LONGSHORE SEDIMENT RANSPORT PATTERN ... - GeoEcoMar

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L_ G i y:zl:!:!:j:tg:!",."- -2 ediment Transport Pattem along Romanian Danube Delta Coast alternate channel switching. The energetic wave with the Black Sea. Reworked deltaic sands along regime in the caused by the preponderance of the shore have built a coastal barrier complex from local waves due to relatively short fetches, by the SfAntu Gheorghe to Midia. The barrier complex high angle of wave attack, and by the ineffective consists mainly of a baymouth barrier which refraction on a relatively steep nearshore. separates the large lagoon complex of Razim- STUDY AREA Geological Setting Sinoe, once open (Panin, 1983), from the sea. The shelf is broadest in the front of Danube delta and narrows to the south. The nearshore gradients to a The Romanian Black Sea coast (Fig.1) 12-15 m depth along the Danube delta coast, stretches over 245 km from the Chilia distributary of the Danube (45"12' N, 29"40' E) at the Romanian-Ukraine border to the town of Vama Veche (43"44' N, 28'35' E) at the border with range between 0.003 and 0.01, with the steepest slopes along the Sakhalin barrier island (Panin, 1985). The beach profile is generally multibarred (Postolache et al., 1992). Bulgaria. The Romanian coast can be divided using geographic and geomorphic criteria into a northern unit and a southern unit (Panin et al., '1979-1994). The northern unit is the low-relief Danube delta coastal zone while the southern unit The modern sediments on the nofthern sector of the coast consist of Danube-borne quartz sands (about 70% silica, Panin, 1989). The heavy mineral content is about 3%. Sands carried by the littoral drift from the region north of Danube delta have is characterised by eroding cliffs and loess, protected in places by narrow beaches (Charlier higher silica content than the Danubian sediments (90%). The subaerial beach sediments are and de Julio, 1985; Panin, 1979-1994). generally medium-fine sands (d50
1g ,m ]X lh nte il A ;t, st n, )d cr IS al al /e ts 'p ) )- n d IS v c )f Ir rl tl r n L- Giosan et al. - Longshore Sediment Transport Paftern along Romanian Danube Delta Coast Driga, 1986). The Danube delta subsides with 1.3- sediment discharge decreased in the last century 2 mmlyear, due to the compaction of deltaic mainly due to damming, the water discharge sediments and regional tectonics. The tide of the increased. Also the 25-30o/o sediment discharge Black Sea along the Romanian coast is reduction after Danube damming was not as semidiurnal, with a range of 7 lo 12 cm. lt has an extreme as in the case of the Nile (98%; Sestini, unnoticeable effect when compared with other 1992) or Ebro (95%; Jimenez and Sanchez-Arcilla, deformational fluctuations, such as seiches or 1993), but added to the previous regressive trend, storm surges which can reach a maximum of 2 m it amounts to 5E% of the total load. The increase in height, and 1 .2 to 1,5 m respectively (Bondar, water discharge can be attributed to climatic 1972; Panin et al., 1979-1994). Other important changes, and partly after damming to marsh mean sea level fluctuations with multiannual and reduction. The lron Gates dams (lron Gates I and seasonal cyclicity are due to river discharge, lron Gates Il) were closed in 1970's in the upper changes in the water exchange through the sector of Danube lower course. Major rivers Bosphorus straight, and precipitation/evaporation draining the Carpathian and Balkan mountains are variations. dammed, and discharge is therefore insignificant. River Discharge The relatively small reduction in the sediment discharge after damming, was due to increased The most significant factor affecting the erosion of the river bottom and islets in the lower hydrologic budget of the Black Sea is the seasonal course of the river (Panin, 1979-1994; Mihailescu, variation of river discharge. The Danube provides 1983), and to meander cut-offs in 1990's, along the 38% of the river water entering the Black Sea distributaries in delta area (Panin, 1979-1994). (Glaskow, 1970). Other important rivers in terms of their water discharge are Dniepr, Southern Bug, Wind And Wave Regime and Dniestr. Danube's annual discharge is highest The average wind speed in the northwestem from April to July. ln response to the variation in Black Sea is between 6.5 and 5 m/s (Bulgakov et river discharge, the sea level fluctuates between al., 1992). The predominant wind directions during 20 and 30 cm from season to season and about 20 the year, as they were measured at meteorological cm from year to year (Bondar, 1972). Between stations on the coast, are from the north, west and 1858 and 1988 the Danube discharged annually south (Ciulache, 1993). The prevalent direction for about 191 km3 of water, of which 63% was the onshore blowing winds is from Northeast. discharged through the Chilia distributary, 17% During the summer months, however, the through the Sulina distributary and 20% through predominant direction is onshore from the souththe SfAntu Gheorghe distributary (Bondar et al., Southeast (Diaconu et al., undated, as cited by 1992). The Danube water discharge increased Panin et al., 1979-1994). Because the Romanian from 178 km'in 1858 to 203 km'in 1988. The sector is relatively short, the wind regime does not mean total sediment discharge of the Danube was vary significantly along the coast especially for the about 52x106 metric tons per year between 1858 onshore winds (Ciulache, 1993). Storms are and 1966 (Bondar et al., 1992). The sediment prevalent from the north and Northeast, with an discharge in 1858 was 65x10o metric tons, but only average wind speed of 9.8 m/s and a duration 38x10" metric tons in 1988 (Bondar et al., 1992). ranging between I to 22 hours (Diaconu et al., This discharge was distributed 55% to the Chilia, undated, as cited by Panin et al, 1979-1994). 214/o to the Sulina and 23% to the SfAntu Waves higher than 0.2 m occur about 50olo of Gheorghe arm. About 25-30% reduction of the the year; 60-85% of them are local wind waves, total sediment discharge has been recorded after and 15-40o/o swells (Bondar, 1972). The wave 1970 when the first lron Gates dam was closed in regime was analysed for this study based on wave the Romanian-Yugoslavian sector of Danube observation made over a 1O-year period between (Panin et al., 1979-1994; Popa, 1992). The bed- 1972 and 1981 at a depth of 11m, offshore the load sediment discharge, that has a median grain southern town of Constantza (see "Methods"). We size between 0.1-0.5 mm, has been estimated to found that waves higher than 0.2 m (the lower limit be between 4.5o/o and 19o/o of the total sediment of the wave height that was measurable using the discharge. Today, ^the Chilia distributaryavailable instrumentation) arrived at the coast from discharges about 3x10o metric tons/year bed-load all offshore directions about 51% of the year sediments (between 57o/o afid 65% of Danube's (Fig.2). The annual average signiflcant wave total bed-load discharge), Sulina about 0.85- height was 0.8 m, with a mean period of 5 seconds 1.3x106 metric tons/yeir' (between 18.5% and (Fig.3). Most waves anived from the NE-SE 24.5o/o of the total dischgrge), and Sfdntu quadrant, the predominant direction (about 3oo/o ot Gheorghe about 0.75-1x10" metric tons/year total waves) and energy flux being from E. The (between 19% and 20.5o/o of the total discharge; waves arriving from NE-ENE sectors occurred Bondar, 1972; Bondar and Harabagiu, 1992). lt more frequently and their mean energy flux was can be noticed from above data that while Danube GEO.ECO- MARINA, 2/1 997 National lnstitute of Marine Geology and Geo-ecology of Romania Proc. lntern. Workshop on'Fluvial-Marine lnteractions" in Malnas, Romania, Oct.1-7, 1996 13
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LONGSHORE SEDIMENT RANSPORT PATTERN ... - GeoEcoMar

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