map of known and potential debris flow pathways in watauga county

map of known and potential debris flow pathways in watauga county

TN
MAP OF KNOWN AND POTENTIAL DEBRIS FLOW PATHWAYS IN WATAUGA COUNTY, NORTH CAROLINA
NORTH CAROLINA GEOLOGICAL SURVEY
DIVISION OF LAND RESOURCES
DEPARTMENT OF ENVIRONMENT AND NATURAL RESOURCES
As
he
Co
u
FOR SHALLOW TRANSLATIONAL SLOPE MOVEMENTS
nty NC 1,200,000 Feet E
By
Richard M. Wooten, Anne C. Witt, Kenneth A. Gillon, Thomas J. Douglas, Rebecca S. Latham, Stephen J. Fuemmeler, and Jennifer B. Bauer
2008
36 22' 30" N
=
GEOLOGIC HAZARDS MAP SERIES 3
SLOPE MOVEMENT HAZARD MAPS OF WATAUGA COUNTY, NC
SHEET 3 OF 4, VERSION: MARCH 18, 2008
36 22' 30" N
=
88
«
¬
EXPLANATION
Inset Map Scale:
1:18,000
NC 1,170,000 Feet E
EE
E SS
N
N
E
T
Map Symbols
Map Unit
Designation
Description
FEET
1,000
Known debris flow
pathways.
Increasing
0
NC 1,200,000 Feet E
81 45' 00" W
=
Areas that lie within the pathways of mapped recent debris flows and other shallow,
translational slope movements (those occuring in 1940 or later).
OTHER MAP SYMBOLS
Initiation zones of shallow,
translational slope movements
on unmodified slopes
Roads
Primary Roads
Secondary Roads
Blue Ridge Parkway
81 37' 30" W
=
£
¤
421
NC 950,000 Feet N
Relative Hazard
Zionville
NC 950,000 Feet N
NC 1,230,000 Feet E
81 52' 30" W
=
Potential debris flow
pathways.
he
As
Areas of past debris
flow activity.
un
Co
Decreasing
ty
¬ Todd
«
194
Areas of no known or
predicted debris flow
activity.
£
¤
TE
NN
ES
SE
E
421
Av e
Inset 1. Map showing the Deep Gap area of Watauga County.
Areas that lie within the predicted flow paths of debris flows and other shallow, translational
slope movements that could potentially initiate within the unstable and upper threshold (high
hazard) predicted stability zones of the Stability Index Map.
Municipal boundaries
Areas of mapped slope movement deposits. Because slope movements have previously
deposited material at these locations, future deposition of slope movement materials could
occur in these areas again.
Additionally, a potential slope stability hazard exists for slope movement deposits because
they typically consist of thick, unconsolidated clay- to boulder-sized material that can be
unstable in high, steep-sided excavations, and in areas over-steepened by stream erosion.
Areas located outside of the predicted flow paths of potential debris flows and other shallow,
translational slope movements, and outside of the mapped extents of slope movement
deposits.
Slope movements may result from the modification of slopes in this map unit.
u
Co
nt
£
¤
y
421
S ou
th
Known debris
flow pathways
<1%
Fo
rk
W
w
N
R iv
er
e
ive
Minor Rivers
PERCENTAGE OF WATAUGA COUNTY WITHIN
EACH MAP UNIT DESIGNATION
ry
u ga R
Major Rivers
Table 1. Definitions and explanation of map unit designations.
Mast
at a
Rivers
r
Sherwood
£
¤
321
Sugar
Grove
36 15' 00" N
=
£
¤
321
55%
Areas of no known
or predicted debris
flow activity
NC 1,260,000 Feet E
41%
Potential debris
flow pathways
NC 920,000 Feet N
81 30' 00" W
36 15' 00" N
NC 920,000 Feet N
=
=
194
«
¬
£
¤
221
£
¤£
¤
321
194
«
¬
421
Ashe C
ou n ty
Deep
Gap
Sout h Fo r k Ne w Rive r
ilk
W
4%
Areas of past debris
flow activity
£
¤
es
421
un
Co
BYP
e
Ri dg
Introduction
r
Pa
105
kw
«
¬
OVERVIEW OF THE POTENTIAL DEBRIS FLOW PATHWAYS MAP
ty
Bl ue
ay
Beech Mountain
ga
u
Wa
ta
Valle
Crucis
Ri v
er
Boone
105
«
¬
£
¤
421
105
«
¬
St
ny
o
o un ty
Aver y C
Cre
rk
k
El
Fo
ek
The slope movement hazard map series for Watauga County consists of four maps,
Geologic Hazards Map Series 3 (GHMS-3) Sheets 1, 2, 3, and 4 designed to be used in
conjunction with each other. This map is Sheet 3. The accompanying maps are: Sheet 1,
Slope Movement and Slope Movement Deposit Map of Watauga County, North Carolina;,
Sheet 2, Stability Index Map of Watauga County North Carolina; and, Sheet 4, Map
Showing the Zone of Potential Rock Slope Instability with the Generalized Bedrock
Geologic Compilation.
81 52' 30" W
=
N
v
Map of Known and Potential Debris Flow Pathways (Geologic Hazards Map Series
3, Sheet 3).
194
«
¬
SCALE 1:36,000
2
3
Map Information:
Datum: North American Datum of 1983
Coordinate System: State Plane, Zone 3200
Projection: Lambert Conformal Conic
Cartography by North Carolina Geological Survey
Produced in a Geographic Information System (GIS) using ArcGIS™.
£
¤
£
¤
221
KILOMETERS
321
105
«
¬
NC 890,000 Feet N
Basemap:
Hillshade derived from 20-foot resolution LiDAR (Light
Detecting And Ranging) digital elevation data provided
by the North Carolina Floodplain Mapping Program using
a sun azimuth of 315° and a sun altitude of 45°.
NC 890,000 Feet N
ty
1
3
Seven
Devils
Based on information and data available as of March 18, 2008 concurrent with the GIS versions of the maps released to Watauga County on this date.
TN
23
£
¤
441
¤
£
64
£
¤
!
E l k C reek
VA
Watauga County
Asheville
Co
un
0
2
es
1
MILES
§
¦
¨
77
§
¦
¨
40
95
§
¦
¨
!
Raleigh
81 30' 00" W
=
§
¦
¨
26
SC
§
¦
¨
40
ar
ge P
R id
kw a
y
Blowing Rock
NC 1,260,000 Feet E
36 7' 30" N
=
321
£
¤
Wilmington
C ou n
LOCATION OF WATAUGA COUNTY, NORTH CAROLINA
221
£
¤
Aver y
Atlantic Ocean
!
The Potential Debris Flow Pathways Map is intended to indicate the distribution of areas
that could potentially be affected by debris flows or other shallow, translational slope
movements. In these locations, further slope stability analysis, including field verification,
is recommended prior to citing facilities or undertaking ground disturbing activities.
Map Production
The Map of Known and Potential Debris Flow Pathways is derived from two sources:
1. Outlines of debris flow tracks and slope movement deposits from the Slope
Movements and Slope Movement Deposit Locations Map; and,
The following sequential steps outline the method used to produce the areas of predicted
debris flow pathways using a 20 ft (6 m) LiDAR-derived DEM.
105
«
¬
Bl ue
GA
This color-coded map portrays areas that could potentially be affected by debris flows or
other shallow, translational slope movements such as debris/earth slides and flows.
Designated units on this map are: known debris flow pathways, predicted debris flow
pathways, areas of past debris flow activity, and areas of no known or predicted debris
flow activity. The Stability Index Map shows areas where shallow translational slope
movements are more likely to originate in response to a 5-inch (125-mm) or greater
recharge event within a 24-hour period. The Map of Known and Potential Debris Flow
Pathways delineate areas likely to be in the path of these slope movements should they
occur, including areas significantly further downslope from where the slope movements
originate. Table 2 shows the color codes used on the map with explanations that
correspond to the map unit designations.
2. High hazard areas of the Stability Index Map.
§
¦
¨
85
Wi
lk
0
In response to the number of slope movements (landslides) and the destruction caused by
the remnants of Hurricanes Frances and Ivan in western North Carolina in September
2004, the North Carolina General Assembly authorized the North Carolina Geological
Survey (NCGS) to produce landslide hazard maps for 19 western counties. Watauga
County was selected as the second county to be mapped after Macon County because of
the large number of landslides and the 14 landslide deaths associated with the August 1314, 1940 storm, and the fast-growing population potentially at risk from slope movements.
The intent of the landslide hazard mapping program is to provide the public, local
government and local and state emergency agencies with a description and location of
areas where slope movements have occurred, or are likely to occur, and the general areas
at risk from these slope movements. The locations of previous slope movements are
important because they often reoccur in the same general areas. This predictive mapping
is not intended to be a substitute for a detailed, onsite analysis by a qualified geologist or
engineer.
ty
81 45' 00" W
36 7' 30" N
ty
Caldwell C ou n
=
NC 1,200,000 Feet E
=
y
Caldwell Cou nt
NC 1,170,000 Feet E
81 37' 30" W
=
NC 1,230,000 Feet E
Caldwell Coun ty
ilk
W
es
un
Co
ty
2. Hydrologic flow paths based on topographic gradients and streams mapped by the
North Carolina Stream Mapping Project were then created in ArcGIS TM from
points placed on a 20 ft (6 m) grid constructed from the high hazard areas on the
Stability Index Map.
3. Flow paths were terminated once they encountered slope gradients less than three
degrees for contiguous areas >0.25 acres (10,893 ft2 or 1,012 m2). Three degrees
was selected as a nominal gradient consistent with the lowermost downslope extent
of most slope movement deposits and tracks delineated in the accompanying Slope
Movements and Slope Movement Deposits Location Map.
4. These flow paths were then buffered to 33 ft (10 m) on all sides to delineate the
predicted debris flow pathways. This buffer approximates the average track width
of mapped debris flows in Watauga County. Note: In some cases the automated
33-ft (10-m) buffering routine may extend the predicted debris flow pathways
slightly upslope and over ridge tops above debris flow source areas.
Manual editing of the digital map was required to adjust the downstream extents of the
predicted debris flow pathways in order to terminate unrealistic flow paths where
warranted. These adjustment procedures are outlined as follows:
1. Flow paths that originated on man-made cut slopes were terminated near the base
of the cut.
2. Flow paths were terminated before flowing over dams.
5. In some instances, errors in the LiDAR data did not represent “bare-earth”
topography. Flow paths that originated from these features were removed.
Map Unit Designations
Known debris flow pathways. Areas that lie within the pathways of mapped recent
debris flows (those occurring in 1940 or later).
Potential debris flow pathways. Areas that lie within the predicted flow paths of
debris/earth slides and flows that potentially initiate in the unstable and upper threshold
predicted stability zones of the Stability Index Map. Slope movements that initiate on
slopes destabilized by modification within these high hazard zones may also follow
similar flow paths.
Areas of past debris flow activity. Areas mapped as slope movement deposits (Slope
Movements and Slope Movement Deposits Map) outside the areas of predicted flow paths
are designated as areas of past debris flow activity. Debris flows and other slope
movements have previously deposited material at these locations. For this reason future
slope movements could also affect these areas; however, it is more likely that future debris
flows will affect areas of known and predicted debris flow pathways. Additionally, a
potential hazard exists for slope movement deposits because they typically consist of deep,
unconsolidated accumulations of clay- to boulder-sized material that can be unstable in
high, steep-sided excavations, and in areas over-steepened by stream erosion.
Areas of no known or predicted debris flow activity. Areas outside of the predicted
flow paths of potential debris/earth slides and flows, and mapped slope movement
deposits. There is a low likelihood of damage in this area from slope movements that
originate from outside of this area. Slope movements may result from the modification of
slopes in these areas.
1. High hazard areas from the Stability index Map greater than 0.25 acres (10,893 ft 2
or 1,012 m2) were designated as the most likely source areas for slope movements
that could affect areas downslope.
ACKNOWLEDGEMENTS
The North Carolina Geological Survey gratefully acknowledges assistance and information furnished by local government agencies in Watauga County. Special thanks go to the residents
of Watauga County for their willingness to provide information and property access. The North Carolina Department of Transportation - Geotechnical Engineering Unit, the North Carolina
Floodplain Mapping Program, the U.S.D.A. Forest Service, and the U.S. Geological Survey supplied much useful data and assistance. Reviews and c omments on the draft maps and in the
field by Louis Acker, Andy Bobyarchick, Bart Cattanach, Jack Callahan, Carl Merschat, Hugh Mills, Loren Raymond, Katherine Scharer, Keith Seramur, Jim Simons and Kenneth Taylor
greatly improved the product. Mike Medina and John Nickerson contributed GIS and cartographic support, and the North Carolina Center for Geographic Information and Analysis,
Department of Environment and Natural Resources provided contractual assistance.