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Numerical study on tidal currents and seawater exchange in the Benoa Bay, Bali, IndonesiaHENDRAWAN I Gede1*, ASAI Koji2
1 Department of Marine Sciences, Faculty of Marine Sciences and Fisheries, Udayana University, Badung-Bali 80361, Indonesia
2 Department of Civil and Environmental Engineering, Yamaguchi University, Yamaguchi 755-8611, Japan
Received 9 May 2012; accepted 6 May 2013
©The Chinese Society of Oceanography and Springer-Verlag Berlin Heidelberg 2014
AbstractA three-dimensional (3-D) finite volume coastal ocean model (FVCOM) was used for the study of water cir-culation and seawater exchange in the Benoa Bay, Bali Island. The M2 tidal component was forced in open boundary and discharge from six rivers was included in the numerical calculation. The M2 tidal elevation produced by the FVCOM has a good agreement with the observation data. The M2 tidal current is also suc-cessfully calculated under the ebb tide and flood tide conditions. The non-linear M2 tidal residual current was produced by the coastline geometry, especially surrounding the narrow strait between the Serangan Is-land and the Benoa Peninsula. The tidal residual current also generated two small eddies within the bay and one small eddy in the bay mouth. The salinity distribution influenced by river discharge could be success-fully calculated, where the numerical calculation and the observation results have a good correlation (r2) of 0.75. Finally in order to examine the seawater exchange in the Benoa Bay, the Lagrangian particle tracking method and calculation of residence time are applied. The mechanism of particle transport to the flushing of seawater is depicted clearly by both methods. Key words: FVCOM, M2 tidal current, M2 residual current, salinity, seawater exchange
Citation: Hendrawan I Gede, Asai Koji. 2014. Numerical study on tidal currents and seawater exchange in the Benoa Bay, Bali, Indonesia. Acta Oceanologica Sinica, 33(3): 90–100, doi: 10.1007/s13131-014-0434-5
1 IntroductionSeawater in the coastal area has an important role on the
sustainability of the coastal ecosystem. For instance, the coastal area is used as a nursery region and feeding ground for a large number of marine species. In the last decades, because of the increased development activities exploiting the coastal areas, the degradation of environmental quality has become a serious issue among researchers. Seawater experiences various con-taminants either directly from pollutant sources or indirectly from river discharges. The knowledge of hydrodynamics pro-cesses in the coastal area has an essential role in any investiga-tion. It can be used to investigate the capability of seawater to assimilate those various pollutants discharged into it. The main flow of seawater in the coastal region is strongly influenced by tidal system (Imasato et al., 1980), in addition to that induced by river flow and wind. Therefore, an accurate study of the tidal flow in coastal waters becomes one of the most important en-vironmental sciences. In recent years, the numerical simulation of the tidal current is widely used for coastal ocean circulation study. However, until now, any study concerning coastal ocean circulation in the Benoa Bay has not been found, particularly studies that describe and try to measure the seawater exchange system.
As shown in Fig. 1, the Benoa Bay is semi-enclosed water sit-uated in the southern part of Bali Island, Indonesia. The lateral dimension of the Benoa Bay is 10 km × 5 km in the inner part of the bay. The Benoa Bay is characterized by a narrow strait in the bay mouth formed by the Serangan Island and the Benoa Pen-
insula. The main vessel harbor in the Bali Island, namely Benoa Harbor, is located in the inner part of the bay. Since 1996, the Serangan Island and Bali Island were connected by bridge, and very limited water could pass through under the bridge into the inside of the Benoa Bay.
The Benoa Bay is important for the Bali Island both from an environmental as well as economic point of view. From the environmental point of view, the Benoa Bay is the area for the mangrove ecosystem, but it is also used for the disposal of gar-bage. From the economic point of view, the Benoa Bay is used for harbor, oil station, and tourism activities. Therefore, in order to conserve the marine environment in the bay, understanding of the characteristics of the seawater circulation is imperative.
Figure 2 shows the bathymetry map that was obtained from Hydro-Oceanography Division, Indonesian Navy (DISHIDROS-TNI AL). The Benoa Bay has a shallow water depth, where the deepest part in the inner part of the bay is less than 15 m, and in the outer part (towards the ocean), less than 50 m. In the inner part of the bay, particularly in the nearest coastline, the water depth varies from about 1 to 5 m. The surroundings of the bay mouth and the east part of Benoa Harbor has stepped a bottom topography with a depth of about 10 to 15 m.
In general, the oceanic circulation within the Benoa Bay is controlled by inflows from the Badung strait and the Indian Ocean. The tides and tidal currents are typical of water circula-tion in the Benoa Bay, which is mostly impacted by seawater from the Badung Strait. The type of tides in the Benoa Bay is semi-diurnal, in which the M2 is the dominant tidal compo-
Acta Oceanol. Sin., 2014, Vol. 33, No. 3, P. 90–100
DOI: 10.1007/s13131-014-0434-5
http://www.hyxb.org.cn
E-mail: [email protected]
Foundation item: The Beasiswa Unggulan program from Ministry of Education and Cultural Republic of Indonesia.*Corresponding author, E-mail: [email protected]
HENDRAWAN I Gede et al. Acta Oceanol. Sin., 2014, Vol. 33, No. 3, P. 90–100 91
nent. During spring tide, the sea level in the inner bay can rise to 2.5 m and drop to about 2.1 m during neap tide.
In addition, the Benoa Bay is under the influence of the six rivers discharges: the Badung River, the Mati River, the Sama River, the Telabah River, the Loloan River and Buaji River. Ac-cording to the report of Environmental Impact Assessment Agency of Bali Province, the fresher water discharged into the
bay is less than 360 m3/h in each river discharge. This low salin-ity of fresher water runoff has a significant effect on the water circulation due to the difference of the salinity with the seawa-ter.
It is important to understand the physical process of the tidal current field in order to discuss the water quality and the ecosys-tem in the Benoa Bay. Particularly, the seawater exchange is one of the important physical processes. In order to clarify the mech-anism of seawater exchange, we undertake a numerical simula-tion by tracking the particles released in the current driven by the M2 tidal component. Additionally, the seawater-flushing rate is estimated by calculating the time consumed by the particles' movement in a box area as a function of the initial condition. The finite volume coastal ocean model (FVCOM) developed by Chen et al. (2003) is used in our study. FVCOM is based on the Finite Volume Method and three dimensional primitive equa-tions. One unique feature of FVCOM is the use of the unstruc-tured grid. The suitability of an unstructured grid approach in FVCOM enables us to reproduce coastal ocean currents with a high resolution in a complex coastal geometry. FVCOM has been successfully used by many researchers for the investigation of the coastal ocean circulation (Chen et al., 2003; Huang et al., 2008) and the physical mechanism for the offshore detachment (Chen et al., 2008). The Lagrangian method was widely used for examining particle tracking and seawater exchange (Chen et al., 2008; Bilgili et al., 2005; Awaji and Kunishi, 1980).
2 Numerical modelThe FVCOM used in this research is FVCOM 2.7.1 series. The
governing three-dimensional equations consist of the momen-tum, continuity, temperature, salinity, and density equations as given by Chen et al (2006). The vertical eddy viscosity and the vertical thermal diffusion coefficients are obtained using a modified Mellor-Yamada level 2.5 (MY-2.5) turbulence closure model (Galperin et al., 1988). The horizontal diffusion coeffi-cients are determined using a Smagorinsky eddy parameteriza-tion method (Chen and Liu, 2003).
The FVCOM is composed of the external and internal modes that are computed separately using two split time steps. The FVCOM subdivides the horizontal numerical computational domains into a set of non-overlapping unstructured triangular meshes. An unstructured triangle is composed of three nodes, a centroid, and three sides. The scalar variables, such as salin-ity (S), water elevation ( ), and vertical velocity (w) are placed at the nodes, and determined by a net flux through the sec-tion linked to the centroid and the mid-point of the adjacent sides in the surrounding triangles (called the tracer control ele-ment or TCE). The horizontal velocities u and v are placed at the centroid and calculated on the basis of the net flux through the three sides of that triangle (the momentum control element or MCE).The FVCOM uses an exact form of the no flux bottom boundary conditions for temperature and salinity. The bottom slope and the gradients of the temperature and the salinity are calculated using Green’s theorem.
In this study, the computational domain was configured with unstructured triangular grids in the horizontal and -level in the vertical (Fig. 3). The grid in the horizontal cases was de-signed with different resolutions, about 200 m in the inner of bay to 600 m in the outer of bay, and the vertical grid was di-vided into ten -levels.
To consider the wet and dry conditions during ebb tide and
Bali Island
Benoa Harbor
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0 00
0
Serangan
Ben
oa P
enin
sula
96.67
72.79
48.92
25.04
1.169
Dep
th/m
Fig.2. Bathymetry map.
Bali Island8.680°
8.760°
9.167°
0.000°S8.840°
100.833° 119.167° 137.500° E
115.120° 115.200° 115.280° E
S
Benoa HarborSerangan(before reclamation)M
ati R
iver
Sam
a Ri
ver
Ben
oa P
enin
sula
Badu
ng R
iver
Tela
bah
Rive
r
Buaj
i Riv
er
Lolo
an R
iver
Fig.1. Benoa Bay map with scribble black lines indicat-ing river discharge. The inset picture is the Indonesia Ar-chipelago, and Bali Island is indicated with a black dash circle.
HENDRAWAN I Gede et al. Acta Oceanol. Sin., 2014, Vol. 33, No. 3, P. 90–10092
flood tide, a wet/dry point treatment method has been incorpo-rated into the calculation. The vertical column thickness is less than 0.05 m in the cell during ebb tide and will be designed as a dry cell and its velocity is set to 0 m/s. During flood tide where the water level rises, a dry cell becomes wet and its velocity and elevation are computed.
In this study, the FVCOM was forced by four river discharges within the bay and two river discharges in the northern part of Serangan Island (Fig. 1). In the open boundary of the computa-tional domain, M2 tidal component was forced for tidal water circulation, and the temperature is determined to be constant. The computational domain is also forced by fresher water from six rivers which are discharged into the Benoa Bay. In the surface boundary, the meteorological parameters are determined to be constant. The constant salinity is used as an initial condition since no observation data was obtained in the Benoa Bay. This assumption would be fairly acceptable since the water depth of the Benoa Bay is shallow. Furthermore, a constant salinity and meteorologica data were used in this study because the climatological condition in the Benoa Bay does not change sig-nificantly for short periods of time (tropical area). The M2 tidal component used in the open boundary was obtained from the tidal model developed by the Ocean Research Institute (ORI),
the University of Tokyo (ORI-Tide) (Matsumoto et al., 1995). The initial setup conditions are summarized in Table 1.
The investigation of seawater exchange in the Benoa Bay was performed using the Lagrangian particle tracking method. The Lagrangian particle tracking solving a nonlinear system of ordinary differential equation (ODE) is as follows (Chen et al., 2006):
d [ ( ), ]dx v x t tt= ,
(1)
where x is the particle position at a time t, dx/dt is the rate of change of the particle position in time; and v[x(t), t)] is the 3-D velocity field generated by the model. The particles released in the model domain are treated using a conserved mass meth-od. In this calculation, the dependence of the velocity field on time has been eliminated since the velocity field is considered stationary during the tracking time interval (Chen et al., 2006). In order to provide a clear analysis of the seawater exchange in the Benoa Bay, the model area was divided into five regions as shown in Fig. 4. In each region, the characteristics of particle transport will be investigated thoroughly to obtain an overall characteristic of the seawater exchange. The seawater exchange in the model domain was also investigated by using the calcu-lated time of particles in residence in the model area.
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Fig.3. Unstructured triangular grids.
Table 1. The initial set up conditions for the model
Items Contents
Grid number of node
number of element
1 298
2 342
Layers uniform layer with 10th sigma layer
Open boundary tide conditions M2 tidal component
temperature and salinity uniform
River discharge Badung River, Mati River, Sama River, Telabah River,
Buaji River and Loloan River
Meteorological condition uniform
Time step 1.0 s
Longitude/UTM
E: oceanD: eastern part of the bayC: western part of the bayB: central part of the bayA: southern part of the bay
Latit
ude/
UTM
304 000
9 03
6 00
09
032
000
9 02
8 00
0
308 000
Fig.4. Benoa Bay divided into five model regions.
HENDRAWAN I Gede et al. Acta Oceanol. Sin., 2014, Vol. 33, No. 3, P. 90–100 93
3 ObservationAn observational investigation was performed to measure
the seawater salinity. The Center for Remote Sensing and Ocean Sciences (CReSOS) of Udayana University-Bali and Kyowa Con-crete Industries Ltd carried out the field observation in May 2005 for the salinity using a compact CTD. The locations of the observation stations are shown in the Fig. 5. Seven observation stations were located in the inner part of the bay and six stations in the outside of the bay. The salinity parameter was measured at every 2 m depth.
Within the bay, there is one tidal observation station that was set up by the National Coordinating Agency for Surveys and Mapping (Bakosurtanal) of Indonesia to measure the tidal elevation. The measurement of tidal level was recorded every hour. In order to validate the model calculation, the M2 ampli-tude was filtered using a least square analysis method.
Unfortunately there is no measurement of tidal current available yet in the Benoa Bay. Therefore, in this research, we are unable to validate the tidal currents produced by numerical calculation with field data.
4 Results and discussion
4.1 Tidal level validation
Figure 6 shows a comparison between the numerical and observation results for the M2 tidal height and phase lag. The comparison between the observed and the simulated ampli-tudes and phase lags of the M2 tidal component at the tidal measurement station in the inner of the Benoa Bay shows a good agreement. Table 2 shows a small discrepancies in ampli-tude and phase lag, which are 0.02 m and −1.2° respectively. The small discrepancy between observation and numerical calcula-tion indicated that FVCOM has a good performance to simulate the seawater level in the model domain.
Figure 7 shows the results of co-amplitude charts for the dominant semidiurnal tide M2. The numerical calculation shows that the amplitude increases from the bay mouth to the inner part of the bay. It is due to the fact that the inner part of the bay is shallower than the outer part. The bay mouth shows somewhat leveling of amplitude that is evidently affected by the narrow strait between the Serangan Island and the Benoa Pen-insula.
4.2 M2 tidal current and M2 tidal residual currentFigures 8 and 9 show the tidal current at the ebb tide and
flood tide, respectively. During the ebb tide, the seawater flows out from the inner bay into the ocean. The narrow strait be-tween the Serangan Island and the Benoa Peninsula causes
Longitude/UTM
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ude/
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N
observation pointtidal station
123
45 6
7 89
10
111213
9 03
0 00
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035
000
310 000
Mati River
Badung River
Telabah River
Buaji River
Loloan River
Sama River
Fig.5. Salinity observation station indicated with black circle and tidal measurement station indicated with tri-angle.
t/h
modelobservationerror
0
−20
20
−40
−60
−80
40
60
80
0 5 10 15 20 25 35 4530 40 50
Elev
atio
n/m
Fig.6. Tidal level verification.
Table 2. Comparison of FVCOM result and observation for M2
amplitude and phase lag
FVCOM Observation Difference
Amplitude/m 0.74 0.72 0.02
Phase lag/(°) 256.8 258.0 −1.2
Longitude/UTM
Latit
ude/
UTM
Co-
ampl
itude
/m
300 000
0.710
0.716
0.722
0.728
0.733
0.739
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09
030
000
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5 00
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040
000
305 000 310 000 315 000
N
Fig.7. Co-amplitude of M2 tidal component.
HENDRAWAN I Gede et al. Acta Oceanol. Sin., 2014, Vol. 33, No. 3, P. 90–10094
an increase in M2 tidal current. At the narrow strait in the bay mouth, the M2 tidal current speed reached 0.46 m/s at the sur-face layer and decreased due to the increased water depth. The maximum of the tidal current was 0.29 m/s near the bottom. In the northern part of Serangan Island as well, the tidal cur-rent was slightly high, which could be caused by the effect of narrow channel formed in that area. During the flood tide, the seawater flowed into the bay and developed a small eddy in the surrounding of Western part of the Benoa Peninsula. This could happen due to the geometric condition of the model area. Dur-ing flood tide, the M2 tidal current speed reached 0.31 m/s in the surface layer and 0.19 m/s in the nearest bottom layer. This tidal current is lower than that in ebb tide condition.
The tidal residual current is well defined in a number of pa-pers (Imasato, 1983; Awaji, 1980). In this paper the M2 tidal re-sidual current defined by Eq. (2) (Imasato, 1983).
r0
1( , ) ( , , )dT
U x y u x y t tT
= ∫ ,
(2)
where Ur is the residual current, T is the tidal period, and u(x,y,t) is the velocity for x and y direction at time t. The weak M2 tidal residual current was revealed in the Benoa Bay as shown in Fig. 10. The maximum velocity of the residual current occurring in the narrow strait of the bay mouth was 0.097 m/s for the sur-face layer and 0.059 m/s for the nearest bottom layer. Despite the weakness of the tidal residual current, it could play a domi-nant role in the distribution of passive contaminants in the ocean (Yasuda, 1980). Two small eddies within the bay and one small eddy in the bay mouth can be seen. These tidal residual circulations are caused by non-linearity that happened due to the coastal geometry formed by a narrow strait in the bay mouth and the bottom topography (Yanagi, 1976; Zhou et al., 2012). Along the central region of the bay until the bay mouth, the tidal residual current was slightly higher and had more complicated stream systems than other regions in the model domain. It could be the case that the central region of the bay is the crossing area for the three-stream regions: the southern part, the western part and the eastern part of the Benoa Bay. The tidal residual current is also obviously influenced by the bottom topography, in which
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a b
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000
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0.3 m/s
0.3 m/s
Fig.8. Tidal current velocity at ebb tide. a. Surface and b. near bottom layer.
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a b
9 03
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000
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0 304 000 306 000
0.3 m/s
0.3 m/s
Fig.9. Tidal current velocity at flood tide. a. Surface and b. near bottom layer.
HENDRAWAN I Gede et al. Acta Oceanol. Sin., 2014, Vol. 33, No. 3, P. 90–100 95
the tidal residual current in the surface layer is relatively higher than in the bottom layer (Yanagi, 1976). The river discharge into the inner of the Benoa Bay could also influence the tidal residual current as suggested also by Zhou (2012).
4.3 Salinity distributionFigure 11 shows the salinity distribution after 60 days of river
discharges. Low salinity appeared in the inner of the bay and in the northern part of Serangan Island. During ebb tide, the low salinity water in the inner bay was distributed toward the bay mouth. Otherwise during flood tide, the higher salinity from the ocean flowed into the bay. It implied that the salinity in the in-ner of the bay increased.
Figure 12 shows the vertical profile of salinity during ebb tide along the cross section that is indicated by a straight black line in horizontal topographical figure in the inset figure. It was
depicted clearly that the salinity was not well-mixed vertically, especially in the inner of the bay, whereas a well-mixed verti-cal distribution was shown in the outer bay. The vertical salinity distribution at the inner of the bay showed low concentration in the surface layer (dash circle line in the Fig. 12a) during the ebb tide. At the same time relatively higher of the meridional current velocity component is indicated at the same location (dash circle line in the Fig. 12c). This meridional current veloc-ity component lead the transportation of fresher water from the Badung River and the Mati River to the inner of the bay. As the zonal current velocity component become weak and the merid-ional current velocity component become strong (dot circle line in the Figs 12b and c), the salinity concentration become high and mixed well in the outer bay (dot circle line in the Fig. 12a). The weak zonal current velocity component at the outer bay (dot circle line in the Fig. 12b) lead the low fresher water flowing
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a b
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000
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ude/
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0.1 m/s 0.1 m/s
Fig.10. Tidal residual current. a. Surface and b. near bottom layer.
Longitude/UTM
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ude/
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000
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31.000
31.121
31.243
31.364
31.486
31.607
31.729
31.850
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31.607
31.729
31.850
9 02
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000
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000
305 000 310 000 315 000
ba
Salin
ity
Salin
ity
N N
Fig.11. Salinity distribution. a. Ebb tide and b. flood tide.
HENDRAWAN I Gede et al. Acta Oceanol. Sin., 2014, Vol. 33, No. 3, P. 90–10096
out from the inner bay. Meanwhile, the strong meridional cur-rent velocity (dot circle line in the Fig. 12c) induced higher sea-water salinity transported from the Badung Strait to the outer of the bay, which leads to a well mixed vertical distribution. The fluctuation of current velocity changes with the time since the meteorological and fresh water inflow also vary with the time (Lu et al., 2012; Lu et al., 2011). This model indicated that the river discharges into the inner bay do not have a significant in-fluence on the seawater salinity in the outer bay.
The relationship between numerical calculation and obser-vation results for salinity are shown in Fig. 13. The correlation determination (r2) given by FVCOM is 0.75. This indicates that the numerical model has a good ability for salinity calculation including fresh water discharges. The salinity discrepancy be-tween the observation and the model caused by the spatial sa-linity distribution in the inner bay was significantly caused only by the rivers discharge by the model, however the observation results are climatologically considered.
4.4 Seawater ExchangeIn order to investigate the seawater exchange in the Benoa
Bay, the 3-D Lagrangian particle tracking was used. The neu-tral buoyant particles were released at surface layer. Initially 144 particles were released uniformly at the maximum flood tide after model calculation become stable. The particle posi-tions were calculated over the length of model time step and re-corded at 60 min interval. Furthermore, the fraction of particle was calculated based on the particles number, and the percent-ages of particle movement are calculated at a specific time. The particles reaching the bottom are treated to be trapped by the bottom.
The overall seawater exchange in the Benoa Bay can be seen in Fig. 14. The particles were transported rapidly to the ocean. Thirty seven percent of particles were exported to the ocean immediately after they were released. Furthermore, 50% of the particles were exported to the ocean after four tidal cycles and increased 70% after nine tidal cycles. Based on exponential de-
305 125303 750302 375301 000 306 500
15
Salinity31.73 31.76 31.79 31.82
129630
Dep
th/m
Longitude/UTM
305 125303 750302 375301 000 306 500
u/m∙s−10.00 0.09 0.17 0.26 0.35
15129630
Dep
th/m
Longitude/UTM
305 125303 750302 375301 000 306 500
v/m∙s−1−0.75 −0.49 −0.23 0.03
15129630
Dep
th/m
Longitude/UTM
a
b
c
Fig.12. Vertical profile of salinity distribution (a), zonal current component (u) (b), and meridional current component (v) (c) along the cross section. The inset pic-ture in the left is the location of cross section, and the inset picture in the right is the tidal condition for the cross section.
31.80
31.80
31.85
31.90
31.95
32.00
31.85 31.90 31.95 32.00Model salinity
Obs
erva
tion
salin
ity
Fig.13. Validity of Salinity distribution.
0 1 2 3 4 5 6 7 8 9 10M2 tidal cycle
100
90
80
70
60
50
40
30
20
10
0
Frac
tion
/%
Fig.14. Overall particle remaining inside of the bay.
HENDRAWAN I Gede et al. Acta Oceanol. Sin., 2014, Vol. 33, No. 3, P. 90–100 97
cay equation of particle movement, more than 95% of particles predicted will be exported into the ocean after 30 tidal cycles. It means that the particles initially laid down within the bay will be nearly completely transported into the ocean after 1 month.
Figure 15 depicts the particles remaining at each model re-gion. As shown in the western part region (black dot-line) and the central part region (green dot-line), the particles were trans-ported rather rapidly to another region than in the eastern part region (blue dot-line) and southern part region (red dot-line). In the western part region, more than 90% of the particles were transported after four tidal cycles and after two tidal cycles for the central part region. However, the particles in the southern part region were transported slower than in both of the previ-ously mentioned regions. More than 90% of the particles were exported to another region after nine tidal cycles. In the eastern part region, the particles were transported very slowly than in other regions. It takes more than ten tidal cycles to transport the particles into other regions.
Figure 16 shows the particles transport for each model re-gions. In the western part region (the upper-left frame), about 40% of the particles initially in western part region were trans-
2 3 4 5 6 7 81 9 10
100
90
80
70
60
50
40
30
20
10
0
M2 tidal cycle
eastern part of baycentral part of baysouthern part of baywestern part of bay
Frac
tion
/%
Fig.15. Particle remaining in each model region.
100
90
80
70
60
50
40
30
20
10
0
Frac
tion/
%
2 3 4 5 6 7 81 9 10M2 tidal cycle
to the central part region
to the southern part regionto the ocean
particle remaining
a
100
90
80
70
60
50
40
30
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0
Frac
tion/
%
2 3 4 5 6 7 81 9 10M2 tidal cycle
to the central part region
to the southern part regionto the ocean
particle remaining
d100
90
80
70
60
50
40
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10
0
Frac
tion/
%
2 3 4 5 6 7 81 9 10M2 tidal cycle
to the western part regionto the eastern part region
to the southern part regionto the ocean
particle remaining
c
100
90
80
70
60
50
40
30
20
10
0
Frac
tion/
%
2 3 4 5 6 7 81 9 10M2 tidal cycle
to the central part regionto the oceanparticle remaining
b
Fig.16. Particles transport characteristics in each region. a. Western part region, b. southern part region, c. central part region and d. eastern part region.
HENDRAWAN I Gede et al. Acta Oceanol. Sin., 2014, Vol. 33, No. 3, P. 90–10098
ported into the ocean after nine tidal cycles, while more than 40% of the particles were transported to the central region just after particles were released, however, after three tidal cycles it decreased to about 20% and increased to 40% after nine tidal cycles. The particles from the western part region were also connected with those in the southern part region. The southern part region received particles from the western part region after one tidal cycle and increased to 40% after four tidal cycles. Some particles from the western part region were transported directly to the ocean connected by the central region, while some par-ticles were transported to the southern part region first before finally transported into the ocean.
As shown in the southern part of the bay (the upper-right frame), more than 80% of the particles were transported to the ocean and less than 20% of the particles remained in the cen-tral part of the bay during nine tidal cycles. The particles from the southern part region were not connected with those in the
western region and the eastern region. It seemed that the parti-cles were directly transported from the southern region into the central part of the bay and transported to the ocean (Fig. 17).
The particles in the eastern part region (the lower-right frame), roughly more than 30%, were transported into the ocean immediately after they were released, and this amount increased to nearly 50% after nine tidal cycles. Particles from this region were also related to those in the southern part re-gion. More than 10% of the particles were transported after four tidal cycles. The eastern part region trapped about 40% of the particles after the sixth tidal cycle. This gives an impression that the particles cannot be transported to another region in the model domain. This can be known by investigating the residual current pattern, suggesting that the residual current in the east-ern part region went north. Additionally, the tidal current in the bay mouth strongly flowed to the west-east direction. There-fore, the seawater in this region will be difficult to exchange to
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Fig.17. Particle tracking, color indicated the different way distribution. a. Western part region, b. southern part region, c. central part region and d. eastern part region.
HENDRAWAN I Gede et al. Acta Oceanol. Sin., 2014, Vol. 33, No. 3, P. 90–100 99
the other region. Finally, more than 50% of particles were directly transported
from the central part of the bay (the lower-left frame) into the ocean immediately after the particles were released, and this amount increased to above 70% after eight tidal cycles. Some particles were also transported into the southern part region, with more than 20% transported after two tidal cycles. It is obvi-ous that the increasing particles transported to the ocean will be accompanied by decreasing particles transported to the southern part region. A few particles were also transported to the western and the eastern part region.
Above results revealed the general characteristics of the seawater exchange in the Benoa Bay. In general the fraction of particles has two ways to find the gate into the ocean. In the first way, the particles are directly transported into the ocean from each region, and in the other way the particles transported to the southern part of the bay and then exported to the ocean (Fig.17). However, the particles in the eastern part of the bay are not transported into the ocean easily compared with other re-gions. It could be caused by the residual current (Zhou, 2012). The residual circulation near the open boundary of the eastern part of the bay can be seen in Fig. 10. The particles in the eastern part would be trapped by this circulation. It would explain why the particles do not move to the ocean easily.
Another way to investigate the seawater exchange in the Benoa Bay is by calculating the residence time. The residence time of the particles could be known by seeing how long the particles reside in a specific area to be flushed. In this study, we used a 300 m × 300 m moving box to estimate the residence time (Bilgili et al., 2005). Figure 18a shows the time particles spend in these box regions as a function of their initial condi-tions. Within the bay the particles spend 1 to 5 h. The figure de-picted that the residence time has a good relationship with the energy of tidal residual current (Fig. 18b). The higher energy of the tidal residual current caused the residence time of the par-
ticles to be short. Waters, surrounding the bay mouth such as the south edge of the eastern part region, the east edge of the southern part region and a few areas in the western part region (red color), have a shorter residence time, which corresponds to a relatively high energy of the tidal residual current. On the other hand, in the rest of the area (green color) with a relatively weak energy of the tidal residual current, waters have a longer residence time.
5 Concluding remarksFVCOM is successfully applied to investigate water circu-
lation in the Benoa Bay-Bali. The narrow strait formed in the bay mouth doesn't result in a high discrepancy value for the tidal elevation, amplitude, and phase lag compared with ob-servation results. A weak M2 tidal residual current is revealed in whole model domain. However, the weakness of residual cur-rent brings about an essential effect on the longtime particles transport. Regarding the impact of fresh water discharge from river, the calculation shows that vertical salinity profiles in the inner bay is significantly affected by fresh water discharge. The salinity computation suggests that FVCOM can predict well the salinity distribution in the Benoa Bay.
The characteristics of the seawater exchange are clearly de-picted by using the Langrangian particle method. Each region in the model domain has a different way to transport the par-ticles into the ocean. Overall, nearly 70% of the particles were exported to the ocean after nine M2 tidal cycles. Furthermore, the residence time of the particles in model domain is useful to investigate the seawater exchange in the Benoa Bay.
AcknowledgementsWe would like to thank Pallav Koilrala and Azizul Moqsud for
their cooperation to check English text.
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Fig.18. Residence time (a) and Energy of residual current (b).
HENDRAWAN I Gede et al. Acta Oceanol. Sin., 2014, Vol. 33, No. 3, P. 90–100100
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