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Variability

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Variability

    Variability

    Availableonlineatwww.sciencedirect.com

    ':ScienceDirect

    nternationalJoumalofSedimentRehash24(2oo9)177-188

    IN'I.ERNrIoNAL

    J0IJllNAI,0F

    SED?ENT

    RESEARCH????????????????????????????一

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    Variabilityinrainfallthresholdfordebrisflowafterthe

    Chi-ChiearthquakeincentralTaiwan,China

    c.L.SHIEH,Y.S.CHEN,,Y.J.TSAI,andJ.H.w

    Absact

    Thepurposeofthisstudyistoanalyzevariabilityinrainfallthresholdfordebrisflow(critical rainfla11f0rdebrisflowtriggering)af

    ertheMT7.3Chi-ChiearthquakeincentralTaiwanin1999.

    Twostudysiteswithdifferentgeologicalconditionsweresurveyedintheearthquakearea.Streambed

    surveyswereconductedtocontinuouslymonitordebrisflowsbetween1999and2006.Duringthe

    7-yearstudyperiod,everydebrisfloweventwasidentified,andthestreambedcharacterized.Results

    showthattherainfallthresholdfordebrisflowwasremarkablylowerjustaftertheChi.Chi Earthquake,butgraduallyrecovered.Todate,thisrainfallthresholdisstilllowerthantheoriginal

    levelpriortotheearthquake.Thisvariabilityinrainfallthresholdiscloselyrelatedtotheamountof

    sedimentmaterialintheinitiationareaofdebrisflow.whichincreasedrapidlyduetolandslides

    resultingfromtheearthquake.Withtheincreaseinsedimentmateria1.therainfallthresholdwas

    loweredseverelyduring

    efi/'styearfollowingtheChi.Chiearthquake.However,heavyrainfalls

    mobilizedthesedimentmaterial,causingdebrisflowsandtransportingsedimentdownstream.With

    thedecreaseinsedimentmateria1.therainfallthresholdrecoveredgrad1Jallyovertime.Furthermore.

    debrisflowsoccurredonlyinthesubbasinsthathadsufficientsedimentmaterialtocausesignificant

    movement.Hence.theseresultsconfirmthatthesedimentmaterialintheinitiationareaofdebfisflow

    isacrucialcomponent0fmerainfallthresholdfordebrisflow.

    KeyWords:Debrisflow,Rainfallthreshold,Sedimentmaterial,Earthquake,Landslide 1Introduction

    TheMI7.3Chi.ChiEa~hquake(23.85~N,120.81oE)of21Sept.,1999,wasthelargestearthquakeon

    TaiwanIslandforthepasthundredyears.Thiscatastrophicearthquakecaused2,4l5deathsandl1,305

    injuries(Jhong,2001).IncentralTaiwanalone(2,400h),over20,000landslides(Wangeta1.,2000)of

    approximatelyl13kmoftotalareaweretriggeredbytheearthquake(Hungeta1.,2000;Lineta1.,2003,

    2006).Sedimentwasgeneratedfromthese1andslides.anddepositedonthestreambanksorstreambeds.

    Inthefollowingrainyortyohoonseasons,heavyrainfallsmobilizedthesedimentmaterialandcaused

    debrisflowseasily.Forexample,655debrisflowsoccurredincentralTaiwanduringTyphoon

Torsiin

    2001.0fthese,244causedseriousdamagetohousesandroadsfShieh,2002),andresultedinmorethan

    240mortalities.ThiswasthelargestsedimentdisasterforthepastrecordsinTaiwan.Asaconsequence

    Prof.,DepartmentofHydraulicandOceanEngineering,NationalChengKungUniversity,Taiwan,China,

    Email:shieh@dprc.ncku.edu.tw

    "Ph.D.Candidate,DepartmentofHydraulicandOceanEngineering,NationalChengKungUniversity,Taiwan,

    China,E-mail:yushiu.chen@gmail.tom(correspondingauthor)

    Ph.D.Student,DepartmentofHydraulicandOceanEngineering,NationalChengKungUniversity,Taiwan,

    China,Email:gbrong@gmail.tom

    C0

    Professor,DepartmentofCivilEngineering,NationalChengKungUniversity,Taiwan,China,E-mail:

    jhwu@mail.ncku.edu.tw

    Note:TbeoriginalmanuscriptofthispaperwasreceivedinDec.2007.TherevisedversionwasreceivedinSept.

    2008.DiscussionopenuntilJune2010.

    InternationalJournalofSedimentResearch,Vo1.24,No.2,2009,PP.177-188.177.

    ofthisdisaster.theSoilandWaterConservationBureauofTaiwandesignatedmountainousareasas

    hazardous,andthegovernmentenhanceditswamingsystemofdebrisflow. ThesignificantincreaseindebrisflowfrequencyfollowingtheChi.Chiea~hquakeandthestrongeflfect

    ofrainfall0nthegenerationofdebrisflowspromptedaredefinitionoftherainfallthresholdforavalid

    warningsystem(Lineta1.,2003:Shiehetal2004;Cheneta1..2006).Inthecourseoftime,the governmentloweredtherainfallthresholdfordebrisflowtriggering.Consequently.governmentand

    engineersbecameincreasinglymoreinterestedinquantifyingandbetterunderstandingthevariabilltyin

    rainfallthresholdfordebrisflow.

    Todate.engineersandresearchershaveappliedprecipitationdatatoestablishrainfallthresholdsfor

    debrisflowtriggering.Bothrainfal1intensityanddurationhavebeenappliedasindicestoestablishthe

    rainfallthresholds(Caine,1980;CannonandEllen,1985;Vandine,1985Wieczorek,1987;Keefereta1.,

    1987;Chen,2005).Takahashi(1981a)andShieheta1.(1995)alsoappliedrainfallintensityand

    cumulativeprecipitationasindicestodefinecriticalrainfallthresholdforwarningsystemsinJapanand

    Taiwan,respectively.AtierTyphoonTorajiin2001.Shieh(2002)alsoappliedclusteranalysisto

    establishtherainfallthresholdforallhazardousstreamsinTaiwan.whichyieldedabetterdisaster

    warningandpreventionsystem.

    Debrisflowsaretriggeredbythreeessentialfactors:amountofavailablesedimentmateria1.surface

    water,andsteepnessofstreamslopesfTakahashi,l98lb).Therelationbetweenrainfa11anddebrisflows

    hasbeenwell

    examinedinthepast.but1eSSattentionhasbeenpaidtothesedimentmateria1factor.New landslidescausedbvtheChi-Chiearthquakeincreasedtheamountofsedimentmaterialonthestreambed

    andchangedthesedimentcondition.Thisnewconditionhintsattheimportanceofidentifying

debrisflow

    thresholdvariabilityfi.e.,cfiticalrainfallconditionsthatcausedebrisflows),aswellastherelation

    betweentherainfallthresholdandsedimentconditions.Inthispaper,thisrelationisanalyzedby

    investigatingpost

    earthquakeprocessesattwostudysitesinmountainouswatershedsofcentralTaiwan. 2Studysites

    Figure1showsthelocationsoftheWushihkengandMt.Ninety-NinewatershedsinTaiwan.andtheir

    nearbyraingaugestations,whichwereestablishedbytheCentralWeatherBureau.Taiwan.Three

    stations(C1F89,COF90,andC1F9H)arelocatedneartheWushihkengandtwo(C1H93andC1H94)near

    Mt.Ninety-Nine.BeforetheChi

    Chiearthquake,onlythreecatastrophicearthquakes(MI.>6.0)occurred incentralTaiwanin1916and1917(MI6.8on28thAug.,1916;MI6.2on15thNov.,1916:MI6.2on

    17thJan.,1917;CentralWeatherBureau,Taiwan).Since19l7,therehavebeennoseveresediment

    disasters(i.e.,largescalelandslides,debrisflows)recordedexceptforthoseassociatedwiththeChi.Chi

    earthquake.

    .

    178

    wE

    S

    0102040

    :Rain-gaugestation

?:EpicenteronChi-Chiearthquake,:Chelungpufault

    Fig.1Thestudycites:WushihkengwatershedandMt.Ninety?Nine

    InternationalJournalofSedimentResearch,Vo1.24,No.2,2009,PP.177_188 TheChi.Chiearthquakein1999wastriggeredbyreactivationoftheChelungpufault.Thesurface

    ruptureswereover100kilometers.Figure1showstheepicenteroftheChi.Chiearthquakeandlocation

    oftheChelungpufault.ThedistancesfromtheepicentertotheWushihkengandMt.Ninety-Nine

    watershedsareonly50and20kilometers.respectively.Themaximumpea1(groundacceleration(PGA)at

    thesesitesreached664and820ga1.respectively.Theimpactoftheearthquakewasimmediateatthetwo

    sites.

    TheWushihkengwatershedislocatedinTaichungCounty.andisamainbranchoftheDa-AnRiver.

    Thewatershedareacovers3.458hectaresandincludesfoursubbasins.Figure2showsthewarershed

    topographyandthefoursubbasins.Theprecipitationisabout2,600to3,100millimetersperyear,and

    mostrainfa11takesplacebetweenMayandSeptember.Thegeologyoftheareaisclassifiedbytwo

    di

    erentrocktypes.RocksinSubbasinIandIVconsistofgraywackeandmetamorphicrocks.whereas

    strataofSubbasinIIandIIlconsistofsandstoneandshale.BeforetheChi

    Chiearthquake,the

    Wushihkengwatershedwasanaturereservewithabundantnaturalbiologicalresources.Mostofthearea

    wascoveredbybroad.1eafforests.andonlyonevillagewaslocatedatthewatershedoutlet.Su

bsequentto

    theChi

    Chiearthquake.manyshallowslopefailuresandlargescalelandslides(633hectares)occurredin

    thearea.Furthermore,debrisflowsbegantooccurfrequentlyinthewatershed,CaBSinganincreasein

    streambedelevationofoverl0meters(Shiehetal,2006a).

    Thesecondsite.Mt.Ninety.Nine.islocatedinNantouCounty.ThewatershedbelongstotheWuRiver

    andincludessevensubbasins.Onlyfiveofthemwereselectedforthisstudy.becausetheothertwowere

    disturbedbysedimentminingduringthestudyperiod.Figure3showsthewatershedtopographyandthe

    fivesubbasins.Theprecipitationatthislocationisabout2,000to2,500millimetersperyear,andmost

    rainfallOCCurSbetweenMayandSeptember.TherockinMt.Ninety-Nineiscomposedmainlyof

    conglomerateSimilartoe

    shihkengwatershedallmountainousareasweredominatedbybroad.1eaf

    andbambooforestsbeforetheChi.Chiearthquake.Severalvillageswerelocatedontheright(north)bank

    ofWuRiver.Onlyafewareasnearthevillagesweretilledland.Followingtheearthquake.1andslide

    areasconstitutedabout48%0ftheentirewatershed.andalmosthalfoftheareabecamebarehillslopesas

    aconsequence.Sedimentmatedalsweretransporteddowns

    camaftertheearthquake.andcausedseveral

    disastersinthevillages(Shiehetal,2006b).Thearea,streamlength,averagestreamslopeandthe

    geologyofeachsubbasinarelistedinTable1.

Fig.2Watershedtopographyandsubbasins

    intheWushihkengwatershed

    3Studymethod

    Fig.3Watershedtopographyandsubbasins

    intheMt.Ninety-Nine

    3.1Debrisflowmonitoringandstreambedsurvey

    AftertheChi.Chiearthquake.continuousfieldsurveillancewasundertakenatbothstudysites.Debris

    flowmonitoringsystemsalsowereestablishedinSubbasinsIandIII.Inordertoindentifytl1edebrisflow

    events,thismonitoringsystemrecordedstreamwaterdepth,precipitationandrea1.timevideoofthe

    channelfFig.4).Thewaterdepthwasmeasuredusinganultrasonic1evelmeter,andpreci[}itation

    InternationalJournalofSedimentResearch,Vo1.24,No.2,2009,PP.177-188179

    recordedbyaself-registeringraingauge.Thereal-timevideorecordedontoadataloggerandtransmitted

    totheresearchinstituteatCheng

    KungUniversitybyadirectnetworkconnection.Figure5showsimages

    takenfromrecordedvideowhenTyphoonMatsaarrivedin2005.Thedebrisflowwasidentifiedbyvideo

    imagerywhenrainfalloccurred,andboththeprecipitationandwaterlevelofdebrisflowwerealso

    recorded.

    StreamAverageslopeSt

    udysiteSubbasinIDArea(ha)Geologylength(kIn)ofstream(degree)

    Subbasin.I1.7579.78.2Gmywacke&

    metamorphicrock

    WushihkengSubbasinII4434.112.5Sandstone&shale

    watershedSubbasin.III1722.810.7Sandstone&shale

Subbasin.IV1.0856.7l3.7Graywacke&

    metamorphicrock

    Subbasin-V1001.712.2Conglomeraterock

    SubbasinVI892.O12.3Conglomeraterock

    Mt.Ninety-NineSubbasin.?Il332.89.0Conglomeraterock.

    Subbasin.?II721.87.5Conglomeraterock

    Subbasin.IX972.05.6Conglomeraterock

    Besidesthedebrisflowmonitotingsystem,additionalfieldsurveillancewasmountedateachsubbasin

    whenheavyrainfallwaspredicted.Ifdebrisflowsoccurred.streambedmeasurementsweretakenwitha

    differentialglobalpositioningsystem(DGPS)receiverandelectronictotalstation.Longitudinalprofiles

    andcross.sectionswereplottedwiththesemeasurements.Sedimentdeposition/erosionwasestimatedby

    comparingprofilesandcross.sectionsbeforeandafteradebrisflow.FollowingTakahashi'Sresearch

    (198la),thecriticalstreamslopefortheonsetofadebrisflowisabout12to15..InthisPaper,thecritical

    streamslopeisdefinedconservativelyas12..andthecatchmentareaabovethisstreamslopeis designatedtheinitiationareaofdebrisflow.

    HStheamountofsedimentmaterialonthestreambedin

    theinitiationareaofdebrisflowcanbeestimated.

    .

    180

    F.g.4Debrisflowmonitoringsystematastudysite

    InternationalJournalofSedimentResearch,Vo1.24,No.2,2009,PP.177-188 Fig.5Real

    timephotosgrabbedfromvideowhenTyphoonMatsacomingin2005(upper-le~ beforerainfallcoming;upper-?right:dischargeincreasedbeforedebrisflow;lower?-left:

    thepeakflowwhiledebrisflowcoming;lower-right:runoffsafterdebrisflow) Thenewlandslideareasandlocationswerealsosurveyedaftertheearthquakeandtyphoonstoidentify

    thesedimentcondition.Setsofsatelliteimages,twobeforetheearthquakeandelevenaftertheearthquake,

    werechosentoanalyzequantitativechangesinlandslideareas.Figure6presentsaflowchartof

    automaticlandslidesurveillance.ThepotentialbareareasintheimagesweremarkedUSingtheNormalize

    DifferenceVegetableIndex(NDVI).whichiSdefinedastheratioofdifferencebetweentheredand

    near=infraredbandstotheirsummation,presentedasthefollowingequation: NDVI=(RedNIR)/(Red+NIR1(1)

    whereREDandNIRstandforthespectralreflectancemeasurementsacquiredintheredandnear-infrared

    bands.respectively.GoodvegetativecoverhasahigherNDVIvalueandbareareashavelowervalues.

    Therefore,NDVIisasimpleindextodistinguishbareareasfromvegetativecover.However,amongthe

    potentialbarearearesults,imagesofstreams,reads,fieldsandvillagesmayresultinimproperlandslide

    iudgments.Therefore,GISreportswereusedtosubtracttheimproperareasfromthevariableresults.

    Streamsweregeneratedautomaticallybyasingleflowdirectionalgorithmwitha5X5mgriddigital

    terrainmode1.Roads,fieldsandvillageswereidentifiedbyfeaturesidentifiedbytheMinistryofThe

    Interior,Taiwan,andconfirmedthroughremotesensingandfieldwork.

    3.2Analysisofrainfal1thresholdfordebrisflow

    Previousstudies(Shieh.2002)haveshownthatrainfallintensityandcumulativeprecipitatio

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