Debris-Flow Hazards within the Appalachian Mountains of
 

the Eastern United States
 

U.S. Department of the Interior 

U.S. Geological Survey 

Fact Sheet 2008–3070

August 2008 

Tropical storms, including hurricanes, often inflict major 

damage to property and disrupt the lives of people living in 

coastal areas of the Eastern United States. These storms also 

are capable of generating catastrophic landslides within the 

steep slopes of the Appalachian Mountains. Heavy rainfall from 

hurricanes, cloudbursts, and thunderstorms can generate rapidly 

moving debris flows that are among the most dangerous and 

damaging type of landslides. This fact sheet explores the nature 

and occurrence of debris flows in the central and southern 

Appalachian Mountains, which extend from central Pennsylvania to northern Alabama. 

Geology and Climate Contribute to Debris Flows 

Debris flows are considered one of the most dangerous 

forms of landslides, which is a general term covering a wide 

variety of mass-movement landforms and processes involving 

the downslope transport of soil and rock material en masse. A 

debris flow is a moving mass of fragmented rock and soil in 

which more than half of the particles are greater than sand size. 

Because a debris flow moves rapidly and with great force, it 

is capable of destroying almost everything in its path. Debris 

flows are most frequently generated on steep slopes during 

heavy rainfall, and often travel along preexisting drainages and 

streams. The soils and poorly consolidated material on these 

slopes may become saturated during rainfall, lose coherence, 

and begin moving downslope in a flowing stream, drawing in 

and transporting additional soil and rock as the flow rapidly 

moves through the drainage network. 

The central and southern Appalachian Mountains are comprised of three physiographic provinces that have had many historic debris flows; the provinces are, from east to west, the Blue 

Ridge, the Valley and Ridge, and the Appalachian Plateau. The 

Blue Ridge province is underlain by igneous and sedimentary 

rocks that have undergone structural transformation. Some of 

the highest peaks in the Appalachian Mountains are in this province. The Valley and Ridge province is underlain by folded sedimentary rocks, typically with inclined sandstone strata beneath 

ridges. The Appalachian Plateau province is underlain by nearly 

flat-lying sedimentary rocks. Debris flows have occurred in all 

of these provinces. When considered on a regional scale, rainfall 

and the steepness of a slope are important parameters governing 

debris-flow distribution, in addition to rock type and structure. 

Extreme rainfall events are the most likely trigger. 

Fifty-one debris flows that occurred in the Appalachian 

region between 1844 and 1985 were documented by Michael 

Clark (Clark, 1987). Figure 1 shows the distribution of these 

flows (also referred to as historical landslide events). Many of 

these events occurred in isolated and rural areas; therefore, the 

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k Event of interest 

Landslide Incidence and Susceptibilty 

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Moderate (1.5 - 15% of area) 

High (more than 15% of area) 

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High Incidence and High Susceptibility 

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Base from U.S. Geological Survey, 2008 0 50 100 200 Miles The map projection is Albers Equal Area Conic
 

with a central meridian of 96o W and standard
 

parallels of 29.5o N and 45.5o N.The latitude
 

of origin is 23o N.The datum is the North 0 50 100 200 Kilometers
 American Datum 1983.
 

Figure 1. Distribution of debris flows within the Appalachian 

Mountains of the Eastern United States (Sources: Clark, 1987; 

Radbruch-Hall and others, 1982). 

data on the number of debris flows, the area affected, or the 

associated rainfall are lacking. Published reports about these 

events have been reviewed by the U.S. Geological Survey, plus 

four additional documented events that have occurred since 

1987. Nineteen examples of debris flows are listed in table 1, 

including the date, location, number of reported flows, type of 

storm, total rainfall, and rainfall duration. These 19 events are 

depicted in figure 1 with small asterisks. The debris flows from 

Clark (1987) and from table 1 are plotted in figure 1. Most of 

the events fall into areas of high incidence and high susceptibility (red), but several events are in areas of much less susceptibility. The map is useful in depicting regional trends, but should 

not be used to predict future occurrences. 

Some Well-Documented Debris Flows 

The most devastating debris flow listed in table 1 occurred 

during the night of August 19–20, 1969, in Nelson County, Va. 

Printed on recycled paper
Table 1. Examples of historical rainfall-induced debris flows within Appalachian Mountains of the Eastern United States. 

[~, approximately; >, more than] 

Location 

number on 

figure 1 

Rainfall 

total, in 

millimeters 

Duration of 

most severe 

rainfall 

Type of rain event 

Number of 

reported debris 

flows 

Dates of debris 

flows 

(month/day/year) 

General location 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

12 

13 

14 

15 

16 

17 

18 

19 

564 

380 

305 

508 

254 

782 

400 

116 

254 

635 

710–800 

300 

320 

115 

350 

770 

635 

513 

636 

24 hours 

2 days 

4 hours 

2 days 

6 hours 

4.5 hours 

1 day 

1 hour 

2 days 

8 hours 

8 hours 

9 hours 

5 days 

14 days 

3 days 

14 hrs 

1 day 

1 day 

11 days 

Hurricane 

Thunderstorm 

Cloudburst 

Thunderstorm 

Hurricane 

Thunderstorm 

Cloudburst 

Cloudburst 

Hurricane Diane 

Hurricane Camille 

Hurricane Camille 

Thunderstorm or cloudburst 

Storm 

Thunderstorm 

Hurricane Juan 

Storm 

Storm 

Hurricane Isabel 

Hurricanes Frances 

and Ivan 

45 

~100 

> 100 

4 on photo, 

but total not 

reported 

763 

Numerous 

466 

100 

4 

1584 

3793 

Several hundred 

24 

62 

3,000 

629 

61 

6 

155 

7/15–16/1916 

6/13/1924 

8/4–5/1938 

7/4–5/1939 

8/10–17/1940 

7/18/1942 

6/17–18/1949 

9/1/1951 

8/18–19/1955 

8/19–20/1969 and 

9/5–6/1969 

8/19–20/1969 

7/19–20/1977 

11/6–7/1977 

8/14/1980 

9/3–5/1985 

6/27/1995 

6/27/1995 

9/18–19/2003 

9/7–17/2004 

Transylvania County, N.C. 

Carter County, Tenn. 

Webb Mountain, Tenn. 

Wolfe and Breathitt County, eastern 

Kentucky 

Watauga County, N.C. 

Elk, McKean, Potter, and Cameron 

Counties, north-central Pennsylvania 

Augusta and Rockingham Counties, 

Va., and Highland County, W. Va. 

Mount Le Conte, Great Smoky 

Mountains National Park, Tenn. 

Brodhead Creek, northeastern Pennsylvania 

Spring Creek, Greenbrier County, 

W. Va. 

Nelson County, Va. 

Johnstown area, western Pennsylvania 

Asheville-Statesville, Transylvania 

County, N.C. 

East Brady, along Allegheny River, 

northern Pennsylvania 

Pendleton County, W. Va., and Highland County, Va. 

Madison County, Va. 

Albemarle County, Va. 

Shenandoah Valley, Va. 

Macon County, N.C. 

Hurricane Camille, having caused great damage along the Gulf 

Coast in Mississippi, moved northeast into Virginia as a tropical depression and stalled in central Virginia. The result was a 

heavy rainfall of at least 710 millimeters (mm) (28 inches (in)) 

within an 8-hour period during the night. Approximately 3,800 

debris flows and associated flooding resulted, destroying houses 

and killing 150 people within Nelson County. 

During the more recent Hurricane Ivan, on September 16, 

2004, in Macon County, N.C., soil and rock material detached 

from the upper slopes of Fish Hawk Mountain and moved as a 

debris flow 3.6 kilometers (2.2 miles) downslope along Peeks 

Creek to the Cullasaja River, causing 5 deaths and 2 injuries, 

and destroying 16 buildings (figs. 2A and B). 

On June 27, 1995, a small but violent summer storm 

dropped 250 to 770 mm (30 in) of rain over a 16-hour period on 

a small area in rural Madison County, Va., resulting in at least 

629 highly destructive debris flows. The storm, which occurred 

during the day in a moderately populated area, was tracked by 

the National Weather Service using Doppler radar. The landslide 

and debris-flow area resulting from this storm was extensively 

photographed and examined in the field. The data that were 

obtained can serve as a model to help predict the potential for 

similar events within the Appalachian Mountains. 

Figure 3 shows an area affected by the June 1995 storm 

near Graves Mill, Madison County, Va. Debris flows are shown 

in dark shading and areas of flooding are indicated by lighter 

shading. Rainfall in this area exceeded 700 mm (27 in). During 

the 1995 event, the majority of debris flows occurred within a 

larger region where rainfall exceeded 500 mm (20 in). 

Figure 4 shows the cumulative rainfall for the Madison 

County event at several locations affected by the 1995 storm. 

A few of the debris flows were witnessed, and their times 

recorded; these observed debris flows are shown as letters 

A–M. What is striking about these data is that the debris flows 

were generated during the 3-hour period of rapid increase in 

precipitation, suggesting that the rate of rainfall is most critical, and even more critical than total storm rainfall. The high 

rate of rainfall is required to sufficiently wet the soil so that the 

internal pressure of contained water breaks down the coherence 

of the soil particles, allowing the entire saturated mass to flow 

downslope. 

Forecasting Debris Flows

Prediction of debris flows based on rainfall thresholds 

remains highly problematic. For example, the relation that prolonged precipitation has on landslides by wetting the soil before 
A B

Figure 2. A. Hurricane Ivan triggered this large and lengthy Peeks Creek landslide and debris flow in Macon County, N.C., on September 16, 

2004. B. Buildings destroyed by the landslide and debris flow at Peeks Creek, N.C., during Hurricane Ivan, September 16, 2004. Photographs by 

Richard Wooten, North Carolina Geological Survey. 

a major storm is entirely unknown. Structural controls such as 

extensive jointing, and the wetting requirements of soils overlying various rock types may also play an important role. Figure 5 

shows the rainfall intensity (in inches per hour) and the duration 

for several storms in the Blue Ridge of central Virginia where 

the resulting debris flows are known. The threshold line shown 

represents the minimum rainfall necessary for triggering debris 

EXPLANATION 

Debris Flow/Flood Effects 

Flood erosion and deposition 

Debris flow erosion and deposition 

1 .5 0 1 KILOMETER 

1 1/2 0 1 MILE 

Figure 3. Map showing debris flows extending from an area of 

flooding within Madison County, Va., June 1995. 

flows. This threshold may be valid for the metamorphic igneous 

and sedimentary rocks of the Blue Ridge province, but may be 

entirely unreliable for the different rocks and soils of the Valley 

and Ridge and Appalachian Plateau provinces. 

Mathematical formulas such as that applied for figure 5 

may also be dependent on climatic factors of temperature and 

annual rainfall. For example, rainfall thresholds for triggering 

debris flows in areas such as California, Hawaii, and Puerto 

Rico are generally lower, suggesting the need to develop threshold models for each specific region in the United States that is 

prone to debris-flow activity. 

Occurrence intervals of debris flows within a broad region 

can be estimated based on historical records as well as on 

I

G

E,F

D

C A,B

H

L

M

J,K

Rainfall Intensity / Times of Debris Flows 

Cumulative Rainfall, in inches 

June 27, 1995 

Cumulative Rainfall, in millimeters 

Figure 4. Rainfall and a few observed times of debris flows are 

shown as letters A–M during the June 27, 1995, storm in Madison 

County, Va. (Wieczorek and others, 2000). 
Intensity and duration threshold for triggering debris flows in the Blue Ridge, central Virginia 

Duration, in hours 

Rainfall, in inches per hour

Rainfall intensity, in millimeters per hour 

Location/Date 

Debris 

flows 

No debris 

flows 

Madison County, June 27, 1995 

Nelson County, Hurricane Camille, August 1969 

Moorman’s River, June 27, 1995 

Madison County, Hurricane Frances, September 1996 

EXPLANATION 

Figure 5. Rainfall intensity and duration for storms in the Blue 

Ridge of central Virginia. The line depicts a predicted threshold that 

represents minimum values necessary for triggering debris flows. 

examination of prehistoric debris flows using carbon-14 dating. 

Within western and central Virginia, the recurrence interval for 

debris flows across the region has been estimated to be approximately 15 years. This estimate depends on the size of the area 

to which it is applied; the recurrence at a single locality or 

drainage basin is more likely in the order of thousands of years. 

Historical data suggest that the recurrence interval for debris 

flows is much higher in the southern Appalachian region of 

North Carolina and may be as frequent as 3 to 5 years. 

Warning the public of potential or impending debris-flow 

hazards will require a concerted effort by local, State, and Federal Governments to inform the general public about these hazards. A network of rain gages that records rainfall in real time 

is available in parts of the southern and central Appalachian 

Mountains and could become more widely developed in the 

region. To be effective as a hazard-warning system, the raingage 

network data should be integrated with radar precipitation data, 

hillslope monitoring information, and radio, television, and 

Internet communications for State Emergency Response Centers 

and public and emergency managers in areas of concern. Longer 

range mitigation of debris-flow hazards can be improved by 

careful placement and strengthening of roads and bridges, and 

by the selection of sites for new construction that are away from 

areas most susceptible to landslide failure and subsequent inundation by debris flows. 

References Cited 

Campbell, R.H., 1975, Soil slips, debris flows, and rainstorms in 

the Santa Monica Mountains and vicinity, southern California: U.S. Geological Survey Professional Paper 851, 51 p. 

Clark, M.G., 1987, Debris slide and debris flow historical events 

in the Appalachians south of the glacial border, Reviews in 

Engineering Geology, v. 7, p. 125–138. 

Eaton, L.S., Morgan, B.A., Kochel, R.C., and Howard, A.D., 

2003, Quaternary deposits and landscape evolution of the 

central Blue Ridge of Virginia: Geomorphology, v. 56, 

p. 139–154. 

Radbruch-Hall, D.H., Colton, R.B., Davies, W.E., Lucchitta, I., 

Skipp, B.A., and Varnes, D.J., 1982, Landslide overview map 

of the conterminous United States: U.S. Geological Survey 

Professional Paper 1183, plate 1. 

Wieczorek, G.F., Morgan, B.A., and Campbell, R.H., 2000, 

Debris-flow hazards in the Blue Ridge of central Virginia: 

Environmental and Engineering Geoscience, v. 6, n. 1, p. 

3–23. Also available online at http://eeg.geoscienceworld.org/ 

cgi/content/abstract/6/1/3. 

Williams, G.P., and Guy, H.P., 1973, Erosional and depositional 

aspects of Hurricane Camille in Virginia, 1969: U.S. Geological Survey Professional Paper 804, 80 p. 

Wooten, R.M., Gillon, K.A., Witt, A.C., Latham, R.S., Douglas, T.J., Bauer, J.B., Fuemmeler, S.J., and Lee, L.G., 2008, 

Geologic, geomorphic, and meteorological aspects of debris 

flows triggered by Hurricanes Frances and Ivan during 

September 2004 in the Southern Appalachians of Macon 

County, North Carolina: Landslides, v. 5, no. 1, p. 31–44. 

Also available online at http://www.springerlink.com/ 

index/2705hw84480m5420.pdf. 

Where To Go for More Information 

1.	 The U.S. Geological Survey Landslide Program has 

information, publications, and educational information on 

its Web site at http://landslides.usgs.gov or call (toll-free) 

1–800–654–4966 

2.	 For general informtion about slides, debris flows, rock falls, 

or other types of landslides in your area, contact your city 

or county geology or planning office. In addition, all 50 

States have State Geological Surveys that can be accessed 

through a link at the USGS Web site, at http://landslides. 

usgs.gov. 

3.	 For an assessment of the landslide risk to an individual 

property or homesite, obtain the services of a State-licensed 

geotechnical engineer or engineering geologist. These 

professionals can be found through the membership listings 

of two professional societies, the American Society of Civil 

Engineers (ASCE), http://www.asce.org and the Association of Engineering Geologists, http://www.aegweb.org. 

Often, personnel in State or country planning or engineering departments can refer competent geotechnical engineers 

or engineering geologists. 

4.	 For more information about the design and construction 

of debris-flow mitigation measures which may include 

debris basins, debris fences, deflection walls, or other 

protective works, consult your city or country engineer, 

local flood-control agency, or the U.S. Department of 

Agriculture, Natural Resources Conservation Service at 

http://www.ncgc.nrcs.usda.gov. 

By Gerald F. Wieczorek and Benjamin A. Morgan