina Geological Survey, 2003).
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Figure 1.2: Threshold precipitation values necessary for producing debris flows in the southern
Appalachian Mountains. Storms likely to start debris flows occur above the 125 mm/d threshold.
Storms with precipitation values higher than 250mm/d are deemed “rare” but do occur in North
Carolina (after Eschner and Patric, 1982).
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CHAPTER 2: PROJECT SETTING
2.1 Introduction
The French Broad watershed is approximately 7330 km² and includes the counties
of Buncombe, Haywood, Henderson, Madison, and Transylvania, and portions of Avery,
Mitchell, and Yancey counties (Figure 2.1). Major tributaries of the French Broad River
include the Nolichucky, Toe, and Pigeon Rivers. The watershed includes large portions
of Great Smoky Mountains and Pisgah National Parks. Two major interstates, Interstate-
40 and Interstate-26, cross the basin, as does the Blue Ridge Parkway. The French Broad
River actually begins in Rosman, North Carolina (35 miles southwest of Asheville, NC)
where four tributaries converge.
Topography within the French Broad basin ranges greatly, from the relatively
gently sloping floodplains along the banks of the French Broad River to steep slopes in
the mountains and along roadcuts. The highest point in the watershed, and the entire
Appalachian mountain chain, is Mount Mitchell at an elevation of 2073 m.
2.2 Geology
Given the large size of the French Broad River basin, 40 geologic units and 17
general soil types have been mapped within the watershed by North Carolina agencies.
The watershed lies within both the Blue Ridge Belt and, to the east, a small portion of the
Inner Piedmont Belt. Bedrock generally consists of sedimentary, metasedimentary, and
intrusive igneous rock of Proterozoic and Paleozoic age (North Carolina Geological
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The Blue Ridge geologic province reaches its greatest east-west extent in the
Carolinas, Tennessee and Georgia. The province is bounded on the northwest by the Blue
Ridge fault systems (Holston-Iron Mountain, Great Smoky, and Cartersville Faults) and
on the southeast by the Brevard fault zone, while the Hayesville and Greenbrier fault
zones intersect in the middle of the French Broad basin (Hatcher and Goldberg, 1994).
These faults transported a series of large crystalline thrust sheets over the Paleozoic rocks
of the Valley and Ridge province, each with different tectonic histories and degrees of
metamorphism (Hatcher, 1987). The western Blue Ridge is composed of a rift-facies
sequence of clastic sedimentary rocks deposited on basement rock. The eastern Blue
Ridge records a series of slope-and-rise sequences associated with rifting and continental
collision (Hatcher and Goldberg, 1994).
The Brevard Fault zone separates the Blue Ridge province from the Inner Piedmont
block to the east. The block is a composite stack of thrust sheets containing a variety of
metamorphic rocks, intrusive granitoids, and sparse ultramafic bodies (Horton and
McConnell, 1994). Within the French Broad Watershed, only a few units from the Inner
Piedmont block are represented i.e., the Henderson gneiss and middle-to-late Paleozoic
granite gneiss. The Brevard fault zone itself represents a linear, southeast-dipping belt of
cataclastic and mylonitic rock, 1-to-2 km wide (Horton and McConnell, 1994).
According to Scott (1972), geologic structure and bedrock orientation play a more
important role in slope stability than rock type in the Southern Appalachians. When soils
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particularly true when fracture surfaces are parallel to the dip surface. It was observed in
the study area that even during a light precipitation event, groundwater flow through
fracture zones was swift. This concentration of groundwater could quickly cause an
increase in pore-water pressure in soils on a slope or create ephemeral channels for debris
flows to follow. A similar correlation between joint orientation, direction of groundwater
flow, and debris-flow initiation was noted in the Coweeta Basin, an experimental forest
and research station just south of the watershed (Grant, 1987). In the SINMAP and
SHALSTAB models, groundwater flow is incorporated by using a value for soil
transmissivity (m²/s).
2.3 Soils
The types of soil in the French Broad watershed reflect the regional geology
because variation in bedrock mineralogy partly controls soil mineralogy (Figure 2.3).
Herein soil will be defined as “unconsolidated mineral or organic material on the
immediate surface of the earth that has been subjected to and shows effect of genetic and
environmental factors” (Jackson, 1997). Steep relief, broad ridges, and humid
temperatures allow for a wide range of soil-forming conditions. On steep side-slopes,
Inceptisols are common whereas Ultisols are found on gently sloping areas (Graham and
Buol, 1990). Inceptisols, particularly Dystrocrepts, are soils that have not developed
many diagnostic features due to rapid erosion rates and downslope movement. Ultisols
develop from acidic parent materials, such as granite, and have an increase in clay
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Mitchell and at very high elevations. Mesic soils have mean soil temperatures of 8° to
15° C (46° to 59° F) during June, July, and August, whereas frigid soils have mean
temperatures less than 8° C (46° F) during the summer months (Buol et al., 2003).
Generally, soils with a high susceptibility of failure tend to have a large mica content and
develop over micaceous schist, slate, and phyllite (Scott, 1972).
Soil cover varies in thickness and development depending upon slope and
weathering and can range from less to one meter to several meters in depth (Clark, 1987).
Steep slopes create a shallow soil veneer held together by plant roots that may overlie a
rock or saprolite layer. More moderate slopes, i.e., those between 30 and 35 degrees,
develop thicker soil profiles and are more prone than shallow soils to debris flows. This
may seem contradictory, but steep slopes are not able to develop and retain soil cover and
can easily be washed away, even during moderate rainfalls. Thus, an increase in soil
depth and a decrease in slope increase the risk of slope instability (Scott, 1972).
In the unglaciated portions of the Blue Ridge, chemical weathering plays an
important role in the breakdown of rock into soil regolith (Grant, 1987). According to
Graham et al. (1990), three types of regolith are found in the Southern Appalachians i.e.,
saprolite, colluvium, and soil residuum. Thick sequences of saprolite, or “rotten rock,”
develop on gentle slopes composed of metasedimentary and metaigneous rock,
promoting stability and chemical weathering (Jacobson, et. al., 1989). Typically, saprolite
is covered by soil residuum. Residuum is separated from saprolite where the visual
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colluvium and alluvium are deposited in valley bottoms and along rivers (Hatcher, 1987;
Otteman, 2001).
2.4 Climate
Due to the variation of altitude (460-to-2073 m) within the French Broad
watershed, temperature and moisture regimes vary greatly from one place to another
within this area. In fact, the mountains have some of the wettest and driest weather in
North Carolina (Daniels et al., 1999). The greatest 24-hour rainfall total in the State (565
mm) was measured in the watershed at Altapass in Mitchell County on July 15-16, 1916
when a hurricane passed through the area. In contrast, the station with the driest weather
on average is located in downtown Asheville in Buncombe County (State Climate Office
of North Carolina, 2003).
Mean annual rainfall in the southern Appalachians ranges from 1000 to 2700 mm
(1-to-2.7 m) with snowfall only contributing 5 percent of the total precipitation (Neary
and Swift, 1987). Rainfall occurs frequently as small, low-intensity rains in all seasons
but localized heavy precipitation is uncommon. Precipitation is usually greater during the
winter and spring, with March being the wettest average month (Table 2.1 and Figure
2.5). The highest maximum precipitation amounts have been recorded in the summer
months when localized, high-intensity thunderstorms and hurricanes are more common.
As a result, a majority of debris-forming rainfall events in the Blue Ridge occur in June,
July, and August (Clark, 1987). Recently, in September 2004, several debris flows were
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Orographic influences generate extremely heavy rainfall in localized mountainous
areas, even in storms with weak pressure gradients and gentle air circulation (Scott,
1972). Generally, rainfall increases with elevation at a rate of 5 percent per 100 m (Swift
et al., 1988), but altitude is not as important as orographic boundaries. The Blue Ridge
produces an elongate area of high values of mean precipitation (Jacobson et. al., 1989).
As can be seen in Fig. 2.6, there is a marked increase in the amount of annual
precipitation in northern Transylvania County and southern Haywood County.
2.5 Vegetation
Like rainfall, vegetation within the watershed varies with the topography. Slope
aspect and shading by adjacent higher mountains also influences the distribution of major
tree species (Daniels et al., 1999). At lower elevations (below 1400 m) hardwoods, oak,
hemlock and pine forests dominate. Hardwoods such as yellow poplar, ash, and black
cherry are found in coves and along steep slopes whereas several varieties of pine and
oak thrive in open areas (Scott, 1972). Except for the most rugged terrain, the region’s
forestland has been cut or burned at least once since European settlement (Clark, 1987).
In the very high mountainous areas of the watershed (above 1400 m) distinctive
ecological systems have been established as a result of the cool year-round temperatures.
Areas are often wind-swept and trees are damaged by ice and winter wind. Red spruce,
mountain ash and Fraser fir are common with the latter dominating above 1890 m
(Daniels et al., 1999). Grass balds and areas dominated by low shrub like rhododendron
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Figure 2.1: Location Map of the French Broad Watershed in western North Carolina.
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28
Figure 2.2: General geologic map for the French Broad Watershed. Individual geologic unit descriptions can be found in Appendix A (adapted from
North Carolina Geological Survey, 1985).
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29
Figure 2.3: General soil map for the French Broad Watershed. Individual soil descriptions can be found in Appendix B (adapted from U.S. Department
of Agriculture, 1998).
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Figure 2.4: The U.S. Department of Agriculture guide for the textural classification of soils. This
guide is only for soils with a particle size of less than 2 mm in diameter. A rock fragment modifier
(gravelly, cobbly, stony, bouldery) prefaces the textural name if particles larger than 2 mm compose
more than 15% of the soil (Buol et al., 2003).
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Table 2.1: Table of the average, median, minimum, and maximum precipitation totals (mm/month)
from 1895 to 2001 for the mountains of North Carolina (NCDC Climate Data Online, 2003).
North Carolina Climate Division 01 - Southern Mountains
Average Total Precipitation (mm/month) 1895-2001
JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC TOTALAverage 117.86 115.82 142.49 114.81 110.74 121.67 136.14 133.60 99.06 92.20 92.71 116.08 1393.18Median 109.98 112.52 131.83 111.76 100.58 118.36 131.06 120.65 91.95 82.55 90.93 110.74 1312.93Minimum 8.38 13.21 20.83 23.11 29.21 37.34 49.02 19.81 8.89 1.27 24.13 9.14 244.35Maximum 279.15 254.00 282.96 208.79 271.02 245.87 315.21 398.27 249.43 257.56 303.02 272.54 3337.81
Monthly Precipitation for the NC Southern Mountains -
1895-2001
0
50
100
150
200
250
300
350
400
J A N
F E B
M A R A P
R M A Y J U
N J U L
A U G
S E P
O C T
N O V
D E C
Month
M i l l i m e t e r s ( m m )
Average
Median
Minimum
Maximum
Figure 2.5: Graph based on the data from Table 2.1 (NCDC Climate Data Online, 2003)
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32
Figure 2.6: Average annual precipitation in inches within the French Broad Watershed. (Adapted from data provided by North Carolina Center for
Geographic Information and Analysis map server (http://204.211.135.111)).
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CHAPTER 3: PRE-HISTORIC AND HISTORIC DEBRIS FLOWS IN WESTERN
NORTH CAROLINA
The Appalachian Mountains have a long history of producing destructive debris flows.
Through the Pleistocene, temperature and moisture fluctuations associated with the transition
from glacial to interglacial ages, destabilized exposed soil and rock. These pre-historic debris
flows have formed prominent modern landforms and a rolling topography (Jacobson et. al.,
1989). Records of flooding in western North Carolina associated with hurricanes and other
strong storms exist back into the late 1700’s. Recorded instances of debris flows and other slope
movements during major rain events began in the late 1800’s, but their generation and mechanics
have been poorly understood until recently. This study is only a part of the ongoing natural
hazards research being conducted by the North Carolina Geological Survey (Wooten et al., 2004)
and North Carolina Department of Transportation. Continued study of the history of debris flows
can help identify triggering mechanisms that are particular to North Carolina and the recurrence
interval of these events.
3.1 Quaternary Debris Flows
The Quaternary history of the Appalachian Mountains consists of slow uplift and
subsequent denudation. Although extensive terrain is mantled with colluvial deposits, individual
ancient mass movements have only recently been identified in the Blue Ridge Province. Studies
of these deposits have elucidated the rate of soil development and erosion, catastrophic debris-
flow frequency and triggering events, and the possible role of periglacial processes in the
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poorly sorted, but may be either matrix-or-clast-supported (Figure 3.1). Typically, fans are
composites of several mass-wasting events with a weathered surface on each colluvial unit in the
sequence. This indicates that there may be great differences in age between the units and
upwards several thousand years have elapsed between debris-flow-forming events (Kochel,
1990). In the Great Smoky Mountains, characteristic recurrence intervals for major debris flows
are on the order of 400 to 1600 years (Kochel, 1990) whereas catastrophic debris flows have
been estimated to occur every 3000-6000 years in Nelson County, Virginia (Kochel, 1984;
Kochel and Johnson, 1984).
Catastrophic geomorphic events, such as debris flows, have been the principal means of
erosion of the central Appalachian mountain chain during the Quaternary. The long-term
denudation rates in the Appalachians average 40mm/kyr (Hack, 1980). In the short term, a
single debris flow can remove enough material to account for 1 kyr of erosion during a single
storm (Mills, et. al., 1987).
Numerous studies (Table 3.1) hypothesize that extreme precipitation events, associated
with major Quaternary climate change and periglacial environmental conditions have encouraged
the formation of a number of debris flows in the Southern Blue Ridge during the Pleistocene
(Liebans and Schaetzel, 1997). During the last 850 kyr there have been at least ten major ice
advances that have brought glaciation to much of the northern Appalachians and periglacial
conditions to the southern Appalachians (Braun, 1989). The term “periglacial” refers to the area
near a glacier that is affected significantly by cold weather and has a landscape appreciably
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During glacial periods, North Carolina experienced a greater frequency of freeze-thaw
cycles and physical weathering. Rock exposed at high elevations decomposed to a thin loose soil
mantle (Mills, 2000). A dry polar climate dominated the region. In modern polar climates,
monthly temperatures average below 10°C (50°F) year-round, resulting in little to no tree
growth (Lydolph, 1985). Although a polar climate can create a ready supply of sediment through
erosion and physical weathering, the lack of precipitation inhibits the formation of debris flows.
In contrast, slow mass movements, such as solifluction and creep, are common (Ritter et al.,
1995). In Virginia, slope wash of material may have proliferated more than debris flows during
the Pleistocene (Eaton et al., 1997; Eaton, 1999). In the Great Smoky Mountains National Park,
block field and slope deposits produced by Pleistocene frost wedging have been identified Clark
and Torbett (1987).
After the late Wisconsin glacial maximum, near the end of the Pleistocene, the northward
migration of the polar front would have allowed tropical moisture to reenter the area during the
summer months (Kochel, 1990). Previously undisturbed soil and rock then became exposed to
heavy precipitation. Slopes that were still sparsely vegetated (due to cold winter temperatures)
apparently became saturated and unstable, creating numerous large debris flows. Repeated
intervals of glacial and interglacial climate on a periglacial landscape probably created episodic
sequences of catastrophic mass-wasting during the Pleistocene and early Holocene (Kochel,
1990).
Hillslopes remained generally unstable during the late Wisconsin glaciation, but
6 3
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high elevations (>1100 m), produced large volumes of colluvium (>106 m3) and may have
transported material as far as 8 km in a single event (Hatcher et al., 1996). Giant Pleistocene rock
and block slides also have been identified in both southwestern Virginia (Shultz, 1986) and the
Great Smoky Mountains (Hadley and Goldsmith, 1963). These slides indicate that large sections
(100-400 million m³) of sandstone detached along bedding planes and joints and slid downslope
(Schultz, 1986). If so, these debris flows and slides were significantly larger than those produced
during modern times.
Studies of pre-historic debris flows and fans tend to use three major dating techniques,
i.e., relative-age dating of soil-horizon development and clast weathering, carbon-14 dating of
organic material, and thermoluminescence dating of colluvium and related sediments (Table 3.1).
These studies quantitatively provide evidence of repeated debris-flow activity during the
Quaternary (Clark, 1987).
Relative dating, when used as the only means for quantifying the age of a colluvial or fan
deposit, may be less reliable than the other two techniques. Nonetheless, relative-age
relationships have been used to correlate the development of debris flows from several different
sites in western North Carolina (Mills, 1982; Mills and Allison, 1995b; Liebens and Schaetzl,
1997). Generally, these studies use weathering-rind development, Munsell soil color, clay
content, and superposition to differentiate between older and younger debris-flow deposits.
Although these studies reach reasonable conclusions, none of them provide a probable date for
the surface deposits being studied, only a generalized time period. Results could be improved if
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unless the weathering conditions in both areas are very similar. Nonetheless, relative dating is a
useful descriptive tool and a good first approximation for modern debris flow hazard areas.
These studies also provide evidence of periods of episodic mass-wasting that was more extensive
than the processes occurring today.
Few radiocarbon-dating studies have been completed in North Carolina due to a lack of
datable organic material in the stratigraphic record and the presumption that some deposits are
too old (Table 3.1; Jacobson et al., 1989; Mills and Allison, 1995b). Existing dates in nearby
Virginia range from as recent as 2,200 kyr to as old as 50,800 kyr (Eaton et al., 1997). Dates for
North Carolina range from 16,000 – 25,000 kyr. Kochel (1990) believes that these older dates
indicate that while debris-flow activity may have been impeded during glacial maxima, tropical
moisture occasionally has invaded the area.
Further radiocarbon dating, where possible, would greatly improve our knowledge of the
Pleistocene environment and pre-historic Holocene mass-wasting in western North Carolina and
the Appalachians. Few historical accounts of debris flows have actually been recorded and this is
a field that could fruitfully be explored in more detail, particularly with new dating techniques
using longer-half-life techniques such as K-Ar dating.
3.2 Modern Flooding and Debris FlowsThe first recorded instance of a major flood in the French Broad watershed occurred in
April 1791, six years before Asheville, NC was incorporated with its present name (Tennessee
3 2 1 J 1876
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3.2.1 June, 1876
The first recorded instance of debris flows affecting the N.C. Blue Ridge occurred on
June 15-17, 1876. The debris flows accompanied flooding that is often called the “June Freshet,”
one of the greatest floods in the upper reaches of the French Broad watershed (Tennessee Valley
Authority, 1960). At the time, a debris flow was generally attributed to a “waterspout”, i.e., a
sudden funnel-shaped cascade of water falling from the sky during a torrential rain event
(Clingman, 1877). It was believed that the force of the falling water ripped away the soil from
the side of the mountain, leaving only solid bedrock. The term “waterspout” was used not only to
describe a meteorological event but also a geomorphic feature. The closest modern term to this
"waterspout" is “microburst.”
Clingman (1877) reports that at least 40-60 waterspouts were reported in portions of
Macon and Jackson counties during the June, 1876 storm. Although Clingman (1877) did not
provide a mechanism for the “waterspouts”, his detailed descriptions of the erosion and
deposition are excellent, e.g., that of two debris flows that occurred in Macon County near the
crest of Fishhawk Mountain and the Tessantee River on the afternoon of June 15, 1876
(Clingman, 1877). There were no known fatalities but the Conley family witnessed the debris
flow across the river from their home:
“They saw a large mass of water and timber, heavy trees floating on the
top, which appeared ten or fifteen feet high, moving rapidly towards them, as if itmight sweep directly across the Tessantee and overwhelm them. Fortunately,however, sixty or seventy yards beyond the creek the ground becamecomparatively level, and the water expanded itself, became thus shallower, andleaving many of the trees strewn for a hundred yards along the ground, entered
ti Fi hh k M t i i th h f l h kill d d 15 h
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noting as Fishhawk Mountain is the same area where four people where killed and 15 houses
destroyed in a debris flow that occurred on Sept. 16, 2004 (see below).
3.2.2 May, 1901
From May 18-to-23, 1901 a series of low-pressure systems passed through western North
Carolina and brought heavy rain, with the heaviest precipitation occurring on May 21-22. The
storm was centered near the Black Mountains of North Carolina. Total precipitation amounts
ranged from 8.99” (22.8 cm) in Marion, North Carolina to 5.04” (12.8 cm) in Asheville, North
Carolina (Myers, 1902). Extreme flooding affected portions of the Nolichucky, Watauga, Little
Tennessee, and Catawba Rivers systems (Myers, 1902; Scott, 1972). Later flooding in the spring
and summer only added to the destruction. Total damage to farms, bridges, highways, and
buildings in the French Broad watershed was estimated to be four million dollars (U. S.
Department of Agriculture, 1902).
Most of the debris flows associated with the 1901 storm occurred in Buncombe,
Henderson, Mitchell and McDowell Counties (Scott, 1972) (Figure 3.2). The Southern Railroad
was particularly affected as a number of slides buried tracks for hundreds of meters or had
portions of track washed away in the associated flooding. A resident of Marion, George Bird,
reported that a number of slides affected the hills near his home and generated large piles of
timber (Holmes, 1917). Landslides and “waterspouts” seemed to have been particularly
prevalent in Mitchell County where as many as 17 slides were observed on one hill by Myers
(1902) (Figure 3.3). Myers (1902, p. 104) describes in detail one of the largest slides he
end of the cavity a sharp and well defined channel led down the hill to the stream
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end of the cavity a sharp and well-defined channel led down the hill to the streamat the base, this channel being from 5 to 6 ft. wide and from 4 to 5 ft. deep withside walls practically vertical cut down though a gravelly clay. …It is estimated
that the excavation has a total content of about 2,500 cu. yds. of earth whichseems to have disappeared utterly.”
The particular slide described by Myers (1902) destroyed a log house that was in the flow
path. Other accounts by area residents describe cloudbursts of extreme intensity accompanying
the “waterspouts” and that water bubbled and then burst from the ground at the head of many
smaller slides (Myers, 1902). It can be assumed from these descriptions that the mass movements
in Mitchell County were debris flows, given their high water-and-debris content, their
characteristic flow path, and their rupture surface.
3.2.3 July, 1916
During July of 1916 two tropical cyclones moved through the French Broad watershed
causing extensive flooding and numerous debris flows. On July 5-6, 1916 a weak hurricane
passed over the Mississippi and Alabama coast and moved northeast, eventually deteriorating
into a tropical depression by the time that it reached western North Carolina (Southern Railway,
1917). This storm produced 4 to 10 inches (10 to 25 cm) of rain but did not create any known
debris flows. On July 14, a hurricane made landfall in Charleston, South Carolina and traveled
rapidly northwest into the mountains of North Carolina (Figure 3.4). By the morning of July 15,
the center of the powerful storm had already reached western North Carolina. The flood of July
14-16, 1916 was the largest recorded flood ever to have affected western North Carolina. It also
where 22 22 inches (56 4 cm) fell in a 24-hour period (Hudgins 2000) This is also the greatest
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where 22.22 inches (56.4 cm) fell in a 24-hour period (Hudgins, 2000). This is also the greatest
24-hour rainfall total ever recorded in North Carolina. Generally, the storms of 1916 produced
two distinct regions of exceptionally heavy precipitation, i.e., one in Mitchell, Avery, and
Caldwell counties, and the other in Transylvania and Henderson counties (Figure 3.5). Runoff
from the second storm was estimated to be as high as 80-90 percent (Southern Railway, 1917).
The first storm had already thoroughly soaked the soil, increasing antecedent moisture
conditions, and filled most streams nearly to flood stage (Scott, 1972). Rainfall from the second
storm exacerbated flood conditions (Figure 3.6).
The July, 1916 storms killed about 80 people and caused $22M in damages (Southern
Railway, 1917). In Asheville, several homes and buildings were destroyed and four of the main
river bridges were washed away (Tennessee Valley Authority, 1960). The Southern Railway
Company suffered extreme financial losses and transportation within western North Carolina
was disrupted for several days. Many railway lines were covered by debris flows, trapping
freight and passenger trains between terminals. The Southern Railway (1917) reported that
almost every mile of track between Asheville and Statesville was covered by debris or washed
out. At some places, track was suspended in mid-air after the fill below was washed away
(Southern Railway Company, 1917).
Generally, debris flows were reported along the Blue Ridge Mountains to the east,
southeast, and south of Asheville (Holmes, 1917; Scott, 1972) (Figure 3.2). Most slides occurred
between 5 pm., July 15 and 7 a.m., July 16. The flows began before dark and could be heard
0 6 to 6 1 m (2 to 20 ft) and averaged 1 5 to 1 8 m (5 to 6 ft) Bedrock was seldom exposed
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0.6 to 6.1 m (2 to 20 ft) and averaged 1.5 to 1.8 m (5 to 6 ft). Bedrock was seldom exposed
anywhere along any slide (Holmes, 1917).
3.2.4 August, 1940
In August of 1940, a pair of storms caused significant flooding and numerous debris
flows in the western mountains of North Carolina; the first occurred from August 11-17 and the
other from August 28-31. These storms also brought record flooding to portions of Virginia,
Tennessee, and South Carolina. Approximately 30-40 lives were lost and there were at least
$30M in damages (U.S. Geological Survey, 1949). The situation was similar to that of 1916,
with two large storms occurring in the same month. The 1940 mid-August storm was strikingly
similar to the to the second 1916 storm in terms of rainfall intensity and storm path (Figure 3.4).
However, unlike the 1916 storm, the antecedent moisture conditions in 1940 were relatively dry,
allowing for increased infiltration, hence lower overall flood discharge levels (U.S. Geological
Survey, 1949).
The first storm in 1940, an unnamed hurricane, made landfall between Beaufort, South
Carolina and Savannah, Georgia on August 11, 1940. Although no wind speeds were recorded,
damage reports indicate that trees were uprooted and broken, many buildings were damaged or
destroyed, and 20 coastal residents were killed. An unusually high tide was reported, reflecting
the storm surge. The storm then moved inland and curved northward following the Savannah
River Valley, weakening significantly. It followed a semi-circular path through Georgia,
Tennessee and Virginia, and then back into North Carolina before it moved offshore on August
slow rate of movement allowed for heavy precipitation for several days over the North Carolina
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y p p y
Blue Ridge, resulting in high rainfall totals. Maximum precipitation totals ranged from 13 to 16
inches (33 to 41 cm) at to as little as 5 inches (13 cm) in Asheville (Tennessee Valley Authority,
1960). A series of well-defined storms centers over the Appalachians Mountains extended
toward the northeast from Blue Ridge, Georgia to Luray, Virginia (Figure 3.7), apparently due to
an orographic influence on the storm precipitation (U.S. Geological Survey, 1949).
The second storm in 1940 occurred during the period of August 28-31 but intense rainfall
did not begin until the morning of August 29. Rainfall continued to fall until August 30 when it
abruptly ended around noon. Only passing showers remained by August 31 (U.S. Geological
Survey, 1949). This storm was a relatively local meteorological disturbance that only affected
the French Broad and Little Tennessee watersheds (Figure 3.8). Precipitation was of both shorter
time duration and aerial extent than the mid-August storm, but of higher intensity. Rainfall
amounts ranged from 8 to 13 inches (20 to 33 cm) on the western slopes of the Blue Ridge in 20
to 30 hours (U.S. Geological Survey, 1949). Given the antecedent moisture conditions due to the
earlier storm, flooding was more severe near the storm center but overall was not as widespread.
The 200-300 debris flows associated with both storms of 1940 attributed greatly to the
devastation wrought by the floods (Scott, 1981). These slides occurred near the centers of both
storms in shallow saturated soils on steep slopes (Figure 3.2). They originated on slopes 300 to
400 feet (91.5 to 122 m) from the tops of mountains, near the slope break. The debris flows
ranged in width from 6 to 8 feet (1.8 to 2.4 m) and 40 to 50 feet (12 to 15 m) long to 200 to 300
During the mid-August storm of 1940, debris flows mainly occurred in the Blue Ridge
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g g y g
Mountains, from the North Fork of the Catawba River northward into Watauga County near the
North Carolina – Virginia border (Figure 3.2). During the late August storm, debris flows
occurred mainly in the Upper Pigeon and Tuskasgee River basins (Figure 3.2). Because of the
concentration of high-intensity rainfall within a small area, more than 200 debris flows occurred
in an area of only 150 mi2 (388.5 km2) (U.S. Geological Survey, 1949).
3.2.5 November, 1977
In early November 1977, a storm system that had formed as a low-pressure system in the
Gulf of Mexico moved northwestward into the Appalachian Mountains (Neary and Swift, 1987).
Rainfall began in western North Carolina in the early morning of November 2 and continued at a
steady rate (20-50 mm/day) until November 5. This steady rain was followed by intense
downpours (102 mm/hr) on the night of November 5-6 during which most of the debris flows
were initiated (Nearly and Swift, 1987). This heavy precipitation, as in 1916 and 1940, was
produced by convection associated with orographic lifting over the southern Appalachians. Four
areas of exceptionally heavy precipitation (200-320 mm) were produced along the southeast
ridges of the North Carolina Blue Ridge. Two of these areas were within the French Broad
Watershed (Neary and Swift, 1987) (Figure, 3.9).
Although the heaviest rainfall in 1977 occurred in the vicinity of Mt. Mitchell, the best
information about debris flows and flooding came from the Bent Creek watershed, located about
15 km southwest of Asheville. A survey was conducted here immediately following the storm
ephemeral creekbeds or along hillslope depressions (Pomeroy, 1991). Scarps occurred in shallow
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residual soils less than 1 m deep over gneissic bedrock (Neary et al., 1986). All of the flows
occurred in undisturbed, forested areas (Neary et al., 1986).
Topography in the Bent Creek watershed is at least partially controlled by the underlying
concentration of tension joints in the bedrock. Where there is a greater amount of jointing,
topographic hollows tend to develop. These joints allow for the infiltration of groundwater,
enhancing breakdown of the rock. This accelerates weathering, providing loose material for mass
wasting (Pomeroy, 1991). Debris flows seem to originate on the bedrock-soil or bedrock-
colluvium interface within these hollows.
The November 1977 flood killed at least thirteen people; sixteen counties in western
North Carolina were declared disaster areas. The most serious flooding occurred along the
French Broad River downstream from Asheville and in Yancey County where nearly every
bridge was washed out (Eshner and Patric, 1982; Stewart et al., 1978). Flooding destroyed 384
homes, 389 miles (622 km) of highway, and 12 dams. In total there was over $50M in damages
associated with this storm (Stewart, 1978).
In 1977, precipitation, slope, and topography all contributed to the initiation of debris
flows southeast of Asheville (Pomeroy, 1981). In comparison to the historical range of rainfall
intensities for the entire Tennessee Valley area, the maximum intensities associated with the
1977 storm were in the middle-to-low range but antecedent moisture was exceptionally high
(177% above normal) for the two months preceding the storm (Neary and Swift, 1987). The
towards the northeast, east, and southeast (Pomeroy, 1991). These slopes would only receive
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sunlight in the morning and thus would have higher soil moisture.
3.2.6 September, 2004
The 2004 Atlantic hurricane season was exceptionally brutal. Fifteen tropical or
subtropical storms formed in the North Atlantic. Nine of these storms became named hurricanes
and six of these struck the United States (National Weather Service, 2004a). In North Carolina,
the remnants of three tropical systems, i.e., hurricanes Frances, Ivan and Jeanne, impacted the
western part of the state in September. Frances and Ivan caused extreme flooding in Asheville
and several debris flows and rockslides in the mountains, affecting Interstate-40. Rainfall totals
for the month over much of western North Carolina ranged from 10 to 25 inches (25 to 64 cm).
This was 2-to-5 times greater than normal (Badgett et al., 2004).
Hurricane Frances struck the east coast of Florida early on Sept. 5, 2004 and quickly
weakened into a tropical storm (National Weather Service, 2004a). The storm then rapidly
moved across the state, through the panhandle of Florida, and northeastward across the eastern
United States (Figure, 3.4). The effects of hurricane Frances could first be felt in North Carolina
on September 6 around 6:00 p.m. (Boyle, 2004) but most of the flooding and mass wasting
occurred on September 8 (Appendix C).
In North Carolina, the heaviest precipitation in 2004 occurred slightly east of the French
Broad Watershed but heavy rain also fell in the eastern portions of Tennessee and Kentucky, and
through most of West Virginia, Virginia, Maryland, Ohio and southwestern Pennsylvania
Sixty miles southwest of Asheville, Lake Toxaway received 14 inches (35.6 cm) of rain (Nowell,
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2004). In total, 17 western counties were affected by flooding (Anonymous, 2004). Hundreds of
people were evacuated from their homes and several had to be rescued from the rising water
(Nowell, 2004). Areas of Asheville located near the Swannanoa River were flooded, particularly
the shopping center near the entrance to the Biltmore Estate, were water stood as much as 5 feet
(1.5 m) deep (Nowell, 2004). In Haywood County, flooding along the Pigeon River also
inundated downtown Canton and Clyde.
The remnants of hurricane Frances caused at least 21 reported incidents of mass wasting
(Appendix C) along several major roadways in seven western North Carolina counties. However,
only three counties within the boundaries of the French Broad Watershed experienced debris
flows, i.e., Avery, Henderson, and Transylvania. The largest reported debris flow occurred east
of Asheville on Interstate-40, near Old Fort Mountain in McDowell County (Figure 3.12). This
slide crossed the westbound lane and the median to block four of the six lanes of a five-mile
stretch of Interstate-40 (Nowell, 2004). In Watauga County, one house was destroyed and eight
others condemned when a debris flow moved through a subdivision (North Carolina Geological
Survey, 2004a). Portions of the Blue Ridge Parkway were closed when at least six debris flows
destroyed the roadway in four areas between Linville Falls and Waynesville (Ball, 2004). About
250 roads became impassable or were closed due to flooding and mass wasting (Barrett, 2004).
Most of that road damage was in Buncombe County (Ball, 2004).
Ivan was an unusually long-lived hurricane that made landfall along the United States
southwestward, crossed over Florida and then into the Gulf of Mexico. By September 23, the
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remnants of Ivan had strengthened into a tropical storm and the “re-born” storm made landfall
for the second time on September 24 over southwestern Louisiana (National Weather Service,
2004a).
The remnants of Ivan moved into western North Carolina early on September 16.
Although Ivan had weakened to a tropical storm by the time it reached North Carolina, it still
packed powerful winds and heavy rain. Rainfall was not as heavy as that which fell during
Frances, mainly because the storm moved rapidly northeastward, but the western portion of the
state still received between 4 to 8 inches (10 to 20 cm) of rain. The heaviest precipitation fell in
Transylvania, Jackson and McDowell Counties at high elevations. Black Mountain (near
Asheville) received 11.5 inches (29.2 cm) of precipitation and Sapphire (in Transylvania County)
reported 15 inches (38 cm) (Figure 3.11) (Badgett et al., 2004).
Although Ivan produced less rain than Frances, high antecedent-moisture conditions and
saturated soils allowed for more slope movements to be produced. A total of 53 slope
movements have been attributed to hurricane Ivan (Cabe, 2004). Those recorded in Appendix C
occurred along major roadways (20 slope movements), but several other slope movements may
also have occurred in undisturbed or rural areas and were not reported by either the North
Carolina Department of Transportation (NCDOT) or major news agencies. Further work will
have to be conducted to obtain a complete record of these slope movements.
Slope movements, downed trees, and flooding obstructed several roads throughout
Further west, near the North Carolina-Tennessee border, a large portion of the eastbound lane of
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Interstate-40 collapsed due to undercutting by the swollen Pigeon River. A major debris flow
also destroyed a home in Candler in Buncombe County (Cantley-Falk, 2004).
The worst damage occurred in the community of Peeks Creek in Macon County. At
around 10:10 p.m. on September 16, a debris flow originated near the peak of Fishhawk
Mountain (Figure 3.13) destroying at least fifteen houses, injuring several people, and resulting
in the deaths of four people (and an unborn baby). The debris flow traveled approximately 2.25
miles (3.6 km), possibly in pulses, dropping nearly 2200 feet (670 m) in elevation as it
progressed down a mountain cove and into the north fork of Peeks Creek (Cabe, 2004). The
velocity of the flow was estimated to be 20.3 mi/hr (32.7 kph) near the scarp and 33.2 mi/hr
(5305 kph) just upstream of the area of major damage (Cabe, 2004). The force of the flow
scoured the streambed, ripped trees down and left other striped of bark; houses were removed
from their foundations (North Carolina Geological Survey, 2004b; Ostendorff, 2004). The flow
probably originated as a debris slide; a slab of cohesive rock, debris and earth the size of a
football field detached from the side of the mountain and quickly disintegrated into a debris flow
as more water mixed in with the slide material (Cabe, 2004).
What is remarkable about the Peeks Creek disaster is that this location is the same area
where a two large debris flows occurred in 1876 (Clingman, 1877). Observations of residents
living in the area were strikingly similar in both incidents. Clingman (1877) describes trees
stripped of bark and limbs, a “clean, broad furrow more than two miles” long carved into the side
transformer, one resident described seeing debris spinning and flying around in the air in a
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circular motion above their house (Biesecker and Shaffer, 2004). In 1876, residents described
seeing funnel-shaped spinning masses of water near the crest of the mountain (Clingman, 1877).
Tornadoes are fairly rare in mountainous areas, but do occasionally develop. While there was
wind damage throughout the Peeks Creek area after the passage of Ivan, this damage was more
consistent with wind shear. So far, the National Weather Service has not been able to conclude if
a tornado actually did touch down on Fishhawk Mountain, but they do not discount the
eyewitness accounts of local residents (Cabe, 2004).
The question remains, "Why did mass movements occur in these Macon County areas as
opposed to elsewhere?" In the Peeks Creek flow, fracture planes in the rock, sloping 35-55
degrees, provided a smooth slip surface. Soil layers over this bedrock were thin, generally less
than three feet (1 m) deep (Cabe, 2004). These physical properties of the terrain probably
facilitated the debris flows on Fishhawk Mountain. Meteorologically, the rainfall rates from the
remnants of hurricanes Frances and Ivan were not unusually intense for either event. However,
the combined rainfall totals were exceptionally heavy. The rainfall produced by Frances initially
saturated mountain soils and slopes. Before the soil had a chance to drain sufficiently, Ivan
moved through the area, bringing even more rain to already soaked areas. Rainfall from Ivan
may have caused even higher soil-water pressure on slopes than during Ivan, explaining why
there was more mass wasting during the second storm. Nonetheless why, exactly, debris flows
occurred in one area and not another, even under similar meteorological and physical conditions
3.2.7 Precipitation Thresholds and Recurrence Intervals
Generally historical rainfall totals from 1901 to 2004 are well within the 125 mm/d to
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Generally, historical rainfall totals from 1901 to 2004 are well within the 125 mm/d to
250 mm/d precipitation thresholds suggested by Eschner and Patric (1982) as necessary for
debris-flow generation (see Figure 1.2). Average rainfall amounts were greater than 125 mm/d in
all cases, i.e., greater than the minimum amount of precipitation necessary to saturate soil and set
the stage for debris flows (Table 3.2). While storms with rainfall totals of 250 mm/d are
described as extremely rare, all but one storm produced maximum precipitation amounts that
exceeded 250 mm/d (Eschner and Patric, 1982). Extreme precipitation does not necessarily
guarantee that debris flows will occur, as during the July 5-6, 1916 storm, but extreme
precipitation certainly increases the risk of slope instability.
Escher and Patric (1982) suggested a return interval of 100 years or less for storms
producing debris flows in western North Carolina. Nonetheless, historical rainfall data from 1901
to 2004 indicates that recurrent intervals may be smaller. According to Cabes (2004), there have
been 14 storms or hurricanes that have triggered slope movements in western North Carolina
since 1901. Based on the occurrence of major debris-flow triggering storms from the historical
record (i.e. those that have extensive amounts of precipitation and debris-flow location data), a
preliminary recurrence interval for slope instability can be estimated. The return interval for
mass wasting from 1876 to 2004 (for six events) averages 25.6 years. This return interval has
varied from as few as 15 years to as many as 37 years. Based on Cabes’ data (2004), the
recurrence interval for slope movements from 1901 to 2004 may be as few as 7.4 years. On
traveled over the area within 10 to 20 days of each other. Antecedent moisture seems to
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play a crucial role in predisposing slope to debris-flow generation. Geoscientists,
emergency management, and citizens living in these mountainous areas, must be vigilant
in monitoring weather conditions, particularly with repeated sequences of heavy rain
events.
Table 3.1: Prehistoric debris flow studies in the southern Blue Ridge and the age-dating techniques
utilized.
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Reference Year Dating Technique Location Age of Features
Kochel 1987 Radiocarbon Davis Creek, VA > 11,000 BP
Jacobson et al. 1989 Radiocarbon West Virginia10,000 - 12,000 BP; 315BP
Behling et al. 1993 Radiocarbon West Virginia 17,000 - 22,000 BP
Kochel 1990 RadiocarbonAppalachian Mountains, NC
16,000 - 25,000 BP
Eaton et al. 1997 RadiocarbonUpper Rapidan River Basin,VA
2,200-50,800 BP
Shafer 1984 Thermoluminescence Flat Laurel Gap, NC Late QuaternaryMills 1982 Relative-age North Carolina ?
Mills and Allison 1995aRelative-age/paleomagnetism
Watauga County, NC 780 ka - 1Ma
Mills and Allison 1995b Relative-age Haywood County, NC ?
Liebens andSchaetzl
1997 Relative-age Macon and Swain Co., NC ?
Mills 2000 Relative-age Appalachians ?
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54
Figure 3.2: Areas of major debris flows and landslides in western North Carolina (after Scott, 1972).
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Figure 3.3: A sketch map of debris flows that occurred along Gouges Creek in Mitchell County,
North Carolina in May, 1901 (Myers, 1902).
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Figure 3.4: Map showing some of the hurricane and storm paths that have affected western North
C li t d b th U S N ti l H i C t d th U S G l i l S W t
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Figure 3.5: Total storm precipitation for July 14-16, 1916.
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Figure 3.6: U. S. Geological Survey hydrograph from the river gauge located on the French Broad
River in Asheville for the month of July, 1916. In early July, there is a large hydrographic spike
generated by rainfall from the remnants of a hurricane that passed through the area. There is also a
second large peak between July 16 and 17 generated by the flooding associated with a hurricane
moving northwestward over the watershed. This is the greatest recorded streamflow for the gauge at
Asheville.
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Figure 3.7: Total storm precipitation for August 14-15, 1940 adapted from U. S. Geological Survey,
1949).
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Figure 3.9: Total storm precipitation for November 2-5, 1977 (adapted from Neary and Swift, 1987).
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Figure 3.10: Total storm precipitation (inches) for the remnants of Hurricane Frances (NationalWeather Service, 2004b).
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Figure 3.12: A debris flow that blocked the westbound lanes of Interstate-40 near Old Fort Mountain
in McDowell County (North Carolina Geological Survey, 2004a).
Figure 3.13: The initiation zone of the debris flow that occurred on Fishhawk Mountain anddevastated the Peeks Creek area of Macon County on September 17, 2004 (Wilett, 2004).
Table 3.2: Major storms within the French Broad Watershed and their minimum, average, and
maximum precipitation amounts.
STORM DATE MIN (in) AVG (in) MAX (in) MIN (mm) AVG (mm) MAX (mm)
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( ) ( ) ( ) ( ) ( ) ( )
1901 5 7 8.9 128 177.8 228.41916 (1)* 4 7 10 101.6 177.8 254
1916 (2) 1 10 22.2 25.4 254 564.4
1940 (1) 4 13 16 101.6 330.2 406.4
1940 (2) 3 8 13 76.2 203.2 330.2
1977 2 8 14 50.8 203.2 355.6
2004 - FRANCES 4 10.3 16.6 101.6 261.6 421.6
2004 – IVAN 4 9.5 15 101.6 241.3 381
*no debris flows produced
CHAPTER 4: METHODOLOGY
In order to quantify debris-flow potential in the French Broad Watershed, two
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deterministic process-based computer-modeling programs have been chosen, i.e.,
SINMAP (Stability INdex MAPping) (Pack et al., 1998) and SHALSTAB (Shallow
Landsliding Stability Model) (Dietrich and Montgomery, 1998). Both use similar
numerical models and steady-state hydrologic assumptions to quantify the influence of
topography on pore pressure. These computer programs, instructions, and examples are
freely available for download from their respective websites.
4.1 Infinite-slope Equation
SINMAP and SHALSTAB are based on the infinite-slope form of the Mohr-
Coulomb failure law, an equation commonly applied as part of a slope-stability model
within the GIS environment (Hammond et al., 1992; Montgomery and Dietrich, 1994;
Wu and Sidle, 1995; Pack et al., 1998). The infinite slope equation is:
where C r is root cohesion, C s is soil cohesion, θ is slope angle, ρ s is soil density, ρ w is the
density of water, g is acceleration due to gravity, D is the vertical soil depth, Dw is the
vertical height of the water table, and Φ is the internal soil friction angle. Ultimately, the
infinite-slope equation calculates a dimensionless stability index, or factor of safety (FS),
by comparing the destabilizing components of gravity with the stabilizing components of
This assumption is reasonable because the drainage barrier, e.g., top of bedrock, and the
ground surface are often parallel on colluvial slopes. The contrast in hydraulic
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conductivity between the overlying soil and the material forming the drainage barrier
causes groundwater to flow nearly parallel to the ground surface, providing a surface for
slope failure (Hammond et al., 1992). Another assumption is that the failure plane is
assumed to be infinite in extent and the resistance to movement along the sides and ends
of the slope movement are so insignificant that they may be ignored (Hammond et al.,
1992). Finally, the soil is assumed to be of uniform thickness.
Although SINMAP and SHALSTAB share similar physical assumptions and
equations, they use different indices to quantify instability. SINMAP uses the infinite-
slope equation to calculate a “factor of safety”, i.e., an estimated potential for instability
whereas SHALSTAB quantifies terrain instability in terms of the effective recharge
required to trigger instability. A comparison of the results and relative performance of
these programs is difficult due to their dissimilar output. To simplify comparison, model
parameters have been kept as similar as possible.
4.2 Parameterization of the Models
SINMAP and SHALSTAB share specific parameters that must be input into the
program to optimize the models for local soil and meteorological conditions. The models
have three necessary components in common, i.e., digital terrain data in the form of a
digital elevation model (DEM) or some other type of grid-based digital topographic data,
4.2.1 Digital Elevation Models
Both SINMAP and SHALSTAB assume that the dominant control on debris-flow
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occurrence is surface topography, specifically the interplay between slope and shallow
subsurface flow convergence (Montgomery and Dietrich, 1994). To calculate topography,
both computer programs require digital elevation data. In this study, DEMs at 10-meter
and 30-meter scales were obtained from the United States Geologic Survey National
Elevation Dataset (http://seamless.usgs.gov/). Both SINMAP and SHALSTAB are highly
sensitive to the accuracy of the DEM. Verification of data quality is advisable prior to
running these models (Zhang et al., 1999; Dietrich et al., 2001; Guimaraes et al., 2003).
As explained by Michael Oimoen, a scientist from the USGS National Center for
Earth and Resources Observations and Science (pers. comm., 2005), USGS DEMs used
for this study were created from the digitized contours of scanned 1:24,000-scale
topographic quadrangle sheets. The contours from these maps are digitized and either a
10 x 10 meter or 30 x 30 meter mesh grid is overlaid on top of the digitized contours. For
each grid cell, inverse distance weighting interpolation is used to calculate the elevation
value of each cell (Maune et al., 2001). Since the 10-meter and 30-meter DEM are
created from the same scale map and source data, neither is more “accurate” than the
other. But as the cell size is smaller for the 10-meter DEM, the 10-meter spacing better
captures the detail in the contours. Given both 10- and 30-meter DEM data are available
for the watershed, SINMAP and SHALSTAB runs were completed in both resolutions
catchment or drainage area, and compute the slope for each grid cell (Dietrich and
Montgomery, 1998; Pack et al., 2001).
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Both programs have procedures for increasing accuracy and eliminating potential
errors inherent in the DEM grid. Pit-filling corrections are automatically executed within
both programs to eliminate artificial sinks or depressions in the DEM. Given that such
grid elements are rare in natural topography, they are assumed to be errors created during
the initial preparation of the DEM (Pack et al., 2001). Both programs use a “flooding”
approach where pits are filled, raising the elevation to that of the lowest neighboring grid
cell.
The processing time for DEMs with large spatial areas, such as a DEM for the
entire watershed, can be extremely long, depending on the speed and memory availability
of the user’s personal computer. Both SINMAP and SHALSTAB seem to have a DEM
size limitation. Although SINMAP could process the entire watershed at the 30-meter
scale, the 10-meter DEM had to be broken down into county-size pieces. SHALSTAB
cannot process DEMs larger than a county at a 30-meter scale without creating run-time
errors and premature termination. At the 10-meter scale, SHALSTAB can run a
maximum of four 1:24,000-scale quad-sized DEM blocks before shutting down. This
significantly increases process time. For this reason, the original 30-meter DEM had to be
clipped to county-size when run in SHALSTAB. In the interest of time, only Haywood
County was chosen for several model runs in SHALSTAB for comparison to the
4.2.2 Debris Flow Inventory
In order to complete the slope-stability model runs for SINMAP and
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SHALSTAB, basic digital-line and polygon coverages were collected and formatted for
use in ArcView 3.x and ArcGIS 8.3. Both programs require the input of landslide
location data to verify the model results; SINMAP requires point data whereas
SHALSTAB requires polygonal shapes of each landslide feature.
Landslide location data for this study was obtained through a combination of
aerial-photography interpretation, previous studies, field investigation, and data provided
by both the North Carolina Geological Survey and the North Carolina Department of
Transportation. Most of this data was not in a digital (e.g., GIS) format and was digitized
into a single landslide-location database for the French Broad Watershed (Appendix D).
Other GIS data, e.g., roads, hydrology, and geology, and digital orthophoto quadrangles
(DOQs) were obtained through the North Carolina State University Geodigital Library
website (http://www.lib.ncsu.edu/stacks/gis/) and the North Carolina Center for
Geographic Information and Analysis map server (http://204.211.135.111).
4.2.3 Soil Data
Various soil parameters, such as cohesion, density, and hydraulic conductivity,
have been estimated from the general soil maps of North Carolina and from the advice of
experienced geologists who routinely work with soils in the study area. Basic soil maps
were downloaded from the State Soil Geographic database (STATSGO) available
that give the proportional extent of the component soils and their properties. There are
eighteen general soil types found within this watershed (see Figure 2.2). The STATSGO
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soils coverage for the State of North Carolina was downloaded as a polygon shapefile,
imported into ArcGIS, and clipped to the extent of the French Broad Watershed.
4.2.4 Soil Density
Both programs require the input of a value for soil density. This value is used to
represent the total bulk density of the soil over the entire study area. In SINMAP the
default value for soil density is 2000 kg/m3
while in SHALSTAB the value is 1700
kg/m3. Otteman (2001) used a value of 1450 kg/m3 for her study area in the Bent Creek
Experimental Forest in Buncombe County, North Carolina whereas in Madison County
Virginia, a value of 1200 kg/m3
was used by Morrissey et al. (2001).
A geologist with
the North Carolina Department of Transportation estimates typical density values of
1441.6 kg/m3to 2082 kg/m
3in soils of western North Carolina (Jody Kuhne, pers.
comm., 2004). Although no independent soil-density analysis was completed, a value of
1922 kg/m³ was chosen as representative value for the entire watershed and was used in
all model runs. This number was suggested by Richard Wooten, a geologist with the
North Carolina Geological Survey who has done extensive slope-stability work in
western North Carolina.
4.3 SINMAP Parameters
SINMAP has simplified the infinite-slope equation (1) originally proposed by
combined into one dimensionless cohesion factor. The SINMAP stability-index equation
is given by:
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where the variables a and θ are the specific catchment area and slope, respectively, and
are derived from the topography determined using a DEM. The other parameters, C
(cohesion), Φ (soil friction angle), R/T (recharge divided by transmissivity), and r (the
ratio of water and soil density) are manually entered into the model. These parameters are
considered more uncertain and are specified in terms of upper and lower boundary values
(Pack et al., 1998b).
The default values for the foregoing parameters are provided in Figure 4.2. Values
may be adjusted to represent local conditions, to identify landslide-prone areas more
precisely, and to assure that a significant amount of the landslides are captured in areas
considered “unstable”. The following text rationalizes the specific input values and
distributions used in this study (T/R, C , and Φ).
For each cell of the DEM, SINMAP calculates a factor of safety (FS), a ratio of
stabilizing-to-destabilizing forces found in equation (2). This is a dimensionless index
number with a value between zero and 10. If the index falls below one, there is a high
probability that the area is unstable whereas high index values (greater than one) indicate
4.3.1 T/R (Ratio of Transmissivity to Effective Recharge)
The ratio of transmissivity of the soil (m2/hr) to the effective recharge (m/hr) has a
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default range of 2000 to 3000 (m). When multiplied by the sine of the slope, the T/R
value can be interpreted as the length of the hillslope (in meters) required to develop
saturation (Pack et al., 1998b).
The recharge rates used in this study have been derived from four precipitation
thresholds, i.e., 50 mm/d, 125 mm/d, 250 mm/d, and 375 mm/d. Two of these
precipitation thresholds, 125 mm/d and 250 mm/d, are described by Eschner and Patric
(1982) as necessary for debris-flow initiation in the Appalachians (see Figure 1.2). The
50 mm/d rate was chosen as a minimum rate, i.e., one where SINMAP should not
indicate a large mapped area of instability. The last threshold amount, 375 mm/d, is used
as a maximum, i.e., an extreme example of the precipitation that can produce debris flows
in the French Broad Watershed. Only a few times have recorded rainfall totals actually
exceeded this rate (see Table 3.2).
The transmissivity rate (m²/hr) was calculated using the basic equation:
T = Kb (3)
where K (m/hr) is the permeability or hydraulic conductivity of the soil and b (m) is the
soil depth. Permeability rates within each soil unit are available in the STATSGO soil
attribute database. The permeability of each of the eighteen soil units within the
watershed was calculated by determining the average permeability of each of the
the recharge rate to find the upper and lower T/R (m) values. The final T/R values used
were an average of each of the eighteen soil units (Table 4.1).
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4.3.2 Dimensionless Cohesion
In SINMAP, root and soil cohesion is combined with soil density and thickness to
calculate a dimensionless cohesion factor, C (Pack et al., 1998b). Conceptually, this is the
ratio of the cohesive strength of the soil and roots relative to the weight of a saturated
thickness of soil, or the contribution of cohesion to the stability of a slope (Pack et al.,
1998b). The equation used to determine dimensionless cohesion is:
C = (Cr + Cs)/(hρsg) (4)
where C r is root cohesion (N/m2), C s is soil cohesion (N/m2), h is the soil thickness (m),
ρ s is the wet soil density (kg/m3), and g is the acceleration due to gravity (9.81 m/s
2).
Various values for cohesion have been suggested in diverse SINMAP studies. The
default values used for cohesion in SINMAP are 0 and 0.25 but Morrissey et al. (2001)
used values between 0 and 1.28. Wooten uses values of 0.6 to .96, suggesting that zero
cohesion is unlikely in the Southern Appalachians due to the considerable amount of low
vegetation growing on mountain slopes (e.g., rhododendron), which would add to the
overall root cohesion found in the soil. Because of the lack of more precise soil cohesion
data, the SINMAP default values were deemed reasonable for the watershed and used in
all of the SINMAP model runs.
S
shearing angle. The angle is constant for a specified material and depends on the size,
shape, and surface roughness of the grains, the density of the soil, moisture content, and
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material saturation (Easterbrook, 1999).
Values for the soil friction angle for certain soil types can be estimated from
tables provided by Hammond et al., (1992). These tables require that the soil be classified
according to the Unified Soil Classification (USC) system (ASTM D-2487-85 and D-
2488-84) which are part of the STATSGO attribute data. Although no independent soil
analysis was completed, the 26° and 45° soil-friction angles used to calibrate the model
were considered realistic for the study area (Hammond et al., 1992). Given that this study
requires that parameters be generalized over large areas, a wide variation encompassing
the properties of several different soil types, from clayey to sandy and gravelly soils, is
more realistic than a small range of values.
4.4 SHALSTAB Parameters
Like SINMAP, SHALSTAB is an extension of ArcView©
and is theoretically
based on the infinite-slope equation. SHALSTAB assumes steady-state, saturated flow
parallel to the slide surface and uses Darcy’s Law to estimate the spatial distribution of
pore water pressure. However, the equation that SHALSTAB uses to formulate a slope-
stability map is quite different from that used by SINMAP. The SINMAP equation solves
for a factor of safety whereas SHALSTAB calculates the ratio of log q/T (effective
precipitation over transmissivity) (Dietrich et al., 2001). Essentially, this is the amount of
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The two hydrologic-slope stability equations solved by SHALSTAB are shown
above. Equation (5) solves for the hydraulic ratio while equation (6) solves for the area
per outflow boundary length. The equations have three topographic terms defined by the
DEM: drainage area (a), outflow boundary length (b), and hillslope angle (θ ). Of the
other four parameters, soil bulk density ( ρ s) and internal friction angle (Φ) can be
assigned by the user. The ratio of transmissivity (T ) and effective precipitation (q) are
solved by SHALSTAB and are given as the final output of the model (Dietrich et al.,
2001).
The authors of SHALSTAB made even more simplifications to the infinite-slope
equation, and their slope-stability model, than did the authors of SINMAP. First, they
decided to set the cohesion to zero and eliminate the variable of root strength altogether.
Although this approximation is clearly incorrect in some situations, the elimination of
root cohesion makes parameterization easier for the user. Root strength varies widely,
both spatially and over time. It is especially hard to estimate over large areas, such as at
the watershed-scale. They also consider setting the cohesion to zero to be a very
“conservative” thing to do as this maximizes the extent of possible instability within the
be “parameter free” when run with the default parameter values. In other words, they
wanted a slope stability model where a user did not have to have any available soil data
t t lib t th d l Wh th d f lt i f SHALSTAB i l
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parameters to calibrate the model. When the default version of SHALSTAB is run, only
the soil density and the internal soil friction angle can be adjusted. Later versions of
SHALSTAB have included an option where the cohesion value and soil depth can be
adjusted if necessary (Figure 4.3).
The SHALSTAB model needed to be calibrated in a way in which it could be
directly comparable with SINMAP. Some of the variables, such as soil density (1922
kg/m³) and soil thickness (2 m), could remain the same between the two models. But
other soil property values, such as cohesion and soil friction angle, vary widely over the
study area. This problem is overcome by SINMAP, which uses a range of values for
cohesion, soil friction angle, and transmissivity over recharge. SHALSTAB only uses a
single input value for cohesion and soil friction angle. As specific soil data is not
available at the watershed-scale, a variety of parameters were tested during several
different model runs to find which values best-approximated landslide susceptibility.
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Figure 4.1: The infinite slope equation as defined by Hammond et al., (1992) and Pack et al., (1998b)
where C r is root cohesion, C s is soil cohesion, θ is slope angle, ρ s is soil density, ρw is the density of
water , g is acceleration due to gravity, D is the vertical soil depth, Dw is the vertical height of the
water table, and Φ is the internal soil friction angle. In the SINMAP model, the ratio of the vertical
soil depth to the vertical soil height is simplifed so that depth is measured perpendicular to the slope
(h). (Diagram after Hammond et al., 1992 and Otteman, 2001)
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Figure 4.2: Default parameters used in the SINMAP model analysis. The values for the gravitational
constant and the density of water were not adjusted in this study.
Table 4.1: Table of the hydraulic conductivity ( K , m/hr), transmissivity (T , m²/hr), and T/R (m) values used for each precipitation threshold (50 mm/d,
125 mm/d, 250 mm/d, and 375 mm/d) in the SINMAP analysis. The numbers in blue are the lower bound values while the numbers in red are the upper
bound values.
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78
SOILMAPPING
UNIT
HYDRAULICCOND (m/hr)
LOWER
HYDRAULICCOND (m/hr)
UPPER
T (m2 /hr)
LOWERT (m
2 /hr)
UPPER
T/R 50 (mm/d)LOWER
T/R 50 (mm/d)
UPPER
T/R 125(mm/d)
LOWER
T/R 125(mm/d)
UPPER
T/R 250(mm/d)
LOWER
T/R 250(mm/d)
UPPER
T/R 375(mm/d)
LOWER
T/R 375(mm/d)
UPPER
NC005 0.024 0.331 0.048 0.662 24.0 331.0 9.6 132.4 4.6 63.7 3.1 42.4
NC006 0.014 0.152 0.028 0.304 14.0 152.0 5.6 60.8 2.7 29.2 1.8 19.5
NC088 0.005 0.038 0.010 0.076 5.0 38.0 2.0 15.2 1.0 7.3 0.6 4.9
NC089 0.003 0.027 0.006 0.054 3.0 27.0 1.2 10.8 0.6 5.2 0.4 3.5
NC090 0.005 0.115 0.010 0.230 5.0 115.0 2.0 46.0 1.0 22.1 0.6 14.7NC091 0.009 0.053 0.018 0.106 9.0 53.0 3.6 21.2 1.7 10.2 1.2 6.8
NC092 0.009 0.091 0.018 0.182 9.0 91.0 3.6 36.4 1.7 17.5 1.2 11.6
NC093 0.01 0.032 0.020 0.064 10.0 32.0 4.0 12.8 1.9 6.2 1.3 4.1
NC094 0.005 0.075 0.010 0.150 5.0 75.0 2.0 30.0 1.0 14.4 0.6 9.6
NC095 0.003 0.021 0.006 0.042 3.0 21.0 1.2 8.4 0.6 4.0 0.4 2.7
NC096 0.008 0.027 0.016 0.054 8.0 27.0 3.2 10.8 1.5 5.2 1.0 3.5
NC097 0.025 0.156 0.050 0.312 25.0 156.0 10.0 62.4 4.8 30.0 3.2 20.0
NC098 0.004 0.015 0.008 0.030 4.0 15.0 1.6 6.0 0.8 2.9 0.5 1.9NC099 0.01 0.081 0.020 0.162 10.0 81.0 4.0 32.4 1.9 15.6 1.3 10.4
NC100 0.01 0.099 0.020 0.198 10.0 99.0 4.0 39.6 1.9 19.0 1.3 12.7
NC102 0.007 0.035 0.014 0.070 7.0 35.0 2.8 14.0 1.3 6.7 0.9 4.5
NC103 0.005 0.025 0.010 0.050 5.0 25.0 2.0 10.0 1.0 4.8 0.6 3.2
NC104 0.012 0.046 0.024 0.092 12.0 46.0 4.8 18.4 2.3 8.8 1.5 5.9
AVERAGE 3.0 331.0 1.2 132.4 0.6 63.7 0.4 42.4
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Figure 4.3: Default values used for SHALSTAB.
CHAPTER 5: RESULTS AND DISCUSSION
5.1 Introduction
The purpose of this chapter is to provide model results for SINMAP and
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The purpose of this chapter is to provide model results for SINMAP and
SHALSTAB for a variety of parameter ranges, to describe the effectiveness of each
model, and to identify the most sensitive model parameters. SINMAP and SHALSTAB
results are compared herein to identify weaknesses and strengths inherent to each model.
Finally, results will be compared with respect to slope, aspect, geology, and soil type.
5.2 Debris Flow Inventory
A total of 142 debris flows have been mapped in the 7330 km² study area (Figure
5.1 and Appendix D). All of these slope movements have been located using historical
documents and maps, aerial photography, field identification, and data provided by both
the North Carolina Geological Survey and the North Carolina Department of
Transportation. Landslides were identified in all counties except for Avery County. Of
the 42 geologic units that occur within the watershed, debris flows have developed on
fourteen. Most of these units are gneiss with a large component of biotite or muscovite
(coded Ybgg, Ymg, Zabg, Zatm, Zatb, Zybn on the geologic map). Debris flows also
have occurred in quartz diorite (Dqd), metagraywacke (Zatw, Zgs), quartzite (Zsl),
siltstone (Zsp), sandstone (Zsr), and slate (Zwc).
The greatest number of debris flows (52) have occurred in soil map unit NC095,
generally a stony fine-to-sandy loam, and NC093 (46), a stony silt-to-sand loam. The
coarse-grained soils may be attributed to rapid infiltration through these soils. In the thin
soils of the Southern Appalachians, soil pore pressure can quickly increase as
groundwater builds above the impermeable bedrock, increasing the risk of slope failure.
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g p , g p
The gradient and aspect of the debris-flow locations have been calculated using
the 10-meter DEM (with ArcGIS Spatial Analyst extension), given that the 30-meter
DEM seems to underestimate slope (Zhang et al, 1999). For the entire watershed, slope
varies from 0 to 74 degrees. Debris flows have occurred on slopes ranging from 10 to 50
degrees, although 88% of the slopes are at least 20 degrees. The average slope on which
debris flows have initiated is 28 degrees.
The aspect slope angle, as calculated in ArcGIS, is the compass direction towards
which a slope faces. The "aspects" of the debris-flow headscarps occur in diverse
directions, but show an affinity toward the east (32), southeast (23), southwest (21), and
south (21). East-facing slopes receive only morning sunlight during the winter and thus
have higher soil moisture than south- and west-facing slopes. During high rainfall events,
this leads to higher levels of antecedent moisture, faster saturation, and greater soil pore-
water pressure, leading to an increased incidence of slope failure. The incidence of debris
flows on south-facing slopes may be due to the direction of storm tracks over the
watershed. Several of the hurricanes that have tracked over western North Carolina have
come from south, out of the Gulf of Mexico (see Figure 3.4).
5 3 SINMAP R lt
summarized in Table 5.1. Although there is an option in SINMAP to create multiple
calibration regions, usually based on the properties of individual soil types, only one
general calibration region has been used in this study. This was deemed appropriate due
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to the general nature of the soil data and the regional scale of the study.
SINMAP defines six different stability-class definitions based upon the stability
index (SI) (Table 5.2). The terms “stable”, “moderately stable”, and “quasi-stable”, are
used to represent areas that should not fail under the most conservative input parameters.
The areas modeled as having “lower threshold” and “upper threshold”, are those that
respectively have a <50% or >50% probability of instability. Areas defined as “defended
slopes” are unstable throughout the range of the specified parameter. Where these slopes
occur in the field, they are only stable due to factors not modeled by SINMAP. For
example, they may be bedrock outcrop (Pack et al., 1998b). Parameters are adjusted so
that the resulting map “captures” the maximum amount of observed landslides in regions
with a low SI (stability index), while minimizing the spatial extent of low-stability
regions (Pack et al., 2002).
Initially, SINMAP model runs for this study involved using 30-meter DEMs,
default parameters, and the four aforementioned precipitation thresholds. At the outset of
this study, only 30-meter DEMs were publicly available for western North Carolina. In
the summer of 2004, 10-meter DEMs also became available. As a result, the SINMAP
model runs were completed at both scales to see if the coarseness of the 30-meter DEM
5.3.1 Results – 30-meter DEM
The default SINMAP values seem to underestimate instability in the watershed.
For the 30-meter DEM, the model predicts that 52% (73) of the observed debris flows
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would have occurred in the lower threshold for instability whereas none were found in
the “upper” and “defended” stability classes (Figure 5.3). All of other 67 debris flows
(48%) occurred in the more stable stability classes.
In the next SINMAP calculation, a precipitation threshold of 50 mm/d was used
with the 30-meter DEM. Soil friction angle, soil density, cohesion and T/R were adjusted
(Table 5.1). Under these conditions, 93.6% (131) of the inventoried debris flows are
predicted to occur in the unstable classes, but only one of these was predicted to fail
unconditionally. In this model run, 62.7% (4431.8 km²) of the study area is predicted to
be unstable.
Thresholds of 125 mm/d, 250 mm/d, and 375 mm/d were subsequently used with
SINMAP. For all three simulated rain events, 131 observed debris flows occurred in
unstable zones whereas 9 slides (6.4%) were predicted to form in stable zones. Both the
total area of instability and the area predicted to be “defended” increased slightly as
recharge increased. For a precipitation event of 125mm/d, the unstable zones accounted
for 4418.1 km² (62.6%) of the study area while 28.2 km² (0.4) of the watershed was
considered unconditionally unstable (Figure 5.4). The defended class contains four
(2.9%) of the inventoried debris flows.
defended zone increases to 61.4 km² (0.9%) and includes eight (5.7%) of the debris
flows.
In the final calculation, the low value for dimensionless cohesion was increased
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slightly from zero to 0.1 for a recharge event of 125 mm/d, with all other properties
remaining the same. This should better model the effects of root cohesion in the
watershed, a factor which often has a strong effect on slope stability, even during heavy
precipitation. With cohesion increased, the area of predicted unstable land decreased to
3170.6 km² or 44.6% of the watershed, but still contained 124 (88.6%) of the inventoried
debris flows. Increased cohesion also increased the stability zones so that 35.7% of the
study area (2534.7 km²) became classified as unconditionally stable.
5.3.2 Results – 10-meter DEM
When comparing the area of the 10-meter grid calculated by SINMAP to the 30-
meter grid, the total area of the 10-meter grid is less than the 30-meter grid (Appendix E).
The 10-meter SINMAP model run seems to have assigned “NO DATA” values instead of
a stability index to many cells in the raster. Moreover, the stability index classes captured
only 122 of the 142 debris flows used in the parameter run. No explanation or repair has
yet been discovered for this problem.
The default results for the 10-meter DEM differ slightly from 30-meter results but
still underestimate instability (Figure 5.5). The same number of debris flows (73)
occurred in unstable stability classes, accounting for 18.4% of the study area (1274.3
As with the 30-meter DEM model, the results for the four precipitation thresholds
are very similar. The extent of the “stable”, “moderately stable”, and “quasi-stable” zones
(2409 km²) and the number of debris flows (7) occurring within these classes are the
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same for all four recharge events (Figure 5.6). Only the “lower”, “upper”, and “defended”
stability classes change slightly for each precipitation threshold, shifting to a more
unstable class as the recharge increases. The number of debris flows that fall in the
unstable classes (115) also remains the same for all four recharge events.
Compared to the 30-meter DEM, a larger portion of the watershed is predicted to
be unconditionally unstable in the 10-meter DEM. For the 50 mm/d threshold, 24.8 km²
of the area is predicted to be “defended” although no debris flows fall into this zone. For
125 mm/d of recharge, the “defended” zone increases to 70.8 km² (1.0%) and includes
eight (6.6%) slides. In the calculation for 250 mm/d and 375 mm/d recharge, the
unconditionally unstable area increases to 126.4 km² and 138.8 km², respectively, and
contains 12 (9.8%) of the debris flows.
As with the 30-meter DEM, when the value for dimensionless cohesion is
increased (0.1), the areas of instability decrease while stability increases. Of the 122
inventoried debris flows, 108 (88.5%) fall within the three unstable zones. This is
equivalent to 3278.7 km² (47.7%) of the watershed. More of the watershed falls within
the “defended” stability zones (50.2 km²) than in the 30-meter DEM, whereas 33.6% of
the area is predicted to be unconditionally stable (2308.1 km²).
parameters used. The areas of greatest instability also match well with the steepest terrain
and generally follow steep ridgelines. Overall, the model does well, accurately modeling
approximately 94% of the inventoried debris flows in unstable zones with an SI less than
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1.0 for both DEM scales and for all four precipitation thresholds. With a slightly
increased cohesion value, accuracy decreases to about 88%, but landslide density
increases as the model minimizes the extent of the areas mapped as unstable.
The 10-meter DEM models a slightly greater percentage of instability in the
watershed than the 30-meter DEM. Slopes that are derived from DEMs vary with the
spatial resolution, becoming lower at larger pixel sizes (Zhang et al., 1999). Given the
coarser resolution of the 30-meter DEM, the elevation data is more generalized and
slopes are typically underestimated. Using the 30-meter DEM, SINMAP could not
predict small areas of increased instability along narrow ridges and valleys. The greater
resolution of the 10-meter DEM allows for better prediction of unstable areas (2.7%
improvement) and a slightly greater percentage of inventoried debris flows within the
three unstable classification zones (an 0.7% improvement).
In this study, SINMAP is not particularly sensitive to changes in the value for
recharge. Even with significant changes in the precipitation threshold, there was little
change in the predicated areas of instability. This is unexpected because one of the most
important factors in triggering debris flows surely is the rate of recharge. SINMAP was
more sensitive to changes in the values for dimensionless cohesion, a value increased
British Columbia so the default parameters reflect representative soil data for that cool-
climate area (Pack et al., 1998b). Model calibration was based on the thick packages of
coarse subangular till and colluvium found in British Columbia. Even with low values of
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hydraulic conductivity ( K ), a large value for soil thickness (b) would result in a greater
transmissivity value. Deeper soils occur in the study areas of Pack et al. (1998b) than in
the present study area, hence their use of T/R default values of 2000 m and 3000 m.
Given these default values with a 2-meter soil thickness and a moderate (125 mm/d)
recharge rate, the hydraulic conductivity lies between 5.21 m/hr and 7.81 m/hr. These
rates are representative of well-sorted sand and gravel but are unrealistic for the French
Broad Watershed (Fetter, 1994). Clearly, there are other factors at work in the Southern
Appalachians that trigger debris flows, factors that are not yet taken into account by
SINMAP.
5.4 SHALSTAB Results
As with the SINMAP model, SHALSTAB was used to model instability using
both a 10-meter and 30-meter DEM. Due to limitations in the SHALSTAB program, only
Haywood County was run in the model (see discussion in Chapter 3). Haywood County
was chosen for its rugged topography and the dispersed nature of the inventoried debris
flows in the county. Slopes range from 0° to 67° with a median of 20°. Some of the
steepest terrain in the French Broad Watershed is located in the northwestern corner of
the county, within the Great Smoky Mountains National Park.
condition. Consequently, if the tangent of the slope (θ ) is greater than or equal to the soil
friction angle (Φ), the slope will be unstable, even under dry conditions (Dietrich and
Real de Asua, 1998). High values of log (q/T ) are the SHALSTAB lower bounds, the
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unconditionally “stable” condition. Shallow slopes do not allow for high enough pore
pressure in the soil, even during full saturation, and are rarely unstable (Table 5.3). For
every grid cell, SHALSTAB calculates the amount of critical effective precipitation
necessary to trigger pore-pressure-induced instability, at a constant rate of transmissivity.
Areas with lower values are considered more unstable because they require less
precipitation to cause them to fail than do areas with higher values (Table 5.4).
SHALSTAB requires that landslide locations be supplied to verify the model
results. Instead of simple point locations, polygonal shapes of each landslide scar are
necessary for this model. In the case of Haywood County, 23 landslide polygons were
interpreted and digitized from digital orthophotos and DEMs using ArcGIS 3-D Analyst.
The landslide location has been placed within a stability class based upon the lowest q/T
value that it intersects. The authors of SHALSTAB consider the model to be successful if
the majority of landslides occur in grid cells with low values of log (q/T ) (Dietrich and
Montgomery, 1998).
The four parameters that can be adjusted in SHALSTAB are soil friction angle,
soil density, soil depth, and cohesion. Whereas SINMAP uses a range of input
parameters, SHALSTAB allows for only a single variable to be adjusted and run at a
upper and lower bounds. The parameters used in each SHALSTAB run are summarized
in Table 5.5.
5.4.1 Results – 30-meter DEM
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The results for all parameters run in SHALSTAB with a 30-meter DEM are
summarized in Appendix F. Even though 23 landslide polygons were digitized for the
county, only 22 of the landslides were actually calculated by SHALSTAB and used in the
final results.
As with SINMAP, the SHALSTAB default parameters seem to underestimate
instability (Figure 5.7). Seven of the mapped debris flows occur in the unconditionally
stable zone, with land characterized by gradients too low to fail even when saturated.
Collectively, this comprises 974.9 km² (31.82% of the county). All of the other debris
flows occur in stability classes between –3.1 and –2.2, approximately 38% (421 km²) of
the county. No landslides fall into the “chronic instability” zone, the category where areas
are defined as potentially unstable even without the addition of significant rainfall
(Dietrich et al., 2001).
To estimate which parameter values best predict instability in the study area, the
number of cells in each log (q/T ) category have been determined for each parameter run.
The resulting cumulative frequency (percent area) of the county falling into each
instability class is plotted on Figure 5.8 (Dietrich et al., 2001). The cumulative percent of
debris-flows-per-instability category also is calculated (Figure 5.9).
(20) assigned to the “chronic,” the “<–3.1,” and the “-3.1 - -2.8” categories. This is
equivalent to 979.3 km² or 59.7% of the county.
The next test involved increasing cohesion slightly to 2000 (N/m³); this value is
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equivalent to a dimensionless cohesion value of 0.1 as used in SINMAP. For soil friction
angles of 26° and 35°, this increase in cohesion decreased the cumulative percentage of
the area and number of landslides predicted to occur in unstable areas. For log (q/T )
values less than –2.8, the area decreased by 13% given a soil friction of 26° and 12%
with a soil friction angle of 35°. Strangely, for a soil friction angle of 45°, there was no
change in the calculated results for values of cohesion between zero and 2000 (N/m³).
This seems to indicate that SHALSTAB is more sensitive to changes in soil-friction angle
than soil cohesion.
The most stable parameters were equivalent to the SINMAP upper bound, i.e., a
soil friction angle of 45° and cohesion equal to 9427.41 N/m³. This high cohesion number
is equal to a dimensionless cohesion value of 0.25 when the soil depth is 2 m and soil
density is 1922 kg/m². All of the landslides and nearly the entire area (99.92%) fell into
the “stable” category.
5.4.2 Results – 10-meter DEM
A benefit of the SHALSTAB program is that the model does not “lose” grid cells,
like SINMAP, during individual calculations. There was a slight loss of grid-cell area
from the 30-meter to the 10-meter DEMs, but not nearly as much as with the SINMAP
the clipping procedure in ArcInfo where some pixels were excluded from the final
clipped DEM in the 30-meter scale due to the coarse nature of the data.
Appendix F summarizes results for a 10-meter DEM and each of the tested
h d f l d i i bili i h
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parameters. The default parameters seem to underestimate instability in the county, even
though more of the mapped landslides are captured in higher stability classes. Only two
landslides (8.8%) fall into the “stable” class but the area is slightly smaller than in the 30-
meter model run, i.e., 870.1 km² or 53% of the county. The rest of the debris flows (21)
are contained in the stability classes between –3.1 to –2.2 that encompass 37% (615.7
km²) of Haywood County.
As with the 30-meter DEM, a variety of parameters were tested with the 10-meter
DEM and both the cumulative percent of the county area and the area of landslides were
calculated (Figure 5.10 and Figure 5.11). The smaller grid size of the 10-meter DEM
increases the predicted extent of unstable ground in these latter calculations. The lower
bound values (26, 0) were found to contain the most debris flows in the category of
“chronic instability”: 20 slides were assigned to an area of 534.8 km² (32.6% of the
county). Slightly less of the area, 23%, is predicted to be unconditionally stable. This is a
difference of only 3% from that defined by the 30-meter DEM.
As with the 30-meter DEM, the influence of cohesion on the model was tested by
increasing the value slightly from zero to 2000 N/m³ for each soil friction angle (26°, 35°,
45°). For log (q/T ) values less than –2.8, the area decreased by 12.7% with a soil-friction
` The upper bound parameters, 45° and 9427.41 N/m³ calculate the most stable
land. A majority of the debris flows (21) fell into the “stable” category that comprises
1633.7 km² of the county. But the areas of instability were also increased by 7.2% from
th f d b th 30 t DEM T lid d i th “ 3 1 2 8” d “ 2 5
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those found by the 30-meter DEM. Two slides occurred in the “-3.1 – -2.8” and “-2.5 - -
2.2”instability classes and unstable zones with a log (q/T ) of greater than –2.2 make up
5.4 km² of the county (8.5%).
5.4.3 SHALSTAB Interpretation
Like SINMAP, SHALSTAB is more sensitive to changes to some parameters than
others. A significant increase in cohesion causes more of the study area to be designated
as unconditionally stable, but overall the model is less sensitive to this parameter.
SHALSTAB seems to be most sensitive to changes in soil-friction angle. With only a 9
degree increase in friction angle from 26º to 35º, the total extent of predicted instability,
for a log (q/T ) less than –2.8, decreases by approximately 29%. Unconditionally stable
areas increase by around 20%. SHALSTAB calculates the unstable terrain depending on
the tangent value of slope. Given that a greater percentage of a county has a lower
gradient when lower soil-friction angle values are used in the model runs, larger regions
will be calculated as having a potential for instability. Accurate topography, as
determined from the DEM, plays an important part in the SHALSTAB calculation.
The authors of SHALSTAB based their interpretation of the success or failure of
the model on the mapped log (q/T ) results. The model is considered successful if the
the interpretation of model effectiveness is based upon the potential for future mass-
wasting in areas that have not yet failed (Guimaraes et al., 2003). This interpretation is
necessary with the lowest tested values, a soil-friction angle of 26o
and cohesion of zero.
Using these parameters SHALSTAB accurately models 90 9% to 100% of the mapped
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Using these parameters, SHALSTAB accurately models 90.9% to 100% of the mapped
debris flows in the upper three instability categories (log (q/T ) less than –2.8). Despite the
fact that these low values could be interpreted as a worst-case scenario for debris-flow
initiation, they seem to overestimate the extent of instability in Haywood County (Figure
5.12).
If one compares the debris-flow density for each instability category, mid-range
parameters capture the most landslides in the smallest area. Using the 10-meter DEM
with a 35° soil-friction angle and soil cohesion value of 2000 N/m³, six debris flows are
captured in the “chronic instability” class within an area of 51.9 km² (3.2% of total area).
This is a landslide density of 0.116 slides per square kilometer, but an accuracy of only
26% for the “chronic instability” region, and 60.9% for the three highest instability
classes (Figure 5.13).
All of these debris-flow scars have been interpreted from DOQs, but field
verification of the landslides may improve the modeled results. SHALSTAB is very
sensitive to the accuracy of mapped debris-flow locations, requiring that the standard for
mapping be higher than normal, particularly when 10-meter or higher-resolution DEMs
are used (Dietrich and Montgomery, 1998b).
comparison of the relative performance of the programs is difficult because they calculate
completely different indices to quantify instability. In order to compare them, the
SINMAP stability index and the SHALSTAB log (q/T ) must be transformed into an
analogous format This can be accomplished using the Spatial Analyst extension of
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analogous format. This can be accomplished using the Spatial Analyst extension of
ArcGIS.
For direct comparison, the parameters used in SINMAP and SHALSTAB must be
identical. A moderate soil-friction angle of 35°, a zero value for cohesion, and a soil
density 1922 kg/m³ were chosen. Transmissivity and recharge were also kept at a
constant rate of 1 m²/d and 125 mm/d, respectively, which set the SINMAP T/R value
equal to 200 m. The SHALSTAB log (q/T ) value for these parameters is equal to –2.28.
This procedure is similar to the one used by Dietrich et al. (2001). Figure 5.14 shows a
comparison of the two programs for the same area in Haywood County using identical
parameters. Both programs calculated 705.4 km² (49%) of Haywood County to be
unstable. SHALSTAB calculated slightly more of the county as unstable (870.3 km² or
61%) than SINMAP (740.5 km² or 52%). Visually, the SHALSTAB results appear more
clustered, while the SINMAP results seem more scattered. These differences are due to
the different ways that the models calculate area and slope (Dietrich et al., 2001). Overall,
the calculated results for both programs are strikingly similar i.e., both programs
predicted 81-95% of the same area to be unstable (Figure 5.15).
5 6 G l d S il
“Zchs”, i.e., slate of the Copperhill Formation (Figure 5.16). This unit is a graphitic to
sulfidic slate-to-phyllite found in the Great Smoky Mountains National Park in Haywood
County (North Carolina Geological Survey, 1985). Rock descriptions of the Copperhill
Formation are scarce but in the geologic map of the Great Smoky Mountains of Hadley
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Formation are scarce but in the geologic map of the Great Smoky Mountains of Hadley
and Goldsmith (1963), "Zchs" corresponds to a map unit called “pЄa”, i.e., the Anakeesta
Formation. According to Merschat (pers. comm., 2005), a geologist with the North
Carolina Geologic Survey, the slate of the Copperhill and the Anakeesta are separate
formations but are nearly identical. The Anakeesta is part of the Smoky Mountain Group
and the Ocoee Supergroup. It is a pyritic “dark, fine-grained argillaceous rock” with
interbedded metasiltstone and coarse sandstone (Hadley and Goldsmith, 1963). Both the
Anakeesta and the Copperhill formations represent repetitive turbidite sequences at
different stratigraphic levels (Merschat, pers. comm., 2005). Topography in this portion
of the Great Smoky Mountains National Park is exceptionally narrow and steep with
“serrate crests and craggy pinnacles,” and thin residual soils (Hadley and Goldsmith,
1963).
Near the bottom of the Anakeesta, sulfidic argillaceous rock commonly
intertongues with sandstone like that within the underlying Thunderhead sandstone. The
Thunderhead, a member of the Smoky Mountain Group (Zgs), also can be extremely
unstable based on the SINMAP and SHALSTAB results (Figure 5.16). It is also known to
produce boulder fields and slope in western North Carolina and Tennessee.
chemical weathering and could account, in part, for the steep weathered topography and
instability associated with this unit. Transverse and strike jointing also cause angular cliff
exposures in the Anakeesta and Thunderhead sandstone in the Great Smoky Mountains
National Park (Hadley and Goldsmith, 1963).
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National Park (Hadley and Goldsmith, 1963).
The most stable geologic units in the French Broad Watershed were found to be
that coded “bz ” on the Geologic Map of North Carolina (1985), i.e., the metamorphosed
rocks of the Brevard Fault Zone, and that coded “CZtp”, i.e., porphyroblastic gneiss of
the Sauratown Formation. Both of these units are located in the southeastern portion of
the watershed and lie within the geomorphologic region known as the Western Piedmont.
The Brevard Fault zone seems to form a dividing line between moderately unstable rock
and stable rock (Figure 5.16).
Both SINMAP and SHALSTAB predict the most unstable soil unit in the
watershed to be that coded NC104, i.e., a general soil group that occurs in northern
Haywood County, generally overlying the Copperhill Formation. This soil unit is
principally composed of the Tanasee (21%), Burton (19%), Oconaluftee (16%), and
Porters (14%) soil series (U.S. Department of Agriculture, 1998) (see Appendix B). The
Tanasee soil series is sandy loam derived from colluvium and often forms toe slopes, fans
and benches on coves in the high elevations of the Appalachians. The Burton and Porters
series are a sandy-to-fine loam derived from residuum on ridge and side slopes. The
Ocanaluftee is a channery loam, also derived from residuum, or from slate, phyllite, or
(U.S. Department of Agriculture, 2005). All of the stable soil units are located on the flat
topography along the floodplain of the French Broad and Pigeon Rivers (Figure 5.17).
5.7 Jointing, Fracturing and Foliation
One factor that neither SINMAP nor SHALSTAB takes into consideration is the
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influence of geologic structure on groundwater flow. Groundwater flow through a
fracture is notoriously difficult to model because the rate of transmissivity is strongly
controlled by the size of the fracture aperture and the connectivity of the fracture network
(Renshaw, 2000). Nonetheless, flow through joints and fractures, and over bedrock
foliation planes parallel to the dip slope, may play a significant role in triggering debris
flows in western North Carolina (Figure 5.18).
In the Great Smoky Mountains National Park, it was noted that the heads of
debris flows originate at the intersection of cleavage and joints or beds, or on the
cleavage or bedding plane (Southworth et al, 2003). Intersecting joints and fractures can
cause groundwater flow to become concentrated in topographic hollows. During a heavy
rainfall event, these hollows could periodically be “flushed out” of weathered colluvial
material, triggering a debris flow. Foliation planes provide a smooth surface on which
sliding may initiate, particularly if the dip angle for bedding is close to the dip of the
foliation.
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Figure 5.1: Landslide inventory map for the French Broad Watershed. The cluster of location pointsin southern Buncombe County is due to the extensive mapping of 1977 debris flows done by Pomeroy
(1991) and Otteman (2001).
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Figure 5.2: A comparison of 30-meter and 10-meter DEMs in Haywood County, North Carolina. In
the coarser 30-meter DEM, there is a great deal of pixelation. Lower resolution can lead to an
underestimation of both the slope and instability in the study area.
Table 5.1: The parameters used in all of the SINMAP model runs.
Precipitation
Threshold
Soil
Density
T/R
Max.
T/R
Min.
Cohesion
Max.
Cohesion
Min.
ΦMax. ΦMin.
Default 2000 3000 2000 0 .25 30 4550mm 1922 331 3 0 .25 26 45
125mm 1922 132.4 1.2 0 .25 26 45
125mm 1922 132 4 1 2 10 25 26 45
Table 5.2: SINMAP stability index definitions (Pack et al., 1998b).
Condition Class Predicted
State
Parameter
Range
Possible Influence
of Factor Not
Modeled
Map
Color
SI > 1.5 1 Stable SlopeZone
Range cannotmodel instability
Significantdestabilizing factorsrequired for
green
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required for instability
1.5 > SI >1.25
2 Moderatelystable slopezone
Range cannotmodel instability
Moderatedestabilizing factorsrequired for instability
blue
1.25 > SI >1.0
3 Quasi-stableslope zone
Range cannotmodel instability
Minor destabilizingfactors could lead toinstability
yellow
1.0 > SI >0.5
4 Lower thresholdslope zone
Pessimistic half of range requiredfor instability
Destabilizing factorsare not required for instability
pink
0.5 > SI >
0.0
5 Upper
thresholdslope zone
Optimistic half of
range required for stability
Stabilizing factors
may be responsiblefor stability
red
0.0 > SI 6 Defendedslope zone
Range cannotmodel stability
Stabilizing factorsare required for stability
tan
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Figure 5.3: SINMAP results for a 30-meter DEM and using default parameters.
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Figure 5.4: SINMAP results for 125 mm/d recharge and using a 30-meter DEM.
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Figure 5.5: SINMAP results for a 10-meter DEM and using default parameters.
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Figure 5.6: SINMAP results for 125 mm/d recharge using a 10-meter DEM.
Table 5.3: Mapped instability classes used in the SHALSTAB model analysis.
Log (q/T ) Interval Color
Chronic Instability Dark red
< - 3.1 Red
Table 5.4: Table comparing q/T and log (q/T ) values and the precipitation rate required to initiate
instability for soils with a transmissivity of 65 m²d and 17 m²/d (after Dietrich and Asua, 1998).
q/T (1/m) log (q/T ) (1/m) Precip for T =
65 m²/d (mm/d)
Precip for T =
17 m²/d (mm/d)
.00079 -3.1 52 14
.00158 -2.8 103 27
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.00316 -2.5 206 54
.00833 -2.2 410 103
.01266 -1.9 818 214
Table 5.5: Parameters used in the SHALSTAB model runs.
Soil Friction
Angle
Density
(kg/m³)
Soil Depth (m) Cohesion
(N/m²)
*45 1700 1 0**45 1922 2 9427.41
45 1922 2 2000
45 1922 2 0
35 1922 2 2000
35 1922 2 0
26 1922 2 2000
***26 1922 2 0
* SHALSTAB default values** SINMAP upper bounded values*** SINMAP lower bounded values
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Figure 5.8 Cumulative percent of Haywood County found in each log (q/T ) category for a variety of soil parameters for the 30-meter DEM (after Dietrich et al., 2001). In the legend, the first number is
the degree if soil friction angle and the second number is the amount of cohesion (N/m³).
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Figure 5.10: Cumulative percent of the area if Haywood County for each log (q/T ) instability
category for a variety of soil parameters for the 10-meter DEM (after Dietrich et al., 2001).
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Figure 5.14: Comparison of the output of SINMAP and SHALSTAB for 125 mm of recharge, 35° soil
friction angle, 1922 kg/m³ soil density, and zero soil cohesion for a location in Haywood County. The
red areas are calculated as unstable by both programs whereas the gray areas are calculated to be
stable. Visually the SHALSTAB results seem to cluster better while the SINMAP results are more
scattered. Overall, the results are very similar.
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Figure 5.16: The mean stability index for each geologic unit in the French Broad Watershed. The
most unstable unit, Zchs, is located in the northwestern portion of Haywood County. The most stable
units, bz and Ctzp, are located in the southeastern portion of the study area.
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Figure 5.17: The mean stability index for each soil unit in the French Broad Watershed. The most
unstable soil unit in the watershed is NC104, which is located in western Haywood County. The most
stable soils are located around the floodplains of the Pigeon and French Broad Rivers.
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Figure 5.18: Picture taken in October 2003 along the Blue Ridge Parkway, near Mt. Mitchell. Even
during this light rain event, water is pouring out from fractures in the rock.
CHAPTER 6: CONCLUSIONS
Debris flows and other forms of mass-wasting in western North Carolina are a
destructive force that deserves more attention from both local and federal-level planners
and citizens living in these mountainous areas. Debris flows are difficult to predict and
have such a low frequency that it is rare for them to recur in the same area within the
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q y
average life span of area residents. Unlike a flood-hazard map, a landslide-hazard map is
inherently imprecise with regard to the extent, location, and timing of future mass-
wasting. After the tragic results of the September 2004 hurricane season, North Carolina
Senate Bill 7, the Hurricane Recovery Act of 2005 was written and passed (available
online at http://www.ncga.state.nc.us/). It recognizes the need for better understanding of
debris flows and provides funds for disaster relief and to identify areas of potential slope
instability.
This study has incorporated historical rainfall data, geology, soil types, and
geomorphology to help predict debris flows in the French Broad Watershed. A
combination of factors have been found to trigger a debris flow:
1. Topography is the most important factor in generating a debris flow. Slopes
with historic incidences of mass-wasting have averaged 28 degrees in this
study area but range up to 50 degrees.
2. Intense precipitation, generally greater than 125 mm/d, tends to increase the
probability of debris flows. This is especially true when a second large storm
pressure, decreases cohesion, and may induce slope failure. Steeply sloping
areas may be of potential concern under these certain weather conditions and
should be closely monitored.
3. Both SINMAP and SHALSTAB predict that certain geologic units are more
prone to failure than others. Sulfidic shale beds of the Copperhill Formation in
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the Great Smoky Mountains National Park are particularly susceptible, largely
due to their steep topography, thin soil cover, and vulnerability to chemical
weathering.
4. Both SINMAP and SHALSTAB predict that certain soil units are more prone
to failure than others. Particularly vulnerable to mass-wasting are those that
form at high elevations on steep slopes, are well drained, have a moderate to
rapid rate of permeability, and develop over less permeable bedrock.
5. The movement of groundwater may be concentrated through joints, fractures,
and foliation, quickly increasing pore pressure and decreasing cohesion and
friction between the soil and underlying bedrock. Areas with thin soils
overlying fractured bedrock may be more prone to slope instability.
SINMAP and SHALSTAB have both proven adequately accurate in the
prediction of slope instability on the regional scale of the French Broad Watershed. Both
ArcView and ArcGIS provide useful visual representations of landslide-hazard maps that
may readily be updated as new information becomes available. The performance of both
both slope and relief. A finer 10-meter grid maximizes the overall extent of
predicted instability, but significantly increases computer-processing time.
Extremely large 10-meter DEMs, covering more than one county, may cause
either computer program to abort.
2. The modeled results for the default SINMAP and SHALSTAB parameter
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values underestimate the extent of instability in the study area.
3. A SINMAP stability index of less than 1.0, designating instability, has been
calculated for 88% -to- 94% of the inventoried landslides whereas the
corresponding stability threshold in SHALSTAB has designated 91% to 100%
of the mapped debris flows. The SHALSTAB threshold is a log (q/T ) value
less than –2.8 for a soil-friction angle of 26° and cohesion of zero. Overall,
these results seem to over-predict the areas of instability.
4. The results from both models are similar, calculating 81-95% of the same area
of Haywood County as unstable. Visually, the SINMAP results seem
dispersed whereas the SHALSTAB results are more clustered.
The advantages and disadvantages of SINMAP and SHALSTAB are discussed
below and summarized in Table 6.1. Neither model was conclusively found to predict
instability better than the other and each should be used with caution and with the most
accurate input data available.
1. Some of the advantages of the SINMAP model are as follows:
• It can run large DEMs, e.g., watershed size, without aborting the program
or causing errors.
• Recharge values are used in the calculation of the T/R values so these
precipitation thresholds can be tested.
2. Some of the disadvantages of the SINMAP model are as follows:
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• During some model runs, some grid cells calculated as “NO DATA,”
effectively changing the area calculated by the model.
• The model was created for study areas that had thick soil packages and
high transmissivity rates, neither of which characterize the French Broad
Watershed.
3. Some of the advantages of the SHALSTAB model are as follows:
• When run with only the default values, the user needs little knowledge of
the actual soil parameters, or even mapped landslides, to produce a map of
relative instability.
• No grid cells are “lost” in the SHATSTAB model calculation, making a
direct comparison of total watershed area between the 10-meter and 30-
meter DEM easier and more accurate.
4. Some of the disadvantages of the SHALSTAB model are as follows:
• To obtain more accurate results, specific soil-parameter data must be
introduced into the model however the model does not allow for a range
5. Neither model takes into account either antecedent moisture or the effect that
geologic structure can have on concentrating groundwater flow. Nonetheless,
both of these factors probably have a significant effect on instability.
Deterministic slope-stability analytical methods, e.g., SINMAP and SHALSTAB,
are generally more useful in areas where ground conditions are fairly uniform throughout
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the study area (Dai et al., 2002). Due to the scale of this study, assumptions and
generalizations about soil parameters had to be made, thus forcing these parameters to be
uniform over a large area. From this broad reconnaissance study, the French Broad
Watershed offers several avenues for further study:
1. Subsequent studies should use a smaller study area, so that a site-specific soil
and geologic analysis may be completed. Precise measurements of hydraulic
conductivity (permeability), soil density, soil-friction angle, and soil cohesion
increase the accuracy of the predicted SINMAP and SHALSTAB results.
2. An accurate and detailed debris flow inventory should be constructed when
using these models. Verification of the modeled results using actual debris-
flow locations is essential.
3. Mapping the extent of “ancient” debris-flow deposits in the study area could
be used to better understand the return interval for such catastrophic events.
4. High-quality (finer-resolution) precipitation data must be acquired. Although
generalized rainfall data is useful, debris flows are often generated by intense,
topographic information. This kind of data would include 10-meter DEMs or
smaller and LIDAR data.
Table 6.1: The advantages and disadvantages of SINMAP and SHALSTAB.
SINMAP SHALSTAB
Advantages Disadvantages Advantages Disadvantages
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Advantages Disadvantages Advantages Disadvantages
Allows for a rangeof input values/
recharge thresholdscan be tested
Requires knowledgeof specific
soil/recharge parameters
Can be run withoutknowing soil
parameters
Does not allow for arange of input
values
Factor of safety easyto interpret
Must adjust parametersrepeatedly tocapture landslides
Does not requirerecharge values, can back-calculate
Log (q/T ) valuedifficult to interpret
Can run large DEMs “NO DATA” gridcells/shifting
Retains calculatedgrid cell area
Aborts when usinglarge DEMs
--
Does not take intoaccount geologicstructure or antecedent moisture
--
Does not take intoaccount geologicstructure or antecedent moisture
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Appendices
Appendix A: Geologic Units
GeologicUnit
Rock Type Formation Name (if given)
Geologic Description Composition
CZab Metamorphic Amphibolite and Biotite Gneiss
Interlayered; minor layers and lenses of hornblende gneiss, metagabbro, micaschist, and granitic rock
Inequigranular, locally abundant potassicfeldspar and garnet; interlayered and
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CZbg Metamorphic Biotite Gneiss and Schist
feldspar and garnet; interlayered andgradational with calc-silicate rock,sillimanite-mica schist, mica schist, and
amphibolite. Contains small masses of granitic rock
CZgms Metamorphic SAURATOWN MOUNTAIN Garnet-Mica Schist Interlayered with amphibole
CZtp Metamorphic SAURATOWN MOUNTAIN Porphyroblastic gneiss
Ccl Sedimentary LOWER CHILHOWEE Arenite
Feldspathic arenite, white to yellowishgray. Minor silty shale, feldspathicsiltstone, and conglomerate in lower part
Ccu Sedimentary UPPER CHILHOWEE AreniteVitreous quartz arenite, white to lightgray; interbedded sandy siltstone andshale
Chg Metamorphic HENDERSON GNEISS GneissMonzonitic to granodioritic,inequigranular
Cr Sedimentary ROME FORMATION Shale and SiltstoneShale and siltstone, variegated red to brown; interbedded fine-grainedsandstone
DSc Metamorphic
CAESARS HEAD GRANITE
GNEISS Granite Gneiss
Equigranular to porphyritic, massive to
well foliated; contains biotite andmuscovite
Dqd Igneous INTRUSIVE Quartz Diorite and GranodioriteContains biotite, muscovite, andxenocrysts
Dsc ? ? ? ?
PzZu Metamorphic INTRUSIVE Meta-Ultramafic Rock
Metamorphosed dunite and peridotite;serpentinite, soapstone, and other alteredultramafic rock
SOgg Metamorphic INTRUSIVE Granite GneissPoorly foliated; interlayered with biotiteaugen gneiss
Ybam Metamorphic UNCONFORMITY Amphibolite
Equigranular, massive to well foliated,interlayered, rarely discordant,metamorphosed intrusive and extrusivemafic rock; may includemetasedimentary rock
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y
Ybgg Metamorphic UNCONFORMITY Biotite Granite Gneiss
Pinkish gray to light gray, massive towell foliated, granitic to quartz
monzonitic; includes variablymylonitized orthogneiss and para-gneiss,interlayered amphibolite, calc-silicaterock, and marble
Ygg Metamorphic UNCONFORMITY Granodioritic Gneiss
Greenish gray to pinkish gray, porphyroclastic to mylonitic; epidote,sericite, and chlorite common
Ymam Metamorphic UNCONFORMITY Amphibolite
Equigranular, massive to well foliated,interlayered, rarely discordant,metamorphosed intrusive and extrusivemafic rock; may includemetasedimentary rock
Ymg Metamorphic UNCONFORMITYMigmatitic Biotite-HornblendeGneiss
Layered biotite-granite gneiss, biotite-hornblende gneiss, amphibolite, calc-silicate rock; locally contains relictgranulite facies rock
Ytg Metamorphic TOXAWAY GNEISS GneissPoorly foliated to well foliated,
equigranular to inequigranular, granitic
Zaba MetamorphicALLIGATOR BACK FORMATION
Amphibolite
Equigranular, massive to well foliated,interlayered, rarely discordant,metamorphosed intrusive and extrusivemafic rock; may include
metasedimentary rock
Zabg MetamorphicALLIGATOR BACK FORMATION
Gneiss
Finely laminated to thin layered; locallycontains massive gneiss and micaceousgranule conglomerate; includes schist, phyllite and amphibolite
Z M hiASHE METAMORPHICSUITE AND TALLULAH A hib li
Equigranular, massive to well foliated,interlayered, rarely discordant,
h d i i d i
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Zata Metamorphic SUITE AND TALLULAHFALLS FORMATION
Amphibolite metamorphosed intrusive and extrusivemafic rock; may include
metasedimentary rock
Zatb MetamorphicASHE METAMORPHICSUITE AND TALLULAHFALLS FORMATION
Biotite gneiss
Interlayered with biotite-garnet gneiss, biotite-muscovite schist, garnet-micaschist, and amphibolite
Zatm MetamorphicASHE METAMORPHICSUITE AND TALLULAHFALLS FORMATION
Muscovite-biotite gneiss
Locally sulfidic, interlayered andgradational with mica schist, minor amphibolite, and hornblende gneiss
Zatw Metamorphic
ASHE METAMORPHIC
SUITE AND TALLULAHFALLS FORMATION
Metagraywacke
Foliated to massive. Locallyconglomeratic; interlayered andgradational with mica schist, muscovite- biotite gneiss, and rare graphitic schist
Zch Metamorphic COPPERHILL FORMATION Metagraywacke
Metagraywacke, massive, graded bedding common; includes dark-grayslate, mica schist, and nodular calc-silicate rock
Zchs MetamorphicSLATE OF COPPERHILLFORMATION
Slate
Slate to phyllite, dark gray, graphitic,sulfidic; includes metagraywacke with
local graded bedding
Zgma MetamorphicGRANDFATHER MOUNTAINFORMATION
Meta-ArkoseSericitic, conglomeritic, locally cross- bedded interlayered metasiltstone andslate
Zgmg MetamorphicGRANDFATHER MOUNTAINFORMATION
Greenstone
Schistose to massive, amygdaloidal;interlayered with metasedimentary rock
Zgs MetamorphicGREAT SMOKY GROUP,UNDIVIDED
Metagraywacke andmetasiltstone
Thick metasedimentary sequence of
massive to graded beds of metagraywacke and metasiltstone withinterbedded graphitic and sulfidic slateand schist
Zlm Metamorphic Lincolnton Metadacite Metadacite
Zm Igneous MAX PATCH GRANITE Granite
Mottled pink and light green, coarsegrained to porphyritic, massive; containsbi tit
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biotite
Zrb Sedimentary RICH BUTT SANDSTONE SandstoneFeldspathic; interbedded with dark argillaceous layers and laminae
Zs MetamorphicSNOWBIRD GROUP,UNDIVIDED
Metasiltstone and sandstone
Feldspathic metasiltstone,metasandstone, and phyllite. Basal schistcontains lenses of quartz-pebbleconglomerate
Zsl Metamorphic LONGARM QUARTZITE Quartzite
Cross-bedded. feldspathic, locallyconglomeratic; includes dark slate andmetasiltstone
Zsp Sedimentary PIGEON SILTSTONE Siltstone
Thin bedded to laminated, commonlycross-bedded, metamorphosed; locallyincludes argillite and calcareous andankeritic metasiltstone grading to siltymetalimestone
Zsr SedimentaryROARING FORK SANDSTONE
Sandstone
Greenish gray, fine to medium grained,locally cross-bedded, metamorphosed;interbedded metasiltstone and phyllite
Zss Metamorphic SANDSUCK FORMATION Slate
Slate and metasiltstone, dark green to black. Metaconglomerate lentils in upper part; calcareous metasandstone, sandymetalimestone, and quartzite in lower part
Zsw MetamorphicWADING BRANCHFORMATION
Slate
Sandy slate to coarse-grained pebblymetagraywacke with local graded bedding. Basal quartz-sericite schist or phyllite
Zwc Metamorphic WALDEN CREEK GROUP,UNDIVIDED
Slate Slate to metasiltstone, local limy bedsand pods; interbedded with quartz-pebblemetaconglomerate and metasandstone
ZYbn Metamorphic Biotite gneiss
Migmatitic; interlayered and gradationalwith biotite-garnet gneiss andamphibolite; locally abundant quartz andalumino-silicates. Stratigraphic positionuncertain
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bz Metamorphic BREVARD FAULT ZONE Schist and phyllonite
Fish scale schist and phyllonite,
graphitic; interlayered with feldspathicmetasandstone, marble lenses
Appendix B: General Soil Data
MUSEQ NAME S5ID
% OF
MAPUNIT
SURFACE
TEXTURE UNIFIED AASHTO
DEPTH
LOW
DEPTH
HIGH
PERM
LOW
PERM
HIGH
HY COND
LOW
HY COND
HIGH
NC005 1 TOXAWAY NC0021 35 SIL CL A-4 0.0 1.0 0.60 20.00 0.21 7.00
NC005 2 ROSMAN NC0024 17 L ML A-4 2.5 5.0 2.00 20.00 0.34 3.40
NC005 3 DELANCO MD0155 26 SIL ML A-4 1.0 2.5 0.20 2.00 0.05 0.52
NC005 5 COMUS MD0050 9 FSL ML A-2 6.0 6.0 0.60 6.00 0.05 0.54
NC005 6 HATBORO PA0016 7 L ML A-4 0.0 0.5 2.00 6.00 0.14 0.42
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NC005 7 BRADSON NC0028 4 GR-L SM A-2 6.0 6.0 0.60 20.00 0.02 0.80
NC005 8 SUNCOOK CT0001 2 LS SM A-2 3.0 6.0 6.00 20.00 0.12 0.40
0.94 13.08 (IN/HR)
0.024 0.331 (M/HR)
NC006 1 CLIFTON NC0015 15 L ML A-4 6.0 6.0 0.60 6.00 0.09 0.90
NC006 2 BRADDOCK VA0054 23 L CL A-2 6.0 6.0 0.60 6.00 0.14 1.38
NC006 3 EVARD SC0083 19 L ML A-4 6.0 6.0 0.60 2.00 0.11 0.38
NC006 4 BRADDOCK VA0231 17 GR-L SM A-2 6.0 6.0 0.60 6.00 0.10 1.02
NC006 5 TOXAWAY NC0021 9 L CL A-4 0.0 1.0 0.60 20.00 0.05 1.80
NC006 6 URBANLAND
DC0035 8 VAR 2.0 2.0
NC006 7 CLIFTON NC0015 9 L ML A-4 6.0 6.0 0.60 6.00 0.05 0.54
0.55 6.02 (IN/HR)
0.014 0.152 (M/HR)
NC088 1 CHESTER MD0001 23 L CL A-4 6.0 6.0 0.60 2.00 0.14 0.46 NC088 2 ASHE NC0186 15 ST-FSL SM A-2 6.0 6.0 2.00 6.00 0.30 0.90
NC088 3 CHESTER MD0001 15 L CL A-4 6.0 6.0 0.60 2.00 0.09 0.30
NC088 4 CHESTER MD0001 6 L CL A-4 6.0 6.0 0.60 2.00 0.04 0.12 NC088 5 CODORUS PA0015 6 L ML A-4 1.0 2.0 0.60 20.00 0.04 1.20
NC088 6 ASHE NC0019 5 FSL SM A-4 6.0 6.0 2.00 6.00 0.10 0.30
NC088 7 CHANDLER NC0263 6 ST-FSL SM A-4 6.0 6.0 2.00 6.00 0.12 0.36
NC088 8 ASHE NC0019 3 FSL SM A-4 6.0 6.0 2.00 6.00 0.06 0.18
NC088 9 CHANDLER NC0263 3 ST-FSL SM A-4 6.0 6.0 2.00 6.00 0.06 0.18
NC088 10 CHESTER MD0001 4 L CL A-4 6.0 6.0 0.60 2.00 0.02 0.08
NC088 11 SUNCOOK CT0001 3 LS SM A-2 3.0 6.0 0.60 20.00 0.02 0.60
NC088 12 TATE NC0025 3 L ML A-4 6.0 6.0 2.00 6.00 0.06 0.18 NC088 13 WATAUGA NC0091 3 L SM A-4 6.0 6.0 0.60 6.00 0.02 0.18
NC088 14 WATAUGA NC0091 3 L SM A-4 6.0 6.0 0.60 6.00 0.02 0.18
NC088 15 FANNIN NC0020 2 SIL ML A-4 6.0 6.0 0.60 6.00 0.01 0.12
0.21 1.52 (IN/HR)
0.005 0.038 (M/HR)
NC089 1 CHESTER MD0001 32 L CL A-4 6 0 6 0 0 60 2 00 0 19 0 64
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NC089 1 CHESTER MD0001 32 L CL A 4 6.0 6.0 0.60 2.00 0.19 0.64
NC089 2 CLIFTON NC0264 13 ST-L SM A-4 6.0 6.0 0.60 6.00 0.08 0.78
NC089 3 CHESTER MD0001 11 L CL A-4 6.0 6.0 0.60 2.00 0.07 0.22 NC089 4 CLIFTON NC0015 9 L ML A-4 6.0 6.0 0.60 6.00 0.05 0.54
NC089 5 CHESTER MD0001 5 L CL A-4 6.0 6.0 0.60 2.00 0.03 0.10
NC089 6 CODORUS PA0015 8 L ML A-4 1.0 2.0 0.60 20.00 0.05 1.60
NC089 7 WATAUGA NC0091 4 L SM A-4 6.0 6.0 0.60 6.00 0.02 0.24
NC089 8 WATAUGA NC0091 4 L SM A-4 6.0 6.0 0.60 6.00 0.02 0.24
NC089 9 CLIFTON NC0015 3 L ML A-4 6.0 6.0 0.60 6.00 0.02 0.18
NC089 10 FANNIN NC0020 3 SIL ML A-4 6.0 6.0 0.60 6.00 0.02 0.18
NC089 11 FANNIN NC0020 3 SIL ML A-4 6.0 6.0 0.60 6.00 0.02 0.18
NC089 12 PORTERS NC0152 3 ST-L ML A-2 6.0 6.0 0.60 6.00 0.02 0.18
NC089 13 CLIFTON NC0015 1 L ML A-4 6.0 6.0 0.60 6.00 0.01 0.06
NC089 14 TATE NC0025 1 L ML A-4 6.0 6.0 0.60 6.00 0.01 0.06
0.11 1.08 (IN/HR)
0.003 0.027 (M/HR)
NC090 1 HAYESVILLE NC0013 25 L SM A-4 6.0 6.0 0.60 6.00 0.15 1.50
NC090 2 HAYESVILLE NC0013 15 L SM A-4 6.0 6.0 0.60 6.00 0.09 0.90
NC090 3 BRADSON NC0028 13 GR-L SM A-2 6.0 6.0 0.60 20.00 0.08 2.60 NC090 4 CODORUS PA0015 11 L ML A-4 1.0 2.0 0.60 20.00 0.07 2.20
NC090 5 BRADSON NC0028 9 GR-L SM A-2 6.0 6.0 0.60 20.00 0.05 1.80
NC090 6 EVARD SC0083 7 SL SM A-2 6.0 6.0 0.60 6.00 0.04 0.42
NC090 7 ROSMAN NC0024 6 L ML A-4 2.5 5.0 0.60 20.00 0.04 1.20
NC090 8 DELANCO MD0155 4 FSL ML A-4 1.0 2.5 0.60 6.00 0.02 0.24
NC090 9 EDNEYVILLE NC0023 4 FSL SM A-2 6.0 6.0 0.20 6.00 0.01 0.24
NC090 10 HATBORO PA0016 4 L ML A-4 0.0 0.5 0.60 6.00 0.02 0.24
NC090 11 TOXAWAY NC0021 2 SIL CL A-4 0.0 1.0 0.60 20.00 0.01 0.40 0.20 4.54 (IN/HR)
0.005 0.115 (M/HR)
NC091 1 DITNEY TN0075 27 CB-SL ML A-4 6.0 6.0 0.60 6.00 0.16 1.62
NC091 2 UNICOI TN0054 18 STV-L GM A-2 6.0 6.0 2.00 6.00 0.36 1.08
NC091 3 JUNALUSKA NC0181 12 CN-FSL SM A-4 6.0 6.0 0.60 6.00 0.07 0.72
NC091 4 BRASSTOWN NC0206 8 CN-FSL SM A-4 6.0 6.0 0.60 6.00 0.05 0.48
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NC091 5 LONON NC0203 10 CB-SL SM A-2-4 6.0 6.0 0.60 6.00 0.06 0.60
NC091 6 NORTHCOVE NC0204 7 STV-FSL GM A-2-4 6.0 6.0 0.60 6.00 0.04 0.42 NC091 7 SOCO NC0180 10 CN-FSL SM A-4 6.0 6.0 2.00 6.00 0.20 0.60
NC091 8 JUNALUSKA NC0181 4 CN-FSL SM A-4 6.0 6.0 0.60 6.00 0.02 0.24
NC091 9 BRASSTOWN NC0206 2 CN-FSL SM A-4 6.0 6.0 0.60 6.00 0.01 0.12
NC091 10 LONON NC0203 1 CB-SL SM A-2-4 6.0 6.0 0.60 6.00 0.01 0.06
NC091 11 NORTHCOVE NC0204 1 STV-FSL GM A-2-4 6.0 6.0 0.60 6.00 0.01 0.06
0.35 2.10 (IN/HR)
0.009 0.053 (M/HR)
NC092 1 SYLCO TN0014 32 SIL GC A-4 6.0 6.0 0.60 2.00 0.19 0.64
NC092 2 DITNEY TN0075 41 CB-L ML A-4 6.0 6.0 0.60 6.00 0.25 2.46
NC092 3 TUSQUITEE NC0158 3 ST-L SM A-2-4 6.0 6.0 2.00 6.00 0.06 0.18
NC092 4 DITNEY TN0075 1 CB-L ML A-4 6.0 6.0 2.00 6.00 0.02 0.06
NC092 5 DITNEY TN0075 2 CB-L ML A-4 6.0 6.0 2.00 6.00 0.04 0.12
NC092 6 CATASKA TN0133 14 STV-L CL-ML A-4 6.0 6.0 2.00 20.00 0.28 2.80
NC092 7 SPIVEY TN0109 2 STV-L GM A-2 6.0 6.0 0.06 6.00 0.00 0.12
NC092 8 HAYWOOD NC0095 2 ST-L SM A-2-4 6.0 6.0 0.06 20.00 0.00 0.40
NC092 9 SYLCO TN0014 1 L GC A-4 6.0 6.0 0.60 2.00 0.01 0.02 NC092 10 UNICOI TN0054 1 STV-L GM A-2 6.0 6.0 0.60 6.00 0.01 0.06
NC092 11 UNICOI TN0054 1 STV-L GM A-2 6.0 6.0 2.00 6.00 0.02 0.06
0.35 3.58 (IN/HR)
0.009 0.091 (M/HR)
NC093 1 ASHE NC0186 53 ST-FSL SM A-2 6.0 6.0 2.00 6.00 1.06 3.18
NC093 2 CHESTER MD0001 15 L CL A-4 6.0 6.0 0.60 2.00 0.09 0.30
NC093 3 CHESTER MD0001 12 L CL A-4 6.0 6.0 0.60 2.00 0.07 0.24 NC093 4 ASHE NC0019 9 FSL SM A-4 6.0 6.0 0.60 6.00 0.05 0.54
NC093 5 ROCK OUTCROP
DC0015 8 UWB 6.0 6.0 2.00 0.00 0.16 0.00
NC093 6 ASHE NC0019 1 FSL SM A-4 6.0 6.0 2.00 6.00 0.02 0.06
NC093 7 CLIFTON NC0264 1 ST-L SM A-4 6.0 6.0 0.60 6.00 0.01 0.06
NC093 8 TUSQUITEE NC0158 1 ST-L SM A-2-4 6.0 6.0 0.60 6.00 0.01 0.06
0.41 1.26 (IN/HR)
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0.010 0.032 (M/HR)
NC094 1 CHANDLER NC0263 23 ST-FSL SM A-4 6.0 6.0 2.00 6.00 0.46 1.38
NC094 2 WATAUGA NC0091 12 L SM A-4 6.0 6.0 2.00 6.00 0.24 0.72
NC094 3 CHANDLER NC0263 11 ST-FSL SM A-4 6.0 6.0 0.60 6.00 0.07 0.66
NC094 4 CLIFTON NC0264 7 ST-L SM A-4 6.0 6.0 0.60 6.00 0.04 0.42
NC094 5 WATAUGA NC0091 7 L SM A-4 6.0 6.0 0.60 6.00 0.04 0.42
NC094 6 ASHE NC0186 5 ST-FSL SM A-2 6.0 6.0 0.60 6.00 0.03 0.30
NC094 7 FANNIN NC0020 5 SIL ML A-4 6.0 6.0 2.00 6.00 0.10 0.30
NC094 8 CLIFTON NC0015 6 L ML A-4 6.0 6.0 0.60 6.00 0.04 0.36
NC094 9 CLIFTON NC0015 4 L ML A-4 6.0 6.0 0.60 6.00 0.02 0.24 NC094 10 TATE NC0025 3 L ML A-4 6.0 6.0 0.60 6.00 0.02 0.18
NC094 11 CHANDLER NC0017 4 L ML A-2 6.0 6.0 0.60 6.00 0.02 0.24
NC094 12 CHESTER MD0001 2 L CL A-4 6.0 6.0 0.60 2.00 0.01 0.04
NC094 13 CLIFTON NC0015 2 L ML A-4 6.0 6.0 0.60 6.00 0.01 0.12
NC094 14 CODORUS PA0015 9 SIL ML A-4 1.0 2.0 0.60 20.00 0.05 1.80
0.18 2.98 (IN/HR)
0.005 0.075 (M/HR)
NC095 1 EVARD SC0083 17 SL SM A-2 6.0 6.0 0.60 6.00 0.10 1.02
NC095 2 FANNIN NC0020 10 SIL ML A-4 6.0 6.0 0.60 6.00 0.06 0.60
NC095 3 PORTERS NC0152 10 ST-L ML A-2 6.0 6.0 0.60 6.00 0.06 0.60
NC095 4 BREVARD NC0012 9 L ML A-4 6.0 6.0 2.00 20.00 0.18 1.80
NC095 5 FANNIN NC0020 8 SIL ML A-4 6.0 6.0 0.60 6.00 0.05 0.48
NC095 6 TUSQUITEE NC0158 8 ST-L SM A-2-4 6.0 6.0 0.60 6.00 0.05 0.48
NC095 7 ASHE NC0186 8 ST-SL SM A-2 6.0 6.0 2.00 6.00 0.16 0.48
NC095 8 PORTERS NC0152 7 ST-L ML A-2 6.0 6.0 2.00 6.00 0.14 0.42
NC095 9 ASHE NC0186 5 ST-SL SM A-2 6.0 6.0 2.00 6.00 0.10 0.30 NC095 10 TUSQUITEE NC0026 5 L ML A-4 6.0 6.0 2.00 6.00 0.10 0.30
NC095 11 EDNEYVILLE NC0023 3 FSL SM A-2 6.0 6.0 2.00 6.00 0.06 0.18
NC095 12 ROCK OUTCROP
DC0015 1 UWB 6.0 6.0 2.00 0.00 0.02 0.00
NC095 13 BREVARD NC0012 2 L ML A-4 6.0 6.0 2.00 20.00 0.04 0.40
NC095 14 CHANDLER NC0263 2 ST-L SM A-4 6.0 6.0 0.60 6.00 0.01 0.12
NC095 15 TUSQUITEE NC0158 2 ST-L SM A-2-4 6.0 6.0 2.00 6.00 0.04 0.12
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NC095 16 EVARD SC0083 1 SL SM A-2 6.0 6.0 0.60 6.00 0.01 0.06
NC095 17 EVARD SC0083 1 SL SM A-2 6.0 6.0 0.60 6.00 0.01 0.06 NC095 18 TUSQUITEE NC0026 1 L ML A-4 6.0 6.0 0.60 6.00 0.01 0.06
0.13 0.82 (IN/HR)
0.003 0.021 (M/HR)
NC096 1 PORTERS NC0152 31 ST-L ML A-2 6.0 6.0 2.00 6.00 0.62 1.86
NC096 2 PORTERS NC0152 16 ST-L ML A-2 6.0 6.0 2.00 6.00 0.32 0.96
NC096 3 CODORUS PA0015 10 SIL ML A-4 1.0 2.0 0.60 20.00 0.06 2.00
NC096 4 CHESTER MD0001 8 L CL A-4 6.0 6.0 0.60 2.00 0.05 0.16
NC096 5 TUSQUITEE NC0158 7 ST-L SM A-2-4 6.0 6.0 0.60 6.00 0.04 0.42 NC096 6 CHESTER MD0001 6 L CL A-4 6.0 6.0 0.60 2.00 0.04 0.12
NC096 7 CHANDLER NC0263 4 ST-FSL SM A-4 6.0 6.0 0.60 6.00 0.02 0.24
NC096 8 PORTERS NC0022 4 L ML A-4 6.0 6.0 2.00 6.00 0.08 0.24
NC096 9 PORTERS NC0022 4 L ML A-4 6.0 6.0 2.00 6.00 0.08 0.24
NC096 10 TUSQUITEE NC0026 3 L ML A-4 6.0 6.0 2.00 6.00 0.06 0.18
NC096 11 TUSQUITEE NC0026 3 L ML A-4 6.0 6.0 2.00 6.00 0.06 0.18
NC096 12 FANNIN NC0020 2 SIL ML A-4 6.0 6.0 2.00 6.00 0.04 0.12
NC096 13 FANNIN NC0020 1 SIL ML A-4 6.0 6.0 0.60 6.00 0.01 0.06
NC096 14 WATAUGA NC0091 1 L SM A-4 6.0 6.0 0.60 6.00 0.01 0.06
0.33 1.08 (IN/HR)
0.008 0.027 (M/HR)
NC097 1 EDNEYVILLE NC0023 73 L SM A-2 6.0 6.0 0.60 6.00 0.44 4.38
NC097 2 EDNEYVILLE NC0023 11 L SM A-2 6.0 6.0 2.00 6.00 0.22 0.66
NC097 3 TUSQUITEE NC0026 9 L ML A-4 6.0 6.0 2.00 6.00 0.18 0.54
NC097 4 ASHE NC0019 3 GR-FSL SM A-2 6.0 6.0 2.00 6.00 0.06 0.18
NC097 5 TUSQUITEE NC0026 3 L ML A-4 6.0 6.0 2.00 6.00 0.06 0.18 NC097 6 TOXAWAY NC0021 1 L CL A-4 0.0 1.0 2.00 20.00 0.02 0.20
0.99 6.17 (IN/HR)
0.025 0.156 (M/HR)
NC098 1 WAYAH NC0188 43 GR-L SM A-2-4 6.0 6.0 0.60 6.00 0.26 2.58
NC098 2 TANASEE NC0197 9 ST-L SM A-2-4 6.0 6.0 2.00 6.00 0.18 0.54
NC098 3 PORTERS NC0152 8 ST-FSL ML A-2 6.0 6.0 2.00 6.00 0.16 0.48
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NC098 4 WAYAH NC0188 8 GR-L SM A-2-4 6.0 6.0 2.00 6.00 0.16 0.48
NC098 5 WAYAH NC0188 8 GR-L SM A-2-4 6.0 6.0 2.00 6.00 0.16 0.48 NC098 6 TANASEE NC0197 7 ST-L SM A-2-4 6.0 6.0 2.00 6.00 0.14 0.42
NC098 7 PORTERS NC0152 6 ST-FSL ML A-2 6.0 6.0 2.00 6.00 0.12 0.36
NC098 8 CULLASAJA NC0237 1 STV-L SM A-1-B 6.0 6.0 2.00 6.00 0.02 0.06
NC098 9 EDNEYVILLE NC0115 3 ST-FSL SM A-4 6.0 6.0 2.00 6.00 0.06 0.18
NC098 10 EDNEYVILLE NC0115 2 ST-FSL SM A-4 6.0 6.0 2.00 6.00 0.04 0.12
NC098 11 CHESTNUT NC0242 1 ST-FSL SM A-2-4 6.0 6.0 2.00 6.00 0.02 0.06
NC098 12 SPIVEY TN0109 1 STV-L GM A-2 6.0 6.0 0.06 6.00 0.00 0.06
NC098 13 TUSQUITEE NC0158 1 ST-FSL SM A-2-4 6.0 6.0 0.60 6.00 0.01 0.06
NC098 14 CHESTNUT NC0242 1 ST-FSL SM A-2-4 6.0 6.0 2.00 6.00 0.02 0.06 NC098 15 TUCKASEGE
E NC0226 1 ST-L ML A-2 6.0 6.0 2.00 6.00 0.02 0.06
0.17 0.60 (IN/HR)
0.004 0.015 (M/HR)
NC099 1 FANNIN NC0020 36 SIL ML A-4 6.0 6.0 2.00 6.00 0.72 2.16
NC099 2 FANNIN NC0020 20 SIL ML A-4 6.0 6.0 0.60 6.00 0.12 1.20
NC099 3 TALLADEGA GA0037 17 SIL SM A-4 6.0 6.0 0.60 2.00 0.10 0.34
NC099 4 IOTLA NC0140 9 L SM A-2 1.5 3.5 2.00 20.00 0.18 1.80
NC099 5 EVARD SC0083 6 SL SM A-2 6.0 6.0 0.60 6.00 0.04 0.36
NC099 6 EVARD SC0083 6 SL SM A-2 6.0 6.0 0.60 6.00 0.04 0.36
NC099 7 TATE NC0025 3 FSL ML A-4 6.0 6.0 0.60 6.00 0.02 0.18
NC099 8 HATBORO PA0016 2 L ML A-4 0.0 0.5 0.60 6.00 0.01 0.12
NC099 9 TUSQUITEE NC0026 1 L ML A-4 6.0 6.0 0.60 6.00 0.01 0.06
0.39 3.22 (IN/HR)
0.010 0.081 (M/HR)
NC100 1 EVARD SC0083 37 L ML A-4 6.0 6.0 0.60 2.00 0.22 0.74
NC100 2 OTEEN NC0107 18 L ML A-4 6.0 6.0 0.60 6.00 0.11 1.08
NC100 3 HAYESVILLE NC0013 4 L SM A-4 6.0 6.0 0.60 6.00 0.02 0.24
NC100 4 SALUDA SC0082 25 L SM A-2 6.0 6.0 0.60 6.00 0.15 1.50
NC100 5 TUSQUITEE NC0158 2 ST-L SM A-2-4 6.0 6.0 0.60 6.00 0.01 0.12
NC100 6 TATE NC0025 1 ST-L ML A-4 6.0 6.0 2.00 6.00 0.02 0.06
NC100 7 BREVARD NC0012 1 L ML A-4 6.0 6.0 0.60 20.00 0.01 0.20
NC100 8 HAYESVILLE NC0151 12 ST L SM A 4 6 0 6 0 0 60 6 00 0 07 0 72
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NC100 8 HAYESVILLE NC0151 12 ST-L SM A-4 6.0 6.0 0.60 6.00 0.07 0.72
0.39 3.92 (IN/HR) 0.010 0.099 (M/HR)
NC102 1 JUNALUSKA NC0181 21 CN-L SM A-4 6.0 6.0 0.60 6.00 0.13 1.26
NC102 2 TSALI NC0179 11 CN-L SM A-4 6.0 6.0 2.00 6.00 0.22 0.66
NC102 3 SPIVEY TN0109 17 STV-L GM A-2 6.0 6.0 0.60 6.00 0.10 1.02
NC102 4 SANTEETLAH
NC0208 11 CN-FSL SM A-4 6.0 6.0 0.60 6.00 0.07 0.66
NC102 5 STECOAH NC0184 6 CN-L SM A-4 6.0 6.0 2.00 6.00 0.12 0.36
NC102 6 SOCO NC0180 5 CN-L SM A-4 6.0 6.0 2.00 6.00 0.10 0.30 NC102 7 JUNALUSKA NC0181 6 CN-L SM A-4 6.0 6.0 0.60 6.00 0.04 0.36
NC102 8 TSALI NC0179 4 CN-L SM A-4 6.0 6.0 2.00 6.00 0.08 0.24
NC102 9 CHEOAH NC0190 8 CN-L SM A-4 6.0 6.0 0.60 6.00 0.05 0.48
NC102 10 SPIVEY TN0109 5 STV-L GM A-2 6.0 6.0 2.00 6.00 0.10 0.30
NC102 11 SANTEETLAH
NC0208 3 CN-FSL SM A-4 6.0 6.0 0.60 6.00 0.02 0.18
NC102 12 JUNALUSKA NC0181 1 CN-L SM A-4 6.0 6.0 0.60 6.00 0.01 0.06
NC102 13 TSALI NC0179 1 CN-L SM A-4 6.0 6.0 2.00 6.00 0.02 0.06
NC102 14 BRASSTOWN NC0206 1 CN-L SM A-4 6.0 6.0 0.60 6.00 0.01 0.06 0.28 1.38 (IN/HR)
0.007 0.035 (M/HR)
NC103 1 EDNEYVILLE NC0115 13 ST-FSL SM A-4 6.0 6.0 0.60 6.00 0.08 0.78
NC103 2 CHESTNUT NC0242 9 ST-FSL SM A-2-4 6.0 6.0 2.00 6.00 0.18 0.54
NC103 3 STECOAH NC0184 8 CN-L SM A-4 6.0 6.0 2.00 6.00 0.16 0.48
NC103 4 SOCO NC0180 5 CN-L SM A-4 6.0 6.0 2.00 6.00 0.10 0.30
NC103 5 STECOAH NC0184 6 CN-L SM A-4 6.0 6.0 2.00 6.00 0.12 0.36 NC103 6 SOCO NC0180 4 CN-L SM A-4 6.0 6.0 2.00 6.00 0.08 0.24
NC103 7 SPIVEY TN0109 5 STV-L GM A-2 6.0 6.0 2.00 6.00 0.10 0.30
NC103 8 TUSQUITEE NC0158 3 ST-L SM A-2-4 6.0 6.0 0.60 6.00 0.02 0.18
NC103 9 PORTERS NC0152 6 ST-FSL ML A-2 6.0 6.0 2.00 6.00 0.12 0.36
NC103 10 COWEE NC0171 6 GR-L SM A-2-4 6.0 6.0 2.00 6.00 0.12 0.36
NC103 11 EVARD SC0135 8 GR-L SM A-2 6.0 6.0 0.60 6.00 0.05 0.48
NC103 12 EDNEYVILLE NC0115 4 ST-FSL SM A-4 6.0 6.0 0.60 6.00 0.02 0.24
NC103 13 CHESTNUT NC0242 2 ST FSL SM A 2 4 6 0 6 0 2 00 6 00 0 04 0 12
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NC103 13 CHESTNUT NC0242 2 ST-FSL SM A-2-4 6.0 6.0 2.00 6.00 0.04 0.12
NC103 14 PORTERS NC0152 6 ST-FSL ML A-2 6.0 6.0 2.00 6.00 0.12 0.36 NC103 15 SPIVEY TN0109 6 STV-L GM A-2 6.0 6.0 2.00 6.00 0.12 0.36
NC103 16 TUSQUITEE NC0158 1 ST-L SM A-2-4 6.0 6.0 0.60 6.00 0.01 0.06
NC103 17 SANTEETLAH
NC0208 2 CN-L SM A-4 6.0 6.0 2.00 6.00 0.04 0.12
NC103 18 SAUNOOK NC0195 3 GR-L SM A-2 6.0 6.0 0.60 6.00 0.02 0.18
NC103 19 STECOAH NC0184 1 CN-L SM A-4 6.0 6.0 0.60 6.00 0.01 0.06
NC103 20 DELLWOOD NC0183 1 GR-FSL SM A-2-4 2.0 4.0 2.00 20.00 0.02 0.20
NC103 21 DILLARD GA0061 1 L ML A-4 2.0 3.0 0.60 2.00 0.01 0.02
0.22 1.00 (IN/HR) 0.005 0.025 (M/HR)
NC104 1 TANASEE NC0197 21 ST-L SM A-2-4 6.0 6.0 0.20 6.00 0.04 1.26
NC104 2 BURTON NC0114 19 ST-L SM A-2 6.0 6.0 2.00 6.00 0.38 1.14
NC104 3 OCONALUFTEE
NC0192 16 FL-L SM A-4 6.0 6.0 2.00 6.00 0.32 0.96
NC104 4 PORTERS NC0152 14 ST-FSL ML A-2 6.0 6.0 2.00 6.00 0.28 0.84
NC104 5 SPIVEY TN0109 7 CB-L GM A-2 6.0 6.0 2.00 6.00 0.14 0.42
NC104 6 CHEOAH NC0190 10 CN-L SM A-4 6.0 6.0 0.60 6.00 0.06 0.60 NC104 7 STECOAH NC0184 2 CN-L SM A-4 6.0 6.0 2.00 6.00 0.04 0.12
NC104 8 TANASEE NC0197 3 ST-L SM A-2-4 6.0 6.0 2.00 6.00 0.06 0.18
NC104 9 BURTON NC0114 2 ST-L SM A-2 6.0 6.0 2.00 6.00 0.04 0.12
NC104 10 SANTEETLAH
NC0208 5 L SM A-4 6.0 6.0 2.00 6.00 0.10 0.30
NC104 11 SOCO NC0180 1 CN-L SM A-4 6.0 6.0 2.00 6.00 0.02 0.06
0.46 1.80 (IN/HR)
0.012 0.046 (M/HR)
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Appendix C: Slope Movement Data for the 2004 Hurricanes Frances and Ivan
HURRICANE FRANCES - SEPTEMBER 6-8, 2004County Location Date Reported Source Description
Avery County, Near Crossnore US-221 Both Directions NCDOT Road is one lane in certain locations
Avery County, Near Crossnore SR-1504 Both Directions,Pineola Rd.
NCDOT HIGH WATER, MUDSLIDE
Blue Ridge Parkway Mileposts: 322, 345, 348,349, 413, and 429
9/9/2004 Asheville Citizen-Times
Most of those between mile 322 and 349 took out major portions of the motor road, from just south of Linville Falls tosouth of Buck Creek Gap at NC 80 near Marion.
Henderson County SR-1710 Both Directions,Bald Rock Rd.
9/10/2004 17:06 NCDOT Trees down and Mudslide, Road has collapsed along with amudslide near waterfall.
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Henderson County SR-1799 Both Directions,Deep Gap Rd.
9/8/2004 NCDOT Deep Gap has many landslides blocking the road
Henderson County SR-1194 Both Directions,Patterson Rd.
9/8/2004 NCDOT Two mudslides into road
Henderson County SR-1613 Both Directions,Slick Rock Rd.
9/8/2004 NCDOT Road is washed out. Mudslide covering entire road
Henderson County, Near Hendersonville
SR-1706 Both Directions,Little Creek Rd.
NCDOT Sinkhole - road washed away about 1 mile off Sugarloaf Mountain Road (SR 1868).
Henderson County Shepard Street portion of
N.C. 191
9/8/04 11:00AM NCDOT One lane closed due to mudslide
Jackson County NC-281 (Mile Marker 13to 17) Both Directions,Little Canada
9/8/2004 NCDOT Road is closed due to slide from SR 1140 (Fanny Mae BrownRoad) to Rock Bridge near Tannassee Gap. Detour is signed.Detour distance is 41 miles.
McDowell County, Near OldFort
I-40 (Mile Marker 72 to67or 69-67) BothDirections
early morning,9/8/04
NCDOT A slide on I-40 at Old Fort Mountain has blocked 2 of 3 laneson westbound I-40 as well as 2 of the 3 eastbound lanes.Emergency crews are working to reopen the rest of the lanesas quickly as possible. Expect delays.
McDowell County, Near Woodlawn
US-221 Both Directions NCDOT There are several mudslides and dangerous areas at the top of US 221 North (Linville Mtn.). The road is closed and will bereopened as soon as the crews can complete the work andmake the road passable again. US 221 North (Linville Mtn.)is closed due to Hurricane Frances. Approximately 70 feet of
the road is washed away at the Mountain ParadiseCampground. It will take a couple of months to rebuild this portion.
Polk County Pearson Falls Rd., near Saluda
9/9/2004 Asheville Citizen-Times
A mudslide closed Pearson Falls Road near Saluda.
Polk County U.S. 176 9/9/2004 Asheville Citizen-Times
between Saluda and Tryon, one lane closed from mudslides
Polk County U.S. 176 9/9/2004 Asheville Citizen- Temporarily closed Tuesday near the Henderson-Polk County
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Times
p y y y
line when mud and trees slide down a bank Transylvania County Old Highway 64/ N.C.
281N9/8/04 11:00AM NCDOT Portion of road closed due to mudslides near the Jackson
County line
Watauga County White Laurel subdivision,near Boone
9/8/2004 Rick Wooten, personalcommunication
This appears to have been an embankment failure. One homewas destroyed, and eight condemned for occupancy
HURRICANE IVAN - SEPTEMBER 16-18, 2004County Location Date Reported Source Description
Avery County N.C. 184 and 194 Asheville CitizenTimes
There was so much rain that high water and mudslidescovered N.C. 184 and 194, shutting Banner Elk off from therest of the region.
Buncombe County Mink Farm Road 9/18/2004 Asheville CitizenTimes
mudslide
Buncombe County Hookers Gap 9/18/2004 Asheville CitizenTimes
mudslide
Buncombe County Gibbs Road 9/18/2004 Asheville Citizen
Times
mudslide
Buncombe County Freedom Farm Road 9/18/2004 Asheville CitizenTimes
mudslide
Buncombe County Newfound Rd 9/18/2004 Asheville Citizen mudslide
Times
Buncombe County North Turkey Creek Rd 9/18/2004 Asheville CitizenTimes
Mudslide at Early's Mtn. Road
Buncombe County Sluder Branch Road - #99 9/18/2004 Asheville CitizenTimes
Mudslide & Pvt. Bridge damaged
Buncombe County N.C. 151 Asheville CitizenTimes
Closed due to major slide near Blue Ridge Parkway access
Buncombe County Arrowood Rd. near Starnes Cove Rd.
9/17/2004 2:00 Asheville CitizenTimes
debris flow occurred on side of mountain near the town of Enka-Candler, destroyed at least one home
Haywood County I-40 mudslide in thewestbound lane at MM 35
9/17/2004 13:44 NCDOT Interstate 40 is closed in Haywood County from Exit 451 inTennessee to Exit 20 in North Carolina due to a slope failure.
Haywood County U.S. 19-23 9/17/2004 0:00 Asheville CitizenTimes
mudslides
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Haywood County Dutch Cove at Turnpike 9/17/2004 Asheville CitizenTimes mudslide
Haywood County I-40, between the TN stateline and MM 20
9/17/2004 NCDOT I-40 is closed in Haywood County between the TennesseeState Line and mile marker 20 due to the road being washedaway.
Haywood County U.S.276 9/17/2004 Asheville CitizenTimes
rockslide south of Waynesville
Henderson County Middle Fork (one laneclosed)
9/17/2004 Asheville CitizenTimes
mudslide
Jackson County, Near Cashiers NC-107 (Mile Marker 18to 18) Both Directions
9/16/2004 23:02 NCDOT mudslide - N.C. 107 is closed form the Thorpe Power Plantsouth to Cashiers. Downed trees, boulders and mudslides are blocking the road, the main access the area.
Jackson County, Near Cashiers US-64 Both Directions 9/16/2004 23:19 NCDOT US 64 is closed between Cashiers and the Transylvania Co.Line due to slides and debris. US 64 is closed betweenCashiers and the Macon Co. Line due to slides and debris.Hwy 64 at Spring Forest Road, three-fourths of highway hasslid off
Macon, near Franklin SR-1310, Wayah Rd. 9/17/2004 7:46 NCDOT Down Trees and Slides
McDowell County, Near Woodlawn
NC-226 ALT BothDirections
9/17/2004 6:21 NCDOT Rockslide on NC226-A will probably take all day to move.
Swain, Near Bryson City SR-1195, Hwy 19a 9/17/2004 8:02 NCDOT Mud Slide and shoulder broke off
Transylvania County, near Brevard
SR-1540, Wilson Rd. 9/18/2004 3:35 NCDOT Due to high water, trees, power lines, and slides road isclosed.
Watauga, near Boone SR-1130, Lee GualtneyRd.
9/17/2004 17:16 NCDOT Slide blocking road
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Appendix D: Debris Flow Inventory
MATERIALDATA
SOURCEREMARKS LAT LONG COUNTY YEAR TYPE GEO UNIT SOIL UNIT SLOPE ASPECT
NCDOT SR 1607, .5mi S of Madison County line 35.7300 82.7000 Buncombe 1990 Slump Ymg NC090 10 N
Cv Scott, R Brevard slide 35.1900 82.8700 Transylvania 1970 Debris Flow Dqd NC093 11 SE
NCGS BRP Mile 357.7 35.7305 82.3082 Buncombe 1995 Debris Flow Zatw NC098 11 SW
Pomeroy 35.4800 82.6500 Buncombe 1977 Slump Zatm NC093 13 E
Rd NCDOT MP 17.6, E of SR 1336WBL
35.6300 82.9900 Haywood 1968 Debris Flow Zch NC006 13 SE
Rd NCDOT Intersection of BRP, 694,SR 2053
35.6500 82.4900 Buncombe 1982 Slump Zatm NC095 13 N
Pomeroy 35 4800 82 6700 Buncombe 1977 Slump Zatm NC093 15 E
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Pomeroy 35.4800 82.6700 Buncombe 1977 Slump Zatm NC093 15 E
Cv NCDOT .6 mi E of SR 1130 WBL 35.4700 82.9500 Haywood 1990 Slump ZYbn NC090 15 SW
Rd NCDOT .2mi N of BuncombeCounty line, US 25-70
35.7600 82.6100 Madison 1989 Debris Flow Ybgg NC100 16 NW
Rd NCDOT Balsam Gap Landslide 35.4400 83.0800 Haywood 1989 Slump ZYbn NC095 17 E
Rd NCDOT Rt. 206 35.6000 82.9400 Haywood 1996 Slump ZYbn NC006 17 W
NCDOT NBL NC 226, .6mi S of 1253
35.9900 82.1600 Mitchell 1990 Slump Zatm NC093 17 SW
Lambe Balsam Gap II Landslide 35.4500 83.0700 Haywood Debris Flow ZYbn NC103 18 NE
Cv NCDOT 600 ft. S of SR 1319 35.8900 82.7500 Madison 1986 Slump Zsr NC092 18 W
Rd NCDOT Fines Creek Slide 35.6700 82.9900 Haywood 1978 Debris Flow Ybgg NC006 19 N
Pomeroy 35.4800 82.6500 Buncombe 1977 Slump Zatm NC093 19 S
NCDOT NC 262, 1.8mi S of TNline
36.0900 82.0900 Mitchell Slump Ymg NC098 19 S
Dockal Off FR475, W of DavidsonRiver, elev. 2500 feet
35.2800 82.8100 Transylvania 1993 Slump Dqd NC094 20 S
FS Possible landslide -Richland Ridge Rd.
35.1800 82.8900 Transylvania 2003 Debris Flow Zata NC006 20 S
Cv? NCGS Maggie Valley debris flow 35.5014 83.0944 Haywood Debris Flow Zgs NC103 21 NE
Dockal Cove Creek Slide 35.2800 82.8100 Transylvania Slump Zatb NC094 21 SE
Pomeroy Number 50 35.4500 82.6600 Henderson 1977 Debris Flow Zatm NC095 21 E
Rd NCDOT N-side of NC19 35.9200 82.0600 Mitchell 1976 Slump Zabg NC093 21 N
Cv? USFS Microburst; dual slides near Dry Branch
35.7700 83.0400 Haywood 1994 Debris Flow Zsl NC103 22 SE
Rd Pomeroy Number 24 35.4700 82.6600 Buncombe 1977 Debris Flow Zatm NC093 22 SE
Pomeroy Number 49 35.4400 82.6600 Henderson 1977 Debris Flow Zatm NC093 22 E
Lambe Waterville Landslide 35.6900 83.0100 Haywood Debris Flow Ybgg NC102 23 SW
Rd NCDOT SBL NC 25-70, S of NC208
35.9100 82.7500 Madison 1986 Slump Zsr NC092 23 SW
Rd NCGS Deadly debris flow (12/11)- Maggie Valley Debris
35.5048 83.8500 Haywood 2003 Debris Flow Ybgg NC103 23 NE
Pomeroy Number 15 35.5000 82.6500 Buncombe 1977 Debris Flow Zatm NC093 23 NE
Pomeroy Number 6 35.4800 82.6900 Buncombe 1977 Debris Flow Zatm NC093 23 NE
Pomeroy Number 41 35.4300 82.6100 Henderson 1977 Debris Flow Zatm NC093 23 SE
Pomeroy Number 43 35.4300 82.6400 Henderson 1977 Debris Flow Zatm NC093 23 S
Pomeroy Number 35 35.4600 82.6400 Buncombe 1977 Debris Flow Zatm NC093 23 SE
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Pomeroy Number 35 35.4600 82.6400 Buncombe 1977 Debris Flow Zatm NC093 23 SE
Pomeroy Debris Torrent 35.4800 82.6800 Buncombe 1977 Debris Flow Zatm NC093 23 W
Dockal Near Maxwell Cove, east of drainage
35.3200 82.7400 Transylvania 1996 Slump Zatb NC095 24 SW
Dockal Cove Creek Slide 35.2800 82.8100 Transylvania Slump Zatb NC094 24 SE
Rd NCDOT SBL NC 215, 200' N of SR 1216, approximate
35.4000 82.9400 Haywood 1989 Debris Flow Zatm NC095 24 E
rk NCDOT I-40, World's Fair Slide 35.7700 83.0900 Haywood 1982 Slump Zsp NC102 24 SW
Cv? NRCS Several debris flowstriggered by trop.
depression
35.7647 82.2656 Yancey 1977 Debris Flow Zatw NC098 25 W
Pomeroy Number 36 35.4700 82.6200 Buncombe 1977 Debris Flow Zatm NC093 25 SE
Pomeroy Number 37 35.4700 82.6100 Buncombe 1977 Debris Flow Zatm NC093 25 S
NCDOT SBL SR 1137 35.9300 82.5000 Mitchell 1990 Slump Zabg NC093 25 SW
NCDOT SR 1318, .55mi W of SR 1334
35.9200 82.6700 Madison 1990 Debris Flow Ybgg NC093 25 W
Rd Pomeroy Number 16 35.4966 82.6504 Buncombe 1977 Debris Flow Zatm NC093 26 E
Pomeroy 35.4800 82.6800 Buncombe 1977 Slump Zatm NC093 26 SW
Pomeroy Number 34 35.4500 82.6500 Buncombe 1977 Debris Flow Zatm NC095 27 NE
Pomeroy 35.4800 82.6500 Buncombe 1977 Slump Zatm NC093 27 S
Dockal Cove Creek Slide 35.2800 82.8100 Transylvania Slump Zatb NC094 27 E
NCDOT WBL SR 1334, 0.2 mi NWof SR 1425
35.9100 82.6900 Madison 1990 Debris Flow Ybgg NC093 27 N
Pomeroy Number 17 35.5200 82.6400 Buncombe 1977 Debris Flow Zatm NC093 27 S
Pomeroy Number 39 35.4400 82.6400 Buncombe 1977 Debris Flow Zatm NC093 27 N
Pomeroy Number 51 35.4500 82.6600 Henderson 1977 Debris Flow Zatm NC095 27 E
Pomeroy Number 34 35.4500 82.6500 Buncombe 1977 Debris Flow Zatm NC095 28 NE
Pomeroy 35.4800 82.6700 Buncombe 1977 Debris Flow Zatm NC095 28 SE
Pomeroy 35.4900 82.6600 Buncombe 1977 Debris Flow Zatm NC095 28 S
Rd NCDOT W of SR 1395 35.8900 82.6600 Madison 1980 Slump Ybgg NC095 28 E
Pomeroy Number 16 35.5200 82.6400 Buncombe 1977 Debris Flow Zatm NC093 28 S
wr NCDOT 35.5400 82.8100 Haywood 1984 Slump ZYbn NC006 28 S
Cv NCDOT MP 8.8, E of Harmon Den 35.7200 83.0400 Haywood 1979 Slump Zsl NC102 28 SW
NCDOT NC 1318, .25 W of 1334 35.9200 82.6700 Madison 1990 Debris Flow Ybgg NC093 28 N
Cv NCGS Hickey Fork Debris Flow 36.0068 82.6977 Madison 1999 Debris Flow Zwc NC092 28 SE
Cv NCGS Allen Stand Debris Flow 35.9865 82.7611 Madison 1999 Debris Flow Zwc NC092 28 E
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Rd Pomeroy No. 23, Runnout length of 2820 ft. 35.4700 82.6600 Buncombe 1977 Debris Flow Zatm NC090 29 S
Pomeroy 35.4800 82.6500 Buncombe 1977 Debris Flow Zatm NC093 29 SE
Pomeroy Number 14 35.4900 82.6600 Buncombe 1977 Debris Flow Zatm NC095 29 NE
Pomeroy Number 7 35.4800 82.6900 Buncombe 1977 Debris Flow Zatm NC093 29 NE
NCGS Blackstone Knob WestSlide
35.7353 82.3174 Buncombe 2004 Debris Flow Zatm NC095 29 S
Pomeroy Number 4 35.4788 82.6863 Buncombe 1977 Debris Flow Zatm NC093 29 NE
Pomeroy No. 31, Chute 1800 ft. long,
into Laurel Branch
35.4600 82.6500 Buncombe 1977 Debris Flow Zatm NC095 30 E
Rd Pomeroy Number 30 35.4600 82.6700 Buncombe 1977 Debris Flow Zatm NC095 30 NE
Pomeroy Number 25 35.4800 82.6700 Buncombe 1977 Debris Flow Zatm NC095 30 E
Cv NCDOT MP 1.0, I-40 35.7700 83.0800 Haywood Debris Flow Zsr NC102 30 SW
Pomeroy Number 10 35.4900 82.6800 Buncombe 1977 Debris Flow Zatm NC093 30 SW
Pomeroy Number 48 35.4300 82.6600 Henderson 1977 Debris Flow Zatm NC093 30 E
Pomeroy Number 12 35.4900 82.6600 Buncombe 1977 Debris Flow Zatm NC095 30 S
Rd/Cv NCDOT MP 7.95 EBL, N of singletunnel
35.7200 83.0300 Haywood 1973 Debris Flow Zsl NC102 30 N
Pomeroy 35.4900 82.6500 Buncombe 1977 Debris Flow Zatm NC093 31 SWWitt Rt. 215 35.2900 82.9100 Transylvania Slump Zatm NC095 31 E
Pomeroy Number 2 35.4700 82.7000 Buncombe 1977 Debris Flow Zatm NC095 31 E
Pomeroy Number 40 35.4400 82.6200 Buncombe 1977 Debris Flow Zatm NC093 31 NE
Pomeroy Number 44 35.4300 82.6400 Henderson 1977 Debris Flow Zatm NC093 31 S
Pomeroy No. 8, Debris Torrent 35.4800 82.6800 Buncombe 1977 Debris Flow Zatm NC093 31 SW
Rd NCDOT NC 63, 5.1mi N of Buncombe County line
35.7000 82.8200 Madison 1980 Slump Ybgg NC095 31 NE
Rd Otteman 35.4967 82.6431 Buncombe 1977 Debris Flow Zatm NC093 31 S
Cv? USFS Microburst; dual slides near Dry Branch
35.7700 83.0400 Haywood 1994 Debris Flow Zsl NC102 32 SE
Pomeroy Number 32 35.4500 82.6500 Buncombe 1977 Debris Flow Zatm NC095 32 E
Rd Otteman Number 29 35.4600 82.6800 Buncombe 1977 Debris Flow Zatm NC095 32 NE
Pomeroy 35.4900 82.6500 Buncombe 1977 Debris Flow Zatm NC095 32 S
Witt identified in 3D Analyst 35.7200 86.0400 Haywood Slump Zsl NC102 32 W
Rd Pomeroy Number 28/29 35.4700 82.6600 Buncombe 1977 Debris Flow Zatm NC095 33 E
rk DOQQ identified from DOQQ 35.7700 83.0900 Haywood Slump Zsp NC102 33 SW
Pomeroy Number 11 35.4900 82.6700 Buncombe 1977 Debris Flow Zatm NC095 33 E
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Pomeroy Number 42 35.4200 82.6200 Henderson 1977 Debris Flow Zatm NC093 33 E
Pomeroy Number 42 35.4200 82.6200 Henderson 1977 Debris Flow Zatm NC093 33 SE
Pomeroy Number 38 35.4400 82.6300 Buncombe 1977 Debris Flow Zatm NC093 33 E
Cv NCGS Blackstone Knob East Slide 35.7338 82.3154 Buncombe 2004 Debris Flow Zatm NC095 33 SW
Pomeroy 35.4700 82.6600 Buncombe 1977 Debris Flow Zatm NC093 34 SE
Pomeroy Number 14 (second head) 35.4900 82.6600 Buncombe 1977 Debris Flow Zatm NC095 34 NE
Pomeroy Number 5 35.4800 82.6900 Buncombe 1977 Debris Flow Zatm NC093 34 E
rk NCDOT I-40, East of Twin Tunnels 35.7600 83.0400 Haywood 1985 Slump Zsl NC102 34 SE
NCGS Pounding Mill Branch Slide 35.9951 82.7325 Madison 2001 Debris Flow Zwc NC092 34 SPomeroy Number 33 35.4600 82.6500 Buncombe 1977 Debris Flow Zatm NC095 35 E
Cv Pomeroy Chute 920 ft. long, debristorrent
35.4600 82.6500 Buncombe 1977 Debris Flow Zatm NC095 35 SE
MGW Pomeroy Number 28/29 35.4700 82.6600 Buncombe 1977 Debris Flow Zatm NC095 35 E
Wooten Mt. Mitchell Slide 35.7300 82.3000 Buncombe 2003 Debris Flow Zatw NC098 35 S
Rd NCDOT SBL NC 25-70, 250 ft. N of SR 1319
35.8900 82.7500 Madison 1986 Slump Zsr NC092 35 W
Pomeroy Number 3 35.4800 82.6900 Buncombe 1977 Debris Flow Zatm NC095 35 E
Pomeroy Debris Torrent 35.4900 82.6800 Buncombe 1977 Debris Flow Zatm NC095 35 Wwr NCDOT Lesser Fines Creek Slide 35.6700 82.9900 Haywood 1950 Slump Zbgg NC006 35 S
Pomeroy Number 33 35.4600 82.6500 Buncombe 1977 Debris Flow Zatm NC095 36 E
Witt Rt. 215 35.2900 82.9100 Transylvania Slump Zatm NC095 36 E
Pomeroy Number 8 35.4800 82.6800 Buncombe 1977 Debris Flow Zatm NC093 36 NW
Pomeroy Debris Torrent 35.4900 82.6600 Buncombe 1977 Debris Flow Zatm NC095 36 SW
Rk NCDOT MP 0.4, I-40 35.7700 83.0900 Haywood 1997 Slump Zsp NC102 36 S
Pomeroy 35.4468 82.6474 Henderson 1977 Debris Flow Zatm NC095 36 S
Otteman number 27 35.4600 82.6700 Buncombe 1977 Debris Flow Zatm NC095 37 E
Pomeroy Number 27 35.4800 82.6700 Buncombe 1977 Debris Flow Zatm NC095 37 NE
Cv NCDOT SBL NC 261 36.1000 82.1000 Mitchell 1977 Debris Flow Ymg NC098 37 SW
Pomeroy Debris Torrent 35.4900 82.6600 Buncombe 1977 Debris Flow Zatm NC095 37 SW
Pomeroy Debris Torrent 35.4900 82.6600 Buncombe 1977 Debris Flow Zatm NC095 37 SW
Cv Otteman number 28 35.4600 82.6700 Buncombe 1977 Debris Flow Zatm NC095 38 NE
Rd NCDOT NBL NC 25-70, S of NC208
35.9100 82.7500 Madison 1989 Slump Zsr NC092 38 NW
Pomeroy Number 46 35.4400 82.6500 Henderson 1977 Debris Flow Zatm NC093 38 SE
Pomeroy Number 45 35.4400 82.6500 Henderson 1977 Debris Flow Zatm NC093 38 SE
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Pomeroy Number 13 35.4900 82.6700 Buncombe 1977 Debris Flow Zatm NC095 38 SW
Witt Rt. 215 35.2900 82.9100 Transylvania Slump Zatm NC095 39 E
Cv NCDOT I-40, may have beenrockfall, not sure
35.7800 83.1000 Haywood 1989 Slump Zsp NC102 39 S
wr NCDOT NC 215 35.3500 82.9100 Haywood 1988 Slump Zatm NC095 39 NE
Rd Pomeroy debris torrent 35.4600 82.6700 Buncombe 1977 Debris Flow Zatm NC095 40 SE
Rd Pomeroy Ext. rock rubble at toe,multi-headed flow, No. 28
35.4600 82.6700 Buncombe 1977 Debris Flow Zatm NC095 40 SE
Pomeroy Number 29 35.4600 82.6700 Buncombe 1977 Debris Flow Zatm NC095 41 EPomeroy Number 27 35.4800 82.6700 Buncombe 1977 Debris Flow Zatm NC095 41 E
Pomeroy Number 9 35.4900 82.6800 Buncombe 1977 Debris Flow Zatm NC093 41 W
Pomeroy Number 9 35.4900 82.6800 Buncombe 1977 Debris Flow Zatm NC093 41 SW
Pomeroy Debris Torrent 35.4900 82.6700 Buncombe 1977 Debris Flow Zatm NC093 41 W
Pomeroy 35.5000 82.6700 Buncombe 1977 Debris Flow Zatm NC095 41 E
Pomeroy Number 26 35.4800 82.6700 Buncombe 1977 Debris Flow Zatm NC095 42 SE
Pomeroy Number 1 35.4700 82.7000 Buncombe 1977 Debris Flow Zatm NC095 42 NW
Pomeroy Number 47 35.4400 82.6500 Henderson 1977 Debris Flow Zatm NC093 46 SE
Pomeroy 35.4900 82.6600 Buncombe 1977 Debris Flow Zatm NC095 46 EPomeroy Number 22 35.4800 82.6600 Buncombe 1977 Debris Flow Zatm NC095 1977 SE
Appendix E: SINMAP Results
Default Parameters - 30m DEM
Stable ModeratelyStable Quasi-Stable LowerThreshold UpperThreshold Defended Total
Area (km²) 3796.6 949.2 1216.3 1114.8 15.3 0.1 7092.3
% of Region 53.5 13.4 17.1 15.7 0.2 0.0 100.0
# of Landslides 12 14 41 73 0 0 140
% of Slides 8.6 10.0 29.3 52.1 0.0 0.0 100.0
LS Density 0.003 0.015 0.034 0.065 0.000 0.000
Recharge 50mm – 30m DEM
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Stable Moderately
Stable
Quasi-Stable Lower
Threshold
Upper
Threshold
Defended Total
Area (km²) 1735.2 358.0 544.1 3112.1 1315.9 3.8 7069.1
% of Region 24.5 5.1 7.7 44.0 18.6 0.1 100.0
# of Landslides 2 2 5 46 84 1 140
% of Slides 1.4 1.4 3.6 32.9 60.0 0.7 100.0
LS Density 0.001 0.006 0.009 0.015 0.064 0.263
Recharge 125mm – 30mDEMStable Moderately
Stable
Quasi-Stable Lower
Threshold
Upper
Threshold
Defended Total
Area (km²) 1735.2 358.0 544.1 3103.3 1286.7 28.1 7055.4
% of Region 24.5 5.1 7.7 44.0 18.2 0.4 99.9
# of Landslides 2 2 5 46 81 4 140
% of Slides 1.4 1.4 3.6 32.9 57.9 2.9 100.1
LS Density 0.001 0.006 0.009 0.015 0.063 0.142
Recharge 250mm – 30m DEM
Stable Moderately
Stable
Quasi-Stable Lower
Threshold
Upper
Threshold
Defended Total
Area (km²) 1735.2 358.0 544.1 3102.1 1278.7 53.6 7071.7
% of Region 24.5 5.1 7.7 43.9 18.1 0.8 100.1
# of Landslides 2 2 5 46 78 7 140
% of Slides 1.4 1.4 3.6 32.9 55.7 5.0 100.0
LS Density 0.001 0.006 0.009 0.015 0.061 0.131
Recharge 375mm – 30m DEM
Stable Moderately
Stable
Quasi-Stable Lower
Threshold
Upper
Threshold
Defended Total
Area (km²) 1735.2 358.0 544.1 3102.1 1278.7 61.4 7079.5
% of Region 24.5 5.1 7.7 43.8 18.1 0.9 100.1
# of Landslides 2 2 5 46 77 8 140
% of Slides 1.4 1.4 3.6 32.9 55.0 5.7 100.0
LS Density 0.001 0.006 0.009 0.015 0.060 0.130
R h 125 C 1 25 30 DEM
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Recharge 125mm, C = .1-.25 – 30m DEMStable Moderately
Stable
Quasi-Stable Lower
Threshold
Upper
Threshold
Defended Total
Area (km²) 2524.7 535.7 844.5 2355.6 811.2 3.8 7075.5
% of Region 35.7 7.6 11.9 33.3 11.5 0.1 100.1
# of Landslides 8 4 4 63 60 1 140
% of Slides 5.7 2.9 2.9 45.0 42.9 0.7 100.1
LS Density 0.003 0.007 0.005 0.027 0.074 0.263
Default Parameters - 10m DEM
Stable Moderately
Stable
Quasi-Stable Lower
Threshold
Upper
Threshold
Defended Total
Area (km²) 3673.9 868.0 1120.2 1229.4 44.3 0.6 6936.4
% of Region 53.0 12.5 16.1 17.7 0.6 0.0 100.0
# of Landslides 14 11 24 69 4 0 122.0
% of Slides 11.5 9.0 19.7 56.6 3.3 0.0 100.0
LS Density 0.004 0.013 0.021 0.056 0.090 0.000
Recharge 50mm- 10m DEM
Stable Moderately
Stable
Quasi-Stable Lower
Threshold
Upper
Threshold
Defended Total
Area (km²) 1591.9 322.7 494.4 2944.9 1514.0 24.8 6892.7
% of Region 23.1 4.7 7.2 42.7 22.0 0.4 100.0
# of Landslides 0.0 2.0 5.0 33.0 82.0 0.0 122.0
% of Slides 0.0 1.6 4.1 27.0 67.2 0.0 100.0
LS Density 0.000 0.006 0.010 0.011 0.054 0.000
Recharge 125mm - 10m DEMStable Moderately
Stable
Quasi-Stable Lower
Threshold
Upper
Threshold
Defended Total
Area (km²) 1591.9 322.7 494.4 2932.5 1455.2 70.8 6867.5
% of Region 23.2 4.7 7.2 42.7 21.2 1.0 100.0
# of Landslides 0.0 2.0 5.0 33.0 74.0 8.0 122.0
% of Slides 0.0 1.6 4.1 27.0 60.7 6.6 100.0
LS Density 0.000 0.006 0.010 0.011 0.051 0.113
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Recharge 250mm - 10m DEM
Stable Moderately
Stable
Quasi-Stable Lower
Threshold
Upper
Threshold
Defended Total
Area (km²) 1591.9 322.7 494.4 2930.9 1434.2 126.4 6900.5
% of Region 23.1 4.7 7.2 42.5 20.8 1.8 100.0
# of Landslides 0.0 2.0 5.0 33.0 70.0 12.0 122.0
% of Slides 0.0 1.6 4.1 27.0 57.4 9.8 100.0
LS Density 0.000 0.006 0.010 0.011 0.049 0.095
Recharge 375mm -10m DEM
Stable Moderately
Stable
Quasi-Stable Lower
Threshold
Upper
Threshold
Defended Total
Area (km²) 1591.9 322.7 494.4 2930.8 1432.4 138.8 6911.0
% of Region 23.0 4.7 7.2 42.4 20.7 2.0 100.0
# of Landslides 0.0 2.0 5.0 33.0 70.0 12.0 122.0
% of Slides 0.0 1.6 4.1 27.0 57.4 9.8 100.0
LS Density 0.000 0.006 0.010 0.011 0.049 0.086
Recharge 125mm, C = .1-.25 – 10m DEM
Stable Moderately
Stable
Quasi-Stable Lower
Threshold
Upper
Threshold
Defended Total
Area (km²) 2306.1 495.1 789.9 2269.2 959.3 50.2 6869.8
% of Region 33.6 7.2 11.5 33.0 14.0 0.7 100.0
# of Landslides 5.0 4.0 5.0 45.0 56.0 7.0 122.0
% of Slides 4.1 3.3 4.1 36.9 45.9 5.7 100.0
LS Density 0.002 0.008 0.006 0.020 0.058 0.139
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Appendix F: SHALSTAB Results
Cohesion 2000, SFA 26 – 30m DEM
PERCENT CUMPERCENT INSTABILITY NUMSLIDES PERSLIDES CUMSLIDES Area km² CUMAREA LS DEN
14.87 14.87 Chronic Instability 10 45.45 45.45 243.8 243.8 0.041
20.50 35.37 < -3.1 5 22.73 68.18 336.2 580.0 0.015
11.40 46.76 -3.1 - -2.8 2 9.09 77.27 186.9 766.9 0.011
10.55 57.32 -2.8 - -2.5 2 9.09 86.36 173.1 940.0 0.012
6.40 63.72 -2.5 - -2.2 0 0.00 86.36 105.0 1044.9 0.000
0.81 64.53 > -2.2 1 4.55 90.91 13.3 1058.2 0.075
35.47 100.00 Stable 2 9.09 100.00 581.8 1639.9 0.003
Cohesion 2000 SFA 45 – 30m DEM
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Cohesion 2000, SFA 45 30m DEMPERCENT CUMPERCENT INSTABILITY NUMSLIDES PERSLIDES CUMSLIDES Area km² CUMAREA LS DEN
0.04 0.04 Chronic Instability 0 0.00 0.00 0.7 0.7 0.000
2.00 2.04 < -3.1 3 13.64 13.64 32.8 33.5 0.092
4.19 6.23 -3.1 - -2.8 2 9.09 22.73 68.7 102.2 0.029
9.63 15.85 -2.8 - -2.5 5 22.73 45.45 157.8 260.0 0.032
9.20 25.05 -2.5 - -2.2 1 4.55 50.00 150.8 410.8 0.007
1.99 27.04 > -2.2 0 0.00 50.00 32.6 443.4 0.000
72.96 100.00 Stable 11 50.00 100.00 1196.6 1639.9 0.009
Cohesion 9427.41, SFA 45 (SINMAP Upper Bound) – 30m DEM
PERCENT CUMPERCENT INSTABILITY NUMSLIDES PERSLIDES CUMSLIDES Area km² CUMAREA LS DEN
0.00 0.00 Chronic Instability 0 0.00 0.00 0.0 0.0 0.000
0.00 0.00 < -3.1 0 0.00 0.00 0.0 0.0 0.000
0.00 0.00 -3.1 - -2.8 0 0.00 0.00 0.0 0.0 0.000
0.01 0.01 -2.8 - -2.5 0 0.00 0.00 0.2 0.2 0.000
0.04 0.05 -2.5 - -2.2 0 0.00 0.00 0.6 0.8 0.0000.03 0.08 > -2.2 0 0.00 0.00 0.5 1.3 0.000
99.92 100.00 Stable 22 100.00 100.00 1638.7 1639.9 0.013
Default Results – 30m DEM
PERCENT CUMPERCENT INSTABILITY NUMSLIDES PERSLIDES CUMSLIDES Area km² CUMAREA LS DEN
0.04 0.04 Chronic Instability 0 0.00 0.00 0.7 0.7 0.000
4.17 4.21 < -3.1 3 13.64 13.64 68.3 69.0 0.044
7.18 11.39 -3.1 - -2.8 4 18.18 31.82 117.7 186.7 0.034
14.33 25.72 -2.8 - -2.5 6 27.27 59.09 235.1 421.8 0.026
12.31 38.03 -2.5 - -2.2 2 9.09 68.18 201.8 623.6 0.0102.53 40.55 > -2.2 0 0.00 68.18 41.4 665.0 0.000
59.45 100.00 Stable 7 31.82 100.00 974.9 1639.9 0.007
Cohesion 0, SFA 26 (SINMAP Lower Bound) – 30m DEM
PERCENT CUMPERCENT INSTABILITY NUMSLIDES PERSLIDES CUMSLIDES Area km² CUMAREA LS DEN
25.59 25.59 Chronic Instability 11 50.00 50.00 419.7 419.7 0.026
24.52 50.11 < -3.1 6 27.27 77.27 402.1 821.8 0.015
9 60 59 71 -3 1 - -2 8 3 13 64 90 91 157 5 979 3 0 019
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9.60 59.71 3.1 2.8 3 13.64 90.91 157.5 979.3 0.019
8.72 68.43 -2.8 - -2.5 0 0.00 90.91 143.0 1122.2 0.000
5.20 73.63 -2.5 - -2.2 2 9.09 100.00 85.2 1207.5 0.023
0.38 74.01 > -2.2 0 0.00 100.00 6.3 1213.8 0.000
25.99 100.00 Stable 0 0.00 100.00 426.2 1639.9 0.000
Cohesion 0, SFA 45 – 30m DEM
PERCENT CUMPERCENT INSTABILIT NUMSLIDES PERSLIDES CUMSLIDES Area km² CUMAREA LS DEN0.04 0.04 Chronic Instability 0 0.00 0.00 0.7 0.7 0.000
2.00 2.04 < -3.1 3 13.64 13.64 32.8 33.5 0.092
4.19 6.23 -3.1 - -2.8 2 9.09 22.73 68.7 102.2 0.029
9.63 15.85 -2.8 - -2.5 5 22.73 45.45 157.8 260.0 0.032
9.20 25.05 -2.5 - -2.2 1 4.55 50.00 150.8 410.8 0.007
1.99 27.04 > -2.2 0 0.00 50.00 32.6 443.4 0.000
72.96 100.00 Stable 11 50.00 100.00 1196.6 1639.9 0.009
Cohesion 2000, STA 35 – 30m DEM
PERCENT CUMPERCENT INSTABILIT NUMSLIDES PERSLIDES CUMSLIDES Area km² CUMAREA LS DEN
1.08 1.08 Chronic Instability 3 13.64 13.64 17.6 17.6 0.170
7.55 8.62 < -3.1 4 18.18 31.82 123.8 141.4 0.032
9.66 18.28 -3.1 - -2.8 5 22.73 54.55 158.4 299.8 0.032
13.97 32.25 -2.8 - -2.5 2 9.09 63.64 229.1 528.9 0.009
9.61 41.86 -2.5 - -2.2 2 9.09 72.73 157.6 686.6 0.013
1.73 43.60 > -2.2 0 0.00 72.73 28.4 715.0 0.000
56.40 100.00 Stable 6 27.27 100.00 925.0 1639.9 0.006
Cohesion 0, STA 35 – 30m DEM
PERCENT CUMPERCENT INSTABILIT NUMSLIDES PERSLIDES CUMSLIDES Area km² CUMAREA LS DEN3.34 3.34 Chronic Instability 3 13.64 13.64 54.8 54.8 0.055
14.04 17.38 < -3.1 9 40.91 54.55 230.2 285.1 0.039
13.00 30.38 -3.1 - -2.8 2 9.09 63.64 213.1 498.2 0.009
14.94 45.31 -2.8 - -2.5 3 13.64 77.27 244.9 743.1 0.012
9.42 54.73 -2.5 - -2.2 1 4.55 81.82 154.5 897.6 0.006
1.49 56.23 > -2.2 0 0.00 81.82 24.5 922.1 0.000
43.77 100.00 Stable 4 18.18 100.00 717.8 1639.9 0.006
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Total 10m DEM Haywood - 26 1922 0 2
PERCENT CUMPERCENT INSTABILIT NUMSLIDES PERSLIDES CUMSLIDES Area km² CUMAREA LS DEN
32.57 32.57 Chronic Instability 20.0 86.96 86.96 534.8 534.8 0.037
17.09 49.66 < -3.1 0.0 0.00 86.96 280.7 815.5 0.000
8.52 58.18 -3.1 - -2.8 3.0 13.04 100.00 139.9 955.4 0.021
9.14 67.32 -2.8 - -2.5 0.0 0.00 100.00 150.1 1105.5 0.000
6.60 73.92 -2.5 - -2.2 0.0 0.00 100.00 108.3 1213.8 0.000
3.05 76.97 > -2.2 0.0 0.00 100.00 50.2 1264.0 0.000
23.04 100.01 Stable 0.0 0.00 100.00 378.3 1642.3 0.000
Total 10m DEM Haywood - 26 1922 2000 2
PERCENT CUMPERCENT INSTABILIT NUMSLIDES PERSLIDES CUMSLIDES Area km² CUMAREA LS DEN
21.35 21.35 Chronic Instability 13.0 56.52 56.52 350.6 350.6 0.037
14.85 36.20 < -3.1 5.0 21.74 78.26 243.7 594.3 0.021
9.28 45.48 -3.1 - -2.8 2.0 8.70 86.96 152.4 746.7 0.013
10.92 56.40 -2.8 - -2.5 2.0 8.70 95.65 179.3 926.0 0.0118.33 64.73 -2.5 - -2.2 0.0 0.00 95.65 136.8 1062.8 0.000
4.02 68.75 > -2.2 0.0 0.00 95.65 66.0 1128.8 0.000
31.26 100.01 Stable 1.0 4.35 100.00 513.3 1642.1 0.002
Total 10m DEM Haywood - 35 1922 2000 2
PERCENT CUMPERCENT INSTABILIT NUMSLIDES PERSLIDES CUMSLIDES Area km² CUMAREA LS DEN
3.16 3.16 Chronic Instability 6.0 26.09 26.09 51.9 51.9 0.116
6.85 10.01 < -3.1 7.0 30.43 56.52 112.6 164.5 0.062
7.26 17.27 -3.1 - -2.8 1.0 4.35 60.87 119.2 283.7 0.008
12.95 30.22 -2.8 - -2.5 4.0 17.39 78.26 212.7 496.4 0.019
12.78 43.00 -2.5 - -2.2 3.0 13.04 91.30 209.9 706.3 0.0146.83 49.83 > -2.2 0.0 0.00 91.30 112.1 818.4 0.000
50.17 100.00 Stable 2.0 8.70 100.00 823.9 1642.3 0.002
Total 10m DEM Haywood - 35 1922 0 2
PERCENT CUMPERCENT INSTABILIT NUMSLIDES PERSLIDES CUMSLIDES Area km² CUMAREA LS DEN
7.02 7.02 Chronic Instability 10.0 43.48 43.48 115.2 115.2 0.087
11.20 18.22 < -3.1 5.0 21.74 65.22 183.9 299.1 0.027
9.83 28.05 -3.1 - -2.8 1.0 4.35 69.57 161.4 460.5 0.006
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14.55 42.60 -2.8 - -2.5 5.0 21.74 91.30 239.0 699.5 0.021
12.46 55.06 -2.5 - -2.2 1.0 4.35 95.65 204.7 904.2 0.005
6.27 61.33 > -2.2 0.0 0.00 95.65 103.0 1007.2 0.000
38.67 100.00 Stable 1.0 4.35 100.00 635.0 1642.2 0.002
Total 10m DEM Haywood - 35 1922 9427.41 2
PERCENT CUMPERCENT INSTABILIT NUMSLIDES PERSLIDES CUMSLIDES Area km² CUMAREA LS DEN
0.00 0.00 Chronic Instability 0.0 0.00 0.00 0.0 0.0 0.0000.11 0.11 < -3.1 2.0 8.70 8.70 1.8 1.9 1.111
0.24 0.35 -3.1 - -2.8 1.0 4.35 13.04 3.9 5.8 0.256
0.92 1.27 -2.8 - -2.5 2.0 8.70 21.74 15.1 20.9 0.132
2.40 3.67 -2.5 - -2.2 3.0 13.04 34.78 39.4 60.3 0.076
2.59 6.26 > -2.2 2.0 8.70 43.48 42.6 102.9 0.047
93.74 100.00 Stable 13.0 56.52 100.00 1539.3 1642.2 0.008
Total 10m DEM Haywood - 45 1922 0 2
PERCENT CUMPERCENT INSTABILIT NUMSLIDES PERSLIDES CUMSLIDES Area km² CUMAREA LS DEN
0.34 0.34 Chronic Instability 0.0 0.00 0.00 5.6 5.6 0.000
2.29 2.63 < -3.1 8.0 34.78 34.78 37.6 43.2 0.213
3.37 6.00 -3.1 - -2.8 2.0 8.70 43.48 55.4 98.6 0.036
8.46 14.46 -2.8 - -2.5 3.0 13.04 56.52 139.0 237.5 0.022
11.85 26.31 -2.5 - -2.2 7.0 30.43 86.96 194.6 432.1 0.036
7.70 34.02 > -2.2 0.0 0.00 86.96 126.5 558.6 0.000
65.99 100.00 Stable 3.0 13.04 100.00 1083.6 1642.2 0.003
Total 10m DEM Haywood - 45 1922 2000 2
PERCENT CUMPERCENT INSTABILIT NUMSLIDES PERSLIDES CUMSLIDES Area km² CUMAREA LS DEN0.09 0.09 Chronic Instability 0.0 0.00 0.00 1.6 1.6 0.000
0.99 1.09 < -3.1 5.0 21.74 21.74 16.3 17.9 0.307
1.69 2.78 -3.1 - -2.8 2.0 8.70 30.43 27.8 45.7 0.072
4.92 7.70 -2.8 - -2.5 3.0 13.04 43.48 80.8 126.5 0.037
8.50 16.20 -2.5 - -2.2 4.0 17.39 60.87 139.5 266.0 0.029
6.39 22.59 > -2.2 2.0 8.70 69.57 104.9 370.9 0.019
77.41 100.00 Stable 7.0 30.43 100.00 1271.3 1642.2 0.006
T t l 10 DEM H d 45 1922 9427 41 2
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Total 10m DEM Haywood - 45 1922 9427.41 2
PERCENT CUMPERCENT INSTABILIT NUMSLIDES PERSLIDES CUMSLIDES Area km² CUMAREA LS DEN
0.00 0.00 Chronic Instability 0.0 0.00 0.00 0.0 0.0 0.000
0.00 0.00 < -3.1 0.0 0.00 0.00 0.1 0.1 0.000
0.01 0.01 -3.1 - -2.8 1.0 4.35 4.35 0.1 0.2 8.598
0.03 0.04 -2.8 - -2.5 0.0 0.00 4.35 0.5 0.7 0.000
0.15 0.19 -2.5 - -2.2 1.0 4.35 8.70 2.4 3.1 0.417
0.33 0.52 > -2.2 0.0 0.00 8.70 5.4 8.5 0.00099.48 100.00 Stable 21.0 91.30 100.00 1633.7 1642.2 0.013
Total Default Values - 10m DEM Haywood
PERCENT CUMPERCENT INSTABILIT NUMSLIDES PERSLIDES CUMSLIDES Area km² CUMAREA LS DEN
0.34 0.34 Chronic Instability 0 0.00 0.00 5.6 5.6 0.000
3.96 4.30 < -3.1 8 34.78 34.78 65.0 70.6 0.123
5.21 9.51 -3.1 - -2.8 2 8.70 43.48 85.5 156.1 0.023
12.38 21.89 -2.8 - -2.5 5 21.74 65.22 203.2 359.3 0.025
15.61 37.50 -2.5 - -2.2 6 26.09 91.30 256.3 615.6 0.023
9.52 47.02 > -2.2 0 0.00 91.30 156.4 772.0 0.000
52.98 100.00 Stable 2 8.70 100.00 870.1 1642.1 0.002