7．ResultsoftheInvestigationofTshoRolpa
Amongthepotentiallydangerousglacierlakesmentionedabove，TshoRolpaglacierlakewas
intensivelyinvestigatedforlongtermmonitoringnotonlybecauseitisgrowingrapidlyandhasan
apparentlygreaterlikelihoodofburstingbutalsobecauseitisatypicalmoraine−dammedglacier
lakeintheNepalHimalaya．Thefollowingaspectswerestudied．
−

GeomorphologicalfeaturesofTshoRolpa，Interiordeadicesoundingintheendmorainearea，− Meteorologicalconditions，− Seasonalvariationofthelakelevelandtheamountofdischarge−−

Massbalanceoflakewater，Limnologicalstudiesofthelake，SuChaswatertemperature，SuSpendedparticles，its

Sedimentationanddensitystratification，−Chemicalcompositionoflakewaterandchemicalbackgroundaroundthelake，Processesandrateofthepresentlakeexpansion，−Presentconditionofthesupra−glacialpondontheTrakardingglacierasanembryo

Ofaglacierlake．
7．1PhysicalSetting・OfTshoRolpa
7．l．1Location

Tsho Rolpaislocated at an elevation of4580m a．s．1．on27O 50，N：86O 28，Ein the

headwatersoftheRoIwalingKhola，OneOfthetributariesofTamaKosiwhichmergesintoSun
KoshiinSaptaKoshiwatersystemasshowninFig．10．ThedirectaerialdistanceisllOkmeastrnortheastfromKathmandu．ThenorthoftheRoIwalingValleyismarginedbyMt．GauriSankar

rangewhichrunsfromwesttoeastandliesastheborderbetweenNepalandTibet，China．TheMt．
GauriSankar（7146ma．s．1．）isthehighestmountaininthevalley．ThevalleyopenstothewestandisconnectedtoKhumbuHimalintheeasternfringe．
7・l・2LongitudinalCrossSectionalongtheRiverDownstreamofTshoRo暮pa

As a typicalexample of a GLOF path along a river，thelongitudinalcross section along

RoIwalingKholaandTamaKosiisshowninFig．11．TheprofileisdrawnfromTshoRolpa（4ma．s．1．）andisextendeddowntotheconfluenceofKhimtiKhola（590ma．s．1．）whereKhimti（TamKosi2）HydroLpOWerProjecthasbeeninitiated．

RoIwalingKholaruns27kmalongremarkablysteepriverbedwithanaverageinclinationofO．

12tomeettheconfluence ofTamaKosiat analtitudeofonly1390ma．s．1．ThegradientoftheriverbedbetweenSuriDhobanandtheconfluenceofKhimtiKhola（590ma．s．1）iscomparative
gentleoftheorderofO．016．

−43−

TheGlacierLakeanditsOutburstFloodintheNepalHimalaya

Fig．10：LocationofTsho Rolpa．
5000

4000
︵∈︶

︵U
︵U
O
2

60

40

20

0
0
0

1

80

ひPコl≡＜30000Distance（km）
Fig．11：LongitudinalcrosssectionalongtheRoIwalingKhola andtheTamaKosi．

7．l．3Drainage Basinofthe Lake
Accordingtothetopographicalmap（Schneider Map on a scale oflto50，000），thedrainage

basinofTshoRolpaliesbetween4580ma．s．1．，Whichisthelakelevel，and6943mofMt．Teng

Ri，thehighestpeak．Itissurroundedby5000smto6000smpeaksinthe fringe．Because ofthe
altitude，thedrainagebasinisdistinctivelydecoratedwithsnowandglaciersasshowninFig．12．
−44一

ResultsoftheInvestigationofTsho Rolpa

Fig．12：Drainagebasin ofTsho Rolpa．

4.5

TheGlacierLakeanditsOutburst Floodinthe NepalHimalaya

Thecompositionsofthesurfacestructureareshowninthefigure．AltitudinaldistributionoftheareasforeachcompositionofthesurfacestructureistabulatedinTable5；55．3％and16．5％ofthe
totalbasin area of77．6km2were covered with debris free glaciers and debris covered glaciers，
respectively．Only26．5％isoccupiedbythebarerocksurface．TheareaofTshoRolpaglacierlake，
1．39km2，OCCupiesl．7％of the basin area．Thelarge ratio of the glacier area to the basin area

indicatesthatmeltwaterofsnowandiceisessentiallyimportantforthesourceofwatersupplied
into thelake．

Table5：AltitudinaldistributionoftheTsho Rolpa drainagebasin．
（）in％RangeofAltitude（m）

from
4580

to
5000

Area（km2）

rock
1．16
（9．74）

5000

5400

5．57
（38．18）

5400

5800

7．81
（31．75）

5800

6200

4．21
（20．58）

6200

6600

1．74
（31．47）

6600

6943

0．03
（7．34）

total

20．53
（26．47）debris free debris covered
glacier
glacier
0．40
9．00（3．39）

6．30（43．16）

15．71（63．82）

16．25（79．40）3．79（68．59）0．41（92．22）

42．87（55．26）（75．50）

total
lake
1．36

11．92

（11．41）

2．72

14．60

（18．64）

24．61

1．09
（4．43）

0．00

20．47

（0．00）

0．00

5．52（0．00）

0．00

0．45（0．00）

12．81（16．52）1．36（1．75）

77．57
（100）

7．l．4End Moraine−dam

AsshownbythecoverphotographandPhoto14，TshoRolpahasbeenformedonthetongue
Of Trakarding glacier，One Of the typicaldebris coveredglaciersin the Himalaya．Thelakeis

dammed by the end moraine as shownin Photo23．The moraineslooks quite fresh with no
Vegetationonthemandareconstitutedofveryloosedebriscarriedbytheglacier．Thehorseshoe−ShapedmorainewasformedduringtheLittleIceAgewhichhappened a fewhundredyearsago
When Trakardingglacier actively flowed down to the end moraine positiorl．The height of themorainedamis150mfromtheriverbedofRoIwalingKholaandtheinclinationsoftheouterslope

are30／100attheupperpartand15／100atthelowerpart．Thelateralmorainesareextremelylo

andsteeptowardthelake．Assuch，COntinualcollapsescouldoccurintothelakeasshowninPho
24．

The morainedamis akin to one of those poorly constructed rockfi11dams composed of
unconsolidateddebris．Therefore，thedamisveryunstable．AsstatedinthenextChapter，deadicemassiscontainedintheendmoraineandbeneaththelake．Moreoverthemoraineispossiblyfrozen
exceptforafewmetersinthesurfacelayerthoughtherearenocleardata．Theicemassesandfrozenconditionofthemorainemayendurethegreathydro−preSSureOfthelakewater．
46

ResultsoftheInvestigationofTshoRolpa

Photo23＝EndmoraineoftheTrakardingglacier，about150minheight．

YnOtO24：Leftlateralmoraine wlth theloose andSteep Slope；the end moraine of an unnamed
glacierbeingalso seeninthephoto．

−47−

The Glacier Lake anditsOutburst Floodin the NepalHimalaya

7．l．5Upstream Endofthe Lake

The upstream end of thelake contacts the Trakarding glacier．The clifトshaped terminus
collapsesperiodica11ybycalvingsothat，SOmetimes，icebergsarefoundasseeninPhoto25・

glacierretreatsyearafteryearbycalving．Thismeansthatthelakeisexpandingupstreamnotby
meltingbutbycalving．Theglacieriscoveredbythickdebrisofaround30to50cmindepthexcept
forthespotsofspoonCutShapedbareiceasseeninthefigure．Thesefeaturesarecommononthe
debris−COVeredglaciertongueintheNepalHimalaya．

Photo25：CliffShapedglacierterminuscontactingwiththelake；theglacierbeing
COVeredwiththick debrisexceptthespotsofspoon−CutShapedbareice．

7．l．6ShapeoftheJake Basin
TheshapeoftheiakebasinwasalreadyshowninFig．6CandispresentedhereagaininFig．

13withvaluablepoints，Whichwi11beconvenientlyreferredtoinSection7．5．Thelakedepth

firstobservedinFebruary1993．ThepreliminaryresultswerepresentedinWECSReport（Mooleal．，1993）．InFebruary1994themeasurementsoflakedepthwerecontinuedtoobtainmoredetailed

COnfiguration of thelake basin．The bathometric map wasimproved using enoughdepth data
（Kadota，1994）．

The surface area and volume of stored water have also been reassessed asl．39km2and76．6

millionm3，reSpeCtivelyasalreadyshowninTable3．Therelationshipsbetweenthelakeareaa
depth，andbetweencumulativevolumeoflakewateranddeptharerespectivelyshowninFig．14−aand−b．Forloweringthelakelevel，forexampleby20m，1akewaterofabout25，000m3woul
haveto be drained out，Whichwouldcause thelake area to decrease tol．05km2．As seenin thebathometricmapofdownlakeareaneartheendmOraine（Fig．15）preparedbyModderandOlden（1995），thebasinshapeisrathercomplexincomparisonwiththebasinshapeofthe mainlake．

lakeleveldecreasesdownonlytolO m，theshoreline retreatremarkably andthe morainedam
becomesthick enough，eSpeCiallyintheleftbanksideofthelake．
−48

ResultsoftheInvestigationofTsho RoIpa

1km

0

■
Fig．13：Bathometric map of Tsho Rolpa as of Feb．1994（after Kadota，1994；
modified）．Arrowshowingasmallstreamfromanunnamedglacierincontact
with thelake．
X：PointsoflimnologlCalmeasurement．
●：Benchmark onthemoraineridgeforthetopographicalsurvey．

▲：Campingsitesontheend morainenearthelakeandinthevalleyoutthe
leftlateralmoraine．

4

1
1

2

1

a

0

q爪こ＜

0

∩︶

0
0

8

0

6
︵c∈︶2⊃一〇＞聖焉t⊃∈⊃U

0 00
0 0 0 0 0 0 0
4 2
87−654321
0b

20 40 60 80 100 120 140

Depth
Fig．14：a）Relationshipbetweenlakearea anddepth．
b）Relationshipbetweencumulativevolumeoflakewateranddepth．

49−

TheGlacier Lake anditsOutburst Floodinthe NepalHimalaya

100

200m

Fig．15：Bathometric map of downlake area near
the endmoraine（after Modderet alリ1995；
Simplified）．

7．l．7lslandsin the Lake

TheIslandsaresituatedintheendpartofthelakeasexhibitedinFig．13andPhoto26，Which

Photo26：Icecoredislandslocatedatthedownlake area ofTsho Rolpa．

一50

ResultsoftheInvestigationofTsho Rolpa

arecoredwithdeadglacierice．Withmeltingofcoredice，theislandsaredecreasinginheight

Willdisappearintothelakeinfuture．
7．l．80utlet of the Lake

Watersuppliedintothelakefromthebasinareaof77．6km2isdrainedoutthroughtheoutlet

formedonthelowestpartoftheendmoraine（Photo27）throughoutayear，eVeninwinterseason

Atleastfourformeroutletscanbefoundonthemoraine．Nonuniformmeltingprocessofthecorediceinthemorainemayhavecreatedtheshiftingoutletsoneafteranother．Asaworkingoutletwas

Photo27：Outlet stream ontheendmoraineviewinga）fromthefootoftheendmoraine；b）fromahelicopter．
−51

TheGlacierLakeanditsOutburst Floodin the NepalHimalaya

changedtogiveplacetonewlycreatedoutlet，theabsolutewaterlevelofthelakemayhavebeen

shiftedtothenewlevel．Infact，Shrubsaregrowingonapartofthemoraineonlybesidethelake
asseeninPhoto．28．Thephotographshowsthecampsiteforthelakestudymarkedby▲inFig・
16．Amongtheshrubs，thosegrowingonthelowlevelalongjustthelakeshorehavewitheredanddied．Thetreeringswerecountedtobe20annualrings．Ontheotherhand，thelivingonesatthe
relativelyhighlevelhad30ringsasofNovember，1994（FujiwaraandGomi，1995）．Thisindicates
thatthelakelevelmayhaveincreaseduptothedeadtreelevelaboutlOyearsago・

Photo28：Shrubgrowingontheend morainenearthecamplngSite．

7．l．9lnflow Streaminto the Lake

OnlyonevisiblesmallstreamfromtheunnamedglacierincontactwiththelakefromtheleftbanksideneartheglacierterminusofTrakardingglaciercanberecognizedasonlythesourceof
surfacewaterinto thelake asshownbyanarrowinFig．13．Theendmoraine ofthisunnamed

glacierstillcontainsthedeadglaciericeunderthickdebris，Whichisnowexposedattheterminus

cliffasseenin Photo29．Sincethe deadiceisexposed，Violent melting occurs during the warm
summermonths．Meltwaterhascausedasmallstreamofsome20−40L／stopourintothelak
showninPhoto30．Nootherwatersourcesarefoundbesidesthissurfacestream．Thismeansthat

almost allwaterin Tsho Rolpa drainage basin，SuCh as meltwater of snow andice as we11as
rainwater，muStbeabsorbedandconcentrateintheenglacialdrainagesystemsoftheTrakarding
glacier andpoursintothelakethroughtheenglacialmouths（see Section7．5．4．1）intheglacierterminus．

52

ResultsoftheInvestigationofTsho Rolpa

Photo29：Exposedice bodyin the end moraine of an unnamedglacierboundingontheleftsideofthelake．

Photo 30：Smallstream of meltwater from theexposediceshownin Photo29．
−53−

TheGlacier Lake anditsOutburst Floodinthe NepalHimalaya
7．l．10Leakage

Sevenleakage points were found on the outer slope of the end moraine asindicated by x

marksinFig．16．Leakingvolumeofwaterwasmeasuredtobeoftheorderof20L／satthelargest
leakage point．Thelevelof theleakage pointis some50mlower than thelakelevel，Whichcorresponds to the water saturated areain the end moraine detectedby the electric resistivlty

surveyasmentionedinfollowing Section7．1．11．The other points ofleakage release out alittwaterbyvolume．Thedevelopment ofsuchpipingmayweakentheendmorainestructure．

0

100

200 m

Fig．16：Mapoftheendmorainearea．
Finesolidlines：Electricalresistivitysurveyline，Twomarks，X，Onthelinel
indicatingthepointsoftheverticalelectricsounding．
Thick solidline：Most downstreamlimit of theice cored area．
Dashedline：Depth measurementline between LamaIsland andInstrument
Island．
Chainedline：Ridgeofthemoraine．
●：Positionofthemeteorologicalstationandlevelgaugestation．
J：Waterlevelbench mark．
▲：CamplngSite．
X：Leakagepoint．

54

ResultsoftheInvestigationofTshoRolpa
7．111DeadJcein the End Moraine Area

Themostessentialcountermeasuretopreventtheoutburstfloodofaglacierlakeistodecrease
the waterlevelofthelakeby making a deep enoughbreachin the moraine−dam not to allow
OVertOppingoflakewater．Itwillhavetheeffectofdiminishingthehydrostaticpressureagainstthe

moraine，dam．Furthermoreevenifthemorainedambreaks，lessamountofreleasedwatercause

arelativelysmallfloodsothatconsiderablylessdamagemaybeexpected．
ExcavationofabreachinthemoraineLdammayexposetheinterioricebodyinthemoraine．
Oncetheicebodyisexposed，StrOngSOlarradiationacceleratesitsmelting，Whichresultsintheendmorainestructurallytoweakenandcauseaspontaneousmoraine−dambreak bywhichaGLOF
maybetriggered．Itisclearthattheinnerstructureofanendmoraineshouldbeinvestigatedbefore
COnductinganycivilLenglneerlngtreatmenttOthemorainerdam．

Forfindingthedepthandlocationoftheicebodyburiedintheendmorainearea，theElectric

ExplorationResistivityMethodwasappliedinthepost−mOnSOOnSeaSOnOf1994（Kita，1995）．The
methodcanprovideustheprofileoftheelectricresistivity（ER）intheinteriorofthemoraine．SincematerialshavetheirownERValuewecanestimateandidentifythematerialonthebasisofthemeasuredERValue・Sinceiceisoneofthegoodelectricinsulators，theER−Valueshouldbever

highamongthematerialsoftheendmoraineareaconsistingofsuchmaterialsasdebris，bedrock
Water andice．The ERValues were measuredin situ as4，000to8，000n−m for the unsaturated

morainewithwater，2000to4000rトmforthesaturatedmoraineand500tol，000ロトmforwater．

Thesurveyedpointsandlinesareshownonthemapoftheendmoraineareaasillustratedin

Fig．16．First，thedepthsoficemassweredeterminedattwopointsonthelakenearLamaIsland

markedby（Ⅹ）onthesurveylinelinFig．16bymeansoftheVerticalElectricSounding（VES）．Then
One−dimensionalverticaldistribution of ER−Value can be obtained by VES method．Thelayer
beneath4to6mfromthesurfaceshowedaveryhighERValueofmorethanlO，000f2¶m．This
indicatedthaticemass existsjustnearthesurface，0nly46min depth，Which means that t
thicknessofdebriscanbe assumedtobe4L6m・ThemeteorologlCalstation，Waterlevelgauge
StationandotherimportantspotsarepresentedinFig．16．

Next，CrOSS SeCtionalprofiles of ERLValues were obtained by means of the2−Dimensional
ElectricProfilingalongthreesurveylinesl（150minlength），2（250m）and3（360m）asshowninFig
16・ThismethodallowstodetectamoredetailedcrosssectionofER−Valuesintherelativelywi

areaalongthesurveylinethantheVESmethod．Thestructureoftheendmoraineestimatedbythe

CrOSSSeCtionalprofileofER−ValueisshowninFig．17．AtsomepointslOOmfarfromtheshorel

inthesurveylinesland3，highERValuemorethanlO，000nmWeredetected，Whichshowst
presenceoficemassatadepthof5tolO munder thelakebottom．

ItwasconfirmedthatdeadicemassiscontainedinthevicinityoftheendmoraineareaofTsho
Rolpa．

ー55−

TheGlacierLakeanditsOutburst Floodinthe NepalHimalaya

n

e

l

i
L

︵∈︶壱dひ口

0 20 40 （iO 80 1（氾 1ZO 140 1（iO 180 200 220 240
Distance（m）︵∈︶卓dリロ

O10 30 50 70 90 110 130 150
Dis也nce（m）
つJ

e
n
i
L

0
川
20
聞 40 50 60︵2︶卓dO凸020 40 60 80 100 120 140 1（iO 180 2（カ 220 240 260 280 3（氾 320 340 360
Dis払nce（m）

Fig・17‥Internalstructureoftheendmoraineareaalongthreesurveylinesshownin
Fig．16（AfterKita，1995；mOdified）・−56

ResultsoftheInvestigationof Tsho Rolpa

7．2PossibleRisksforTshoRolpaGLOF
AsregardstheinternaltriggersforTshoRolpaGLOF，theexistenceoftwokindsoftriggers

areidentified．Oneisthepossibilityofaslopecollapseandtheotherisaniceavalanche．Ass

inPhotos29and30，theice−COredendmoraineoftheunnamedglaciercontactswiththelake．Wh

thefeaturesareshowninPhoto31．Aslopecollapseofthisverysteepmorainewithhugeamount
Ofmassmaytriggerthelakeburst．HangingglaciersasshowninPhoto320nthelakesideslopes

OfMt．Tsoboje（6689ma．s．1．）intherightbankhaveapossibilityofgeneratinghugeiceavalanches

SinceablationLValleyintherightbankiscompletelyburiedbydebrisfallingdownfromthesesteep

SlopesofMt．Tsoboje，theavalanchedirectlyfallsdownintothelake．Anearthquakeshoulda
beconsideredasanexternaltriggercontributingtolake−burst．

AsmentionedinpreviousSection7．1．10，thereareseveralleakagepointsintheendmorai（x marks on Fig．16）．The development ofconduits and continuous melting ofthe deadice can
Weakentheendmoraine．Thus，theotherpossibilityofmoraine−dambreakisaslidingand／or

COllapseoftheendmorainespontaneouslyduetothepipingandmeltingofthecoredice．Nobody
Can SCientifically predict the timing of the burst，becauseitis fully unpredictable destruCtion
phenomenonasthesame asearthquakes．

Photo31：Endmorainewiththesteepslopeformedbyanunnamedglacier−57

TheGlacierLakeanditsOutburstFloodintheNepalHimalaya

Photo32‥HangingglaciersonthesteepslopeofMt・Tsobojeintherightbanksideof thelake．

7．3MeteorologicalStudiesofTshoRolpa
ThemeteorologicalconditionhavebeencontinuouslyobservedbymeansofAanderaaAuto−maticWeatherStation（AWSr2700）fromJune1993toAugust，1996attheInstrumentIsland（468ma．stl．）asshownby●markontheinstrumentislandinFig・16（Yamada，1995）・
Airtemperature，humidity，winddirectionandairpressurearemeasuredathourlyintervals・
ThevaluesofprecipitationandglobalradiationaretheintegratingvaluesduringonehourLThe
rainfallsensorofbuckettypehasacatchmentfunnelwithaprecise200mmdiameterorifice・Soliprecipitationduringwinterarenotfullyreliable・Precipitationbysnowmaybeunderestimated・
windspeedsensorhastwooutputsignals，aVerageWindspeedandwindgusti・e・maXimumwind
speedduringthesamplingintervalofonehour・MonthlyvaluesandtheextremevaluesofthemeteorologicalelementsaretabulatedinTable
6．Maximumandminimumvaluesindicaterespectivelytheextremehourlyvaluesduringthemonthin Table6．
58−

ResultsoftheInvestigationofTshoRolpa

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喜 喜lしn LC〉 く＝ Lrつ l し仁〉（＝ L∫つ L仁〉 く＝ し【つ く⊃ ⊂⊃ ⊂＝〉 〈＝⊃ しrつ し∫つ ⊂⊃ L⊂〉 く＝）⊂＝〉 く＝〉 く＝ しゎ ⊂⊃ ⊂⊃ ⊂⊃ しr） し∩ ⊂⊃ く＝ Lrつ L「）】 l■寸m⊂つNm Lrつ ⊂つ て一 寸 ■寸 寸 m ⊂⊃ N ⊂⊃

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59

TheGlacierLakeanditsOutburstFloodintheNepalHimalaya
7．3．1Air Temperature

SeasonalvariationofdailymeanairtemperatureisshowninFig．18．Dailymeanairtempera

turekeepsaboveOOCduringthemidrMaytothemid−OctoberandbelowOOCduringthemidOctobertomidMayofnextyear．Asseeninthefigures，airtemperatureisalmostconstantaround56OC

duringthemonsoonseason：1argeperiodicfluctuationwiththecycleofaboutlOdayscharacterizes

the dry season from November to May．Annualaverage air temperaturein1994and1995arerespectively−1・2andrl・90C・AsseeninTable6，thewarmestmonthismostlyJulywithmonthaverageof5・3−5・50Cin19931996：thecoldestisJanuaryorFebruarywith9．20CinJan．，1995and
11・5℃inFebリ1996；themaximumandtheminimumhourlyvaluesofairtemperatureduringthre
yearsare，reSPeCtively，＋14．6DCon6Jul．，1993and−23．20C on16Feb．，1995．
8

0

・4

−8
2

︵00︶聖コl巴乳∈￠こ葡Uq￠∈ゝ■馬凸

4

6

寸﹁︑の○︑∞のの﹁￠﹁︑の○︑りののL∽ミND︑盟Wの﹁の﹁︑Lし︑のののL

ON︑の○︑のののr

NN︑の○︑のののL

LN白○︑Sののr

Cqしミ寸の￠L∽N︑の○︑寸功のrトq∽e寸∞m▼︑のののT￠NaO︑寸ののL

父竿

OC︑の○︑CののL

岩︑∽古︑のののr

Fig・18：Seasonalvariation ofdailymeanairtemperature．

7・3・2Precipitation

DailyamountofprecipitationisshowninFig．19．ThoughappreciableprecipitationoccurseveninthedryseasonfromJanuarytoMay，64to72％ofannualprecipitationareconcentratedinthe
monsoonseasonfromthemidJunetothemidSeptemberin1994andin1995，reSPeCtively．T
annualprecipitationisrecordedas414・5mmin1994and694・Ommin1995・Themaximummonthly
precipitation of243．5mm occurredinAugust，1993．Thelocalized heavy rainfalldid not occur

duringtheobservedperiodbecausethemaximumhourlyprecipitationwasrecordedasonly13．O
mminAugust，1993asshowninTable6．
7．3．3Humidity

DailymeanhumidityisshowninFig．20．Relativehumiditydatashowsthatthisregionbelon
tothetypicalmonsoonclimate二Considerablehighvaluesandrelativelysmalldiurnalvariati

Characterizethe monsoonseason form midJuneto midSeptember．The maximum ofmonthly
averageis97％inAugustbothin1993and1994．Dailymeanhumidityrapidlydecreasesafterthe

monsoonseasonfromtheendofSeptember．DecemberandJanuaryarethedriestmonthsinayear
60−

ResultsoftheInvestigationofTsho Rolpa

0
■○
▲U
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︵∈∈︶u O 焉 l 亘 0 巴 d 倉 巾 凸

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寸Lあ○︑父器﹁りL読○︑りの功L￠LaO︑￠◎のLの▼︑L﹁︑∽仇仇﹁OSO︑S采L

Nqの○︑∽宋L

LqNO︑S仇の▼︹qLL︑蓋﹁のSO︑寸虎の︻トqの○︑寸仇の▼￠N内○︑寸の∞▼のq︹L議偶の︹○のあS史W▼﹁○︑父︶あの∞▼■0
0

Fig．19：Seasonalvariationofdailyprecipitation．

O
U▲

8

▲U

O
4

︵邑倉p盲コ工∪芯∈倉巾凸OO2O王≦岩︑m訪00▼

竺S岩︑∞のの︻￠﹁aO︑￠のの†のミ﹁ミの￠仇﹁Oqの○︑∽￠の﹁NNaO︑墨の︻▼ロコNOぁのの†のq†▼壷﹁のSO︑寸功のrトq巾e寸￠の▼￠qNO︑寸の￠一

頃甲−こ申望ご○のaOあのの﹁†○︑父︶あ￠の▼Fig．20：Seasonalvariationofdailymeanhumidity．

withmonthlyaveragevalueofonly33−44％，andtendstograduallyincreasetothenextmonsoon

season．DryseasonfromtheendofSeptembertotheendofMayischaracterizedbylargediurnal
anddailyvariation．
7．3．4Solar Radiation

DailyamountofsolarradiationisshowninFig．21．Theannualmeansolarradiationis269W／
m2in1994and237．6W／m2in1995whichcorrespondsto23．2and20．5MJ／m2／day，reSpeCtively．The
solarradiationreaches aminimuminDecember／January；themonthlymean of158−180W／m2，

whilethemaximumisobservedinMayjustbeforethemonsoonseasonwithavalueof356−364
m2asshowninTable6．Themaximumhourlysolarradiationwasrecordedof454W／m2（39．2MJ／
m2／day）onMay，1994．
一61−

TheGlacierLakeanditsOutburst Floodinthe NepalHimalaya

王≦岩︑器の﹁￠ミの○︑￠のの﹁︑∽￠か︻り†内○︑￠の功一の▼︑

OSO︑ののの﹁Nqの○読の功一

LqNO︑の宋︻CqL﹁︑寸のの﹁崎Nあ○︑蔑の﹁トq∽○︑寸訪仇▼￠N内○︑寸功の▼のqL﹁︑C￠￠L

LO︑m5︑Cのの▼Oのあ○︑のの∽r

0
0
0
0
O
5
0
■〇 ▲U
〇
■
〇0540043
300205211

ー0 1 ∪ ⊃ 0 ∈ 竺 ざ 占︵N∈︑主︶ u O 毒 P 聖 賢 e き t O 工 の▲5U

Fig．21：Seasonalvariationofdailyamountofshortwaveradiation．
O

7・3・5WindSpeed

Windspeedwasmeasuredforaverageandthemaximum（gust）valuesduringlhourinterval
asshowninFigs．22aand−b．Dailymeanvaluesareratherweakandquitestableatl．7to2．9m／
throughoutayearandtheannualaverageisonly2．1m／sin1994and2．4m／sin1995，reSpeCtively．

∪宕∈書占︵UひS︑∈︶pぉdspu盲543210a）−62−

寸丁︑￠○︑￠ののL∽▼︑の○︑況のL①▼aO︑￠ののrの﹁︑ 義挙望ONaO︑のののL

NN︑の○︑のののL

さ卓望

LN︑NO︑ののの▼CN︑のqの○︑芸のL

トN︑のロ︑寸肋の﹁￠N︑NO︑寸￠のrのN︑r﹁︑どののL

O爪︑の○︑Cののr

LO︑￠○︑Cのの﹁6420864つ▲∩﹀

きコ○≡○彗一e＞∪認∈書芸︵Uむぞ∈︶召乳spu盲∈⊃∈一議∈

Fig．22：Seasonalvariationofa）dailymeanwindspeed；b）dailymeanvalueofhourlymaximumwindspeed．

ResultsoftheInvestigationofTsho Rolpa

ThemaximumwindspeedisratherstableintheperiodfromApriltoNovemberataround5m／s
butfluctuatesconsiderablyinthedrywinterseasonfromDecembertoMarchasshowninFig．22

b．Stronggustsoftenoccurredatmorethan20m／sinthisdryseasonandtheextremevalue
recordedas32．8m／son17March，1994．

Theperiodicdiurnalvariationcanbeclearlyseenthroughoutayearexceptwinterseasonin

DecembertoFebruaryasshowninFigs．23−a，b，Candd；gentleandalmostconstantwindspeed
lessthan2m／scontinuesduringthenightinmostseasonsandalittlebitmorethan2m／sduring
winter．Windspeedstartstoincreaseataround8：00to9：00andreachesadailymaximum
around5m／sat12noonto14：00：thengraduallydecreasesandreachesagaintothegentleand
COnStantnightwindat18：00to20：00．

76543210︵の︑∈︶p宕dspu叫≧

76543210
76543210
76543210

0 3 6 9 12 15 18 21 24
Fig．23：Diurnalvariationofhourlymeanwindspeedmonthlyaveraged：a）MarchtoMay．
b）JunetoAugust．
C）Septemberto November．
d）DecembertoFebruary．

ー63−

TheGlacierLakeanditsOutburstFloodintheNepalHimalaya
7．3．6Wind Direction

WindatRoIwalingValleyischaracterizedbyevidentup−Valleywind・AsseeninFigs・24−a，b，c，andd，WindindaytimeatRoIwalingValleycorrespondstotheup−Valleywindinthedirectio
NW（3150）．TheaxisofthevalleyisinNWtoSEasseeninFig・12・Theup−Valleywinddevelops
clearlyfromMaytoNovember（Fig．24−a，b，C）andisobscuredinwinterfromDecembertoApr（Fig．24LdandLa）．Onthecontrary，thedown−Valleywindinnightisnotclear・DirectionofnightwindisnotdownJValleydirectionofSE（1350）butmostlyinbetweenStoW：onlyinwinte
seasonofDecembertoApriltobeSEtoS．

360
￠ 270
180
q〉 【⊃ 90
0
0
0 0 0
6
3 2
0
0
0
0
0
0
6
3
2

360
270
180
90
0

0 3 6 9 12 15 18 21 24
Fig．24：Diurnalvariationofhourlymeanwinddirectionmonthlyaverageda）Marchto May．
b）JunetoAugust．

c）Septemberto November．
d）DecembertoFebruary．

−64−ResultsoftheInvestigationofTshoRolpa
7．3．7Air Pressure

TheannualmeanairpressureatTshoRolpa（4580ma．s．1．）is585．8hPain1994and585．5
in1995，reSpeCtively・Airpressureshowsaseasonalfluctuation‥Monthlyminimumis580583hPa
inJanuary／Februaryandthemaximumis590hPainSeptembers，1994and1995．Italsoshowsabou
lOdaysperiodicfluctuationasshowninFig・25；theamplitudeoftheperiodicfluctuationissma11
duringthemonsoonseasonandlargeindryseason・Airpressureseasonallyfluctuateinbetween574（hourlyminimum）and593hPa（hourlymaximum），Whichdifferenceisonly19hPaasseeninTab
6・Airpressureissurprisinglystablethroughoutayear．Airpressureshowsacleardiurnalbi−peakvariation．

88
86
糾
82
80

︵e皇︶巴⊃S∽巴d﹂葡u認∈ゝニ巾凸90
78
寸この○︑りの仇L

りL︑の○︑￠ののL∽﹁aO︑￠のの﹁のL︑rL︑ののかL

ON︑￠○︑のののr

Nqの○︑のののL

LN︑NO︑Sのの†

CN︑LL︑嘉Lのqの○︑寸ののL

トqの○︑寸ののL

りqNO︑寸のの﹁のN︑LL︑CののL

555555555

O巾︑の○︑CののL

刊

LO︑￠○︑ののの﹁76Fig・25：Seasonalvariationofdailymeanairpressure．

7・4HydrologicalStudiesofTshoRolpa

Asalreadystated，theglacierlakeTshoRolpareceivesallmeltwaterandrainwaterfromthe
drainagebasinof77・6km2・Suchwaterpouringintothelakeoverflowsthroughtheoutletcutting
theendmoraine・Inordertoobtainthehydrologicalfeaturesofthelake，thefollowinghydrologicalelementssuchaswaterlevelandrelationshipbetweenwaterlevelanddischargewereobserved．
7．4．1Water Level

Waterlevelwasautomaticallyandcontinuouslymeasuredattheintervalofeverylhourby

usingapressuretypewaterlevelgaugeoftheresolutionoflmm（Yamada，1996）．Photo33show

the overlooking view of the outlet；the bridge was temporally constructed for the discharge

measurement；awaterlevelgaugewasinstalledontheleftbankjustupstreamofthebridge．The

Sizeofthestreamisabout5minwidthand2mindepthatthedeepestpoint．Waterlevelwasalso

measuredmanuallyineachfieldvisitwithreferencetowaterlevelbenchmark（WLBM）asshown

byJmarkinFig・16・Themeasurementaimsatobtainingtheseasonalvariationofdischa
throughtheoutletbyusingtheobservedrelationshipbetweenwaterlevelanddischarge．
65−

The Glacier Lake and its Outburst Flood in the Nepal Himalaya

Photo33：0verlookingviewoftheoutlet．Levelgaugebeinginstallednearabridge
temporally construCted．

TheseasonalvariationofdiurnallyaveragedlakelevelreferredtoWLBMisshowninFig・26・

The minimum waterlevelof the base flow suddenly starts toincreasein the beginning of themeltingseasonattheearlyMayandreachesthemaximumlevelinJuly・Aftertherainyseason，

1akeleveldecreasesgraduallyandmonotonouslytowardtheminimumlevelrealizedinmidMarch・
ThusitisclearthatthehydrologlCalyearofTshoRolpadrainagebasinstartsfrommidrMarch・
Highwaterlevelis retainedin the monsoon of1995・Thisis coincident with high monsoon
precipitationin1995asshowninFig・19・Themonsoonin1995ismoreactivethanthosein1993an
1994．Waterlevelannuallyfluctuatesin approximately2mbetween132cm（hourly minimum）

︵U

8

O
6
0

4

0
0
0

0

4
2
6

0

一
l

︵∈0︶一望芯二塵扇ミ

■ソー

−80
−100
−120

Fig．26：Seasonalvariationofwaterlevelofthelake

66一寸︻＼∞○＼竺望讐＼の○＼gのの︻gL＼NO＼冨竺の︻＼ ＼のかの︻ON＼00○＼嬰望

NN＼の○＼の霊︻︻N＼NO＼ののの︻CN＼ ＼亨望ごのN＼の○＼甘か■ご

トN＼の○＼寸のの︻＼冨竺￠N＼NO＼寸言l∞N＼

OC＼∞○＼芸巴石＼￠○＼C霊︻−140ResultsoftheInvestlgationofTsho Rolpa

and＋79cm（hourlymaximum）．Thehourlyhighestlevelsareof65．5cmon17July，1993，56Ac

On2July，1994and78．7cmon6June，1995intherainyseasonandthehourlylowestlevelsof−13
3cmon12March，1994and−123．9cm on23March，1995，Ll17．5cmon12March，1996．

CleardiurnalvariationsinthelakelevelareobservedinthemeltingseasonasshowninFig．
27asanexample．Thediurnalvariationscharacterizedischargefromthedrainagebasincovered

bysnowandglacier．Thisiscausedbythediurnalvariationofmeltwatersupplyinto thelake：

Snowandglaciermeltingarecloselyrelatedtothediurnalvariationofairtemperatureandsolar
radiation．Afterthemeltingseason，thediurnalvariationdisappears．

Unexpectedfluctuations oflakelevelsometimeshappeneddueto thecalvingoftheglacierterminus orsuddendrainage ofa supra−glacialpondinto thelake．Usually the amplitude ofthe

fluctuationwaswithin±30cm．Thephenomenaoccurredatleastonceineachfieldvisitof1993t
1996．

dO642

0
0

︵∈0︶一￠＞心﹂﹂むle≧

∩︶
246

0
0
の○︑○こCのの︹のN︑の○︑CののLのミの○︑Cのの﹁の0︑の○︑Cのの﹁のN︑の○︑Cの竺のミの○︑Cのの﹁の○︑の○︑Cの竺

OC︑ト○︑CののL

ON︑ト○︑ののの﹁○ミト○︑Cのの﹁OC︑∽○︑Cのの﹁ON︑￠○︑のののL

0Oミ￠○︑Cののr0

Fig．27二Waterlevelclearlyshowingdiurnalvariationinthemeltingseason．
0

7・4・2Discharge

Discharge measurements were carried out by means of flow velocity measurement mainly
duringthefieldvisitsofJulyandNovember1994forobtainingtherelationshipbetweenwaterlevel

anddischarge．Aflowvelocitysensorwasattachedtoasteelrod．Whendischargewasatmoretha

about4m3／s，0nlyflowvelocityatthesurface（Vs）wasmeasuredbecausethesteelrodwasnot

StrOngenOughtostandthehighflowvelocity；theaverageflowvelocitywasassumedasO．88Vs．

TherelationshipbetweenwaterlevelanddischargeisshowninFig．28．TheemplrlCalequat
derivedis：

Q＝4．9247exp（0．0211h）
WhereQisdischargeinm3／sandhiswaterlevelreferringtoWLBMincm．
−67

TheGlacier Lake anditsOutburst Floodinthe NepalHimalaya▲U O O
6 4 2
O︵∈0︶ 工 盲 ＞ ￠ ﹂ J ￠ l e ‡O O O n︶
々 ■ 裾 ■8
0

2

6

4

12

10

8

14

16

Discharge，Q（m3／sec）
Fig．28：Relationshipbetween waterlevel（referring to waterlevelbenchmark）anddischarge．

Byusingthisequation，thewaterlevelisconvertedintodischarge．Theseasonalvariatio
dailymeandischargeanddailyrunoffar・eShowninFig．29．Themaximumdischargewas19．6m3／
son17July，1993，16．2m3／son2July，1994and25．9m3／son6June，1995；theminimumdischwaso．30m3／son12March，1994，0．36m3／son23March，1995andO．41m3／son12March，1996

coefficientofriverregimeisonlyaround60，Whichisarathersmallvaluecomparedtothelarge

SeaSOnalvariationofprecipitation．Theannualamount oflakedischargeiscalculatedtobe92．6
millionm3from15March，1994to14March，1995and128．9millionm3from15March，1995

March，1996，reSpeCtively；theannualaveragedischargeof2．9m3／sin1994hydrologicaiyearand
4．1m3／sin1995−hydrologicalyear，equivalentto3．2mm／dayin1994and4．5mm／dayin1995．This
COrreSpOndstotheannualtotalrunoffofl193mmin1994and1661mmin1995．

6
4

28，6

2
0

26，4

2

24．2

2
9 7 5 3 1．8

1

︻∠

1 1

6 4

■− 1
6 4 8
2 0
の○＼爪○︑り仇∽−の○︑Nロ︑ののか▼

ト○︑Lr︑ののの▼の○︑の○︑のののr

こ︑の○︑ののの▼O﹁︑NO︑ののの▼

NL︑Lミ寸のの▼

寸ミの○︑寸￠の▼￠Uの○︑曾ののr

︑Cののr

れ︻︑Ne寸の仇−トのr︑の○︑Cの仇︻

4 2﹁N︑SO︑Cのの︹

▲8 6

6 4 2

2 0

刀u⊃○↓二∃∃︑day︶nU 6 4 2 ．∩︶

8

2︵s︑の∈︶乱J空US占22．0

Fig．29ニSeasonalvariatjon ofdailymean djschargeand dajlyrunoff．

−68

ResultsoftheInvestigationofTshoRolpa

Annualmassbalanceinthe Tsho Rolpa drainagebasincanbe expressedby the followlng
equation：
P＝Q＋E＋△G＋△S，

WherePistheannualprecipitationoverthedrainagebasin；Qistheannualamountofdischarge

throughtheoutletofthelake：Eisannualevaporation：△Gisthechangeintheglaciermassand△Sisthechangeinthevolumeofstoredwaterinthedrainagebasin．ThevalueEisassumed
benearlyzerobecauseoflowtemperatureeveninsummer，and△Sisalsoregardedaszerobecause
Oftheperiodofoneyear．Iftheprecipitationin1994−and1995−hydrologicalyearof424．3mmand
696・5mm，reSpeCtively，ObservedattheMeteorologicalStation（Section7．3．2）areassumedasP，△G

arecalculatedas4241193＝一769mmin1994and964mmin1995．Theseamountgivethetotal

annuallossoftheglaciermassintheTshoRolpadrainagebasinin1994−and1995Lhydrologicalyear・Thesevaluesaretoolargefortheglacierseventosufferunusuallylargemelting．Pmayb
morethantheobservedprecipitation，i．e．moreprecipitationmayhave occurred at thehigher

altitudesinthedrainagebasin．

7■4・3ChangeoftheLakeLevelbyMeansofArtificialDrainageofLakeWater
Employlngthedataofanannualhydrographattheoutlet，thechangeoflakelevelduetoan

artificialdrainingoutofthelakewateriseasilycalculated・Byinterpolatingdischargeintheperiod
betweenSeptemberandNovember，1994，Whenthedatawaslacking，thedailymeandischargein

One hydrologicalyearis estimated as a typicaland moderate annualhydrograph of thelake

drainage・Thehydrologicalyearis defined as a period from15March，1994to14March，19
referrlngtOthelowestwaterlevel・ThelakelevelchangeswithlapseoftimeareshowninFig．3
5

0

−5
︵∈0
警じこUl﹄くr■−J O 5 0 ︻〇 〇 5 0 ■−︶ O
T ■2 ●2 ■3 ■3 ■ ↓ ●5一5−6

−10

＼＼

＼ ＼

／／

＼

H ／（＼

＼

H ＼

＼

＼

喜 §

N

Fig．30：Lakelevelchangewithlapseoftimeinthecasesoftheartificial
drainageamountofO（Natural），0．5，1，2，3，4m3／s，reSpeCtively．Thedrainage starting from15March，the beginnlng Of the hydrological
yearwhenthelakelevelisthelowest．

−69−

TheGlacierLakeanditsOutburstFloodinthe NepalHimalaya

inthecasesoftheartificialdrainageinO（Natural），0．5，1，2，3and4m3／s，reSpeCtively，Whichstarted

from15March，thebeginningofthehydrologicalyear．Thelakeleveldecreasesremarkablydur

thedryseasonwhennorainwaterandnomeltwaterareproducedbecauseoflowtemperaturebelow

freezingpoint．Duringthemonsoonseason，however，hugeamountsofrainwaterandmeltwater

pouredintothelakeandthelakelevelquicklyrecoveredfromtheendofMaywhenmeltingstarts．
Thelakelevelchanges arecalculatedfortwo years．Afterthetwoyearsthe fluctuation oflake
levelisrepeatedinthefirstfivecasesoftheartificialdrainagecondition．Onlyinthecasewitha
dischargeof4m3／s，thewaterlevelisnotrecoveredexhibitingaquickdecreasingtrend．

7．5LimnologicalStudiesofTsho Rolpa
7．5．］ThermalCondition of the Lake∴．・．／．／l・リ／／・・．／／ノ一ハ！J7／＝！／−1l′／／−J●ハ叫＝′／／仙、

Toclarifythethermalstructureofthelake，theverticalprofilesofwatertemperaturewere

Observedduringeachfieldvisit．Themeasurementsweremadeatseveralpointsalonganapproxi

matecenterlineinthelongitudinaldirectionofthelakefromneartheendmorainetotheproximity
Oftheglacierterminus．
SeasonalvariationofverticaltemperatureprofilesataroundthedeepestpointisshowninFig．
31．Watertemperatureatthesurfacefluctuatesfrom6．OOCinsummertoOOCinwinter．Theseasonal
Temperature（℃）
0

1

3

2

4

5

6

︵∈︶壬d心□00
8
6

Fig．31：Seasonalvariationoftheverticalprofile
Of water temperature at around the
deepestpoint．
−70−ResultsoftheInvestigationofTshoRolpa

fluctuationgraduallydecreasesinamplitudewithincreasingdepthdownto50m．Belowthisdep
thetemperatureseasonallyfluctuatesonlywithin2．6L3．1DCinspiteofthemaximumdensityofpure

Waterbeingrealizedat around40C．Tsho Rolpaisthus characterizedby such a uniquethermal
StruCture aS Shownby the upper activelayersin1ess than50min depth and the deeper quasi−isothermallayer．

Thedeadicebeneaththelakebottommaynotdirectlycontactwithlakewaterbutwiththelakesedimentputtingonthedebris．Thesurfacetemperatureofthesedimentincontactwithlake
Watermayberetainedataround30Cthroughoutayear，Whileboundarytemperaturebetweenthe

debrisandthedeadiceiskeptexactlyatthefreezingpoint、Thedeadicemaybemeltbyconductiv

heatflowthroughthesedimentandthedebris．
AfterlakeicehasdisappearedattheendofApril，1akewaterabsorbssolarradiationatthe
Surface and the surfacelayersinless than around15m are quickly heated up during the pre−mOnSOOnSeaSOnfromMay．TheheatingcontinuesuntilearlyJunejustbeforemeltingseasonan

WarmSuptOarOund40mindepth．Asagreatamountofmeltwaterflowsintothelakeduringthe
monsoonseason，thelayersfromlOmto40mindeptharecooledbytheintrusionofcoldwater

fromtheglacier．1n Septemberneartheend ofmeltingseason，theclearthermocline appear

around15mindepth．Asairtemperaturecontinuouslydecreasesandfallsdownbelowmeltingpointat midpOctober as wellas the decrease of solar radiation，Water temperature also decreases
monotonously．ThelakestartstobefrozenfromtheshoreinearlyNovemberandthesurfaceis
entirelycoveredbyice possiblyin earlyDecember．In February when theice thickness reaches
maximumthicknessofaround50cm，ClearthermoclinecouldappearintheupperlayerinlessthanlOmindepth，Sincetheupperlayeriscooledenough．
Horizontaldistributions of the water temperature profile are shownin Figs．32La and−b，respectivelyobtainedon25June，1994andon3November，1994atthepoints＃Oto＃5inFig．13
Exceptforthepoints＃OandN＃0，theprofilesinJune（Fig．32−a）indicatesimilardistributionswith
adifferenceoflessthanlOCatequivalentdepths．Theprofilesat＃OandN＃0，bothneartheglacierterminus，eXhibitagreattemperaturechangeataround40mindepth．Thesourceofcoldwater
belowthisdepthwi11bediscussedinSection7．5．4．1．TheprofilesinNovember（Fig．32−b）indicate

thattheisothermalsurfacelayersaremadeupduetothewindmixingandcooling，Withaslightdifferenceintemperaturealongthelakelongitudinally．Theseseasonalvariationsintemperature
arerelatedtoheatingand cooling at the surface and dynamic behaviors oflake water．Further
Studiesshouldbeconductedinviewofthelakehydrodynamicsforunderstandinghowtodevelop
SuCh thermalstructure．

∴丁子．J．ごIli／／げれ、川♪tソ血〃∵（J／〟け／一 Jkゴ／†タカ〔・t，Withtheobjectiveofcollectingthebasicdataforalongwaveradiationinaheatbalancestudy
at thelake surface，Water temperature at thelake surface was continuously measuredin the
resolutionofO．lOCduringtheperiodfromtheendofJunetothebeginningofNovember，1994．
Atimeseriesofsurfacewatertemperaturemeasuredatthepoint＃5isshowninFig．33．The

extremelylargediurnalvariationbutrelativelysmalldailyfluctuationwithadailyaverageof5−6−71−

TheGlacierLakeanditsOutburstFloodintheNepalHimalaya

Temperature（℃）

Temperature（℃）
3

4

5

6

7

0

3

2

1

−＃1
一難−・S＃0＝−‥＃2
一一−−＃0…−−・＃2−＃3−＃3

・一◆−＃4一−●一糾＃5■−・・−−＃5

・■

】

0 1 2

Fig．32：Verticalprofilesofwatertemperaturealongthecenterlineofthelake‥a）June，1994．b）November，1994．

︵p︶聖⊃l空乳∈￠ト00U64
NN︑ ︑寸かの▼NO︑Lミ寸ののL

CミOL︑寸のの︻MN︑の○︑寸のの▼CO︑の○︑寸のの†王≦岩︑寸￠の︻のN︑ト○︑寸￠￠︻の○︑ト○︑寸ののLの︻︑￠○︑寸￠の︻Fig．33：Seasonalvariation of water temperature on thelake surface
measuredatthepoint＃5．

−72−

4

ResultsoftheInvestigationofTdlORolpa

OC areseenduringthemonsoonseason．ThedailyaveragestartstodecreasefromearlyOctoberWithdecreasingdiurnalamplitude．
ここ了．7．．フItiJ／けれソ〃♪川〟〃Jで（そ／■0〃坤）〃・ J／仙イ）J′／／（イWatertemperatureattheoutletwasmeasuredtoestimatetheheatlossfromthelake．Dueto
thetopographicalcharacteristicsofthedrainagebasin，almostallthesurfacewatersintheTsho

Rolpabasin，nOtOnlymeltwaterbutalsorainwater，areOnCeabsorbedintotheTrakardingglac
SyStem．Thuswaterwithfreezingpointissuppliedintothelake．Sinceonlyaround20−40L／s
WaterissuppliedintothelakeasasurfacestreamasmentionedinSection7．1．9，temperaturealmostallthesuppliedwatercanberegardedtobeatfreezingpoint．Dischargeattheoutlet，Q
Withtemperature，T，maylosetheheatenergyofcQTfromthelake，Where cisspecificheatof
lakewater．WatertemperaturewasmeasuredinalmostoneyearfromlateJune，1994tolateMay，
1995．

ThewatertemperatureshowscleardailyandseasonalvariationsasshowninFig．34corre−
SpOnding to the variation of air temperature（Fig．18）and solar radiation（Fig．21）．The daily

amplitudeislargerinsummerthaninwinter．Thewatertemperatureiscomparabletoorslightly

lowerthanairtemperatureduringthemonsoonseason，buthigherindryseasonwhenairtempera・

turedecreasesbelowfreezingpoint．EveninthewinterseasonwatertemperatureshowsabovelO
Cbecauseoftheenoughheatstorageinthelakeduringsummerandtheinsulationfromcoolingby
thethickicecoverinwinter．Whilethe water temperature duringthe rainyseasonexhibits analmostconstantvalueof4．5to5．50C，itdrasticallydecreasesduringOctobertomid−Novemberfrom
around50C to around2．50C due to the abrupt decrease of air temperature and solar radiation．
Accordingly，Withthedecreaseofdischargetoaminimuminthenextmid−March，Watertempera

turealsoreachesaminimumofaroundlOCandthenrapidlyincreasesafterlakeicemeltsaway
WhichmayoccurintheendofApril．

■〇
4︵p︶巴⊃l巴乳∈￠ト

6
つJ
0

−73−

トN︑の○︑のの￠L

トN︑†○︑ののの▼のqCO︑のの￠▼りNaO︑のか￠LトN︑岩︑のかのLのN白ミ寸のの▼のN︑；︑寸￠の︻のN︑0ミ寸♂のr￠N︑仇○︑寸￠のr

Oeの○︑寸のの▼†eト0︑寸のの†LO︑ト○︑寸功の︻

1﹁0︑￠○︑寸のの▼2

Fig．34：Seasonalvariationofwatertemperatureattheoutlet．

TheGlacierLakeanditsOutburstFloodintheNepalHimalaya

Itisevidentthattheoutflowwaterisslightlycoolerthanthelakesurfacewaterincomparison

Fig．33withFig．34duringtheendofJunetothemiddleofOctober．Thisisbecausethelakesurf

isheatedbysolarradiationbuttheoutflowwaterpartiallyincludesdeepercoldwaterofthelake．
7．5．2Albedo of the Lake Surface

Albedo ofthelake surface was measuredin the condition ofsurface water SSC（suspended

particleconcentration）of157mg／LinJune，1994atthepoint＃5（seeFig．13）includingfine，Cloand rainy period at the same point as the measurement of surface water temperature．Solarelevationatthetimewhenalbedowasobservediscalculated．Therelationshipbetweenalbedo，α，
andsolarelevation，h，indegreeisshowninFig．35．Theempiricalequationderivedisexpressedas
α＝0，785h▲0・451．ThevalueofalbedowastheorderofsomeO．15inaverageduringdaytime．
0．35
0．30
0．25

年0．20

0

て】
q〉

⊇ 0．15
＜
0．10
0．05
0．00
0

10

20

30

40

50

60

70

80

90

Solarelevation，h（degree）
Fig．35：Relationshipbetweenalbedo andsolarelevation．

7．5．3AnnualCycleoftheThermalRegimeonthe Lake

Thethermalregimeofthelakeischaracterizedbythelimitedperiodofheatabsorptionin】a
Water．Astheresults ofheatbalancestudy ofthelake（Sakaiand Yamada，1997），1ake wateris

mainlyheatedupbysolarradiationinthepremOnSOOnperiodbetweenlateApril，justafterlake
iceformedduringcoldwinterseasonmeltingawayandopenwaterexposedundersunshine，and
mid−June，justbeforetheheavymeltingseasoncoming．Solarradiationisthemainheatsourcefor

lakewater：otherheatbalancetermsarenegligiblysmallthroughoutayear．Lakewaterstores

greatamountofheatduringpre−mOnSOOnperiod，thenmonotonouslylosesthestoredheatinthe
Other seasons．

Thoughthehotsummerseasonprovidesalotofheattolakewaterbyrelativelystrongsolar
radiation，agreatdealofmeltwater（0℃）suppliedintothelakeconsiderablycontributestocool

lakewaterenough，Thissituationstartsfrommid−JuneandcontinuestolateSeptemberbythet

WhensolarradiationdecreasesandairtemperaturefalIsbelowfreezingpoint．Thelakestartsto−74一

ResultsoftheInvestigationofTshoRolpa

freezein early November andis completely covered byice bylate November．Ice thickness

increasesmorethan50cmbytheendofnextMarch（Kadota，1994）．FromlateSeptemberthroughWinterlakewateriswarmenoughcomparingwithsurroundingclimate．Lakewatersurelyloses

StOredheatinone−Sided．LakeicestartstomeltinearlyAprilandcompletelydisappearsbyla
April（personalcommunicationwithDr．T．Kadota）．Storedheatinthelakeisminimizedint
SeaSOn．

7・5・4SuspendedParticIesinthe Lake

TransparencyoflakewaterislessthanlOcmbecauseofhighturbidityduetofinesuspended
particles．Ifahandisinsertedverticallyintothelaketothewrist，thefingertipsarenotvisible．

Sincethesuspendedparticlesinlakewatermightstronglyinfluencethephysicalpropertiesof
thelake，SuSpendedparticleconcentration（SSCmg／L），itsparticlesizedistribution，Sedimentation

rateandparticlesizedistributionofbottomsedimentwereinvestigatedatvariouspointsinthelake
lnVar10uSSeaSOnS．

WaterwassampledbytheVanDorn−typeWaterSampleranditsturbiditywasmeasuredby
Turbidity Meter oflighttransmissiontype．The turbiditywas convertedinto SSC obtainedby
filteringthesampledwaterwithanopeningofOA5JLmm．
75．4．J50〟作g〆鋤e乃（わdfもγJグcわざ

Therearetwosourcesofthesuspendedparticles；oneisturbidwaterpouringintothelake
fromtheen−glacialmouth（s）ofTrakardingglacierandtheotheristhelateralmorainesinducing

almostincessantcollapseasseenclearlyinPhoto24．

AsshownpreviouslyinFig．32−a，thesharpdiscontinuityinwatertemperaturewasfoundatthe
depthofca．40matthepoints＃O andN＃OinJune，1994．Atthesametime，SSCmeasurements
WeremadeatN＃0・Infact，aSShowninFig．37innextSection，SSCabruptlychangedfrom260mg

Latadepthof40mto900−1200mg／Lbelow50m．Thisclearlyindicatesthatcold（00C）andhighturbidwaterflowsintothelakethroughtheen−glacialmouth（s）andoccupiesthelowerlayerbelow
40m・Themouthisprobablysituatedinalocallylimitedpositionbecausesuchacoldandhigh1yturbidwaterwasfoundatonlytwopointsamongthefivepointsobservedtransversallyalongthe

glacierterminus．Accordingtotheresultofdepthsoundingneartheglacierterminus，abotto

Channelcanbedetected；aropeorastringsuspendingasensorwasofteninclineddownstream−Wardduringtheobservations．Unfortunately，thesizeofthechannelwasnotobservedbutsurely

morethanatleastlOmindepthandafewtenmeterinwidth．
The cold and turbid water supplied from the glacier included relatively coarse suspended
particlesasshowninFig．39dofSection7．5．4．3．
こJ・J・ご11リノ／（、（J／丹（帝／ −∫（！／■∫J吋九リノ ／（、（7∫ ・√／／〃け〃／（、りJJ汀〃／＝晶JJ〟〃（／βJ′仏几一昭巾・

TheverticalprofilesofSSCwereobtainedinthevariousseasonsataroundthedeepestpoint

asshowninFig．36．TheSSCincreasesalmostlinearlywithdepthinanyseason，Whiletheverti

profile slightly fluctuates seasonally．Absolute value of SSC clearlyincreases fromJune to−75−

The Glacier Lake and its Outburst Flood in the Nepal Himalaya
SuspendedSedimentConcentration（mg／L）

0 100 200 300 400 500 600 700 800−−○−02／22爬叫−−■−06／09／94

Fig．36：Seasonalvariationoftheverticalprofile
Ofsuspendedsedimentconcentration（SSC）
ataroundthedeepestpoint．

November・SSCmayincreaseduringthemonsoonseasonduetotherelativelygreatsupplyofturbid

Waterfromtheglacierandmorainematerialssuppliedbycollapseofthelateralmoraines．During
Winterandbeforemeltingseason，COnSiderablenetdepositionofsuspendedparticlesonthelake
bottomcouldoccurbecauseofaverylittlewaterinflowandnoactivecollapseofthefrozenlateralmoraines・ThusSSCgraduallydecreasesduringwinterandthenisretrievedtothelevelofprevious

Juneafterthestartofthemeltingseason．

ThehorizontalvariationofSSCprofileisshowninFig．37．TheprofilesweremeasuredinJune，
1994atthepointsN＃0，i2and＃3andNovember，1994at＃0，＃2，＃3and＃5（seeFig．13）．

profileinJuneshowsacleardifferencefrompointto pointbut thosein Novemberindicateno

remarkabledifference・TheuniqueSSCdistributionatthepointN＃Oisalreadydiscussedinjus

the previous Section．Those horizontalvariations may be understood by considering seasonal
Variationsinユakedynamicsandtheseasonalsupplyingprocessesofsuspendedparticユes．

Thoughthedensityofpurewaterisdeterminedbyonlywatertemperature，bulkwaterdensity
Ofthelakeisconsiderablyinfluencedbysuspendedparticles・Thebulkdensitypwiscalculatedby
thefollowingequation：
β山＝（1C・10￣6／βg）β∂十C・10￣6，−76−

Results of the Investigation of Tsho Rolpa
SuspendedSedimentConcentration（mg／し）

0 200 400 600 800 100012001400

Fig．37：SSC profiles measuredlongitudinallyalong thelakeinJune and November，
1994．

WhereCisaSSCinmg／L，Psistheparticledensitying／cm3andp。isthepurewaterdensityat

temperatureofeOC．SincewatertemperaturewassimultaneouslymeasuredwithSSC，thedensit
profilewaseasilycalculated．Theparticledensitypg wasassumedtobe2．65g／cm3．
ThebulkdensityprofilesareshowninFig．38．Theprofilepatternsarequitesimilartotho
OfFig．36becausethebulkdensitypd，isstronglyinfluencedbySSC．Thebulkdensitythusexhibits

amonotonousincreasewithdepththroughoutayear，irrespectiveofthelargeseasonalfluctuations
Ofwatertemperatureinthelayerslessthan50mindepthasshowninFig．31．Itshouldbenoted

thatthestablestratificationoflakewaterthroughoutayearisremarkablecharacteristicsofhighturbidlake．Suspended particles act as the obstacle ofwater convectioninthelake．The stabl

Stratificationthroughalltheseasonsputsusonequestion：Howcantheheatstoredinthesurface
layersbetransportedtothelakebottomto keeptemperaturealmostconstant around3OC？The
bottomwaterisconsideredtobecooledbydeadicemelting．Suchheattransportmayneedwind
drivenactionoflakewatersinceaheatsourcetomeltthedeadicebeneaththelakebottomisonly

the surplus of heat balance at thelake surface，Whichis mainlygiven by solar radiation．Th

dynamicprocessesoflakewatermaybethekeytounderstandtheheattransportfromsurfacetothebottom．

一77−

TheGlacierLakeanditsOutburst Floodinthe NepalHimalaya

WaterDensity（pw−1）xlO5
0

10

20

30

40

Fig．38：Seasonalvariationoftheverticalprofile
Ofbulkdensity．

75．4．3滋おム廟減毎払乃〆5毎匝加如り物砿滋AseasonalvariationofparticlesizedistributionwithdepthisshowninFigs．39−atO−d．Figure39−aWaSObtainedataroundthedeepestpointon26June，1994；Fig．39−band−CarOundthela
Centeratthepoint＃3′，On13November，1993andon22February，1994，reSpeCtively．Figure39−d
WaSObtainedatthepointN＃00n27June，1994，WheretheprofilesofwatertemperatureandSSC
WereSimultaneouslymeasuredasalreadyshowninFigs．32−aand37．

Theparticlesizedistributions areshown asrelationsbetweenparticlesizeinphiscaleandfrequencypercent（wt．％）ofeachsizefraction．Thephiscaleisdefinedas少＝一log2d，Wheredis
a particle diameterinmm．Thisscalemeansthat thelarger the−Value thesmaller the particle
diameter・Theparticleswiththediameterof2mm（少＝−1．0）tol／16mm（Q＝4．0）aredefinedas
Sand，thosewithl／16mmtol／256mm（Q＝8．0）assiltandthosesmallerthanl／256mm（orlarger
than8in少）as c物′．

Suspendedparticlesmostlyconsistofsiltandclay．Thisindicatesthatsandandmorecoarser
particlesaresettledoutonthelakebottomimmediately，becausetheirsettlingvelocityistoolarge

tobesuspendedinwater．AttheendofJunewhenmuchmeltwaterissuppliedintothelake，ShowninFig．39−a，thelakehasadistincttwo1ayerstructuresuchastheupperclay−richandsilt−pOOrlayerabove40mindepthandthelowerclayeysiltlayerbelow40m．Thisstructuremaybe

retainedduringtheheavymeltingseasoninthemonsoonperiodwhenmanysuspendedparticlesare−78一ResultsoftheInvestigationofTshoRolpa
％
20
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C）atthepoint＃3 inFebruary，1994；d）atthepointN＃0，neartheglacierterminusinJune，1994．

Suppliedintothelake，AttheendofNovemberwhenairtemperaturefallsbelowfreezingpointand
theparticlesupplydiminishes，thetwolayerstructuredisappearsandalmostthesamedistribution

patternscanbeseenthroughallthedepthasshowninFig．39b．Intheice−COVeredperiod，aSShowninFig・39LC，relativelycoarseparticlesintheupperlayerareeasilysettleddowntothedeeperlayer，SOthattheupperthelayerthefinertheparticles．
Theparticlesizedistributionsneartheglacierterminusshowcomplexfeatureswithdepthas

ShowninFig・39−d・Mostoftheparticlesinthelayersabove40mareclayorsiltyclay，Whichi
thesimilartothoseataroundthedeepestpoint（Fig．39−a）．Thissuggeststhatthesizedistributio

intheupperlayeratN＃O aredeterminedbytheadvectivesedimentdiffusioninducedbywind−drivencurrents（Chikitaetal．，1997）（seeFigs．23and24）．Theparticlesatthedepthsof50ma

60mareclayeysiltwithasmallamountofsand・Thesecoarseparticleswereclearlysuppliedby
Sedimenト1adenunderflow，akindofdensitycurrents，Originatingfromtheen−glacialmouth（s）．
−79−

TheGlacierLakeanditsOutburstFloodintheNepalHimalaya

Suspendedparticlesinoutflowwaterattheoutletconsistalmostofclay（Q＞8）asshownin
Fig．40−a，Sincecoarseparticlesaresettledoutonthelakebottomonthewaytotheoutlet・Intheleakagewaterflowingoutonthe outerslope oftheendmoraine（ⅩmarksinFig・16），thesize
distributionissimilartothatattheoutletasshowninFig．40−b．Itsuggeststhatleakagewaterdoes

notcomefromthemeltofthedeadiceburiedinthemorainebutfromlakewaterdirectly．

a）

n
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⊂） の 0 1∫） ⊂） l∫） ○ の ⊂〉 の ⊂〉 の ⊂＞ の ⊂〉 の ⊂〉 寸 寸 の 頂 くD くD 卜＿ 卜、 の の の ○） ⊂〉 ⊂） ▼ ▼− N▼￣▼￣【▼￣▼￣Particlesize（d））
Fig．40：Particlesizedistributionofa）outflowwaterattheoutletこb）1eakagewater．

7．5．5Bottom Sediment of theJake

Thebottomsedimentwassampledatthepoints＃1to＃5．At＃1，Onlyafewgravelweretaken
outfromthebottom．Nobottomsedimentwasfound．Thatmaybeduetonotenoughtimetocau

sedimentation after this areais converted to become thelake，Or the existence of too strong

underflowcomingfromtheglaciertoaccumulateparticlesofsand，Siltandclay．Infact，1akewatervelocityofaboutO．5m／swasmeasuredatthebottomlayerneartheglacierterminus（personalcommunicationwithK．Chikita，HokkaidoUniversity）．Thesedimentoftheotherpointsconsistsof
sand，SiltandclayasshowninFig．41−a．Thesamplingpoints＃1to＃5areapproximatelylocatedato．5km，1km，1．5km，2kmand2．5kmfarfromtheglacierterminus，reSpeCtivelyas
Fig．13．Asfarawayfromtheglacierterminus，theratioofsandandcoarsesilt（Q≦5）decreases
andthatofthefinesiltandclay（Q＞6）increases．Itisadistinctcharacteristicsthatthemedium
siltof5＜少≦6isverypoororlackinthebottomsedimentatallthepoints．Itisevidentthatth
sedimentsortingoccursaccordingtothedistancefromtheglacierterminus・Sincethesedimenta−
tionofsandandcoarsesiltalmostdisappearsinbetweenthepoints＃4and＃5，theconsiderabl
sedimentsortingisterminatedinbetween2to2．5kmfromtheglacierterminus・−80−ResultsoftheInvestigationofTsho Rolpa

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7．5．6Sedimentation Rate

Suspendedparticlesarecontinuouslydepositedonthelakebottom．Thesedimentationrate（g／Cm2）wasmeasuredbyfixingcylindricalsedimenttrapswiththediameteroflOcmnearthebottom
Ofthepoint＃5during8to13November，1993．Thedailysedimentationrateisobservedas6．18×

10￣3g／cm2．SincetheverticalprofilesofSSCdonotshowgreatseasonalandhorizontalvariation
asshowninFigs．36and37，theannualsedimentationrateatthepoint＃5couldbeestimatedas2．26
g／cm2．Ifthisratecanbeadoptedoverthelakebasinwiththeareaofl．39km2，theannualamoun
Ofdepositisderivedtobeoftheorderof3×104tonoverthelakebasin．Sincehighersedimentation

rateis expected near the glacier terminus area due to sedimentation by the sediment−1aden
underflow，theannualamountoftotaldepositinthelakebasinisexpectedtobeactuallylargerthan
the above value．

Theannualamountofsuspendedparticlesintheinflowwaterisestimatedtobeatleastofthe

OrderoflOXlO4ton，SinceSSCofinflowwaterismorethanlOOOmg／LasmentionedinSection7．
5．4．1andtheannualwaterdischargethroughtheoutletwasalreadyestimatedastheorderoflO

millionm3whichisexactlyequivalenttotheannualinflowfromtheglacier．Thesedimentsupplied

from thelateralmoraines andthe unnamedglacier shouldbe takeninto account．Thelatteris，
however，negligiblysmall．TheformerisestimatedtobeoftheorderoflxlO4tonifthesurface
Ofthelateralmorainecollapsingisassumedas450，000m2；itsrateoferosionisannually oft
Orderoflcm；thedensityofthemoraineitselfisabout2g／cm3．SSCoftheoutflowwateratthe
Outletislessthan200mg／L．Thusannualsedimentlossfromthelakeisevaluatedaslessthan2×

104ton．Finallyannualnetbalanceofsuspendedparticlesisevaluatedtoamounttomorethan（
L2）×104ton．Itbringsanaveragedepositof6．5g／cm20rabout3．2cminthicknessoverthelake
bottom ofl．39km2in area．
−81−TheGlacierLakeanditsOutburstFloodinthe NepalHimalaya

Theannualamountofdeposit，2．26g／cm20btainedfromthesedimenttrapatthepoint＃5is
toosmallcomparedwith6．5g／cm20btainedabove．Thisindicatethatnearertheglaciermoret
deposition．Largedepositionofsuspendedparticlesaswellassand／coarsesiltparticlesma

bemadeinuniformbutoccurhigherrateintheuplakearea．

Theparticlesizedistributionofsedimenttrappedatthepoint＃5isshowninFig．41pb．Th
sediment少＝6．Otollinsizeisdominantwithunimodality．Thisdistributionpatternisreasonably
similartothatofbottomsedimentat＃5asshowninFig．41a．NotethephiscaleinFig．41−bis
differentfromthatinFig．41−a．
7．6ChemistryofWater，SnowandIceRelatedtoTshoRolpa

Inordertoobtainthebasicinformationonthechemicalbackgroundofthelakeandaroundthelake，WaterSampleswerecollectedinJuneandJulyof1994andtheirchemistrywasexamined．Th
Samplings were madefrom thelake water，pOnd water on the Trakardingglacier，SnOWin theaccumulationareaoftheTrambauglacier，aSupra−glacialstreamwaterontheTrambauglacier
Trambauglacierice，Outflowwaterattheoutlet，leakagewaterformtheendmoraine，rainwat

intheraingaugeinstalledontheInstrumentIslandandriverwaterofRoIwalingKholaandTama
Kosi．Theionicconcentrations，electricconductivityandpHweremeasuredinthelaboratory．The
results are shownin Table7．

Itis evident that precipitationin this areais relatively acidic butionically pure withlow

electric conductivity（EC）．Lake wateris relatively richin HCO3￣，Ca2＋and SO42J，and also
relativelyhighinECwithavalueabout40JLS／cm．Thisiscommontooutletwater，1eakagewate
andriverwater．ThatisprobablybecauseterrestrialmaterialswithionsrichinHCO，P，Ca2＋and
SO．2−inducethewaterswithsuchionicconcentrationandhighvalueofEC．
TheionofPO。3．isnottracedinallthesamplesandNH4＋concentrationisverypoorexcept
inthepondwater，SnOW，deadiceandrainwater．ThelakeandriverwatersarenearneutralatpH
Of6Ato6．9，Whiletheothersamplesindicateaslightlyacidicproperty．
7．7Present Growth Rate ofthe Lake

TshoRolpahasbeencontinuouslygrowingsinceabout45yearsagoasdiscussedinSection6．

3．3．Thelakeexpandsinthelongitudinaldirectionowingtotheretreatingoftheglacieranda
deepeningbyvirtueofdeadicemelting．Thelakehasnospacetoexpandforthelateraldirection

Sinceboththesidesofthelakeconfinedbythelateralmoraines．Presentgrowthrateisdiscussed
in this Section．

Thelongitudinalgrowthofthelakeisnotduetomeltingofthecliff−Shapedterminusbutrather

duetothecollapseofthecliff，i．e．calvingasmentionedinSection7．1．5．Thecalvingresultsint

lake growing upstreamLWard．Survey ofthe contactline between thelake and the terminus ofTrakardingglacierispracticallydifficultbecauseitisnotpossibletoapproachtheglaciertermi−

nus，eitherfromthelakesideorfromtheunstableglaciersidewithoutincurringveryhighrisk
Therefore，thephotographsofthecontactlinehadbeentakenfromtheleftsidebank．AsshowninPhoto34，thecontactlinehasshiftedupstreamyearafteryear．Thegrowthrateofthelakeis−82−

ResultsoftheInvestigationofTsho Rolpa

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TheGlacierLakeanditsOl】tburstFloodintheNepalHimalaya

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ddtO出〇一S↑︸OResults oftheInvestigation of Tsho Rolpa

Photo34：Expansion ofthelakeandretreat oftheglacierduetocalvlng：a）19November1993．b）40ctober1995．

estimatedatsome200mduringabout2yearsfrom19November，1993（Photo34−a）to40ctober，1995（Photo34b），Whichwasobtainedbysurveyingthepositionsofcharacteristiclandmarkssuch

ascracksonthebedrock，talusetc．ontherightsidebankbehindthecontactline；annualgrow

rateisroughlyestimatedaslOOm．Sincethelakewasborn45yearsagoandhasgrownto3．

inlengthsofar，theaverageannualrateofexpansionisabout70m，SOthatrecentgrowthratema
be rather accelerated．

AsdiscussedbyChikitaet al．（1997），aCtivecalvingmaybecausedbytheeffectivemeltingof

the cliffrOOt at the glacier terminus due to wind driven surface current of wellwarmed water．
SurfacewaterwarmedbysolarradiationisdriftedtothecliffShapedglacierterminusbytheup
Valleywindduringdaytime．
ThedeepeningrateofthelakecanberneaSuredbydepthmeasurementsatthefixedpointsor
fixedlinessetoutinthelake．Itistheoreticallyeasybutpracticallynotpossibleexceptintheareas
near the end moraine and islands due to the difficulty of positioning the fixed points and of re
meaSuringthedepthatthesamepoint，becausethelakeshoresatthelateralmorainesarevery
riskytoapproach．Thelakeisboundedonbothsidesbythelooseandunstablelateralmoraineswith
COnSiderablysteep slopes（Photo24），Whichiscollapsing almostcontinuouslyinto thelake，eSpe−

CiallylnSummermOnSOOnSeaSOn．Theglacierterminusisalsoverydangerousduetoitscalvlng．
85

TheGlacier Lake anditsOutburst Floodin the NepalHimalaya

Onecannotfindanysuchfixedpointsinthelakeexceptonlyinthewinterseasonwhenthelake
becomescoveredbythickice．
Duetotheabovelimitations，thedepthprofilesweremeasuredalongafixedlinebetweenLama
IslandandMiddleIslandasillustratedbydashedlineinFig．16．Thedepthprofilesindicatethatthe

deepenlngisstilloccurrlngaSShowninFig．42andthedeepeningrateof48．1cminaveragethro

themeasurementlineisobtainedduringtheperiodfrom14November，1993to30ctober，1995．I

COrreSpOndstotheannualdeepeningrateof25．5cm．Theislandsareconfirmedtobealsosinkin

intothelakeduetothemeltingofdeadicebeneaththeislands．
Themeasureddeepeningratebetweenislandsisrathersmallascomparedtothatatthedeepest
pointasdiscussedinSection6．3．3．Thatisbecausethedebrisactingasaeffectiveheatinsulatoris

thicker nearthedownlake area than atthe deepest point．The significant fact derived from thedeepenlngrate meaSurementSeStablishesthat，beyonddoubt，thelakebottomisstilldeepening．

Lama lsland
O

Middlelsland

Distance（m）
lO

20

30

40

50

60

70

80

46

0
0
0
0

︵∈0︶壬d￠□Fig．42：Deepening of thelake by means of bottom melting along the surveyline
Shownbythedashedlinein Fig．16．

7．8．Investigation ofaSupraglacialPondasan Embryo ofa Glacier Lake

Supraglacia11akeshavedevelopedfrompondsontheglacier．Thepresentphysicalstatusof
thepondsandtheirchangewiththelapseoftimeareessentiallyimportantforunderstandingthe
quickdevelopmentmechanismofsuchglacierlakes．ThepondsontheTrakardingglacierlocatedneartheglacierterminuswereinspectedonJune，1994．Sevenpondswerefoundwithinl．5kmfr

theglacierterminuswithdifferentcolorandturbidityduetothedifferenceintheirageandexisting

COndition．AtthepondchosenastyplCalamongthem，Observationswereconductedonthesizean
depth ofthepond，Watertemperature Ofthepondsurface，Verticalprofilesofwatertemperature，
SuSpendedsedimentconcentrationandelectricconductivity．Thepondhadunfortunatelydisappear，

eddueto drainingout throughsomecrack（s）intheicebodyconstitutingthepondbasinwhenr−86−ResultsofthelnvestigationofTshoRolpa

meaSurementWaSattemptedinOctober，1994．Fortheconvenienceoffutureinvestigations，Only

meaningfuldata areshownhere．
Thepondislocatedafewhundredmetersupstreamoftheglacierterminusandsurroundedby
exposediceWallwiththearea of4075m2．Themaximumdepthismeasuredtobe9m andthe
averageisroughlyestimatedas5．5mandthewaterstoredinthepondiscalculatedtobe22，400
m3．Therewerenovisiblesurfaceflowsintoandoutofthepond．
Duringtheobservationperiodfrom19to23June，1994，thepondlevelincreasedalmostlinearlyby31cm，Whichcorrespondsto316m30fmeltwaterinthedailyaveragesuppliedintothepondthroughthe en−glacialchannel（s）．The changein the volume of water during those4daysis
equivalentto5．6％oftotalwaterstoredinthepond．Sincethetemperatureofsuppliedwateri

thepondisassumedtobeatfreezingpoint，itcouldconsiderablyaffectthethermalregimeofthe
pond．
The surface temperature of the pondis shownin Fig．43．In comparisonwith the surface

temperatureofTshoRolpa（seeFig．33），Whichisaround6OC，itshowsaremarkablylowvalue

3・5±20Cbecauseofmuchinflowofmeltwaterwithfreezingpointandalsothecoolingeffectof

glaciericeunderandsurroundingthepond．Thetemperatureprofilesatthedeepestpointisshownin Fig．44；average water temperaturethroughdepthincreasedfrom3．30C on20June12：46to
3・40Con23June14：30．Duringthisperiod，heatbalanceofthepondisevaluatedbyassumingt

pondtoberectangularinshape，uniformicemeltingtooccuroverthepondbottomandtheheat
receivedinthepondimmediatelytobeconsumedbyicemelting．Theheatbalanceisevaluatedto
bepositive from the meteorologicaldataconcurrently obtained．Daily meltingrate of5．5cmis

estimated．Iftherateretainsthroughthehotsummerseasonforabout4months，6．6mofbottom

icemaybemeltedout．Therealmeltingrateshouldbedirectlymeasuredtovalidatethe above

meltingrateandassumptions・Themechanismofheattransfertothepondbottomshouldalsobe
Studiedbyfurtherinvestigation．Itis，atleast，Clarifiedthatapondisaneffectiveheatabsorber．

1 0︵UD︶巴コle﹂監∈￠ト

5432
CN︑￠○

○？00

00‖00

NN︑￠○

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○？00

00⁚00

0N︑￠○

00﹂00

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Fig．43：Surfacewatertemperatureofapond onthe Trakardingglacier．

ー87

TheGlacierLakeanditsOutburst Floodinthe NepalIiimalaya

Temperature（OC）
0 1 2 3 4 5︵∈︶￡dむ凸
5

⊥
6

7

8

9

Fig．44：Verticalprofilesofwatertemper−
atureofthepondontheTrakarding
glacier．

Thepr

Ofiles ofsuspended sediment concentration（SSC）measuredin the pond areshownin

Table8asanexampleforfuturereference．ThevalueofSSCisremarkablylessthanthatofTsho

Rolpa．ThusbulkdensityratherdependsmoreonwatertemperaturebutlessonSSC，Whichderive

active convection and then effective heat transfer from the surface to the bottom．
Table8：Verticalprofileofsuspendedsedimentconcentrationinpondwaterof
the Trakardingglacier．
Depth（m）

0

1

2

3

4

5

5．3（bottom）

Turbidity（mg／L） 18．8 18．4 17．8 17．4 17．O 17．2 16．5

As mentionedin Section6．3．5，if a pondlastslong enoughand some thermalregime goesbeyondthecriticalcondition，apOndstartstogrowupwideranddeepertobecomeasupraTglacia1
1ake．Existenceofwaterwithfreesurfaceontheglacierdrasticallychangesthethermalcondition
oftheglaciersurface．Itcanabsorbmoreheatthanthatonthedebriscoveredsurface．

88一