Debris-Flow Hazards in the

Blue Ridge of Virginia

The foothills of the Blue Ridge

province in central Virginia are dotted with working farms along meandering

rivers. The setting is peaceful to the casual

observer, but on closer inspection there are

signs of past violent weather and geologic

events. Massive boulders found at the base

of the foothills are vivid reminders of catastrophic large landslides and debris flows

that have sculpted the local landscape.

Evidence of past landslides, like these boulders, can be found in the foothills of the

Blue Ridge from Pennsylvania to Georgia.

In the summer of 1995, a major landslide event occurred in Madison County,

Virginia. During an intense storm on June

27th, 30 inches of rain fell in 16 hours. In

REDUCING LANDSLIDE HAZARDS IN THE UNITED STATES

UNITED STATES GEOLOGICAL SURVEY

Flow path

Source of large

debris flow

Inundated

farm

Numerous debris flows scarred the foothills of the Blue Ridge in Madison County, Virginia, after the devastating storm of June 27, 1995. These debris flows,

which began on steep slopes, moved rapidly down channels in the mountains. Small flows joined to form larger flows and spread mud, boulders, and other

debris in valleys, inundating residents' homes and farms. (Photograph copyright by Kevin Lamb, 1995; published with permission.)

the mountainous areas of the county, rainsaturated landslides known as debris flows

were triggered by the hundreds. The

Rapidan, Robinson, and Conway Rivers

flooded the lowlands, wreaking widespread

destruction (see map on page 3). Because

of the severity of the storm’s effects, rural

communities such as Graves Mill were isolated when bridges, roads, and power and

telephone lines failed. The full extent of

the damage was not recognized until aerial

surveys were made several days later. The

county was then declared a Federal disaster area by the President. The Federal Emergency Management Agency, U.S. Army

Corps of Engineers, National Park Service,

Virginia Department of Transportation, the

U.S. Geological Survey, and other agencies offered assistance in recovering from

the devastation and are studying ways to

reduce losses from future such events.

The most severe effects of the storm

occurred in the areas of heaviest rainfall.

In addition to flooding along the main rivers and streams, landslides on steep hillsides rapidly transformed into fast-moving

debris flows. Ultimately scientists identified hundreds of individual debris flows.

One flow near Graves Mill, Virginia, traveled nearly 2 miles, and an eyewitness estimated that it moved at a speed approaching 20 miles per hour. Damage to manmade

structures commonly occurred where debris flows emerged from mountain
2

channels, spreading large quantities of

rocky material. Houses and barns were inundated or crushed by the debris. Pastures

and cornfields were covered, and livestock

perished. Many trees, stripped of their bark,

branches, and leaves, were carried into

swollen streams and rivers and acted as

battering rams against buildings and

bridges. Such debris greatly increased the

destructive power of the flood waters. In

addition, debris from the mountain channels was carried into streams and rivers,

contributing to downstream flooding. One

woman was killed by a debris flow near

Criglersville, Virginia, and seven other fatalities were caused by flooding elsewhere

in the State. Total damage from the storms

was estimated at over $100 million.

Historical records tell us that destructive landslides and debris flows in the Appalachian Mountains occur when unusually heavy rain from hurricanes and intense

storms soaks the ground, reducing the ability of steep slopes to resist the downslope

pull of gravity. During Hurricane Camille

in 1969, such conditions generated debris

flows in Nelson County, Virginia, 90 miles

south of Madison County. The hurricane

caused 150 deaths, mostly attributed to

debris flows, and more than $100 million

in property damage. Likewise, 72 hours of

storms in Virginia and West Virginia during early November 1985 caused debris

flows and flooding in the Potomac and

Cheat River basins that were responsible

for 70 deaths and $1.3 billion in damage to

homes, businesses, roads, and farmlands.

Scientists have documented 51 historical debris-flow events between 1844 and

1985 in parts of the Appalachians—most

of them in the Blue Ridge area. Recent

studies of deposits exposed in stream channels during the 1995 storms in Madison

County found evidence of prehistoric debris flows. Radiocarbon dating of plant

remains from debris-flow deposits near

Graves Mill indicates that these processes

have occurred there repeatedly over the last

34,000 years.

Scientists from the U.S. Geological

Survey and from cooperating universities,

including Princeton University and the

University of Virginia, have developed an

inventory of landslides, debris flows, and

flooding from the storm of June 27, 1995,

by using aerial photography, field investigations, rainfall measurements from rain

gages, and National Weather Service radar observations. This inventory and a debris-flow hazard map that was prepared

from it will be used to help understand the

conditions that can lead to disasters like

the floods and debris flows caused by the

1995 summer storms in Virginia and to

suggest methods of mitigating the effects

of such events in the future.

What Are Debris Flows?

Some landslides move slowly and cause damage gradually, whereas

others move so rapidly that they can destroy property and take lives

suddenly and unexpectedly. Debris flows (also referred to as

mudslides, mudflows, or debris avalanches) are a common type of

fast-moving landslide that generally occurs during intense rainfall

on water-saturated soil. They usually start on steep hillsides as soil

slumps or slides that liquefy and accelerate to speeds as great as 35

miles per hour or more. They continue flowing down hills and into

channels and deposit sand, mud, boulders, and organic material onto

more gently sloping ground. Their consistency ranges from watery

mud to thick, rocky mud (like wet cement), which is dense enough to

carry boulders, trees, and cars. Debris flows from many different

sources can combine in channels, where their destructive power may

be greatly increased.

This debris flow near Graves Mill was triggered during the storm of June 27,

1995, in Madison County, Virginia. It emerged from a mountain channel (at

upper right) and deposited rocky debris on this farm. At 11:30 am the

powerful debris flow moved the two-story farmhouse (arrow) more than 30

feet from its foundation. The family in the house survived because they took

refuge in the second story. Several other structures on the farm were

destroyed. (Photograph copyright by Kevin Lamb, 1995; published with

permission.)
3

An intense storm on June 27, 1995, dropped 30 inches of rain on the foothills of the Blue Ridge in Madison County, Virginia. This storm triggered hundreds

of debris flows and caused widespread flooding. Some rural communities were isolated when bridges, roads, and powerlines failed. Devastation was so

great that the President declared the county a Federal disaster area. The track of the storm's center during its most damaging phase closely matches the

areas where most of the debris flows occurred. As can be seen on this map, small debris flows move downslope following drainage channels and often

merge to form larger flows.

SHENANDOAH

NATIONAL

PARK

Skyline

Drive

Graves


Mill

Wolftown

Storm Track & Time
 River or Stream Debris Flows Flooded Areas

Madison

KIRTLEY MTN

SAG TOP

DOUBLETOP MTN

OLD RAG MTN

BLAKEY RIDGE

BLUFF MTN

Robinson

River

Rapidan

Conway

River

Big Meadows

Criglersville

Syria

Banco

Etlan

662

649

231

231

643

670

600

29

FLATTOP RIDGE

0 1 2 MILES

River

Rapidan

230

1:11 PM

9:11 AM

NC

Ohio

Pa

NJ

Del

Md

Ky

Tenn

W. Va

MADISON 


COUNTY

VIRGINIA

Atlantic


Ocean

2:15 PM

9:11 AM

8:12 AM

+

11:14 AM

10:15 AM

+

+

+

+

2:45 PM

12:12 PM

+

+

+

+
Paula L. Gori and William C. Burton

U.S. Geological Survey Fact Sheet-159-96

1996

For more information, please contact:

National Landslide Information Center

U.S. Geological Survey, Mail Stop 966

Box 25046, Federal Center

Denver, CO 80225

1-800-654-4966

highland@gldvxa.cr.usgs.gov

U.S. DEPARTMENT OF THE INTERIOR • U.S. GEOLOGICAL SURVEY

4

What You Can Do If You Live Near Steep Hills

Prior to Intense Storms During Intense Storms

1. Become familiar with the land around you. Learn whether

debris flows have occurred in your area by contacting local

officials, State geological surveys or departments of natural

resources, and university departments of geology. Slopes

where debris flows have occurred in the past are likely to

experience them in the future.

2. Support your local government in efforts to develop and

enforce land-use and building ordinances that regulate construction in areas susceptible to landslides and debris flows.

Buildings should be located away from steep slopes, streams

and rivers, intermittent-stream channels, and the mouths of

mountain channels.

3. Watch the patterns of storm-water drainage on slopes near

your home, and note especially the places where runoff water converges, increasing flow over soil-covered slopes.

Watch the hillsides around your home for any signs of land

movement, such as small landslides or debris flows or progressively tilting trees.

4. Contact your local authorities to learn about the emergency

response and evacuation plans for your area, and develop

your own emergency plans for your family and business.

1. Stay alert and stay awake! Many debris-flow fatalities occur when people are sleeping. Listen to a radio for warnings

of intense rainfall. Be aware that intense short bursts of rain

may be particularly dangerous, especially after longer periods of heavy rainfall and damp weather.

2. If you are in areas susceptible to landslides and debris flows,

consider leaving if it is safe to do so. Remember that driving

during an intense storm can itself be hazardous. If you remain at home, move to a second story if possible.

3. Listen for any unusual sounds that might indicate moving

debris, such as trees cracking or boulders knocking together.

A trickle of flowing or falling mud or debris may precede

larger landslides. If you are near a stream or channel, be

alert for any sudden increase or decrease in water flow and

for a change from clear to muddy water. Such changes may

indicate landslide activity upstream, so be prepared to move

quickly. Don't delay! Save yourself, not your belongings.

4. Be especially alert when driving. Embankments along roadsides are particularly susceptible to landslides. Watch the

road for collapsed pavement, mud, fallen rocks, and other

indications of possible debris flows.

U.S. Geological Survey scientists measuring a fresh debris-flow scar near

Graves Mill in Madison County, Virginia, after the storm of June 27, 1995.

Sources of Technical Information

Landslide and debris-flow hazards caused by the June 27, 1995, storm in Madison County,

Virginia (includes discussion of mitigation options), by G.F. Wieczorek, P.L. Gori, R.H.

Campbell, and B.A. Morgan. U.S. Geological Survey Open-File Report 95-822, 1995,

33 p.

Preliminary inventory of debris-flow and flooding effects of the June 27, 1995, storm in

Madison County, Virginia, showing time sequence of positions of stormcell center, by

G.F. Wieczorek, B.A. Morgan, R.H. Campbell, R.C. Orndorff, W.C. Burton, C.S.

Southworth, and J.A. Smith. U.S. Geological Survey Open-File Report 96-13, 1996, 8 p.,

1 map (1:24,000 scale).

When the Earth moved in Madison County, by W.C. Burton. Washington Post, June 12,

1996.

Geomorphic studies of the storm and flood of November 3–5, 1985, in the Upper Potomac

and Cheat River basins in West Virginia and Virginia, by R.B. Jacobson, U.S. Geological

Survey Bulletin 1981, 1993, 207 p.

Debris flows/avalanches: Process, recognition, and mitigation, edited by J.E. Costa and

G.F. Wieczorek. Geological Society of America, Reviews in Engineering Geology, v.

VII, 1987, 236 p.

Landslide loss reduction: A guide for State and local government planning, by R.L. Wold,

Jr., and C.L. Jochim. Federal Emergency Management Agency Earthquake Hazards Reduction Series 52, 1989, 50 p.