Wave climate, coastal sediment budget and shoreline changes for the Danube Delta

Marine Geology 262 (2009) 39–49

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Marine Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a r g e o

Wave climate, coastal sediment budget and shoreline changes for the Danube Delta Sebastian Dan a,b,⁎, Marcel J.F. Stive a, Dirk-Jan R. Walstra c, Nicolae Panin b a b c

Delft University of Technology, Faculty of Civil Engineering and Geosciences, 2600 GA, Delft, The Netherlands National Institute of Marine Geology and Geoecology–GeoEcoMar, 23-25, Dimitrie Onciul Street, RO-024053, sector 2, Bucharest, Romania Deltares, Marine and Coastal Infrastructure, P.O. Box 177, 2600 MH, Delft, The Netherlands

a r t i c l e

i n f o

Article history: Received 10 April 2008 Received in revised form 2 March 2009 Accepted 4 March 2009 Communicated by J.T. Wells Keywords: Danube Delta Wave climate Alongshore transport Closure depth Sediment budget Shoreline predictions

a b s t r a c t Since its formation, approximately 12 000 years ago, the coastal zone of the Danube Delta experienced alternating periods of accretion and erosion related to the natural process of lobe abandonment caused by river avulsion. Over the last 150 years, the effects of human interventions modified these natural trends. Regulation of the Danube River (e.g. the Iron Gates barrages facilitating navigation and controlling river discharge), construction and extension of the Sulina entrance jetties and dredging of the Sulina mouth bar for navigational purposes are examples of human interventions that influenced and still influence the actual state of the Danube Delta coastal zone. Most importantly, these interventions diminished the quantity of sediments reaching the coast and, consequently, led to a severe retreat of the shoreline, especially in the Sulina–Sf. Gheorghe coastal sector. To investigate and quantify the recent sediment budget and associated shoreline changes along the Danube Delta coastal zone we constructed a hindcast of the wave climate, the alongshore transport capacity and the resulting sediment budget using a numerical coastline model. On a local scale, it was found that overwash plays an important role; i.e. the observed evolution of Sahalin spit island shows a cross-shore landward migration that is much larger than could be explained by the calculated alongshore sediment transport gradients. On a delta-scale however, the resulting alongshore sediment transport gradients and associated sediment budgets comply very well with the observations, confirming the dominant role of wave-induced alongshore transport and giving confidence in our predictions regarding the shoreline position for the next 25 and 50 years. The results of this study indicate the potential for broader applicability of the model to other deltas, especially those in wave-dominated settings. © 2009 Elsevier B.V. All rights reserved.

1. Introduction At present many deltas globally are subject to a strong pressure due to increasing human impacts (Syvitski et al., 2005). Although the Danube Delta (Fig. 1) is relatively untouched by urbanization, human intervention such as the construction of the Sulina branch jetties, offshore disposal of the sediment dredged from the Sulina canal mouth and the intervention works along the Danube River (Panin,1997; Giosan et al., 1999; Ungureanu and Stănică, 2000; Stănică et al., 2007) have led to major changes in the state of the Danube Delta coastal zone by decreasing the amount of sediments reaching the coast. With its origin in Germany, the Danube River course ends in Romania forming the Danube Delta, the second largest delta in Europe. Its formation was initiated approximately 12 000 years ago when the actual Danube Delta was a gulf. After this gulf was filled in

⁎ Corresponding author. National Institute of Marine Geology and Geoecology– GeoEcoMar, 23-25, Dimitrie Onciul Street, RO-024053, sector 2, Bucharest, Romania. Tel./fax: +40 21 252 25 94. E-mail addresses: [email protected], [email protected] (S. Dan), [email protected] (M.J.F. Stive), [email protected] (D.-J.R. Walstra), [email protected] (N. Panin). 0025-3227/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2009.03.003

with sediments, different lobes formed and evolved shaping the present day Danube Delta (Panin, 1997, 1998, 2005; Panin et al., 1997; Panin and Jipa, 2002). The Danube River discharges into the Black Sea through three branches (from north to south): 1) Kilia, which transports approximately 58% of the water and sediment discharge, 2) Sulina, the major waterway, with a contribution of 19% and 3) Sf. Gheorghe, with a contribution of 23%, (Bondar and Panin, 2001). While the tidal regime is rather small (average amplitudes of 7–11 cm, Bondar et al., 1973), a larger role in the creation of sediment accommodation space was and still is due to relative sea level variations. Relative sea level rise is between 2.8 and 3.1 mm/year consisting of eustatic sea level rise, estimated for the past century at about 1.3 mm/year (Malciu, 2000) and subsidence, with estimated values of 1.5–1.8 mm/year (Panin, 1999). The Kilia secondary delta is prograding, favoured by the relatively large supply of sediment, viz. more than 50% from the total sediment discharge of the Danube River (Bondar and Panin, 2001). The shoreline in the Sulina area is retreating rapidly and the process is enhanced by human intervention, as we will highlight and discuss below. Presently, the Sf. Gheorghe lobe is still advancing moderately, forming and reworking the Sahalin spit-like island (Fig. 1). The sediment reworking due to overwash results in a landward migration of the spit island

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Fig. 1. Romanian littoral. The dotted line polygon shows the study zone.

developing the asymmetric lobe of Sf. Gheorghe. Eventually this will result in a merge of the spit island with the mainland. There is geological evidence that this is part of a cyclic pattern which occur a number of times during the evolution of the delta. The Danube Delta coastal zone has been adversely affected by construction of various structures along the river and the coast. The dams built along the Danube River and its tributaries to regulate the water discharge and to produce hydropower, have dramatically decreased the amount of sediment reaching the delta. Among these structures the most important are the barrages Iron Gates I and II (approximately 900 km upstream from the Black Sea), constructed in 1970 and 1983, respectively. These two barrages alone lead to a decrease of 35–50% of the discharged sediments (Panin, 1996). Locally, the Sulina jetties (Fig. 1) built for navigation purposes also have a profound impact on the evolution of the coastal zone. They block the alongshore sediment transport and remove the riverine sediments as a source for the delta system by discharging the sand far into Black Sea. Starting in 1858, works were undertaken to transform the Sulina branch into a navigational channel including meander cut-offs. Without anticipating further consequences, the strategy was adopted to successively elongate the Sulina jetties in order to avoid or at least reduce the accumulation of sediments at the seaward opening of the channel. As a result the jetties now extend 8 km into the sea, completely blocking the alongshore drift from the north resulting in large sedimentation of the bay north of Sulina and causing down drift erosion (south of Sulina). A minimum water depth of 7 m is maintained at the Sulina arm mouth by regular dredging of the mouth bar and dumping the sand in offshore locations. This is an additional cause for beach erosion in the Sulina–Sf. Gheorghe area.

The Romanian deltaic coast extends from the border with Ukraine (north) to Cape Midia (south) having a total length of 140 km. The study area is part of this Danube Delta littoral zone, being confined to the area from the Sulina jetties northward to the Portita inlet southward and covering a range of depths from the shoreline (west) up to 35–40 m (east). The total length of the shoreline of the study zone is 94 km, including the open sea side of Sahalin spit island (Fig. 1). The Sulina–Sf. Gheorghe section (34 km) is characterized by an alongshore current towards the south and a retreat of the shoreline. Just south of the Sulina jetties, due to the sheltering of northern waves, a northward directed alongshore sediment transport occurs which results in local accretion over an 8 km stretch. Near Sf. Gheorghe (over a stretch of 6 km, just north of it) the coast is in dynamic equilibrium, with periods of erosion alternating with periods of accretion. However, the section in between (a stretch of 20 km) is heavily eroding with rates ranging from 5 to 20 m/year (Panin 1996, 1999; Ungureanu and Stănică, 2000; Stănică et al., 2007). At Sf. Gheorghe mouth an arcuate mouth bar, called Sahalin, formed. The sand bar emerged in 1897, after an exceptional flood, and continuously stretched towards south-west reaching its present length of 17 km. Simultaneously, processes such as overwash caused a shoreward migration of the sand bar, sometimes with a rate of 70 m/ year (Panin, 1996). In the late 1970s, the northern part of the spit-like bar connected to the mainland. Behind the spit island the shoreline is advancing due to the sheltered environment and the sediment supplied by secondary distributaries of Sf. Gheorghe lobe. In some parts of the section Ciotica – Perisor (about 20 km long), south-westward from Sahalin (Fig. 1), the coastline retreated more than 500 m since 1950, due to merging of the lakes from this area with

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the sea. Southward, down to Portita Inlet over a distance of 23 km the shore is reported to be in dynamic equilibrium with episodes of alternating erosion/accretion of 5 to 10 m/year (Panin, 1996). In the whole study zone the sediment that forms the active beach and surf zone is well-sorted fine sand, mostly quartzitic, in places enriched with heavy minerals. The main sedimentary source is the alluvial sediment delivered by the Danube River. 2. Objectives The main objectives of the present study are to (1) obtain a better understanding of the processes governing the evolution of a large part of Danube Delta coastal zone and (2) determine a quantification of the alongshore sediment transport and associated erosion/accretion rates. While the dynamics of the major part of the study area are mainly dictated by the alongshore current and sediment transport gradients, Sahalin spit island is a more complex system. Besides alongshore sediment transport gradients overwash plays an important role that needs to be taken into account. As a final objective, predictions for the shoreline position over the next 25 and 50 years are made, taking sea level rise into account. To address the main uncertainties and to obtain a comprehensive morphological model of the Danube Delta coastal zone, a typical engineering approach was employed. Starting with wind records and a bathymetric map as input for a wave generation and propagation model, the nearshore wave climate was derived. Furthermore, the wave climate and the associated closure depth served as the main inputs for the computation of the alongshore sediment transport capacity. The concept of closure depth was used in order to delineate the active beach and surf profile. The closure depth is the depth beyond which net cross-shore sediment-transport rates are so small, that changes in morphology become insignificant on the time-scales of our interest (Bruun, 1962; Hallermeier, 1981). Assuming invariance of the active profile, sediment budgets are determined by alongshore sediment transport gradients alone, i.e. we have adopted the one-line coastal modelling approximation. A numerical model, with a relatively large alongshore resolution, was used to derive detailed alongshore sediment transport estimates. The resulting gradients of the alongshore transport together with the closure depth, based on local wave characteristics, were used to derive shoreline changes. Finally, predictions of the shoreline positions for the next 25 and 50 years were made based on numerical simulations including the effect relative sea level rise. The detailed wave climate and the alongshore sediment budget, along with the observed rates of migration were used to analyze the sediment circulation in the Sahalin domain. It turns out that the sediment budget of the Sahalin spit island is not determined by the alongshore sediment transport gradients alone. Overwash is playing a major role in the landward migration of the spit island. Although the above approach was taken to investigate a microtidal deltaic coast where data are scarce (especially wave data), it can easily be applied to other deltas where quantitative investigation is required. 3. Methods 3.1. Wind analysis To obtain reliable alongshore sediment transport estimates, local wave climates are imperative as sediment transports along the Danube Delta coast are primarily induced by wind and wave generated currents. The only continuous series of wave data available in Romania have been collected near Constanta, approximately 100 km southward of our study zone and they are not representative for our study area. In front of the Danube Delta the available wave data are based on visual observations and the records are not continuous, with blackouts occurring especially during extreme events. However,

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reliable local wind measurements are available which, as the waves are almost entirely generated by local wind, were used to determine the nearshore wave climate. Collected wind data consist of two sets. The first set covers a 10 year period (from January 1991 to December 2000) and is recorded at Sulina meteorological station. This station is situated at the landward base of the Sulina jetties. The second set covers one year (2002), but the measurements were made simultaneously at Sulina and the Gloria Platform (situated offshore, 30 km eastward from Portita Inlet, Fig. 1). For both datasets, Sulina and Gloria, records were taken four times per day containing speed (m/s) and direction (nautical convention, 16 directions). At Sulina the records were made at a height of 10 m, but at Gloria Platform at 36 m height above the mean sea level. The Gloria dataset was converted to the standard elevation of 10 m above the mean sea level (using U10 ≈ U36/1.15 (Shore Protection Manual, 1984)). In order to find out if the first dataset (Sulina 1991–2000) is representative for the offshore wind conditions a comparison was made between the nearshore (Sulina 2002) data and the offshore (Gloria 2002) data. The correlation between the two locations turned out to be fairly good, almost 71% of records being identical or very similar. However, wind directions crossing land (from SSW to NNW clockwise) did not correlate very well. To solve this problem a land vicinity correction was applied. An empirical method was used, obtained from a study of winds in the Great Lakes area, USA, (Shore Protection Manual, 1984) resulting in a dimensionless coefficient of 1.2 as a multiplication factor for the recorded wind speed. As a result of the correction a correlation was obtained between the wind speed data from the Sulina 2002 and Gloria 2002 stations. Based on this result, only the wind speeds of the wind directions crossing land from the first set (Sulina 1991–2000) were corrected for the land vicinity. The final result was a set of wave data recorded close to the shore, but corrected to be representative for the offshore conditions. The directions starting from W to NNW, clockwise, were not taken into account in the wave computations because these winds cross land and generate offshore-directed waves having negligible influence on sediment transports. However, there are three directional bins (SSW, SW and WSW) that cross the land which can drive alongshore sediment transports. These were corrected and included in the wave simulations. Wind speeds below 3 m/s were not considered as they constitute an insignificant contribution to the wave driven alongshore sediment transports.

3.2. Wave climate simulations The bathymetric map used as input for the wave simulation was issued by GeoEcoMar in 1995. The distance between bathymetric profiles is approximately 3 km and the measurements were made using the Hi-Fix system (Sheriff, 1974). Data from navigation maps were used to improve the accuracy of the map for the nearshore area. Along with wind and bathymetric data another important variable for the simulation of a wave climate is the fetch, the distance over which the wind conditions generate waves. The fetch was chosen to be 100 km from Sahalin Island towards the north, east and south in accordance with the spatial extent of the typical storm systems in the north-western part of the Black Sea (Ginsburg et al., 2002). SWAN (Simulating WAves Nearshore) (Booij et al., 1999), the numerical model used for simulating the wave climate, is a thirdgeneration numerical wave model to compute random, short-crested waves in coastal regions with shallow water. It accounts for processes such as refraction, diffraction, wave-wave interactions and dissipation processes (bottom friction and depth-induced wave breaking). The model is based on a formulation of the discrete spectral balance of action density that accounts for refractive propagation over arbitrary bathymetry and current fields and it is driven by boundary conditions

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Fig. 2. The onshore ends of the profiles used for computation of the alongshore sediment transport capacity are shown by black dots. Capital letters indicate the sectors used to compute the sediment budget. The grey arrows show the direction of the sediment transport, the length of arrows being proportional with the volume of transported sediment.

and local winds. In SWAN the waves are described with the twodimensional wave action density spectrum. Sixty six onshore wind conditions (direction and speed classes) were used for the simulation of the wave climate. The total duration of these conditions was 220.6 days per year. The remaining 144.4 days were offshore directed or below 3 m/s wind speed. The averages of significant wave heights discussed further will only refer to these 220.6 days/year. The simulation of the generation and propagation of the waves was made by running the numerical model twice for every directional and speed class. First, the simulation was made for a large and coarse grid (2 × 2 km cells) and second for the so-called “nested grid”, containing only the littoral area, with a finer resolution of computation (200 × 200 m cells). 3.3. Alongshore sediment transport capacity 3.3.1. Closure depth As explained above, the closure depth concept is used in our oneline modelling approach to derive coastline changes from alongshore transport gradients. Since no systematic measurements of the submerged active beach profiles are available we employed the method proposed by Hallermeier (1980) determining the transition between motion and no motion of bed sediment: A = D



8ðs−1Þg ω2 D

0:5

ð1Þ

where A is the wave excursion, D the diameter of the spherical grains, s is the relative density (ρs/ρ), g the acceleration of gravity, and ω is the angular frequency (2π/T). In our derivation we applied the following physical values: density of the sediments ρs = 2.65·103 kg/m3,

density rof the water ρ = 1·103 kg/m3, g = 9.81 m/s2 and a grain size D = 0.2 mm. For the computation of the closure depth 25 representative points were selected (Fig. 2). The closure depth is defined as the depth where equality between the left side term and right side term in Eq. (1) is reached. For different depths the characteristics of the water and sediments were considered to be constant. The wave characteristics used to compute the closure depth were extracted from the computed wave climate as they vary with depth and geographical location. It is generally suggested to select the extreme wave condition that occurs at least 12 h per year (Sabatier et al., 2004) as the characteristic condition. In our case, the highest waves are produced by winds from north at 40 m/s and from south-west at 30 m/s. We took into account the same ten years of wind records at Sulina used as a base for computing the wave climate. 3.3.2. Unibest-CL+ computation The UNiform Beach Sediment Transport Coast Line (Unibest-CL+) model (e.g. Tilmans, 1991; Szmytkiewicz et al., 2000) was used to obtain a continuous and detailed computation of the alongshore sediment transport capacity. The model is based on the one-line theory, introduced by Pelnard-Considère (1956). The basic assumption of the model is that the profile of the active beach does not change during accretion/erosion processes and neglecting cross-shore processes. Unibest-CL+ simulates the cross-shore wave heights, and determines the wave-driven alongshore current and associated sediment transports for a range of transport formulations along a cross-shore profile. These alongshore sediment transports are subsequently used to drive the coastline change model. Twenty-five profiles were selected (Fig. 2) to cover the whole study zone from Sulina (north) to Portita Inlet (south). For each

S. Dan et al. / Marine Geology 262 (2009) 39–49

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Fig. 3. Computed significant wave heights and their spatial distribution for four wind directions and speed classes. The black arrows show the direction of the waves and the colours indicate the significant wave heights (to view this figure in colour see the online version).

location the model uses local data. The bottom profiles were interpolated from the local bathymetry, the seaward wave boundary conditions were interpolated from the wave field predictions and the corresponding closure depths were used. The sediment transport formulae used for the sediment transport computation were CERC (Shore Protection Manual, 1984) and Bijker (1971).

4. Results 4.1. Wave climate

3.4. Predictions of the future position of the shoreline By assuming that the wave climate will not change Unibest-CL+ was used to simulate the coastal evolution over the next 25 and 50 years based on the alongshore sediment transport predictions for the same 25 locations used in the hindcast computations. To account for sea level rise the Bruun rule (Bruun 1962, 1983) was used: R ¼ a ⁎ l= h

the cross-shore distance between the dunes (the landward limit of the active beach) and the point where the closure depth is reached (the seaward limit of the active beach).

ð2Þ

where R is the shoreline retreat, a is the relative sea level rise, l is the cross-shore extension of the active nearshore system and h is the total depth of the active system, being the summation of the dune height and the closure depth. We adopted a relative sea level rise of 3 mm/ year (assuming that the present day rate will not change), an average dune height of 1.5 m (Panin, 2005) and the closure depth as derived above. The length of the closed nearshore system, l, was computed as

Using the wind analysis as an input, the wave climate simulations indicate three main wave directions. The northern directions, from N to ENE, (Fig. 3a and b) have an occurrence of 102 days/year, mostly in wintertime. Waves from these directions produce the largest average wave heights and are found to dominate shoreline behaviour. The maximum wind speeds (34 and 40 m/s north) from the entire set of ten years wind records are in the same category of northern directions. The eastern directions, from E to SE (Fig. 3c) have the lowest average wave heights and they occur only 27.8 days/year. The maximum wind speed from these directions is 20 m/s (from the east direction). The southern directions, from SSE to WSW (Fig. 3d) produce medium average wave heights and occur 90.8 days/year. The maximum wind speed from these directions is 30 m/s (from south-west direction) and they occur mostly in summertime. The local wave climates depend on the shoreline orientation and location (e.g. due to wave shielding). Locally, wave data were extracted

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Fig. 4. Wave distributions for four representative points along the study zone (point locations in Fig. 2). a) point 2; b) point 7; c) point 14 and d) point 21.

for representative locations depths ranging between 7.5 and 25 m (always larger than local closure depth). Average significant wave height and occurrence of different wave directions for these representative points are presented in the following. a) Sulina–Imputita Channel sector. Just south of the jetties (point 1, Fig. 2) the coast is shielded from waves from the northern directions. As a consequence, waves from southern and eastern directions are dominant. The average significant wave height here is 0.66 m, the lowest from the entire studied zone. For point 2, about 8 km south of the jetties, the northern waves are not shielded anymore as can be seen in Fig. 4 were the local wave distribution is presented. There are two major direction sectors: NNE–ENE and SSE–S for all the wave heights. b) Imputita Channel–Sf. Gheorghe mouth sector. In this sector the northern directions dominate the wave climate. The average significant wave height is gradually increasing from 0.84 m at point 2 to 0.94 m at point 10 (Fig. 2). The distribution of wave directions in case the of point 7 (Fig. 4) shows the same major sectors (northern and southern) as for the previous sector. Waves over 1 m height are predominant from the north–east direction, which already suggests that the dominant alongshore current is oriented towards the south. c) Sahalin Island sector. A maximum average wave height of 0.98 m is reached in this sector at point 12 (Fig. 2), as this sector is exposed to all wave directions. Southward, close to the tip of the island the wave heights are lower, only 0.73 m at point 15 (Fig. 2) due to the decreasing influence of the northern directions. The distribution of the wave direction for point 14 (Fig. 4) shows similar trends as point 7, but the weight of the waves over 1 m height is larger. As the bay behind the Sahalin spit is sheltered against waves from the dominant directions the waves are very low (Fig. 3), even when the wind is

from south-western directions which correspond to the orientation of the bay aperture, which is probably caused by the very low water depth. d) Ciotica–Portita Inlet sector. This sector is sheltered against the northern wave directions by Sahalin Island, so eastern and southern directions are dominant here. The sector is experiencing rather small values of the average wave heights, ranging from 0.70 m (point 21) to 0.77 m at point 25 (Fig. 2). The wave graph for point 21 (Fig. 4) indicates that for waves over 1 m height the dominant direction is from south.

4.2. Alongshore sediment transport capacity 4.2.1. Closure depth Computed closure depth values show a large variability along the Danube Delta coastal zone, ranging from 5.0 m at profile 20 to 11.6 m at profile 11 (Table 1). Sheltering against northern wave directions due

Table 1 The closure depth computed using Hallermeier's formula (profile locations in Fig. 2). Profile Profile Profile Profile Profile Profile Profile Profile Profile Profile

1 2 3 4 5 6 7 8 9

Closure depth (m)

Profile

6.9 6.8 6.7 8.4 8.7 8.4 9.6 9.4 10.0

Profile Profile Profile Profile Profile Profile Profile Profile Profile

10 11 12 13 14 15 16 17 18

Closure depth (m)

Profile

10.5 11.6 10.8 9.0 8.0 5.9 5.5 5.6 5.4

Profile Profile Profile Profile Profile Profile Profile

Closure depth (m) 19 20 21 22 23 24 25

5.1 5.0 5.3 5.6 5.6 6.4 6.6

S. Dan et al. / Marine Geology 262 (2009) 39–49

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Fig. 5. The alongshore sediment transport capacity computed with UNIBEST CL+. The segment between profile 11 and 16 corresponds to the Sahalin spit island domain (point locations in Fig.2)

to the Sulina jetties and Sahalin spit island is the cause of the rather small values of the closure depth in the northern and southern parts of the study zone. Maximum values occur in the front of Sahalin mainly due to the full exposure to both dominant wave directions. Because the shoreline in the study zone changes orientation from north–south to east–west the closure depth is determined by waves from two directions. The maximum wave heights that occurred at least 12 h per year were generated by 40 m/s winds from the north direction. For the area between Imputita Channel and the middle of Sahalin (profiles 3 to 12, Fig. 2) the closure depth is determined by these northern waves. Due to the wave shielding the northern waves do not reach the nearshore system in the areas Sulina–Imputita Channel (profiles 1 and 2) and Sahalin–Portita inlet (profiles 13 to 25). The highest waves reaching the nearshore system in these two areas are generated by south-western winds with speeds of 30 m/s, which we have used to determine the closure depth.

4.2.2. Unibest-CL+ computation In Fig. 5 the results of the one-line coastal modelling computation are presented. Close to the Sulina jetties (profiles 1 and 2 in Fig. 2) the transport is oriented towards north. In the area of Imputita Channel– Sf. Gheorghe (Fig. 2) the alongshore transport is southward oriented and continuously increasing. Between profile 2 and profile 4 the transport is increasing with a very steep gradient, suggesting intense erosion. The next sector (profiles 4 to 9, Fig. 2) has the same trend, but with a milder gradient. In the Sf. Gheorghe mouth area (points 9 to 11, Fig. 2) the transport is approximately constant. This is the sector where the maximum value of the alongshore sediment transport capacity for the whole studied area is reached, 1 080 000 m3/year (CERC) and 775 000 m3/ year (Bijker). Because the shoreline orientation is changing southward (profile 11, Fig. 2), the sediment transport is found to decrease with a strong gradient reaching almost zero toward the southern part of the island sector (profile 16, Fig. 2). In front of Sahalin spit island the wave climate is the strongest in the whole area. However, the southern waves compensate the influence of the northern waves, explaining lower magnitudes of the net sediment transport. Around profile 15 (Fig. 2) a new peak in the sediment transport is reached, 210 000–360 000 m3/year. The sector between profile 16 and profile 25 (Figs. 2 and 5) is characterized by a low sediment transport from south-west toward north-east, the maximum being 55 000–85 000 m3/year (profile 23, Fig. 2). This sector, with a mild slope of the submerged

beach, is controlled by the waves from southern directions that approach the shore at low incidence angles, explaining the relative small values of the sediment transport. 4.3. The sediment budget The alongshore sediment transport, closure depth and additional sources of sediment were taken into account for the computation of the sediment budget. We describe 7 sectors (Fig. 2) based on alongshore transport gradients and morphological characteristics of the coastal zone. The erosion/accretion values are expressed with a minimum (corresponding to the Bijker formula) and a maximum value (corresponding to the CERC formula). In Table 2 computed rates of erosion or accretion are shown along with the observed rates (Panin, 1996). Sector E corresponding to Sahalin spit island (Fig. 5) is not included because of the more complex dynamics and it will be discussed separately. Based on the results the coastal zone in front of Danube Delta can be divided into three sections, corresponding to the characteristics of the sediment budget. a) Sulina–Sf. Gheorghe (Sectors A – D). The Sulina jetties, delimiting this section in the north extend into the sea for 8 km up to a depth of 8 m, which is larger than the computed closure depth in the area. The assumption that these jetties are impermeable to the alongshore sediment drift from the north is therefore valid. For the first sector, A (Fig. 2), the computed average advance of the shoreline is between 5 and 8 m/year. The formation of an eddy-like current in this sector has been previously suggested (Panin, 1996; Stănică et al., 2007); the same authors mention an observed mean annual shoreline advancement rate

Table 2 The comparison between the computed and the observed rates of erosion/accretion (sector locations in Fig. 2). Sector

Computed

Observed

A B C D F G

Accretion 5–8 m/year Erosion 9–13 m/year Erosion 2 m/year Erosion less than 1 m/year Stable Erosion less than 1 m/year

Accretion up to 10 m/year Erosion over 10 m/year Erosion Stable Erosion/stable Erosion

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Fig. 6. a) The location of the cross section (AB) used to estimate the volume of accreting sand per year. b) Cross section, s is the slope (not to scale).

ranging between 8 and 10 m. The largest erosion rate of the whole study area is computed in sector B (Fig. 2), between 9 and 13 m/ year. The observed erosion rate here is 6–11 m/year (Panin, 1996). In the sector Imputita Channel–Casla Vadanei (points 2–7, Fig. 2) an average shoreline retreat of 7–10 m/year was recorded in the period 1952–1985 (Panin, 1996; Dan et al., 2007). The computed rate in the present study for the same sector is between 5 and 7 m/ year. In sector C (Fig. 2) the erosion rate decreases to values between 2 and 2.5 m/year. The erosion rate is reaching 0.5 m/year in the sector D, sector, which coincides with the segment described as stable (Ungureanu et al., 1996). b) Sahalin Island (Sector E). The Sahalin Island section is corresponding to sector E in Fig. 2. Apart from alongshore sediment supply from the north, overwash induces a significant shoreward migration and the combined processes provoke a continuous elongation to the south-west (Panin, 1999; Giosan et al., 1999). The alongshore transport is supplying this sector with 775 000–1 080 000 m3/year. Another sediment contributor is the Sf. Gheorghe branch discharging approximately 800 000 m3/ year of sand (Panin and Jipa, 2002). The decrease of the computed sediment transport suggests accretion, but according to Panin (1999) and Giosan et al. (1999) the eastern side of the island is strongly eroding.

Sahalin spit island elongates towards south-east with an average rate over the last century ranging between 100 m/year (Giosan et al., 1999) and 140 m/year (Vespremeanu-Stroe, 2004). Not only the emerged part of the island is increasing, but also the associated submerged domain, while the width of the island remains almost constant. Since at the end (south) of the Sahalin section the sediment transport is 0, we hypothesize that the sediment input from the north is deposited in this area feeding the island's elongation. Hence we checked whether the volume of the sand brought into system by the alongshore transport and the Sf. Gheorghe sediment source is of the same order of magnitude as the volume of sand deposited at the southern tip of the island. The location of the cross-section (A–B) chosen for quantification is shown in Fig. 6a. The general direction for the elongation of the island is towards south-west, but the tip of the island is moving towards west, so the cross-section is oriented almost north–south. This section starts from the north at 3 m depth (the largest depth of the bay) and ends, southward, at 8 m depth, the closure depth in this area (Fig. 6b). Based on these figures, the estimated volume of the sand stored every year at the end of the Sahalin spit island is 1.40–1.96 millions m3/year, which is very close to the volume of the sand brought together by the alongshore transport and Sf. Gheorghe branch (1.58–1.88 millions m3/year).

S. Dan et al. / Marine Geology 262 (2009) 39–49

5. Discussions and conclusions

Table 3 The predicted shoreline position after 25 years. Bruun rule effect Erosion/accretion rates (m/year) after 25 years computed with Unibest-CL+ (m/year) (without Bruun rule effect) Sector

CERC formula

Predicted shoreline retreat/ advance after 25 years including the Bruun rule effect

Bijker formula

CERC formula

Bijker formula

Advance 155 m Retreat 300 m Retreat 80 m Advance 7 m Retreat 13 m Retreat 31 m

Advance 93 m Retreat 210 m Retreat 60 m Retreat 13 m Retreat 18 m Retreat 31 m

A

− 0.80

7.0

4.5

B

− 0.40

− 11.6

− 8.0

C D F G

− 0.50 − 0.61 − 1.00 − 0.72

− 2.7 0.9 0.5 − 0.5

− 1.9 0.1 0.3 − 0.5

47

Location of the sectors in Fig. 2.

c) Sahalin–Portita inlet (Sectors F–G). This section contains two sectors (Fig. 2): F, with a computed rate of erosion of 0.4 m/year and G, with a computed rate of accretion of 0.6 m/year. The eastern half of the sector F (Zatoane Lakes area in Fig. 2) and the sector G are computed to be erosive, but the western part of sector F is on average stable (Panin, 1998). The erosion of the barrier that separates the lakes from the sea can explain the observed erosion (10 m/year) in the Zatoane lakes area.

4.4. Predictions In Tables 3 and 4 shoreline position changes are quantified for the next 25 and 50 years, respectively. The predictions for the sector E corresponding to Sahalin spit island are not considered since this sector is under the influence of more complex processes, such as overwash, which will be discussed further on. It can be observed that the erosion/accretion rates are similar to the rates computed for the present situation. However, the magnitudes of the erosion/accretion rates decrease especially after 50 years. The effect of sea level rise (determined by the Bruun rule) is not having a significant influence for the sectors rapidly eroding or accreting, such as sectors B and A (Fig. 2), respectively. For the sectors where the erosion/accretion rates are not very large the Bruun rule effect is dominant for the future position of the shoreline. For sector F (Fig. 2) the accretion trend will even be reversed due to relative sea level rise. The explanation for alongshore varying rates of erosion/accretion due to sea level rise is the slope gradient, which is different for every sector. In the sectors where the active beach is more dynamic, such as in B and C, the beach slope has a steep gradient and therefore the retreat rate caused by sea level rise is the lowest of the entire study zone. In contrast, for sectors F and G the retreat of the shoreline is almost exclusively caused by sea level rise. There are two sectors that form an exception. First sector A, where despite a rather large rate of retreat due to sea level rise the shoreline will advance fast because of the large quantity of sediments deposited in this sector. Secondly, sector D is expected to preserve the same equilibrium as today, the retreat caused by sea level rise being compensated by a slight deposition due to a change in the alongshore transport gradients. It has to be mentioned that the retreat/advance rates of the shoreline were computed separately for the effect of sea level rise and changes in sediment transport rates. The simple summation of these two yields only a first order estimation only, but we expect that it is sufficient to predict the order of magnitudes and trends for future evolution of the study zone.

The main contribution of this work is the quantitative determination of a near shore wave climate, of the alongshore sediment transport and of a sediment budget for a large part of the Danube Delta coastal zone. Moreover, predictions for the evolution of the deltaic coastal zone in the next 25 and 50 years were made. The simulated wave climate based on wind records proved to be reliable despite the lack of precise data. Wave data (computed and recorded) are presented in a report regarding the rehabilitation of the southern Romanian littoral zone (JICA/ECOH report, 2006). The area where these records and computations were made is situated 100 km south of our study zone, offshore. The local visual observations and waves computed with the WAM model and fed by European Centre for Medium-Range Weather Forecasts wind data were analyzed and correlated resulting in an average significant wave height of 0.95 m. Our computed average significant wave heights range from 0.66 m to 0.98 m. Two sectors (point 1 and points 17–23, Fig. 2) are sheltered against the northern wave directions, therefore the average significant wave heights are lower in these sectors. In the sector between points 2 and 12 in Fig. 2, which is fully exposed to all the three main wave directions northern, eastern and southern, the average significant wave height is between 0.80 and 0.98 m, so very similar with values from the JICA/ECOH report. Vespremeanu-Stroe (2004) computed a wave climate for a point situated 10 km south of Sulina and 6 km offshore, based on the same 10 year wind records as used in the present study. The average wave height computed using wind-wave correlation nomograms was 0.95 m. The computed wave climate in the present study covers a large part of the Danube Delta coastal zone, reflecting local particularities of the different beach sectors. Results of the alongshore sediment transport capacity computations clearly indicate the variable characteristics of different parts of the study zone. The alongshore sediment transport capacity computed with Unibest-CL+ shows steep gradients for the sector Sulina–Sf. Gheorghe. In this sector the transport is oriented southward, except for a small sector close to the Sulina jetties. The explanation of these steep gradients (between points 2 and 11, Fig. 2) is the exposure of the sector to waves from northern directions and the relative sheltering against southern directions. The opposite situation occurs in the sector confined by Sahalin and Portita Inlet (points 16 to 25, Fig. 2), where southern wave directions are controlling the evolution of the shoreline. The magnitude of the net sediment transport in the Sahalin–Portita sector is much lower than that in the Sulina - Sf. Gheorghe sector. The gentle slope of the submerged beach and the milder wave climate cause this. The waves from southern directions are not high and do not occur as often as those from northern directions. The waves approach the shore almost perpendicular (points 16 to 22, Fig. 2), causing a very low sediment transport. Another explanation for the low values of the net sediment Table 4 The predicted shoreline position after 50 years. Bruun rule effect Erosion/accretion rates (m/year) after 50 years computed with Unibest-CL+, without Bruun rule effect (m/year) Sector

CERC formula

Bijker formula

CERC formula

Bijker formula

Advance 265 m Retreat 555 m Retreat 170 m Advance 75 m Retreat 30 m Retreat 56 m

Advance 175 m Retreat 400 m Retreat 125 m Retreat 5 m

A

− 0.80

6.1

4.3

B

− 0.40

− 10.7

− 7.6

C

− 0.50

− 2.9

− 2.0

D

− 0.61

2.1

0.7

F G

− 1.00 − 0.72

0.4 − 0.4

0.3 − 0.4

Location of the sectors in Fig. 2.

Predicted shoreline retreat/ advance after 50 years including the Bruun rule effect

Retreat 35 m Retreat 56 m

48

S. Dan et al. / Marine Geology 262 (2009) 39–49

transport in this sector is the action of the waves from eastern directions which counterweight the action of those from southern directions. In the Sahalin sector the sediment transport is southwards oriented, the gradients suggesting deposition between points 11 and 14, then alternative erosion and deposition at the southern tip of the island (points 14 to 16, Fig. 5). Since Unibest-CL+ is not designed to model spit islands, the results of the computations in this sector are therefore unreliable and need additional consideration. The sediment transport in front of the Danube Delta (between Sulina and Cape Midia) has been evaluated previously Giosan et al. (1999) based on observed wave data (although not continuous, and with visual estimation of the wave direction) and an estimated closure depth of 9 m. A sediment budget constructed using bathymetric maps from different years and observed erosion/accretion rates complete the computation. Although the trends for the alongshore sediment transport are similar, our computed volumes are larger for the Sulina– Sf. Gheorghe sector and lower for the Sulina–Portita Inlet sector than the computations by Giosan et al. (1999). The probable explanation for these differences lies in the more accurate wave climate and closure depths used in the present study. In the case of Sulina–Sf. Gheorghe area larger values are due to the wave climate, which includes all the extreme events. For Sahalin–Portita Inlet area lower values of the sediment transport can be explained by the less energetic wave climate and, subsequently, lower values of the closure depth. Both sediment transport formula (CERC and Bijker) produce results in line with the observations in terms of trends and gradients of the alongshore sediment transport. The sediment budget, constructed in accordance with the particularities of each beach sector, covers the whole study area, in contrast with earlier studies that were made at different times for rather short beach sectors. The computed rates of erosion or accretion resulting in the sediment budget are very close to the observed ones, especially the rates computed with the CERC formula. This is somewhat unexpected since comparisons with other bulk transport formulae like that of Kamphuis (2000) suggest that the CERC formula tends to overestimate the alongshore sediment transport capacity (personal communication Kamphuis, 2008). Although we have no handle on this issue, we expect that the dominance of one direction in the greater part of the domain (viz. the north) may play a role here, and suggest that a generic study of this issue is recommendable. The Sahalin spit island sector is a sector in which other processes than alongshore sediment transport processes alone play a role. In this sector the wave climate and, subsequently, the gross alongshore transport reach the highest values indicating a relative dynamic environment. The two important factors contributing to this are the full exposure of the western side of the island to both important wave directions (northern and southern) and the rather steep slope of the submerged beach. In contrast, behind the island, in the Sahalin Bay, the environment is very quiet allowing the deposition not only of sand, but of silt and mud as well (Panin, 1998). The sediment accumulated in Sahalin Bay (behind the spit island) has three main sources. First, the bay is supplied with sediments finer than sand (mud and silt) by the Sf. Gheorghe lobe through its secondary branches. Because of the sheltering by the spit island this environment allows these fine sediments to settle. The second source of sediments for Sahalin Bay is overwash, which we hold responsible for the erosion of the eastern side of the Sahalin island. This process is promoted by the narrow width and low elevation (1.0–1.5 m) of the island and the steep slope of the submerged domain eastward. The third source, cross-shore reworking of sediment, is active during storms. The sediment accumulating at the southern tip of the island is reworked by waves from southern directions and transported into the bay. The past evolution of the island (Giosan et al., 1999) indicates that the length of the island is growing constantly since is formed, meantime the width of the island is, on average, slightly increasing. Continuous erosion of the island due to the alongshore transport will

lead, eventually, to disappearance of the island, since there is no accumulation on the landwards side of the island. The only process, which can explain the movement of the island and the absence of a decrease in width, is overwash. The constant sediment input into Sahalin Bay induced by waves in both alongshore and cross-shore processes play a crucial role in the evolution of Sahalin spit island. Our hypotheses regarding the evolution of the Sahalin spit island explain the elongation by constant input sediment from the northern sector and retreat by cross-shore processes. In support of our hypotheses it is useful to divide the gross alongshore sediment transport in its two main directions: northward and southward for the sector between points 11 and 16 (Figs. 2 and 5). When the drift is southward oriented, the current has sufficient sediment available to reach full transport capacity (points 2 to 11). The alongshore current is saturated in terms of transported sediments, thus a significant volume of sediments is deposited once the shoreline changes its orientation. When the drift is northward oriented the alongshore sediment transport capacity is virtually unsaturated when reaching the Sahalin island, therefore the volume of sediments transported northward is small. In terms of capacity of alongshore sediment transport the computed situation shown in Fig. 5 (points 11 to 16) is suggested to be realistic. In terms of the actual quantity of transported sediments the situation is different. Because the northward alongshore current is not able to compensate the transport of the alongshore current southward oriented, a large part of the sediments coming in the Sahalin domain from north will be transferred to the southern tip of the island and deposited there, contributing to the permanent elongation of the island. The good match between the volume of sediment brought into the system and the volume stored in the elongation suggests no significant sediment loss offshore. However, due to the limited data, the hypothesis that some of the sediment is lost from the nearshore system being carried to larger depths than the closure depth cannot be excluded. The results of the present work are relevant for wave-dominated deltas, formed in micro tidal seas. Although large parts of the Danube Delta coastal zone are relatively pristine, coastal environments in the Sulina area littoral sediment circulation was and still is strongly affected by human interventions, with similar situations occurring in European (Rhone Delta–Bird, 1988; Ebro Delta–Jimenez et al., 1997; Po Delta–Simeoni and Bondesan, 1997) and African (Nile Delta–Fanos, 1995) deltas. In many deltas (e.g. the deltas of the Ebro and the Apalachicola rivers) the typical curvature of the spit islands closely resembles that of Sahalin spit-like island, which leads us conclude that our results are more generally applicable. The comprehensive morphological model used in this work can thus be applied successfully to investigate deltaic coasts, which are heterogeneous from both a morphologic (shoreline orientation, beach slope etc.) and a human interventions point of view.

Acknowledgements The authors want to thank Adrian Stănică (National Institute of Marine Geology and Geoecology–GeoEcoMar, Romania) and Alfred Vespremeanu-Stroe (Faculty of Geography, University of Bucharest) for supporting the study with valuable data and useful observation. The computations were made during the doctoral fellowship in the frame of “European Centre of Excellence for Environmental and Geo-ecological Studies on River–Delta–Sea Systems in Europe: case study River Danube, its Delta, Black Sea System”, Project number EVK3-2002-00503, European Commission–FP5. The authors would like to express their gratitude to one of the reviewers, Prof. J.W. Kamphuis, for a detailed discussion on the performance of bulk

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