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This section introduces you to landslides, one of the most common
geologic hazards in North Carolina and shows selected examples.
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Landslide contents
The landslide section consists of several pages to facilitate on-line
user viewing. This page provides information on how to recognize landslides
and how to deal with them and provides selected landslide examples.
The pages are:
Selected landslide examples
The following images show selected examples of the destructive effects
of landslides. One example of the direct and indirect cost of landslides
is the Pigeon River Gorge rockslide (see below). Road repair and stabilization
costs exceeded $10 million. Indirect costs, such as interruption of
business, commerce and tourism because of lengthy detours, probably
exceeded $5 million.
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(Left) -- Boulders on the Blue Ridge Parkway form
the April 24, 2003, rockslide near Potato Field Gap, northeast of
Asheville. The 165-ton boulder is being broken down to fit into
a dump truck. |
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(Right) -- Scar from the 1997 rockslide along I-40 in the Pigeon
River Gorge. The rockslide closed the highway for more than 20
days. In this image, clean-up efforts have removed most of the
debris pile, a retaining wall has been constructed and I-40 has
been reopened.
Landslides can be very small , like those on the side of a ditch,
or they can be as large as mountainside like this. This rockslide
was caused, in part, by the nature of the underlying rock. Most
rock formations have layers similar to a stack of books. If you
tip a stack of books they will slide off each other just as tilted
layers of rocks can slide off a mountainside. The layers of rock
are planes of weakness where the landslide separates from the
side of the mountain. Geologic maps show how these planes are
oriented and the maps can be used for route planning.
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| (Above) -- Apartment complex in Maggie Valley, North Carolina,
built on debris fan deposits (green outline). The apartment complex
is built on an old landslide and stream deposits at the base of
a mountain. The landslide deposits have accumulated because of recurring
landslides and where they have happened in the past, they can happen
in the future. Inset image on right of image shows general elements
of a debris flow -- described in more detail below. |
The trained eye of a geologist can discover hints that the land is
currently moving and uncover clues as to where it has moved in the past.
Evidence that the land is moving include cracks that appear when the
land is slowly pulling apart from the hillside. These cracks are called
tension cracks. With time, the ground on one side of the tension crack
may slide downhill forming a scarp (see illustrations below). If the
ground moves far enough, it will leave a mark called a scar. A fresh
scar will usually have a lighter color and no vegetation compared to
the surrounding slopes.
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(Left) -- Main parts of a general landslide. |
| (Right) -- This is where a debris flow, a type of landslide, was
triggered in Henderson County by heavy rainfall from the remnants
of Hurricane Francis in 2004. Tension cracks indicate areas on the
ground have pulled away from the hillside. Scarps indicate areas
that have already dropped down; with time, tension cracks in the
pavement will likely become scarps. |
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Another clue that the land is moving is trees growing at an angle or
with bent trunks as shown in the two images below. Trees growing on
the side of a hill normally grow straight up. If the land is slowly
moving downslope, the trees will lean, but keep trying to grow straight
towards the sun. This process results in their bent growth pattern and
indicates the trees are slowly sliding down the hill on top of the landslide.
This should be a warning that the land may move substantially with the
next large rainstorm.
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(Left) -- Tree showing curved growth. The curved growth pattern
(yellow line) indicates the tree is growing on top of a slowly moving
landslide. |
| (Right) -- A landslide can carry a lot of debris. This debris
flow top the bark off the tree 10 feet above the ground indicating
the flow as about 10 feet deep at that point. It also carried the
large boulder downhill, bringing it to rest next to the tree. A
landslide can leave a trail of destruction along its path and a
pile of debris at the end of its journey. |
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Hurricanes in the Mountains -- Hurricanes not only affect the coast,
they can move inland and drop tremendous amounts of rain. A single hurricane
can substantially affect a large area. If consecutive storms hit an
area within a short period of time, the results can be devastating.
such an occurrence happened when the remnants of Hurricanes Frances
and Ivan passed through the North Carolina mountains within a two-week
time span in September 2004.
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(Above) -- In 2004 the Peeks Creek Community in Macon County,
North Carolina experienced on of the largest landslides to hit
the North Carolina mountain region in decades. Tons of water,
mud, rocks, trees and other debris traveled for more than two
miles. The debris flow as 30 feet deep and 250 feet wide at some
points and traveled downhill as fast as 33 miles per hour. (North
Carolina Geological Survey staff geologist, low-left corner of
image for scale.)
The large amount of rain falling on steep, unmodified slopes
in such a short period of time triggered the more than two-mile-long
debris flow. Ivan dropped between 4 to 6 inches of rain, with
higher elevations receiving 10 or more inches of precipitation.
This rainfall was in addition to the remnants of Hurricane Frances
that dropped 6 to 10 inches of rain, with many areas receiving
greater than 10 inches. This landslide was most likely because
of steep slopes, shallow soil nd large amounts of rain. Mountain
counties were hard hit by these back-to-back events with descruction
to property and transportation routes throughout the region.
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(Left) -- The Peeks Creek debris flow destroyed or damaged 15
homes, and five fatalities, including an unborn child, were reported.
The debris flow pushed this two-story home 30 feet off its foundation.
The piles of debris to the right of the house are the remnants of
an adjacent house that was destroyed. |
| (Right) -- In a subdivision in Watauga County, one home was destroyed
and eight others condemned for occupancy by landslides related to
Hurricane Frances in 2004. |
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(Left) -- This landslide, caused by Hurricane Ivan in 2004, destroyed
this home in Starnes Cove, Buncombe County. Photo courtesy of the
Asheville Citizen-Times online edition. |
References
North Carolina Geological Survey, 2005, When the Earth Moves,
Randy Bechtel, editor: Information Circular 32: North Carolina Geological
Survey, Raleigh, North Carolina, 24 p.
Varnes, D. J., 1978, Slope movement types and processes: In: Landslide
Analysis and Control: In Schuster, R. L., Krizak, eds. Transportation
Research Board Special Report No. 176, National Academy of Sciences,
Washington, D. C., P. 11-33.
For additional information
The contact for additional information about geologic hazards in North
Carolina is Mr. Richard Wooten, P.G.; his e-mail is Rick.Wooten@ncmail.net.
He is located in the Swannanoa, North Carolina office (western North
Carolina) and can be reached by telephone at 828.296.4500. His mailing
address is: 2090 U. S. Highway 70, Swannanoa, North Carolina 28778.
An alternate North Carolina Survey staff geologist contact is Dr. Jeff
Reid, P.G., 512 North Salisbury Street, Raleigh, North Carolina, 27699-1612.
His telephone number is 919.733.2423 x403. His e-mail is Jeff.Reid@ncmail.net.
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