Marine Geology. Processes controlling the development of a river mouth spit

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1 Marine Geology 280 (2011) Contents lists available at ScienceDirect Marine Geology journal homepage: Processes controlling the development of a river mouth spit Sebastian Dan a,b,, Dirk-Jan R. Walstra a,c, Marcel J.F. Stive a, Nicolae Panin b a Delft University of Technology, Faculty of Civil Engineering and Geosciences, 2600 GA, Delft, The Netherlands b National Institute of Marine Geology and Geoecology GeoEcoMar, 23-25, Dimitrie, Onciul Street, RO , sector 2, Bucharest, Romania c Deltares Delft Hydraulics, Hydraulic Engineering, P.O. Box 177, 2600 MH, Delft, The Netherlands article info abstract Article history: Received 4 December 2009 Received in revised form 16 November 2010 Accepted 5 December 2010 Available online 14 December 2010 Communicated by J.T. Wells Keywords: spit Sahalin Danube Delta Delft-3D sediment transport overwash Spits are among the most dynamic features in the coastal zones. Their stability is, very often, the result of a fragile equilibrium between the availability of sediments and the forcing hydrodynamics. Due to the complex interactions between the processes shaping such geomorphologic features the investigation is difficult and requires separate analysis for each of the processes. A typical example of a spit is Sahalin, which emerged one century ago at the mouth of the Danube Delta's southernmost distributary, and has continuously evolved through elongation and lateral migration. In order to investigate and quantify separately each of the main processes shaping a spit we divide our research in two stages. First, wave induced sediment transports were simulated and analyzed using a complex processes based on a numerical model for an idealised spit. This schematized spit was based on the shape of a number of spits. Secondly, the findings were used in a similar approach for a real case: the Sahalin spit. Results show convergence of the wave fields towards the spit and large transport rates for the dominant wave directions. The sediment budget, derived from the predicted transport and the historical maps of the spit, show that the evolution of the spit is the result of a continuous interaction between along- and cross-shore sediment transport. Furthermore, a good match was obtained between the volumes of sediment supplied to the spit system and those feeding the expansion of the spit. The final output is a conceptual model that includes four stages (submarine accumulation, emerging, evolution and merging with the mainland of the spit) based on the findings from the present study as well as on the findings of previous authors. Although the model was constructed to explain the evolution of Sahalin spit, it is suggested that it can be applied more generally for spits formed in wave-dominated deltas, in a microtidal environment and with a wave climate dominated by one direction Elsevier B.V. All rights reserved. 1. Introduction The morphologic features called spits are defined as an accumulating form attached at one end to the mainland which usually appears where the coast makes a sudden change in its orientation (Petersen et al., 2008). Spits are very dynamic coastal features steered by complex formation and evolution processes. The many processes involved in their evolution and, in many cases, the lack of reliable historical data, especially in relation to the associated submerged domain, hamper our understanding. We conjecture that spits form under the influence of two main processes: wave induced along- and cross-shore sediment transport (e.g. Leatherman, 1979). One of the most important concepts used for the study of a spit is the equilibrium coastline. The main condition for a coast to be in equilibrium, in the case of a uniform wave climate and no loss or input Corresponding author. National Institute of Marine Geology and Geoecology GeoEcoMar, 23-25, Dimitrie, Onciul Street, RO , sector 2, Bucharest, Romania. Tel./fax: addresses: sebi@geoecomar.ro (S. Dan), Dirkjan.Walstra@deltares.nl (D.-J.R. Walstra), M.J.F.Stive@tudelft.nl (M.J.F. Stive), panin@geoecomar.ro (N. Panin). of sediments from onshore and/or offshore, is a zero gradient of the alongshore transport, since this is one of the main processes determining the long term erosional or accretive state of a coast (May and Tanner, 1973). One possible situation is that the coast shape is a straight line with zero gradients for the alongshore transport (Deigaard and Fredsøe, 2005). However, a circular coastline shape is proposed by Bruun (1954) for two cases: an equilibrium island and an equilibrium bay with a non-zero alongshore gradient. For the case of the island, the front of the island is eroded to both sides resulting in down drift migration of the whole island. In the case of the bay, a coast down drift of an erosion resistant point (Hsu et al., 1989; Silvester, 1970), subject to an obliquely wave climate will cause sediment transport gradients. The coast will respond by a reorientation perpendicularly to the dominant wave direction and, consequently reducing the alongshore transport to zero. Deigaard and Fredsøe (2005) state that an accumulating spit is an example of an equilibrium coastline because it is a coastal feature which migrates while maintaining its shape. A spit shaped by gradients in alongshore transport will tend to align to an equilibrium orientation (Zenkovich, 1967), depending on the dominant wave direction. An alignment at a smaller or larger angle than that of the /$ see front matter 2010 Elsevier B.V. All rights reserved. doi: /j.margeo

2 S. Dan et al. / Marine Geology 280 (2011) mainland will lead to accumulation or erosion of the spit, respectively. Another cause for formation of spit-shape shoreline features can be the instabilities of the sediment drift (Ashton et al., 2001; List and Ashton, 2007) induced by the waves approaching quasi-parallel the straight coastlines with relative large incidence angles. Spits formed in deltas can be extremely dynamic involving large sediment transport in both along- and cross-shore direction with rapid advancing and migration rates, such as is the case for Sahalin spit in the Danube Delta (Dan et al., 2009), Trabucador La Banya spitbarrier system in Ebro Delta (Jiménez and Sánchez-Arcilla, 2004) or Goro spit in Po Delta (Simeoni et al., 2007). Giosan (2007) explains the evolution of the Sahalin spit as part of the development of St. Gheorghe lobe from the Danube Delta. This lobe is highly asymmetric in down drift direction making it a typical case of the evolution of wave dominated deltas (Bhattacharya and Giosan, 2003). Giosan (2007) proposed a model for the formation and evolution of Sahalin spit based on a morphodynamic feed-back. The sediments brought by the southernmost distributary of the Danube Delta cannot be redistributed by the waves and the associated currents thereby promoting the building of a submarine platform. The growth of the platform is stimulated by refraction and shoaling of the waves (due to the shallower water) which favour the entrapment of the sediments. Once emerged, the spit evolves through a continuous elongation in down drift direction and landward migration. This evolution is explained by a barrier steering effect (Giosan, 2007) of the alongshore current and by the instabilities which can occur when the waves approach the shoreline at high angles of incidence (Ashton and Murray, 2005). Kraus (1999) discusses the processes governing the spit evolution, and highlights the importance of overwash processes. For some spits such as Trabucador La Banya spit (Ebro Delta), overwash can be the main driving force for their evolution due to both low elevation of the spit and storm surges. A primary factor determining the overwash intensity is the wave climate, especially the wave period (Kraus, 1999). A secondary factor is related to climate: weather cycles and intermittencies in sediment supply for the spit due to variation of the river discharge. Another factor is anthropogenic, building of dams on the river which restricts sediment supply to the river and transport to the coast. Jiménez and Sánchez-Arcilla (2004) used a one-line model to investigate and quantify the changes in shoreline position (at the seaside of the spit) induced by the residual alongshore transport gradients and a model for the cross-shore processes at the bay-side of the spit. A comprehensive study (Petersen et al., 2008) presents the investigation of the growth of spits driven by gradients in alongshore transport. The main findings of the study, based on numerical modelling and experiments, were that a spit is likely to reach equilibrium under a constant wave climate with waves approaching the spit at angles larger than 45 and that the width of the spit is proportional to the width of the surf zone. However, the study does not take into account cross-shore processes such as overwash. 2. Objectives The main objectives of this work are 1) to investigate the evolution of Sahalin spit, and 2) to derive generic conclusions based on the evolution of Sahalin for spits in general, extending the findings of previous studies (e.g. Petersen et al., 2008). We hypothesize that two main processes controlling spit evolution are the gradients in alongshore sediment transport (given a sufficient sediment supply) and cross-shore sediment transport processes, especially overwash. A natural geomorphologic feature such as Sahalin spit formed and evolved under the influence of these two main factors and many others such as: extreme events (floods, storms, etc.) resulting in significant sediment input and water level variations; relative sea level rise; irregularities of the bathymetry caused by spatial variation of the sediment characteristics; development of vegetation and others. In order to explore the relationship between the two main processes (along- and cross-shore transports) shaping a spit we have simplified the natural setting by designing an idealized spit. The emerged and submerged domain of this idealised case of a spit has quasi-parallel depth contours, one type of sediment and no sediment input from the river or up drift alongshore current. Wave field distributions and the induced sediment transport (along- and crossshore) were investigated for wave directions varying 270 around the sea side of the ideal spit. A next step was to evaluate a representative wave climate for the Sahalin spit. The sediment transport for both along- and cross-shore directions were computed and analyzed. The results of the investigation on the ideal spit served as a better understanding of spit dynamics in general as well as for setting up the simulations for the Sahalin case. Based on the results of the sediment transport computations and supplementary information such as the past evolution of the Sf. Gheorghe lobe, relative sea level rise and different sediment sources, we constructed a sediment budget for Sahalin spit. Finally, using the information derived from the present study as well as the findings of previous authors we propose a conceptual model for the evolution of a spit formed at a river mouth. This model is developed as a generic concept applicable to spits formed in similar condition as Sahalin spit. 3. Idealised spit 3.1. Methods The typical shape of different spits from Mediterranean and Black Sea (Table 1) was used to construct an idealised spit shape and subsequently used to investigate both the along- and cross-shore sediment transport as well as the relationship between them under various wave directions. Delft-3D, a state-of-the-art numerical model (Lesser et al., 2004), was used to compute the sediment transport along and across the spit. This numerical model simulates fluid flow, waves, sediment transport and morphological changes at various timescales (e.g. Edmonds and Slingerland, 2007; Van Rijn et al., 2007). Two modules were used in the present study: SWAN for the wave formation and propagation and FLOW module to simulate the wave-driven currents and the subsequent sediment transport. The wave module, SWAN (Simulating WAves Nearshore), is designed to simulate random, short-crested waves in coastal regions with shallow water. The main processes included in the model are: refraction, wave wave interactions and dissipation processes due to 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 and local winds (for details see Booij et al., 1999). Although SWAN does not account for diffraction it was used in the present applications because refraction is the dominant processes along the spit. Table 1 Examples of spits analyzed for designing the ideal spit. Spit Length (m) Width (m) Elevation (m) Reference Sahalin, Danube Delta 17, b2 Tiron, 2010 Trabucador, Ebro Delta N1 Jiménez and Sánchez- Arcilla, 1993, 2004 Spits from the western Low Ashton et al., 2001 side of the Azov Sea 35, Goro spit, Po Delta 5450 N500 Low Simeoni et al., 2007 La Gracieuse spit, Rhone Delta Low Sabatier et al., 2009

3 118 S. Dan et al. / Marine Geology 280 (2011) The Delft-3D-Flow module is based on finite differences grid and solves the unsteady shallow-water equations in two or three dimensions. The system of equations includes the horizontal momentum equations, the continuity equation, the transport equation and a turbulence closure model. Since the vertical accelerations are assumed to be small compared to gravitational acceleration and therefore not considered, the vertical momentum equation is reduced to the hydrostatic pressure relation. The Delft-3D-FLOW module includes sediment as constituents which can be computed. The suspended sediment is computed by taking into account the density effects, settling velocity, sediment exchange with the bed, vertical diffusion coefficient for sediment, suspended sediment correction vector and the bed load sediment with the transport components adjusted for bed-slope effects.this module can be used in many environments characterized by shallow water and complex dynamics because it accounts for the majority of the processes controlling these environments: wind shear, wave forces, tidal forces, density-driven flows and stratification due to salinity and/or temperature gradients, atmospheric pressure changes, drying and flooding of intertidal flats and others (for details see Lesser et al., 2004; Van Rijn et al., 2007; van der Wegen et al., 2008; Tung et al., 2009). Delft3D is a robust process-based 3D model which has been applied in a range of alluvial and marine environments. Several hydrodynamic validation studies exist in which the tide, wave and combined forcing were tested (e.g. Sutherland et al., 2004; Walstra et al., 2000). Delft3D includes the well-known SWAN wave model for which a range of validation studies have been carried out. The model has successfully been applied in the coastal environment to study nearshore morphology (Hartog et al., 2008; Ruggiero et al., 2009), shoreface nourishments (Grunnet et al., 2004, 2005; Van Duin et al., 2004) and offshore tidal sand waves (Tonnon et al., 2007). In several tidal inlet studies (van der Wegen et al., 2010) the model showed good agreement with well-known empirical relations of Jarrett (1976) and the closure curve of Escoffier (1940). Delft-3D was also validated for a number of processes related to hydrodynamics, sediment transport and morphological changes in different environments (Hibma et al., 2004; Lesser et al., 2004) including for geomorphologic features (Tung et al., 2009; van Maren, 2005) similar to spits. In the present study overwash transport is estimated for fully submerged spits to enable us to use the same hydrodynamic formulations in the entire model domain. Local wave set-up induces water level gradient driven flows which combined with breaking waves result in a landward directed flow across the spit. The resulting transports are calculated with the Bijker transport formula (Bijker, 1971). This approach was adopted as it results in a consistent set of model formulations in the entire model domain. When using the Bijker transport formulation oscillatory wave transport due to wave asymmetry is not accounted for. In the presented applications (both idealised and Sahalin spit) only advective cross-shore transport processes are included which originate from the wave-induced undertow using a GLM approach (see Walstra et al., 2000 for details). As presented in Table 1 the characteristics of the representative spits have large variations. However, for the simplification of computations we choose rather minimum length, width and elevation for the idealised case. The bathymetry of the idealised spit was generated as a series of parallel ellipses starting from the same centre with the long axis twice that of the short axis, hence the isobaths are quasi-parallel. The sector containing the ideal spit represents a quarter of these ellipses (Fig. 1). The rectangular grids used for simulations have a cell size of m. The length of the spit at the sea side is 5750 m and the width gradually increasing from 250 m at the connection with the mainland (point 1) and 650 m at the tip (point 13, Fig. 1), while the elevation ranges between 1.0 and 1.2 m and the grain size of sediments was set D 50 =0.2. The elevation and the flooding level of the idealized spit were set in such a way to obtain overwash and inundation of the island because these two processes generate the maximum transport of sediments from the seaside towards land side of the ideal spit (Sallenger, 2000). Three scenarios for water level were used: 0 m, when no transport over the ideal spit is generated, +0.5 m when mostly overwash occurs and +1.0 m when inundation affects large parts of the island generating maximum sediment transport over the island transport. For ease of comparison, wave fields and associated sediment transport distributions were chosen to be always driven by H sig =2 m, T=7 s for different wave directions ranging from 270 to 180 clockwise (Table 2). In order to simulate the sediment transport during storm surges (in particular the overwash induced sediment transport over the spit) we made runs for three scenarios of water level elevation: 0, +0.5 and +1.0 m. The lateral flow boundaries for all the runs were of the Neumann type with a water level gradient (Roelvink and Walstra, 2004), and the offshore boundary is set as a constant water level condition. Fig. 1. Ideal spit. The dry land is indicated by hachured area and the water depth is indicated by grey shades. The black arrows indicate the wave directions used in simulations.

4 S. Dan et al. / Marine Geology 280 (2011) Table 2 Boundary conditions and the type of results derived from the computations using the idealised spit island. Boundary conditions Wave direction (degrees, nautical convention) Significant wave height (m) Wave period (s) Results Wave climate Alongshore transport Yes Yes Yes Yes Yes Yes 360/0 2 7 Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes No Yes Yes No Crossshore transport 3.2. Results As expected the wave field distributions for the five main directions (Fig. 2a e), indicate convergence towards the spit, already suggesting large sediment transport gradients and the probability of breaching and overtopping of the spit. The results of the wave simulations served as main input for wave driven currents and associated sediment transports simulations. Sediment transport computations for the idealised spit were conducted for seven representative wave conditions and computed separately for along- and cross-shore directions. In Figs. 3 and 4 the variation of the sediment transport capacity along the spit are presented. Spits develop in the direction of the dominant waves, therefore we will call the 270 and 315 wave directions the up drift or dominant conditions, the wave directions 360, 45 and 90 the middle conditions and the wave directions 135 and 180 the down drift wave directions. The up drift wave directions show an increase in the sediment transport in the first part of the spit (near the mainland) gradually decreasing in the second part (near the tip). The down drift wave directions usually produce low sediment transport except for point 13 situated just at the tip of the spit. The middle wave directions induce large alongshore sediment transport and play an important role in the morphodynamics of the spit (Figs. 3 and 4). The cross-shore sediment transport computations (Fig. 5) indicate considerable differences for the three water level elevation scenarios. For the first scenario (no elevation) there is no sediment transport over the spit, but for the other two scenarios (+0.5 and +1.0 m elevation) there is a significant cross-shore transport, on average five times more for 1.0 m than for 0.5 m, from the sea side towards the mainland side of the spit. As the middle wave directions approach the spit more perpendicular to the general orientation of the spit they produce the largest cross-shore sediment transport (except 90 wave direction). The largest volume of sediments transported cross-shore over the spit is caused by the 360 wave direction since is the largest cross-shore component integrated over the alongshore spit domain. Although the wave angle plays a significant role for the sediment transport, in this case the water layer on top of the spit is determinant for the magnitude of the sediment transport because the thickness of this layer is proportional to the sediment volumes transported over the spit. The ratio between the largest volumes of sediments transported alongshore (Q) and the total volume of sediment transported over the idealised spit (C) is different for the two considered cases. As a general rule, the Q/C ratio decreases from the direction 270 towards the direction 90. For the water level +0.5 m the average ratio is 0.75, ranging from 0.1 to 1.58, while for the water level +1.0 m the average ratio is approximately 4.2 and ranging from 0.95 to 10 (Fig. 6). Fig. 2. The spatial distributions of the wave fields for five different directions: a) 270 ; b) 315 ; c) 360 ; d) 90 and e) 180. The dry land is indicated by hachured area and the water depth is indicated by grey shades.

5 120 S. Dan et al. / Marine Geology 280 (2011) Fig. 3. Alongshore sediment transport capacity for an ideal spit (see Fig. 1 for location of the points). Positive values indicate down drift transport, while negative values indicate up drift transport. For this idealized spit shape we explored a variety of wave directions to detect the contributions of each direction to its overall morphological development. The global effect of different wave directions on the ideal spit dynamics is better understood if they are discussed in the three group directions described earlier. The dominant, up drift wave directions (270, 315 ) produce, on average, moderate overwash (especially at the beginning of the spit) and favour the transfer of sediments towards the tip. The middle wave directions (360, 45, 90 ) generate significant alongshore sediment transport, but compensate each other due to the opposite directions of the wave induced currents, while the cross-shore transport is very large. The down drift wave directions (135, 180 ) induce very low sediment transport, both along- and cross-shore, playing an important role just locally for the typical recurving of the spit. The elevation above mean sea level of the idealised spit was set constant for all three scenarios ( m), but the extent of flooding and overwash varies, mainly due to the alongshore width variation of the spit, the wave set-up and the different water level elevations, in the three considered scenarios. For the first scenario, with water level set to 0, the entire spit remains dry during simulations. During the simulations for the second scenario, with a water level elevation of +0.5 m, approximately 10% of idealised spit surface remains dry, 35% is flooded with low water depth, on average 0.35 m, and 55% is flooded with larger water depth above the spit, on average 0.5 m. Finally, the third scenario with the maximum water elevation of +1.0 m, the distribution of dry and flooded parts of the spit is the same as in the second scenario, the only difference being the average water depth: 0.65 m for the low water depth and 1.0 m for the larger water depth. The spatial distribution of flooding, and consequently the overwash intensity, is proportional to the spit width since the down drift tip remains always dry while the portion close to the mainland is almost completely flooded in the case of the second and the third scenario. Although included in the model, the wave setup does not play an important role since the flooding extent is similar for all the wave scenarios. 4. Sahalin spit 4.1. Historical evolution of Sahalin spit Sahalin spit formed at southernmost Danube Delta's distributary Sf. Gheorghe (Fig. 7). Danube River, Europe's second largest river, discharges into Black Sea through Danube Delta. This delta has three branches from north to south: 1) Kilia, which transports approximately Fig. 4. Variation of the alongshore sediment transport capacity with the wave directions for a number of points around the ideal spit (see Fig. 1 for location of the points). Positive values indicate down drift transport, while negative values indicate up drift transport.

6 S. Dan et al. / Marine Geology 280 (2011) Fig. 5. Sediment transport over the ideal spit for two water level scenarios: +0.5 m (dotted line) and +1.0 m (solid lines) (see Fig. 1 for location of the points). Positive values indicate transport from offshore directions, while negative values indicate transport from the mainland directions. 58% of the water and sediment discharge, 2) Sulina, the major waterway, 19% and 3) Sf. Gheorghe, 23%, (Bondar and Panin, 2001). There are several theories on the Danube Delta formation, but the majority of authors converge towards the hypothesis that a former bay or gulf was filled with sediments and after a succession of lobe formation the Danube Delta took its actual shape (Giosan et al., 2006; Panin, 1997, 1998, 2005; Panin and Jipa, 2002; Panin et al., 1997). The sediments started to accumulate in the bay placed in the present day Danube Delta's position approximately 11,700 years BP. The deposition was possible due to the presence of a sand barrier called initial spit at the sea side opening of the bay which created a low energy environment. After the infilling of the bay with sediments brought mainly by the Danube River the sand barrier was breached and the delta St. George I formed and develop between 9000 and 7200 years BP. The active sedimentation moved to Sulina lobe for the next 5000 years BP and this lobe reached a maximum extension into the sea from all the Danube Delta's lobes, the shoreline being 10 to 15 km more offshore than today. Approximately 3500 years ago Kilia secondary begun its development. In the last 2000 years the Sulina lobe eroded at a rate of 5 to 8 m per year and the sedimentation changed to Kilia (which became the largest distributary in terms of volumes of transported water and sediments) and to the newly formed St. George II delta (Panin, 1997, 1998, 2005; Panin and Jipa, 2002; Panin et al., 1997). The last evolution cycle of Sf. Gheorghe lobe (St. George II delta), still active today, was initiated 2800 years BP. The lobe is highly asymmetric, the down drift (southern) wing being much larger than the up drift (northern) one. The presence of many fossil beach ridges resembling former spits (Fig. 8) suggests that the recent evolution of Sf. Gheorghe lobe was probably controlled by the formation of a succession of spits. Although the aeolian sand transport is suggested to be important (Vespremeanu-Stroe and Preoteasa, 2007) for the area north of Sf. Gheorghe river mouth, there are no studies on the Sahalin spit regarding this process. However, there are two factors suggesting low aerial transport: vegetation covering the majority of the spit and the relative high moisture content of the sand due to the frequent overtopping. The Danube Delta coastal zone is wave dominated except for the Kilia secondary delta where the large quantities of sediments discharged by the Kilia distributary supply the advancement of the shoreline. The beach sector Sulina Sf. Gheorghe (Fig. 7) is on average eroding due to both natural trends and human interventions. The area just south of the Sulina jetties (8 km long) is advancing due to the eddy-like current generated by the jetties. The southern part of this sector (6 km long) is in dynamic equilibrium, episodes of shoreline retreat alternating with advancing ones. 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; Stănică et al., 2007; Ungureanu and Stănică, 2000). The alongshore sediment transport between Sulina and Sf. Gheorghe is southward oriented, excepting a short part just south of Sulina jetties. In the beach sector confined by Sahalin spit (north) and Portita Inlet (south) (Fig. 7) the shoreline is on average retreating at low rates, except for some short sectors where the shoreline retreat was higher (5 10 m/year) due to merging of the sea with local lakes. The alongshore sediment transport is northward oriented and it has low amplitudes, maximum values being between 55,000 and 85,000 m 3 / year (Dan et al., 2009). Fig. 6. The ratio between along- and cross-shore transport for each scenario of wave direction water level elevation used for the ideal spit.

7 122 S. Dan et al. / Marine Geology 280 (2011) Fig. 7. The Danube Delta. The study area, confined by the dotted rectangle, is detailed in Fig. 8. The main component of the active beach sediments is well-sorted fine sand, mostly quartzitic, in places enriched with heavy minerals with the origin in the Danube River basin, with an average grain size D 50 =0.2. Sahalin spit formed in 1897 (Panin, 1996) and developed by constant elongation towards south and migration towards mainland (west). The spit is frequently breached by storms (enhanced by the low elevation bellow 2 m) in the northern half and episodically it experiences large elongation and retreat rates (500 m/year and 70 m/ year, respectively). The spit's domain is highly dynamic due to the full exposure to major wave directions and large quantities of sediments supplied by alongshore transport and ( million m 3 /year, Dan et al., 2009) and Sf. Gheorghe distributary discharging approximately 0.8 million m 3 /year of sand (Panin and Jipa, 2002). The average elongation rates range between 125 and 165 m/year, depending on the considered time interval (Bondar et al., 1983; Giosan et al., 1999; Tiron, 2010; Vespremeanu-Stroe, 2007) and the average migration rate is over 20 m/year. In the last century the sediment supply for Sahalin spit constantly decreased due to two main causes. First, the water discharge on the Danube River and its tributaries was increasingly regulated by construction of embankments and dams since the middle of the nineteenth century. These structures have caused a gradual decrease of sediment supply to the delta, but a dramatic decrease in sediment supply occurred after the construction of Iron Gates I and II barrages, built in 1970 and 1983 approximately 900 km upstream from the Black Sea. The barrages alone caused a decrease of 35 50% of discharged sediments (Panin, 1996). Second, the volumes of sediments eroded and transported by alongshore current from Sulina Sf. Gheorghe area towards Sahalin were larger in the first half of the twentieth century. This hypothesis is supported by the different shoreline orientation and large erosion rates between 1909 and 1952 (Panin, 2001). Considering this sediment supply variation, the evolution of Sahalin spit (Table 3) can be divided into two time periods: the pre-damming period with relative natural rates for the sediment input ( ) and the post-damming period with Fig. 8. Sf. Gheorghe lobe and successive positions of the Sahalin spit in the last century (positions 1911 to 1993 after Giosan et al., 1999). The black dashed line indicates fossil beach ridges (after Panin, 1996).

8 S. Dan et al. / Marine Geology 280 (2011) Table 3 Length and average width variations for Sahalin spit island in the last century ( after Bondar et al., 1983; after Tiron, 2010). Year Length (m) ,100 12,600 14,700 14,800 16,700 17,400 19,200 Average width (m) reduced sediment input ( ). For the pre-damming period the average elongation is 165 m/year (Bondar et al., 1983), the average rates of lateral migration are 30 m/year ( ) and 22 m/year ( ), while for the post-damming interval (Tiron, 2010) the rates are smaller: 125 m/year average elongation rate and 20 m/year average rate of lateral migration. There are few possible explanations for the lower rates of elongation and lateral migration after 1970 for Sahalin spit. Most important are: the decrease of sediment quantities reaching the system, change in spit orientation and larger length of the spit. Possible inaccuracies related to the measurements of the Sahalin position (after or before important storms or floods) and to the calculations made in order to make comparable maps or aerial photos taken at different times could also alter the estimation for the rates of elongation and migration of the Sahalin spit Methods Previous work suggests the importance of cross-shore processes (overwash) along with alongshore processes (Dan et al., 2007, 2009; Giosan, 2007; Giosan et al., 1999, 2005; Panin, 1996, 1998, 1999, 2005) for Sahalin spit formation and evolution, but the information about the volumes of sand involved in spit dynamics is scarce. Dan et al. (2009) used the one line model UNIBEST-CL+ (Tilmans, 1991)to compute the alongshore sediment transport capacity for a large part of the coastal zone of Danube Delta, but for such a complex geomorphologic feature as Sahalin spit the one line modelling concept is unable to provide reliable predictions due to the fact that crossshore processes and the complex currents around the tip of the spit cannot be accounted for. Since direct measurements of the along- and cross-shore sediment transport are not available, we used the numerical model Delft-3D (a description of this model was provided in the Section 3.1) to compute the sediment transport. The main input for sediment transport capacity computation data were: a bathymetric map issued by GeoEcoMar in 1995, the simulated wave climate for the Danube Delta, wind measurements and physical characteristics of the water and sediments. The bathymetric map was obtained by the interpolation of bathymetric profiles with an equidistance of 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 near shore area. The wind data covering eleven years ( and 2002) was divided in 66 speed and direction classes, containing wind directions from north to west south-west (clockwise) and wind speeds from 5 to 40 m/s. The wind climate was schematised to reduce computational efforts to 12 representative wind conditions (Table 4). Three model grids were used to convert the wind climate to a near shore wave climate which subsequently can be used to determine the local hydrodynamics and sediment transport for the spit. First, a rectangular coarse grid ( km total size and 1 km grid cell size) was used to obtain the coastal wind induced wave fields. The size of this grid was chosen in such a way to correspond to spatial extension of the typical storm systems in the Black Sea (approximately 100 km, Ginsburg et al., 2002). Following the general distribution of the depth contours a second curvilinear grid was designed, nested in the first one and provided with boundary conditions from the first simulation. This grid is finer near shore ( m average grid cell size) and coarser offshore ( m grid cell size) and was used for the simulation of the wave climate in the near shore area. The near shore simulation of the water flow and sediment transport required a third grid nested in the second one using boundary conditions extracted from the wave simulations. This curvilinear grid was built to reflect the detailed morphology of the submerged beach, with a high resolution in the near shore region and cross-shore direction (14 80 m grid cell size) and coarser offshore the offshore region and alongshore direction ( m grid cell size). As in the case of the ideal spit, the lateral flow boundaries were of the Neumann type and the offshore boundary was the still water level condition. The overwash was estimated in the same manner as for the ideal spit. The Sahalin spit elevation relative to the mean sea level ranges between 1 m (northern part) and 2 m (southern part). The river input was not included since the simulations were run in stationary mode, but the river sediment input was considered when the sediment budget was constructed. Finally, equidistant profiles (parallel and perpendicular to the shore) were used to extract the along- and cross-shore sediment transport. The sediment transport in low lying coastal areas is highly influenced by the storm surges. To account for storm surges a water level elevation is prescribed based on a correlation between observed sea level variations and wind conditions at Sf. Gheorghe (Vespremeanu-Stroe, 2007) ranging from 0 to 0.9 m (Table 4) Results Consistent with the idealised case, the wave fields generated for all 12 wind conditions indicate convergence towards the spit and consequently intense sediment transport is expected. In Fig. 9 four examples of wave distribution are shown under various wind directions and speeds. Two extreme events (Fig. 9a and d) with winds from northern and southern directions at 40 and 30 m/s, each occurring once in the period , were plotted. The other two examples (Fig. 9b and c) with winds from north east and east southeast at speeds of 24 and 15 m/s, respectively are more common events occurring at least ten times per year. For all four wave events the extension of overtopping of the spit is related to the combined surge levels and wave conditions. As expected, the resulting sediment transport show that large volumes of sand are involved in the dynamic of Sahalin spit, consistent with sand volumes derived from bathymetry surveys. The net alongshore sediment transport is southward oriented and Table 4 Wind characteristics and water level elevation for the 12 conditions use to simulate wave climate and sediment transport for Sahalin spit island. No. Wind conditions Water Direction (nautical convection, degrees) level Speed (m) (m/s) 1 NNE NNE NE NE NE ESE S S SSW SSW SW SW

9 124 S. Dan et al. / Marine Geology 280 (2011) Fig. 9. The spatial distributions of the wave fields for: a) wind from north at 40 m/s; b) wind from north-east at 24 m/s; wind from east south-east at 15 m/s and c) wind from south at 30 m/s. The length of the black arrows is proportional to the significant wave heights and the grey shades indicate the water depth. rapidly increasing in the up drift (northern) half of the spit and gradually decreasing down drift to approximately zero at the southern tip (Figs. 10 and 11). Dan et al. (2009) compute the alongshore sediment transport capacity using the one-line numerical model UNIBEST-CL+ for the coast confined by Sulina jetties (north) and Portita Inlet (south) using two formulas, CERC (Shore Protection Manual, 1984) and Bijker (Bijker, 1971). The results indicate larger volumes for CERC formula and better correlation for the computed erosion/accretion rates with the observed ones. An approximately constant ratio of 1.4 was found between the volumes of sand computed with the CERC formula and those computed with Bijker formula for the entire considered coastline. Because Delft-3D does not include the CERC formula, the results from Bijker formula were scaled with this factor. The cross-shore sediment transport simulations indicate that large volumes of sand are transported across the Sahalin spit, the total volume of sand transported in a year ranging between 0.8 and 1 million m 3. The volume vary with the transport formula and the place where the extraction of data was made, either onshore (at the sea side) or inshore (at the bay side of the spit) (Fig. 12). The crossshore overwash volume primarily depends on the local width of the spit. The volumes of sediments transported over the spit decrease along the Sahalin from the connection with the mainland (north) towards the tip (south), the same as in the idealised case. The probable explanation for such large volumes of sediment overtopping the spit every year lies in the low elevation of the spit (below 2 m), storm surge amplitudes (up to 1 m), convergence of the majority of Fig. 10. The location of the points and sectors used to analyze the sediment transport and budget. The black arrows show the sediment transport induced by an extreme event (waves generated by wind from north at 40 m/s Fig. 9a). The grey shades indicate the water depth.

10 S. Dan et al. / Marine Geology 280 (2011) Fig. 11. The net alongshore sediment transport capacity for Sahalin spit computed with two different numerical models: UNIBEST CL+ (Dan et al., 2009) and Delft-3D (see Fig. 10 for location of the points). Inland refers to the landward side of the spit while onshore refers to the sea side. the wave fields towards the spit and the relative steep slope of the submerged active beach Sediment budget Obtaining a reliable estimate of the sediment budget for the entire depositional system of Sahalin spit is hampered by the lack of data about the evolution of the submerged parts of the spit. However, the general evolution of the emerged area of Sahalin system over the last century (Table 3) can be used to derive estimates of the sediment volume entering the spit system. Although, the evolution of Sahalin is governed by episodic events such as storms and floods, the use of multi-annual average transport rates provide good understanding on the evolution at century scale. A common method to derive the sediment budget of a coastal feature is to compare bathymetric maps (e.g. Rhone Delta, Sabatier et al., 2006) from different years. In the case of Sahalin, this method would give unreliable results due to lack of precise and compatible data about the submerged domain. Another option for computing the volumes of sediments involved in the dynamics of Sahalin would be to assume that beach profiles remain parallel during the migration of the spit. This assumption cannot be sustained since the Sahalin system does not keep a constant geometry because the submerged domain flattens (Bondar et al., 1983; Giosan et al., 1999). The probable explanation lies in the faster response of the upper shoreface to forcing conditions than the middle and lower shoreface (Stive and de Vriend, 1995). The large majority of the sediments entering the system are deposited near the spit tip, resulting in elongation of the spit. Previous studies (Dan et al., 2007, 2009) computed the rate of deposition due to elongation using a cross section at the southern tip of the spit confined by the largest depth in the Sahalin Bay and the closure depth at the sea side to be approximately 14,000 m 3 /m/year. At an average elongation rate of 125 m/year for the last several decades, approximately 1.75 million m 3 of sand is deposited annually. This volume is in good accordance with the volume of sediments entering the system: the net alongshore transport ( million m 3 /year, Dan et al., 2009) and Sf. Gheorghe sediment input (0.8 million m 3 /year, Panin and Jipa, 2002). In terms of a classical sediment budget the Sahalin system can be divided in three sectors (Fig. 10) with two sources (river input and alongshore transport from north) and one sink (spit elongation). The first sector, A, which stable in average (Dan et al., 2009) is controlled only by the gradients in the alongshore transport. In the second sector, B, the transport capacity of the alongshore current increases enough to include the river discharge and to transport the sediments towards the third sector, C. Here the transport capacity of the alongshore current gradually decreases and the sediments start to deposit, feeding the constant elongation of the spit. If the present day rates of Sahalin spit elongation and migration remains constant into the future, the spit is expected to merge with the mainland in approximately two centuries. The balance between along- and cross-shore (overwash) sediment transport can be strongly influenced by changes in climate or/and sediment supply resulting in acceleration, deceleration or even disappearance of the spit. Due to expected climate change (Meehl et al., 2007) which implies an increased number and intensity of extreme events and accelerated sea level rise, it is probable that the evolution of the spit will accelerate. If this is the case, then due to larger transport capacity Fig. 12. The cross-shore sediment transport variation along Sahalin spit computed with Delft-3D (see Fig. 10 for location of the points).

11 126 S. Dan et al. / Marine Geology 280 (2011) the sediments will be transferred to the tip of the spit more rapidly and the rate of elongation will be higher. This will result in thinning of the spit and, along with an accelerated sea level rise, make the spit more vulnerable to already stronger storms. As an immediate effect the lateral migration rate will increase and large breaches or even disappearance of the spit will be highly probable. The prevalence of the cross-shore transport over the alongshore transport can also be caused by a decrease of sediment supply to the spit system. The sediment supply can decrease mainly due to human interventions such as structures built on the river or/and on the shore, both resulting in sediment retention upstream and up drift, respectively. The only probable process resulting in deceleration of the spit evolution is the shoreline change, determined by the lateral migration. This will have the same effect as lower alongshore sediment transport and consequently lower rates of elongation. The volumes of sediments available for the cross-shore transport will be larger and, probably, the rates of lateral migration will be lower and the spit's width will increase. 5. Conceptual model To synthesise and explain the findings of the present work as well as the findings of previous authors (Dan et al., 2007, 2009; Giosan, 2007; Giosan et al., 1999, 2005; Panin, 1996, 1998, 1999, 2005) we propose a conceptual model for the formation and evolution of a spit. The model describes the most important four stages (Fig. 13) for the formation, evolution and disappearance of a spit formed at a river mouth, in a microtidal environment and a wave climate dominated by one direction: a) Submarine accumulation The submarine accumulation (Giosan et al., 2005) formed, primarily, due to the large volume of sediments discharged by the river into the sea. The waves and the wave-induced currents cannot redistribute the sediments alongshore, therefore the water depth decreases. The change in shoreline orientation enhances the sediment accumulation by decreasing the alongshore current transport capacity. The processes are causing a morphodynamic feed-back (Giosan, 2007): the water depth decrease due to sediment accumulation and this lower depth is the main cause for wave shoaling and therefore more sediment accumulation. Typically, this stage is taking place at decadal time scales. b) Emerging spit The deposition process described in the first stage continues and the submarine platform is fed by two major sources of sediments: the river input and the alongshore transport. The local wave conditions are influenced by the decreased water depths causing wave asymmetry resulting in upslope transport. The combined processes of alongshore supply of sediment and upslope wave asymmetry transport will confine the sediment deposition to a relatively narrow cross-shore area, which eventually will emerge. For the formation of the spit it is necessary that the river discharges relatively large volumes of sediments in a short period. Because the river sediment input varies even over short time spans mainly due to the yearly climate variation, but also to anthropogenic influences of the river basin (e.g. agriculture, river embankments, etc.) it is probable that an exceptional period of large sediment discharge during a river flood would lead to the formation of the spit. In this case the emergence of the spit would take place in a few years. c) Spit evolution The elongation of the spit domain (emerged and submerged) is supported by large volumes of sand entering the system. As shown in both the idealised and Sahalin case, the waves converge towards the spit inducing, locally, water level set-up. The cross-shore Fig. 13. The main four stages describing formation and evolution of a spit: a) submarine accumulation; b) emerging spit; c) intermediary stage and d) final stage.