Marine Geology 280 (2011) 116–129
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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
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
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 – Delft Hydraulics, Hydraulic Engineering, P.O. Box 177, 2600 MH, Delft, The Netherlands
a r t i c l e
i n f o
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
a b s t r a c t
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.
© 2010 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-024053, sector 2, Bucharest, Romania.
Tel./fax: +40 21 25 2 25 94.
E-mail addresses: [email protected] (S. Dan), [email protected]
(D.-J.R. Walstra), [email protected] (M.J.F. Stive), [email protected] (N. Panin).
0025-3227/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.margeo.2010.12.005
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
S. Dan et al. / Marine Geology 280 (2011) 116–129
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
117
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)
Sahalin, Danube Delta 17,000
Trabucador, Ebro Delta 5500
Spits from the western
side of the Azov Sea
Goro spit, Po Delta
La Gracieuse spit,
Rhone Delta
5500–
35,000
5450
4500
Width
(m)
Elevation
(m)
Reference
80–300
160–
3500
100–
5000
N500
150–
690
b2
N1
Low
Tiron, 2010
Jiménez and SánchezArcilla, 1993, 2004
Ashton et al., 2001
Low
Low
Simeoni et al., 2007
Sabatier et al., 2009
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S. Dan et al. / Marine Geology 280 (2011) 116–129
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
100 × 100 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 D50 = 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 Hsig = 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.
S. Dan et al. / Marine Geology 280 (2011) 116–129
Table 2
Boundary conditions and the type of results derived from the computations using the
idealised spit island.
Boundary conditions
Results
Wave direction
(degrees, nautical
convention)
Significant
wave height
(m)
Wave
period
(s)
Wave
Alongshore Crossclimate transport
shore
transport
270°
315°
360/0°
45°
90°
135°
180°
2
2
2
2
2
2
2
7
7
7
7
7
7
7
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
No
No
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.
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S. Dan et al. / Marine Geology 280 (2011) 116–129
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 (1.0–1.2 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.
S. Dan et al. / Marine Geology 280 (2011) 116–129
121
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 m3/
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.
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S. Dan et al. / Marine Geology 280 (2011) 116–129
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
D50 = 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 (0.8–1.1 million m3/year, Dan
et al., 2009) and Sf. Gheorghe distributary discharging approximately
0.8 million m3/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 (1900–1970) 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).
S. Dan et al. / Marine Geology 280 (2011) 116–129
123
Table 3
Length and average width variations for Sahalin spit island in the last century (1900–1960 after Bondar et al., 1983; 1970–2000 after Tiron, 2010).
Year
1900
1923
1928
1935
1960
1970
1980
1990
2000
2006
Length (m)
Average width (m)
3200
200
7800
250
7600
300
10,100
350
12,600
350
14,700
315
14,800
290
16,700
320
17,400
275
19,200
310
reduced sediment input (1980–2006). 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 (1927–1960) and
22 m/year (1960–1990), 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.
4.2. 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 (1991–2000 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 (200 × 200 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
(75 × 425 m average grid cell size) and coarser offshore (100 × 650 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 (48 × 320 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).
4.3. 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 1991–2000, 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.
1
2
3
4
5
6
7
8
9
10
11
12
Wind conditions
Direction (nautical convection, degrees)
Speed
(m/s)
NNE – 22.5°
NNE – 22.5°
NE – 45°
NE – 45°
NE – 45°
ESE – 112.5°
S – 180°
S – 180°
SSW – 202.5°
SSW – 202.5°
SW – 225°
SW – 225°
11
15
15
19
24
15
8
11
11
15
8
11
Water
level
(m)
+ 0.1
+ 0.25
+ 0.35
+ 0.55
+ 0.9
+ 0.3
+ 0.1
+ 0.1
0
0
0
0
124
S. Dan et al. / Marine Geology 280 (2011) 116–129
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 m3. 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.
S. Dan et al. / Marine Geology 280 (2011) 116–129
125
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.
4.4. 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 m3/m/year.
At an average elongation rate of 125 m/year for the last several
decades, approximately 1.75 million m3 of sand is deposited annually.
This volume is in good accordance with the volume of sediments
entering the system: the net alongshore transport (0.8–1.1 million m3/year, Dan et al., 2009) and Sf. Gheorghe sediment input
(0.8 million m3/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).
126
S. Dan et al. / Marine Geology 280 (2011) 116–129
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.
S. Dan et al. / Marine Geology 280 (2011) 116–129
processes (breaching and overtopping), promoted by higher levels
of the water as well as by low elevation of the spit, causes a lateral
migration of the newly formed spit. As the spit length is increasing
and the alongshore sediment supply has to be distributed over a
larger area, the balance between the transports shifts gradually
from alongshore to cross-shore. This will slowly shift the balance
from the prevalence of the alongshore elongation of the spit to
lateral migration. As a result, the orientation of the spit is gradually
changing from the same orientation of up drift stretch of coast to
almost parallel with the mainland down drift of the river mouth.
Probably, the evolution of the spit is made trough a succession of
storm events. During storms, mainly from the up drift directions,
the overwash cause the lateral migration, while during calmer
periods the alongshore transport realigns the spit by filling up the
breaches and supply with sediments on the sea-side of the spit,
especially the down drift parts.
Behind the spit, a sheltered bay is forming and sediments ranging
from sand to mud starts to settle in this bay. The secondary
branches of the river discharge into the bay mainly silt and mud
and their deposition is possible due to the protected environment.
The other important source of sediment for the filling up of the bay
is the wave action which transports sand by two mechanisms:
indirectly, through wave-induced overwash and directly, when
their direction is perpendicular to the bay opening.
The curvature of the down drift tip of the spit is caused by the
intense wave refraction and diffraction and by the sediment
transport from the down drift directions. This is the longest of the
stages, occurring at century scale.
d) Final stage
Initially, the spit's orientation is comparable to the up drift stretch of
coast and the dominant waves approach the shore at an angle of
approximately 45° implying a maximum alongshore sediment
transport (Ashton et al., 2001; Deigaard and Fredsøe, 2005). The
elongation of the spit continues, but at lower rates and the lateral
migration due to the cross-shore processes becomes the main
process controlling the dynamics of the spit. The migration changes
the orientation of the spit and increases the incident angle of the
dominant waves, leading to a continuous decrease of the alongshore
transports. At this time, the spit is rather long and a decreased
volume of sediments is redistributed over a longer stretch of coast
such that the volume of sediments reaching the down drift tip of the
spit is smaller than in the previous stage, and the lateral migration is
becoming even more important. The bay behind the spit is narrow,
very shallow and almost filled up with sediments. Due to the
prevalence of the cross-shore over the alongshore processes, the spit
will merge with the mainland transforming the bay into a lagoon.
Since the alongshore current cannot transport all the sediments
entering the system, they will deposit close to the river mouth. This
process along with the flattening of the submerged active beach due
to the rapid lateral migration will form a new submarine platform,
representing the first stage of a new spit formation cycle. The time
span of this final stage is estimated at decades.
Based on the possible future evolution of the Sahalin spit a number of
factors were identified as playing an important role for the evolution
127
of a spit formed at a river mouth, in a microtidal environment with
one direction of dominant waves (Table 5).
In the future it is possible that the conditions allowing the formation
of a spit such as Sahalin will change. The engineering works along
Danube River and its tributaries have already decreased the volume
of the sediments reaching the coast (Panin, 1996; Panin and Jipa,
2002), and it is highly probable that this trend will continue. A
decreased sediment input along with an anticipated climate change
will make the accumulation of the sediments at the river mouth less
probable. This will result in a deceleration of the cyclic process of spit
development significantly and/or the emersion of a new spit less
probable. However, initially an acceleration of the lateral migration
of the present spit may occur due to the increasing relative
importance of cross-shore transport. After the merging of the
present spit with the mainland the cycle will decelerate or might
even stop.
Similar features were investigated in the Mississippi Delta and
conceptual models for their evolution were proposed. Penland et al.
(1988) proposed a three stage conceptual model for barrier island
evolution. In the first stage marine processes transformed the
discharged river sediments on the inner shelf into an erosional
headland with flanking headland barriers and recurved spits. A lack
of sediment supply combined with rising sea levels separated this
barrier shoreline from the mainland in the second stage forming a
barrier island arc, the Louisiana barrier islands. The final stage of the
model is submergence, when ongoing sea level rise and storm
processes transform the barrier island into a sand rich marine shoal
that is detached from the deltaic coastline. The evolution of the
barrier islands from Mississippi Delta was explained in more detail
by Campbell (2005) who proposed a dynamic morphosedimentary
model in four stages. In the first stage a barrier island with a sand cap
that sits on top of mixed (sand, silt and clay) deltaic sediments that
also underlie relatively wide backbarrier marshes is subject to
erosion. In the second stage the mixed deltaic sediments are eroded,
while in the third stage the remaining sediments (backbarrier
marshes) are also removed by the waves and currents. In the fourth
stage the sand released into the system in previous stages form a
new protective sandy beach. Although there are some similarities
with our conceptual model (migration of the barriers, cyclic
character of the processes) the differences are significant. The
evolution of a spit as proposed in our model does not imply the
submergence, but merging with mainland of the spit. As a result our
model describes a mechanism of progradation for an asymmetric
deltaic lobe, while Penland et al. (1988) and Campbell (2005)
proposed models for a retreating delta lobe.
6. Discussion and conclusions
The main results of the present study are the investigation of the
relationship between along- and cross-shore sediment transport for
an idealised spit, the simulation of wave climate, sediment transport
computation and a sediment budget for Sahalin spit and a conceptual
model for the formation and evolution of a spit in general.
Table 5
The main causes for the acceleration or deceleration of a spit evolution.
Spit
evolution
Wave and wave induced Natural causes
currents intensity
Acceleration
Higher
Deceleration Lower
•
•
•
•
•
Higher energy wave climate
Larger rates of the relative sea level rise
More extreme events (floods and storms)
Modification of the shoreline orientation
Lower energy wave climate
Sediment status
Directly human induced causes
Less sediment in the spit
system
• Structures built on the river (e.g. barrages)
• Sediment extraction from the spit system
• Structures built on the shore up drift of the spit system
• Shortcuts on the river
• Sediments deploy on the spit system
More sediment reaching
the spit system
128
S. Dan et al. / Marine Geology 280 (2011) 116–129
The idealised spit proved useful when just changes of sediment
transport along- and cross-shore were investigated avoiding interferences such as the river sediment input or irregularities of the
submarine depth caused by variable processes. The wave climate for
the selected directions suggests important sediment transport due to
the convergence towards the spit. As expected, the majority of wave
directions generate alongshore sediment transport which can modify
the morphology of the spit dramatically if one of them is dominant.
In terms of dynamics of the spit, the influence of wave directions
can be described for three situations. First, the sediment transport
induced by the “up drift” wave directions is feeding the elongation of
the spit by transferring the sediments towards the tip, contributing to
the thinning of the spit (especially the part close to mainland) and
generating low (270°) and intense (315°) overwash. Second, the
“middle” directions generate intense overwash (360° and 45°) being
the main driving force for the lateral migration of the spit. Third, the
“down drift” directions (135°, 180°) do not generate significant
sediment transport, being important only for their local contribution
to the curvature of spit's tip, supporting the findings of other authors
(Kraus, 1999) regarding the dynamics of spits.
The numerical model Delft-3D was found to provide realistic
estimates when the wave climate, water flow and sediment transport
for Sahalin spit were simulated. The results indicate distributions and
volumes of sediment consistent with values derived from old maps of
Sahalin area. The wave climate indicates the same wave convergence
for Sahalin as for the idealised case. Although the circulation of
sediments around Sahalin was described previously (Dan et al., 2009;
Giosan, 2007; Giosan et al., 1999, 2005; Panin, 1996) the hypothesis
that the net alongshore sediment transport capacity is able to transfer
the sediments brought by the up drift current and the river (Sf.
Gheorghe) towards the southern part of the spit was supported. This
is based on the results of the sediment transport computations
showing large volumes of sand transported alongshore, the maximum
reaching 1.6 million m3/year. In theory, the deposition of the
sediments would take place for the entire down drift part of the spit
since the transport capacity starts to decrease in this section of the
spit. There are two main arguments supporting the hypothesis that
the majority of the sediments are transported down to the tip of the
spit. First, if the net sediment transport is divided in its two main
components southward and northward it is most probable that the
northward transport is not saturated with sediments since it is
generated over a very short distance, while the southward (dominant) one is generated over a long distance and therefore saturated
with sediments. Second, the southward oriented drift current is very
intense mainly due to the relative steep slope of the submerged beach.
In the Sahalin domain there is an average input of sand every year
of 1.6–1.9 million m3 from two main sources: the up drift alongshore
current and the river supply. The large majority of the sand entering in
the system supplies the elongation of the spit (Dan et al., 2007, 2009).
The rapid lateral migration of Sahalin spit, with observed rates over
20 m/year is caused mainly by large volumes of sand transported over
the spit into Sahalin Bay. The simulation of cross-shore transport
indicates an irregular distribution of this process along the spit, the up
drift half of the spit being more affected and the total volume of sand
transported over the spit is 0.8 to 1 million m3/year. Moreover, the
cross-shore processes (overtopping and breaching of the spit) are
identified as the main cause for the lateral migration of the spit.
The volume of approximately 1 million m3 transported on average
every year over the spit finds enough accommodation space in the bay
behind. If the assumption is made that the deposition is uniform all
throughout Sahalin Bay, then a layer of sand 2.3–3.2 mm thick is
accumulating every year due to the cross-shore sediment transport.
This deposition rate is of the same order of magnitude as relative sea
level rise, estimated to be 2.8–3.1 mm/year (Malciu, 2000; Panin,
1999). The good match between the volume of sediments transported
into the bay and the accommodation space created by relative sea
level rise is a possible explanation for the rather slow decrease of the
water depth in the bay.
The sediment budget shows that the large quantities of sediment
transported into the bay behind Sahalin spit are of the same order of
magnitude with the accommodation space created by relative sea
level rise and provides a supplementary argument for the division of
the along- and cross-shore sediment transport effects over the spit
dynamics. Since its formation Sahalin spit elongates at fast rates, while
the width is rather constant. Significant erosion due to the alongshore
drift will lead to disappearance of the spit such that only the crossshore processes can explain the lateral migration.
Apparently, a geomorphologic feature such as Sahalin spit does not
comply with the equilibrium assumption stated by Zenkovich (1967)
since it is changing its orientation gradually from parallel to nearly
perpendicular to the drift current. The natural trend of the spit is to
find an equilibrium orientation, but with every episode of overwash
this orientation is perturbed. The next relatively calm period leads to
restoration of the equilibrium orientation, but also to a new position
of the spit closer to the mainland due to the lateral migration
generated by overwash.
The conceptual model for the formation and evolution of a spit at
river mouth, in a micro-tidal environment and as part of a wavedominated (from one primary direction) deltaic lobe development, is
the generic outcome of our work. The model, constructed using the
results of our simulations and the findings of previous authors
emphasise the importance of the interaction between along- and
cross-shore processes when a spit is studied, and it can be applied to
other spits formed in similar environments as Sahalin spit.
Although this study does not deal with climate change, it has to be
mentioned that the natural factors (especially those related to the
water dynamics) listed as natural causes in Table 5 are part of the
present global increase in number and intensity of extreme events.
These changes are arguably considered as induced also by human
activities. The formation of a spit at a river mouth can be strongly
influenced by the variation of the sediment supply and such a spit will
emerge only if a consistent submarine sediment accumulation takes
place over a relatively short period of time (few years).
The formation and evolution of a spit at a river mouth is possible
when the supply of sediments exceeds the capacity of wave induced
transport (both along- and cross-shore), and the succession of
formation-evolution-merging cycles of a spits represents a mechanism of progradation for a deltaic lobe. Rapid reduction in sediment
input along with an increased dynamics of the water (especially larger
waves and higher water levels) strongly reduces the possibilities for
the formation of a spit.
Acknowledgements
The financial support for this study was granted by Lamminga
Foundation and EC FP6 no. 044122 project Concepts and Science for
Coastal Erosion Management (CONSCIENCE). The authors would like
to thank to Ştefan Constantinescu and to the two reviewers François
Sabatier and one anonymous for their valuable suggestions and
comments which greatly improve the quality of the manuscript.
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