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Home , Debris, Debris flow, Flood

DDebrebrisis--FlowFlow HazardsHazards inin thethe UnitedUnited StatesStates

WhatWhat AreAre DDebrebrisis Flows?Flows?

ome move slowly ebris flows are fast-moving landslides that occur in a wide variety SS and cause damage gradual- DDof environments throughout the world. They are particularly dan- ly, whereas others move so gerous to and because they move quickly, destroy objects in rapidly that they can destroy their paths, and often strike without warning. U.S. Geological Survey property and take sudden- (USGS) scientists are assessing -flow in the ly and unexpectedly. Debris and elsewhere, and developing real-time techniques for monitoring flows, sometimes referred to as hazardous areas so that road closures, evacuations, or corrective actions mudslides, , , or can be taken. debris , are common types of fast-moving landslides. These flows generally occur during periods of intense - fall or rapid . They usually start on steep hillsides as shallow landslides that liq- uefy and accelerate to speeds that are typically about 10 mph, but can exceed 35 mph. The consistency of debris flows This occurred in the ranges from watery to Columbia gorge near the town of Dodson, Oregon, during a thick, rocky mud that can carry rainfall and snowmelt event in large items such as boulders, February 1996 (Photo Inset: S. Cannon, USGS). The combina- trees, and . Debris flows tion of rainfall and snowmelt from many different sources can resulted in severe flooding and landsliding throughout the Pacific combine in channels where Northwest. The debris flow shown their destructive power may be here was deposited in a number of separate events over a period of 2 days. The flows originated high on the palisade and traveled through the steep . The occupants of the house escaped with their lives greatly increased. They continue when they heard a loud rumbling and crashing of trees and saw the approaching material through their flowing down hills and through kitchen window. Boulders, mud, and debris from these and several nearby debris flows were deposited across the east-bound lanes of Interstate Highway 84, and blocked the highway for 5 days (Aerial Photo: channels, growing in volume D. Wieprecht, USGS). with the addition of , , mud, boulders, trees, and other materials. When the flows Dangerous, Fast-Moving Landslides reach canyon mouths or flatter Fast-moving flows of mud and , trees, and obstructing and road- ground, the debris spreads over called debris flows or mudslides, are ways with thick deposits of mud and a broad area, sometimes accu- among the most numerous and dangerous rocks. Debris flows are typically associat- mulating in thick deposits that types of landslides in the world. They ed with periods of heavy rainfall or rapid can wreak havoc in developed are particularly dangerous to life and snowmelt and tend to worsen the effects areas. property because of their high speeds and of flooding that often accompanies these the sheer destructive force of their flow. events. Finally, in areas burned by These flows are capable of destroying and brush , a lower threshold of pre- homes, washing out roads and bridges, cipitation may initiate debris flows. sweeping away vehicles, knocking down

U.S. Department of the Interior U.S. Geological Survey U.S. Geological Survey Fact Sheet 176-97 WhereWhere dodo debrdebrisis flowsflows ooccur?ccur?

Debris Flows in the Western United States Highly destructive debris flows occur in enced damaging debris-flow episodes of extreme northern , Idaho, many areas across the United States. throughout this century. El Niño, the Oregon, and Washington. The mountains Hilly areas subject to prolonged, intense -warming phenomenon that can of Colorado and the Sierra Nevada of rainfall are particularly susceptible. Areas produce heavier-than-usual rainfall in cer- California have also experienced debris throughout southern California are fre- tain areas of the United States, was asso- flows in areas receiving high rates of quently beset by debris-flow problems, ciated with countless debris flows in rainfall, rapid snowmelt, or a combina- and public agencies have expended vast Utah, when El Niño’s increased rainfall tion of these. As more people populate on massive debris-protection effects were felt during the early 1980’s. hilly areas of the west, the potential for systems for more than 65 years. The San Hilly areas of experience much damage from debris flows increases. Francisco Bay region also has experi- destruction from debris flows, as do areas

Damage from a March 1995 debris flow that issued from Peña Canyon, Malibu, California. View is looking east along California State Highway 1 (Pacific Highway) (Photo: California Department of Transportation).

During late May and early June of 1983, a series of debris flows emerged from Rudd Canyon into the community of Farmington, Utah. Although no one was injured, a number of houses were inundated and destroyed by debris flows. Due in part to USGS studies, debris basins were constructed here and elsewhere along the Wasatch Front north of Salt City to prevent subsequent damage. Utah was particularly hard hit by landslides and debris flows during the El Niño events that struck the United States in the early 1980's (Photo: S. Ellen, USGS).

Debris Flows in the Eastern United States Debris flows are not limited to areas of the Atlantic Ocean in 1969. During an flow near Graves Mill, Virginia, traveled the Western United States. Many debris- intense on June 27, 1995 in nearly 2 miles, and an eyewitness esti- flow have also occurred in hilly Madison County, Virginia, 30 inches of mated that it moved at a speed approach- and mountainous regions of the Central rain fell in 16 hours. Hundreds of debris ing 20 miles per hour. The combined and Eastern United States, particularly in flows occurred in the mountainous areas and debris-flow devastation prompt- the Appalachian Mountains. Thousands of the county amid widespread flooding. ed a Federal declaration for the of debris flows in several eastern States Many houses and barns were inundated or county. were caused by heavy rainfall from hur- crushed by debris; pastures and cornfields ricane Camille as it moved inland from were buried; and livestock perished. One Hazardous Areas

ebris C DD flows start on steep D slopes—slopes steep D enough to make walking difficult. Once started, howev- Canyon bottoms, chan- Roadcuts and other altered or er, debris flows can travel even nels, and areas near the outlets excavated areas of slopes are over gently sloping ground. A of or channels are par- C particularly susceptible to The most hazardous areas are ticularly hazardous. Multiple debris debris flows. Debris flows and other canyon bottoms, stream chan- flows that start high in canyons common- landslides onto roadways are common ly funnel into channels. There, they during rainstorms, and often occur during nels, areas near the outlets of merge, gain volume, and travel long dis- milder rainfall conditions than those canyons, and slopes excavated tances from their sources. needed for debris flows on natural slopes. for buildings and roads. Debris flows commonly begin Areas where is in swales (depressions) on steep channeled, such as along road- B slopes, making areas downslope D ways and below , are from swales particularly hazardous. common sites of debris flows and other landslides. and Debris Flows Wildfires can also lead to destructive debris-flow activity. In July 1994, a severe swept Storm King Mountain west of Glenwood Springs, Colorado, denuding the slopes of . Heavy on the mountain in September resulted in numer- ous debris flows, one of which blocked Interstate 70 and threatened to the Colorado River. A 3-mile length of the highway was inundated with tons of rock, mud, and burned trees. The closure of Interstate 70 imposed costly delays on this major transcontinental highway. Here, as in other areas, the USGS assisted in analyzing the debris-flow threat and installing moni- toring and warning systems to alert local safety officials when high-intensity rainfall occurred or debris flows passed through a susceptible canyon. Similar types of debris Debris flows like these near Glenwood Springs, Colorado, are a consequence of heavy rainfall on flows threaten transportation corridors and burned hillsides. In addition to personal injuries and damage to 30 vehicles engulfed by these flows, other development throughout the West in transportation along the Interstate 70 corridor was brought to a standstill for a day, and business and and near -ravaged hillsides. operations in the Glenwood Springs area were seriously impeded (Photo: Jim Scheidt, U.S. Bureau of Management). Debris Flows and Volcanoes

Among the most destructive types of debris flows are those that accompany volcanic eruptions. A spectacular example in the United States was a massive debris flow resulting from the 1980 eruptions of Mount St. Helens, Washington. Areas near the bases of many volcanoes in the Cascade Mountain Range of California, Oregon, and Washington are at risk from the same types of flows dur- ing future volcanic eruptions. In areas with vulnerable populations, such as the valleys near Mt. Rainier in Washington, scientists are producing maps that delineate debris-flow dangers. In many cases, the USGS works with other agencies installing hazard-detection and warning systems, and developing means for communicat- ing hazards and warnings about volcanic eruptions and debris flows. During the 1980 eruption of Mount St. Helens, a debris flow traveled about 14 miles down the of the North Fork Toutle River. It destroyed nine highway bridges, many miles of highways and roads, and about 200 homes on the flood of the Toutle River (Photo: D. Crandell, USGS).

o help prepare for debris What can you do if you live near steep hills? TTflows and lessen their impacts, scientists from the Prior to Intense : During Intense Storms: USGS Hazards Become familiar with the land around you. Learn Stay alert and stay awake! Many debris-flow fatalities Program conduct research on 1 whether debris flows have occurred in your area by 1 occur when people are sleeping. Listen to a radio for contacting local officials, State geological surveys or warnings of intense rainfall. Be aware that intense short debris-flow processes. An departments of natural resources, and university depart- bursts of rain may be particularly dangerous, especially ments of . Slopes where debris flows have occurred after longer periods of heavy rainfall and damp . example of this research is the in the past are likely to experience them in the future. assessment of rainfall thresh- If you are in areas susceptible to landslides and debris olds for debris-flow initiation. Support your local government in efforts to develop 2 flows, consider leaving if it is safe to do so. Remember 2 and enforce land-use and building ordinances that regu- that driving during an intense storm can itself be hazardous. These thresholds are used for late construction in areas susceptible to landslides and debris flows. Buildings should be located away from steep Listen for any unusual sounds that might indicate mov- local and regional debris-flow slopes, streams and , intermittent-stream channels, 3 ing debris, such as trees cracking or boulders knocking warning systems. Additional and the mouths of mountain channels. together. A trickle of flowing or falling mud or debris may precede larger flows. If you are near a stream or , Landslide Program activities Watch the patterns of storm-water drainage on slopes be alert for any sudden increase or decrease in water flow include evaluation of debris- 3 near your home, and note especially the places where and for a change from clear to muddy water. Such changes runoff water converges, increasing flow over -covered may indicate landslide activity upstream, so be prepared to flow susceptibility at national slopes. Watch the hillsides around your home for any signs move quickly. Don't delay! Save yourself, not your belong- and regional scales and emer- of land movement, such as small landslides or debris flows ings. or progressively tilting trees. gency response to debris-flow Be especially alert when driving. Embankments along and disasters, Contact your local authorities to learn about the emer- 4 roadsides are particularly susceptible to landslides. 4 gency-response and evacuation plans for your area and Watch the road for collapsed pavement, mud, fallen rocks, both in the United States and develop your own emergency plans for your family and and other indications of possible debris flows. in cooperation with foreign business. countries.

Lynn M. Highland, Stephenson D. Ellen, Sarah B. Christian, and William M. Brown III

COOPERATING AGENCIES For more information, contact:

The USGS cooperates with many Federal, State, local, and international agencies on debris-flow U.S. Geological Survey hazards assessments, warning systems, and disaster analyses. Principal cooperators for events National Landslide Information Center discussed in this Fact Sheet include: Federal Center, Box 25046, MS 966 Denver, CO 80225-0046 USA California Division of Mines and Geology Madison County, Virginia Tel: 1-800-654-4966 California Department of Transportation San Mateo County, California Colorado Department of Transportation U.S. Army Corps of Engineers email: [email protected] Colorado Geological Survey U.S. Bureau of http://geohazards.cr.usgs.gov Federal Agency U.S. Forest Service U.S. Garfield County, Colorado Graphic design by Pamela S. Detra