Watauga County, NC Real Estate Landslide Hazards

8/3/2019 Watauga County, NC Real Estate Landslide Hazards

1/10

14 Journal o Student Research in Environmental Science at Appalachian

Landslide hazards in Watauga County,North CarolinaDave Christie

Environmental Sciences Program, Appalachian State University, Boone, [email protected]

Abstract

The purpose o this research is to analyze the terrain o Watauga County, North Caro-

lina and develop models predicting slope instability and landslide probability. Devel-

opment in Boone, NC was also analyzed to determine i any existing buildings and

inrastructure were at risk. Slope and soil type, as well as previous landslide occurrence

were all analyzed and reclassied using Geographic Inormation System (GIS) sotware

to produce predictive models o slope instability and, consequently, landslide hazards.

In Watauga County the area o highest risk was the northwestern region, including

the towns o Vilas and Sugar Grove, curving towards the center (near Boone), with an

additional high-risk area in the southeast around the town o Deep Gap. There were

a ew isolated areas in Boone (southwestern and central, as well as Howards Knob to

the north) that posed some landslide risk to a ew buildings and roads, but as a whole

the town was not at risk.

Introduction

There are many types o slope ailures, all o which

can be hazardous. Landslides occur throughout

the United States, primarily in mountainous, hilly

regions and along the Pacic coast [1]. Land-slides cause over $1 billion in damages and are

responsible or between 25 and 50 deaths in the

nation annually [1, 2]. Landslides can damage

buildings, property, utilities, roads and bridges

[3]. Landslides can be triggered in several ways,

including heavy rains, earthquakes, blasting, and

development on steep slopes [3, 4]. Landslide

risk models and maps are essential to inorm

governments, planning committees, developers,

and the general public about the potential haz-ards o slope movement and to provide ways to

mitigate the impact o such disasters, especially

i the models can predict which areas are most

likely to experience slope instability.

In Watauga County North Carolina, more

development is occurring on unstable slopes,

which may increase the risk o landslides [2].

Since Watauga County is mountainous and re-

ceives a signicant amount o rainall, approxi-

mately 150 cm annually, the risk o landslidesand slope instabilities may be high, especially

in areas with steep slopes and areas undergoing

development [5]. For Watauga County, the over-

whelming cause o landslides historically has

been intense rainall. The Southeast Hurricane o

1940 was devastating to the East Coast, aecting

Georgia, South Carolina, North Carolina, Virginia,

and parts o Tennessee [6]. Watauga County re-

ceived between 25.4 and 38.1 cm o rain in just

a ew days, triggering an overwhelming majorityo the landslides on record.

The dominant type o landslide during the

1940 hurricane was the debris fow. A debris

fow is typically a rapid mass movement o mate-

rial, which includes loose soil, rock, and organic

matter [1, 3]. This type o landslide combines

with water to orm a slurry, which can range in

consistency rom thin and watery to rather thick

[1]. Debris fows tend to occur on steep slopes

and can reach speeds up to 35 mph [1, 3]. SinceWatauga County is in a mountainous region, it

is not surprising that so many o the landslides

have been debris fows due to the many steep

slopes in the area. The second most common

orm o landslide in Watauga County has been

the debris blowout which is not surprising be-

cause this area tends to receive high velocity

winds on a airly regular basis.

In this study, historical landslide data in

Watauga County is combined with current slopeand soil type data to assist in the prediction o

landslide hazards in Watauga County and the

Town o Boone, North Carolina.


2/10

Figure 1. Percent Slope Map o Watauga County, North Carolina.

Volume 1, 1st Edition Spring 2011 15

Methods

The data or this study was obtained through

secondary sources, the majority o which came

rom Geographic Inormation Systems (GIS) data

gathered primarily rom North Carolina (NC)

OneMap, an online GIS clearinghouse [7]. The

data was downloaded and opened in ESRI ArcGIS

(ESRI, 380 New York Street, Redlands, CA) sot-ware. ArcMap was used to generate the maps o

Watauga County, North Carolina, and ArcScene

was used to create the 3-dimensional visual-

izations. The boundary shapele or Watauga

County was downloaded rom the Soil Data Mart

[8]. A shapele or the municipal boundaries or

Boone was also obtained rom the North Caro-

lina Department o Transportation (NCDOT) [9].

A Watauga digital elevation model (DEM) in ras-

ter data ormat was downloaded rom a remoteGIS database as a base map or the research [10].

The Watauga area DEM was clipped to match the

county boundary. From this model a slope map

was generated, which showed the percent slope

o the terrain throughout Watauga County (Fig-

ure 1). Percent slope is calculated using the ol-

lowing conversion:

%slope = 100 tan(angle) (1)

Here, tan(angle) is the ratio o slope height to

horizontal length. Percent slope is more com-

monly used in slope regulations than degrees

[2, 4]. This slope map was divided into our main

categories: low, moderate, steep, and very steepslopes. The low slope range included values rom

0-25%, the moderate slope range included val-

ues rom 25-75%, the steep slope range included

values rom 75-150%, and the very steep slope

included values between 150-325%. In general,

the range o minimum slope or landslides to oc-

cur is between 15-25%, so 25% was chosen as

the limit or the low slope category [2, 4, 11]. A

hillshade map was also derived rom the DEM to

provide a visual aid or the layout o the terrain,resulting in a map o terrain that resembles a LI-

DAR (LIght Detection And Ranging) image.

A dataset containing the recorded landslides

o Watauga County through 2006 was down-

loaded, which contained inormation includ-

ing the location, date and nature o the past


3/10


landslides since 1940 [12]. Spatially detailed soil

data or Watauga County was obtained rom the

Soil Data Mart [8]. The Soil Survey o Watauga

County, North Carolina, a document describing

the soil characteristics such as soil composition

and elevation range, was used to correlate the

map unit symbol abbreviations with their actual

map unit symbols [8]. Data or buildings in andaround Boone were obtained through a GIS re-

mote database on the Appalachian State Univer-

sity Geography and Planning network.

The historical landslide data was added to

the map le and was displayed over the slope

map. The raster values o the slopes on the map

were joined to the landslide attribute table to

provide a method o classiying the landslides.

These were then classied using a quantile

method, placing approximately equal amountso landslides in each category. The soil map

(SSURGO) [8] or Watauga County was overlaid

with the Watauga DEM le [13]. The soil map

was then spatially joined to the landslides. The

soil polygons that contained landslides were se-

lected rom the new soil-landslide layer. These

selected soil types were placed into a new layer.

Each soil type was assigned a count o how many

landslides were present in its layer. This soil type

layer was converted rom vector to raster data inorder to allow or reclassication. Each soil type

then had its number o landslides converted into

a percentage, and was given a value rom low-

est percentage to highest. Any soil types that

had the same percentage were assigned the

same value. For example, the lowest percent-

age, 0.048%, was given a value o 1 so all the

soil types with this percentage value were as-

signed a 1. These values were assigned to each

soil type and the layer was reclassied to createa standard o comparison. There were 29 dier-

ent classes or the reclassied soil layer. The slope

map was then reclassied evenly into 29 classes

in order to match the number o classes or soil

type to ensure proper reclassication. The range

o slopes was rom between 0% to 325%, with

one o the classes being just 0% slope.

The reclassied soil layer was added to the

reclassied slope layer to produce a layer that

showed which o the soil types was most suscep-tible to re-activating the ormer landslides. This

same method was applied to the entire soil map

o Watauga County to produce a map o land-

slide risk using:

Landslide risk = [Reclassifed soil layer] +(2)

[Reclassifed slope layer]

The output layer adds the values o both othese layers to create a nal ranking. The higher

the ranking, the more likely a landslide will oc-

cur. The output layer was grouped into our cat-

egories o landslide risk: low, moderate, high and

very high. These categories were divided evenly

so that they contained 14 ranks each. This new

map layer was overlaid on top o the hillshade

map to give a good visual representation o the

landslide risk throughout Watauga County.

There was not enough data available to pro-duce a actor o saety the ratio o the resisting

orce o the slope to the disturbing orce placed

on it or this scale o slope analysis. Many vari-

ables can be taken into consideration, including

the angle the slope can ail, the unit weight o

the soil, and the cohesion o the soil. More so-

phisticated calculations using a wider range o

parameters would serve to more condently

determine slopes stability and landslide prob-

ability [14]. Factors such as soil cohesion and unitweight were unavailable, so more precise mea-

surements could not be made.

Data

Table 1 shows the slope range in both percent

Table 1. The number o landslides ound in

each range o slopes rom historical landslide

data o Watauga County, North Carolina, in both

percent and degrees.

Slope Range

(percent)

Degrees Number of

Landslides

0-18 0-10.2 7

18-30 10.2-16.7 2

30-50 16.7-26.6 27

50-75 26.6-36.9 82

75-95 36.9-43.5 88

95-120 43.5-50.2 319

120-155 50.2-57.2 755

155-180 57.2-60.9 425

180-205 60.9-64.0 238

205-230 64.0-66.5 103

230-255 66.5-68.6 28

255-300 68.6-71.6 18

>300 >71.6 2


4/10


and degrees, and the number o landslides that

have occurred in each group, based on available

historical landslide data or Watauga County,North Carolina.

According to the United State Department

o Agriculture (USDA), the Chestnut-Edneyville

complex (CkE) soil has an overall loamy texture

but is also described as stony. The CkE soil type

is classied as being the most susceptible soil

type or landslides in Watauga County. The next

most susceptible soil to water erosion, the Ashe-

Chestnut complex (AcF), is classied as generally

loamy but is also listed as very rocky. The thirdmost susceptible soil to water erosion, Chestnut-

Ashe complex (ChF, more Chestnut than Ashe

soil in composition), is also classied as loamy [8].

Table 2 shows the map unit symbols or each soil

type, the number o historical landslides associ-

ated with each soil type, and the percentage o

landslides that all into each category.

Figure 1 (p. 15) shows the dierence in per-cent slope throughout Watauga County. Figure 2

(pg. 18) shows the distribution o historical land-

slides categorized by slope. The areas prone to

landslide reactivation are illustrated in Figure 3

(pg. 19) and Figure 4 (pg. 20), which shows the

total landslide risk or Watauga County. Figure 5

(pg. 21) shows the landslide risk or Boone. Figure

6 (pg. 22) shows the number o historical land-

slides that all into dierent slope ranges. Figure

7 (pg. 22) is a graph o the number o landslidesthat occur in each soil type. Figure 8 (pg. 23)

shows the distribution o Chestnut-Edneyville

complex that has a slope between 120-155%.

Table 2. The association between soil type, landslide requency, and the percentage o landslides

corresponding to the Map Unit Symbols.

Map Unit

Symbol

Number of

Landslides

Percentage

of Total

Map Unit

Symbol

Number of

Landslides

Percentage

of Total

AcD 10 0.5 CtD 6 0.3

AcE 50 2.4 CtE 38 1.8

AcF 272 13.0 EvD 5 0.2

ArF 24 1.1 EvE 13 0.6

BaE 6 0.3 FaD 1


5/10

Figure 2. Total number o historical landslides in Watauga County, North Carolina. Each landslideis classied based on the percent slope on which it occurred. The slope classication is based on a

modied quantile method in which each class contains roughly the same number o values.


Analysis

The southeastern portion o Watauga County

has experienced a high concentration o slope

movements along moderate to very steep slopes

(Figure 1, pg. 19, and Figure 2). The western/

northwestern portion o the county, near the

North Carolina/Tennessee border, also has hada high concentration o historic landslides on a

range o moderate to very steep slopes. These

were the two areas with the most pronounced

grouping o landslides. There has been some

scattering o landslides in the southeastern por-

tion o the county, more sporadically placed than

the other two areas but requent nonetheless

(Figure 2). The Town o Boone has only had a ew

landslides that have occurred within its munici-

pal boundaries, but several additional landslideshave occurred just outside o the border, includ-

ing several noticeable occurrences on Howards

Knob.

The Chestnut-Edneyville complex soil type

has had the highest landslide requency, and was

thereore classied as the most susceptible soil

or slope movement in the study area, account-

ing or 23.6% o all landslides. The Ashe-Chest-

nut complex was the next most susceptible soil

type, covering 13% o the total landslides. The

third most susceptible soil type or landslide oc-

currence was the Chestnut-Ashe complex, with10.8% o landslides occurring on it (Figure 6).

O the 2087 landslide occurrences, 2067 were

composed o debris. O these debris type land-

slides, 292 were blowouts, 106 were composites

(a combination o a slide and a fow), 19 were

general slides, and 1650 were fows. The remain-

ing 20 landslide types were earth, with 2 blow-

outs, one slide, and 17 fows.

There is a large extent o slope steepness in

Watauga County, ranging rom 0.833% up to324.96% slope, according to the slope map gen-

erated rom the Watauga DEM le. The results

rom the reclassication o soils shows that the

area rom the northwest curving towards the

middle o the county (near the towns o Vilas


6/10

Figure 3. Landslide Re-occurrence Probability in Watauga County, North Carolina, based on soiltypes and slope. The areas are ranked by value based on probability o landslide occurrence.


and Sugar Grove) has the most risk or landslide

occurrence, in addition to the southeast portion

o the county (near the town o Deep Gap) (See

Figure 4, pg. 20). There were a ew isolated areas

in Boone that showed a airly high risk or land-

slides, with risk appearing in the southeastern

portion o the town, the center, and the easternportion, as well as some risk just north o Boone

including Howards Knob (Figure 5, pg. 21).

For landslide reoccurrence, the most likely

regions to experience such an event were the

northwest portion o the county and a small

part o the southeast. An area o lower risk was

established in the northeastern part o the map.

A small part o the southwestern portion o

Boone had a moderate risk o landslide reacti-

vation (Figure 3). Chestnut-Edneyville complexcontained the highest percentage o landslides.

The slope range that had the highest number o

landslides was between 120% and 155%. Figure

8 (pg. 23) shows the locations o these soil types

that all within this slope range, indicating the

most susceptible soil type to slope ailure.

Conclusions

Slope steepness and soil type play major roles

in landslide susceptibility. There are quite a ew

properties o soil that aect stability includ-

ing composition, texture, and other actors. TheChestnut-Edneyville complex is the soil type

most likely to produce a landslide, and the slope

range o 120-155% is the slope range with the

highest potential to cause a landslide.

Based on slopes, soil types, and historical

data, the area west and northwest o Boone (near

the towns o Vilas and Sugar Grove) tended to

have the most risk or landslides. The area to the

southeast o Boone (Deep Gap) also had a large

area o high risk. Boone itsel had a ew areasthat posed risk to slope instability, primarily the

southwestern and central portions o the town,

with notable risk just north o Boone around

Howards Knob. As a whole, the inrastructure in

Boone remained largely unthreatened by land-


7/10

Figure 4. Risk Assessment or Landslides in Watauga County, North Carolina, based on landsliderequency, soil type, and slope. These areas are ranked based on probability o landslide occurrence.

The urban areas are visible in blue, with the municipal boundary or Boone visible in the center.


slides. The areas that were aected included a

neighborhood in the southwest and perhaps

a ew streets in the central area. The buildings

and streets just south o Howards Knob showed

some risk o being damaged by landslides, but

the majority o the town appeared to be airly

stable.It should be noted that the methods used

and the results shown herein should be inter-

preted as strictly a high-level exercise to possi-

bly direct urther inquiry planning and policy

decisions should be guided by a more thorough

analysis o landslide hazard probabilities.

References

[1] Landslide Types and Processes. USGS. July

2004. Accessed 8 April 2011. http://pubs.usgs.gov/s/2004/3072/s-2004-3072.html

[2] Planning or Natural Hazards. Landslide

TRG. Oregon Department o Land Conser-

vation & Development, Community Plan-

ning Workshop. Eugene, OR. July 2000. Ac-

cessed 18 April 2011. http://www.oregon.

gov/LCD/HAZ/docs/landslides/05_land-

slide.pd

[3] Highland, L. M., and Bobrowsky, Peter. The

Landslide Handbook A Guide to Under-

standing Landslides. Reston, VA. U.S. Geo-

logical Survey Circular. 2008. Accessed19 March 2011. http://pubs.usgs.gov/

circ/1325/pd/C1325_508.pd

[4] Inrastructure and Sustainability. The Law-

rence Group. Oct. 2009. Accessed 19 March

2011. http://www.thelawrencegroup.com/

boone2030/inalDocs/InrastructureSus-

tainability.pd

[5] Watauga County, NC. Commerce Eco-

nomic Development Center. North Caro-

lina. February 2011. Accessed 24 April 2011.https://edis.commerce.state.nc.us/docs/

countyProle/NC/37189.pd

[6] Southeast Hurricane August 10-18 1940.

NOAA. National Climatic Data Center.

Asheville, NC. Accessed 4/15/11. http://


8/10

Figure 5. Potentially hazardous landslide areas in Boone, taking into account landslide requency,soil type, and slope. Previous landslides are also visible in the image. Existing inrastructure shows

which parts o the town may be damaged in the uture by landslides.


www.hpc.ncep.noaa.gov/tropical/rain/seh-

rcn1940.html

[7] Richard M. Wooten assisted by Rebecca

Latham and Jeery C. Reid. Landslides

shapele. North Carolina Geological Sur-

vey, Asheville, NC. Downloaded rom:

http://www.nconemap.com/Deault.aspx?tabid=286

[8] Soil Survey o Watauga County, North Caro-

lina. USDA, Natural Resources Conserva-

tion Service. 1999. Accessed 10 April 2011.

http://soildatamart.nrcs.usda.gov/manu-

scripts/NC189/0/Watauga_NC.pd

[9] Municipal Boundaries shapele. NCDOT.

14 Feb. 2011. Accessed 12 March 2011.

http://www.ncdot.org/it/gis/DataDistribu-

tion/DOTData/deault.html[10] Watauga DEM le. USGS. Appalachian State

University Geography and Planning De-

partment.

[11] Landslide. City o Roseville 2011 Multi-Haz-

ard Mitigation Plan. City o Roseville, Cali-

ornia. Accessed 14 April 2011. http://www.

roseville.ca.us/civica/ilebank/blobdload.

asp?BlobID=19073

[12] Landslides. NC Geological Survey. Ac-

cessed 5 March 2011. http://www.geology.

enr.state.nc.us/Landslide_Ino/Watauga-

County.html[13] Soil Survey Geographic (SSURGO) Database

or Watauga County, North Carolina. USDA,

Natural Resources Conservation Service.

For Worth, TX. 2009. Accessed 12 April 2011.

http://soildatamart.nrcs.usda.gov/

[14] Krishnamoorthy, Agrahara. Factor o Saety

o a Slope Subjected to Seismic Load. EJGE.

2007. Accessed 30 April 2011. http://www.

ejge.com/2007/Ppr0751/Ppr0751.pd


9/10

Landslide Frequency by Slope Range

0

100

200

300

400

500

600

700

800

0 -1 8 1 8-30 3 0-50 5 0-75 7 5-95 9 5-12 0 1 2 0-15 5 1 55 -1 80 1 80 -2 05 2 05 -2 30 2 30 -2 55 2 55 -3 00 >30 0

Percent Slope

Landslide Frequency per Soil Type

0

100

200

300

400

500

600

AcDAcEAcFArFBaE

BoDBoEBoFBrF

BuDBuF

BwECaCCaECdEChF

CkDCkE

CoDCoECoFCsCCsDCsECtDCtEEvDEvEFaDFaE

NkAPuDPuEPuF

SkDSnC

SnDSoDSoEUd

UkC

UkDUkE

UkF

UnF

WaC

WaD

Soil Type

Figure 6. The number o historical landslides (vertical axis) based on their slope range. It is evident

that the middle range o slope (between 95% and 205%) has been the most susceptible to

landslides.

Figure 7. The number o historical landslides (vertical axis) or each soil type.



10/10

Figure 8. The distribution o Chestnut-Edneyville complex, the most susceptible soil type, which allswithin the range o 120-155% slope.


Watauga County, NC Real Estate Landslide Hazards

Documents

Transcript of Watauga County, NC Real Estate Landslide Hazards