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Assessing the Risk of Glacial Lake Outburst Floods in the Cordillera Blanca, Peru: A Case Study at Lake Palcacocha Rachel Chisolm, Daene McKinney, Marcelo Somos, Ben Hodges Center for  Research  in  Water  Resources   The  University  of  Texas  at  Austin:  Department  of  Civil,  Architectural  and  Environmental  Engineering rachel.chisolm@gmail.com,  daene@aol.com

Hydrodynamic+Lake+Model+

Avalanche)Model)

The$overall$goal$of$this$research$is$to$study$the$flood$risk$for$communi7es$living$downstream$from$ glacial$lakes$in$the$Cordillera$Blanca,$Peru.$As$these$lakes$con7nue$to$grow,$they$pose$an$ increasing$risk$of$catastrophic$glacial$lake$outburst$floods$(GLOFs).$For$many$decades$Lake$ Palcacocha$has$posed$a$threat$to$ci7zens$living$in$the$watershed$below$and$was$the$source$of$the$ deadly$1941$GLOF$that$nearly$destroyed$Huaraz.$$ The$most$common$GLOF$triggers$are$avalanches$or$ice$calving$into$the$lake,$which$result$in$a$large$ wave$that$is$propagated$across$the$lake,$causing$moraine$overtopping.$This$work$focuses$on$the$ upper$part$of$the$glacial$watershed$system,$from$the$glacier$to$the$terminal$moraine$damming$ the$lake,$but$it$is$part$of$a$broader$effort$to$assess$the$GLOF$risk$from$Lake$Palcacocha.$The$results$ from$this$work$will$be$used$as$input$to$a$downstream$GLOF$and$risk$assessment$model.$$ A$flow$hydrograph$for$each$scenario$of$moraineMovertopping$events$is$produced$through$the$ following:$$ •  Run$avalanche$simula7ons$for$three$scenarios$ •  Determine$ini7al$characteris7cs$of$the$avalancheMgenerated$impulse$waves$ •  Model$wave$propaga7on$and$moraine$overtopping$with$a$hydrodynamic$lake$model$

The$RAMMS$avalanche$module$(Bartelt,$et$al.$1999)$was$used$to$simulate$avalanches$into$Lake$ Palcacocha$from$the$Palcaraju$Glacier$for$three$different$scenarios:$small,$medium$and$large$ events.$The$characterisHcs$for$the$simulated$avalanches$and$the$results$of$the$avalanche$ simulaHons$are$given$in$Table$1.$$ $ $$ Avalanche) Avalanche) Avalanche) Volume)(m3)$ Thickness)(m)$ Velocity)(m/s)$ )$ Small$Event$ 500,000$ 6$ 20$ Medium$Event$ 1,000,000$ 15$ 32$ Large$Event$ 3,000,000$ 20$ 50$ Table)1:)CharacterisLcs)of)simulated)avalanches)at)Lake)Palcacocha)for)three)scenarios.))The) avalanche)thickness)and)velocity)are)given)at)the)point)of)impact)with)Lake)Palcacocha.))

(a)$

(b)$

(c)$

The'Fine'Resolu%on'Environmental'Hydrodynamics'(FREHD),'a'descendent'of'the'PC2'Matlab' model'(Hodges'and'Rueda'2008)'is'used'to'model'the'wave'propaga%on'across'Lake'Palcacocha' and'the'overtopping'of'the'moraine.'The'model'is'run'at'a'5'm'grid'resolu%on'with'square'grid' cells.'The'lake'bathymetry'data'were'taken'from'a'bathymetric'survey'done'by'UGRH'(2009).'The' model'domain'extends'downstream'of'the'terminal'moraine'to'the'point'where'the'original' terminal'moraine'was'destroyed'by'the'1941'GLOF.''

100

distance (m)

Research(Objec,ves(

4600

200

Loca%on'of'Slide'Impact'

300

4550

Terminal'Moraine'

4500

400 500

Bo^om'Eleva%on'(m)'

250

500

750

1000 distance (m)

1250

1500

1750

2000

Figure+5:+Bathymetry+data+for+Lake+Palcacocha+used+in+the+hydrodynamic+simula=on+and+ terrain+data+for+the+downstream+area+located+below+the+terminal+moraine+

Figure)3:)Profiles)of)the)maximum)avalanche)velocity)for)the)small)(a),)medium)(b)) and)large)(c))event)scenarios) Avalanche$Simula7on$

The'wave'characteris%cs'of'the'ini%al'impulse'wave'for'the'medium'event'(Table'3)'are'reflected' in'the'ini%al'free'surface'eleva%on'(Figure'6),'and'the'FREHD'model'simulates'the'wave' propaga%on'from'its'ini%al'posi%on.'' 4580

distance (m)

Ini7al$Impulse$Wave$

Hydrodynamic$Lake$Model$

Figure)4:)ThreeGdimensional)representaLon)of)the)maximum)height)(leQ))and) maximum)velocity)(right))for)the)large)avalanche)scenario)

Moraine$Breach$Model$

100

4575

200

4570

300

4565

400

4560

500

Water'Surface' Eleva%on'(m)'

250

500

750

1000 distance (m)

1250

1500

1750

2000

Figure+6:+Free+surface+eleva=on+of+ini=al+impulse+wave+for+mediumCsized+avalanche+

Impulse)Wave)Model)

Downstream$GLOF$Model$

Figure(1:(Schema,c(diagram(of(the(processes(to(be(studied(at(Lake(Palcacocha(

Study(Area( Lake$Palcacocha$lies$at$the$base$of$the$Palcaraju$Glacier$and$the$head$of$the$Paria$River$basin.$$ Palcacocha$contains$approximately$17.3$million$m3$of$water$and$has$a$maximum$depth$of$73$m.$ The$average$water$surface$eleva7on$is$4562$m.$$The$steep$slope$of$the$Palcaraju$glacier$directly$ above$Lake$Palcacocha$and$the$small$amount$of$freeboard$make$the$lake$suscep7ble$to$outburst$ flooding.$There$have$been$efforts$to$control$the$lake$level$using$siphon$tubing,$and$there$is$a$ permanent$drainage$pipe$that$maintains$the$current$lake$level.$$

Palcaraju$Glacier$

Empirical$equaHons$are$used$to$determine$the$iniHal$characterisHcs$of$the$avalancheLgenerated$ impulse$wave$according$to$the$method$outlined$by$Heller$and$Hager$(2009).$The$avalanche$ characterisHcs$presented$in$Table$1$are$used$as$inputs$to$this$empirical$model$along$with$the$ dimensions$of$the$lake$and$the$densiHes$of$the$slide$material$and$water$(Table$2).$$$ Variable) SHll$water$depth$at$slide$impact$

Value) 64$m$

Slide$width$ Density$of$slide$material$

350$m$ 900$kg/m3$

Slide$impact$angle$(α)$

30˚$

Table)2:)Values)of)input)variables)in)the)impulse)wave)model) The$characterisHcs$of$the$avalancheLgenerated$impulse$wave$are$determined$as$a$funcHon$of$the$ impulse$product$parameter$(P),$which$is$based$on$the$slide$Froude$number$(F$=$slide$velocity/$$$$$),$ gh relaHve$slide$mass$(M),$and$the$relaHve$slide$thickness:$$ 1 1 1 2 ! # 2 4 6 $$ P = FS M cos" α 7 $ The$empirical$impulse$wave$model$produces$characterisHcs$for$the$iniHal$impulse$wave$that$can$ then$be$used$as$inputs$to$the$hydrodynamic$lake$model.$$The$iniHal$impulse$wave$characterisHcs$ for$each$of$the$three$scenarios$are$presented$in$Table$3.$$

{ ( )}

Lake$Palcacocha$

Terminal$Moraine$

Lake$Palcacocha$

Cordillera$Blanca$ Peru$ Figure(2:(Loca,on(of(Lake(Palcacocha(in(the(Cordillera(Blanca,(Peru(

Maximum)Wave) Downstream)distance)to) Height)(m)) )$ Wavelength)(m)) maximum)wave)heightG)xm)(m)) Small$Event$ 9$ 253$ 147$ Medium$Event$ 21$ 468$ 254$ Large$Event$ 42$ 793$ 392$ Table)3:)Results)of)impulse)wave)model)for)three)scenarios.)))

Conclusions+ A'threeYdimensional'hydrodynamic'lake'model'will'allow'for'more'accurate'representa%on'of'an' outburst'flood'event'than'has'previously'been'possible.'The'work'presented'here'will'give'a' reasonable'es%mate'of'the'flow'hydrograph'and'total'volume'for'a'moraineYovertopping'event' and'will'also'give'an'idea'of'the'response'%me'available'aZer'an'avalanche'occurs.'AZer'the'ini%al' moraineYovertopping,'the'moraine'will'likely'begin'to'erode,'allowing'for'complete'lake'drainage.' This'event'will'be'modeled'through'a'separate'process,'but'the'results'of'the'hydrodynamic'lake' model'will'influence'the'modeling'of'the'ensuing'moraine'breaching'event.''While'the'work' presented'here'will'provide'the'ini%al'flow'hydrograph'for'a'GLOF'event,'it'can'also'be'used'to' determine'the'%me'to'begin'applying'the'model'of'the'moraine'erosion'and'lake'drainage'to' determine'the'subsequent'flood'hydrograph.'The'two'hydrographs'can'then'be'combined'to'be' used'as'inputs'to'the'downstream'GLOF'model.''' ' The'downstream'communi%es'that'could'be'affected'by'a'GLOF'event'are'the'mo%va%on'for'this' research,'and'the'ul%mate'goal'is'to'determine'what'effect'a'GLOF'event'may'have'on'these' communi%es.''Because'each'process'in'the'chain'of'events'leading'to'a'GLOF'affects'the'end' result,'it'is'important'to'represent'all'the'physical'processes'accurately.''Improved'representa%on' of'the'upstream'processes,'such'as'the'avalanche'model,'impulse'wave'model,'and'lake'model' presented'here'will'be^er'inform'the'models'of'downstream'processes,'such'as'moraine' breaching,'GLOF'propaga%on'downstream,'and'risk'assessment'for'affected'communi%es.''

References Bartelt, P., B. Salm, U. Gruber; 1999; Calculating dense-snow avalanche runout using aVoellmy- fluid model with active/passive longitudinal straining; Journal of Glaciology; vol. 45; no. 150; pp. 242-254. Heller, V., Hager, W. H., and Minor, H.-E; 2009; Landslide generated impulse waves in reservoirs—Basics and computation; VAW-Mitteilung; Vol. 211; R. Boes, ed.; ETH Zurich; Zurich. Hodges, B.R., and F.J. Rueda; 2008; Semi-implicit two-level predictor-corrector methods for non-linearly coupled, hydrostatic, barotropic/baroclinic flows; International Journal of Computational Fluid Dynamics; 22:9: 593-607. UGRH (2009). Unidad de Glaciologia y Recursos Hidricos, Autoridad Nacional de Agua (ANA) de Peru.

Chisolm: Assessing the risk of glacial lake outburst floods, Palcacocha Lake, Peru  

Poster presented at the High Mountains Adaptation Partnership conference in Huaraz, Peru, July 2013.

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