DESCRIPTION:
Hydrologic Hazards at Volcanoes

Mudflows or debris flows composed mostly of volcanic materials on the flanks of a volcano are called lahars. These flows of mud, rock, and water can rush down valleys and stream channels at speeds of 20 to 40 miles per hour and can travel more than 50 miles. Some lahars contain so much rock debris (60 to 90% by weight) that they look like fast-moving rivers of wet concrete. Close to their source, these flows are powerful enough to rip up and carry trees, houses, and huge boulders miles downstream. Farther downstream they entomb everything in their path in mud.

Historically, lahars have been one of the deadliest volcano hazards. They can occur both during an eruption and when a volcano is quiet. The water that creates lahars can come from melting snow and ice (especially water from a glacier melted by a pyroclastic flow or surge), intense rainfall, or the breakout of a summit crater lake. Large lahars are a potential hazard to many communities downstream from glacier-clad volcanoes, such as Mount Rainier.

Lahars (also called volcanic debris flows or mudflows) are mixtures of water-saturated rock debris that flow downslope under the force of gravity. For simplicity in the discussions and compilations in this report, we have followed the usage of Crandell and others (1984) and used the term lahar to include both true lahars (Crandell, 1971), and downstream lahar-runout flows (Scott, 1985). Lahar-runout flows are hyperconcentrated streamflows that form by downstream transformation of lahars through loss of sediment and dilution by streamflow (Pierson and Scott, 1985; Scott; 1985, 1986). Additional dilution downstream may result in transformation of hyperconcentrated flows into normal streamflows, or floods.

Rock debris in lahars ranges in size from clay to blocks several tens of meters in maximum dimension. When moving, lahars resemble masses of wet concrete and tend to be channeled into stream valleys. Lahars are formed when loose masses of unconsolidated, wet debris become mobilized. Rocks within a volcano may already be saturated, or water may be supplied by rainfall, by rapid melting of snow or ice, or by a debris-dammed lake or crater lake. Lahars may be formed directly when pyroclastic flows or pyroclastic surges are erupted onto snow and ice, as apparently occurred in November 1985 at Nevado del Ruiz, in Colombia, where about 23,000 people lost their lives (Herd and Comite\" de Estudios Vulcanologicos, 1986). Lahars may be either hot or cold, depending on the temperature of the rock debris they carry.

Lahars can travel great distances down valleys, and lahar fronts can move at high speeds--as much as 100 km/hr. Lahars produced during an eruption of Cotopaxi volcano in Ecuador, in 1877, traveled more than 320 km down one valley at an average speed of 27 km/hr (Macdonald, 1972). Lahars that descended the southeast flank of Mount St. Helens in 1980 had initial flow velocities that exceeded 100 km/hr; average lahar flow velocities were about 67 km/hr over the 22.5 km traveled before the lahars entered a reservoir (Pierson, 1985). High-speed lahars may climb valley walls on the outside of bends, and their momentum may also carry them over obstacles. Lahars confined in narrow valleys, or dammed by constrictions in valleys, can temporarily thicken and fill valleys to heights of 100 m or more (Crandell, 1971).

The major hazard to human life from lahars is from burial and impact by boulders and other debris. Buildings and other property in the path of a lahar can be buried, smashed, or carried away. Because of their relatively high density and viscosity, lahars can move and carry away vehicles and other large objects such as bridges.

An inverse relation exists between the volume and length of lahars and their frequency; that is, large lahars are far less frequent than small ones. For this reason, lahar hazard progressively decreases downvalley from a volcano, and at any point along the valley, hazard from lahars decreases with increasing height above the valley floor.

Lahars have occurred repeatedly during eruptions at snow-covered volcanoes in the northwestern U. S. during Holocene time. Large lahars originating in debris avalanches have occurred at Mounts Shasta, Hood, St. Helens, Rainier, and Baker, and some have been caused by the failure of debris- or moraine-dammed lakes. Small lahars are frequently generated at ice-covered volcanoes by climatic events such as heavy rainstorms and periods of rapid snowmelt due to hot weather (Miller, 1980).

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Floods related to volcanism can be produced by melting of snow and ice during eruptions of ice-clad volcanoes, by heavy rains that may accompany eruptions, and by transformation of lahars to stream flow. Floods carrying unusually large amounts of rock debris can leave thick deposits at and beyond the mouths of canyons and on valley floors leading away from volcanoes. Eruption-caused floods can occur suddenly and can be of large volume; if rivers are already high because of heavy rainfall or snow melt, such floods can be far larger than normal.

Danger from eruption-caused floods is similar to that from floods having other origins, but floods caused by eruptions may be more damaging because of an unusually high content of sediment. The hydrology of river systems may be altered for decades following the rapid accumulation of great quantities of sediment (e.g., U.S. Army Corps of Engineers, 1984). Subsequent reworking of this sediment may lead to further channel aggradation, and aggravate overbank flooding during high river stages. Floods can also be generated by waves in lakes that overtop or destroy natural or man-made dams; such waves can be produced by large masses of volcanic material moving into the lake suddenly as a debris avalanche, lahar, or pyroclastic flow.

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Some smaller lahars may be triggered by the sudden release of water from cavities within or beneath the ice. We call these events glacial outburst floods, or jökulhlaups (an Icelandic term pronouced Yo-kul-hloips). ... Jökulhlaups often become lahars when they incorporate the rock debris that lies within their path. ...

Because outburst floods are unpredictable, you should be alert when visiting valleys with glacier-fed streams, particularly on unusually hot or rainy days. If you are near a stream and hear a roaring sound coming from upvalley, or note a rapid rise in water level, move quickly up the stream embankment, away from the stream channel and to higher ground. Do not try to escape by moving downstream; debris flows move faster than you can run. ...

Glacial outburst floods at Mount Rainier result from sudden release of water stored within or at the base of glaciers. Outburst floods and the debris flows they often trigger pose a serious hazard in river valleys on the volcano. The peak discharge of an outburst flood may be greater than that of an extreme meteorological flood (such as the 100-year flood commonly considered in engineering practice) for any given stream valley. ...

Glacial outburst floods at Mount Rainier are unrelated to volcanic activity. The best-studied outbursts those from South Tahoma Glacier are correlated with periods of unusually high temperatures or unusually heavy rain in summer or early autumn. The exact timing of outbursts is unpredictable, however.

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Emplacement or flowage of hot pyroclastic rock debris on or into thick snowpacks on volcanoes can trigger hazardous rapid flows of sediment (including ice grains) and water. Such rapid flows can extend far beyond the flanks of a volcano, as has been observed at volcanoes in many parts of the world (Major and Newhall, 1989). Commonly these sediment-water flows achieve discharges and velocities that produce catastrophic consequences more than 100 kilometers downstream from source areas. The hazardous nature of such flows was most recently demonstrated in 1985 at Nevado del Ruiz volcano in Colombia, where snowmelt-generated sediment-water flows killed about 23,000 people (Lowe and others, 1986; Naranjo and others, 1986; Pierson and others, 1990).

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Given the current, relatively quiet, eruptive behavior of Mount St. Helens, debris flows and floods at present constitute the greatest hazards related to volcanic activity. The potential for mudflows and floods was increased by the existence of new ponds and lakes formed when the debris avalanche of May 1980 blocked parts of the preexisting drainage to serve as natural dams. As these natural dams are composed of loose, easily erodible volcanic debris, they are structurally weak and could fail, which would trigger mudflows and floods.

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