那位英语高手帮翻译一下~小弟急用~

Deploymentanddisposalofsedimentinareassuitablefortheirremovalandwhere

interferencewiththemaintenanceoperationsforthedamisreduced.

Thechoiceofthebestdesiltationstrategyisgenerallymade,havinginmindthesedimentload,granulometry,besidestothephysicalandhydrauliccharacteristicsofthereservoir.

Dredgingormechanicalexcavationremovessedimentdepositsfromreservoirsby

hydraulicdredgingordryexcavation,evenifthesemethodsareexpensivebecauseofthelargevolumesofinvolvedmaterialandthedifficultytofindasuitabledisposalarea.However,onesalargesedimentvolumeisdepositedinareservoir,oftenexcavationistheonlyavailablemanagementoption.

Hydraulicflushinginvolvestheloweringofthewaterlevelandthesubsequentremovalofsedimentthroughtheopeningofthebottomoutlettoestablishaflowalongtheimpoundedreach,formingaconeoferosionwhoseeffectisshortintime.Suchaprocedureisveryeconomicalsinceitdoesnotrequireanyparticularapproachandexploiteshydraulichead.

Neverthlessitpresentssomelimitations.Itbringstodrawdownortoemptythereservoir.

Besides,flushingcausesthesedimenttobereleasedfromthereservoiratamuchhighersedimentconcentrationthaninanaturalcondition.

Siphondredgesaredifferentfromaclassicaldredgebecausethepumpisnotpresent;inthiscasethesedimentmovesinapipelinebymeansofanhydraulicheadbetweenthewatersurfaceandthedischargepoint,thatisgenerallylocatedatthelowestavailablepointdownstreamthedam.Thechoiceoftheremovalandtreatmenttypologyofthesedimentinareservoiriscomplexanddependsontheinteractionmultipleoffactors.Inpractice,therearemanyvariablestobetakenintoaccountandthecorrectchoiceisstrictlyrelatedtothevolumetoberemovedandtothereservoircharacteristics.

Inordertobetterexplainthelimitsofapplicability,ashortlistofexamplesofdesiltation

operationsarereportedinthefollowing.

3.1WarsakDam(Pakistan)

Thehighsedimentconcentrationpassingthroughthepowerhouseproducedasevere

damagetotheturbines,whichsuggestedtheintroductionofasemi-circularskimmerwallinfrontofthereservoir(KingandStevens,2001).Thepurposeoftheskimmingwallwastodivertalargepercentageofthematerialfromthepowerfacilities.Thematerialwasthenremovedbyflushing.Theinvestigationhighlightedthatflushingoperationsaresufficientlyefficientwhentheyarecarriedoutfor30daysinayearwithareductionoftheloadcloseto30%,otherwiseitispossibleduringtheflushingoperationtosimplyshutdownthepowerhouseandopenthebottomoutletsfor20daysduringthefloodpeaktoobtainareductionof58%.Theskimmingwallissuitabletoreducebedloadbutthereisnotpositiveeffectforthesuspensionloads,

evenifan“indirect”benefitcouldbeobtainedbyshuttingdownthepowerhouseduringthehigh-suspendedsedimentconcentration.

3.2Tarbela(Pakistan)andAmbuklao(ThePhilippines)Dams

Evaluatingsomeprojects(Tarbela,inPakistan,andAmbuklao,inthePhilippines),Alam(2002)suggestastep-by-stepsolutiontothesedimentationproblemofthereservoirthroughtheseparationandextractionofsediment.Thepartialremovalofthecoarsematerialintheupstreamdam,byemployingflowdiversiondams;theremovalofabrasivesedimentinstages,usingthesettlingbasinandsmallreservoirsbeforeitreachestheturbinesand,finally,theremovalofthefinermaterialthroughflushing,sluicingandtheuseofmultipledamsandreservoirs.

3.3MauvoisinArchDam(Switzerland)

Durand(2001)reportssomeconsiderationsaboutpossiblestrategiesforthesolidremovalintheMauvoisindaminSwitzerland.Thisdam,builtin1951onaRhoneaffluent,LaDransedeBagnes,withaheightof250mandavolumeofabout203Mm3isthehighestarchdamoftheWesternEurope.Theupstreamriverstransportapproximately300,000m3ofalluvialmaterialperyear.In30yearsofoperationabout10Mm3ofsedimentshavebeentrappedinthelake.

Themanagementcorporation(LesForcesMotricesdiMauvoisinS.A.)proposedvarioussolutionstotheproblem:thesurgeofsedimentclosethebottomoutlet,thecreationofapredamtodredgeandtheraisingofthebottomoutletandsluicebyabout20m.Theeffectivenessandthefeasibilityoftheabovementionedstrategieswerepartiallycompromisedbytopographicalandstructuralfactorssuchasanhardentrytothedamandthepresenceofatunnelofsmalldimensions;thecurvingofthedam,thedepthofthesedimentlevel(approximately175m),thepresenceoficeandanavalancherisk.Theadoptedsolutionwastoraisethelevelofthebottomoutletandsluice.

3.4VerboisDam(Switzerland)

TheVerboisdam,whichinterceptstheRhodeandtheArverivers(Switzerland),since1965,haslostabout17%ofthewatercapacity(Hofmannandetal.,2001).Theaverageannualsedimentvolumeisestimatedinabout415,000m3,removaloperationsarenecessaryinordertoguaranteetheperfectfunctionalityofthepowerhousefacilitiesandalsoofkeepingfreethechannel,usedbythebargestotransporttheGenevasolidwastetotheincineratoratCheneviers.UptonowtheperiodicemptyingoftheVerboisreservoirhasbeentheonlytechniqueadoptedinordertoavoidthesedimentationprocessofthereservoir.Thisoperationisnormallycarriedouteverythreeyears,inJune,whengenerallythefloodeventsoccurinthe

Rhode.Everyflushingoperationevacuatesapproximately370,000m3ofsediment.

3.5GénissiatDam(France)

ThedesiltationoftheGénissiatdam(France),100mhighandwithastoragevolumeofabout60Mm3,iscarriedouteverythreeyearsbymeansofemptyingprocedureconductedwithadischargeof450m3•s-1inaseasoninwhichtheRhodehasanaverageflowrateof360m3•s-1(Bouchard,2001).ThesimultaneousemptyingoftheupstreamSwissreservoirsmakehardertheoperations.Theflushingiscarriedoutintwophases:aninitialloweringofthewaterleveltoevacuatesedimentfromGénissiatreservoir,followedbyapartialfillingupinordertofacilitatethepassageofsediment,comingfromtheSwissreservoirs.Thewholeoperationissubordinatetosomelimitations,whichdonotallowtheGénissiatdamtoexceedasuspendedsolidconcentrationof10g•l-1,evenifthereleasesoftheSwissdamsproduceahigherupstreamconcentration.

3.6AlgerianDams

ThefirstdamsbuiltinAlgeriabefore1900werecompletelyfilledupbysedimentand

actuallytheycannotbeused;thetotalwatercapacityofover70reservoirsisabout4,340Mm3ofwhich20%isalreadylostduetothesedimentload.Itcanbepredicted,usingtheactualtrend,thatattheendof2010morethan¼ofthetotalwatercapacitywillbelost(Benblidia,2001).ThemoreimpressivecasesarerespectivelytheFergougdamwhichwaspartiallydestroyedin1927bythethrustduetofloodwaterandmud,thed’OuedFoddadamwherethebottomoutletisunder40mofmudandthed’IghilEmdadamwhere,duringtheextractionof17,5Mm3ofmaterialbywayofthedensitycurrentmethod,between1965and1982,350Mm3ofwaterwerelost,about20m3ofwaterforeverym3ofextractedsediment.Actually,inAlgeria,theusualtechniqueisthemechanicaldredges,whichcanbedissembledandtransportedfromonesitetoanother.Between1962and1980approximately19Mm3wereextractedfrom4reservoirs,usingadredgewithacapacityof340,000m3•(month)-1,butsuchamethodneeded5m3-6m3ofwaterforeverym3ofextractedmud.Attheendofthe80sanewmodelofdredgewasintroduced.Itwascharacterizedbyaextractionratioequalto1,whichallowedthisdredgetoextractabout4Mm3ofmudfromFergougdamin6months;themuddeliveredintotheriver,wasdirectedtothesea.Thegainedbenefitswerecomparedwiththoseofthedesiltationoperations,the

damraisingandeventheconstructionofanewdam.Bearinginmindthecostperm3ofrecoveredwater,itturnedoutthatthecostofthedredgingoperation,whilebeingthesameasthatoftheconstructionofanewdam,isbyfarinferiortoadamraising.

3.7JarnsjonLake(Sweden)

AsurveyontheJarnsjonLake(Sweden)putinevidenceasignificantpresenceof

contaminatedsediment.Theadoptedsolutioninordertoreducethespreadingofthecontaminatedsedimentisaspecialdredging.Indetail,adryexcavationwithinaprotectivegeotextilescreen,abletocontainmostofthecontaminatedsediments.Thetreatmentofthedredgedmaterialincludesafurthermechanicaldewateringandthefinaldisposalinalandfill,wherethehighlycontaminatedsedimentareseparatedfromtheslightlytoxicbywayofageotextilescreen.Beforereachingsuchachoice,apossiblesolutionwastodeviatetheriverandtodepositthedredgedmaterialinacompletelyisolatedarea;however,theriverischaracterizedbyveryhighdischargeandthisrequirestherealizationofthediversionchannelbeforethemaximum

dischargeoccurs;moreoverthelakedredgingwouldhavecreatedfurtherdifficultiesbecauseitisinanareawithglacial-fluvialdeposits,characterizedbyanlargemobilityofthegroundwater.

3.8CachìReservoir(CostaRica)

The54Mm3CachìhydropowerreservoirhasbeenconstructedontheRaventzonriver

(RicaCoast)in1966.In1973itwascompletelyemptiedusingtheflushingoperationandinthefollowing18yearsitwassubjectedto14flushingoperation.Inthisway,thesedimentvolumewasreducedfrom82%to27%(MorrisandFan,1997).

Theflushingoperationswereaccuratelyoverseenanddocumentedtoassesstheir

effectiveness.Theoperationswereorganizedinthreedifferentphases:aslowloweringofthewaterlevelfollowedbyarapidloweringlasting5-10hours,andfinallyafreedischargephaselastingtwo-threedays.Obviouslythereleasedsedimentvolumewasstrictlyrelatedtotheflowrate,theincomingsolidtransportandtheperiodwhentheflushingwascarriedout.Inparticularitwasfoundthatthehighervaluesofsolidtransporttakeplacebetweentheendofthephaseofrapidloweringandthatoffreedischargewithapeakconcentrationof400mg•l-1.

3.9PiaganiniDam(Italy)

Bartolettietal.(1990)describedtheoperationplannedtoremoveasedimentvolumeof500,000m3takenfromPiaganinidamontheVomanoriver(CentralofItaly),sotorestoretheoriginalwatercapacity.TheremovalofsedimentbymeansofadryexcavationandthedirectdisposalofwatersolidmixtureintheVomanoriverhadsomedifficultiesrelatedtotheneedofensuringthatthenaturalflowratehadturbiditycharacteristicscompatiblewiththeItalianlawrequirements,evenifsuchastrategywasfavorabletodevelopmentofbeachesclosetothedeltariver.Thestrategyadopted,instead,wasthehydraulicdredgingofsedimentafteranopportunedewateringprocessanditsfollowingtransporttoadisposal.Theuseofwatertighttruckswas

imposedinordertoavoidanywaterormudloss.

Thedewateringprocessinvolvedtheuseof:

-Arevolvingscreen,adoptedtoseparateparticlesofdimensionsover20mm-25mm;thesewerethentransportedtothedumpbytrucks;

-Acentrifugalmudcleanerinordertoseparatethesandhavingagranulometryover

74μm;

-Winnowdewatering,bywhichwaterwasseparatedthroughvibrationfromsand.Thesandcharacterizedbyahumiditylevellowerthan30%,wascarriedtoabinbymeansofaconveyerbeltand,thendumpedinalandfillareabytrucks;

-Aflocculationtank,wheretheincomingsedimentflowfromthecentrifugalcleaner,

wasaddedtoachemicalsubstancesinordertoimprovetheseparationofsolidparticles.

-Twoclarifiers,inparallel,toseparatemud;theclearwaterwassendedbacktothelakeandthemudwascarriedtothelandfillarea.

3.10PozzilloReservoir(Italy)

Tamburinoetal.(1993)reportastudyregardingtheremovaloperationsonPozzillo

reservoir,situatedontheSalsoriver(Sicily,Italy).Theusualmanagementinvolved

remarkablerangesofthewaterlevel(upto30m)overaveryshorttimethatinducedsmallbankmudslideswithconsequentsedimentdepositionsclosetothegates.

Atthattimethewatercapacitywasabout123.7Mm3incomparisonwith140.5Mm3

effectivelyavailableatthestartofthedamoperation.Thesolidleadtothereservoirwasestimatedatapproximately29Mm3between1959and1984,correspondingto2,000m3•yr-1perkm2ofcatchment.

Theoperationscarriedoutinordertoreducethesolidcontributionhaveconcernedtherivernetworkupstreamofthedam,bymeansofmaintenanceworksoftheupstreamcatchment(520checkdamsand50bottomsteps).

Settlementoperationswerealsocarriedoutinordertocontrolthesiltationprocess.Thefirstwasaimedatremovingthedepositsclosetothebottomoutletsinordertoallowthepassageofanaturalincomingflowthroughthegates.

Thedisadvantagesofsuchoperationarerelatedtothelownaturalflows,theriskofthedisfunctioningofthegatesinthecaseoffloodandtheelevationoftheriverbeddownstreamofthedam.Thesecondoperationhadtheobjectivetolimitthesolidloadclosetothebottomoutlets,inordertoaddressthesedimentationandaccumulationinthecenterofthelake.Suchaprocedurewascarriedoutbymeansofasmallvolumenearthegates;howeversuchanexpedienthasonlyashortvalidity.

3.11RendinaDam(Italy)

TheAbateAloniadamontheOliventoriver(SouthernItaly);wasrealizedinthefiftiesandhasaupstreamcatchmentof400km2andalakewithawatercapacityofapproximately23Mm3.Thesedimentationdynamics,intheseventies,madedesiltationnecessary,throughdredgingduringwhich,asedimentvolumeofabout2Mm3-2.5Mm3wasremovedfromthelake.Currentlyanotherdesiltationusingthesamemethodhasbeenplannedtorecoveravolumeofapproximately1.5Mm3buthasnotyetbeencarriedout.SuchanoperationneverthelesswillbeaccompaniedbysubsequentsedimenttreatmentofthesametypesuchasthePiaganinidam.

3.12PontecosiCatchment(Italy)

Lucignani(2002)reportstheprocesses,whichwerecarriedoutduringthedesiltationofthePontecosicatchment(NorthernItaly).Theadoptedprocedurewasthatofflushingthroughthegatesafterwhichthesedimentwasreleasedatdifferentintervalsinordertopreventandtominimizetheharmfuleffectsontheaquaticandfluvialenvironment.Actually,theadoptedflushingprocedurewasrealized,bywayofawaterrelease,somemonthsbeforethecompleteemptying,throughthebottomgatesforshortintervalsoftime,butinconcomitancewiththefloodperiod.Thereforesuchanoperationtakestheshapeofanextensionintimeofanaturalevent,tobedeterminedaccordingtothecharacteristicsofeachevent.Itisanticipatedtherefore,byakindof“preparation”,inordertoreachtheplanneddateforthetrueemptyingwiththecatchmentrelativelyclearofsedimentandwithamodestecologicalimpact.Thepreliminaryphaseofemptyingofthereservoiristhechemicalandeco-toxicologicalanalysisofthesedimentlawsoastoexcludethepresenceoftoxic

substances,whichcouldcompromisetheriverecosystem.

Theoperationsinvolvedaflowratevariablebetween70m3•s-1and100m3•s-1witha

removalsedimentvolumeequalto29,000m3.

4FINALREMARKS

Eventhoughatfirstglance,themoreefficientmethodsappeartobethe“freeflow

flushing”andthe“flushingunderpressure”,itisclearthattheseapproachessufferof

problemsrelatedtheheavysolidconcentrations.Flushingisthereforelimitedand,insomecases,rarelyusedinthemanagementofdamsintheMediterraneanareaandsemiaridclimatebecauseofthereducedwateravailability.

FrenchandSwissexperienceshaveshowedhowdifficultcouldbethecontrolofthe

sedimentconcentrationduringtheflushingoperation.Infacttheaveragesediment

concentrationslevelverified,morethan5g•l-1,couldbedangerousfortheecosystem.

AmongtheItalianexperiencesofdesiltationwithflushing,themostinterestingwasrelatedtothePontecosireservoir,evenifthetotalsedimentvolumeremovedwasverylow(29,000m3).

Asfarasthelegislativestandardsareconcerned,thebestexamplewasconductedonthePiaganinidam:thedesiltingoperationshavebeenperformedbyinsertingsomesectionsforchemical-physicaltreatmentintotheoperationlinesinordertoreducethesedimentconcentrationinagreementwiththelawandtoachievethemaximummoisturevalueforsolidwaste.Moreover,suchtechnologyisusedinthepresenceofanysubstancethatcancausethewastetobeclassifiedas“special”,howeverinthiscase,asforharbourdesiltation,itisnecessarytointroduceatreatmentlinetoclarifywater.Thelimitofapplicabilityofthistechnologyisrepresentedbythehighcostofthetreatmentsince,inthatofthePiaganinireservoirthecostwasequalto15€/m3.Itisimportanttospecifythatthistreatmentalsoinvolvesapartialrecoveryofselectsediment(gravelandsand)tobereusedasaggregateproductionandthatthetransportcostisabout40%ofthetotalvalue.Notwithstandingthehighcosts,thesameactivitiesarenecessary,eventorestoretheefficiencyofthebottomoutlet,

bypass,etc.Anywayacaseacost-benefitanalysismustbeperformedinordertoverifythecost-efficiencyofthispracticecomparedtoWatershedscouldbecompetitive:withtheseremovaltechniques.

Somestudies(Balaccoetal.,2003)putinevidencethatmanydesiltationtechniquestorecoverwaterresourcesareuneconomic:therecoveredwatercapacity,infact,willbelostagainafterthefollowingfillingupphenomena,thereforethecostofthatrecoveredvolumewillbehigherthan1,5€/m3.

Theshortanalysisreportedsuggeststhatasiltedupreservoiranditsrehabilitationareopenproblems,thecomplexityofwhichisrelatednotonlytothetechnicalaspectbutalsotolegislativerestrictions.

Thechoiceofstrategycouldbeapproachedbytwodifferentstages:itisnecessaryto

improvethestudiesandtofindapproachesabletoreducethesedimentcontribution,butalsothetechnicallegislationhavetolookintotheproblemheavingmoreattentioninordertoestablisharealdifferencebetweenthesestrategiesandthegeneraldisposaloperations.