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 Mount Rainier, Washington; view toward NNE |
A two-year cooperative pilot project is now under way to develop, deploy, and begin operation of an automated system to detect the occurrence of a lahar in the Puyallup River valley. The U.S. Geological Survey Volcano Hazards Program and the Pierce County, Washington, Department of Emergency Management are full partners in the pilot project. The Puyallup River drains the west flank of Mount Rainier, and the densely populated Puyallup valley extends about 70 km diagonally across Pierce County from Mount Rainier to the Port of Tacoma on Puget Sound. Upon detection of a lahar in the valley, the system is intended to issue an automatic notice to County emergency-management officials that would trigger immediate, preplanned emergency-response actions. |
Careful study of the deposits in the large valleys that drain Mount Rainier shows that, over the past 10,000 years, Mount Rainier has been the source of numerous lahars (volcanic debris flows) that buried now densely populated areas as far as 100 km from the volcano. Lahars are flowing mixtures of water and sediment that contain such a high concentration of rock debris that they look and behave like flowing wet concrete. They are capable of destroying buildings, bridges, and other man-made structures by battering, dislodgement, and burial. See effects of lahars for more information and photographs.
Prehistoric lahars originated on the steep flanks of the volcano and were channeled into the big valleys that carry water and sediment westward to Puget Sound or the Columbia River. Evidence from their deposits combined with observations of modern debris flows suggest that they traveled at speeds as fast as 70-80 km/hr at depths of 30 m or more in the confined parts of the valleys but slowed and thinned in the more distant, wider parts. During the past few thousand years, lahars that spanned valley floors well into the now densely populated Puget lowland have recurred, on average, at least every 500 to 1,000 years. There is every reason to expect that future lahars from Mount Rainier will be similar in behavior and frequency of occurrence to past lahars.
Mount Rainier and Puyallup River valley |
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Aerial view of Puyallup River valley and the growing community of Orting; Carbon River on left and Puyallup River on right. The most recent large lahar to rush down this valley occurred about 500 years ago when part of Mount Rainier's west flank collapsed. The resulting lahar swept through the Puyallup valley, which contained an old-growth forest, and eventually reached Puget Sound. The lahar knocked down trees as large as 2-3 m in diameter and encased the logs as well as the lower parts of still-standing trees in muddy rock debris about 5 m thick. Some trees and stumps in the lahar deposit were unearthed during recent construction of new homes on the valley floor. |
Geologic evidence indicates that many of the large prehistoric lahars from Mount Rainier originated as surges of meltwater initiated by rapid melting of snow and ice during eruptions. The meltwater torrents transformed to lahars by incorporation of loose sediment from the volcano flanks. Such a lahar, initiated by a small eruption in 1985 at a Colombian volcano, Nevado del Ruiz, took more than 20,000 lives in Armero, a valley-floor community located about 75 km from the volcano's summit. The lahar took about 2.5 hours to reach Armero.
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People who perished in Armero and other towns around the volcano could easily have been spared if only they had known that the lahar was coming and that safety was within an easy walk only a few hundred meters away. Buildings in the middle of Armero were completely swept away by the lahar. See description and photographs of this tragic volcanic disaster. |
Mount Rainier is carefully monitored for signs of volcanic reawakening, and an eruption that could produce a catastrophic lahar initiated by vigorous release of meltwater is expected to follow days, weeks, or even months of readily detected symptoms of volcanic unrest. Thus, it is likely that there will be opportunity for citizens and communities to prepare for an impending eruption.
However, deposits of some of the large prehistoric lahars from Mount Rainier are rich in clay, implying that they contain abundant hydrothermally altered debris from within the volcano. Therefore, they are inferred to have originated as huge avalanches of water-saturated, clay-rich debris from massive gravity-driven failures of the volcano's flanks. Absence of geologic evidence substantiating coincidence of some of these large, clay-rich, prehistoric lahars with eruptions raises concern that some may have occurred with no attendant eruptive activity. They may have been triggered by intrusion of magma into the edifice, which would show symptoms like those observed before eruptions. On the other hand, they may have been triggered by earthquakes or hydrothermal-system explosions, or a volcano flank may simply have collapsed when it became sufficiently destabilized by progressing hydrothermal alteration. Such events could generate a massive lahar with no recognized precursory warning. A reliable lahar-warning system designed to detect such sudden events can provide notification to people downstream that a lahar is underway.
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A mass of sliding rocks, snow, and ice swept down the northeast side of Mount Rainier about 5,700 years ago, removing the entire summit of the volcano and creating a deep horseshoe-shaped crater. This volcanic landslide was accompanied by a small explosion, and it transformed into an enormous lahar that swept into Puget Sound more than 50 km downstream. Original images appear in a video program, Perilous Beauty -- the hidden dangers of Mount Rainier. |
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Inasmuch as lahars seek valley bottoms, people can quickly climb or drive to safety in many cases by simply evacuating the floor of a well-defined valley before the lahar arrives; they need go no farther than high ground adjacent to the valley. A critical issue is to know when evacuation is necessary. Travel time for a large lahar from Mount Rainier may be an hour or less to Orting, the city closest to Mount Rainier in the Puyallup valley, and possibly as little as 30 minutes may be available from detection of a large lahar to its arrival. Successful evacuation there will depend on detection of an approaching lahar, clear warning, public understanding of the hazard, and practiced response by citizens. Decreased lahar velocity as the valleys broaden downstream gives more time--about an additional hour--for response in the larger urban areas of Puyallup and Sumner, which are closer to Puget Sound.
It is critical that the lahar-detection system be completely automatic. Except during volcanic unrest when intense around-the-clock monitoring by a team of volcanologists is underway, the time from initiation of a lahar to its arrival in a populated valley-floor area is insufficient for analysis of the data by scientists before notices are issued. Thus the system must be designed to unfailingly detect a lahar with minimum opportunity for false alarms.
Lahars will be detected by networks of five acoustic flow monitor (AFM) stations that have been placed within tens to hundreds of meters from the active flood plain in the upper reaches of both the Puyallup and Carbon River valleys. The network in each valley is located about 25 km upstream from Orting, which is near the confluence of the two valleys. Each AFM station consists of a microprocessor-based data logger that measures the amplitude, frequency, and duration of ground vibrations detected by an exploration-class geophone. When measurements exceed programmed thresholds, the data are radioed to the base-station computer.
Two AFM stations in each valley are located above flood level but within the expected inundation zone of a significant lahar. Those stations, then, will serve as "deadman" devices whose destruction by a major lahar would be noted by the system. The other three stations in each valley are located above the anticipated lahar-inundation limit with the expectation that they will monitor ground vibrations and transmit data throughout passage of a lahar. Data from all stations are transmitted by radio to duplicate base-station computers located at the Law Enforcement Support Agency, City of Tacoma and Pierce County Emergency 9-1-1 Center in Tacoma and at the Washington State Emergency Operations Center at Camp Murray. Software, currently under development, analyzes the incoming data and triggers an automatic unequivocal notice when a significant lahar is detected.
Ground-vibration response of an acoustic-flow monitor |
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 Comparison of response to ground vibration between a typical seismometer and an acoustic-flow monitor |
An acoustic-flow monitor (AFM) is a geophone sensitive to ground vibration with higher frequencies than those generated by earthquakes and most volcanic activity. An AFM is most sensitive to ground vibrations between approximately 10 and 300 Hz, whereas a typical seismometer, which records earthquakes and volcanic eruptions, has a frequency response of 0.5-20 Hz. Ground vibration generated by lahars (debris flows) is predominantly in the frequency range of 30-80 Hz close to a stream channel and vibration generated by earthquakes, volcanic tremor, and explosive eruptions is predominantly < 6 Hz. This difference in ground-vibration frequency combined with the difference in frequency response between AFMs and seismometers enables scientists to distinguish between lahars and other natural ground-vibrating events. |
AFMs detect lahar at Redoubt Volcano, Alaska |
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 Comparison of response to ground vibration between a typical seismometer and three acoustic-flow monitors |
A lahar at Redoubt Volcano triggered by an eruption on April 6, 1990, was detected by 3 AFMs that had been installed in Drift River valley a few months earlier. The graph shows the amplitude of a seismic signal (top curve) that was caused by the eruption at 5:23 p.m. Note the delay in time before the AFMs (bottom 3 curves) recorded an increase in ground vibration caused not by the eruption, but by a lahar that moved progressively downstream. Note the change in scale for each sensor. The AFMs were insensitive to the low-frequency ground vibrations caused by the eruption, and thus unambiguously recorded the passage of the lahar at each station. |
 in September 1998 |
The USGS Volcano Hazards Program and the Pierce County Department of Emergency Management entered into their formal two-year cooperative agreement in early summer 1998. Working closely together through the first summer, they selected the station sites and installed the station housings (55-gallon drums), the station hardware (geophones, radios, antennas), and telemetry repeater stations sufficient to ensure a high level of redundancy. At that stage, Pierce County largely managed site preparation and installation of the station housings, while the USGS took responsibility for procuring and assembling the station hardware. Left: Orting Fire Chief (right), USGS scientist (center) and volunteer of the Pierce County, Washington, Explorer Search & Rescue Unit install an antenna for a telemetry repeater above the Puyallup River valley. Mount Rainier in background. As the AFM telemetry requires line-of-sight radio transmission, it is necessary to use repeaters to link the AFM stations to the base stations at Tacoma and Camp Murray. In order to assure redundancy and minimize the chance for a disabling telemetry failure, every AFM is capable of transmitting to at least two repeaters, and each repeater can transmit the AFM data to both of the base stations. |
During the remainder of the two-year project, the USGS role will be primarily to test and evaluate the stations for sensitivity and durability; set sensitivity parameters so as to filter out ground vibrations from normal floods, wind, or passing log trucks; develop and test the software that analyzes data from the stations and governs lahar detection and automatic notification; and train Pierce County personnel to take over full operation of the system. The Pierce County Department of Emergency Management will be responsible for preparing to assume full control and operation of the system and for developing effective emergency-response actions once a lahar is detected.
Upon mutual agreement at the end of the pilot project, it is expected that Pierce County will assume ownership and full control of the Puyallup valley lahar-detection network.
Members of the Pierce County, Washington, Explorer
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Pierce County, Washington,
Department of Emergency Management
Steve Bailey, Director
(253) 798-7470; sbailey@co.pierce.wa.us
U.S. Geological Survey, Volcano Hazards Program
webmaster
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URL of this document: http://volcanoes.usgs.gov/About/Highlights/RainierPilot/Pilot_highlight.html
Last modified: February 12, 1999