longshore sediment transport pattern along romanian danube delta coast

7/27/2019 Longshore sediment transport pattern along Romanian Danube delta coast

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L. Giosanet al. LongshoreSedimentTransport atternalong RomanianDanubeDelta Coast

LONGSHOREEDIMENTRANSPORTATTERNALONGROMANIANANUBE ELTACOAST

Liviu rosnru',enry oxulrtewtczl,Nicolae ANrN',lulianPosroLAcHE31NewYork StateUniversity,MarineGeosciences enter,StonyBrook,USA,E-mail: [email protected] tute f MarineGeology ndGeo-ecologyf RomaniaGeoEcoMar,DimikieOnciuStreet, o.23-25, ucharest0318,RomaniaTel/Fax: 4AJ 252.25,94, -mail: [email protected] iruc.ro3Romanian arineResearchnstitute MRI,Bd .MamaiaNo.304, onstanta 700,RomaniaTel +49-41643 288,Fax.+40-4183127 ',E-mail: mri@alpha mri. o

Abstract. The paper presents he littoralsediment drift patternsalong the RomanianDanube Delta coast. Using sand budgettechniques nd simulations i th he "ShoreModell ing ystem" oftware ackage f U.S.ArmyCorps of Engineershe longshoresediment ransportwas computed or each of the six sections aken nto consideration ithin he DanubeDeltacoast zone (about140 Km-long). The current state of the beach dynamics along the coast of the Delta are presented herein and predictiveobservations re made. Some actionsare proposed o be taken n order o impr ove he monitoring f the Romanian Black Seacoaslz0ne.Key words: littoral edimentssedlmentransport, imulation,oast one,BlackSe a

INTRODUCTIONMany deltas around the world have beenexperiencinga drastic change in their evolution

due to anthropogenic lteration f both water andsediment discharge, coupled with vigorouslongshore sediment transport. In the case ofDanube delia, coastal erosion s threatening heecosystem of wetlands, and coastal lakes.Furtherrnore he chronic sand deficit extendsdownstreamof the delta coast, negativelyaffectingth e important tourist-economyof the seasideresorts n southernsector of the RomanianBlackSea shore.Several actorshave been dentifiedoexplain he recessional ehaviour f Danubedelta:(1) alternate hannelswitchingwhich divertsmostof the river sediment discharge to a singledistributarystarving the earlier built lobes; (2)generaldecrease n the river sedimentdischargedu e to damming, dredging and agriculturalpractices in the Danube drainage basin; (3)engineering tructureswhich disrupt he longshoresediment ransportpattern;and (4) relativese alevel rise. The tidal processesar e not importantsince the mouth of the Danube is a microtidalenvironment.

This situation s not unique but also found inothermicrotidal eltaiccoasts ike hoseof the Nile(Sestini , 992;Stanley, 996), he Po (Capobiancoet al . ,1995), nd he Ebro Jimenez ndSanchez-Arci l la,1993;Palanques nd Gui l len, 995).Theconstruction of dams and other river controlstructures n the drainagebasin, dredgingalong

the river, he entrapment f water and sediments nthe drainage asinand deltaicplain or agriculturalpurposes,he decrease n the rainfall attern,havebeen variously cited as important factors indecreasing he rivers sediment discharge andfurthermoreavouring he change rom a generalprogradationalcharacter of a delta into arecessionalone. The river sediment dischargehave been reduced95% from the total oad in theEbro case (Palanques nd Gui l len,1995), and98% for the Nile (Sestini,1992), even though insome places, ike the Ebro extensiveagricultureand deforestation have had contrary effects(Palanques nd Gui l len, 995).At the same t imeth e acuteorientationo the principalwavedirectionof somedeltaiccoastalsegments end to produceintense ongshore ediment ransport.This is thecase of Nile delta coast where net longshoresediment ransport atesup to 1,200,000m'/yearwereestimatedQuelennicndManohar, 997), rVolturno,a small cuspate delta on the westerncoast of ltaly, where the maximum net rate wasestimated o be 1,760,000mt/year (Benassaieta l . ,1995) .

The objective f this studywas to analyse hecurrentbehaviour f the RomanianDanube deltacoast. We will show the importance of the waveinduced longshore sediment transport andanthropogenically modified by engineeringstructuresn controll ing horelinedynamicsalongthe Danubedeltaic coast partially naciivatedby

GEO-ECO-MARIN , 2/1997National nstitute of Marine Geology and Geo-ecologyof RomaniaProc. lntern- Workshopon 'Fluvial-Marinelnteractions" n Malnas,Romania, Oct.1-7, 1996


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L_Gy:zl:!:!:j:tg:!",."- -2edimentTransport attemalongRomanianDanubeDelta Coastalternate hannelswitching. he energeticwaveregime n the caused by the preponderanceflocalwaves due to relatively hort etches,by thehigh angle of wave attack,and by the ineffectiverefraction n a relatively teepnearshore.STUDYAREA

GeologicalSettingThe Romanian Black Sea coa st (Fig.1)stretchesover 245 km from the Chiliadistributaryof the Danube (45"12' N, 29"40' E) at theRomanian-Ukraineorder o t he town of VamaVeche (43"44' N, 28'35' E) at the border withBulgaria.The Romaniancoast can be dividedusing geographic nd geomorphic ri teria nto anorthernunit and a southernunit (Paninet al . ,'1979-1994).The northern unit is the low-relief

Danube eltacoastal onewhi le he southern ni tis characterisedby eroding cliffs and loess,protected n places by narrow beaches(Charlierandde Ju l io , 985;Pan in , 979-1994) .

Fig.1 RomanianBlack Sea coast. DanLtbeDelta coast(includinghe adjacent, enetical lyelated, aymouth arrier nfrontof Razelm-Sinoeagoons) xtendsromChil iao Midia.The Romaniandeltaic coast extends rom theborderwith Ukraine o Midia n the south.Thestretchof shorel ine hich ncludesMusura ay inthe north. and ext ends to the southernmostdistr ibutaryf the Danube,Sfintu Gheorghe, i ththe adjacent Sakhal in barrier is land (f ig.1)constituteshe major contactof the Danubedelta

with he BlackSea.Reworked el taic andsalongthe shorehavebuilta coastalbarrier omplex romSfAntuGheorghe o Midia. The barrier complexconsists mainly of a baymouth barrier whichseparates he large lagoon complex of Razim-Sinoe, nceopen Panin, 983), rom he sea.Theshelf s broadest n the front of Danubedelta andnarrows o the south.Th e nearshore radientso a12-15 m depth along the Danube delta coast,rangebetween .003and 0.01,with the steepestslopesalong he Sakhal inbarrier sland (Panin,1985).The beachprofi le s general lymult ibarred(Postolachet al . ,1992).

The modernsediments n the nofthernsectorof the coastconsistof Danube-borneuartzsands(about 0%si l ica, anin, 989). he heavymineralcontent s about 3%. Sands carriedby the l i ttoraldrift from the regionnorth of Danube delta havehigher i l ica ontent han he Danubian ediments(90%). The subaerial beach sediments aregeneral ly medium-fine sands (d50


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L- Giosan t al. Longshore ediment ransport aftern longRomanian anubeDeltaCoastDriga, 986). heDanube elta ubsides ith1.3-2 mmlyear, due to the compaction f deltaicsediments nd regionalectonics.he ideof theBlack Sea along the Romanian coast issemidiurnal,ith a rangeof 7 lo 12 cm. t hasanunnoticeable ffect when comparedwith otherdeformationalluctuations,uch as seichesorstormsurgeswhichcan reacha maximum f 2 mheight,and 1 2 to 1,5 m respectivelyBondar,1972;Paninet al., 1979-1994). ther mportantmeansea evel luctuationsith multiannualndseasonal cyclicity are due to river discharge,changes n the water exchange hrough heBosphorus traight,and precipitation/evaporationvariations.

RiverDischargeThe most significant factor affecting the

hydrologicudget f heBlackSea s he seasonalvariation f riverdischarge.he Danube rovides38% of the riverwater entering he BlackSea(Glaskow, 970).Other mportantivers n ermsoftheir waterdischarge re Dniepr,Southern ug,andDniestr. anube's nnual ischarges highestfrom April o July. n responseo the variationnriver discharge,he sea level luctuates etween20 and 30 cmfromseasono season ndabout20cm from year to year (Bondar, 972).Between1858and 1988 he Danube ischargednnuallyabout 191 km3 of water, of which 63% wasdischargedhrough he Chiliadistributary,7%through he Sulina distributary nd 20% throughthe SfAntuGheorghe istributaryBondar t al.,1992).The Danubewater dischargencreasedfrom178km' in 1858 o 203 km'in 1988. hemean otalsediment ischargef theDanube asabout52x106metric ons per year between 858and 1966 Bondar t al., 1992).The sedimentdischargen 1858was65x10o etricons,bu tonly38x10"metric ons n 1988 Bondar t al., 1992).Thisdischarge as distributed5% o the Chilia,214/o o the Sulina and 23% to the SfAntuGheorghearm. About 25-30% educt ion f thetotalsediment ischarge asbeen ecorded fter1970when he irst ronGates amwasclosednthe Romanian-Yugoslavianector of Danube(Panin t al., 1979-1994;opa,1992). he bed-loadsediment ischarge,hat hasa median rainsize between .1-0.5mm, has beenestimatedobe between .5o/o nd 19o/o f the totalsedimentdischarge. Today, ^the Chilia distributarydischargesbout3x10ometricons/year ed-loadsediments between57o/o fid 65% of Danube'stotal bed-loaddischarge),Sulina about 0.85-1.3x106metric ons/yeir' between18.5%and24.5o/oof the total dischgrge), and SfdntuGheorghe about 0.75-1x10"metric tons/year(between 9% and 20.5o/of the totaldischarge;Bondar,1972;Bondarand Harabagiu, 992). tcan be noticed romabovedata hatwhileDanube

sediment ischarge ecreasedn the lastcenturymainly due to damming, he water dischargeincreased. lso the 25-30o/oediment ischargereduction fter Danubedammingwas not asextreme s in the case of the Nile (98%;Sestini,1992) rEbro 95%; imenez ndSanchez-Arcilla,1993), ut added o thepreviousegressiverend,it amountso 5E% f he otal oad.The ncreasenwater dischargecan be attributed o climaticchanges,and partly after damming o marshreduction.he ronGates ams lronGates andlronGates l) wereclosedn 1970'sn the uppersector of Danube ower course.Major riversdraininghe CarpathianndBalkanmountainsredammed, nd discharges thereforensignificant.The relatively mall reductionn the sedimentdischarge fter damming,was due to increasederosion f the riverbottomand islets n the lowercourse f the river Panin, 979-1994; ihailescu,1983), nd o meander ut-offsn 1990's, long hedistributariesn delta rea Panin, 979-1994).

Wind And WaveRegimeThe averagewind speed n the northwestemBlackSea s between .5 and 5 m/s (Bulgakov tal.,1992). hepredominantinddirectionsuringtheyear, s heyweremeasured t meteorologicalstations n the coast,are rom he north,west andsouth Ciulache,993). heprevalentirectionorthe onshoreblowingwinds is from Northeast.During the summer months, however, the

predominantirections onshore rom the south-SoutheastDiaconu t al., undated, s cited byPaninet al., 1979-1994).ecausehe Romaniansector s relativelyhort, he wind regime oesnotvarysignificantlylonghe coastespeciallyor heonshorewinds (Ciulache,1993). Storms areprevalentrom the north and Northeast,with anaveragewindspeedof 9.8 m/s and a durationranging etween to 22 hours Diaconu t al.,undated,s cited y Panin t al, 1979-1994).Waveshigher han 0.2 m occurabout50olo fthe year;60-85% f themare localwind waves,and 15-40o/owells (Bondar,1972).The waveregimewasanalysedor thisstudybasedon waveobservationadeovera 1O-yeareriodbetween1972and1981at a depthof 11m,offshorehesouthernownof Constantzasee "Methods"). efound hatwaves igherhan0.2m (the ower imitof thewaveheighthatwasmeasurablesing heavailablenstrumentation)rrived t the coast romall offshoredirectionsabout 51% of the year(Fig.2).The annual average signiflcantwaveheight as0.8m, witha meanperiod f 5 seconds(Fig.3). Most waves anived from the NE-SEquadrant,he predominantirectionabout oo/ottotal waves)and energy lux being rom E. Thewaves arriving rom NE-ENE sectors occurredmore requently nd heirmean energy lux was

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L. Giosanet al . - LongshoreSedirnent ransport atternalongRomanianDanubeDelta Coast

the

Uz()eU-tr

zpUa

zp

higher than those from ESE-SE sectors.Nearshorewave climate s controlled y the beachslope, thus implicitly by the shelf width whichnarrows southward o Sf6ntu Gheorghe mouth,and widens farther south (see Fig.7). Thestrongestwave activity s expected longSakhalinisland,due o poor refraction nd wave shoaling n

N NN E Nt r DN E E DSE SD SSD S1VAVE DIRECTIONFig.ZFrequency f occurrence f wave height s a functionof heirdirection f approacho the coast.

350h3001,25Ya20'l50hl0'A50aoh

2 3 4 5 6 1 8 9PERIODseconds)

Fig,3Frequency f occurren ce f waveperiodEngineeringWorksTw o jetties started to be built at the Sulinamouth Fig.1) n 1856and now extendas far as 8km offshore.These jetties were built to protectSul ina navigation hannelagainstshoal ingwithsediments rift ingrom he nodh.The Sulinaetties

stronglyaffect he evolution f the beachesarthersouth, by discharging he distributary edimentload offshore, s well as interceptin ghe longshoredrift of sand from the north (Sp5taru, 990;Paninet al., 1579-1994).During he second half of the19'ncentury, hannels were cut to shorten thenavigation oute on Sulinadistributary.When thiswas done, the southernar m of SfAntuGheorghefost about 3O o of its water discha rge andconsequently ts sediment transport capacity nfavour of the shortenedar m of Sulina, (Sp5taru,1990).Sul inabed-load ccumulates t the end ofth e jetties as a mouth bar, which is periodicallydredged.The dredged sedimentsare dischargedoffshore, and thus are lost from the nearshoretransport ystem Paninet al . , 1979-1994). ul ina

beach situated just south of th e jetties wa snourished it hsandextractedrom Sulinaharbour,an d wo groinswere also built.A revetment was recently built along the

southernbarrierbeachesbetweenPeritescaandChituc (Postolache t al . , 1995). In order tomitigate he beach erosion on these beaches,some Romanian hydrotechnical ngineershaveproposed o deviatepart of the SfAntuGheorghedistributary ater and sediment oad arthersouth,through a channel dischargingbetween CioticaandPerisor Hangu t al . , 1992).Midiaharbour sprotected y ettiesextending bout5 km offshore;these etties restrict he amount of sand carriedsouthwardby the longshoredrift by redirecting toffshore Fig.1).Shorel ineDynamicsThe northern art of delta coastwhich ncludesChiliadeltaand MusuraBay beaches,most of it onthe Ukrainian territory, has been intenselyprograding n the last centuries Panin, 1985;Mikhailova,1995), ,After he Sulina jetties werebui l t at the beg inning f th e centu ry, he deltaiccoast arlherwa s partly solated rom the sedimentdrift coming from northern Chilia coast. Thisisolation increased in time with the jettiesextension.Th e jetties can be considerednow asimpermeableo the sediment rift.Beachprofi leswere measuredon a networkof

benchmarks long h e deltacoastsouthof Sul ina,including ne andmark n Sakhal in sland.Basedon these, shorelinechanges were analysed byBondar t al . , 1983) or the periodbetween 962and 1978, and by Vespremeanu nd Stefanescu(1988; ig.4, ) or theperiod etween 962and

0 5 1 0Snlin! IloxlI

d ^6 l? Ea r5 20 25 30 35 40 45 50

SLCbcotghc Mo!(h Srttrlir lillnd

F : o r; r o ' l3 0 - l - , 'u-ro l3 -20-la* :0 r--H 3 0 4

Fig.4Shorel ine hange ates 1962 o 1987) rom Sulina oSf6ntuGheorghe from Vespremeanu nd Stefdnescu, 978)an d or Sacalin sland 1 62 o 1993; ompiled rom Breieran dTeodor, 979and Panin t al.,1979-1994\.

50 60 70 80 90 I 00 110 120 r3 0ciollc! Prrisor rorlirn Midia

ALONGSgORE DISTANCD (kB)

Fig,SShorel ine hange atesbetween1962 and 1987 or th edeltaic coast situated south of Sacalin lsland (data fromVespremeanundStefdnescu,978).

steepbe

1 4 GEA.ECO.MAR NA,2J1 97National nstitute f MarineGeology nd Geo-ecology f RomaniaProc. ntern. Workshop n'Fluvial-Marinenteractions"n Malnas,Romania,QcL1-7,1996


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L. Giosan et al. - Longshore SedimentTransportPattem along RomanianDanube Delta Coast

1987. The actual shoreline Dositionswere notpublished but only the calculated end-pointshorel ine hange ates.Panin et al . (1979-1994)collected nnualbeachprofilemeasurements fter1978within he framework f the RomanianMarineGeologyan d Geoecologynstitute "Danube-BlackSea System Monitoring Program". GAstescu(1979,1986)publ ished horel ine hange nalysesfo r most of the Danubedelta coast basedon mapcomparisons.

The shoreline s retreating over almost theentireDanubedeftacoastsituated outhof Sulinamouth(Fig.4,5) . The maximum etreat ate or theSulina-SfdntuGheorghesector is over 2A mlyr.,which s ound at the latitude f Rosulet ake,about15 km southof Sulinaetties.Almazovet al. (1963)suggested hat the highest retreat s due to thedivergentof the longshoresediment drift in thiszone which may be caused by an anticyclonaleddy attached o the south ee side of the Sulinajetties, generated by a quasipermanent oastalcurrentdirectedsouthward.The existenceof thisdivergence zone is also recognised n othersediment drift studies based on radioactiveorfluorescent racers (Dragota,1973; Bondar andCraciun, 970), al ini ty istr ibut ionBondar, 964),and physicalmodell ing Sp6taru,1971).Howeverthe divergence zone ma y be explained byshelteringand diffractionof the waves coming romthe Northeastsector. Shoreline recession atesover 10 m/year were also registeredalong th eSakhalinbarrier sland,and south of CioticaandbetweenPortitaand Chitucon the southern arrierbeaches. The shoreline advances immediatelysouth of Sulina ettieswhere t is accompanied yintenseshoalingof th e submerged rofile Bondarand Harabagiu,1992). The shorel ineadvancesbehind Sakhalin sland where Sfdntu Gheorghesecondary elta is activelyprograding, nd slowlyat Perigor,south of the Ciotica retreating one,where th e shore orientation changes fromapproximately E- W to a NE-SW direction.Shoreline dvancewas registered n the southernChitucspit du e th e sediment ransportobstructionexerted by Midia harbour etties.The submarineslopealso shoals on th e si des of Midia structures(Panin t al . ,1979-1994).

NearshoreProcessesalong SakhalinBarrierBratescu (1922), Panin et al. (1979-1994),Breierand Teodor (1979),Vespremeanu1983),and Bondaret al , (1984)analysed akhal inslandgenesis and evolut ion.Sakhal in sland (Fig.1)formed n 1897, rom a bar developed outhof th eSf6ntu Gheorghe mouth (Bratescu, 1922).Sediment discharged by SfAntu Gheorghedistributary, and also drifted from north,accumulate as a bar at th e mouth of SfAntu

Gheorghe. Under the influence of mainlysouthward edimentdrift he mouthbar sandsar esupposedly transfened south during stormscausing continuingncreasen lengthof Sakhalinisland Paninet al., 1979-1994). t the same ime,as observed from successive maps and aerialphotos,Sakhalin slandshore is retreating,moreintenselyn its centraland southernparts Paninetal., 1979-1994).The northern half has alreadybeen connectedo the mainlandmarshes n 1975du e to both island retreatand SfAntu Gheorghesecondary delta intensive progradation,transforminghe island in a spit. Due to its lowrelief (0.5-1.5m), the retreat of the island isapparently ontrolled y ovenrvash nd breachingprocesses as suggested by the numeroussuccessivebreaches which appeared over theyears ollowingmajor storms (Panin et al., 1979-1994).The breachesweregenerally apidlyclosedas the wind-induced, nd secondarily idal flows,between he lagoonan d the sea are not strongenougho keep hemopen.The southernip oftheisland recurves into the lagoon due to waverefraction attern ypical or this type of areas andalsobecause he low reliefbeachat th e tip evolvesin an ovennrash ode (as conceptually escribedin Kana,1996). he analysis f is land volut ionorthe 1858-1968 eriod Bondar t al . , 1983)showsthat the islandplatformbuilt ntensivelyn offshoredirectionbetween 1850 and 1923. This processceased practicallyafteruards leaving the profileunaltered elow10 m depth unti l1968.Above hisdepth the bathymetriccontours show a retreat forthe sameperiod.Consequentlyhe beachprofi leo10-12 m depth has flattened while the islandretreated.Also bathymetriccharts comparisonsshow ha t the islandplatformha s been building nthe axial (longshore) irection Jianuand Selariu,1970).This process xtends o a depthof about12m and it was uninterrupt ed hough cyclical,between 858and 1968(Bondar t al . , 1983). nsummary, he islandplatformbuilding n axialan dpossiblyoffshoredirection,and also ovenrvash ndbreaching are the processes that extractsediments rom the Sakhalin nearshoresystem.Using he maps of Breieran d Teodor (1979)an dPanin et al. (1979-1994; Fig.6), the islandshorel ine hanges or the 1962-1993 eriodwerecomputed or hisstudy Fig.4).

Longshore SedimentTransportThere are several direct indicators of thelongshore sediment transport direction andmagnitude including sand accumulation onoppositesides of engineering tructures uch asjetties and groins, and the longshoregroMh ofspits an d barrier islands, methods which were

previouslyapplied on the Danube delta coast.These previous studies indicated that themagnitude f the net ongshoreransport sGEO.ECO-MARINA, 1997National lnstituteof Marine Geologyand Geo-eaologyof RomaniaProc- ntern. Workshop n'Fluvial-Marinenteractions"n Malnas,Romania,Oct.1-7,1996

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direction s also a direct indicator of the netlongshoretransport rate. By using previouslyestimated hoaling ates o 9 m depth (Bondaretal., 1983), he transport s directed outhward t arate of about 1,080,000m'/year. Farther southalongChitucspit ,Bondaret a l . (1980)estimatedthe longshore ransportby using syno ptic winddata.The driftwas directed outhward t a rate ofabout900,000-1,110,0003lyear own o the 9 misobath.

METHODSSedimentBudgetA sand budget was used to estimate of thelongshore ediment ransportpattern.The activebeach, th e longshoretransport, an d the riversediment input were assum ed to be the only

sourcesor sinks or sediments or the entirestudyarea.The aboveassumptions, owever,would notbe validalongbarrierbeachof Sakhalin he island,since he cross-shore edimentatio n y ovenirrashand breaching Panin et al . , 1979-1994), ndpossibly ffshoreoss (Bondar t al . , 1984;Jianuand Selariu, 970)are ikely o be important. hecalculationshereforewere not done for Sakhalinislandbut specialconsideration as given to thebudgethere.The remaining tudy areawas divided nto twocoastal cells based on the changes of thelongshoretransport direction and on Danubedistributaries ediment discharge. The northerncell was taken betweenSulina mouth and SfAntuGheorghemouth. The Sulina mouth etties wereconsidered to be impermeable to sedimenttransport because they extend almost to theestimated depth of closure. Thus the northernlateral boundary for this cell was consideredclosed,while he southernone was left open. t isreasonableo assume hat the Sf6ntu Gheorghedistributary edimentdischarge s largelydirectedto the south, n such a way therewa s no significantcontribution ro m this source to the northernSul ina-Sfdntu heorghecel l . The southem cel l

was between Ciotica, situated on the deltaicmainland hore,and Midia.The southern ateralboundarywas assumed o be closedbecause hejetties protecting Midia harbour disrupt thelongshoresediment transport by redirecting toffshore.The northernboundaryof this cell wasleftopen.Ne t ongshoreransport atternsor eachcell were derived from the calculated volumechangesby integratinghem startingat the closedboundaries.The active beach volume changes werecalculated asedon shoreline hange ates or theperiodbetween 962and 1987 Vespremeanu ndStefanescu,988;Fig.2and Fig.3).Because hesedata do not extend as farther south as Midiaharbour, the shoreline progradationrate was

Fig.6 Consecutive posiiions of Sacalin barrier island(compiledrom Breierand Teodor;1979,Gdstescu, 979;an dPanin t a l . ,1994) .extremely igh. From north o south, he sedimentdrift s directed outhward longChilia obe,showsa reversalsouth of Sulina etties, an d continuessouthward o Midia afterwards.Shuisky (1984)estimated he net sediment ransportalong Chiliadelta o be between120,000m'/year and 850,000mt/year,dependingon the localorientation f thecoast. The southwarddrift along Chil ia can beestimated rom he shoaling ateson northern ideof Sul ina ett ies.Bondar and Harabagiu 1992)proposed a^ value between 1 100,000 and1,900,000m'/year or the shoal ing ate but theperimeteror which t was calculateds not clearlyspecified. hg shoaling ate of 500,000m3/yearo1,300,000m'/year Bondarand Harabagiu, 992)on the southern side of the jetties is a directestimate of the net longshore ransport n thereversal one southof Sulina.The natural hoalingin this ar ea was clearly overestimate d ince anunknownbut substantial mountof sand retrievedfrom Sul inaharbourwas discharged n the beachin 1980s. South of the reversalzone to SfAntuGheorghe,^the ongshoredrift was estimated o700,000m'/year Panin t a l . ,1979-1994)rom hebeach sand deficit. Shuisky (1985) suggestedanet longshore sediment transport of 800,000-900,000 m'/year along the same sector. TheSakhalin arrier slandplatform uilding n th e axial

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considered o be constantsouthwardof the lastmeasurement oint and have the same value asmeasured at that last transect . Although themeasurement station used for collecting theshoreline hange data were widely spacedalongthe coast they were considered o representwellthe actualshoreline ehaviour eingverysimilar othe estimates presented by others (Gdstescu,1986and 1993; UCN,1992:Spdtaru, 992).Theshoreline change rates were corrected for sealevel hanges y applying ruun's u le lnmanandDolan,1989),assuming 3 mm/year ea-level isefor the active deltaic shore (i.e. Sulina-Sf6ntuGheorghecell) and a 1.2 mmlyearsea-level isefor the Ciotica -Midia ell.An averageberm heightof 0.5 m was use d for the entire coast as thelandwardboundary.Becausesuitablesubmergedbeach profi le measurement series wereunavailable or the Romaniancoast, an averagedepth of closure of 9 m was estimated usingHalermeier'smethod (1981 a, b ) based on theavailablewave data. The annual values for theestimated depth of closure, between 1972 and1981, ranged between 6 and 12 m. Previousstudiesof the long-term ubmergeprofi leevolutionsuggest a closure depth between 10 and 12 m(Jianu nd Selariu, 970,Bondar t al . ,1984).

Volumechangeswere calculated singa hybridmodelas appropriate, itherunderassumptionhatan equil ibriumong erm beachprofi les conserved("one-l ine" model; Pelnard-Considere, 956;Hanson nd Kraus, 989), rthatthe beachprofi legeneralshape s non-conserv ativeuringshorelineretreat. During delta formationphase, both theshorelineand the subaqueousdelta advance atthe same rate,while n the reduction rocess,hedelta shoreline s retrea tingmore rapidly han thesubaqueous elta (Refaatand Tsuchiya,1991).This latter behaviour s due to the highererosionrates n the sur f zone compared o fartheroffshorefollowing a decrease in the river sedimentdischarge, which must occur to maintain thepotential ongshore ediment ransport ate.Thusan equil ibrium beach profi le is not conservedduring he delta eduction rocess, ince he profi leslope becomes more gradual. Jimenez andSanchez-Arcilla (1993) confirmed the aboveexperimentalesults n the case of Ebrodelta.The"one ine"modelseemsappropriateo describe hebeach volume changes on a delta coast in theformationphaseonly. When a deltaicshoreline sretreating, s is the case of the Danubedeltacoastunderstudy, t is more appropriateo compute hebeach volume changes using a wedge-shapederosionalprism. However he accreting ector ofthe shoreline south of Sulina jetties seems toconserve an equil ibrium during its advance(G6stescu,1993).Therefore t was reasonableouse he "one-line"model or this sector,as well as

for he otheradvancinghorelineectorsPerisor,andChituc). ongshoreet sand ransport atternwas derivedromvolume hangesor the depthsof closure f 6, 9, and 12 m. The result or the 6and 12 m depth were used to estimate hemagnitudeange f henet ransport. owever,hemostplausiblestimates ereconsideredo be orthe average epthof closure f g m, and heywillbe used n urther iscussions.

Potential ongshore ediment ransportRateWave characteristicsave been recordedatConstantza,n the southern ar tof the RomanianBlackSea coast,since 1964 by the RomanianMarineSciencesResearch nstitute.Significantwaveheight,meanperiod, nd meandirection t6-hour ntervalover a 1O-year eriodbetween1972and 1981were used n the current tudy(Fig.2,3). The measurementsnvolvedvisualobservationsf a buoy ixedat a water epthof 11m. The measured haracteristicsncludemeanwaveheight, eriod, nddirectionhree imesdailyduring the daylight.The wave direction tpestimatedisually, ithan accuracy f about20".Gaps n the record ue o periods f poorvisibilitywere filled with statistically ignificant alues(Kraus ndHarikai, 983).Wavedataweresparsefor othersectors f the coast.Becausehe windregime does not vary significantly long therelativelyshort coast, the wave climate atConstantzaas assumedo representhat alonefor heentire oast.The bathymetry as obtainedby digitising1:50,000calemapspublishedy the RomanianNavyHydrographicervice, asedon their 1979survey.Then he observedwaveswere backwardrefractedn the realbathymetryround he pointwere heyweremeasuredsing teratively wavetransformationrogramprovided y U.S. ArmyCorpsof EngineersRCPWAVE; ialoneet al.,1992),n an attempto transformhewavedata ordeepwater onditions-30m). However, ecause

the angleof approachs poorly esolved n theoriginal igures, he transformed ata were notsignificantlyifferent.Thepotentialongshoreedi.r-.nentransport asestimated singa nearshore ediment ransportmodelbased n the waveenergy lux (NSTRAN;U-S.ArmyCorps f Engineers,984; nmanandDolan, 989). heconstant, which escribesheefficiencyf ongshoreomponentf waveenergyflux n transportingediments, as considerednadjustableparameter.A value of k=0.256,calibratedsinghesandbudget omputationso adepth of 9 m, was used. Both NSTRANand

RCPWAVEare componentprogramsof the"Shoreline odelling ystem"SMS) tandardisedsoftwarecllectionrealed y he U.S.Army orpsGEO-ECO-MARINA,1997National nstitute of Marine Geology and Geo-ecologyof RomaniaProc. ntern.Workshop n'FluviaLMarinenteractions"n Malnas,Romania,Oct.1-7,1996

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L. Giosanet al. - LongshoreSedimentTransport attemalongRomanianDanubeDelta Coast

of Engineers Hanson nd Kraus,1989),The 140km-longarea extending rom Sulina o Midia wasdivided n si x sectorsaccording o the shorelinemean orientation(Fig.7) and the potential ne tsediment transport was computed for each ofthem.The resulting atternswere compared o th etransport atescomputedro m he sand budget.

RESULTSThe general pattern of the net longshoretransport provided by th e sediment budgetcompared avourably o that calculatedby waveenergy lux method (Fig.8,9). The net longshoresediment ransport long he entire tudied oast shigh on average, mainly as a result of both theprevailing E-NE waves superimposedon NNE-SSE generalorientation f the coast.Sulina-SfdntuGheorgheFrom Sulina o SfAntuGheorghe, he result ingne t sediment transport scheme was basicallymade up of two cells (Fig.8, 10): a sector ofnorthward irected ransportsituated mmediatelysouth of the Sulina ettieswith an average ate of190,000 m"/year (ranging between 130,000m"/yearat 6 m depth,and 250,000m'/year at 12m depth) and a longer sector of southwardtransport arther south to SfAntuGheorghewhichhad an average rate of 620,p00 m'/year (with arafige between 415,000 m'/year and 830,000m"/year).Th e magnitudeof the transport n the

northern el l does not explain he shoaling ate ofat least 500,000 m'/year south of Sulina etties(Bondar et al., 1992). Presumably he artificialnourishment of the beach with an unknownquantityof sand, is th e main factor affecting hebudget here. The potential ransportcalculationalsoshoweda two-cellpattern, ut or the northerncell both he lengthand the net ransportwere essthan those obtained rom the sand budget.Thetwo-cell pattern s the result of the shelteringofwaves coming from the NE quadrant, an ddiffractionaround he jetties. The poor qualityofwave data, poor performance of the waverefractionmodel adjacent o jetties, he corruptedsand budget, and probably he existenceof anattachedanticycloniceddy in this shadow zone(Almazov t al . , 1963),al l concur n creatinghedifference between patterns. Other investigatorshave found hat closedattachededdiesmay fromin the lee of capes, etties,or other topographicindentations of the coast, with consequentmodifications o local distributionsof sedimentdeposit ionFerentinosndCol l ins, 980;Davies tal . , 1995). n the southern el l , he net transportincreased rom zero at the nodal point, up to800,000mt/yearat CAslaVSdanei, emainingairlyconstantup to the cell end, at Sf 6ntu Gheorghe(Fig.8).The sediment ransport ate northof C6slaVSdanei s lower relative o the rate of transport

farther south becauseof an increase n thesubmergedeach lope Fig.7) nd he shelteringeffect f Sulinaetties.3doo'

Fig.7 Bathymetry f the study area, and the averageorientationof differentcoast sectors used in computing thepotentialongshoreedimentransport.

-/--'---.4-t

c-,

r0 15 20 2s 40 45 50Sncdin ledAIONGSHOnE DISTANCE km)

Fig.8 Patterns of net longshor e transport (circles) andpotentialransport l ine)betweenSulinaand he southern ip ofSacalinsland.

d0 s0 60 70 80 90 10 0 ll 0 110 I30 14 0ciltritr

^ron,rr,r",j,ln"lrrr^".o,"r,Porxh ctrihcrPil llhlir

Fig.9Patterns f net ongshore ediment ransport or thedeltaic coast south of Sacalin barrier island (using sameconventions s in fig.8).Sakhal in slandSakhalin island shoreline situated south ofSfAntu Gheorghe mouth receded everywherebetween1962 and 1993, reaching he highestretreat ateon the Danubedeltacoast 66m/yr.)atthe islandcentre Fig.4).Consequentlyhe islandconvexityhad been reduced(Fig.6).During thisperiodno t onlydid the islandmigrate andward utits lengthalso increasedwith about 1.5 km at thenorthern ip and 5.5 km at the southernend. t was

0 5SuhrflMDurh

l*;-l;,-

t

longshore sediment transport pattern along romanian danube delta coast

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