Karst flash floods: an example from the Dinaric karst (Croatia)


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Karst flash floods: an example from the Dinaric karst (Croatia)

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Karst flash floods: an example from the Dinaric karst (Croatia)
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Natural Hazards and Earth System Science
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Bonacci, O.
Ljubenkov, I.
Roje-Bonacci, T.
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Karst ( local )
Flash ( local )
Floods ( local )
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serial ( sobekcm )

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Flash floods constitute one of the deadliest and costliest natural disasters worldwide. This paper explains the karst flash flood phenomenon, which represents a special kind of flash flood. As the majority of flash floods karst flash floods are caused by intensive short-term precipitation in an area whose surface rarely exceeds a few square kilometres. The characteristics of all flash floods are their short duration, small areal extent, high flood peaks and rapid flows, and heavy loss of life and property. Karst flash floods have specific characteristics due to special conditions for water circulation, which exist in karst terrains. During karst flash floods a sudden rise of groundwater levels occurs, which causes the appearance of numerous, unexpected, abundant and temporary karst springs. This paper presents in detail an example of a karst flash flood in the Marina bay (Dinaric karst region of Croatia), which occurred in December 2004.
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Natural Hazards and Earth System Science, Vol. 6 (2006-01-01).

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Nat.HazardsEarthSyst.Sci.,6,195– 203 ,2006www.nat-hazards-earth-syst-sci.net/6/195/2006/Authors2006.ThisworkislicensedunderaCreativeCommonsLicense. NaturalHazards andEarth SystemSciences Karstashoods:anexamplefromtheDinarickarstCroatia O.Bonacci1,I.Ljubenkov2,andT.Roje-Bonacci11FacultyofCivilEngineeringandArchitecture,Univ.ofSplit,MaticeHrvatskestr.15,21000Split,Croatia2FacultyofCivilEngineeringandArchitecture,Univ.ofSplit,MaticeVukovarska35,21000Split,CroatiaReceived:13September2005–Revised:15February2006–Accepted:16February2006–Published:31March2006 Abstract.Flashoodsconstituteoneofthedeadliestandcostliestnaturaldisastersworldwide.Thispaperexplainsthekarstashoodphenomenon,whichrepresentsaspecialkindofashood.Asthemajorityofashoodskarstashoodsarecausedbyintensiveshort-termprecipitationinanareawhosesurfacerarelyexceedsafewsquarekilometres.Thecharacteristicsofallashoodsaretheirshortdura-tion,smallarealextent,highoodpeaksandrapidows,andheavylossoflifeandproperty.Karstashoodshavespe-ciccharacteristicsduetospecialconditionsforwatercircu-lation,whichexistinkarstterrains.Duringkarstashoodsasuddenriseofgroundwaterlevelsoccurs,whichcausestheappearanceofnumerous,unexpected,abundantandtempo-rarykarstsprings.ThispaperpresentsindetailanexampleofakarstashoodintheMarinabayDinarickarstregionofCroatia,whichoccurredinDecember2004. 1IntroductionDuringthelastfewdecadesashoodshaveconstitutedoneofthedeadliestandcostliestnaturaldisasterswhichcanoc-curalmosteverywhereintheworld.Inrecenttimes,greatattentionhasbeendevotedtoashoods,promptedbymanycatastrophiceventsoccurringallovertheworld.Flashoodsareoneofthemostimpressivehazardousmanifestationsoftheenvironmentwhichdirectlyaffectshu-manactivityandsecurityandwhoseoriginsanddevelop-mentsarenotyetwellenoughunderstoodandcontrolledbyhumankind.TherearemanywaystopreventashoodsbutnomatterhowwellanyonemethodworksitseffectisalwayslimitedLin,1999.Theobjectiveofthispaperistodescribeindetailandex-plaintheexistenceofonespecialkindofashoodcalled Correspondenceto:O.Bonacciobonacci@gradst.hr thekarstashood.Watercirculationinkarstareasisdif-ferentthaninnon-karstareas,whichisthemainreasonforthestronglydifferentcharacteristicsofkarstandnon-karstashoods.ThispaperpresentsindetailanexampleofkarstashoodwhichoccurredinDecember2004inMarinabaylo-catedinadeepandbareDinarickarstneartheAdriaticSeacoastofCroatia.2FlashoodcharacteristicsFlashoodsareoftentheresultofconvectionalstormsorofhigh-intensityraincellsassociatedwithfrontalstorms.Thesestormsdroplargeamountsofrainwithinabriefpe-riod,oftenmeasuredinminutesratherthanhours.Flashoodsmaybealsocausedorintensiedbyrapidsnowmelt-ing.Itmayindicatethatastormhasoccurredonasteep,bare,impermeablesurfacesuchasanarrowmountainval-leyoraheavilybuilt-upurbanarea,orinasmallcatchmentthroughwhichtheresultingoodpeakpassestoorapidlyforadequateoodwarningstobegivenSmithandWard,1998.Steepterrain,excessiveantecedentprecipitationandthin,bareand/orimpermeablesoilorcoverconspiretocreateconditionshighlyconductivetoashooding.Overlandowtendstoplaythedominantroleinashoodformation.Lowinltrationcapacityisthemostim-portantfactorforoverlandowdevelopment,whichresultsfromphysical,chemicalandbiologicalfactorsSmithandWard,1998.Thesefactors'suddenappearanceanddisappearanceinheadwaterstreamsledtotheterm“ashood”Ward,1978.Flashoodsoccurwithlittleornowarningandcanreachtheirfullpeakinonlyafewminutes.Flashoodhydro-graphshavesharppeaks.Theirrisingandfallinglimbsareverysteepwithanalmostequalduration.Becauseoftheshort-livedanddestructivenatureofashoods,their PublishedbyCopernicusGmbHonbehalfoftheEuropeanGeosciencesUnion.

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196O.Bonaccietal.:Karstashood dischargemeasurementsareextremelycomplexveryoftenremainanunsolvedtask.Thespecicpeakdischargeofashoodsisgreaterthan10andcanreachavalueof100m3s)]TJ/F36 7.57 Tf 5.91 0 Td[(1km)]TJ/F36 7.57 Tf 5.9 0 Td[(2.Flashoodwatersmoveatveryfastspeedsandcankillpeople,rollboulders,tearouttrees,destroybuildings,oblit-eratebridgesandincreasethepotentialforlandslidesandmudslides.Archercalledashoodswallsofwater.Theycanreachheightsof3to6mandgenerallyareaccom-paniedbydeadlycargodebris.Whiletheashoodspeakdischargesaremuchhigherthanfornormaloods,thetotalhydrographvolumesofashoodsarequitesmall.Duetothisreasontheashoodvolumeisnotnecessarilyimpor-tant.Flashoodsareoftendiscussedespeciallyinthecontextofaridandsemi-aridenvironments.Toanevengreaterextentthaninhumidregions,oodsinaridandsemi-aridareasaregeneratedononlyasmallfractionofadrainagebasin.Thispartlyreectsthewayinwhichoverlandow,generatedonsmoothorsteeplyslopinggroundsurfaceshavingsparseveg-etationcover,maydeliversudden-onsetoodowsandre-sultinrapidlyrisinghydrographsSmithandWard,1998.CohenandLaronne05statethatsuspendedsedimenttransportedbyashoodsindesertsismuchhigherthanwhatistypicalofperennialuvialhumidenvironments.Thesedimentyieldofindividualeventsislarge,butthesmallnumberofoodslimitsthemeanannualsedimentyieldtolowvaluesinthesearidenvironments.Thesuddennessandunexpectednessaccompaniedbycatastrophicconsequencesarethemaincharacteristicofallashooding.ThepredictionandforecastingofashoodsisanextremelydifcultandunreliableissueHall,1981;Lin,1999.Thekeyfeatureofashoodforecastingistoiden-tifyquicklywhentheforecastoodisabovethethresholdratherthantheexactpeakdischargeandtimeofoccurrence.Hence,ashoodforecastingdoesnotrequireacomplexmodel.TheUSNationalWeatherServiceidentiedaneedforimprovingashoodguidanceprocedures.Carpenteretal.describeaproceduredevelopedtoprovideim-provedestimatesofthresholdrunoff.Thresholdrunoffhasbeendenedastheamountofrainfallexcessofagivendu-rationnecessarytocauseoodingonsmallstreams.ForpracticalandoperationalreasonsLin1999distin-guishesnaturalfromarticialashoods.Articialashoodshavenothingtodowithabnormalclimaticchanges,exceptinsomeveryspeciccases.Theyarecausedbystructuralfailures.Forexample,dam-breakashoodsarisewhenastormoccurswithamagnitudeoverandabovethedesignlimitsonthestructureorbecauseofafailureindamconstruction.Inthiscasetheirkineticenergyisgreatandtransportcapacityisstrong.3Karst,karstaquifer,watercirculationinkarstandcatchmentinkarstKarstisdenedasaterrain,generallyunderlainbylimestoneordolomite,inwhichthetopographyischieyformedbythedissolvingofrock,andwhichmaybecharacterisedbysinkholes,sinkingstreams,closeddepressions,subterraneandrainageandcaves.Awiderangeofclosedsurfacedepres-sions,awell-developedundergrounddrainagesystem,andastronginteractionbetweenthecirculationofsurfacewaterandgroundwatertypifykarstareas.Itrepresentsterrainwithdistinctivehydrogeology,hydrologyandlandformsarisingfromacombinationofhighsolubilityandwell-developedsecondaryporosity.Duetoveryfastinltrationrates,es-peciallyinbareDinarickarst,overlandowandtheexis-tenceofpermanentopenstreamowsinkarstterrainsarerareBonacci,1987.Trudgill85statesthatwhilethegeneralfunctionsofuvialsystemsinkarstandnon-karstareasmaybesimilar,theirgeomorphologicaleffectsaredifferent.Karstlandformscanbeseeninthecontextofauvialsystem.Verticallyori-entedkarstfeaturescollectandtransportveryfastwaterfromthesurfacetotheundergrounduvialsystem.Thewaterrechargedintokarstaquifersmovesdown-gradientthroughthemusingacombinationofhighlyanisotropicpathways.Karstaquifersareatripleporositysys-temconsistingof:1Matrixpermeability;2Fractureper-meability;3Conduitpermeability.Matrixpermeabilityisacomplexofvoidsinasmallrockfragmentwithavolumeofabout10to100cm3.Thematrixconsistsofnotonlyinter-granularporesbutalsomicrossures,andsmallkarstvoidsChoquetteandPray,1970;Motyka,1998.Fractureperme-abilityisformedfrommechanicaljoints,jointswarmsandbeddingplanepartings,mainlyenlargedbysolution.Con-duitpermeabilityisrepresentedbypipe-likeopeningswithaperturesrangingfrom1cmtoafewtensofmetersWhite,2002.Theaveragevaluesoftheeffectiveporosityofkarstmassifgenerallyvaryfrom0.1to1%.Karstaquifershavecomplexandoriginalcharacteristicswhichmakethemverydifferentfromotheraquifers:highheterogeneitycreatedandorganisedbygroundwaterow,largevoids,highowvelocitiesandhighowratespringsBakalowicz,2005.Themostimportantfeaturesofkarstaquifersaretheconduitswhichprovidelowresistancepath-waysforgroundwaterow.Whitestatesthatcon-duitowoftenhasmoreincommonwithsurfacewaterthangroundwater.Forhimkarsthydrologyrequiresamixofsurfacewaterconceptsandgroundwaterconcepts.Karstaquifersaregenerallycontinuous.However,numeroussub-surfacemorphologicfeaturescaves,chasms,conduits,frac-tures,impermeablelayers,etc.stronglyinuencethecon-tinuityoftheaquifer,andcommonlytheaquiferdoesnotfunctionasacontinuuminacatchmentBonacci,2001.Karstaquiferswithahighdegreeofintegratedconduitper-meabilitytransmitwateronatimescalethatisshortwith Nat.HazardsEarthSyst.Sci.,6,195– 203 ,2006www.nat-hazards-earth-syst-sci.net/6/195/2006/

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O.Bonaccietal.:Karstashood197 Fig.1.Schematicpresentationofthezero-stateofkarstsystembeforeakarstashoodandafterthelonglastingdryperiod. Fig.2.Schematicpresentationofkarstsystemduringtheashoodinmomentwhenkarstconduitsarefullwithwater,andwhengroundwaterlevelisnotstarttorise. respecttothetimerequiredforchemicalreactionsbetweenwaterandthelimestonebedrocktoreachequilibrium.Asaresult,thechemistryofthewateructuatesrapidlyduringthepassageofstorminputsHessandWhite,1988.Watercirculationoverandthroughoutakarstterrainsig-nicantlydiffersfromthatinothertypesofterrain,primarilyduetorapidratesofinltrationandtheinuenceofthesolu-tionallyenlargedundergroundaquifer,clearlydistinguishingitfromporousaquifersandothertypesoffractureaquifersBonacci,2004.Inkarstaquiferswaterisbeingcollectedinnetworksofinterconnectedcracks,joints,cavernsandcon-duits.Thehydraulicpermeabilityofkarstaquifersisessentiallycreatedbyowingwaterandhasananisotropiccharacter.Oneofthemostimportantcharacteristicsofkarstaquifersisthehighdegreeofheterogeneityintheirhydraulicprop-erties.Duetothecomplexityofthekarstmedium,manyexpertsthinkthateverykarstaquifershouldbeconsideredasbeingrepresentativeofitself.Thisisonlythepartialtruth. Fig.3.Schematicpresentationofkarstsystemduringthekarstashoodwhengroundwaterlevelinkarstaquiferisinitsmaximum. Fortunately,commoncharacteristicsinkarstaquiferstruc-tureandingroundwaterowallowtheiranalysis,modellingandexploitation.Thedirectexposureofkarstterrainsfacilitatestheini-tiationofkarstdrainage,anditslaterevolutionthroughsinkholesatthemainpointsofwaterinltrationFordandWilliams,1989.Scaleissuesareparticularlyimportantforunderstandingandmodellingkarstwatercirculation.Condi-tionsinakarstiedmediumarestronglydependentonspaceandtimescales,especiallyinadeepandmorphologicallycomplexvadosezone.Thiszonewithakarstaquiferformatwo-componentsysteminwhichthemajorpartofstor-ageisintheformoftruegroundwaterinnarrowssures,wherediffuseorlaminarowprevails.Ontheotherhand,themajorityofwateristransmittedthroughthekarstunder-groundbyquickorturbulentowsinsolutionallyenlargedconduits.Conduitandmatrixowoccurbothinvadoseandphreaticzones.Kiraly1994explainstheconceptof“karstduality”asadirectconsequenceofkarst'sextremelyhet-erogenicstructure.Dualityexistsin:inltrationprocesses,thegroundwateroweld,anddischargeconditions.Theminimumdiameterofkarstconduitsinwhichturbulentowcouldexistshouldbegreaterthan5to15mmFordandEw-ers,1978.Interactionbetweenthetwoabovementionedtypesofowissignicantandpermanentlypresent.Inkarstterrainscatchmentboundariesandareasarefrag-mentallyunknown.Thecatchmentareainkarstdenedfromsurfacemorphologyi.e.topographiccatchmentrarelycorrespondstothekarsthydrologicalorhydrogeologicaldrainagebasin.Thedifferencesbetweenthetopographicandhydrologicalcatchmentsinkarstterrainare,asarule,solargethatdataaboutthetopographiccatchmentareuselessinpractice.Thisfactcausesalotofdifcultyinkarstashoodforecastingandcontrolling.Thepositionofthewatershedlimitdependsuponthegroundwaterlevel,whichcanchangeveryquickly,espe-ciallyafterhigh-intensityprecipitationwhichmaycauseashooding.Generallyatveryhighgroundwaterlev-elsfossilandinactiveconduitsandspringsareactivated, www.nat-hazards-earth-syst-sci.net/6/195/2006/Nat.HazardsEarthSyst.Sci.,6,195– 203 ,2006

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198O.Bonaccietal.:Karstashood Fig.4.aMapofstudyarea;bTopographiclocationmapindicatingshafts,groundwaterowdirections,karstspringsandsubmarinekarstsprings. whichcauseintercatchmentoverowandtheredistributionofcatchmentareasBonacci,1995.Forunderstandingthekarstsystemitiscrucialtoexplaintheinteractionofground-waterandsurfacewater.Thisisofspecialimportanceforkarstashoodsanalysis.4KarstashoodcharacteristicsThepreviouslyestablishedfactsaboutashoodingandwa-tercirculationinkarstareasclearlyindicatethatthecharac-teristicsofashoodinginkarstterrainsshouldbestronglyanddenitelydifferentthaninnon-karstterrains.Karstashoodshavespeciccharacteristicscausedbydistinctcondi-tionsforwatercirculationandstorageoverandinakarstiedmedium.Therootcausesofkarst-ash-ood-speciccharacteris-ticsare:1Highinltrationrate;2Rareornonexistenceofoverlandowandopenstreams;3Stronginteractionbe-tweenthecirculationofsurfacewaterandgroundwaterinkarstareas;4Smallstoragecapacityofthekarstmedium;5Fastgroundwaterowthroughkarstconduits;6Stronganddirectconnectionsbetweensurfaceinowthroughdifferentkindsofswallow-holesponorsandoutowthroughperma-nentandintermittentkarstsprings;7Existenceofmanyun-knownundergroundandsurfacekarstfeaturessuchascaves,jamas,conduits,sinkholes,karrensetc.;8HighandfastoscillationsofgroundwaterlevelsinkarstareasJourdeetal.,2005;9Interbasinoverowand/orredistributionofthecatchmentareascausedbygroundwaterrising;9LimiteddischargecapacityofmanykarstspringsBonacci,2001;10Limitedcapacityofswallow-holesBonacci,1987.Aspecialcaseofashoodingwasthecatastrophicood-ingofthetownofCetinjeandtheCetinjekarstpoljeSer-biaandMontenegroinFebruary1986.Thiscatastrophickarstashoodisanexampleoftheunfortunatecoinci-denceofenvironmentalextremesextremelyintensivepre-cipitationandfastsnowmelting,thelimitedswallowcapac-ityofponorsandineffectivehumaninterventioninthekarstmassifbythecuttingofanarticialtunnelBoskovicandZivaljevic,1986;Mijatovic,1987;BonacciandZivaljevic,1993;Bonacci,2004.Forkarstashoodsthevolumeofthehydrographismuchmoreimportantthanincasesofnon-karstashoods.Duetoveryfastinltrationrates,overlandowandtheex-istenceofopenwatercoursesonkarstterrainsarerare.Prac-ticallyallrainfallquicklypenetratesthekarstundergroundwhereitllskarstvoidsofdifferentdimensions,andatthesametimeowsundertheinuenceofgravity.Astheca-pacityofkarstvoidsisnotlarge,groundwaterlevelrisingisveryfast.Thisisespeciallymanifestedduringshort-termintensiveprecipitation.Inthistimethesystemofkarstcon-duitsbecomespressurized.IntheDinarickarsttheintensityofgroundwaterlevelrisecanexceedavalueof30mh)]TJ/F36 7.57 Tf 5.91 0 Td[(1Bonacci,1987,1995.Becauseofthepreviouslymentionedreasonsfossilandinactivekarstconduitsandspringsareactivated,and Nat.HazardsEarthSyst.Sci.,6,195– 203 ,2006www.nat-hazards-earth-syst-sci.net/6/195/2006/

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O.Bonaccietal.:Karstashood199 groundwaterbreakthroughonthesurfaceoccursinmanyun-expectedplaces.Theunexpectedandrushedoccurrenceofmanyabundantintermittentkarstspringsisoneofthemaincharacteristicsofkarstashooding.Oneconsequenceofthisisthatgenerallyitisnotpossibletomeasuredischargesduringkarstashoods.Thisrepresentsoneofthegreatestobstacles,whichmakesitimpossibleforunderstandingkarstashoodphenomenaanditsmodellingandcontrolling.AnattempttopresentschematicallythreephasesofkarstashooddevelopmentisgiveninFigs.1,2and3.Figure1showstheso-calledzerostate,i.e.thesituationofakarstsys-temimmediatelybeforethestartofintensiverain,andafteralongdryperiod;thegroundwaterlevelislow,thewaterta-bleisbelowthelowestterrain,andthegroundwaterdoesnotowthroughthekarstconduits.Someofthekarstconduitsarecloggedbyclayorstones,andintheanalysedareatherearenofunctioningkarstsprings.Figure2showsthesituationimmediatelyafterthebegin-ningofintensiveprecipitation.Thegroundwaterlevelisstillatthesamepositionasduringthezerostatebuttheexistingkarstconduitsarefullwithwater.Theowwithinisunderpressureandthegroundwaterinthekarstconduitsisveryhigh.Duetogroundwaterpressuresomeoftheclogsintheconduitsareremovedandintermittentspringsoccurredontheslopesandinthelowlands.Iftheintensityandquan-tityofprecipitationarelargeenough,overlandowonsomepartsoftheterraincanoccur.Duetothismanifestationoffactorstheashoodstarted.Thedurationofthisphasewasnotlong:itlastedfromtenminutestoafewhoursBonacci,1995.Duringthistimelargeandwellconnectedkarstcon-duits,inwhichfastturbulentowsexisted,werelled.Theowunderpressurewasformedonlyinthekarstconduits,whereassmalltectonicandkarstssures,whichformmatrixandintergranularpermeability,werenotyetlledbyground-water.Smallkarstssuresstartedtollbyaslowlaminarortran-sitionalowregimeAtkinson,1977.ItshouldbenotedthatmaximumgroundwaterlevelamplitudeindeepandbareDinarickarstcanreachmorethan120mBonacci,1987;BonacciandRoje-Bonacci,2000.Theepikarstzonelledwithahugequantityofwater.Figure3showsthenextphaseofthesamekarstashoodwhengroundwaterinakarstaquiferreacheditsmaximumvalue.ThisgroundwaterlevelislowerthaninthephaseshowedinFig.2wherethegroundwaterlevelroseonlyinkarstconduits.IntheDinarickarstthisdifferencecanbeconsiderable,morethanafewtensofmeters.Veryoftentheconsequencesofkarstashoodsarelandslidesandmud-slides.ThephaseshowedinFig.3lastedlongerthanthephaseshowedintheprecedingFig.2,andtheirdurationde-pendsonlocalhydrological,geomorphologicalandhydroge-ologicalcharacteristics.Thedurationofanyoodsinkarstareasstronglydependsonthecapacityofnaturalandarti-cialevacuationorgans,especiallyswallow-holes. Fig.5.aSituationoftheRomanwellgallery;bCross-sectionthroughtheRomanwellshaftindicatingminimumandmaximummeasuredgroundwaterlevel. 5ThekarstashoodinDecember2004On6December2004astrongkarstashoodaffectedMa-rinabayandthevillageofMarinasituatedonthecentralpartoftheCroatianAdriaticSeacoast,whichbelongstothecen-tralpartofthebareanddeepDinarickarstregion.Figure4acontainsamapofCroatiashowingthepositionofthestudiedarea.Figure4bshowsadetailedtopographicmapofMarinabayindicatingthepositionsoftwowatersupplyshaftsRo-manwellandDolac,groundwaterowdirections,perma-nentkarstsprings,submarinekarstspringsvruljasandtwocross-sectionsA-AandB-B.TheareaunderstudyconsistsofCretaceousandEocenematerialconsistingofmostlylimestoneandonlypartiallyofdolomitesRoje-Bonaccietal.,2000.Theirinltrationco-efcientispracticallyendless.Duetothisreasoninlargerareastheredonotexisttheconditionsfortheformationofneitherpermanentnorintermittentopenwatercourses www.nat-hazards-earth-syst-sci.net/6/195/2006/Nat.HazardsEarthSyst.Sci.,6,195– 203 ,2006

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200O.Bonaccietal.:Karstashood Fig.6.Studyarealocationmapindicatingpositionofnineraingaugingstationsandprecipitationfeltfrom7hon5December2005to7hon6December2005. Fig.7.GroundwaterhydrographmeasuredintheRomanwellshaftfrom1December2005at0hto10December2005at12h. Figure5ashowsthesituationofaRomanwellgallerywithalengthof250m.Figure5bpresentsthecross-sectionofaRomanwellshaftwithadepthofabout80m.Figure5bdes-ignatestheminimumandmaximummeasuredgroundwaterlevelsinthisshaft.TheRomanwellshaftis2.3kmfarfromtheAdriaticSeacoast,whiletheDolacshaftislocatedabout3kmaway.ThegalleryofDolacshafthasalengthof440m,anditsdepthis32m.Theaveragegroundwatertemperatureis14.7C,anditvariesfrom9.9to21.8C.Theaverageairtemperatureisabout14C.Thesalinityofgroundwaterinbothgalleriesvariesfrom8to700withanaveragevalueof187mg1)]TJ/F36 7.57 Tf 5.91 0 Td[(1ofchloridesStambuk-Giljanovic,1997.Theriseofsalinityiscausedbytheintrusionofseawaterduringpumpingindrysummerperiodswhengroundwaterlevelsarelow.Figure6representsamapofthestudiedareawiththeindicatedpositionsofnineraingaugingstationsandareasoodedbythekarstashoodof6December2004.Nearthenameofeachraingaugingstationthequantityofprecip-itationmeasuredduring24hfrom7hon5December2004to7hon6December2005isgiven.Unfortunately,notoneofthesenineraingaugingstationsisautomatic.Itshouldbestressedthatmainpartsoftheprecipitationfellduringonlythreehoursfrom20to23hon5December2004.FromthesedataitisveryprobablethatthecentreoftherainstormcellwasoverMarinabayandthevillageofMarina.TheaverageannualprecipitationmeasuredattheMarinaraingaugingstationduringtheperiodof1964is905.6mm, Nat.HazardsEarthSyst.Sci.,6,195– 203 ,2006www.nat-hazards-earth-syst-sci.net/6/195/2006/

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O.Bonaccietal.:Karstashood201 Fig.8.Cross-sectionsA-AandB-BseeFig.4withindicatedpositionoftwoshafts,minimumandmaximumgroundwaterlevelsduringtheanalysedashoodandoodedareas. withaminimumof419mmin1983andamaximumof1332.1mmin2004.Theaverageannualmaximum24hpre-cipitationmeasuredatthesamegaugingstationintheperiodof1964is72.6mm.Theminimumandmaximumval-uesintheperiodof1964were28.7mmin1983and144mmin1993.UsingGumbelandlog-normaldistribu-tionsthereturnperiodof24hprecipitationof207.9mmhadanestimated800and1000years,respectively.Figure7showsapartlymeasuredandpartlyreconstructedgroundwaterhydrographintheRomanwellshaft.Themax-imumgroundwaterlevelwas60mabovesealevelma.s.l..Until6December2004themaximummeasuredgroundwaterlevelintheRomanwellshaftwas22.5ma.s.l.IntheDolacshaftthemaximumgroundwaterlevelwasabout30ma.s.l.Theintensityofrisinggroundwaterwas2mh)]TJ/F36 7.57 Tf 5.9 0 Td[(1,whiletheintensityofitsfallingwas0.5mh)]TJ/F36 7.57 Tf 5.91 0 Td[(1.InFig.8twocross-sectionsdesignatedinFig.4arepresented.ItisseenthatthegroundwaterlevelcoincideswiththesurfaceoftheterrainontheA-Across-section.Hillslopesorientedinthedirec-tionoftheAdriaticSeawerecompletelysaturatedbywaterfromtheheightofabout30ma.s.l.,whichmadetheforma-tionofoverlandowpossible.Waterdischargingfromkarstundergroundwashighlysaturatedwithsuspendedsediments.DuringabouttenhoursofthekarstashoodinMarinabayandvillagemanyintermittentkarstspringsappeared.DischargefromsubmarinespringsinMarinabaywashuge,butmeasurementswerenotpossible.TherapidrisingofgroundwatercausedbreachesinalltheMarinahouses'cel-lars.Anopenwatercourse.5minheightwasformedthroughthevillagestreet.Thenationalroadwasoodedby0.2mheighwater,whichinterruptedtrafcforafewhoursseeFigs.9and10.Itshouldbestressedthatsimilarphe-nomenahaveuntilnowneveroccurredinthisregion.Damagecausedbythiskarstashoodwasestimatedatabout1.5millionUSdollars.Itisagreatamountforsuchasmallvillage,whichhasnomorethan500permanentin-habitants.Perhaps,somehumanactivitiesacceleratedthehazardousconsequencesofthiskarstashood.Accord-ingtotheexistingdataitisnotpossibletomakeanyreliableconclusionsaboutanthropogenicinuencesonthekarstashoodanalysed.6ConcludingremarksThefactspresentedinthispapergivedeniteproofthatkarstashoodsbelongtoaspecialkindofashoods.Asitisarelativelynewconceptitisnecessarytodevelopadequatemethodsfortheirinvestigationandcontrol.Itshouldbestressedthatindifferentkarstregionskarstashoodswillmanifestindifferentways.Anykarstsystemshowstheextremeheterogeneityandvariabilityofgeologic,morphologic,hydrogeologic,hydrologic,hydraulic,ecolog-icalandotherparametersinspaceandtime.Suchacomplex www.nat-hazards-earth-syst-sci.net/6/195/2006/Nat.HazardsEarthSyst.Sci.,6,195– 203 ,2006

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202O.Bonaccietal.:Karstashood Fig.9.Karstashoodon6December2004intheupperpartoftheMarinavillage. Fig.10.Karstashoodon6December2004inthelowerpartneartheAdriaticSeacoastoftheMarinavillage. systemneedsaninterdisciplinaryapproach.Themostim-portantcharacteristicofkarstashoodisthestronginter-actionbetweensurfaceandgroundwater.Duetothisreasoncloseco-operationbetweentheinvestigationsofhydrologyandhydrogeologyisnecessary.Thesynthesisofhydrologi-calandhydrogeologicalapproachescouldexpediteprogressinunderstandingkarstashoodingandinitscontrol.Anespeciallycomplexanduntilnowunsolvedproblemistheestablishmentofmonitoringsystemsfortheforecastingandwarningofkarstashoods.Itisnecessarytoincreaseeffortstowardsdevelopingasolutioninthenearfuture.Thisislikelytobemanifestinallaspectsofhydrologicandhy-drogeologicsciences,rangingfromimprovedmeasurementsmethods,toadvancedconceptualmodelsofwaterowun-dervaryingkarstconditions.Thesooneritstartsthebetterpreparedexpertsandmanagerswillbetofacethechallengesoftheuncertainsoon-to-comefuture. Acknowledgements. TheauthorswouldliketothankJ.MilkovicandJ.GoretaforprovidingpermissiontopublishprecipitationdataandthephotosofthekarstashoodinMarinaandtheMarinabay.Editedby:F.GuzzettiReviewedby:J.MotykaandanotherrefereeReferences Archer,D.:Wallsofwater,Circ.BritishHydrol.Soc.,44,1,1992. Atkinson,T.C.:DiffuseowandconduitowinlimestoneterraininMendipHills,SomersetGreatBritain,J.Hydrol.,35,93–100,1977. Bakalowicz,M.:Karstgroundwater:achallengefornewresources,Hydrogeol.J.,13,1,2005. Bonacci,O.:KarsthydrologywithspecialreferencetoDinarickarst,SpringerVerlag,Berlin,1987. Bonacci,O.:Groundwaterbehaviourinkarst:exampleoftheOmblaSpringCroatia,J.Hydrol.,165,113,1995. Bonacci,O.:Analysesofthemaximumdischargeofkarstsprings,Hydrogeol.J.,9,4,3288,2001. Bonacci,O.:Hazardscausedbynaturalandanthropogenicchangesofcatchmentareainkarst,4,655,2004. Bonacci,O.andRoje-Bonacci,T.:Heterogeneityofhydrologicalandhydrogeologicalparametersinkarst:examplesfromDinarickarst,Hydrol.Processes,14,14,2423–2438,2000. Bonacci,O.andZivaljevic,R.:Hydrologicalexplanationoftheowinkarst:exampleoftheCrnojevicaspring,J.Hydrol.,146,405,1993. Boskovic,M.andZivaljevic,R.:HidroloskoosvrtnapoplavuCetinjaod18.do20.02.1986.Hydrologicreviewofthe1986Cetinjeood,Vodoprivreda,18,10203,259,1986. Carpenter,T.M.,Sperfslage,J.A.,Georgakakos,K.P.,Sweeney,T.,andFread,D.L.:Nationalthresholdrunoffestimationutiliz-ingGISinsupportofoperationalashoodwarningsystems,J.Hydrol.,224,21,1999. Choquette,P.W.andPray,L.C.:Geologicnomenclatureandclassi-cationofporosityinsedimentarycarbonates,Bull.Am.Assoc.Pet.Geol.,54,20750,1970. Cohen,H.andLaronne,J.B.:HighratesofsedimenttransportbyashoodsintheSouthernJudeanDesert,Israel,Hydrol.Pro-cesses,EarlyView,2005. Hall,A.J.:Flashoodforecasting,OperationalHydrologyReport,no.18,WorldMeteorologicalOrganisation,Geneva,1981. Hess,J.W.andWhite,W.B.:Stormresponseofthekarsticcarbon-ateaquiferofsouthcentralKentucky,J.Hydrol.,99,235,1988. Ford,D.C.andEwers,R.O.:Thedevelopmentoflimestonecavesystemsinthedimensionsoflengthanddepth,Can.J.EarthSci.,15,11,1783798,1978. Ford,D.C.andWilliamsP.W.:Karstgeomorphologyandhydrol-ogy,UnwinHyman,London,1989. Jourde,H.,Roesch,A.,Guinot,V.,andBailly-Compte,V.:Dynam-icsandcontributionofkarstgroundwatertosurfaceowduringMediterraneanood,Proc.Int.Symp.WaterResour.Environ.ProblemsinKarst,BelgradeandKotor,133–138,2005. Kiraly,L.:Groundwaterowinfracturesrocks:modelsandreal-ity,14thMintropSeminaruberInterpretationsstrategieninExNat.HazardsEarthSyst.Sci.,6,195– 203 ,2006www.nat-hazards-earth-syst-sci.net/6/195/2006/

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O.Bonaccietal.:Karstashood203 plorationundProduktion,RuhrUniversitatBochum,159,1–21,1994. Lin,X.:Flashoodsinaridandsemi-aridzones,IHP-VTechnicalDocumentsinHydrology,no.23,1999. Mijatovic,B.:CatastrophicoodinthepoljeofCetinjeinFebruary1986,atypicalexampleoftheenvironmentalimpactofkarst,Proc.ofthemultidisciplinaryconf.onsinkholesandtheenviron.impactsofkarst,Orlando,Balkema,Rotterdam,299,1987. Motyka,J.:Aconceptualmodelofhydraulicnetworksincarbonaterocks,illustratedbyexamplesfromPoland,Hydrogeol.J.,6,4,469–482,1998. Roje-Bonacci,T.,Miscevic,P.,andTurkovic,A.:Reconstruc-tionofverticalshaftonpartpassingthroughclayinkarstlime-stone,Proc.EUROROCK2000Symp.,Aachen,22328,27March2000. Smith,K.andWard,R.:Floods–physicalprocessesandhumanimpacts,JohnWileyandSons,Chichester,1998. Stambuk-Giljanovic,N.:RimskibunaruGustirnikrajTrogiraRo-manwellinGustirnanearTrogir,HrvatskaVodoprivreda,VI,57,494,1997. Trudgill,S.:Limestonegeomorphology,Longman,London,1985. Ward,R.C.:Floods:ageographicalperspective,Macmillan,Lon-don,1978. White,W.B.:Karsthydrology:recentdevelopmentsandopenquestions,Eng.Geology,65,85–105,2002. www.nat-hazards-earth-syst-sci.net/6/195/2006/Nat.HazardsEarthSyst.Sci.,6,195– 203 ,2006


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