Lahars and Their Effects

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Lahars and Their Effects--Pathways of Destruction

A lahar carries away a bridge spanning the ToutleRiver about 55 km downstream from Mount St.Helens volcano on May !" #!$. %efore arriving atthe bridge" the lahar swept through a logging campand picked up thousands of neatly cut and stackedlogs from along the river. This lahar originated fromthe huge landslide that started the eruption at !&'( inthe morning.

What is a Lahar?

)ahar is an *ndonesian term that describes a hot or cold mi+ture of water and rockfragments flowing down the slopes of a volcano and ,or- river valleys. hen moving" alahar looks like a mass of wet concrete that carries rock debris ranging in si/e from clayto boulders more than $ m in diameter. )ahars vary in si/e and speed. Small lahars lessthan a few meters wide and several centimeters deep may flow a few meters per second.)arge lahars hundreds of meters wide and tens of meters deep can flow several tens ofmeters per second00much too fast for people to outrun.

As a lahar rushes downstream from a volcano" its si/e" speed" and the amount of waterand rock debris it carries constantly change. The beginning surge of water and rock debrisoften erodes rocks and vegetation from the side of a volcano and along the river valley itenters. This initial flow can also incorporate water from melting snow and ice ,if present-and the river it overruns. %y eroding rock debris and incorporating additional water"lahars can easily grow to more than $ times their initial si/e. %ut as a lahar movesfarther away from a volcano" it will eventually begin to lose its heavy load of sedimentand decrease in si/e.

Numerous terms are used by scientists to describe the properties of lahars ,for e+ample"mudflows" debris flows" hyperconcentrated flows" and cohesive and non0cohesive flows-.

What triggers a lahar?
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1ruptions may trigger one or more lahars directly by 2uickly melting snow and ice on avolcano or e3ecting water from a crater lake. More often" lahars are formed by intenserainfall during or after an eruption00rainwater can easily erode loose volcanic rock andsoil on hillsides and in river valleys. Some of the largest lahars begin as landslides ofsaturated and hydrothermally altered rock on the flank of a volcano or ad3acent hillslopes.

)andslides are triggered by eruptions" earth2uakes" precipitation" or the unceasing pull ofgravity on the volcano.

Mount Rainier,Washington

)ahars almost always occur on or near stratovolcanoes becausethese volcanoes tend to erupt e+plosively and their tall" steepcones are either snow covered" topped with a crater lake"constructed of weakly consolidated rock debris that is easilyeroded" or internally weakened by hot hyrothermal fluids. )aharsare also common from the snow0 and ice0covered shieldvolcanoes in *celand where eruptions of fluid basalt lava

fre2uently occur beneath huge glaciers ,for e+ample" at4atna3okull" *celand -.

The scenarios listed below illustrate most of the mechanisms by which lahars aregenerated. or convenience" we6ve grouped the mechanisms according to whether avolcano is erupting" has erupted" or is 2uiet. 1ach mechanism is illustrated with one ormore case studies.

Lahars During Eru tions

Melting of snow and ice by pyroclastic flows and lava flows Lahars !fter Eru tions

Heavy rainfall can lead to erosion and laharsSudden release of water caused by lake breakouts

Lahars Without Eru tions

Sudden landslides at volcanoes can trigger lahars

Effects of lahars

)ahars racing down river valleys and spreading across flood plains tens of kilometersdownstream from a volcano often cause serious economic and environmental damage.The direct impact of a lahar6s turbulent flow front or from the boulders and logs carried

by the lahar can easily crush" abrade" or shear off at ground level 3ust about anything inthe path of a lahar. 1ven if not crushed or carried away by the force of a lahar" buildingsand valuable land may become partially or completely buried by one or more cement0likelayers of rock debris. %y destroying bridges and key roads" lahars can also trap people in
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areas vulnerable to other ha/ardous volcanic activity" especially if the lahars leavedeposits that are too deep" too soft" or too hot to cross.

After a volcanic eruption" the erosion of new loose volcanic deposits in the headwaters of rivers can lead to severe flooding and e+tremely high rates of sedimentation in areas far

downstream from a volcano. 7ver a period of weeks to years" post0eruption lahars andhigh0sediment discharges triggered by intense rainfall fre2uently deposit rock debris thatcan bury entire towns and valuable agricultural land. Such lahar deposits may also blocktributary stream valleys. As the area behind the blockage fills with water" areas upstream

become inundated. *f the lake is large enough and it eventually overtops or breaksthrough the lahar blockage" a sudden flood or a lahar may bury even more communitiesand valuable property downstream from the tributary.

Lahars can"""

"""destroy #y directim act"

"""lead to increasedde osition of sediment"

"""#loc$ tri#utarystreams"

"""#ury %alleys andcommunities withde#ris"

&ase studies of historical lahars

hat6s that cloud upriver8 An eyewitness account of a lahartriggered by rainfall in 9uatemala.

Mount St. Helens" ashington

Muddy River" May !" #!$ Redoubt 4olcano" Alaska " #!#0#$ Mount :inatubo" :hilippines

Heavy rain triggers lahars )ake breakout causes lahar " ##;


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4illarica volcano" =hile " #!;

Real-time warning of lahars

?S9S )ahar detection system and how it works irst test case& Redoubt 4olcano ##$0 ## :ilot :ro3ect& Mount Rainier lahar0warning system

erms 'ften (sed to Refer to Lahars

Scientists often use more specific terms than lahar when referring to moving masses ofwater and rock debris. The various terms are defined by the relative amounts of rockdebris ,sediment- and water in the flow" often as a ratio of sediment to water. And

because a flow of water and sediment can erode or deposit sediment and incorporatewater along its 3ourney downstream" this ratio changes with increasing distance from avolcano. So" for even a single flow" scientists will commonly use different terms whendescribing the flow.

De#ris )low

@ense flows that consist of a relatively high percentage of coarse rock particles are debrisflows. The si/e of sediment transported by debris flows ranges in si/e from clay and silt,less than $.$ mm- to boulders as large as $ m in diameter. A typical debris flowconsists of about ( parts sediment for every one part water. Thus" debris flows mayconsist of more than !$ percent sediment by weightB

Mudflow

A debris flow composed of relatively small rock particles" dominantly sand and silt0si/ed particles ,less than ( mm in diameter-" is often called a mudflow. 1ven though mudflowscan transport large boulders and can have sediment concentrations as great as debrisflows" their sediment composition typically consists of at least 5$ percent sand" silt" andclay0si/e particles ,CmudC refers to silt0 and clay0si/e particles-. Mudflow is probably themost familiar and commonly used term by nonscientists to describe dense mi+tures offlowing sediment and water.

*y erconcentrated +treamflow

A flow containing between ;$ and !$ percent sediment by weight is often referred to ashyperconcentrated streamflow. @ebris flows and mudflows represent the most dense and
http://volcanoes.usgs.gov/Hazards/What/Lahars/VillaricaLahars.htmlhttp://volcanoes.usgs.gov/Hazards/What/Lahars/VillaricaLahars.htmlhttp://volcanoes.usgs.gov/About/What/Monitor/Hydrologic/LaharDetect.htmlhttp://volcanoes.usgs.gov/About/What/Monitor/Hydrologic/AFMRedoubt.htmlhttp://volcanoes.usgs.gov/About/What/Monitor/Hydrologic/AFMRedoubt.htmlhttp://volcanoes.usgs.gov/About/Highlights/RainierPilot/Pilot_highlight.htmlhttp://volcanoes.usgs.gov/Hazards/What/Lahars/VillaricaLahars.htmlhttp://volcanoes.usgs.gov/About/What/Monitor/Hydrologic/LaharDetect.htmlhttp://volcanoes.usgs.gov/About/What/Monitor/Hydrologic/AFMRedoubt.htmlhttp://volcanoes.usgs.gov/About/Highlights/RainierPilot/Pilot_highlight.html


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concentrated mi+tures of flowing sediment and waterD they commonly are composed ofmore than !$ percent sediment by weight.


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+cience nstitute, (ni%ersity of celand'cto#er 0, 0112, 03456 "m"

Two cauldrons a#o%e an eru ting fissure #eneath the 7atna89$ull glacier, located in

&entral celand" An eruption started beneath the glacier during the evening of September'$. hen this image was taken on 7ctober " part of the glacier6s surface had subsided toform two cauldrons" each about 0( km wide. This part of the glacier" located on the northflank of 9rEmsvFtn volcano" was about ;$$0 $$ m thick before the eruption. A ;0km0longfissure eruption beneath the glacier 2uickly melted the iceD the resulting meltwater drainedinto the 9rEmsvFtn caldera. @uring one ;0hour period" scientists observed the surface subside

by about 5$ mB


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:hotographs of the eruption& ; 'cto#er 0-5 ; 'cto#er 1-01 ; Resulting 89$ulhlau ;

Lahars Triggered #y Melting +now and ce

Mount +t" *elens, Washington

@ark pathways created by lahars streak the sides of MountSt. Helens during its catastrophic eruption on May !"

#!$. The lahars were triggered by the sudden melting ofsnow and ice from hot volcanic rocks e3ected by the initiale+plosive activity and subse2uent pyroclastic flows.

Many of the largest and most destructive historical lahars accompanied eruptions fromvolcanoes mantled by a substantial cover of snow and ice. :yroclastic flows are the mostcommon volcanic events that generate lahars00even relatively small pyroclastic flows can2uickly melt large 2uantities of snow and ice. The hot flowing rock debris erodes and mi+eswith snow and ice to form water and trigger snow avalanches on steep slopes. )ava flowsmoving slowly across snow usually do not melt snow and ice rapidly enough to form largelahars but the eruption of lava beneath a glacier can result in substantial ponding of water"which may lead to enormous outpourings of water. The largest historic lahars in terms ofdischarge ,volume of material per second- have occurred in *celand" where these glacialoutburst floods are called >Fkulhlaups.
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Photogra h #y une 5" ## . *n the first three monthsafter the enormous eruption" more than ($$ lahars swept Mount :inatubo" destroyed roadsand bridges" and buried farmland and towns with sediment. Thousands of lahars haveoccurred since ## " and nearly all were triggered by intense rainfall.
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Pyroclastic )lows )ill 7alleys

7iew southeast u Maraunot Ri%er> =une 2, 0110

Landsca e #efore and aftereru tion" :yroclastic flowserupted by Mount :inatubo

profoundly changed the landscapearound the volcano by filling river valleys with hot volcanic rocks asthick as ($$ m and killingvegetation on nearby hillslopes.The pre0eruption landscape ,upper

photograph- consisted ofunconsolidated prehistoric

pyroclastic0flow deposits thatwere deeply eroded.

:yroclastic flows from the >une5 eruption swept down each of

the volcano6s river valleys as faras ( to km ,light0coloredareas in lower photograph-. ithrainfall averaging between ( to ;m per year in this part of the:hilippines" most of it falling inthe monsoon season from >une to7ctober" thousands of small butdestructive lahars originated fromthese pyroclastic0flow deposits.7iew northeast u Marella Ri%er %alley> =une 31, 0110

Water Erodes .ullies and New +tream &hannels

New gullies in yroclastic-flow de osits

@uring heavy rain after theeruption" water first collected insurface depressions on the

pyroclastic0flow deposits and thenspilled into ad3acent low0lyingareas. ithin months" flowingwater had eroded an elaboratenetwork of deep gullies andstream channels into the deposits.Since the erosion took place instill0hot pyroclastic0flow deposits"sudden CsteamC e+plosions were
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common in the first few years andthe resulting lahars downstreamwere dangerously hot ,;$0G$ =-.

+ediment Raises Ri%er &hannels

)looding along !#acan Ri%er

Heavy rain and erosion causedlahar after lahar downstream fromMount :inatubo. @uring therainiest periods in ## " three tofive lahars a day were common.As each new lahar carriedsediment from the pyroclastic0flow deposits downstream" thevolcano6s river channels filledwith sediment. Rising riverchannels caused fre2uentoverbank flooding" even duringnormal streamflow. Here in thelower Abacan River east of Mount:inatubo" floods and lahars spreadinto ad3acent communities andvaluable farmland.

Lahars and )loods Dis lace Thousands of Peo le

)looded community Destroyed home
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The human conse2uences of these post0eruption lahars at Mount :inatubo have beenenormous. *n the first five years following the eruption" lahars destroyed the homes of morethan $$"$$$ peopleD each year during the rainy seasons" the threat of lahars forced another

$$"$$$ people to evacuate their homes and farmland. )ahars also covered about ($"$$$hectacres ,roughly '$$"$$$ acres- with sediment to an average depth of about one meter" and

floods spread rock debris over an area at least several times larger.

More images of lahars at Mount Pinatu#o

*mages and video of post0eruption lahars available from the webserver atMichigan Technological ?niversity

References

Newhall, C.G., )unongba$an, *.S. +eds. , 199(, -ire and !ud /ruptions and lahars o Mt. )inatubo, )hilippines, )hilippine 0nstitute o #olcanolog$ and Seis!olog$, ue2onCit$ and 3niversit$ o 4ashington )ress, Seattle, 11&5 p. Online version of Fire and

Mud

Pierson, T.C., and Janda, R.J., 1992, Immediate and long-term a!ards from la arsand e"#ess sedimentation in rivers draining Mt. Pinatu$o, P ili%%ines& '.(. )eologi#al

(urve* +ater-Resour#es Investigations Re%ort 92- 9.

Lahars &aused #y La$e rea$outs

)ake breakouts commonly occur weeks to months after a river or one of its tributaries become blocked by a landslide or other volcanic deposits" especially pyroclastic flowsand lahars. The most fre2uent cause of a lake breakout is the overflow of water across thenewly formed dam and subse2uent erosion and rapid downcutting into the loose rockdebris. As more water rushes from the lake" the initial channel grows deeper and wider"which allows even more water to surge downstream. %y eroding the blockage and riverchannel downstream" the initial surge of water will incorporate a tremendous volume ofsediment and increase in volume two to four times or more as it races downvalley.
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Photogra h #y +" rantley+e tem#er 31, 0113

&astle La$e, Mount +t" *elens, Washington

%locked by the massive landslide that slid from MountSt. Helens during an eruption on May !" #!$" =astle=reek 2uickly filled with water to form a lake ,centerleft-. *n #! " concern among scientists and publicofficials about the possibility of a sudden lahar caused

by the breakout of water from =astle )ake led the ?.S.Army =orps of 1ngineers to e+cavate an outlet channeland line it with heavy wire mesh and boulders. The newchannel prevented the lake from overtopping its newdebris dam. The water level and stability of the

blockage is still monitored by scientists of the ?.S.9eological Survey.

La$e #rea$out causes lahars at Mount Pinatu#o, Phili inesHundreds of lahars triggered by heavy rain formed after the ## eruption of Mount:inatubo. The rainwater eroded loose" hot pyroclastic flow deposits ,shown here- that filledriver valleys around the volcano to depths of (($ meters. The lahars not only covered large

areas downstream with sediment"destroying homes and farmland" but also temporarily blocked tributary streams.

Lahars loc$ Tri#utary

:yroclastic0flow deposits from the ## eruptionfilled the south fork of the :asig0:ortrero River"

which flows from right to left. After theformation and breakout of several lakes from thisvalley" a new lake began to form in ##; duringthe :hilippine6s summer monsoon season.

=uly @, 011:
http://volcanoes.usgs.gov/Imgs/Jpg/MSH/30410914_048_large.jpg


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La$e )orms ehind Lahar De osits

ull development of the lake. The primary cause

of this new lake was the deposition of sediment by lahars along the main channel of the riverduring the ##; rainy season. The sediment

prevented water flowing into the south fork fromentering the main river.

!ugust 56, 011:

La$e rea$out .enerates Lahar

After a moderate rainfall" water flowing over the blockage severely eroded through the lahar and pyroclastic0flow deposits during the night. Theresulting lake breakout generated a lahar thatswept downstream and killed appro+imately (5

people.

+e tem#er 33, 011:

&ommunity efore La$e rea$out

Town of %acolor ,foreground- along the levee0 bounded :asig0:ortrero River" downstream ofthe lake. The gray area in the upper left iscovered with sediment deposited by lahars andnormal streamflow in ##(. uly ##; had begun to

encroach at the top right.

=uly 3A, 011:

&ommunity !fter La$e rea$out


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)ahars from September ##; though and afterthe date of this photograph buried %acolor todepths of 5 meters in the town and more than $meters in some outlying villages.

+e tem#er 2, 011@

Lahars and Their Effects

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