Journal
Journalof
ofCoastal
CoastalResearch
Research
SI 64
pg -- pg
460
463
ICS2011
ICS2011 (Proceedings)
Poland
ISSN 0749-0208
Modelling of sediment transport of Akyaka Beach
L.Balas†, A.Inan‡ and E.Yılmaz†
†Civil Eng. Dept.,
Gazi University,
06570 Ankara Turkey
[email protected]
‡ Gazi University,
06570 Ankara Turkey,
[email protected]
ABSTRACT
Balas, L., Inan, A. and Yılmaz, E., 2011. Modelling of sediment transport of Akyaka Beach. Journal of Coastal
Research, SI 64 (Proceedings of the 11th International Coastal Symposium), 460-463. Szczecin, Poland, ISSN
0749-0208
The long-term evolution of the beach of Akyaka, Gökova Bay (city of Mugla) located at the Southern Egean Sea
coastline of Turkey in response to imposed circulation, wave conditions and coastal structures has been
numerically modelled. Akyaka Beach is a Specially Protected Area, and located at the East end of Gökova Bay.
The main agent of the circulation is the wind shear in Gökova Bay where the tidal range is only in the order of 25
cm. There is a groin at the south and there exists a breakwater at the north of the beach. When the present
coastline has been compared with the coastline in 1945, nearly 60 meters of erosion has been observed between
these two structures. Currents and longshore sediment transport cause significant shoreline changes along the
beach. The current pattern has been modelled numerically by three dimensional baroclinic model HYDROTAM3D that consists of hydrodynamic, transport and turbulence model components. Sediment transport is modelled
by numerical model GENESIS which is a generalized model for simulating the shoreline change. Shoreline
change of Akyaka Beach has been analysed under the effects of existing coastal structures and precuations for
coastal erosion have been discussed with the help of numerical models. It has been concluded from the numerical
study that to prevent the coastal erosion it is necessary to remove the previously constructed groin at the south of
the beach. To well understand the hydrodynamics and sediment transport at the site is very important before the
construction of any coastal structures.
ADDITIONAL INDEX WORDS: Circulation, Shoreline change, Erosion
INTRODUCTION
The morphology of the beach changes under the effects of
coastal structures, wind, current and waves. Coastal structures
significantly affect the natural balance of sediment transport and
cause erosion in the coastal areas. Longshore and offshore
sediment transport leads to shoreline changes. An accurate
prediction of sediment transport is possible only if the wave and
current hydrodynamics of the coastal area is well understood.
Many researchers work on the development of modeling of the
coastal sediment and changes in beach morphology. Margvellashi
et al. (2008) developed one-dimensional vertical and three
dimensional fine-resolution numerical models of sediment
transport. Nam et al. (2009) proposed a two dimensional
numerical model of nearshore waves, currents and sediment
transport. A mathematical approach and numerical model that
simulates beach and dune change in response to cross-shore
processes of dune growth by wind and dune erosion by storms and
gradients in longshore sand transport that will alter shoreline
position are presented by Hanson et al. (2010). For further
literature survey, the review of numerical models of the sediment
conservation is useful (Callaghan et al., 2006).
In this study, beach of Akyaka, Gokova Bay (city of Mugla)
located at the Southern Aegean Sea coastline of Turkey has been
chosen as the study area. As coastal structures, there exist a groin
and a breakwater along the beach. To investigate the circulations
and current pattern in Gokova Bay, three dimensional
hydrodynamic and transport model HYDROTAM-3D has been
applied to the coastal area (Balas and Özhan, 2000).
HYDROTAM- 3D is a three dimensional baroclinic numerical
model which consists of hydrodynamic, transport and turbulence
model components. It includes wind, tide or density induced
currents, water levels, salinity and temperature variations,
transport of pollutants, forced flushing like sink or source induced
currents. In the hydrodynamic model component, the NavierStokes equations are solved with the Boussinesq approximation. A
composite finite difference-finite element method is applied to the
governing equations. Finite difference method and finite element
method are commonly used in various types of problems of
computational fluid dynamics Equations are solved numerically
by approximating the horizontal gradient terms using a staggered
finite difference scheme. In the vertical plane however, the
Galerkin method of finite elements is utilized. Water depths are
divided into the same number of layers following the bottom
topography. At all nodal points, the ratio of the length (thickness)
of each element (layer) to the total depth is constant. To increase
the vertical resolution, wherever necessary, grid clustering can be
applied in the vertical plane. Grids can be concentrated near the
bottom, surface, or intermediate layers. The mesh size may be
varied in the horizontal plane (Balas and Özhan, 2001; Balas and
Özhan, 2002). The main agent of the circulation is the wind shear
in Gokova Bay where the tidal range is only in the order of 25 cm.
Wind rose analysis and longterm wave statistics of the Bay have
been performed. The circulation pattern in the eastern part of
Gokova Bay has been modeled under the effect of wind shear.
Journal of Coastal Research, Special Issue 64, 2011
460
Coastal Morphological Modeling
The longshore sediment transport along Akyaka beach has been
modeled considering the existent coastal structures with the use of
GENESIS numerical model (Hanson and Craus, 1991). GENESIS
model was developed by US Army Corps of Engineers and it is
one of the most common used sediment model in the literature. It
is applied for the sediment transport due to wave effects. Several
numbers and combinations of groins, breakwaters, seawalls can be
examined by GENESIS model. The effects of combined T and Y
type groins can be investigated and the sand erosion and
deposition around groins are taken into account for different wave
heights, wave periods and incident wave angles. But GENESIS
model has some limitations such as neglecting wave reflection on
the structures, tides.
PROJECT AREA
Gökova Bay is located along the West-East direction, at the
South Aegean Sea coastline of Turkey. Its mathematical
coordinates are 28-29 degrees East Longitude and 37-38 degrees
North Latitude. Akyaka Beach is located at the East end of the
Gökova Bay along the North-South direction and shown in Figure
1. The coastline in year 1945 and the present coastline in year
2010 are given in Figure 2 and in Figure 3 respectively. Today,
there exist a groin (G) that is nearly 150 m. in length at the south
and a breakwater (B) that is almost 200 m. in length at the north of
the beach. There are two rivers with low discharge rates and no
sediment loads, discharging into the coastal area. Firstly the
breakwater was constructed at the North of the beach to protect
the fisherman boats and sand accretion started in the west side of
the breakwater. Then groin was constructed at the South of the
beach and erosion was started along the North-south direction of
the beach, between the structures. Erosion caused almost 60 m.
inland movement of the coastline since 1945. At the south of the
groin there occurs accretion, but since there is a river and boats are
entering into this river, there occurs a regular dredging work of
sand to prevent the sand deposition. This can be defined as a
regular human work performed against the nature. At the west of
the breakwater located at the North of the beach there occurs sand
deposition as well, and that sandy beach is used as a municipality
public beach.
Figure 2. Akyaka Coastline in 1945
Beach
Accreation
G
Beach Erosion
B
Figure 3. Akyaka Coastline in 2010 where G: Groin and
B:Breakwater
Figure 1. Gökova Bay and Akyaka Beach
To investigate the sediment transport and shoreline changes in a
coastal area, it is necessary to understand the coastal
hydrodynamics, wave and current system in the area. For this
purpose, the wind rose of the area has been analyzed using the
wind data between the years of 1987-2009 obtained from the State
Meteorological Station in Bodrum. It is seen that the dominant
wind direction is in between South-west and West directions that
results in significant wave approaches and strong circulation
patterns. Wind rose is given in Figure 4. Circulation pattern in
Gökova Bay is modeled by the three dimensional hydrodynamic
transport model HYDROTAM-3D [Balas and Özhan, 2000]. The
average steady state circulation patterns in the Bay under the wind
blowing from South-West and from West, with a speed of 15 m/s
are shown in Figure 5 and in Figure 6 respectively. It is seen that,
under the effect of southerly winds, in the study area there occur
currents from south to north direction whereas under the effect of
Westerly winds, along the beach, currents from the north meet
with currents from the south and they turn towards the deep waters
(to west direction ) almost in the middle of the beach. Current
speed is about 20-35 cm/s.
Journal of Coastal Research, Special Issue 64, 2011
461
Balas, Inan and Yılmaz
LONGSHORE SEDIMENT TRANSPORT
N
25000
NNW
NNE
20000
NW
NE
15000
WNW
10000
ENE
0-3 m/s
5000
3-5 m/s
0
W
E
5-7 m/s
7-10 m/s
>10 m/s
WSW
ESE
SW
SE
SSW
SSE
S
Figure 4. Wind rose between the years of 1987-2009 in Gökova
Bay
0
Wind
Rüzgar (SW) 15 m/s
SW
15 m/s
2000
4000
6000
y (m)
8000
Longshore sediment transport along Akyaka beach considering
the effects of coastal structures has been analyzed by GENESIS
(GENEralized Model for SImulating Shoreline Change) numerical
model (Hanson and Kraus, 1991). The modeled coastline is nearly
2500 m. in length. Waves are approaching to the coastline from
the interval South-West to North-West. Southerly waves cause
sediment to move from south to north, whereas northerly waves
cause sediment to move from north to south along Akyaka beach.
Considering the longterm wave statistics of the Bay, the
probabilities of waves approaching from every direction with
different wave heights and wave periods are considered in the
modeling and sediment transport rate has been computed by
GENESIS model. It is seen that based on the longterm wave
statistics, sediment transport from North to South is less than the
transport from South to North. So the net sediment transport rate
is from South to North along the beach, nearly 1.42 x 104m3/year.
Simulations have been started using the coastline in 1945, then
simulations have been continued with the addition of the
breakwater and the groin, and almost present coastline has been
obtained in year 2010. It is seen that, the most effective wave
directions are the south-west (SW) and west-north-west (WNW)
directions. If groin and breakwater stay on the beach, simulations
show that erosion will continue in between the structures. The
predicted coastline in year 2015 is given in Figure 6. Using model
predictions, different precautions to prevent erosion have been
tested. Tests show that, it is necessary to remove the groin, and to
have sand deposition along the beach, it would be proper to
construct a groin at the North of the end of the beach. In this case,
erosion would be stopped and there would be sand deposition
along the beach in time. Model prediction is given in Figure 7.
10000
Velocity cm/s
HIZ (cm/s)
12000
0
30
Coastline in 2010
Coastline in 2015
SW
0
2000
4000
6000
8000
10000
WNW
12000
x(m)
Figure 5. Average steady state circulation pattern (wind speed 15
m/s, SW)
0
W
Wind
Rüzgar (W) 15 m/s
W 15 m/s
2000
N
S
E
4000
Figure 6. Predicted coastline in year 2015 with present structures
6000
y (m)
8000
10000
Velocity cm/s
HIZ (cm/s)
12000
0
0
2000
4000
6000
8000
10000
30
12000
x(m)
Figure 6. Average steady state circulation pattern (wind speed 15
m/s, W)
Journal of Coastal Research, Special Issue 64, 2011
462
Coastal Morphological Modeling
W
N
S
E
WNW
SW
analyzed, it is seen that, currents from the north meet with currents
from the south and they turn towards the deep waters (to west
direction ) almost in the middle of the beach and the net sediment
transport is from South to North. Numerical studies showed that
the construction of the groin at the south of the beach was a wrong
decision. It is the main cause of the erosion of the beach since it
disturbs the sediment transport from south to north that feeds the
beach. It has been concluded from the numerical study that to
prevent the coastal erosion it is necessary to remove the previously
constructed groin. To well understand the hydrodynamics and the
sediment transport at the site is very important before the
construction of any coastal structures.
LITERATURE CITED
Figure 7. Predicted coastline in year 2015 with suggested
structures.
CONCLUSIONS
The morphology of the beaches changes under the effects of
coastal structures, wind, current and waves. Coastal structures
significantly affect the natural balance of sediment transport in the
coastal areas. To investigate the sediment transport and shoreline
changes, it is necessary to well understand the coastal
hydrodynamics, wave and current system in the area. The coastal
circulations and long-term shoreline changes of Akyaka Beach
located at the east end of Gökova Bay (city of Mugla, Turkey)
extending in the South-North direction have been modeled by
numerical models. Two already calibrated and widely used
numerical models, namely HYDROTAM-3D and GENESIS, have
been applied to the coastal area. Although Akyaka Beach is a
Specially Protected Area, today there is a groin at the south and
there exists a breakwater at the north of the beach. In year 1945
there was no coastal structure in the coastal area. When the present
coastline has been compared with the coastline in 1945, nearly 60
meters of erosion has been observed between these two structures.
At the west of the breakwater located at the North of the beach
there occurs sand deposition and that sandy beach is used as a
municipality public beach. However this breakwater holds the
sediment transported from North to South, causing erosion along
the beach lying in North-South direction. At the south of the groin
there occurs accretion as well, since this groin holds the sediment
transport from South to North, and causing erosion again along the
beach lying in North-South direction. But since there is a river
inlet and boats are entering into this river, the deposition of sand at
the south of the groin (which is the river inlet), is not wanted and
there occurs a regular dredging work of sand to prevent its
deposition. When the circulation pattern in Gökova Bay has been
Balas, L. and Özhan E., 2000. An implicit three dimensional
numerical model to simulate transport processes in coastal
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Fluids, 34, 307-339.
Balas, L. and Özhan, E., 2001. Applications of a 3-D numerical
model to circulations in coastal waters, Coastal Engineering
Journal, 43(2), 99-120.
Balas, L. and Özhan, E.,2002. Three dimensional modelling of
stratified coastal waters, Estuarine, Coastal and Shelf Science,
56, 75-87.
Callaghan, D.P., Saint- Cast, F., Nielsen, P., Baldock, T.E., 2006,
Numerical solutions of the sediment conservation law; a
review and improved formulation for coastal morphological
modeling, Coastal Engineering, 53 (7), 557- 571.
Hanson, H. and Kraus, N.C., 1991. GENESIS: Generalized model
for simulating shoreline change: Report 2, Technical
reference, CERC-89- 19, Vicksburg.
Hanson, H. H., Larson, M., Kraus, N.C., 2010., Calculation of
beach change under interacting cross- shore and longshore
processes, Coastal Engineering, 57 (6), 610-919.
Margvelashvili, N., Saint- Cast, F., Condie, S., 2008, Numerical
modeling of the suspended sediment transport in Torres Strait,
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ACKNOWLEDGEMENT
Authors would like to thank The Scientific and Technical
Research Council of Turkey, TÜBİTAK, for financial support to
the research project entitled as “Determination of Sediment
Transport Rate and Shoreline Changes on Akyaka Coast.”
Journal of Coastal Research, Special Issue 64, 2011
463