DDebrebrisis--FlowFlow HazardsHazards inin thethe UnitedUnited StatesStates WhatWhat AreAre DDebrebrisis Flows?Flows? ome landslides 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 life and property 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 lives sudden- (USGS) scientists are assessing debris-flow hazards in the United States 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, mudflows, lahars, or can be taken. debris avalanches, are common types of fast-moving landslides. These flows generally occur during periods of intense rain- fall or rapid snowmelt. 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 debris flow occurred in the ranges from watery mud to Columbia River 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 cars. 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 canyon. 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 water, sand, mud, boulders, trees, and other materials. When the flows Dangerous, Fast-Moving Landslides reach canyon mouths or flatter Fast-moving flows of mud and rock, trees, and obstructing streams 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 forest These flows are capable of destroying and brush fires, 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 California, Idaho, many areas across the United States. throughout this century. El Niño, the Oregon, and Washington. The mountains Hilly areas subject to prolonged, intense ocean-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 resources 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 Hawaii 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 Coast 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 Lake 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 storm on June 27, 1995 in nearly 2 miles, and an eyewitness esti- flow disasters 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 flood and debris-flow devastation prompt- the Appalachian Mountains. Thousands of the county amid widespread flooding. ed a Federal disaster 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, stream 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 canyons 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 surface runoff is in swales (depressions) on steep channeled, such as along road- B slopes, making areas downslope D ways and below culverts, are from swales particularly hazardous. common sites of debris flows and other landslides. Wildfires and Debris Flows Wildfires can also lead to destructive debris-flow activity. In July 1994, a severe wildfire swept Storm King Mountain west of Glenwood Springs, Colorado, denuding the slopes of vegetation. Heavy rains on the mountain in September resulted in numer- ous debris flows, one of which blocked Interstate 70 and threatened to dam 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 fire-ravaged hillsides. emergency operations in the Glenwood Springs area were seriously impeded (Photo: Jim Scheidt, U.S. Bureau of Land 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 hazard 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 valley of the North Fork Toutle River. It destroyed nine highway bridges, many miles of highways and roads, and about 200 homes on the flood plain of the Toutle River (Photo: D.