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International Journal of Civil Engineering and Technology (IJCIET)
Volume 7, Issue 3, May–June 2016, pp. 316–328, Article ID: IJCIET_07_03_031
Available online at
http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=7&IType=3
Journal Impact Factor (2016): 9.7820 (Calculated by GISI) www.jifactor.com
ISSN Print: 0976-6308 and ISSN Online: 0976-6316
© IAEME Publication
NUMERICAL MODELLING OF BRINE
DISPERSION IN SHALLOW COASTAL
WATERS
Y. B. Patel
Research Scholar, Dept. of Civil Engineering,
Bharati Vidyapeeth University College of Engineering,
Pune-411043, Maharashtra, India
P. T. Nimbalkar
Professor, Dept. of Civil Engineering,
Bharati Vidyapeeth University College of Engineering,
Pune-411043, Maharashtra, India
T. Nagendra
Scientist–E, Central Water & Power Research Station,
Pune-411024, Maharashtra, India
V. K. Shukla
Scientist–B, Central Water & Power Research Station,
Pune-411024, Maharashtra, India
ABSTRACT
Fresh water is a limited finite resource, vital for the existence of every life
on earth. It is becoming a scarce commodity. This is due to population growth,
climatic changes with more frequent extreme events such as droughts and
floods, increased water contamination of existing supplies, inefficient use of
water etc. To overcome this scarcity, creation of fresh water from sea water by
the process of desalination is a effective and reliable way. Hence desalination
plants are being widely used in coastal areas.
However, desalination process is associated with the rejection of highly
concentrated brine waste which should be disposed off in the coastal
environment with minimum adverse impact. Also, for better efficiency of the
desalination plant, it is essential to locate intake/outfall of the desalination
plant suitably under the prevailing site conditions so that recirculation of the
saline water in the intake is the minimum. Dispersion of brine water at a
typical project site is assessed for the suitability of locations of intake and
outfall of a desalination plant using numerical modelling technique. This study
confirmed that discharge sites with adequately strong flushing currents
Numerical Modelling of Brine Dispersion In Shallow Coastal Waters
http://www.iaeme.com/IJCIET/index.asp 317 editor@iaeme.com
inducing high advection and ambient mixing and also with the optimum
distance from the shore are preferable for locating the outfall.
Key words: Brine Dispersion, Coastal Area, Intake and Outfall, Numerical
Model, Sea Water Desalination
Cite this Article: Y. B. Patel, P. T. Nimbalkar, T. Nagendra and V. K. Shukla,
Numerical Modelling of Brine Dispersion In Shallow Coastal Waters.
International Journal of Civil Engineering and Technology, 7(3), 2016,
pp.316–328.
http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=7&IType=3
1. INTRODUCTION
About 97.5 per cent of the world’s available water reserves consist of saline seawater
and hence are not suitable for drinking. Fresh Water is a limited finite resource, vital
for the existence of every life on earth. It is becoming a scarce commodity. This is due
to population growth, improvements in lifestyle, increased economic activity,
increased contamination of existing water supplies, inefficient use of water, climatic
changes with more frequent extreme events such as droughts and floods etc. To
overcome this scarcity, creation of fresh water from sea water by the process of
desalination is a effective and reliable way. Hence desalination plants are being
widely used in coastal areas [1]. Since large coastline and abundant sea water is
available, desalination plants are coming up in India to produce fresh water from sea
water for industrial as well as domestic use.
During the past 50 years, the desalination of sea and brackish water has gained
importance. In the near future, desalination demand in India is likely to expand at an
annual rate of up to 15%. The country is also looking to use membrane-based
solutions, particularly in inland areas, where there are high levels of salt, fluoride,
arsenic, nitrates and iron in the groundwater.
However, desalination of seawater produces brine water containing high salt
concentration which is normally disposed off into the environment. It is common
practice that the sea water is taken in the desalination plant through an intake and the
brine waste water from the desalination plant is discharged back into the sea through
an outfall [2]. Management of Brine Discharges to Coastal Waters is becoming
important and guidelines given in Jenkins et al [3] should be followed. Detailed and
basic theory of dispersion of waste water into water bodies is given by Fischer et al
[4].
For better efficiency of the desalination plant, it is essential to locate intake/outfall
of the desalination plant suitably under the prevailing site conditions so that
recirculation of the saline water in the intake is the minimum. Also it is important to
understand dispersion of the brine water in the marine environment so as to minimize
its potential impact.
Numerical modelling is a very much useful tool to simulate the dispersion of brine
water at the project site. It is efficient, cost effective and it can consider large area and
different design conditions with ease. Many sophisticated software tools like MIKE21
FM of Danish Hydraulic Institute [5], are available to carry out simulations. It is
proposed to analyze the dispersion of brine water at a typical project site and to assess
the suitability of locations of intake and outfall of a desalination plant using this
numerical modelling technique.
Y. B. Patel, P. T. Nimbalkar, T. Nagendra and V. K. Shukla
http://www.iaeme.com/IJCIET/index.asp 318 editor@iaeme.com
2. STUDY AREA
The city of Chennai is located on the East coast of India i.e. on the coast Bay of
Bengal. It has a chronic water problem as the city depends extensively on ground
water, replenished by an average rainfall of 1,276 mm. The water demand of the
coastal city is 1200 MLD. The city receives about 985 MLD of water from ground
and surface water sources. The demand is expected to increase to about 2,700 MLD
by 2031. Due to an increase in groundwater usage, the underground aquifers are being
depleted at an alarming rate.
To alleviate the freshwater problems, initially, a 100 MLD plant was planned at
Minjur (about 25 km north of the City), followed by another plant with equal capacity
at Nemmeli (about 35 km south of the city) and a third plant with a capacity of
200 MLD has been planned at Pattipulam, south of Nemmeli. The Minjur
Desalination Plant is a Reverse Osmosis (RO), water desalination plant at Kattupalli
village. It is the largest desalination plant in India. The plant produces 100 MLD of
desalinated water from 273 MLD of sea water.
2.1. Site Conditions
2.1.1. Bathymetry
The model region is shown in Fig. 1. It is about 9 x 12 km. It covers about 6 km on
the either side of the intake and outfall locations and extends up to about -30 m depth
contour. The average depth in the model region is of the order of 10 -12 m.
2.1.2. Tidal Levels and Currents
Tidal observations observed at 8 m depth in March 2008 for a duration of about 15
days showed that the tides are semi-diurnal with low amplitude. The minimum and
maximum tidal ranges were 0.3 m and 1.2 m respectively.
The currents were found to be more influenced by the bay circulations generated
by southwest and northeast monsoons. The average tidal currents were of the order of
0.25 m/sec. The current direction generally has the tendency towards north and south,
parallel to the shoreline. The coastal currents were northerly during February to
September and southerly during September to January.
2.1.3. Water Temperature and Salinity
The seawater surface temperature distribution at the study area indicated presence of
warm water (300C) near the coast and relatively cold water (28.5
0C) offshore. The
near bottom temperature varied from 29.30C at the coast to 28.5
0C offshore. Salinity
in the model region was observed to vary from 25 to 35 ppt.
2.1.4. Wind
The wind data of the site indicated that in general, during SW monsoon, wind blows
predominantly from SW with the maximum intensity of about 10-15 m/s and during
NE monsoon it blows predominantly from NE direction with the same intensity.
3. METHODOLOGY
One of the most advanced and comprehensive two dimensional modeling software
available for applications within coastal region is the MIKE21-FM model [5]. This
software is developed by Danish Hydraulic Institute (DHI), Denmark. It is new
modeling system based on flexible mesh approach. The sub-module MIKE21-HD is
Numerical Modelling of Brine Dispersion In Shallow Coastal Waters
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used to simulate the hydrodynamics and the sub-module MIKE21-AD/Transport is
used to simulate dispersion of brine discharge from the desalination plant in coastal
environment at the study area. The hydrodynamic module is based on the following
non-linear vertically integrated 2-D equations of conservation of mass and
momentum.
+
+
= (1)
+
+
=
-
-
–
-
+
+
(2)
+
=
-
-
–
-
+
+
(3)
The overbar indicates a depth average value. For example, and are the depth-
averaged velocities defined by
,
(4)
where, x, y = Cartesian co-ordinates in two dimension;
t = time;
η = surface elevation;
d = still water depth;
h = η + d equal to the total water depth;
, = depth averaged velocity components in x and y directions;
f = 2Ωsin φ is the Coriolis parameter (Ω is the angular rate of revolution
and φ the geographic latitude);
g = gravitational acceleration;
ρ = density of water;
pa = atmospheric pressure;
ρo = reference density of water;
S = magnitude of the discharge due to point sources;
us , vs = velocity by which the water is discharged into the ambient water;
τsx ,τsy = x and y components of the surface wind;
τbx ,τby = x and y components of bottom stresses;
s xx , s xy , syx and syy = components of the radiation stress tensor .
The lateral stresses Tij include viscous friction, turbulent friction and differential
advection. They are estimated using an eddy viscosity formulation based on the depth
average velocity gradient as follows:
,
(5)
Y. B. Patel, P. T. Nimbalkar, T. Nagendra and V. K. Shukla
http://www.iaeme.com/IJCIET/index.asp 320 editor@iaeme.com
where A= horizontal eddy viscosity. MIKE21-AD/Transport module is based on the following non-linear vertically
integrated 2-D equations of conservation of salinity which takes into account
advection, dispersion, source and sink.
+
= (6)
where, = Depth average salinity, = the horizontal diffusion term defined by
=
+
] (7)
and = horizontal diffusion coefficient.
In the 2D transport equations, the low order approximation uses simple first order
upwinding, i.e., element average values in the upwinding direction are used as values
at the boundaries. The higher order version approximates gradients to obtain second
order accurate values at the boundaries. Values in the upwinding direction are used.
To provide stability and minimize oscillatory effects, a TVD-MUSCL limiter is
applied [5].
4. MODEL SIMULATION
4.1. Model Region
The region considered for the model is about 9 km X 12 km covering about 6 km on
the either side of the proposed port and extending up to about -30 m contour as shown
in Fig. 1. The western side of the model region consists the shoreline while the
remaining sides are open to sea. The region is discretised by 11947 triangular
elements and 6241 nodes (Fig.2). It is observed in the open sea at the site that the flow
is unidirectional (either northerly or southerly). Hence to get the northerly flow, a
tidal variation at southern boundary and velocities at northern boundary were
considered. To get southern flow, the boundary conditions are reversed. At the
seaward boundary, no flow was assumed.
4.2. Model Calibration
Before using the model for predictive purpose, it is calibrated. The Fig. 3A and 3B
show calibration of the hydrodynamic model, particularly the observed and computed
variation of
Numerical Modelling of Brine Dispersion In Shallow Coastal Waters
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Figure 1 Bathymetry in the model region
Figure 2 Flexible computational mesh
Y. B. Patel, P. T. Nimbalkar, T. Nagendra and V. K. Shukla
http://www.iaeme.com/IJCIET/index.asp 322 editor@iaeme.com
Figure 3 A Time history of observed and computed tidal level
Figure 3B Time history of observed and computed current
water level and current. Considering the limitations of the model and observed
data the comparison is considered to be satisfactory.
No salinity data in the existing situation at the site were available for calibration of salinity dispersion. Therefore, based on the past experience and referring similar
studies from literature, the calibrating parameters of the transport model i. e.
dispersion coefficient is selected.
4.3. Criteria for Recirculation
The efficiency of the desalination plant is important and to maintain it the
recirculation of salinity in the intake should be avoided by selecting proper locations
of intake and outfall. For avoiding the recirculation the limiting salinity at the intake
above ambient is considered as 0.1 ppt.
Numerical Modelling of Brine Dispersion In Shallow Coastal Waters
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4.4. Model Operation
The ambient temperature of sea water is assumed to be 270C. Ambient salinity of sea
water is assumed to be 32 ppt. The rise in salinity of brine water to be discharged at
the outfall is given to be 31 ppt above ambient.
For the salinity model, as the open boundaries have been taken sufficiently away
from the outfall locations, zero rise in salinity is assumed at the open boundaries. It is
observed that in the open sea at the site, the flow is unidirectional, either northerly or
southerly generally parallel to the coastline. During the period February-September
the flow is northward, while during the period October-January the flow is southward.
The model is operated for different current conditions namely northward 0.25 m/s,
southward 0.25 m/s and weak current i.e. 0.05 m/s towards. The direction of the weak
current is considered to be northward or southward depending on the locations of
intake and outfall to simulate the critical condition.
4.5. Model Operating Cases
The following cases of the intake and outfall were considered for the numerical model
simulations.
Intake and outfall locations on south of south breakwater of L & T port as shown
in Fig.1 are considered. Intake is taken at 550 m from shore. The outfall distance from
shore is varied from 1000 m to 1300 m. Discharge of 1.25 m3/s ( 100 MLD) is
considered at outfall while inflow at intake is taken twice that of outfall.. The
following alternatives of outfall locations are considered.
1. With outfall at 1000 m from shore
2. With outfall at 1100 m from shore
3. With outfall at 1200 m from shore
4. With outfall at 1300 m from shore
4.6. Results and Discussion
The model is operated under northward, southward and weak current conditions in
each of the above cases of outfall location. Typical results, namely, contour plots and
time histories of rise in salinity are shown in Figs. 4A, 4B, 5A and 5B. The salinity
rise at the intake is nearly zero in case of average northward current of 0.25 for all the
outfall lengths. The salinity rise obtained at the intake for different outfall lengths and
for the remaining two current conditions (namely, southward and weak current) are
tabulated in Table 1A and 1B. It is seen that for the average northward current of 0.25
m/s the rise in salinity above ambient at the intake is well below 0.1 ppt. The salinity
rise results from the tables are also plotted in the Fig. 6. From the tables and the Fig. 6
it is seen that it is also less than 0.1 ppt for average southward current of 0.25 m/s for
the outfall distance greater than 1000 m and it is more than 0.1 ppt for outfall distance
less than about 975 m.
Y. B. Patel, P. T. Nimbalkar, T. Nagendra and V. K. Shukla
http://www.iaeme.com/IJCIET/index.asp 324 editor@iaeme.com
Figure 4A Contours of rise in salinity for intake at 550 m, outfall at 1000m from shore,
current =0.05 m/s southward
Numerical Modelling of Brine Dispersion In Shallow Coastal Waters
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Figure 4B Contours of rise in salinity for intake at 550 m, outfall at 1300m from shore,
current =0.05 m/s southward
Y. B. Patel, P. T. Nimbalkar, T. Nagendra and V. K. Shukla
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Figure 5A Time history of salinity rise at intake and outfall for intake at 550m outfall at 1000
m from shore, current =0.05 m/s southward
Figure 5B Time history of salinity rise at intake and outfall for intake at 550m outfall at 1300
m from shore, current =0.05 m/s southward
AT OUTFALL
AT OUTFALL
AT INTAKE
AT INTAKE
Numerical Modelling of Brine Dispersion In Shallow Coastal Waters
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Table 1A Rise in salinity at intake for southward current(0.25 m/s)
S. No. Outfall distance
from shore (m)
Rise in salinity (ppt) above ambient
Range Average
1 1000 0.070-0.110 0.09
2 1100 0.040-0.075 0.06
3 1200 0.020-0.040 0.03
4 1300 0.015-0.030 0.02
Table 1B Rise in salinity at intake for weak southward current (0.05 m/s)
S. No. Outfall distance
from shore (m)
Rise in salinity (ppt) above ambient
Range Average
1 1000 0.12-0.16 0.14
2 1100 0.09-0.12 0.11
3 1200 0.06-0.10 0.08
4 1300 0.05-0.08 0.07
As the southward weak current (of 0.05 m/s) is more critical than the northward
weak current, southward weak current is considered in the simulations. For the
southward weak current, the salinity rise is more than 0.1 ppt for the outfall distance
less than 1200 m. Thus considering the worst situation of weak southward current the
outfall distance should be greater than 1200 m for avoiding recirculation in the intake.
Figure 6 Variation of salinity rise at intake w.r.t distance from outfall
The model studies are carried out with many conservative conditions and
limitations. The model is vertically averaged. It is generally far field model. It is not
capable to simulate the near field dilution properly. There is a limitation of non-
availability of reliable observed data on salinity near the intake and outfall locations
because of which, calibration of model for dispersion parameters was not possible.
With these limitations and considering the abnormal sea conditions such as storm
situations etc. it would be desirable to locate the outfall further away in the deeper
zone with adequately strong flushing currents inducing high advection and ambient
mixing. The outfall location with nearby bathymetric depressions or bounded by
barriers like breakwaters etc. should be avoided. It should be located beyond the
Y. B. Patel, P. T. Nimbalkar, T. Nagendra and V. K. Shukla
http://www.iaeme.com/IJCIET/index.asp 328 editor@iaeme.com
effective penetration of breakwaters of L & T port into the sea. Also considering the
optimum distance from the shore, the outfall should be preferably at about 1300 m
from the shore. Thus this study confirms the suitability of the present outfall location.
5. CONCLUSIONS
For better efficiency of a desalination plant, it is essential to locate intake/outfall of
the desalination plant suitably under the prevailing site conditions so that recirculation
of the saline water in the intake is the minimum. Simulation of dispersion of brine
water discharge from a desalination plant at a typical project site is carried out to
assess the suitability of locations of intake and outfall of the desalination plant using a
two dimensional numerical model based on flexible mesh. The model is operated for
different current conditions prevailing at the site. Also it is operated for different
outfall locations. The main conclusions of this study are given below.
1. This study confirmed that discharge location with adequately strong flushing
currents inducing high advection and ambient mixing and also with the optimum
distance from the shore are preferable for locating the outfall. Thus the
suitability of intake at 550 m and outfall at 1300 m from the shore is
confirmed for the minimum recirculation.
2. MIKE21 FM model is found to be useful for such far field dispersion studies
in the complex and well mixed coastal region.
3. Extensive observations on salinity at the existing site of desalination plant are
very much essential for calibration of the model. Hence extensive monitoring
of dispersion of salinity at such sites would be helpful.
ACKNOWLEDGEMENTS
The authors express their deepest gratitude to Dr. A. R. Bhalerao Principal and Dr.
Mrs. V.S. Sohoni, H.O.D (Civil), Bharati Vidyapeeth University College of
Engineering, Pune, for their support and encouragement. The authors are also thankful
to authorities of CWPRS, Pune for their valuable cooperation for providing necessary
facilities and information.
REFERENCES
[1] Tsiourtis, N.X., Desalination and the environment, Desalination, 2001, 141: 223–
236.
[2] Purnama, A. and Al-Barwani, H. H., Spreading of brine waste discharges into the
Gulf of Oman, Desalination,2006, 195(1-3):26–31.
[3] Jenkins S, Paduan J., Roberts P. (Chair), Schlenk D. and Weis J. (Panel
Members), ‘Management of Brine Discharges to Coastal Waters
Recommendations of a Science Advisory Panel’ Technical Report 694, submitted
at the request of the California Water Resources Control Board by the Southern
California Coastal Water Research Project Costa Mesa, CA, 2012.
[4] Fischer, H.B., List E. J., Koh R. C. Y. and Imberger J., (1979). Mixing in Inland
and Coastal Waters. Academic Press. San Diego, CA, 1979.
[5] Danish Hydraulic Institute (DHI), Denmark, ‘MIKE 21 Flow Model,
Advection/Dispersion Module – User Guide, 2009.
[6] Eluozo. S. N and Ode T, Mathematical Model to Predict Compression Index of
Uniform Loose Sand in Coastal Area of Degema, Rivers State of Nigeria.
International Journal of Civil Engineering and Technology, 6(12), 2015, pp.86–
103.
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