MAPPINGHAZARDSIN

NEPAL’SMELAMCHIRIVER

CATCHMENTTOENHANCE

KATHMANDU’SWATERSECURITY

NOVEMBER2023

ASIANDEVELOPMENTBANK

MAPPINGHAZARDSIN

NEPAL’SMELAMCHIRIVER

CATCHMENTTOENHANCE

KATHMANDU’SWATERSECURITY

NOVEMBER2023

ASIANDEVELOPMENTBANK

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?2023AsianDevelopmentBank

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ISBN978-92-9270-467-4(print);978-92-9270-468-1(electronic);978-92-9270-469-8(ebook)

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DOI:

/10.22617/TCS230541-2

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CoverdesignbyColorsPrint&Production,Bhotahity,Kathmandu,Nepal

Coverphoto:“MelamchiHeadworkssiteaftertheMonsoon2021floodevents”.

(PhotobyMelamchiWaterSupplyDevelopmentBoard(MWSDB)

EditedbySophieHares.

CONTENTS

Tables,Figures,andMapv

Forewords

vii

Acknowledgments

x

Abbreviations

xi

ExecutiveSummaryxiii

1.Introduction

1

ProjectBackground1

EventsandTheirImpacts1

ObjectivesandScopeofAssessmentStudy4

SummaryofMethodology6

2.FieldSurveysandDataCollection7

SiteReconnaissanceVisits7

FieldSurveys8

LiteratureReview13

3.MelamchiCatchmentAnalyses

16

RegionalGeologyoftheCatchment1

6

Geomorphology2

0

LandslideDistribution2

3

GlacierDistributionMap2

5

LandDisplacementAnalysesthroughSARSatellites2

6

LandslidesCausedby2015GorkhaEarthquake2

6

LandslideSusceptibility2

7

4.MelamchiRiverAnalyses

32

HydrologicalSurveysandModeling3

2

HydrologicalAnalysisof2021FloodEvents

40

RiverHydraulicsandSedimentTransportModelSetup

41

RiverHydraulicsandSedimentTransportSimulations4

6

AnalysisofDifferentInfluxScenarios

49

C0NTENTS

iv

5.EstablishmentofNewDesignBasis5

2

Multi-HazardRiskAssessment5

2

DecisionTreeAnalysisforMulti-HazardAssessment

55

ResultsofMulti-HazardRiskAssessment5

7

ResiliencethroughCombinationsofRiskMitigationMeasures6

0

NewCriteriaandApproach6

2

LandslideDamLakesandGlacialLakeOutburstFloods6

3

FlashFloodsfromCloudbursts6

7

ProbableMaximumFlood6

7

SummaryofMaximumDischargeCalculations6

8

ImpactsofClimateChange

69

6.StabilizationandMitigationMeasures

72

SlopeandLandslideMonitoring

72

ManagementofLandslideHazards7

6

StabilizationofLandslides

78

RiverChannelMonitoring

86

ManagementofFlow,Flood,Sediment,andDebris

86

StabilizationofRiverChannel

88

PreliminaryHeadworksDesignConsiderations

89

SummaryandRecommendations

90

Summary

90

Recommendations

90

References9

3

TABLES,FIGURES,ANDMAP

Tables

1ScopeofConsultingServices,Phasing,andKeyTasks

5

2EstimatedLong-TermAverageMonthlyHeadworksStreamflow(m3/s)

36

3EstimatedLowFlows(m3/s)atMRWDSHeadworksSite

36

4EstimationofSedimentBudget,BhemathanKnickpointtoMelamchiBazaar

42

5SimulatedSedimentTransportUpstream,DownstreamofHeadworksArea

51

6CombinationsofMitigationMeasurestoBeFurtherInvestigated

60

7ReservoirStorageVolumewithDamBreakOutburstFloodCalculation

65

8SummaryofPMOFCalculations

68

9LandslideMonitoringintheMelamchiCatchment

75

10ExtendedMonitoringatDifferentSitesinCaseofEarlyHazardWarning

77

11RequirementsofMorphologicalMonitoringofMelamchiRiver

84

12Hydro-Meteorological,MorphologyMonitoringParameters

85

13ManagementOptionsagainstAdverseRiverMorphologyEvents

87

Figures

1EventsLeadingUptothe2021MelamchiBasinFloods

2

2AerialImagesofFloodImpactsattheBhemathanDeposit

8

3ParticleSizeDistributionofSedimentSamplesfromHeadworksArea

10

4SeismicRefractionTomographySurveyinBhemathan

12

5RegionalGeologicalMapofBagmati-GosainkundRegion

17

6GeologicalMapoftheMelamchiCatchment

19

7GeomorphologicalMapofMelamchiRiverUpperCatchment

20

8DownstreamViewofGorgeAreaandHeadworks

22

9UpstreamViewofHeadworksandGorgeArea

23

10MelamchiCatchmentLandslideInventoryMap

24

11FourGlacialLakesintheUpstreamMelamchiCatchment

25

12GlacialLakesInventoryMap

26

13SomeoftheFactorMapsUsedforLandslideSusceptibilityModels

29

14MelamchiCatchmentLandslideSusceptibilityMap

29

15Identificationof15CriticalMelamchiRiverLandslides

30

16MelamchiCatchmentTopographyandHydro-MeteorologicalStations

33

17ObservedandSimulatedHydrograph,DailyAveragedValues,1993-1996

35

TABLES,FIGURES,ANDMAP

vi

18ObservedandSimulatedHydrograph,MonthlyAveragedValues,1993-1996

35

19ProbableMaximumFloodatMelamchiHeadworks,HourlyValues

37

20ComparisonofPMFBetweenProjectsinNepal

38

21ProposedPMOFApproachforEstimatingMaximumDischarge

39

22RainfallDatafromJunetoAugust2021inMelamchiCatchment

40

23ErroneousBed-LevelChangesfromDEMvsImprovedFieldEstimates

42

24ModelDomainfor1DSedimentTransportModeling

44

25LongitudinalProfileofKeyRiverStretches

45

26LongitudinalSimulatedBed-LevelChange,HeadworkstoNakote

47

27ErosionalandDepositionalTrendTriggeredbythe2021DebrisFloodEvent

47

28HeadworksShownPartiallyBuriedafterJune2021Flood

48

29SimulatedBed-LevelChanges(Deposition)NeartheHeadworks

49

30SimulatedWaterDischarge(Influx)Scenarios

50

31Headworks-NakoteBed-LevelChangewithDifferentDischargeScenarios

50

32CumulationSedimentTransportwithDifferentInfluxScenarios

51

33FloodWaveat2,700m3/s-NarrowGorgetoDownstreamofHeadworks

52

34EventTreeAnalysisofPotentialMelamchiRiverBasinHazardScenarios

54

35DecisionTreeAnalysisofMelamchiCatchmentDisasterSusceptibility

55

36LongitudinalProfileofPredictedMelamchiMulti-HazardIndex

56

37CatchmentMulti-HazardZonationMapUsingDecisionTreeAnalysis

57

38VulnerabilityofElement-at-RiskasaFunctionofHazardIntensity

59

39RiskMatrixIllustratingHazardFrequencyandImpacts

59

40GlacierattheUpperReachoftheMelamchiRiver

64

41DischargefromSimulatedBhemathanLakeOutburst

66

42ExtentofPermafrostintheMelamchiCatchment

71

43UsingaLong-RangeLIDARScannerandInclinometertoDetectMotion

73

44TensionCracksAbovetheMelamchiLeftBankLandslide

74

45TheBhemathanKnickpointSeenfromDownstream

79

46MelamchiRiverProfilefromBhemathanKnickpointBasedonDEM

79

47TypicalCross-SectionofGabionWalltoBeConstructedatLandslideToe

81

Map

LocationoftheMelamchiRiverBasin

xiv

Forewords

Nepalhasalonghistoryofdisasterscausedbyearthquakes,

floods,andlandslides.The2021dualfloodevents,triggeredby

theimpactsofclimatechangeandexacerbatedbyvulnerable

Himalayangeology,ledtomassivelandslidesanddebrisfloodsin

theMelamchiRivercatchmentarea.Approximately17peoplewere

killedanddownstreamsettlementsandinfrastructuredevastated,

includingtheheadworksoftheMelamchiWaterSupplyProject

(MWSP)thatwasreadytobecommissionedinSeptember2021.

TheMWSPplaysacentralroleinensuringthedrinkingwater

security,health,andwell-beingofthepeopleoftheKathmanduValley.Damagetothelong-awaitedprojecthighlightedthevulnerabilityofNepal’sinfrastructureandunderscoredtheeffortsoftheGovernmentofNepaltoreducedisasterriskandprotectitsdevelopmentgains.

ItisvitaltoensurethatallnecessarymeasuresaretakentoinsulateimportantprojectsliketheMWSPagainstfuturecatastrophichazards,particularlyasclimatechangetakeseffect.

ThistechnicalreportisanimportantstepinunderstandingthecausesandeffectsofdisasterslikethoseseenintheMelamchiRivercatchment.ItidentifiestherootcausesoftheMelamchidebrisflowsandlaysdownthegroundworkforeffortstorepurposeorreconstructthecriticalinfrastructuredestroyedbythe2021events.

Apartfromsupportingthegovernment’seffortstoquicklyrestoretheMWSP,thisreportwillserveasausefulguidethatoutlinesthemeasuresthatneedtobeconsideredtoensuretheresilienceofkeyinfrastructurebothinNepalandneighboringcountries.Itsfindingsandrecommendationswillresonatewithawiderangeofpolicymakers,practitioners,engineers,andresearchersontheneedtodesignresilientinfrastructureprojectsthathelpdrivesustainabledevelopment.

IwouldliketothanktheAsianDevelopmentBank(ADB)forsupportingandpromotingthisimportantreportandmycolleaguesattheMinistryofWaterSupply(MWS)andtheMelamchiWaterSupplyDevelopmentBoard

(MWSDB)fortheirhelpandguidance.IhopeitwillmotivateNepal’sgovernmentdepartments,agencies,districts,andlocalmunicipalitiestofullyconsidertheimpactsofhazardsinalltheiractivitiesandstrategizesuitablemeasurestobestmitigatethem.

IbelievethisreportwillhelpdecidethebeststepstotaketobothprotectthelivesandlivelihoodsofpeoplelivingintheMelamchicatchmentandensurethelong-termdeliveryofessentialwatersuppliestoKathmandu.

RamAdharSah

Secretary

MinistryofWaterSupply

GovernmentofNepal

Forewords

Forthefast-growingpopulationoftheKathmandu

Valley,theprojectthatbringsdrinkingwaterfromthe

MelamchiRivertotheirhomes,publicbuildings,and

businesses,isavitallifelinethatplaysacrucialrolein

equitableandsustainabledevelopment.Assuch,itis

importanttoprioritizetheresilienceoftheMelamchi

WaterSupplyProject(MWSP)andprotectitagainst

theimpactsofmyriadhazardssuchasearthquakes,

floods,debrisflows,andlandslides.

The2021floodeventsdestroyedMWSP’scritical

headworksstructuresthatwerebuilttodivertwater

toKathmandu,buryingthemunder15-20metersof

debris.Thisnotonlyhighlightedriskstothecapital’s

watersecurityasclimatechangethreatensto

complicateconditions,thefloodsalsoemphasized

theurgentneedtounderstandthreatsfromtheriver’s

catchmentanddevelopmitigationmeasures.

Withthisobjective,theAsianDevelopmentBank(ADB)hasworkedcloselywiththeGovernmentofNepal,theMinistryofWaterSupply(MWS),andtheMelamchiWaterSupplyDevelopmentBoard(MWSDB)tocarryoutthisfirstpartofacomprehensivetwo-stagestudy.ThisinitialreportidentifiesthemultiplehazardspresentintheMelamchiRivercatchmentareaandanalyzesfactorsthattransformedthemintocascadingdisastersduringthe2021monsoon.

Thereportrecommendsmitigationmeasurestostabilizetherivercatchmentandreducethevulnerabilityofkeyinfrastructurealongtheriver’scourse.However,itshowsthatfocusingoninfrastructurealoneisnotenoughandunderlinestheneedtoworkwithawiderangeofstakeholderstoinstitutionalizeevidence-basedmulti-hazardriskassessmentsfromtheprojectplanningstage.Italsorecommendstherolloutofcomprehensiveearlywarningsystemstohelpaffordablymitigatetheimpactsofclimate-induceddisastersinthehighlyfragileHimalayanregion.

TheMelamchiincidentillustratedthat"buildingbackbetter"cannotremainamerecatchphrase.The2021floodsexceededthedesignfloodoftheoriginalMelamchiheadworks,andnewdesignconsiderationsmustensurethattherepurposedorrehabilitatedheadworkscancopewithsimilaraswellasgreaterevents.

WorkinginremotepartsoftheHimalayanregionischallenging,andIwouldliketoextendmythankstotheGovernmentofNepal,myADBcolleagues,andthetechnicalassistanceteamforhavingundertakentheextensiveresearchthatformedthebasisforthisstudy.

IhopethiscomprehensiveandpracticalreportwillguideandassistallstakeholdersinoperationalizingtheMWSP,bothtoguaranteeKathmandu’swatersecurityandshowhowtoincorporateresilienceintofutureHimalayanprojects.

ArnaudCauchois

CountryDirector

NepalResidentMission

AsianDevelopmentBank

Acknowledgments

ThistechnicalreportwasinitiatedbytheGovernmentofNepal,theMinistryofWaterSupply(MWS),andtheMelamchiWaterSupplyDevelopmentBoard(MWSDB),withthetechnicalassistanceoftheAsianDevelopmentBank(ADB)inthewakeofthe2021MelamchiRiver

basinfloods.

ProductionofthereportwasledbySaugataDasgupta,seniorprojectmanagementspecialist,ADB;andSujanRajRegmi,projectanalyst,ADB.ItwaspreparedundertheguidanceofRamAdharSah,secretary,MWS;ArnaudCauchois,countrydirector,ADBNepalResidentMission.

ThereportbenefitedfromthesupportandinputofspecialistsatNepal′sNationalDisasterRiskReductionManagementAuthority,theDepartmentofHydrologyandMeteorology,andtheDepartmentofMinesandGeology.ValuablesupportandguidancewerealsoprovidedbytheInternationalCentreforIntegratedMountainDevelopment,andaspecialistWorldBankteam.

ADB’stechnicalassessmentteamwasledbyHansEnggrob,rivermorphologyexpert.

TeammembersincludedSanjivShah,deputyteamleaderandstructuralengineer;EckhardSchn?cker,engineeringgeologist;RaghunathJha,geographicinformationsystemexpert;

RanjanKumarDahal,geologicalexpert;GyanendraLalShrestha,geotechnicalexpert;Bishnu

RajBaral,sedimenttransportexpert;VishnuPandey,hydrologyexpert;MaheshBhattarai,watersupplyexpert;andShaliniPatwal,ADBconsultant.

ThereportwaspeerreviewedbyBGCEngineering,aninternationalconsultingfirm.

ABBREVIATIONS

ADB

AsianDevelopmentBank

CCTV

closed-circuittelevision

DEM

digitalelevationmodel

DGSD

deep-seatedgravitationalslopedeformation

DHM

DepartmentofHydrologyandMeteorology

DMG

DepartmentofMinesandGeology

DRM

disasterriskmanagement

EOP

Emergencyoperationprocedure

ERT

Electricalresistivitytomography

EWS

earlywarningsystems

GCP

groundcontrolpoint

GIS

geographicInformationSystem

GLOF

glaciallakeoutburstflood

HEC-RAS

HydrologicEngineeringCenter-RiverAnalysisSystem

ICIMOD

InternationalCentreforIntegratedMountainDevelopment

IRBM

integratedriverbasinmanagement

km

kilometer

kN/m2

kilonewtonpersquaremeter

LDOF

landslidedamoutburstflood

LEWS

landslideearlywarningsystem

LIDAR

LightDetectionandRanging

masl

metersabovesealevel

m3

cubicmeter

m3/s

cubicmeterpersecond

mm/hr

millimetersperhour

MRWDS

MelamchiRiverWaterDiversionSubproject

MWS

MinistryofWaterSupply

MWSDB

MelamchiWaterSupplyDevelopmentBoard

MWSP

MelamchiWaterSupplyProject

NDRRMA

NationalDisasterRiskReductionandManagementAuthority

O&M

operationandmaintenance

PMOF

probablemaximumoutburstflood

xii

ABBREVIATIONS

PMP

probablemaximumprecipitation

PSD

particlesizedistribution

SAR

SyntheticApertureRadar

SRT

seismicrefractiontomography

SWAT

SoilandWaterAssessmentTool

TA

technicalassistance

WMO

WorldMeteorologicalOrganization

WTP

watertreatmentplant

EXECUTIVESUMMARY

Nepal’sMinistryofWaterSupply(MWS)launchedtheMelamchiWaterSupplyProject(MWSP)almost2decadesagotoeasechronicwatershortagesinKathmandu.SupportedbytheAsianDevelopmentBank(ADB),itwasdesignedtodivert170millionlitersofrawwateradayfromtheMelamchiRiverthrougha26-kilometer(km)watertunnel,whereitwouldbetreatedandsuppliedtothecapital.However,beforefullprojectoperationscouldbegininSeptember2021,twocatastrophicdebrisfloodsdamagedtheheadworksoftheMelamchiRiverWaterDiversionSubproject(MRWDS).Thisrenderedthesystemthatcontrolstheextractionofwaterfromtheriverunusable.

TriggeringFactors

ThedebrisfloodsresultedfromaseriesofcascadingeventsinandaroundtheMelamchiRivercatchmentarea.In2015,the7.8-magnitudeGorkhaearthquaketriggerednumerouslandslidesinthecatchmentandincreaseditssusceptibilitytoslopeinstabilities.InJune2021,followingrainfallandasharptemperaturerise,aglaciallakeoutburstflood1(GLOF)intheupperrivercatchmentcausedmassiveerosionofamajorsedimentdepositatBhemathan,16kmupstreamoftheheadworks.

Combinedwithprecipitation,thedebrisfloodsgeneratedamajorlandslideatMelamchiGhyangwhichblockedtheriver.Thebuildupofwaterbehindthisnaturaldamcausedalandslidedamoutburstflood2(LDOF)whichsparkedtheformationandsubsequentcollapseofaseriesofnewnaturaldams.Asaresult,vastquantitiesofdebriswereerodedfromtheupstreamriverchannelanddepositedattheheadworksandfurtherdownstream.Thefloodsalsoaggravatedanexistinglandslideattheheadworks.

ImpactsofDebrisFloods

Extremefloodingin2021causedanumberofdeaths;displacedhundredsofpeople;andwashedawayhomes,roads,andbridges.Attheheadworksandintheupstreamcatchment,theeventscausedthefollowing:

.Triggeringofnewlandslidesandreactivationofexistinglandslidesinthecatchment.

.Significantbankerosionandriverbedscourintheupperriverstretches,majorsedimentdepositionattheheadworksandfurtherdownstream.

.Burialoftheentireheadworksareaunder15-20metersofdebris.

.Extensivedamagetotheheadworksstructuresandequipment.

.Aggravationofanexistinglandslideattheheadworks.

1Causedbythefailureofanaturaldamofice,soil,orrockcontainingmeltwaterfromaglacier.

2Triggeredbythefailureofadamthatwasformedbyalandslide.

MAPPINGHAZARDSINNEPAL’SMELAMCHIRIVER

xiv

Asaconsequence,thefragilecatchmentbecamemorevulnerabletofuturelandslidesanderosion,andtheriverisnowexposedtofurthernaturaldammingandoutburstfloods.Theheadworkslandslideposesaseriousthreattothestructuralsafetyoftheheadworks,andamajorrunoutofthelandslidecouldblocktheriverandcauseoutburstflooding.

Map1:LocationoftheMelamchiRiverBasin

NuwakotDistrict

Yangri

River

Melamchi

River

0510

Kilometers

28o00’N

Extension

Headworks

Ribarma

Ambathan

Timbu

MelamchiBazaar

SindhupalchokDistrict

SindhuRiver

KathmanduDistrict

Indrawati

River

Sundarijal

Kirtipur

Lamidanda

27o40’N

27o40’N

Lalitpur

85o20’E

N

LarkeRiver

LangtangNationalPark

28o00’N

85o30’E

ShivapuriWatershed

andWildlifeReserve

Bhaktapur

Thimi

LalitpurDistrict

BhaktapurDistrict

KabhrePalanchokDistrict

Kathmandu

GyalthumRiver

TalamarangRiver

GohareRiver

MahankalWater

TreatmentPlant

NationalPark

NationalCapital

City/Town

Intake

WaterTreatmentPlant Highway

AccessRoad

BulkDistributionTunnelAlignmentTunnelExtensionRiver

DistrictBoundary

BagmatiRiver

Source:AuthorsandMelamchiWaterSupplyDevelopmentBoard.

EXECUTIVESUMMARY

xv

TheStudies

The2021eventsunderscoredtheMWSP’svulnerabilitytoextremeevents.Inviewofthis,ADBinitiatedadetailedtechnicalassessmenttoidentifythecausativefactorsandimpactsofthefloodsanddevelopmeasurestoensuretheresilienceofKathmandu’sdrinkingwatersupplies.Themajorfindingsoftheassessmentwerethefollowing:

.Thefloodeventswereassociatednotonlywithrainfallrunoffbutmultiplehazardsthatcombinedandinteractedwitheachothertodominointocascadingeffects.

.Thefloodshadanestimatedpeakdischargeof2,700cubicmeterpersecond(m3/s),withfloodwavesthattravelledat22meterspersecond(m/s)andreachedthe

headworksin12minutes.

.TenglaciallakesandatleasttwoglaciersarepresentintheMelamchicatchment,butonlyafewlakesposeanoutburstthreat.

.Fifteenlandslidesinthe