longshore sediment ransport pattern along

L. Giosan et al. - Longshore Sediment Transport Pattern along Romanian Danube Delta Coast

LONGSHORE SEDIMENT TRANSPORT PATTERNALONG ROMANIAN DANUBE DELTA COAST

Liviu Grosnru', Henry Boxulrtewtczl,Nicolae PANrN',

lulian PosroLAcHE3

1 New York State University, Marine Geosciences Center,Stony Brook, USA, E-mail : [email protected]

2 National Inst i tute of Marine Geology and Geo-ecology of Romania GeoEcoMar,Dimikie Onciu Street, No.23-25, Bucharest 70318, Romania

Tel/Fax: +4AJ -252.25,94, E-mail : panin-geomar@rolink. i i ruc.ro3 Romanian Marine Research Inst i tute RMRI,

Bd. Mamaia No.304, Constanta 8700, RomaniaTel : +49-41 -643 .288, F ax. +40-41 -831 .27 4', E-mail: rmri@alpha rmri. ro

Abstract. The paper presents the littoral sediment drift patterns along the Romanian Danube Delta coast. Using sand budgettechniques and simulat ions with the "Shore Modell ing System" software package of U.S.Army Corps of Engineers the longshoresediment transport was computed for each of the six sections taken into consideration within the Danube Delta coast zone (about140 Km-long). The current state of the beach dynamics along the coast of the Delta are presented herein and predictiveobservations are made. Some actions are proposed to be taken in order to improve the monitoring of the Romanian Black Seacoasl z0ne.

Key words: littoral sediments, sedlment transport, simulat ion, coast zone, Black Sea

INTRODUCTION

Many deltas around the world have beenexperiencing a drastic change in their evolutiondue to anthropogenic alteration of both water andsediment discharge, coupled with vigorouslongshore sediment transport. In the case ofDanube delia, coastal erosion is threatening theecosystem of wetlands, and coastal lakes.Furtherrnore the chronic sand deficit extendsdownstream of the delta coast, negatively affectingthe important tourist-economy of the seasideresorts in southern sector of the Romanian BlackSea shore. Several factors have been identified toexplain the recessional behaviour of Danube delta:(1) alternate channel switching which diverts mostof the river sediment discharge to a singledistributary starving the earlier built lobes; (2)general decrease in the river sediment dischargedue to damming, dredging and agriculturalpractices in the Danube drainage basin; (3)engineering structures which disrupt the longshoresediment transport pattern; and (4) relative sealevel rise. The tidal processes are not importantsince the mouth of the Danube is a microtidalenvironment.

This situation is not unique but also found inother microtidal deltaic coasts like those of the Nile(Sest ini , 1992; Stanley, 1996), the Po (Capobiancoet al . , 1995), and the Ebro (Jimenez and Sanchez-Arci l la, 1993; Palanques and Gui l len, 1995). Theconstruction of dams and other river controlstructures in the drainage basin, dredging along

the river, the entrapment of water and sediments inthe drainage basin and deltaic plain for agriculturalpurposes, the decrease in the rainfall pattern, havebeen variously cited as important factors indecreasing the rivers sediment discharge andfurthermore favouring the change from a generalprogradational character of a delta into arecessional one. The river sediment dischargehave been reduced 95% from the total load in theEbro case (Palanques and Gui l len, 1995), and98% for the Nile (Sestini, 1992), even though insome places, like the Ebro extensive agricultureand deforestation have had contrary effects(Palanques and Gui l len, 1995). At the same t imethe acute orientation to the principal wave directionof some deltaic coastal segments tend to produceintense longshore sediment transport. This is thecase of Nile delta coast where net longshoresediment transport rates up to 1,200,000 m'/yearwere est imated (Quelennic and Manohar, 1997), orVolturno, a small cuspate delta on the westerncoast of ltaly, where the maximum net rate wasestimated to be 1,760,000 mt/year (Benassai eta l . , 1995) .

The objective of this study was to analyse thecurrent behaviour of the Romanian Danube deltacoast. We will show the importance of the waveinduced longshore sediment transport andanthropogenically modified by engineeringstructures in controlling shoreline dynamics alongthe Danube deltaic coast partially inaciivated by

G EO-EC O-MARIN A, 2/1 997National lnstitute of Marine Geology and Geo-ecology of Romania

Proc. lntern- Workshop on 'Fluvial-Marine lnteractions" in Malnas, Romania, Oct.1-7, 1996

L_ G i y:zl:!:!:j:tg:!",."- -2ediment Transport Pattem along Romanian Danube Delta Coast

alternate channel switching. The energet ic waveregime in the caused by the preponderance oflocal waves due to relatively short fetches, by thehigh angle of wave attack, and by the ineffectiverefraction on a relatively steep nearshore.

STUDY AREA

Geological Setting

The Romanian Black Sea coast (Fig.1)stretches over 245 km from the Chilia distributaryof the Danube (45"12' N, 29"40' E) at theRomanian-Ukraine border to the town of VamaVeche (43"44' N, 28'35' E) at the border withBulgaria. The Romanian coast can be dividedusing geographic and geomorphic cr i ter ia into anorthern unit and a southern unit (Panin et al . ,'1979-1994). The northern unit is the low-reliefDanube delta coastal zone whi le the southern unitis characterised by eroding cliffs and loess,protected in places by narrow beaches (Charlierand de Ju l io , 1985; Pan in , 1979-1994) .

Fig.1 Romanian Black Sea coast. DanLtbe Delta coast( including the adjacent, genetical ly related, baymouth barrier infront of Razelm-Sinoe lagoons) extends from Chil ia to Midia.

The Romanian deltaic coast extends from theborder with Ukraine to Midia in the south. Thestretch of shorel ine which includes Musura bay inthe north. and extends to the southernmostdistr ibutary of the Danube, Sf intu Gheorghe, withthe adjacent Sakhal in barr ier is land (f ig.1)constitutes the major contact of the Danube delta

with the Black Sea. Reworked deltaic sands alongthe shore have built a coastal barrier complex fromSfAntu Gheorghe to Midia. The barrier complexconsists mainly of a baymouth barrier whichseparates the large lagoon complex of Razim-Sinoe, once open (Panin, 1983), f rom the sea. Theshelf is broadest in the front of Danube delta andnarrows to the south. The nearshore gradients to a12-15 m depth along the Danube delta coast,range between 0.003 and 0.01, with the steepestslopes along the Sakhal in barr ier is land (Panin,1985). The beach prof i le is general ly mult ibarred(Postolache et al . , 1992).

The modern sediments on the nofthern sectorof the coast consist of Danube-borne quartz sands(about 70% si l ica, Panin, 1989). The heavy mineralcontent is about 3%. Sands carried by the littoraldrift from the region north of Danube delta havehigher si l ica content than the Danubian sediments(90%). The subaerial beach sediments aregeneral ly medium-f ine sands (d50<0.5 mm).Enlarged contribution of shell detritus increasesthe grain size local ly, especial ly on the Razim-Sinoe barr ier complex beaches (Panin et al . , 1979-1994; G iosan, 1993) .

The evolut ion of the Danube delta has beenextensively studied by Panin (1974, 1989) andPanin et al . (1983). The delta development hasbegun during the Quaternary and it was stronglyinf luenced by the sea-level changes during thisperiod. During the WiJrm regression, the level ofBlack Sea was about 100 m lower than today. Thisposition favoured the erosion of the UpperPleistocene deposits. The delta formed during thesucceeding transgression by an alternate channelextension process (\tt/right, 1985). One to fourdistributaries were alternately orcontemporaneously act ive, each bui lding their owndeltaic lobes. Today there are three main activedistr ibutar ies. Chi l ia, Sul ina and SfAntu Gheorghe.Only Chi l ia, the nofthernmost distr ibutary lobe, andthe secondary delta of Sfdntu Gheorghe arm,which has been bui l t in a quiescent environmentbehind Sakhal in ls land, are prograding. Almost al lthe other coastal sectors are retreating. The rate ofretreat seems to be enhanced by the influence ofthe recent coastal engineering structures (Panin etal", 1979-1994), and by the general decrease ofthe Danube sediment discharge during the lastcentury (Panin et al . , 1979-1994; Bondar et al . ,1 992).

Sea Level Variations

The long term sea level change estimatesindicate afal l of-2.5 mm/year at Varna, Bulgaria inthe southern part the coast, whi le the level r ises inthe northern segment with rates of 1 .2-1.8mm/year at Constantza, and 3.3 mm/year at Sul ina(Bondar cited by Sp5taru, 1990; G6stescu and

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Driga, 1986). The Danube delta subsides with 1.3-2 mmlyear, due to the compaction of deltaicsediments and regional tectonics. The tide of theBlack Sea along the Romanian coast issemidiurnal, with a range of 7 lo 12 cm. lt has anunnoticeable effect when compared with otherdeformational fluctuations, such as seiches orstorm surges which can reach a maximum of 2 mheight, and 1 .2 to 1,5 m respectively (Bondar,1972; Panin et al., 1979-1994). Other importantmean sea level fluctuations with multiannual andseasonal cyclicity are due to river discharge,changes in the water exchange through theBosphorus straight, and precipitation/evaporationvariations.

River DischargeThe most significant factor affecting the

hydrologic budget of the Black Sea is the seasonalvariation of river discharge. The Danube provides38% of the river water entering the Black Sea(Glaskow, 1970). Other important rivers in terms oftheir water discharge are Dniepr, Southern Bug,and Dniestr. Danube's annual discharge is highestfrom April to July. ln response to the variation inriver discharge, the sea level fluctuates between20 and 30 cm from season to season and about 20cm from year to year (Bondar, 1972). Between1858 and 1988 the Danube discharged annuallyabout 191 km3 of water, of which 63% wasdischarged through the Chilia distributary, 17%through the Sulina distributary and 20% throughthe SfAntu Gheorghe distributary (Bondar et al.,1992). The Danube water discharge increasedfrom 178 km' in 1858 to 203 km' in 1988. Themean total sediment discharge of the Danube wasabout 52x106 metric tons per year between 1858and 1966 (Bondar et al., 1992). The sedimentdischarge in 1858 was 65x10o metric tons, but only38x10" metric tons in 1988 (Bondar et al., 1992).This discharge was distributed 55% to the Chilia,214/o to the Sulina and 23% to the SfAntuGheorghe arm. About 25-30% reduction of thetotal sediment discharge has been recorded after1970 when the first lron Gates dam was closed inthe Romanian-Yugoslavian sector of Danube(Panin et al., 1979-1994; Popa, 1992). The bed-load sediment discharge, that has a median grainsize between 0.1-0.5 mm, has been estimated tobe between 4.5o/o and 19o/o of the total sedimentdischarge. Today, ^the Chilia distributarydischarges about 3x10o metric tons/year bed-loadsediments (between 57o/o afid 65% of Danube'stotal bed-load discharge), Sulina about 0.85-1.3x106 metric tons/yeir' (between 18.5% and24.5o/o of the total dischgrge), and SfdntuGheorghe about 0.75-1x10" metric tons/year(between 19% and 20.5o/o of the total discharge;Bondar, 1972; Bondar and Harabagiu, 1992). ltcan be noticed from above data that while Danube

sediment discharge decreased in the last centurymainly due to damming, the water dischargeincreased. Also the 25-30o/o sediment dischargereduction after Danube damming was not asextreme as in the case of the Nile (98%; Sestini,1992) or Ebro (95%; Jimenez and Sanchez-Arcilla,1993), but added to the previous regressive trend,it amounts to 5E% of the total load. The increase inwater discharge can be attributed to climaticchanges, and partly after damming to marshreduction. The lron Gates dams (lron Gates I andlron Gates Il) were closed in 1970's in the uppersector of Danube lower course. Major riversdraining the Carpathian and Balkan mountains aredammed, and discharge is therefore insignificant.The relatively small reduction in the sedimentdischarge after damming, was due to increasederosion of the river bottom and islets in the lowercourse of the river (Panin, 1979-1994; Mihailescu,1983), and to meander cut-offs in 1990's, along thedistributaries in delta area (Panin, 1979-1994).

Wind And Wave RegimeThe average wind speed in the northwestem

Black Sea is between 6.5 and 5 m/s (Bulgakov etal., 1992). The predominant wind directions duringthe year, as they were measured at meteorologicalstations on the coast, are from the north, west andsouth (Ciulache, 1993). The prevalent direction forthe onshore blowing winds is from Northeast.During the summer months, however, thepredominant direction is onshore from the south-Southeast (Diaconu et al., undated, as cited byPanin et al., 1979-1994). Because the Romaniansector is relatively short, the wind regime does notvary significantly along the coast especially for theonshore winds (Ciulache, 1993). Storms areprevalent from the north and Northeast, with anaverage wind speed of 9.8 m/s and a durationranging between I to 22 hours (Diaconu et al.,undated, as cited by Panin et al, 1979-1994).

Waves higher than 0.2 m occur about 50olo ofthe year; 60-85% of them are local wind waves,and 15-40o/o swells (Bondar, 1972). The waveregime was analysed for this study based on waveobservation made over a 1O-year period between1972 and 1981 at a depth of 11m, offshore thesouthern town of Constantza (see "Methods"). Wefound that waves higher than 0.2 m (the lower limitof the wave height that was measurable using theavailable instrumentation) arrived at the coast fromall offshore directions about 51% of the year(Fig.2). The annual average signiflcant waveheight was 0.8 m, with a mean period of 5 seconds(Fig.3). Most waves anived from the NE-SEquadrant, the predominant direction (about 3oo/o ottotal waves) and energy flux being from E. Thewaves arriving from NE-ENE sectors occurredmore frequently and their mean energy flux was

GEO.ECO- MARINA, 2/1 997National lnstitute of Marine Geology and Geo-ecology of Romania

Proc. lntern. Workshop on'Fluvial-Marine lnteractions" in Malnas, Romania, Oct.1-7, 1996

1 3

L. Giosan et al. - Longshore Sedirnent Transport Pattern along Romanian Danube Delta Coast

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Engineering Works

Two jetties started to be built at the Sulinamouth (Fig.1) in 1856 and now extend as far as 8km offshore. These jetties were built to protectSul ina navigat ion channel against shoal ing withsediments drifting from the nodh. The Sulina jettiesstrongly affect the evolution of the beaches farthersouth, by discharging the distributary sedimentload offshore, as well as intercepting the longshoredrift of sand from the north (Sp5taru, 1990; Paninet al., 1579-1994). During the second half of the19'ncentury, channels were cut to shorten thenavigation route on Sulina distributary. When thiswas done, the southern arm of SfAntu Gheorghef ost about 3Oo of its water discharge andconsequently its sediment transport capacity infavour of the shortened arm of Sulina, (Sp5taru,1990). Sul ina bed-load accumulates at the end ofthe jetties as a mouth bar, which is periodicallydredged. The dredged sediments are dischargedoffshore, and thus are lost from the nearshoretransport system (Panin et al . , 1979-1994). Sul ina

beach situated just south of the jetties wasnourished with sand extracted from Sulina harbour,and two groins were also built.

A revetment was recently built along thesouthern barrier beaches between Peritesca andChituc (Postolache et al . , 1995). In order tomitigate the beach erosion on these beaches,some Romanian hydrotechnical engineers haveproposed to deviate part of the SfAntu Gheorghedistributary water and sediment load farther south,through a channel discharging between Cioticaand Perisor (Hangu et al . , 1992). Midia harbour isprotected by jetties extending about 5 km offshore;these jetties restrict the amount of sand carriedsouthward by the longshore drift by redirecting itoffshore (Fig.1).

Shorel ine Dynamics

The northern part of delta coast which includesChilia delta and Musura Bay beaches, most of it onthe Ukrainian territory, has been intenselyprograding in the last centur ies (Panin, 1985;Mikhailova, 1995), ,After the Sulina jetties werebui l t at the beginning of the century, the deltaiccoast farlher was partly isolated from the sedimentdrift coming from northern Chilia coast. Thisisolation increased in time with the jettiesextension. The jetties can be considered now asimpermeable to the sediment drift.

Beach profiles were measured on a network ofbenchmarks along the delta coast south of Sul ina,including one landmark on Sakhal in ls land. Basedon these, shoreline changes were analysed byBondar et al . , (1983) for the period between 1962and 1978, and by Vespremeanu and Stefanescu(1988; Fig.4, 5) forthe period between 1962 and

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steep be

1 4 G EA.ECO.M AR INA, 2J1 9 97National lnstitute of Marine Geology and Geo-ecology of Romania

Proc. lntern. Workshop on'Fluvial-Marine lnteractions" in Malnas, Romania, QcL1-7, 1996

L. Giosan et al. - Longshore Sediment Transport Pattem along Romanian Danube Delta Coast

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1987. The actual shoreline Dositions were notpublished but only the calculated end-pointshorel ine change rates. Panin et al . (1979-1994)collected annual beach profile measurements after1978 within the framework of the Romanian MarineGeology and Geoecology institute s "Danube-BlackSea System Monitoring Program". GAstescu(1979, 1986) publ ished shorel ine change analysesfor most of the Danube delta coast based on mapcomparisons.

The shoreline is retreating over almost theentire Danube defta coast situated south of Sulinamouth (Fig.4, 5). The maximum retreat rate for theSulina-Sfdntu Gheorghe sector is over 2A mlyr.,which is found at the latitude of Rosulet lake, about15 km south of Sulina jetties. Almazov et al. (1963)suggested that the highest retreat is due to thedivergent of the longshore sediment drift in thiszone which may be caused by an anticyclonaleddy attached to the south lee side of the Sulinajetties, generated by a quasipermanent coastalcurrent directed southward. The existence of thisdivergence zone is also recognised in othersediment drift studies based on radioactive orfluorescent tracers (Dragota, 1973; Bondar andCraciun, 1970), sal ini ty distr ibut ion (Bondar, 1964),and physical modelling (Sp6taru, 1971). Howeverthe divergence zone may be explained bysheltering and diffraction of the waves coming fromthe Northeast sector. Shoreline recession ratesover 10 m/year were also registered along theSakhalin barrier island, and south of Ciotica andbetween Portita and Chituc on the southern barrierbeaches. The shoreline advances immediatelysouth of Sulina jetties where it is accompanied byintense shoaling of the submerged profile (Bondarand Harabagiu, 1992). The shorel ine advancesbehind Sakhalin island where Sfdntu Gheorghesecondary delta is actively prograding, and slowlyat Perigor, south of the Ciotica retreating zone,where the shore orientation changes fromapproximately E-W to a NE-SW direction.Shoreline advance was registered on the southernChituc spit due the sediment transport obstructionexerted by Midia harbour jetties. The submarineslope also shoals on the sides of Midia structures(Panin et al . , 1979-1994).

Nearshore Processes along SakhalinBarrier

Bratescu (1922), Panin et al. (1979-1994),Breier and Teodor (1979), Vespremeanu (1983),and Bondar et al , (1984) analysed Sakhal in ls landgenesis and evolut ion. Sakhal in ls land (Fig.1)formed in 1897, from a bar developed south of theSf6ntu Gheorghe mouth (Bratescu, 1922).Sediment discharged by SfAntu Gheorghedistributary, and also drifted from north,accumulate as a bar at the mouth of SfAntu

Gheorghe. Under the influence of mainlysouthward sediment drift the mouth bar sands aresupposedly transfened south during stormscausing a continuing increase in length of Sakhalinisland (Panin et al., 1979-1994). At the same time,as observed from successive maps and aerialphotos, Sakhalin island shore is retreating, moreintensely in its central and southern parts (Panin etal., 1979-1994). The northern half has alreadybeen connected to the mainland marshes in 1975due to both island retreat and SfAntu Gheorghesecondary delta intensive progradation,transforming the island in a spit. Due to its lowrelief (0.5-1.5 m), the retreat of the island isapparently controlled by ovenrvash and breachingprocesses as suggested by the numeroussuccessive breaches which appeared over theyears following major storms (Panin et al., 1979-1994). The breaches were generally rapidly closedas the wind-induced, and secondarily tidal flows,between the lagoon and the sea are not strongenough to keep them open. The southern t ip oftheisland recurves into the lagoon due to waverefraction pattern typical for this type of areas andalso because the low relief beach at the tip evolvesin an ovennrash mode (as conceptually describedin Kana, 1996). The analysis of is land evolut ion forthe 1858-1968 period (Bondar et al . , 1983) showsthat the island platform built intensively in offshoredirection between 1850 and 1923. This processceased practically afteruards leaving the profileunaltered below 10 m depth until 1968. Above thisdepth the bathymetric contours show a retreat forthe same period. Consequently the beach profile io10-12 m depth has flattened while the islandretreated. Also bathymetric charts comparisonsshow that the island platform has been building inthe axial (longshore) direction (Jianu and Selariu,1970). This process extends to a depth of about 12m and it was uninterrupted though cyclical,between 1858 and 1968 (Bondar et al . , 1983). Insummary, the island platform building in axial andpossibly offshore direction, and also ovenrvash andbreaching are the processes that extractsediments from the Sakhalin nearshore system.Using the maps of Breier and Teodor (1979) andPanin et al. (1979-1994; Fig.6), the islandshorel ine changes for the 1962-1993 period werecomputed for this study (Fig.4).

Longshore Sediment Transport

There are several direct indicators of thelongshore sediment transport direction andmagnitude including sand accumulation onopposite sides of engineering structures such asjetties and groins, and the longshore groMh ofspits and barrier islands, methods which werepreviously applied on the Danube delta coast.These previous studies indicated that themagnitude of the net longshore transport is

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GEO.ECO-MARINA, U1997National lnstitute of Marine Geology and Geo-eaology of Romania

Proc- lntern. Workshop on'Fluvial-Marine lnteractions" in Malnas, Romania, Oct.1-7, 1996

1 5


direction is also a direct indicator of the netlongshore transport rate. By using previouslyestimated shoaling rates to 9 m depth (Bondar etal., 1983), the transport is directed southward at arate of about 1,080,000 m'/year. Farther southalong Chituc spi t , Bondar et al . (1980) est imatedthe longshore transport by using synoptic winddata. The drift was directed southward at a rate ofabout 900,000-1,110,000 m3lyear down to the 9 misobath.

METHODS

Sediment Budget

A sand budget was used to estimate of thelongshore sediment transport pattern. The activebeach, the longshore transport, and the riversediment input were assumed to be the onlysources or sinks for sediments for the entire studyarea. The above assumptions, however, would notbe valid along barrier beach of Sakhalin the island,since the cross-shore sedimentation by ovenirrashand breaching (Panin et al . , 1979-1994), andpossibly offshore loss (Bondar et al . , 1984; Jianuand Selar iu, 1970) are l ikely to be important. Thecalculations therefore were not done for Sakhalinisland but special consideration was given to thebudget here.

The remaining study area was divided into twocoastal cells based on the changes of thelongshore transport direction and on Danubedistributaries sediment discharge. The northerncell was taken between Sulina mouth and SfAntuGheorghe mouth. The Sulina mouth jetties 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 the southern one was left open. lt isreasonable to assume that the Sf6ntu Gheorghedistributary sediment discharge is largely directedto the south, in such a way there was no significantcontribution from this source to the northernSul ina-Sfdntu Gheorghe cel l . The southem cel lwas between Ciotica, situated on the deltaicmainland shore, and Midia. The southern lateralboundary was assumed to be closed because thejetties protecting Midia harbour disrupt thelongshore sediment transport by redirecting itoffshore. The northern boundary of this cell wasleft open. Net longshore transport patterns for eachcell were derived from the calculated volumechanges by integrating them starting at the closedboundaries.

The active beach volume changes werecalculated based on shoreline change rates for theperiod between 1962 and 1987 (Vespremeanu andStefanescu, 1988; Fig.2 and Fig.3). Because thesedata do not extend as farther south as Midiaharbour, the shoreline progradation rate was

Fig.6 Consecutive posiiions of Sacalin barrier island(compiled from Breier and Teodor; 1979, Gdstescu, 1979; andPan in e t a l . , 1994) .

extremely high. From north to south, the sedimentdrift is directed southward along Chilia lobe, showsa reversal south of Sulina jetties, and continuessouthward to Midia afterwards. Shuisky (1984)estimated the net sediment transport along Chiliadelta to be between 120,000 m'/year and 850,000mt/year, depending on the local orientation of thecoast. The southward drift along Chilia can beestimated from the shoaling rates on northern sideof Sul ina jet t ies. Bondar and Harabagiu (1992)proposed a^ value between 1 ,100,000 and1,900,000 m'/year for the shoal ing rate but theperimeter for which it was calculated is not clearlyspecified. Thg shoaling rate of 500,000 m3/year to1,300,000 m'/year (Bondar and Harabagiu, 1992)on the southern side of the jetties is a directestimate of the net longshore transport in thereversal zone south of Sulina. The natural shoalingin this area was clearly overestimated since anunknown but substantial amount of sand retrievedfrom Sul ina harbour was discharged on the beachin 1980s. South of the reversal zone to SfAntuGheorghe,^the longshore drift was estimated to700,000 m'/year (Panin et al . , 1979-1994) from thebeach sand deficit. Shuisky (1985) suggested anet longshore sediment transport of 800,000-900,000 m'/year along the same sector. TheSakhalin barrier island platform building in the axial

GEO.ECO.MARINA, A1997National lnstitute of Marine Geolagy and Geo-ecology of Romania

Proc. lntern. Workshop on 'Fluvial-Marine lnteractions" in Malnas, Romania, Oct.1-7, 1996


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considered to be constant southward of the lastmeasurement point and have the same value asmeasured at that last transect. Although themeasurement station used for collecting theshoreline change data were widely spaced alongthe coast they were considered to represent wellthe actual shoreline behaviour being very similar tothe estimates presented by others (Gdstescu,1986 and 1993; IUCN,1992: Spdtaru, 1992). Theshoreline change rates were corrected for sealevel changes by applying Bruun's rule ( lnman andDolan, 1989), assuming a 3 mm/year sea-level risefor the active deltaic shore (i.e. Sulina-Sf6ntuGheorghe cell) and a 1.2 mmlyear sea-level risefor the Ciotica-Midia cell. An average berm heightof 0.5 m was used for the entire coast as thelandward boundary. Because suitable submergedbeach profile measurement series wereunavailable for the Romanian coast, an averagedepth of closure of 9 m was estimated usingHalermeier's method (1981 a, b) based on theavailable wave data. The annual values for theestimated depth of closure, between 1972 and1981, ranged between 6 and 12 m. Previousstudies of the long-term submerge profile evolutionsuggest a closure depth between 10 and 12 m(Jianu and Selar iu, 1970, Bondar et al . , 1984).

Volume changes were calculated using a hybridmodel as appropriate, either under assumption thatan equilibrium long term beach profile is conserved("one-l ine" model; Pelnard-Considere, 1956;Hanson and Kraus, 1989), orthatthe beach prof i legeneral shape is non-conservative during shorelineretreat. During delta formation phase, both theshoreline and the subaqueous delta advance atthe same rate, while in the reduction process, thedelta shoreline is retreating more rapidly than thesubaqueous delta (Refaat and Tsuchiya, 1991).This latter behaviour is due to the higher erosionrates in the surf zone compared to farther offshorefollowing a decrease in the river sedimentdischarge, which must occur to maintain thepotential longshore sediment transport rate. Thusan equilibrium beach profile is not conservedduring the delta reduction process, since the profileslope becomes more gradual. Jimenez andSanchez-Arcilla (1993) confirmed the aboveexperimental results in the case of Ebro delta. The"one line" model seems appropriate to describe thebeach volume changes on a delta coast in theformation phase only. When a deltaic shoreline isretreating, as is the case of the Danube delta coastunder study, it is more appropriate to compute thebeach volume changes using a wedge-shapederosional prism. However the accreting sector ofthe shoreline south of Sulina jetties seems toconserve an equilibrium during its advance(G6stescu, 1993). Therefore it was reasonable touse the "one-line" model tor this sector, as well as

for the other advancing shoreline sectors (Perisor,and Chituc). Longshore net sand transport patternwas derived from volume changes for the depthsof closure of 6, 9, and 12 m. The result for the 6and 12 m depth were used to estimate themagnitude range of the net transport. However, themost plausible estimates were considered to be forthe average depth of closure of g m, and they willbe used in further discussions.

Potential Longshore Sediment TransportRateWave characteristics have been recorded at

Constantza, in the southern part of the RomanianBlack Sea coast, since 1964 by the RomanianMarine Sciences Research Institute. Significantwave height, mean period, and mean direction at6-hour interval over a 1O-year period between1972 and 1981 were used in the current study(Fig.2, 3). The measurements involved visualobservations of a buoy fixed at a water depth of 11m. The measured characteristics include meanwave height, period, and direction three times dailyduring the daylight. The wave direction tpestimated visually, with an accuracy of about 20".Gaps in the record due to periods of poor visibilitywere filled with statistically significant values(Kraus and Harikai, 1983). Wave data were sparsefor other sectors of the coast. Because the windregime does not vary significantly along therelatively short coast, the wave climate atConstantza was assumed to represent that alonefor the entire coast.

The bathymetry was obtained by digitising1:50,000 scale maps published by the RomanianNavy Hydrographic Service, based on their 1979survey. Then the observed waves were backwardrefracted on the real bathymetry around the pointwere they were measured using iteratively a wavetransformation program provided by U.S. ArmyCorps of Engineers (RCPWAVE; Cialone et al.,1992), in an attempt to transform the wave data fordeepwater conditions (-30 m). However, becausethe angle of approach is poorly resolved in theoriginal figures, the transformed data were notsignificantly different.

The potential longshore sedi.r-.nent transport wasestimated using a nearshore sediment transportmodel based on the wave energy flux (NSTRAN;U-S. Army Corps of Engineers, 1984; lnman andDolan, 1989). The constant k, which describes theefficiency of longshore component of wave energyflux in transporting sediments, was considered anadjustable parameter. A value of k=0.256,calibrated using the sand budget computations to adepth of 9 m, was used. Both NSTRAN andRCPWAVE are component programs of the"Shoreline Modelling System" (SMS) standardisedsoftware ccllection crealed by the U.S.Army Corps

GEO-ECO-MARINA, A1997National Institute of Marine Geology and Geo-ecology of Romania

Proc. lntern. Workshop on'FluviaLMarine lnteractions" in Malnas, Romania, Oct.1-7, 1996

1 7


of Engineers (Hanson and Kraus, 1989), The 140km-long area extending from Sulina to Midia wasdivided in six sectors according to the shorelinemean orientation (Fig.7) and the potential netsediment transport was computed for each ofthem. The resulting patterns were compared to thetransport rates computed from the sand budget.

RESULTS

The general pattern of the net longshoretransport provided by the sediment budgetcompared favourably to that calculated by waveenergy flux method (Fig.8, 9). The net longshoresediment transport along the entire studied coast ishigh on average, mainly as a result of both theprevailing E-NE waves superimposed on NNE-SSE general orientation of the coast.

Sulina-Sfdntu Gheorghe

From Sulina to SfAntu Gheorghe, the resultingnet sediment transport scheme was basicallymade up of two cells (Fig.8, 10): a sector ofnorthward directed transport situated immediatelysouth of the Sulina jetties with an average rate of190,000 m"/year (ranging between 130,000m"/year at 6 m depth, and 250,000 m'/year at 12m depth) and a longer sector of southwardtransport farther south to SfAntu Gheorghe whichhad an average rate of 620,p00 m'/year (with arafige between 415,000 m'/year and 830,000m"/year). The magnitude of the transport in thenorthern cell does not explain the shoaling rate ofat least 500,000 m'/year south of Sulina jetties(Bondar et al., 1992). Presumably the artificialnourishment of the beach with an unknownquantity of sand, is the main factor affecting thebudget there. The potential transport calculationalso showed a two-cell pattern, but for the northerncell both the length and the net transport were lessthan those obtained from the sand budget. Thetwo-cell pattern is the result of the sheltering ofwaves coming from the NE quadrant, anddiffraction around the jetties. The poor quality ofwave data, poor performance of the waverefraction model adjacent to jetties, the corruptedsand budget, and probably the existence of anattached anticyclonic eddy in this shadow zone(Almazov et al . , 1963), al l concur in creat ing thedifference between patterns. Other investigatorshave found that closed attached eddies may fromin the lee of capes, jetties, or other topographicindentations of the coast, with consequentmodifications to local distributions of sedimentdeposit ion (Ferent inos and Col l ins, 1980; Davies etal . , 1995). In the southern cel l , the net transportincreased from zero at the nodal point, up to800,000 mt/year at CAsla VSdanei, remaining fairlyconstant up to the cell end, at Sf6ntu Gheorghe(Fig.8). The sediment transport rate north of C6slaVSdanei is lower relative to the rate of transport

farther south because of an increase in thesubmerged beach slope (Fig.7) and the shelteringeffect of Sulina jetties.

3doo'

Fig.7 Bathymetry of the study area, and the averageorientation of different coast sectors used in computing thepotential longshore sediment transport.

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

c-, a

r0 15 20 2s 40 45 50Sncdin hled

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d0 s0 60 70 80 90 100 l l0 110 I30 140

ciltritr

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Fig.9 Patterns of net longshore sediment transport for thedeltaic coast south of Sacalin barr ier island (using sameconventions as in fig.8).

Sakhal in ls land

Sakhalin island shoreline situated south ofSfAntu Gheorghe mouth receded everywherebetween 1962 and 1993, reaching the highestretreat rate on the Danube delta coast (66 m/yr.) atthe island centre (Fig.4). Consequently the islandconvexity had been reduced (Fig.6). During thisperiod not only did the island migrate landward butits length also increased with about 1.5 km at thenorthern tip and 5.5 km at the southern end. lt was

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National lnstitute of Marine Geology and Geo-ecology of RomaniaProc. lntern. Workshop on 'Fluvial-Marine lnteractions" in Malnas, Romania, Oct.1-7, 1996

L. Giosan et al. - Longshore Sediment Transporl Pattern along Romanian Danube Delta Coast

not possible to compute a sediment budget for theSakhalin island because measurements ofsubmerged beach profile were not available.However, the pattern of potential longshoretransport was estimated using the same value for k(i.e.0.256) as for the entire study area. The nettransport directed to the south with an averagemagnitude of about 1,230,000 m"/year, and arange between 820,000 m'/year and 1,640,000m'/year. This value was, not unexpectedly, thehighest figure on the entire Danube delta coast(Fig.10). Sakhal in average shorel ine or ientat ionincreases the wave angle of attack, and the steepnearshore gradient minimises wave refraction. Thetransport increased from about 900,000 m'/yearsouth of the SfAntu Gheorghe mouth to about1,750,000 mtlyear in the is land median sect ion(Fig.8). For the southern half of the is land, thetransport showed a decrease to 750,000 m"/year.This indicates that the southern half of the islandshould have been accreting but the shorelineretreated instead (Fig.4). Therefore, the shorelinevariation must have been controlled bymechanisms other than the longshore sedimenltransport. Cross-shore sediment transferprocesses such as ovenryash, breaching (Panin etal. , 1 994), and offshore sediment transport(Bondar et al . , 1983 and Bondar et al . , 1984)during storms could be more likely to have beenresponsible for the overall recession of the coast inthe southern part of the Sakhalin lsland. Thenorthern half of the island was wider and had afield of relatively high foredunes (Panin et al.,1979-1994), which would inhibit cross-island sandtransport. The southern half of the island wasnarrower with a lower relief and more likely to beovertopped and breached during storms.

There is a convergence iq net potentialtransport of about 500,000 m"/year of sandbetween the southern tip of the Sakhalin island(Fig.8) and Ciot ica on the mainland (Fig.9). Thisamount of sand was apparently not transportedfarther south along the coast but accumulated onthe axial zone of island platform. Bondar et al.(1983b) estimated this shoaling rate, to a depth of9 m, at 1,400,000 m"/year, but for the periodbetween 1858 and 1968. l t is also possible thatpart of the sand entered the bay behind Sakhalinisland. The bay behind Sakhalin is known to beshoaling due to Sfdntu Gheorghe secondary deltaprogradat ion (Panin et al . , 1979-1994; GAstescu,1979 and 1986; Vespremeanu, 1983), but becauseno quantitative studies were done in the area,there are no estimates for the sediment fluxpossibly coming from the offshore.

Giotica-Midia

The net transport continued to be mainlysouthward in the Ciotica-Midia sector of the

Romanian coast (Fig.9, 10). The general pattern ofnet transport calculated from the sand budgetresembled closely the pattern of the potentialtransport. In particular, the positions for localminima and maxima in the transport rate were thesame. lt was expected the cross-shore transport tobe significant, where barrier beach separating theRazim-Sinoe lagoon complex from the sea wasnarrowest, between Portita and Periboina (Fig.1),and therefore to alter the accuracy of the sandbudget, Estimates of the overurash processes inthis area would increase the accuracy of themodel. However, it is expected that the importanceof ovenruash will diminish as a result of the recentlybuilt revetment.

A notable discrepancy is a 10 km-long reversalof the transport, along Periteasca beach (Fig.9),which was not found in calculation of the potentialsediment transport. The magnitude of the reversetransport, however, was relatively small (8,000m"/year on average using the average 9 m depthof closure; taken as negl igible, i .e. -0 in Fig.10). l tmay have been merely due to uncertainties in thetransport magnitudes calculated from the sandbudget. Therefore the reversal zone at Perisor asresulted from sediment budget should beconsidered with caution until new data will improvethe accuracy ofthe budget.

Between Ciotica and Perisor, the net potentialtransport was directed southward at an averagerate of about 270,000 m"/year (180,000 m'/year to6 m depth, and 360,000 m'/year to 12 m depth;Fig.10). The average rate of the net transportsouth of the reversal zone to Midia was about660,000 m3/year_to the south (440,000 mt/yearand 880,000 m'/year to 6 and 12 m depthrespectively; Fig.10). The rate increased from 0 toa maximum of 1,075,000 m'/year on the northernChituc spit and decreased aftenruards to 775,000m"/year at Midia (Fig.9).

The potential transport rate was southward forthe entire Ciotica-Midia stretch. A decrease in therate between km.78 and km.85 indicated that theshoreline in Perisor sector was advancing (Fig.9),but it underestimated both the intensity ofaccretion, and the alongshore extension of theaccretionary zone. Further calibration based on amore accurate sand budget would be needed toclarify the discrepancies.

DrscusstoNChil ia lobe is the only part of Danube delta

intensely prograding" SfAntu Gheorghe distri-butary is in equilibrium on its northern side,progradation being active in the secondary deltabehind Sakhal in is land. The earl ier lobe of Sul inacontinues lo be reworked in an accelerated regimeunder anthropogenic inf luence ( i .e. dredging,jettied mouth). Furthermore the entire coast south

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GEO-ECO-MARINA, A1997National Institute of Marine Geology and Geo-ecology of Romania


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represent advancing and retreating sectors respectively.

45"00

44'00

GEO.ECO-MARINA, Y1997National lnstitute of Marine Geology and Geo-ecology of Romania

Proc. lntem. Workshop on'Fluvial-Maine Interactions" in Malnas, Romania, Oct.1-7, 1996


of Sulina is isolated form Chilia lobe by Sulinajetties. The coast between Sulina and Midia hasretreated at very high rates in the last decades,under very energetic waves due to their acuteangle of attack, and poor refraclion on relativelysteep nearshore. Since this part of the coast isclosed laterally to north and south by impermeablestructures, shoreline changes were due to thelongshore transport that redistributed sand from anarea to another. Cross-shore landward sedimenttransfer processes appear to be important for theSakhal in barr ier is land, and potent ial ly for thenarrow sandy barrier beach between Peritesca andChituc. Sediment transfer to the offshore isprobably active on the steep Sakhalin beach, andis also induced by the jetties at Midia.

About 800,000 m3/year of sand are lost for thenearshore system by maintenance dredging ofSul ina mouth. This amount would be enough tokeep the shorel ine south in equi l ibr ium. l fbypassed south of the reversal zone adjacent tothe jetties, it will possibly stop shoreline retreat.The sediment transport reversal is responsible fortrapping sand in the side of the Sul ina jet t ies,increasing downstream beach starvation.

Sf6ntu Gheorgle distributary also dischargedabout 800,000 m' lyear of sand. The potent ialsediment transport indicates that the average rateof beach retreat on northern Sakhalin, which wasabout 35 m/year, would more than double if thissediment input would disappear. The sand passingthe southern tip of Sakhalin island contributes tothe island platform building, and it is not able tofeed the beaches farther south since the shorelineorientation changes abruptly. Continued retreat ofthe island will enhance bypassing of sediments tothe south.

The change in coast orientation south atPeriteasca, under the present wave climate,favours longshore transport convergence, and thusbeach accretion. This sector acts as a buffer forthe sand transport farther south, increasing evenmore sand starvation of beaches. The proposedchannel to deviate some of the Sf6ntu Gheorghedistributary discharge farther south, betweenCiotica and Perisor, as a solution to reduce theerosion, would probably reduce the shorelinelocally in a zone of relatively reduced economicand ecological importance, but the Sakhalin beachwill have to compensate the loss via erosion. Theisland beach, already has the highest shorelineretreat rate on the Romanian coast, would becameadditionally sand-starved by applying this plan,endangering the rich Sakhalin bay wildlife habitat.Furthermore, the solution does not seem viable forstabilising beaches south of the buffer zone.

The area with highest potential risk is thenarrow barrier between Periteasca and Periboina.

even though the sand accumulation zoneupstream the Midia jetties may eventually providesome positive effects on a longer time scale. Onthe other hand, the sand trapped upstream Midiaharbour starves of sand the southern Romaniancoast where most of public beaches and touristobjectives were developed. lt can be anticipatedthat the time when the trapped sand will beartiflcially bypassed south of Midia is not too faraway since other protection solution appliedMamaia beach were not particularly successful.Alternately, the development pressure mayrelocate to the northern beaches on the Danubedelta coast.

CONCLUSIONS and RECOMMENDATIONS

The long-term recessional character of thebeach between Sulina and Midia is prescribed bythe alternate channel switching process,characteristic for Danube delta evolution. As aconsequence in the last centuries, Danubedischarged mostly through the northern distributaryof Chilia. The shoreline of previously built lobe ofSulina retreats, as it is being reworked under thelongshore sand transport induced by the highenergetic level and acute angle of wave attack.SfAntu Gheorghe lobe has ceased to prograde,exceot for the sheltered area behind Sakhalinisland. The shoreline evolution was negativelyinfluenced by the engineering works at Sulinamouth and Midia harbour, and most probably, bythe natural and human induced decrease of theDanube sediment discharge. The negative effect ofdamming on the Danube sediment load wasattenuated by erosion of the river bottom and isletsin the lower course and by artificial meander cut-offs in the della. Because the sand deficit ispotentially high, the restoration of the entire coastbetween Sulina and Midia may be financiallyprohibitive, and a system of priorities must beestablished. An array of restoration strategies withminimum negative effect for adjacent beaches,including beach nourishment, sand bypassing, anddetached breakwaters, are needed.

lncreased coverage and accuracy of wavemeasurements would probably provide thegreatest improvement in future studies. Three newgauges are planned to be installed along the coast.At least one of them should be situated betweenCiotica and Periteasca where shoreline orientationchanges abrupt ly. For the highly act ive Danubedelta coast, monitoring would include aerialcoverage of the area at least once a year; beachprofiling to the depth of closure on an more closelyspaced landmark network, at least twice a year inorder to estimate both the annual and seasonalvariations; and bathymetric surveys for the highlydynamic areas such as Sulina and Sf6ntuGheorghe mouths, and Sakhal in is land at least

G EO-ECA.MARINA, 2/1 9 97National lnstitute of Marine Geology and Geo-ecology of Romania

Proc. lntern. Workshop ot1 'Fluvial-Marine Interactions" in Malnas, Romania, Act.1-7, 1996


once per decade. A potentially promising methodfor estimating send transport to be used in themodel calibration would be the monitoring of the

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longshore sediment ransport pattern along

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