2011 2nd International Conference on Environmental Science and Development
IPCBEE vol.4 (2011) © (2011) IACSIT Press, Singapore
Hydrodynamic Modelling for Salinity of Singapore Strait and Johor Strait using
MIKE 3FM
Rohit Goyal
Prameet Rathod
Environmental Science and Engineering
Indian School of Mines
Dhanbad, India
E-mail: [email protected]
Environmental Science and Engineering
Indian School of Mines
Dhanbad, India
E-mail : [email protected]
•
Abstract—Numerical models provide quick and better
understanding of the behavior of ocean waves, currents and
sediment transport. Hydrodynamic modelling uses the concept
of scales of motion, dimensionality of flow, physical processes
and forcing mechanisms. The present study aims at modelling
the flow patterns prevailed in the Singapore Strait and Johor
Strait due the tidal forcing from South China Sea and Melaka
Strait. The domain was forced with the measured tidal signal
from the artificially truncated boundaries for a selected period
from 29/03/2009 to 02/04/2009. Measurements available for the
above seasons have been used for the validation of model
results. Further, the hydrodynamic results were used to carry
out the Water Quality Modelling to study the temperature and
salinity variation. Based on the simulation results, flow off
Singapore Strait was observed which was used to simulate the
Temperature and Salinity of Singapore Region.
To check the variations of Temperature and Salinity
in the domain.
II. STUDY REGION:
I. INTRODUCTION
Coastal zone, the interface of land, ocean, and
atmosphere, is clearly of major economic and social
importance. It is defined as the region from the 200m water
depths at the sea to the 200m elevations on the land. Coastal
landscapes consist of beaches, cliffs, dunes, sand pits,
barriers, islands, tidal flats, deltas, tidal inlets etc. These are
results of sediment transport generated by hydrodynamic
forces. The coastal region has acquired greater attention in
the recent days due to increasing utilization of its resources.
But, the vulnerability of the coastlines and coastal resources
due to adverse impact from natural extreme events, pollution
due to industrial discharges, etc. remind us to have a wise
and sustainable management of coast and coastal resources.
Thus, the computational hydrodynamic study becomes more
complex and tricky to bring out the real physics.
Hydrodynamic (HD) modelling[1] is a prerequisite to
environmental/ecological modelling, as it influences the
biological and chemical processes. A good understanding of
scales of motion, dimensionality of flow, physical processes
and forcing mechanisms are essential for HD modelling.
Singapore has two main monsoon seasons The Northeast
monsoon Season (December –March) and the Southwest
monsoon season (June -September).
The objectives of the study are framed as follows:
• To simulate tide-driven currents along the coast of
Singapore.
• To simulate water level and flow off Singapore.
• To quantify the flow velocity.
FIG:1 Study area (locaion map)
III.
COMPUTATIONAL TOOLS AND METHODS
The three-dimensional MIKE 3 FM model developed by
the Danish Hydraulic Institute was used to study the
hydrodynamics of the region for 3D free-surface flows.
MIKE 3[2] is applicable to the simulation of hydraulic and
related phenomena in lakes, estuaries, bays, coastal areas
and seas where stratification or vertical circulation is
important. MIKE 3 Flow Model FM is a modelling system
based on a flexible mesh approach. The flexible mesh is
most suitable for irregular boundaries of the water body. The
modelling tool has been developed for applications within
oceanographic, coastal and estuarine environments.
3.1) METHODOLOGY
MIKE3 FM is capable of taking irregular domains
(unlike in case of MIKE 3) [3]. The process starts from the
generation of mesh i.e., defining model domain and grid size.
295
Specifications required generating the domain and
thereafter meshing involved:
• Maximum element area: 0.0003 ^2
• Smallest allowable angle: 26
• Maximum number of nodes:100000
• Interpolation Method: It has been divided into three
parts;.
a. The Natural Neighbour method.
b. The linear method.
c. The inverse distance weighted method.
• Size of Bounding: 1000% beyond convex hull
• Extrapolation: 00
• Time taken: 47 sec
Once the domain was formed (after defining the open and
closed boundaries) thereafter mesh was generated .Mesh can
be made smooth by increasing the number of
iterations .Scatter data of depth or the bathymetry values was
imported on the mesh and was interpolated by either using
method of Natural
Neighbour or Linear method. After Interpolation it was
exported as either .mesh file or .dfsu file .The exported file
contains the actual bathymetry of the region.
Once the field work for the model was done the
generated bathymetry was given as an input to the model.
According to the needs module was selected and simulation
time and date was given. After that various conditions like
Bed resistance, Flood[4] and Dry, density, Eddy Viscosity,
Solution Technique etc can be given according to the quality
of required output data. Thereafter boundary conditions were
specified and model was made to run and outputs were
specified as required.
3.1.1) DATA USED
Data on currents, sea level, and winds were collected off
Singapore. The data was collected for the period March 29,
2009 to April 2nd, 2009.
3.1.2) GENERATION OF MESH AND BATHYMETRY
Setting up a mesh includes appropriate selection of the
area to be modelled, adequate resolution of the bathymetry,
flow, wind and wave fields under consideration and
definition of codes for open and land boundaries. Mesh file
was generated with the MIKE Zero Mesh Generator [5]. The
mesh file was an ASCII file (.mesh extension) that includes
information of the geographical position and water depth at
each node point in the mesh. The file also includes
information about the node connectivity of the triangular and
quadrangular elements.
Fig 3: Interpolated Bathymetry used for computation
3.1.3) BOUNDARIES
The water levels at the west, south and east open
boundaries of the model were specified based on the tidal
elevations predicted using MIKE3 FM tidal prediction
module. The rest of the domain was interpolated by means
of MIKE3 FM interpolation tool. The Northern boundary
was closed by land therefore it was considered as a no flow
boundary. A constant water level and zero velocity were
used as initial conditions at all grid points [6]. The tidal level
at the open boundaries was used as the boundary condition
and the flow direction at the open boundary was considered
to be perpendicular to the boundary.
3.1.4) CALIBRATION FACTORS
Model calibration is defined as fine tuning of parameters
until the numerical model results and the field measurements
were within an acceptable tolerance by modifying the
boundary conditions and improving the hydrometeorological forcing input[7].Two parameters adjusted
during calibration were:
(i) Roughness coefficient – used in the bottom friction
formulation
Fig 2: Mesh file
296
(ii) Eddy viscosity – parameterized horizontal mixing of
momentum
3.2) MODEL VALIDATION BY WATER LEVEL
The validation process gives an indication of the model's
sensitivity and confidence that the results it produced were
consistent with measurements [8]. The zonal (u) and
meridional (v) components of the currents derived from the
measured elevation of March, 2009 and April, 2009 were
used for the validation of model results. Water level
measurements were available for the period 29th March to
2nd April, and for that period simulated values were
validated . Comparison between measured and simulated
water levels is presented in Fig 4 for all the stations . The
results indicated that modelled water levels matched very
well with measured water levels throughout region as shown:
Fig:4 Comparison of water level at different stations
3.3)TEMPERATURE SALINITY MODULE
The temperature/salinity (TS) module was invoked from
the specification of the density since baroclinic density
(density depends on temperature and/or salinity) was
selected. The TS module sets up additional transport
equations for temperature and salinity[9]. Additionally the
calculated temperature and salinity are feed-back to the
hydrodynamic equations through buoyancy forcing induced
by density gradients. Various parameters considered were:
•
•
•
•
•
•
•
•
•
Temperature Range: 20 – 30 degrees
Salinity Range: 25psu – 35psu
Solution Techniques: Low order, fast algorithm
Scaling Factor: 1
Drying depth: 0.005 m
Flooding Depth: 0.05m and
Wetting depth: 0.1 m
Reference Temperature: 29 degrees
Reference Salinity: 30 psu
3.4) DOMAIN AND MESH GENERATED
Having validated the model , we redefined the whole
domain to a comparatively smaller one so as to increase the
model speed to a good extent .For doing this in the mesh
generator module points, lines and polygons were deleted
collectively and a smaller domain was generated
.
Thereafter with the new domain a mesh was created taking
the parameters as follows:
•
•
•
•
•
•
297
Interpolation Method: Natural Neighbour
Time Taken: 40 sec
Simulation time: 1.5 days
Number of scatter points: 80976
Number of Elements: 6453
Mesh Nodes: 4728
IV. RESULTS AND DISCUSSION
The model was run without wind for the period
29/03/2009 to 30/03/2009
4.1) WATER LEVEL
The surface elevations had been simulated for every 15
minutes. The simulated water level at Singapore showed that
the surface elevation was primarily contributed by tides. It
was assumed that wind contribution to water level variation
was negligible in the context. The presence of higher water
level at the east end of the domain compared to the south
and the west showed that water level along the Singapore
Strait was increasing eastwards. That increase may be due to
the funnelling effect and/or the bathymetric effects [11]. The
currents were varied with the water level variation. Higher
the water level, stronger was the current speed.
Fig 5: Modified domain used to calculate salinity module
3.5) BOUNDARIES (NEW DOMAIN)
The New domain consisted of 7 open boundaries .The
values for the boundaries were obtained from the simulated
outputs of the earlier larger domain which was used for the
validation of the model also. The data at these node point
were extracted using Data Extraction FM tool of MIKE
Zero[10].
Fig 7.Time series plot for surface elevation without using the wind.
4.2) CURRENTS
Tidal currents were significant in this region. The current
speed during ebb tide was almost similar as compared to
flood tide since the currents were considered without wind.
During ebb tide, the predominant flow was towards south
east and during the flood tide flow was towards North West
Tidal currents oscillate mainly in the longshore direction
with little net cross-shore current. The onshore and offshore
currents (u-component) were meagre compared to the
alongshore currents (v-component), irrespective of the
period of simulation. The current slows down during the
tidal slack that was, just before current reversal takes place.
There was no contribution by the fresh water discharges as
such, but some contribution can be expected later, but that
would be very minimal.
Fig 6: Shows location of open boundaries of the new domain
298
Fig 9: Velocity vectors depicting the instantaneous flow during ebb tide
4.3) SALINITY
Point 2 was located at the South West corner of the
domain where boundary value of west side was set to 33 psu
and south was also set to 33 psu. So we see an expected
increase in the value from 32 psu to 33 psu .While point 3
was on the South east side of the domain where boundary
value of east side was set to 32.7 psu and south as said was
set to 33 psu. So here also there was expected increase of
value from 32ap approaching towards 32.7 psu
Fig.8: Velocity vectors depicting the instantaneous flow during flood
tide
The current meter value and the simulated output
showed that, the alongshore component of velocity goes on
decreasing towards the coast. That may be due to some
difference in bed friction. The magnitude of southward flow
was similar to that of the northward flow. That indicated no
role of wind, which could not force the surface water. At
times, the u-component of simulated current showed some
variation with the measured current. The residual currents
were estimated after removing the tides from the measured
data. The upwelling phenomenon [12] was absent during this
period. The result was in agreement with the results of
modelling that the coastal current would influence the tidally
driven flow, though marginally. The simulated current
showed that the currents were having distinct variability
corresponding to the tide. It may be noted that in the pole
ward flow, density driven flow was the existent force that
drives the surface water northwards.
Fig 10: Shows variation in salinity at point 2 and point 3
V.
CONCLUSION
The study examines the hydrodynamics of Singapore
strait and Johor strait using MIKE 3 FM HD model. Results
obtained from model simulation matched very well with the
measurements. Hence, the model was further used for the
simulation of hydrodynamics for other days and points also.
The simulated hydrodynamics reasonably agreed with most
of the earlier studies. The tidal flows were modelled
accurately on coarse grids since they were large-scale
processes. The water level showed very marginal variation.
But, it was the current that have marked particular pattern of
flow. The circulation along the nearshore region was purely
south west during NE monsoon period. The model has
provided a general understanding of the surface flows off the
Singapore and Johor Strait. The reason for variability of
currents from the actual measurements may be due to the
usage of lower resolution bathymetry[13] and limitations of
the model to generate precise HD of the region. The
Hydrodynamic results can be further used to study a wide
range of phenomena related to hydrodynamics, such as
water quality, heat and salt transport and sediment transport
processes[14]. The reason for initial fluctuations in the result
of water level was due to the warm up period of the model
which was around 6 time steps i.e., 1 hrs 30 minutes.
The Model also provided specifications of the
Temperature and salinity off the Singapore region and Johor
Strait. Few variations were observed in the data. An almost
continuous increase of Salinity was observed from the North
to South West (or south east) direction during the first 6 hrs
of flow (The period for which salinity was
calculated).Salinity results were in according to the
measured value).
299
REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
Numerical simulation of flow off Ratnagiri, west coast of India by Dr
P Vethamony and Jaffer Shariff.
MIKE 3 FM, user guide and reference manual, (2001). Reference
manual. Denmark: 70 pp.
MIKE 3 FM, user guide and reference manual, (2003). Reference
manual. Denmark: 40 pp.
Pond, S., Picard GL. (1983). Introductory dynamical oceanography,
Pergamon Press.
Shetye, S. R., Govea AD, Shenoi SSC, Micheal GS, Sundar D,
Almeida AM, Santanam K. (1991). “The coastal current off western
India during the northeast monsoon.” Deep-Sea Research 38: 15171529.
Shetye, S. R., Shenoi SSC, Antony MK, Krishna
Kumar V.
(1985). “Monthly-mean wind stress over along the coast of north
Indian Ocean.” Proc. Indian Acad. Sci. (Earth Planet. Sci 94: 129137).
Shankar, D., Vinayachandran PN, Unnikrishnan AS. (2001). “The
monsoon currents in the north Indian Ocean.”.
Shankar, D. (2000). “Seasonal cycle of sea level and currents along
the coast of India.” CURRENT SCIENCE 78.
MIKE 3 FM tidal analysis and prediction module environment
(2007). Danish hydraulic institute.
HD, U. G. a. R. M. f. M. (2007). Reference Manual. DHI Software
2007. Denmark: 90 pp.
David Huntley, A., Eduardo Siegle, Mark Davidson A. (2002).
“Modelling water surface topography at a complex Inlet System –
Teignmouth, UK.” Journal of Coastal Research 36: 675-685.
Environment and Pollution Law Manual by S.K.Mohanty.
Waves, Tides and Shallow-Water Processes by the Open University.
Hydrodynamics and Transport of Water Quality Modelling by James
L.Martin.
300