circulation and mixing processes in chilika...

Indian Journal of Marine Sciences

Vol. 38(2), June 2009, pp. 205-214

Circulation and mixing processes in Chilika lagoon

P. K. Mohanty1 & B. U. S. Panda

Department of Marine Sciences, Berhampur University, Berhampur- 760 007, Orissa, India

[E-mail: [email protected]]

Received 11 March 2008; revised 21 October 2008

Chilika lagoon, along the east coast of India, is the largest brackish water lagoon in Asia. Hydrodynamics of an aquatic

system like Chilika is extremely important. It has significant impact on sediment and pollutant transport, distribution of

nutrients in water and sediments, and the productivity of the system. Data on salinity, temperature and wind procured during

summer and winter season have been utilized to demonstrate circulation and mixing processes in Chilika lagoon. Density

and wind driven circulation of this lagoon have been examined and depicted for the above period. Mixing of water masses

in the lagoon are studied using straight line as well as triangle mixing methods. The present study infers the inherent

seasonal variability in circulation (wind and density driven) and mixing pattern and also the variability due to opening of

new mouth in Chilika lagoon .

[Keywords: Chilika lagoon, water mass, circulation, mixing, salinity]

Introduction

Chilika lagoon is a shallow water body located in

the east coast of India. It have complete mixing of

water column in the vertical direction but

considerable horizontal gradients of hydrographic and

biological parameters. Therefore, studies on

circulation and mixing in Chilika is essential to

address the major ecological problems that results the

degradation of the lagoon ecosystem. The studies

regarding the above processes are a few 1-3

. The above

studies confines on distribution of current,

improvement of Chilika lake tidal inlet and transfer of

heat and momentum fluxes. Seasonal and interannual

variability in hydrographic parameters after the

opening of the new inlet mouth were studied4

from

2001 to 2004. The above study elucidates high

salinity values during summer. The present study

depict the wind and density driven circulation in

Chilika lagoon during summer and winter season. It

also delineates the proportions of mixing of water

masses between different sectors of Chilika lagoon.

Material and Methods

The Chilika lagoon (N 19° 28′-19° 54′; E 85° 06′-

85° 35′) on the east coast of India (Fig. 1) is separated

from the Bay of Bengal by a long stretch (~60 km) of

sand spit formed by wave and wind action. The water

area of the lagoon is variable from a maximum of 992

km2 during rainy season to 815 km

2 during summer

5.

The shallow (average depth 1.7 m) pear shaped

lagoon to the coastline, is about 65 km in length and

20.1 km (maximum) in width. Large inflow of fresh

water into the lagoon occurs through the tributaries of

Mahanadi river such as Daya, Luna, Ratnachira,

Bhargavi and Makara in north-eastern side; Kusumi,

Malagoni, Mangalajodi, Kansari, Kantabania etc. in

the western side and many water channels (Jora)

during monsoon and post-monsoon season. The

lagoon at present is connected with the Bay of Bengal

through an inlet mouth opened on 23 September,

—————— 1Corresponding author

Fig. 1—Chilika lagoon with its physical settings and four

different sectors.

INDIAN J. MAR. SCI., VOL. 38 NO. 2, JUNE 2009

206

2000 near Sipakuda along the outer channel, at a

distance of about 7.262 km from Satapara6. A new

inlet mouth at a distance of 1.06 km to the north of

the inlet mouth at Sipakuda opened on 1st August,

2008. It has a width of 236 m and depth of 3 m. The

old mouth at Arakhakuda, located approximately at a

distance of 24 km from Satapada, was closed during

the period November, 2003 due to prevalence of

strong northward long shore transport7. Therefore, the

number of inlet mouths, its position and configuration

play a vital role in deciding the flushing time scale

and intensity, salinity distribution and circulation and

mixing pattern in a coastal lagoon like Chilika.

Data on water temperature, salinity and wind fields

prior to the opening of the new mouth were collected

from the Chilika lagoon expeditions conducted by the

Estuarine Biological Station, Zoological Survey of

India, Berhampur8 during summer (11-06-1986 to

29-06-1986) and winter (22-11-1985 to 5-12-1985)

covering 71 and 45 stations respectively. Data after

the opening of the new mouth were collected by one

of the authors during May and December, 2006

covering 30 stations over the whole body of the

lagoon. Station locations covered during each survey

are presented in Fig. 2. A close matching (~80%) of

stations were observed between pre and post mouth

opening period. Rainfall data for Puri, one of the

nearest meteorological observatories to the north of

Chilika and close to the fresh water sources of the

lagoon, were collected from India Meteorological

Department. Rainfall data includes the monthly

rainfall for the years 1985-1986, 2006-2007 and the

monthly mean rainfall climatology (1961-2007).

Fresh water input to the lagoon is mostly from the

Mahanadi Delta and the Western Catchments, and the

data for 2006 were collected from the CDA. Data on

diurnal observations of wind speed, direction, salinity

and surface water temperature for summer and winter

were collected from a previous study9.

Surface water temperature was measured using

bucket method with a standard thermometer of 0.1 °C

accuracy. Salinity was measured using salinometer

and titrimetric method. Wind speed and direction

were observed on the deck of the boat at a height of

2 m from the water level using a portable anemometer

(Propeller type).

The limitation of the present study lies with

observation time which usually spanned from

morning to evening and thus the inherent temporal

variability in surface water temperature and wind

field from one station to the other. In order to

minimize the error due to above effect the station

observations were averaged over specific sector

(Table-1) and have been considered for computation

of density field.

The equation of state defined by the Joint Panel of

Oceanographic Table and Standards10

has been used

to compute the density field from the characteristic

temperature and salinity values. It may be mentioned

that the equation fits available measurements with a

Table 1: Salinity and Temperature in four different sectors of the Chilika lagoon

Pre-mouth Post-mouth

Summer Winter Summer Winter

T (0C) S (ppt) T (0C) S (ppt) T (0C) S (ppt) T (0C) S (ppt)

Southern sector 32.14 12.05 26.61 8.05 30.59 16.25 25.06 6.03

Central sector 29.90 16.41 25.2 6.35 30.09 22.59 23.92 3.08

Northern sector 26.72 14.41 25.26 2.46 30.32 16.99 23.37 0.57

Outer channel 27.03 18.36 24.85 2.34 29.63 35.07 24.17 3.57

T: Temperature (0C) and S: Salinity (ppt)

Fig. 2—Station locations covered during each survey in Chilika

lagoon.

MOHANTY & PANDA: CIRCULATION AND MIXING PROCESSES IN CHILIKA LAGOON

207

standard error of 3.5 ppm for pressure up to 1000

bars, for temperatures between freezing and 40 °C,

and for saltiness between 0 and 42 ppt11,12

. The

density ρ (kg m-3

) is computed in terms of pressure P

(bars), temperature T (°C) and salinity S (ppt)

following the equation of state.

The required density field (ρ) was obtained in a

sequence of steps as given below:

i. The density ρw of pure water was computed for

S=0, and for given T (°C).

ii. The density at one standard atmosphere

(effectively P=0) was computed with given S

(ppt) and T (°C ) and using the ρw value

computed in step (i).

iii. Then the value for sigma-t (σt) was obtained

following Gill12

as

σt = (ρ-1000)+0.025

Wind and density driven circulation were studied

by plotting the wind field and isopycnals

corresponding to the values at each grid points of

observation. Proportions of mixing of different water

masses were studied following Mamayev13

.

“Mixing” is a process where two or more water

masses mix together to give rise to a new water mass

having different density as compared to the original

water masses. The straight line mixing represents the

mixing of two water masses whereas the triangle

mixing (area) is necessary for the analysis of the

mixing of three water masses.

The present study is based on our observations of

temperature and salinity values in southern, central

and northern sectors and outer channel regions of the

Chilika lagoon (Fig. 1). Each of these is considered as

water mass and has characteristic T, S values which

show seasonal variation (Table 1). The movement of

these water masses could be advective or diffusive. In

order to assess the dominant transport processes, the

dimensionless Peclet ‘Pe’ number was estimated

following Zimmerman14

. It is observed data pe values

are <<1 for transport between the inlet and the

channel region, between the discharge points of river

water and the northern sector due to higher water

current (~ 50 cm/s) in the region. Thus, advective

mixing is the dominant process in these regions. As

compared to the inlet mouth, water current in the

three main sectors of the lagoon (1-5 cm s-1

) is very

low15

and the Pe number is <1. Thus the transport

between the three major sectors, viz. southern, central

and northern, is also advective. Therefore, it would be

appropriate to adopt the straight line and triangle

mixing methods to assess the proportions of mixing

of different water masses in Chilika lagoon.

Straight Line Mixing

A and B are two homogenous water masses in

different proportions (Fig. 3). Let the temperature and

salinity of the first water mass equal T1, S1 and of the

second water mass; T2 , S2. The temperature and

salinity of the resultant water mass O (T,S) was

determined by the formulae of mixing.

T=T1 m1 +T2 m2 … (1)

S=S1 m1 +S2 m2 … (2)

Where m1 and m2 are the proportions of the water

masses A and B respectively in the resultant water

mass.

In these equations the proportions are expressed in

parts of a unit, which implies

m1 + m2= 1 … (3)

However, in the present study the proportions m1 and

m2 have been determined for given values of T and S

and with the following two equations.

T1 m1 +T2 m2 = T … (4)

m1 + m2= 1 … (5)

The solution of equation 4 and 5 in the matrix

notation is ;

Fig. 3—Straight line mixing


208

2

1

2 1

1

11

T

m T

m T

− ∆ ∆

= − ∆ ∆

or;

[m1]=2 1

2,T T T T

m− −

= ∆ ∆

where ∆=1 2

1 1

T T

is the determinant of equations 4

and 5.

Triangle Mixing

In case of mixing of three homogenous water

masses A, B and C, having temperature and salinity

(T1 S1); (T2 S2) and (T3 S3) respectively, triangle

mixing can be considered (Fig. 4). The equations for

triangle mixing are:

T=T1 m1 +T2 m2+ T3 m3 … (6)

S=S1 m1 +S2 m2+ S3 m3 … (7)

m1 + m2 + m3 = 1 … (8)

Where m1, m2 and m3 are the proportions of the three

water masses A, B and C respectively.

In Figure 4, D, E, F represent the midpoints of the

line BC, AB and AC with co-ordinates

++=

2,

2

3231 TTSSD

++=

2,

2

2121 TTSSE

++=

2,

2

3131 TTSSF

The resultant water mass formed out of the mixing

of the three water masses is denoted by O. The

salinity and temperature of the resultant water masses

are respectively denoted as S and T.

The equations for straight lines AD and CE are

respectively given as

T – T1= M1 (S-S1), … (9)

T- T3 = M2 (S-S3) … (10)

Where M1 and M2 are slopes of the straight lines AD

and CE and can be calculated as

( )

+−

+−=

2/

2

321

3211

SSS

TTTM

+−

+−=

2/

2

)( 213

2132

SSS

TTTM

Using the two equations of straight line AD and

CE, we can find the value of S as

)/()]().([ 12113231 MMSMSMTTS −−+−= … (11)

)( 111 SSMTT −−= … (12)

For determination of the three unknowns m1, m2

and m3 (the proportion of mixing of the three water

masses) using the equation 6, 7, and 8, we have the

solution in matrix notation as:

( )

( ) ( )

( )

∆

−

∆

−−

∆

−∆

−−

∆

−

∆

−−

∆

−

∆

−−

∆

−

=

112212121

13313131

23323232

3

2

1

S

T

STSTTTSS

STSTTTSS

STSTTTSS

m

m

m

Where,

=∆

111

321

321

SSS

TTT

is the determinant of equation 6,

7, and 8

Fig. 4—Triangle mixing


209

Solving for the above matrix we can have

2 3 2 3 2 3 3 21

1 2 3 1 2 3 2 3 3 2

3 1 1 3 1 3 3 12

1 2 3 1 2 3 2 3 3 2

1 2 1 2 1 2 2 13

1 2 3 1 2 3 2 3 3 2

( ) ( ) ( )

( ) ( ) ( )

( ) ( ) ( )

( ) ( ) ( )

( ) ( ) ( )

( ) ( ) ( )

T S S S T T T S T Sm

T S S S T T T S T S


T S S S T T T S T S


T S S S T T T S T S

− − − + −=

− − − + −

− + − − −=

− − − + −

− − − + −=

− − − + −

Using the above formulations, proportions of mixing

of different water masses in Chilika lagoon were

determined both by straight line and triangle mixing

methods.

Results and Discussion

The results are presented under three sections, 1)

Wind Driven Circulation 2) Spatio-temporal

variability in salinity, temperature and density driven

circulation and 3) Mixing.

Wind driven circulation

Advection and diffusion are considered as two

important physical transport processes in coastal

lagoon. Locally induced wind drift currents in the

lagoons have direct effect on the advective transport

whereas the diffusive transport processes are mostly

due to tidal currents. While the wind drift produces

enhanced mixing in the lagoon, irregular bottom

topography and the presence of different islands

inside the body of the lagoon creates circulation cells.

Fig. 5 presents the wind-induced circulation in

Chilika lagoon. The wind fields depicted in the figure

correspond to the observations taken during day time

(0800 hrs-1700hrs), when the sea breeze plays a

dominant role.

Summer

During summer, winds are predominantly

southerly and southwesterly over the lagoon and

indicate the strong influence of sea breeze as the sea

is to the south-east of the lagoon. In order to

understand the wind drift in details we studied the

wind roses (Fig. 6).

In pre-mouth opening condition, 50% of the time

winds are southerly followed by south westerly

(25.7%) and south easterly (12.9%). Wind speed is

mostly in the range 5-10 and 10-15 ms-1

. The

percentage of wind observed from specific direction

has some bias due to our observation time limited to

8-17 hrs of the day. Because, the diurnal observation

of wind during June, 1987 shows that there is a

gradual shift in the wind direction from south-east in

the morning to west-southwest in the evening9 and the

range of diurnal wind speed is 1-6.8 ms-1

. In the post-

mouth opening condition, 45.5% of the time winds

Fig. 5—Wind driven circulation in Chilika lagoon (a) Pre-mouth

summer (b) Post- mouth summer (c) Pre-mouth winter and (d)

Post-mouth winter

Fig. 6—Wind rose diagram (a) Pre-mouth summer (b) Post-mouth

summer (c) Pre-mouth winter and d) Post-mouth winter. (The

direction is divided up into bins, and each bin contains separation

of the data into wind speeds. Therefore, the number of times wind

comes from a given direction and the speed of the wind is given in

each bin.)


210

are south-westerly followed by southerly (39.4%) and

south-easterly (12.1%). Wind speed during post-

mouth opening period are mostly in the range 0-5 ms-1

and 5-10 ms-1

. Low saline water in the pre-mouth

opening period are heated fast and enhance the wind

speed over the lagoon while in the post-mouth

opening period, relatively slower heating of the high

saline water body results in lower wind speed over

the lagoon. In summer, sea breeze generates wind

drift which mostly helps in the advection of water

from south to north before noon and from southwest

to northeast, the orientation of the lagoon, in the

afternoon. During summer, the circulation in the

lagoon is mostly controlled by the wind and the tidal

forcing. Wind forcing acts on the whole body of the

lagoon except in the outer channel, while tidal forcing

is limited within the inlet mouth, the outer channel

and to some extent in the central sector. Formation of

eddies in the central and northern sectors are evident

both in the pre and post mouth opening conditions.

Study with numerical experiments4 confirmed that

depth gradients in these sectors are responsible for

the observed eddies.

Winter

During winter, the climatology of wind is either

northerly or north easterly representing the north east

monsoon. However, in a coastal lagoon like Chilika,

where the land and sea breeze dominates, the change

in wind direction from summer to winter is very little.

In the pre mouth opening period, as in summer, the

winds are mostly from south (47.3%) and southwest

(28.6%) and the wind speed range between 5-10 and

10-15 ms-1

. However, wind speed during winter is

relatively less than summer and could be attributed to

the weaker land-sea heating contrast. Variability of

wind direction is large in the post-mouth opening

period. The winds are from south (32.4%), south-west

(24.3%), west (16.2%), east-southeast (13.55%) and

north east (10.8%). The wind speed is within the

range 0-5 ms-1

and is weaker than the summer and pre

mouth winter wind speed. During winter, the

circulation in the lagoon is controlled by wind, tide as

well as the freshwater influx. The impact of fresh

water influx on circulation and salinity variation is

discussed in the following section.

Spatio-temporal variability in salinity,

temperature and density driven circulation

Spatial differences in the heating and cooling of

water as well as salinity changes due to precipitation,

river inflow, evaporation and tidal influx cause density

differences in coastal lagoon that may produce

horizontal pressure gradients and currents. Besides,

current velocities and the rate of water exchange are

also functions of the size of the lagoon, its shape,

number and length of inlets14

. It is well known that in

tropical ocean both temperature and salinity changes

alter the water density. But in a tropical coastal lagoon

(e.g. Chilika) the situations are different due to strong

seasonal variability in the forcing functions such as

wind, tide and freshwater inflow. Therefore, it entails a

discussion on temperature and salinity prior to the

discussion on density.

Seasonal variation in temperature and salinity

during post-mouth opening condition are presented in

Fig. 7. Summer temperature is more than winter

temperature in all the sectors of the lagoon. However,

the difference in temperature is more pronounced in

the northern, central and outer channel (Table. 1 and

Fig. 7). This difference arises due to the freshwater

input in the northern and western sectors of the

lagoon through rivers and canal systems mostly

during monsoon through winter. The freshwater input

to the lagoon starts in the month of June, attains the

peak during August and declines thereafter to reach

the minimum in December. From January to May

there is no input of fresh water into the lagoon (Fig.

8). The influx of fresh water and its subsequent

passage to the sea through central and outer cannel,

which commences from mid monsoon months and

continues till December, significantly reduces the

Fig. 7—Surface water temperature (°C) and salinity (ppt) in

Chilika lagoon during 2006 a) Summer temperature, b) Winter

temperature, c) Summer salinity and d) Winter salinity


211

salinity of the lagoon during winter as compared to

summer (Fig. 7). Furthermore, due to the discharge of

fresh water from the lagoon to the sea, the ebb current

dominates over the flood current at the inlet mouth15

.

As a result, the tidal influx into the lagoon is weaker

and hence a general reduction in salinity of the lagoon

is observed during winter (Fig. 7). On the other hand,

during summer absence of fresh water discharge

results in greater tidal influx through the inlet mouth

and thereby enhances the salinity of the lagoon. The

evaporation is distinctly more during summer than

during winter7 and also contributes to the relatively

higher salinity during summer. The seasonal

variability in fresh water influx is associated with the

rainfall locally and also in the catchments of the

rivers. Rainfall records of Puri (Fig. 9), the nearest

meteorological station on the northern side of the

lagoon and close to the sources of fresh water input,

during pre (May, 1986; December, 1987) and post

(May and December, 2006) mouth opening periods

corroborates the above view. Besides, the rainfall

anomalies and their interannual variability observed

during pre and post mouth opening conditions

justifies the comparison of climate and the circulation

and mixing processes in the pre and post mouth

opening periods.

The annual cycle of fresh water input (Fig. 8) into

the lagoon displays a net positive water balance from

monsoon16

to winter season. Further, an estimate of

the residence time of the water in the lagoon16

indicates that it is shorter during monsoon season and

longer during post-monsoon and winter seasons.

Therefore, the net positive balance of water and its

residence time, which, besides controlling the mixing

pattern, decides the intensity and discharge of water

from lagoon to the sea through the inlet mouth. The

pathway for the discharge of water is from the north

and western sectors (close to the input of freshwater)

to the sea through the eastern part of the central

sector (Muggarmukha area), the outer channel and the

inlet mouth and thus the regions experience

significant reduction in salinity as compared to other

sectors of the lagoon.

Density in summer

Following the salinity distribution, the density

during pre-mouth summer is less than the post-mouth

summer (Fig. 10). The density maxima lies in the

central sector followed by the northern and southern

sectors and also in line with the salinity distribution

Table 1: Salinity and Temperature in four different sectors of the Chilika lagoon


Summer Winter Summer Winter

T (0C) S (ppt) T (0C) S (ppt) T (0C) S (ppt) T (0C) S (ppt)

Southern sector 32.14 12.05 26.61 8.05 30.59 16.25 25.06 6.03

Central sector 29.90 16.41 25.2 6.35 30.09 22.59 23.92 3.08

Northern sector 26.72 14.41 25.26 2.46 30.32 16.99 23.37 0.57

Outer channel 27.03 18.36 24.85 2.34 29.63 35.07 24.17 3.57

T: Temperature (0C) and S: Salinity (ppt)

Fig. 8—Freshwater input (Million Cu M) to the Chilika lagoon

during 2006.

Fig. 9—Rainfall (mm) climatology and anomalies during 1985-86

and 2006-2007 of Puri, the nearest meteorological station to

Chilika lagoon.


212

in the three main sectors of the lagoon. Thus, the

density gradient from central to northern sector is in

the direction of the wind drift whereas from central to

southern sector it is in the opposite direction to the

wind drift. This opposite pattern of circulation in

wind and density between central and southern sector

and similar pattern between central and northern

sector is bound to influence the advection and mixing

pattern and is discussed in the following section.

Salinity in the outer channel is highest both during

pre and post-mouth summer as compared to other

sectors. Therefore, the density gradient exists from

outer channel to central sector. Advection and mixing

of water from(to) outer channel to(from) central

sector and to(from) other sectors of the lagoon,

which, besides being controlled by the density

current, is predominantly influenced by the tidal

forcing under flood (ebb) condition in the absence of

freshwater input during summer. As the ebb current is

stronger than flood current even during summer4 , it

helps to create a net advective transport from the

central sector to the outer channel and facilitates

better discharge of water than tidal influx.

Density in winter

Spatial variability pattern in salinity during winter

is different than summer with highest salinity in the

southern sector and lowest in the northern and outer

channel. The magnitude of salinity during winter is

also much less as compared to summer in all sectors

(Fig. 7). The severe reduction in salinity values and

the change in spatial distributional pattern during

winter are due to fresh water input which acts as an

important forcing concurrent with wind and tidal

forcing. Low salinity gradient exists from southern to

central and from central to northern sector both

during pre and post mouth opening conditions.

Therefore, the density distribution also follows the

above salinity gradients. Although the wind drift

during winter is weaker than summer, it is in the same

direction as the density gradient for the three main

sectors and hence helps in the advection of water

from south to north and the mixing process. However,

the freshwater input in the north-western sector

further complicates the mixing pattern with advective

transport from northern to central, and from central to

outer channel. In order to quantify the proportions of

mixing of different water masses, both straight line

and triangle mixing methods were employed and the

results are discussed in the following section.

Mixing Process

Table-2 represents the results of mixing of

different water masses for different seasons in pre and

post mouth opening periods.

Summer

Considering the straight line mixing, proportions

(%) of mixing of water masses between different

sectors are examined. Between southern and central

sectors, where the wind drift and density gradients are

opposite to each other, proportions of water masses of

Fig. 10—Density field (kg m-3) in Chilika lagoon (a) Pre-mouth

summer (b) Post-mouth summer (c) Pre-mouth winter and d)

Post-mouth winter.

Table 2: Proportions (%) of mixing of water masses in different

sectors of Chilika lagoon


Straight line Summer Winter Summer Winter

mixing

Southern Sector 48.4 35.0 40.4 41.2

Central Sector 51.6 65.0 59.6 58.8

Central Sector 72.7 54.2 50.6 50.1

Northern Sector 27.3 45.8 49.4 49.9

Central Sector 69.6 74.4 77.0 76.9

Outer Channel 30.4 25.6 23.0 23.1

Triangle Mixing

Northern Sector 20.7 38.0 42.6 43.5

Central Sector 55.2 46.6 44.7 43.4

Outer Channel 24.1 15.4 12.7 13.1


213

central sector are more than the southern sector

suggesting the predominant role of salinity gradient

(density current) over wind drift. However, higher

wind drift during pre-mouth than post-mouth period

results in higher proportion of mixing of southern

sector water mass during pre-mouth period. Between

central and northern sectors, the density and wind

drifts are in the same direction and hence contribution

of central sector water mass is maximum. Again, due

to higher wind speed during pre-mouth, the

proportion of mixing of central sector water mass is

more than those during post mouth period. Numerical

experiments with wind alone and tide alone15

suggest

that wind forcing has effect only in the main body of

the lagoon while tidal forcing has effect only in the

channel area. Therefore, for mixing of water between

central sector and outer channel, tide is the only

forcing during summer. It is observed that the

proportion of mixing of central sector water mass is

more both during pre and post mouth opening period.

In the triangle mixing also, the contribution of central

sector water mass is dominant as compared to

northern sector and outer channel. These results

suggest that during summer, when there is no fresh

water input, ebb current is stronger than flood and

allows a greater proportions of central sector water

mass to mix with outer channel. In the post-mouth

opening period, contributions of central and northern

sectors are respectively enhanced in the straight line

and triangle mixing while the contribution of outer

channel is reduced both in the straight line and

triangle mixing. The above results further indicate

that the impact of new inlet mouth is more on

discharge of water from the lagoon to the sea than on

tidal influx. This could be due to the gradient in depth

that exists from the central sector, through outer

channel to the inlet mouth, which built up a pressure

head in the flood phase and generates a stronger slope

current during ebb phase.

Winter

As in summer, the contribution of central sector is

maximum as compared to other sectors both in the

straight-line and triangle mixing. Further, due to fresh

water input in the north-western sector, the additional

forcing during winter, the contributions of northern

sector is distinctly more during winter than in summer

in the pre-mouth opening condition. It is also

apparent that the new inlet mouth has minimized the

seasonal variability in freshwater discharge and

facilitated better exchange of water between the

lagoon and the sea due to reduction in the length of

the outer channel by about 17 km.

Conclusions

Circulation in Chilika, a coastal lagoon, has

distinct seasonality. During summer, wind and tide

are the important forcing controlling the circulation

and salinity distribution, while in winter, wind, tide

and fresh water input control the circulation and

mixing pattern. However, wind driven circulation in

Chilika lagoon is mostly due to sea breeze which

predominantly blows from south and southwest both

during summer and winter. The stronger land -sea

heating contrast during summer results in relatively

stronger wind speed than in winter. The seasonal and

interannual variability in salinity, which, besides

being controlled by evaporation and tidal influx, is

predominantly influenced by the fresh water input.

Due to absence or negligible fresh water input during

summer, the salinity in all the sectors is remarkably

more in summer than in winter. Gradients in density

follow the gradients in salinity. During summer, the

gradient in wind drift and density are in the same

direction between central and northern sector, while

they are opposite between southern and central sector.

However, during winter, both wind drift and density

gradients are in the same direction between southern

to central and central to northern sectors. Fresh water

input during winter, mostly in the northern sector, is

opposite to the direction of wind drift and density

current. These seasonally varying wind drift and

density currents and their spatial distributional pattern

influence the mixing process significantly. Both in

straight line and triangle mixing, contribution of

central sector is predominantly more as compared to

other sectors. Advection and mixing of water between

central sector and outer channel are mostly due to

tidal forcing during summer and due to both tidal

influx and fresh water discharge during winter. The

mixing of central sector water mass in greater

proportions as compared to outer channel in both the

season confirms that ebb current is stronger than

flood in Chilika lagoon. The proportion of mixing and

their variation between pre and post mouth opening

period infers that the impact of the new inlet mouth is

more on discharge of water from the lagoon to the sea

than on tidal influx. Thus, it is evident that in addition


214

to the external boundaries, the existence of other

factors such as different water masses, shallow and

deep, fresh and salt, aerobic and anaerobic, etc. lead

to different gradients in Chilika lagoon. The nature

and number of these gradients may make the lagoon

ecosystem more productive and therefore should be

maintained at all cost. Continuous monitoring of

water quality, hydrology, wind, currents and tides is

essential to accomplish the same.

Acknowledgement

Authors are thankful to the Chilika Development

Authority for sharing the river discharge data One of

the authors (PKM) wish to thank the Estuarine

Biological Station (ZSI), Berhampur for providing an

opportunity to participate in the Chilika lagoon

expedition conducted during 1985 and 1986. The

author (USP) acknowledges the financial assistance

offered by the Council of Scientific and Industrial

Research (CSIR), Government of India in terms of

Senior Research Fellowship.

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