Lahar Flashcards
Lahars
mudflows or debris flows composed mostly of pyroclastic sediments eroded from the flanks of stratovolcanoes
on steep slopes lahar speeds can exceed 200 km/hr (120 mi/hr); decelerate in lowland areas (> 30 km/hr), and eventually begin to deposit their sediment load
lahars may contain so much rock debris (60 to 90% by weight) that they look like fast-moving rivers of wet concrete
lahars may travel for more than 80 km down river valleys and stream channels
Primary Lahars “Hot Flows”
volcanic eruptions may trigger lahars by rapidly melting snow and ice
pyroclastic flows can generate lahars when extremely hot, flowing rock debris erodes, mixes with, and melts snow and ice as it travels rapidly down steep slopes
“cold flows” secondary lahars
explosive volcanic eruptions may remove all vegetation around a volcano creating landscapes that are vulnerable to mass movements
intense (i.e., high-volume) or long-duration rainfall events occur during or after an eruption
on steep slopes, rainwater can easily erode and transport pyroclastic sediments generating lahars
Volcanic Hazards
close to their source, lahars are powerful enough to rip up and carry trees, houses, and huge boulders many km downstream
farther downstream they entomb everything in their path in mud
Example #1 Nevado del Ruiz volcano, Colombia
March 12, 1595 lahar event killed more than 600 people in the region
February 19, 1845 lahar event had killed the entire population of 1400 people in the region
volcanic activity had been monitored beginning in November, 1984; November 13, 1985 eruption generated a massive lahar
community of Armero, 50 km down-valley, was buried by lahar deposits 3-8 m thick; more than 23,000 people from a population of 30,000 were killed within minutes
infrequent volcanic activity contributes to poor hazard awareness and community preparedness
Example #2 Mt. Meager, British Columbia
at least 54 eruptions have occurred at Mount Meager in the last 2,600,000 years
eruptions associated with rock avalanches, ash fall, debris flows and lahars
debris flows and lahars, mainly from Mount Meager, have filled Meager Creek valley to a depth of 250 m
remains a major volcanic hazard capable of producing highly explosive eruptions
Pemberton, B.C., a community 65 km downstream from Mt. Meager, faces high risk
ash fall, debris flows and lahars destroy forest and fishery resources
mass movements will block Lillooet River and Meager Creek resulting in potentially catastrophic flooding
potential road closures require evacuation of population
sediment deposition within Lillooet River raising bed of the river; increased risk of flooding during periods of high rainfall
Minimizing risks for pemberton, b.c.
Monitor the volcano
Remotely-sensed data to detect small ground movements that initiate mass movements
Minimizing risks for pemberton, b.c.
Install a warning system
In situ sensors to detect mass movements; communities lack capacity to install and maintain an early warning system
Minimizing risks for pemberton b.c.
Dredge Lillooet River Channel
Prevent overtopping of dykes that provide flood protection
Minimizing risks for pemberton, b.c.
Additional flood protection
raise level of dykes; raise levels of houses vulnerable to flooding
Example #3 Mount Rainier, Washington
Osceola lahar - ~5600 years ago; moved 2-3 km3 of material up to 80 km away from summit; deposits cover 550 km2
National lahar – 2,300-2,200 years ago; moved materials up to 1oo km away from summit
Electron lahar – 500 years ago; moved materials 100 km away from summit
Osceola and National events initiated by explosive eruptions; Electron event initiated in saturated volcanic rocks
Lahar-Human Interactions
100,000’s of people now live on these lahar deposits
there are no guarantees that similar lahar events will not occur again
most of the people living in the lahar hazard zone would have between 40 minutes and 3 hours to respond to a large lahar warning
Monitoring Lahars
an automated network of small sensors called acoustic flow monitors embedded underground to measure ground vibrations made by passing lahars
upon detection of a lahar, computer base stations alert local 24–hour emergency monitoring and notification centers, who initiate the warning component of the system
warning messages trigger immediate, preplanned emergency-response actions
