NUMERICAL MODELLING OF BRINE DISPERSION IN SHALLOW … · the process of desalination is a effective and reliable way. Hence desalination ... Numerical modelling is a very much useful

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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

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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,


T. Nagendra

Scientist–E, Central Water & Power Research Station,


V. K. Shukla

Scientist–B, Central Water & Power Research Station,


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


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


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

https://en.wikipedia.org/wiki/Reverse_osmosis

https://en.wikipedia.org/wiki/Water_desalination

https://en.wikipedia.org/wiki/Kattupalli

https://en.wikipedia.org/wiki/Kattupalli



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)



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



Figure 1 Bathymetry in the model region

Figure 2 Flexible computational mesh



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.



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




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.



Figure 4A Contours of rise in salinity for intake at 550 m, outfall at 1000m from shore,

current =0.05 m/s southward



Figure 4B Contours of rise in salinity for intake at 550 m, outfall at 1300m from shore,

current =0.05 m/s southward



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



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



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.

Documents

NUMERICAL MODELLING OF BRINE DISPERSION IN SHALLOW … · the process of desalination is a effective and reliable way. Hence desalination ... Numerical modelling is a very much useful