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GEOMOR-02145; No of Pages 16

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Geomorphology xx (

Beach–dune interactions on the dry–temperateDanube delta coast

Alfred Vespremeanu-Stroe ⁎, Luminiţa Preoteasa

Faculty of Geography, Bucharest University, Bucharest, 1 Bd. N. Balcescu, 01004, Romania

Received 30 January 2005; received in revised form 1 September 2006; accepted 5 September 2006

Abstract

Beach–dune seasonal elevation changes, aeolian sand transport measurements, bathymetric surveys and shoreline evolutionassessments were used to investigate annual and seasonal patterns of dune development on Sfântu Gheorghe beach, the Danube deltacoast, from 1997 to 2004. Dune volume increased consistently (1.96 m3 m−1 y−1 to 5.1 m3 m−1 y−1) over this 7-year period withhigher rates in the southward (downdrift) direction. Dune aggradation is periodically limited by storms, each of which marks a newevolutionary phase of the beach–dune system. As a consequence of the variable beach morphology and vegetation density during ayear, foredune growth occurs during the April–December interval while between December and April a slightly erosive tendency ispresent. The pattern of erosion and deposition shown by the topographical surveys is in good agreement with the sand transportmeasurements and demonstrates the presence of a vigorous sand flux over the foredunes which is 20–50% smaller than on the beach.This high sand flux, due to low precipitation and sparse vegetation cover, creates an aerodynamically efficient morphology on theseaward dune slope. The seaward dune face accretes during low to medium onshore winds (5.5–12 m s−1) and erodes during highwinds (N12 m s−1).© 2006 Elsevier B.V. All rights reserved.

Keywords: Foredune; Beach–dune interactions; Aeolian sand transport; Microtidal beaches; Temperate dry climate; Black Sea

1. Introduction

Although foredunes and the adjoining beachesrespond differently to short-term events they exhibit along-term coupled evolution. Several recent studies ofbeach–dune interaction recognized that the two environ-ments often have correlated morphodynamics withstrongly coupled sedimentary fluxes (Short and Hesp,1982; Pye, 1983; Psuty, 1988; Davidson-Arnott andLaw, 1990; Sherman and Bauer, 1993; Sherman andLyons, 1994; Saye et al., 2005). In this context, the main

⁎ Corresponding author. Tel.: +40 21 3153074.E-mail address: fredi@geo.unibuc.ro (A. Vespremeanu-Stroe).

0169-555X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.geomorph.2006.09.011

Please cite this article as: Alfred Vespremeanu-Stroe, Luminiţa Preoteasa,Danube delta coast, Geomorphology (2006), doi:10.1016/j.geomorph.200

challenges in understanding coastal dune geomorphol-ogy are:

to investigate and quantify the role of nearshore pro-cesses on beach–dune system behaviour (e.g.Aagaard et al., 2004);to establish the quantitative role of the short time-scale events in long-term evolutionary trends (e.g.Orford et al., 1999);and to develop high accuracy monitoring methods(e.g. Andrews et al., 2002; Woolard and Jeffrey,2002). For example, several studies have investigatedforedune development on different coasts using asediment budget approach (Hesp, 1988; Goldsmith

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et al., 1990; Psuty, 1992, 1993;Arens, 1997;Hellemaa,1998; Hesp, 2002).

Coastal dune studies have concentrated on the fore-dune development of humid temperate coasts. Most ofthese studies deal with either aeolian sand transportinvestigations important in understanding the presentmorphodynamics (Arens, 1996a; Gares et al., 1996;Bauer and Davidson-Arnott, 2003; Aagaard et al., 2004),or documenting the interactions between dune andbeaches (Sherman and Bauer, 1993; Battiau-Queney etal., 2002; Hesp, 2002; Saye et al., 2005) while othersassess the alongshore changes of the foredunes (Psuty etal., 1988) or present foredune evolutionary models

Fig. 1. Location of study area: A) position of Sfantu Gheorghe beach alonGheorghe shown on an ASTER satellite image — May 2002.

Please cite this article as: Alfred Vespremeanu-Stroe, Luminiţa Preoteasa,Danube delta coast, Geomorphology (2006), doi:10.1016/j.geomorph.200

(Short and Hesp, 1982; Hesp, 1988; Psuty, 1988).Studies of beach–dune interactions in temperate dryclimates are comparatively rare; the same paucity ofcoastal dune studies applies to the Black Sea basin.

This paper examines the meso-scale behaviour of abeach–dune system from the Danube delta coast in atemperate dry climate environment with a microtidal,dissipative beach characterised by medium waveenergy. The main research objectives are: (i) todocument the influence of beach dynamics on fore-dune behaviour, (ii) to estimate the aeolian sedimen-tary flux and evaluate its role in controlling theforedune morphology, and (iii) to detrend the impact ofthe short-term changes (storm-induced and seasonal)

g the Danube delta coast; B) the benchmark network on the Sfantu

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Fig. 2. The shoreline evolution in the last three decades [A) PN, B) R 48, C) Parid]; for comparison, the distance from benchmark to waterline isreported to a constant length of y-axis (80 m) in all graphs.

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on the multi-annual evolution of the beach–dunesystem.

2. Study area

The entire study site lies within a protected area, with avery low anthropic pressure, which facilitates the mon-itoring of natural processes responsible for sedimentaryexchanges between beach and dune. Sfântu Gheorghe(Saint George) is a 7.5 km-long sandy beach situated inthe central part of the Danube delta coast, stretchingnorthward of Sfântu Gheorghe channel, one of thethree main distributaries of the Danube (Fig. 1). SfântuGheorghe beach fronts the southern half of the lateHolocene (2500 yr BP–present) Sărăturile beach ridgeplain (Panin, 1989; Giosan et al., 2006). During the 19thcentury, when most of the Sulina–Sfântu Gheorghe

Fig. 3. The main factors controlling the beach–dune system evolution: A)(superimposed black lines) computed as multi-annual averages at Constanta stin the study area; C) sand roses for Sfantu Gheorghe and Sulina meteorologiccoefficient of the vector lengths to obtain a better graphical representation.

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interdistributary coast was subject to strong erosion,Sfântu Gheorghe beach prograded continuously, form-ing beach ridges and a small dune field, which graduallyevolved into a deflation plain. Comparison of historicalmaps shows that progradation ceased at the beginningof the 20th century and the 1923 shoreline is very closeto the present shoreline position. According to surveysmade by the Romanian Hydrological Service in the last30 years, the shoreline of the southern and centralsectors of Sfântu Gheorghe beach displays a metastableposition (Fig. 2), indicative of a beach–dune system indynamic equilibrium (Vespremeanu et al., 2004).

The orientation of the Sfântu Gheorghe beach is N96° and the prevalent northerly winds blow parallel tothe shoreline. The foredunes have moderate heights(2.5–3 m) and an asymmetrical transverse profile withgentle slopes (2–8°). Subaerial beaches fronting the

monthly sea level oscillations (grey columns) and standard deviationation gauge, for the 1933–1990 interval; B) typical bathymetric profilesal stations; the number from the inner circle represents the multiplying

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foredunes have a seasonally controlled morphology,displaying 15–25 m widths at the beginning of thespring and 30–50 m widths at the end of the summer,respectively. Our investigations show that the differ-ences in seasonal signal are induced mainly by waveclimate but also by sea level oscillations dependent onthe Danube discharge (Fig. 3A), both of which favourdeposition in the late summer and autumn and erosionduring the winter and spring.

The Sfântu Gheorghe submerged beach has a slightlyconvex cross-shore profile, with a very uniform mor-phology in the offshore zone. In the nearshore zone(300–400mwidth), the submerged bars diverge seawardfrom north to south to join the Danube mouth bar(Fig. 3B). In general, the barred beach has a dissipativehydrodynamic regime (Vespremeanu-Stroe, 2004a),offering an efficient protection against wave attackduring storms. The area is virtually tideless, with amaximum tidal range of 0.12 m at spring tide (Bondaret al., 1973), the wave energy climate is medium, with asignificant wave height of 0.9 m, and the longshore cur-rent is vigorous, transporting three times more sedimentsouthward than in a reverse direction (Vespremeanu-Stroe, 2004b). The frequent northeasterly waves induce a0.85–1×106 m3 y−1 net longshore sediment transport(LST) (Giosan et al., 1999).

The climate is temperate and continental. TheDanube delta coast is the driest region in Romania andone of the driest around the Black Sea, with a meanannual rainfall of 345 mm. Temperature rises during thesummer to 21.8 °C (the mean in July–August)promoting high rates of evapo-transpiration (Stoenescu,1965). According to the Peguy climogramme (Peguy,1961), the summer months (June–August) and the mid-autumn (October) are considered arid months. Thebeach is exposed to winds from all directions, althoughnortherlies clearly dominate. Of all winds which are ableto mobilize the sand (≥6 m s−1), those from thenorthern sector (ENE–WNW) account for 73.2% ofthe total. Wind climate is high wind energy with 75 vu(vector units), according to Bullard's (1997) adjust-ments to Fryberger's (1979) classification. Thedirectional variability of the strong winds is small,resulting in a southward aeolian drift: N 197° at SulinaandN 182.4° at Sfântu Gheorghemeteorological stations(Fig. 3C). The directional distribution of the potentialdrift is acute bimodal (Fryberger, 1979). Monthlyanalysis of the aeolian drift shows the existence of twoseasons with different potential transport intensities: thecold active season (October–April), with high rates ofpotential transport (83% of the total annual sandtransport) and the warm moderately active season

Please cite this article as: Alfred Vespremeanu-Stroe, Luminiţa Preoteasa,Danube delta coast, Geomorphology (2006), doi:10.1016/j.geomorph.200

(May–September) (Preoteasa and Vespremeanu-Stroe,2004). During the period of investigation, the mostimportant marine event occurred at the beginning (21–25 January 1998) when a major northeastern storminduced significant erosion of the beach–dune system,with shifting of shoreline position and the isolateddevelopment of wash-over fans in the southern extremityof Sfântu Gheorghe beach.

The foredune vegetation did not include woodyspecies until 1971, when an experimental forestry plan-tation inland of the Sărăturile beach ridge plain led to theestablishment of the buckthorn (Hippophae rhamnoides)in the back-dune (Traci et al., 1988). The dune crest andthe foredune seaward slope are covered by grassy veg-etation (Eryngium maritimum, Salsola kali, Spergularemaritima) and sandy bindweed (Convulvulus persicus),all of them with low sand trapping capacity. The onlygrassy species that enables rapid dune growth is Elymussabulosus, which is rare and confined to low surfaceswith high carbonate concentrations.

3. Methodology

In the present study we integrated short and medium-term beach–dune system monitoring. Short-term mon-itoring (pre- and post-storm topographical surveys andinstantaneous sedimentary flux measurements) has beenundertaken in order to quantify the role of factors op-erating over the short-term in the medium-term evolution(years to decades) of the beach–dune system. Integratingmicro-scale processes into a meso-scale evolutionaryapproach can be perceived as being inappropriate; butthe effects of the short-term processes may last for longertimescales, thus influencing the general evolution of theforedune system (Sherman, 1995).

3.1. Topographic surveys

Accurate topographical surveys (±0.1 cm elevation)were undertaken with a total station (Sokkia 610) onindividual transverse profiles (2D) across the beach–dune system. Digital elevation models (DEM) wereused for 3D interpretation. All measurements were setto a 6-benchmark network along the study area. Forfive benchmarks, the measurements covered theinterval December 1997–December 2004, while thePN benchmark was included in our measurementssince July 2000. Measurements were made at approx-imately 3–4 month intervals, except for the winter of2003, when bad weather compromised the fieldcampaign, resulting in an 8-month data hiatus (August2002–April 2003). The two DEMs were started in

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April 2003 and surveyed with the same frequency asthe ordinary cross-shore profiles, but due to the shorttime that elapsed since the start of the measurementperiod, only preliminary results are presented here. Onepolygon is located at the NR 48 benchmark and extends200 m alongshore consisting of 9 cross-shore profilesspaced at 25 m. The second DEM is placed at the R 48benchmark where more detailed surveys were made.The DEM from R 48 extends 40 m alongshore, beingderived from 5-m spaced profiles at which 200–250intermediate survey points are added every time,resulting in a total of about 500 survey points. Forboth DEMs, kriging interpolation methods were used(Golden Software's Surfer), with a grid cell size basedon the results of the comparative analysis of theresiduals error (Andrews et al., 2002). A 1-m grid cellsize was found to be the optimum size for our datadensity for the R 48 DEM and 2-m for the cell size of theNR 48 grid. Data from topographic surveys were usedto assess the morphological and volumetrical changes.

The bathymetric surveys were performed with anechosounder (Garmin 188) for depths greater than 1 m,accompanied by topographic measurements for depthsless than 1 m.

In order to explore the long-term dynamics of thebeach and to integrate the present behaviour within thebroader scale evolution, the shoreline dynamics of the Sf.Gheorghe beach for the last 7 years was compared withthat recorded for the last three decades (Stănică, 2003;Vespremeanu et al., 2004).

3.2. Aeolian sand transport measurements

Instantaneous sedimentary flux measurements wereundertaken mainly to investigate the effects of the winddirection on the foredune development. The aeoliansand flux was measured simultaneously on the beachand on the foredune. During the experiments a windmast with four cup-anemometers mounted at 0.5, 1.05,2.05 and 3.75-m elevation and a wind vane at 2.05 mwas used. It is generally accepted that the averaging ofthe wind speed values for long time periods increasesthe errors in modeling of aeolian transport rates(Namikas et al., 2003). Accordingly, 3-second intervalaverages of 1-second wind data were used in order toobtain shear velocities, transport rates, the meansediment flux and the potential sediment flux. Thus,the transport induced by gusts was better assessed. Thepotential transport is roughly estimated as for idealsurfaces (horizontal, dry and unvegetated) withouttaking into account the slope, the vegetation density orthe water content. In this study we used sediment

Please cite this article as: Alfred Vespremeanu-Stroe, Luminiţa Preoteasa,Danube delta coast, Geomorphology (2006), doi:10.1016/j.geomorph.200

prediction models of Bagnold (1941), Kawamura (1964)and Lettau and Lettau (1977):

q ¼ C

ffiffiffiffidD

rqg

� �u3⁎ ð1Þ

(Bagnold, 1941)

q ¼ Cqgðu⁎−u⁎tÞðu⁎ þ u⁎tÞ2 ð2Þ

(Kawamura, 1964)

q ¼ C

ffiffiffiffidD

rqgðu⁎−u⁎t Þu2⁎ ð3Þ

(Lettau and Lettau, 1977)where q is the sand transport rate (kg m−1 s−1), C is anempirical constant (1.8 for naturally graded dune sands inEq. (1), 2.78 in Eq. (2) and 4.2 in Eq. (3)), d is the meangrain diameter (mm), D is a reference grain diameter of0.25 mm, ρ is the density of the air (kg m−3), g isacceleration due to gravity (m s−2), u⁎ is the shear velocity(m s−1) and u⁎t is the threshold shear velocity (m s−1).

The threshold shear velocity is estimated using theterm of Bagnold (1941):

u⁎t ¼ Affiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffigdðqs−qÞ=q

pð4Þ

where A is a constant (0.1 at the fluid threshold) and ρsis the sand density (2650 kg m−3).

The shear velocity was computed from the gradientof the vertical velocity profiles using von Karman's ‘lawof the wall’:

uz ¼ u⁎kln

zz0

� �ð5Þ

where uz is the wind speed measured at an elevation, z,above the surface, k is von Karman's constant (0.4) andz0 is the roughness length.

The sedimentary flux was measured using cylindricalsand traps buried into the sand with the trap rim at thelevel of topographic surface. The traps are omnidirec-tional with a diameter of 0.46m and 0.1m depth. In orderto increase the efficiency, the traps were designed with a3-cm high detachable grill. Trapping rates are normalisedusing the size of trap diameter and expressed of kg m−1

s−1. Moisture content was determined from additionalsand samples collected from the upper surface (b5 mm)which were weighed, oven-dried and then re-weighed(Sherman and Hotta, 1990).

3.3. Meteorological and water level data

Wind regime was analysed using the Fryberger tech-nique (Fryberger, 1979). The hourlymeanwind speed and

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Fig. 4. Sand transport measurements: A) sand flux versus beach fetch during the experiment 1; B) sand transport rates measured in experiment 1compared with the potential transport; C) sand fluxes measured simultaneously on the beach and foredune (solid line) during experiment 3, comparedwith potential transport (Bagnold, 1941) expressed as coefficient of achievement (dotted line); D) sand transport rates measured in experiment 2compared to the potential transport.

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direction data, measured at 10m height, were obtained forSulina and Sfântu Gheorghe meteorological stations forthe 1991–2000 period. Wind frequencies were recordedin sectors of 22.5°. Although in this paperwe analyse onlythe Sfântu Gheorghe beach, data from Sulina meteoro-logical station are important in estimating the beachmicroclimate. Relative to the shoreline position, Sulinameteorological station is 3 km seaward on the Sulina armjetties, at about 27 km north of the centre of the study area,while Sfântu Gheorghe meteorological station is 1.5 kminland, which explains the important differences in winddata from the two stations (Fig. 3C). Dailywater level datafor the Sfântu Gheorghe channel provided by RomanianHydrological Service were processed in order to findintervals with low and high water level that affect thebeach dimensions.

4. Results

4.1. Aeolian sand transport and associated topographicchanges

Understanding of current foredune morphodynamicsrequires basic information on aeolian sand transport

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characteristics within the component units of the fore-dune. Six field experiments were undertaken to measurethe sediment fluxes both on the beach and foredune; thethree most relevant experiments that paid specialattention to the interaction of aeolian transport and to-pography are presented below.

4.1.1. Onshore winds — September 2003The first two experiments were performed on

September 11, next to the R 48 benchmark, during amoderate onshore storm. Meteorological conditionsduring the first experiment were conducive for criticalfetch investigations; the wind blew from the east (81°)with 10.5–12 m s−1 computed for 10 m height. Foursand traps were deployed on the beach along the winddirection, at 1.6 m, 4.2m, 6.6 m and 9.7 m (Fig. 4A) fromthe maximum swash limit, in order to assess the in-fluence of beach fetch on the sand transport rate. Becauseof the wave set-up, the beach narrowed. During theexperiment the distance between the dune toe and theupper limit of the swash zone measured 10 m, in com-parison with about 30 m before the storm. The measuredsand flux rapidly increased from the waterline reachingits maximum capacity at T3 and T4, for a beach fetch of

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7–9 m (Fig. 4A). Assuming that the values recorded atT3 (0.022 kg m−1 s−1) and T4 (0.021 kg m−1 s−1) arerepresentative of the maximum transport encountered onthe beach, we compared them with the results of severaltransport equations (Bagnold, 1941; Kawamura, 1964;Lettau and Lettau, 1977). The best fit of the experimentaltrapping rates to theoretical estimates was recorded forthe Bagnold equation followed by the Lettau and Lettauequation (Fig. 4B).

During the second experiment four sand traps weredeployed on the foredune in a cross-shore profile: T7wasplaced on the dune crest, T6 in the middle of the seawardslope, T8 into a small wind sluice channel on the seawardflank whereas the fourth trap (T5) was placed close to thedune toe within the shadow zone of an Eryngiummaritimum bush. The difference between wind speeddatasets recorded for sand traps is due to slightly dif-ferent recording intervals; a strong wind gust occurredduring the deployment of T7 and T8. Changing the windmast position into the dunes, the computed roughnesswas found to be considerably greater than on the beach,

Fig. 5. Storm-induced level changes across beach–dune system: A) the easterfrom 21 to 25 January 1998. Solid lines show the cross-shore topographical pstorm.

Please cite this article as: Alfred Vespremeanu-Stroe, Luminiţa Preoteasa,Danube delta coast, Geomorphology (2006), doi:10.1016/j.geomorph.200

with 0.4 mm in the foredunes and 0.045 mm on thebeach. The high transport rate recorded by T8 is ex-plained primarily by its position into a wind sluicechannel. These ephemeral aeolian landforms readilydevelop on the upper part of the seaward slope duringperpendicular onshore winds; the sand blown in this waycontributes to a high sand flux on the dune crest reflectedby T7 trapping rate. The transport equations overesti-mate by four to five times the T5 and T6 trapping rates,three times the T7, and by 50% the T8 trapping rate(Fig. 4D). An inhibiting role on transport was played by alight rain which started at the beginning of the secondexperiment. The mean water content of active surfaceduring the experiment was 1.4%.

During the same onshore storm (September 11–13,2003) topographic surveys were made at the beginningand after the storm in order to assess the surface levelchanges. A clear delimitation of the swash zone duringstorm peakmakes possible the analysis of changes due toaeolian erosion and deposition and the correlation withmarine-induced changes. At R 48, inland from the limit

n storm from 11 to 13 September 2003; B) the great northeastern stormrofile while dotted lines mark the surface level changes induced by the

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Table 1Sediment flux changes over the beach–dune system during onshoreand longshore winds expressed as percentage from the local potentialtransport computed after Bagnold (1941)

Morphological unit Onshore winds(30°–90°) % ofpotential transport

Longshore winds(0°–30°) % ofpotential transport

Beach (winter berm) 35–100 50–120Seaward slope 15–40 10–25Dune crest 20–50 15–25Landward slope b15 b10

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of storm waves, small positive changes (7–8 cm) wererecorded. Further landward, at about 10 m from theswash limit, where vegetation density increases, the sandsuddenly accumulates on the dunefoot forming a 3-mwide and 0.3-m high depositional landform that islaterally continuous alongshore (Fig. 5A). Excepting thedunefoot, the alternation of slight erosion (on steepersectors) and deposition (on flatter sectors) characterisesthe seaward slope. These changes are consistent with themeasured sedimentary fluxes which reached a maximumon the beach, close to the vegetation boundary (T3 andT4), while an important decrease in transport capacitybehind the dune toe (T5) is reflected in the localtopographic changes by a net deposition (Figs. 4A, C,5A). The large difference between sand fluxes over thedune crest and landward slope created an importantdepositional sector just behind the dune crest. The samepattern of erosion and deposition and sediment budget isrecorded on the other profiles (OGA53,NBuival, NR48).

For comparison, a very strong oblique onshore stormduring 21–25 January 1998 (U10=15–28 m s−1, winddirection=67.5°–45°) at OGA53 produced lowering ofthe subaerial beach by 0.6–1.2 m and changes in level ofthe foredune of between −0.4 and +0.25 m (Fig. 5B).The topographical changes induced by strong obliquelyonshore winds consist of an overall erosion of theseaward slope with erosion peaks occurring in steepersectors and the adjacent swash zone, and in discontin-uous deposition on the lee slope.

4.1.2. Longshore winds — October 2003Longshore aeolian transport was observed during a

moderate wind event (U10=9.2 m s− 1, wind di-rection=12°) on October 17. At the OGA 53 benchmark,five traps were simultaneously deployed across a beach–dune transverse profile: T9 on the upper beach at 8.3 mseaward from the dune toe, T10 at 3.8 m seaward from thedune toe, T11 in the middle part of the seaward slope, T12on the dune crest and T13 in the mid-part of the landwardslope. Under the same wind conditions, the traps locatedonly a few m apart on the upper beach (T9 and T10)captured different amounts of sand (Fig. 4C). The max-imum transport rate (0.0074 kg m−1 s−1) at T10, in frontof the dune toe, slightly exceeded the potential sedimentflux predicted by the Bagnold equation. Under the sameconditions (the dry upper beach) at just 4.5 m seawardfrom T10, T9 trapping rate was considerably smaller(0.0046 kg m−1 s−1), probably because of the differentbeach fetch distance and conditions. For T9, the computedbeach fetch is about 240 m, mostly developed on the wet,gently sloping lower beach, while the computed beachfetch for T10 is 290 m, of which 65 m consists of the dry

Please cite this article as: Alfred Vespremeanu-Stroe, Luminiţa Preoteasa,Danube delta coast, Geomorphology (2006), doi:10.1016/j.geomorph.200

winter berm. The highest recorded transport rate in theforedune (0.0023 kg m−1 s−1) occurred on the dune crest,representing 25% of the potential sediment flux. Thelowest trapping rate was on the landward slope (T13).

The topographical changes during this longshore eventwere very small (≤0.05m)with awell-balanced sedimentbudget of only +0.23 m3 m−1. The rapid alternation oferosional and depositional cross-shore sectors suggeststhe re-distribution of the sand over short distances. Theonly exception is represented by the dune toe wheredeposition of 2–5 cm was recorded.

4.1.3. Aeolian flux changes on foredunesA rough estimation of sediment flux changes over the

foredune and upper beach was obtained by computingthe values recorded during six experiments comprisingperpendicular, obliquely onshore and longshore windevents (Table 1). The values are only indicative as theinvestigations (performed in May–October interval,U10=7–13 m s−1) did not include high-energy eventsand winter conditions.

4.2. Longshore and cross-shore foredune dynamics

Under conditions of an average net southward Long-shore Sediment Transport (LST) of 0.85–1×106 m3 y−1

(Giosan et al., 1999; Vespremeanu-Stroe, 2004b) which isprevalent on the Sfântu Gheorghe beach, the foredunedisplays a continuos alongshore distribution. Dune ex-tension at the distal part of longshore transport cell and inthe proximity of the Danube's distributary mouth (themouth bar acts as a sink area for longshore-moving sand)emphasizes the major role of these sedimentary sourcesfor dune development. Typical of the study area, is the co-existence within a short distance of several beach sectorswith different patterns of evolution: i) a 2.5 km long sector(PN – OGA 53) at the northern part of Sfântu Gheorghebeachwhich is affected bymoderate shoreline retreat; ii) astable sector (OGA 53 – Parid: 4.5 km) with ahomogenuous foredune belt; and iii) the southern 1 km

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of Sfântu Gheorghe beach (Parid – Cape Buival) whichhas a slightly prograding trend (Vespremeanu-Stroe andConstantinescu, 2000).

4.2.1. The northern sector (PN – OGA 53)This sector has generally narrow beaches (20–30 m

wide) and foredunes (20–25 m wide) with significantdifferences between the northern and southern sides. Thenorthern part exhibits a steady erosive trend (Fig. 2A)while the southern extremity has been stable for severaldecades (Vespremeanu-Stroe, 2004a). Foredunes with

Fig. 6. Cross-shore profiles showing the evolution of the beach–dune system

Please cite this article as: Alfred Vespremeanu-Stroe, Luminiţa Preoteasa,Danube delta coast, Geomorphology (2006), doi:10.1016/j.geomorph.200

small heights and widths that are periodically destroyedby high magnitude storms are typical in this sector. Sincethe great storm of January 1998, aeolian activity on highbeach deposits and wash-over fans has not been interrup-ted by any large storm, allowing for the development of acontinuous dune belt. The dunes in place are relativelyrecent, forming since the summer 1998 along theboundary between the beach and the wash-over fans.The dune build-up was associated with a decrease in theshoreline retreat rate between 1998 and 2002. Presently,the foredune ridge is continuous along the entire area,

at the benchmarks: PN (A), OGA 53 (B), NR 48 (C), NBuival (D).

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Fig. 7. Foredune evolution in connection with the beach width A) PN, B) OGA 53, C) NR 48, D) R 48, E) Parid, F) NBuival. The dashed line withcircles marks the foredune volume evolution while the thin solid line represents the beach width measured as distance between dune toe and waterline.Foredune evolutionary trends after the January 1998 storm; for the comparison of the foredune volume changes the volumes were normalised to thatrecorded in February 1998.

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with occasional scarp features along its seaward flank.The base of the landward slope rests on a horizontalgrassy vegetated surface with small hummock dunes thatare currently being overwhelmed by the landwardmigration of the foredunes. At the PN benchmark (located7.8 km north of Sfântu Gheorghe mouth), foreduneaccretion induced an increase in volume from 12.8 m−3

m−1 to 23 m−3 m−1 between 2000 and 2002, when thebeach was 30–40 m wide (Figs. 6A, 7A) providingsufficient wind-exposed surface to reaching the criticalbeach fetch. The progressive growth of the foredunescontinued despite an erosive phase of the beach since July2001. The period of time inwhich the foredune developedindependently of the beach behaviour lasted no longerthan 1 year, after which the beach and the foredunes

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followed the same pattern of evolution. After a stormyinterval between February and April 2003, the stoss slopeof the foredune was eroded, being replaced by acontinuous scarp over a distance of 5 km alongshore.As a result, the dune belt started to retreat consistentlyat a rate of about 0.5–2 m y−1 (Fig. 6A).

4.2.2. The central sector (OGA 53 – Parid)Along the central part of the Sfântu Gheorghe beach

(1 to 5 km north of Sfântu Gheorghe mouth) the fore-dunes show the greatest consistency in the entire studyarea, due to its interdistributary position south of thesandy coasts which are the source of sediment forlongshore drift. The coast here comprises an extensivebeach, a well-developed dune belt and three nearshore

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Fig. 8. A) R 48 DEM (April 2003); B) level changes resulted from the subtraction of the April 2003 and August 2004 DEMs.

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bars which ensure an efficient protection against stormwave attack (Fig. 3B). The medium-term (decadal)evolution of the shoreline shows a metastable dynamicequilibrium (Fig. 2A).

Profiles measured between 1997 and 2004 at the OGA53, N R 48 and R 48 benchmarks, show a distinctivedynamic pattern of behaviour in the beach–dune system.The seaward dune face records a high activity, compa-rable to that of the upper beach, despite the differentnature of the processes that are predominantly acting onthem. The landward side of the foredune records aconsiderably slower activity while the crest is stable.

Analysis of the topographical surveys from this sectorreveals a seasonal morphological pattern, with reversiblechanges of the shoreline position of about ±15 m but nolong-term trend of erosion or accretion (Figs. 2B, 7D).During the analysed period (1997–2004) the foredunesdisplay steady aggradation with different speeds at thethree benchmarks (1.9 m−3 m−1 y−1 at OGA 53 to3.8 m−3 m−1 y−1 at R 48). The foredune volume at NR48 increased with a regular rate of 4 m−3 m−1 y−1 until

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autumn 2001 when the trend become stationary. Theforedune evolution seems to be strongly connected withoscillations in beach width. Thus, the stationary intervalcoincided with a period of decreasing beach width(c. 15 m narrower) (Figs. 6C, 7C).

Unlike the other parts of the Sf. Gheorghe shore, thedirect impact of the storm waves is not obvious here, theonly exception being represented by the small ephemeralscarps locally produced on the seaward side of theforedunes during the January 1998 storm. The largesttopographic changes of the seaward side are mainlywind-induced. For the entire period of measurements,the seaward face of the dunes increased in height.

Recent detailed measurements performed at the R 48monitoring polygon (75×40 m) (Fig. 8A), showed thatlandward slope dynamics are controlled by interactionsbetween the local topography and the woody vegetation(H. rhamnoides) that traps sand. The lee slope is shortand rectilinear, exhibiting a steep gradient locallyranging from 25° to 33° depending on the backdunevegetation density. Steeper depositional fronts are present

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in the proximity of very dense vegetation, which producesa slow but constant landward migration of the sand andgenerally promotes higher dune elevations. Foreduneheight in this sector measures 3.5–4 m while the width isbetween 40 and 50 m. The net sand volume changeresulting from topographical changes in foredunes at theR 48 polygon, between April 2003 and April 2004, is212 m3, with gains up to 307 m3 and losses up to 95 m3.For this 1-year interval, a mean depositional rate of0.07 m m−2 was recorded, but this value hides the vari-ability of depositional patterns. Sand accumulating onthe lee slope represented about 90% of the entire volumeof sand deposited on the foredunes (Fig. 8B). Once depo-sited here by wind, the sand is totally decoupled fromfurther exchanges between dunes and beach, thus explai-ning the aggradational process of the foredunes. Thevolume changes in the foredunes between April 2004 and

Fig. 9. The analysis of the NR 48 DEMs: A) level changes recorded bysubtractions of chronologic different mean profiles across theforedunes on NR 48 DEM; the mean profile was obtained for everysurvey by averaging of all 9 cross-shore profiles which represented thebasis of the NR 48 DEM. The circles point to the middle of every 7-mlong segment on which the subtractions were made; B) representationof the April 2003 cross-shore mean profile; C) the bar graph shows thereworked sand quantities in the functional units of foredune fordifferent time intervals.

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August 2004 reveal another net positive volume of 59 m3

with a gain of 113 m3 and a loss of 54 m3 during thisperiod.

Two km to the north of R 48, in the same coastalsector, recent measurements performed at the NR 48monitoring polygon show a slight erosional trend,despite the large amount of sand that travels in thebeach–dune system (Fig. 9A). As the vegetation coverdensity is similar to that on the R 48 experimental site,local morphology is probably the main controlling factoron the sediment deposition pattern. A particular in-fluence is exerted by the beach narrowing since 2002,restricting the sediment flux in the dune. The measure-ments performed from April 2003 to August 2004 reveala net volume of sand eroded from the foredune equal to161 m3 with 550 m3 gain and 711 m3 loss. A continuoussand input occurred only at the base of the landwardslope with a overall quantity of 45 kg m−2 whereas at thedune toe sand accumulation was recorded just betweenApril 2003 and December 2003 resulting a total input of15 kg m−2. A continuous sand depletion producedduring the entire period of measurements on the seawardslope (−102 kg m−2) and dune crest (−96 kg m−2). Themost intensive erosional processes occurred mostly onthe steep sectors (points G and D from Fig. 9A).

4.2.3. Parid – Cape Buival sectorIn this sector measurements were performed at the

NBuival (800 m N of Sf. Gheorghe mouth) and Parid(1000 m N of Sf. Gheorghe mouth) benchmarks (Fig. 1).

This sector has a characteristic multi-decadal cyclicbehavior. The dynamics are characterised by a highsensitivity to storm surges as well as the rapid post-stormrecovery enabled by a large sediment input in the area.After the January 1998 storm, this sector recorded asteady aggradational trend although it experienced themost erosion during the storm surge; the storm waveslocally overwashed the dunes and re-deposited thesand as wash-over fans. Where the vegetation cover wasdenser, the sand was preserved as residual dunes. Theaverage volume of beach sand lost on this occasion atNBuival was of 36 m−3 m−1. After this event, a newforedune belt was re-built on a landward alignment com-pared to the previously destroyed belt at about 60 m fromthe water line and about 40 m behind older dune line. Thewinter berm began to be overwhelmed by incipient sandaccumulations initiated around Salsola kali shrubs. Ananalysis of the topographical profiles showed that deposi-tional processes were continuous and the dune surfacerose by about 0.4 to 1m (Fig. 6C). All thesemorphologicalchanges can be related to the lack of any major stormevents after January, 1998. During the post-storm period

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Fig. 10. Alongshore view of the coastal dune: A) NR 48 benchmark — northward view; B) R 48 – Parid sector — southward view; C) R 48benchmark — northward view.

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(7 years), the beach showed a slow but constant progra-dation rate, presently displaying the greatest widths of 50–65 m. The total sand volume deposited in this sectorbetween February 1998 and December 2004, comprisingboth foredune and subaerial beach, was about 82 m3 m−1

of beachwidth, of which 30–35m3m−1 were deposited inthe foredunes (Fig. 7 E, F). The two locations where thetopographical surveys were undertaken show slightdifferences in foredune accretion rate: (5.1 m3 m−1 y−1

to 4.7 m3 m−1 y−1). The overall accretionary trend of theforedune includes important seasonal differences, with aprevalent erosion of−0.2m3 m−1 month−1 (NBuival) and−0.16 m3 m−1 month−1 (Parid) during December–April,and accretion of 0.65 m3 m−1 month−1 (NBuival) and0.7 m3 m−1 month−1 (Parid) between April and Decem-ber. The central and the landward parts of the newforedune belt show high depositional rates surpassingthose affecting the seaward face of the dunes or the upperbeach (Fig. 6D).

5. Discussion

The results of a 7-year investigation of the beach–dune system on Sfântu Gheorghe coast suggest a high

Please cite this article as: Alfred Vespremeanu-Stroe, Luminiţa Preoteasa,Danube delta coast, Geomorphology (2006), doi:10.1016/j.geomorph.200

sensitivity of foredune morphodynamics to aeoliansedimentary fluxes and to seasonal and annual shorelinechanges. Generally, the pattern of erosion and depositiondeduced from topographical surveys is in good agree-ment with the sand transport measurements. They bothindicate the presence of a vigorous sand flux over theforedune (Stănică et al., 2004) within the same order ofmagnitude (20–50%) of that on the beach. (Fig. 4C, D).This is largely ascribed to the sparse vegetation coverspecific to dry–temperate environmental conditions. Thevegetation density is uniformly distributed over theforedunes and varies between c. 10% during the winterand 20–30% during the summer. In a dune landscape, theaeolian processes are generally controlled by plantbiomass production (van der Meulen, 1990) whichhere, on the Sf. Gheorghe beach (sensu stricto) or intemperate dry climate (sensu lato), remains very low.

Independently of the wind direction, the dune crestshows the highest transport rates in the foreduneswhereas the seaward slope shows larger transport ratesthan the landward slope. This fact particularly accountsfor moderate heights (2.5–3 m) and a general asym-metrical profile of foredunes which comprises a longgentle seaward slope (2–4°), a relatively stable crest and

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a short, moderate-steep landward flank (3–9°) (Fig. 10).Besides sparse vegetation, another control on foreduneelevation is the high incidence of obliquely offshorewinds which contribute to the longshore direction of theaeolian drift (Fig. 3C): oblique and onshore winds aremost effective for foredune development (Arens, 1996b).Unlike the foredune evolution pattern in a temperatehumid climate where, for onshore wind conditions, highwind speeds are necessary to move the sediments acrossthe transverse profile, the sediment dynamics in the studyarea need less vigorous aeolian forcing to enable surfacelevel changes on the landward slope under the sameonshore conditions. The topographical changes of theseaward slope are positive during low and mediumonshore winds (U10=5.5–15 m s−1) and negative duringhigh wind energy conditions (U10N15 m s−1). Duringperiods with higher vegetation density, the dune toe canencounter deposition rates higher than other foredunesubunits, but in the cold season the sand is re-distributed.Thus, the dune toe does not function as the mainaggradational unit for the meso-scale foredune evolution.

Storm impact on the foredune evolution was investi-gated as such events control the long-term morphologicalpattern of beach–dune interactions. The net volumechanges during two onshore storms with differentintensities, January 1998 (high) and September 2003(low), yield different responses in the beach–dune systemwithin the three sectors. The January 1998 storm causedobliteration of coastal dunes from the northernmost 2 kmof the study area and the loss of large sand volumes in thesouthern sector both from the beach (∼ 15 m3 m−1) andforedunes (∼ 21m3 m−1) (Fig. 7A, E, F). The central partwas well defended against wave attack by nearshoretopography and wide beaches. Here, the foredune did notlosemore than 8m3 m−1 beachwidth (Fig. 7B, C,D). Themain differences in beach–dune system response towaves attack are due to distinct shoreline dynamics overthe 3–5 years immediately preceding the storm. Thesignificant dune erosion from the southern sector is as-signed to a vulnerable position of the foredune, very closeby the waterline, as the shoreline of this sector en-countered a significant retreat between 1992 and 1998which diminished dramatically the beachwidth. The post-storm recovery of foredunes from the southern sector isoccurring at a rate of 4.6–5.1 m−3 m−1 y−1 at a position40 m landward of the former dunes.

The general response to low magnitude onshorestorms (September, 2003) includes a positive foredunebudget with erosion occurring on lower beach and partlyon the seaward slope and deposition on the dune toe, theflatter areas of seaward slope and on the landward slope(Fig. 5A). The general effect of this type of storm is to

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re-distribute sand from the lower beach to the upperbeach and dune toe, and from here to the foredune.Generally, during a storm, when the beach is narrow orcompletely flooded, the aeolian critical fetch extendsto the seaward side of the foredune, thus explainingthe erosion that occurred on the steeper areas andforedune crest and deposition at the base of landwardslope (Fig. 5).

The Sfântu Gheorghe coast contains three distinctsectors whose morphology and dynamic patterns arecontrolled by their position relative to the southwardlongshore transport and the Danube mouth. The northernsector (PN – OGA 53) is moderately erosive, currentlyacting as a source for the longshore sediment transport;the central sector (OGA 53 – Parid) is in a dynamicequilibrium, whereas the south of the study area, Parid –Cape Buival, has a multi-decadal cyclic evolution with aprograding behaviour during the study period (Vespre-meanu-Stroe and Constantinescu, 2000) (Fig. 2). Forthe entire period of measurements the overall evolu-tion shows a continuous increase of dune volume withhigher rates in the downdrift direction varying from1.96 m3 m−1 y−1 (OGA 53) to 5.1 m3 m−1 y−1 (Parid).The foredune growth is relatively homogenous and fitswell to a linear trendline (Fig. 7G), except for the sectorswhere changes in beach morphology caused thenarrowing of the subaerial beach during distinct intervals(2003–2004 for NR 48, see Fig. 7C) when the foredunegrowth ceases. A different evolutionary pattern occurs atthe northern site, where shoreline retreat causes narrowbeaches and the dune volume fluctuates depending onthe beach width.

The positive trend of the foredune budget is tem-porarily limited by the occurrence of a major storm eventwhich produces a new evolutionary phase. The pre-storm volume of the foredune (December 1997) wasreached again after 1–3 years on the central sector (OGA53 – Parid) while 5–6 years were necessary for the mosteroded sector (Parid – Cape Buival). These recoveryintervals show that on the stable beaches the long-termtrend can be positive despite the changeable nature of theconditions favourable to foredune building, but if wetake into account their limited volume (b100 m3 m−1

beach width) the secular trend seems to be beach steady– dune steady state. According to Sherman and Bauer's(1993) beach–dune interaction model based on sedimentbudget, modified after Psuty (1992), the general state ofSfântu Gheorghe beach sectors can be classified asfollows:

i) PN – OGA 53 sector: beach negative – dunesteady

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ii) OGA 53 – Parid sector: beach steady – dunepositive state

iii) Parid – Cape Buival sector: beach positive – dunepositive state.

The beachmorphodynamics of all sectors has a strongseasonal pattern with deposition during later spring–autumn and the prevalence of erosion in early winter–spring. Due to the microtidal regime, the beach width isinfluenced mainly by wave climate and wave-inducedcurrents. As a consequence of the variable beach mor-phology and vegetation density during a year, the sea-sonality of processes affects also the foredune. In theApril–December interval the foredune volume increasesand the subaerial beach develops a wide summer berm,while in the December–April interval the foredune areslightly erosive due to the beach narrowing and to theextension of critical fetch on seaward slope duringfrequent storm events.

6. Conclusions

Themorphological evolution of a beach–dune systemin a temperate dry climate indicates high sensitivity toaeolian dynamics represented by the maintenance of asignificant sand flux over the foredunes, at smaller rates(20–50%) compared with that on the beach surface. Thishigh sand flux is due to specific climatic and bioticconditions and facilitates the preservation of an aerody-namic morphology of the foredunes with slopes that dipgently seaward (2–4°) and moderately landward (3–9°)within an asymmetric profile.

The seasonal variation of wind and wave energyinduces different beach morphologies that affect thedune development. During the cold season, with a highfrequency of storms, the beaches get 15–20 m narrower,resulting in transport-limited conditions; as a conse-quence the foredune sediment budget is slightly negativein the December–April interval. Conversely, due to thewider beaches and the denser vegetation cover theforedune grows in the April–December interval.

The medium-term evolution shows a continuousincrease of dune volume with higher rates in the down-drift direction, varying from 1.96 m3 m−1 y−1 (north) to5.1 m3 m−1 y−1 (south). A general evolutionary patternof the studied foredunes, consists of a continuous ag-gradational trend interrupted by major storm surges.Depending on the multi-decadal shoreline dynamics andthe resulting morphology, the beach–dune systems reactdifferently to storm wave attack. The minimum lossesare recording on the stable sectors, where the post-stormrecovery interval lasts only for 1–3 years and enables

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positive trends for multi-decadal periods whilst in theerosive sectors, post-storm foredune growth is limited byrapid landward migration. The storm induced changesare of significant impact considering the long recoveryperiod (years).

Acknowledgements

This work was funded by the National UniversityResearch Council (CNCSIS research grant 346999) andby the Sfântu Gheorghe Marine and Fluvial ResearchStation of Bucharest University. We would like to thankDr. Ştefan Constantinescu for a constant field assistanceand discussions on this topic and also to Bogdan Iancu,Florin Filip, Ionuţ Ovejanu, Vasile Cârlan, Florin Tătui,Mihaela Fîstac, Janina Vlad and Alina Pătrulescu forhelping us with the fieldwork measurements. We aregrateful to Dr. Liviu Giosan, Prof. Andrew Cooper andtwo anonymous referees for their valuable comments onan earlier version of the manuscript. Special thanks aredue to Prof. Emil Vespremeanu and Prof. Mihai Ieleniczfor guiding us at the beginning of our study.

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Beach–dune interactions on the dry–temperate6.09.011