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Numerical Simulation of Circulation and Salinity Structure in Chilika Lagoon Girija Jayaraman†, Amabarukhana D. Rao†, Anumeha Dube†, and Pratap K. Mohanty‡ Centre for Atmospheric Sciences Indian Institute of Technology Hauz Khas New Delhi 110016, India
[email protected] †
Department of Marine Sciences Berhampur University Berhampur 760 007, India ‡
ABSTRACT JAYARAMAN, G.; RAO, A.D.; DUBE, A., and MOHANTY P.K. 2005. Numerical simulation of circulation and salinity structure in Chilika Lagoon. Journal of Coastal Research, 22(0), 000–000. West Palm Beach (Florida), ISSN 07490208. Chilika Lagoon (198289–198549N, 858069–858369E), on the Orissa coast, India, is one of the world’s unique ecospheres. It is the largest brackish water lagoon with estuarine character. Because of its rich biodiversity and socioeconomic importance, it was designated as a ‘‘Ramsar site’’—a wetland of international importance—in 1981. Interest in detailed analysis of the circulation, biotic, and abiotic factors affecting the lagoon and its limnology is a result of the opening of the new mouth on 23 September 2000 to resolve the threat to its environment from various factors—eutrophication, weed proliferation, siltation, industrial pollution, and depletion of bioresources. This paper describes the development of a two-dimensional depth-averaged hydrodynamic model for Chilika Lagoon and the resulting simulation of currents and salinity corresponding to (i) the Southwest and Northeast monsoon seasons and (ii) pre- and post–mouth-opening conditions. Numerical experiments are performed to understand the different factors—wind, tides and freshwater influx—and their independent and collective roles. Comparison of results obtained with one or both of the openings is made to quantify and validate the changing salinity pattern, which has improved the productivity of the lagoon. Our study shows the salinity levels to be much lower during the southwest monsoon compared to the northeast monsoon, which is validated by observations. The decrease in salinity is attributed to more freshwater influx during the southwest monsoon. There is a significant increase (14%–66%, depending on the sector) in salinity after opening of the new mouth, the maximum change being observed in the channel that connects the lagoon to the sea. The constriction in the lagoon that blocks the tidal effects entering the lagoon is found to be responsible for maintaining the main body of the lagoon with low salinity. The dynamic model will be an important input for our on going work in seasonal study of the ecology of Chilika Lagoon. ADDITIONAL INDEX WORDS: Lagoon dynamics, hydrodynamic model, depth averaging.
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INTRODUCTION Wetlands play a central role in regional hydrologic and biogeochemical cycles, in maintaining biodiversity, and in a wide range of human activities. Over the past two centuries, industrialization, urbanization, and deforestation have led to wetland loss, resulting in the extinction of countless species and the alteration of the relationship of wetlands with the regional environment. The present study is concerned with the seasonal circulation and salinity structures in the Chilika Lagoon (198289– 198549N, 858069–858369E) on the east coast of India, the largest brackish water tropical lagoon with estuarine character in Asia (Figure 1). The interest and driving force in studying the ecology of the lagoon comes from increasing threats to the lagoon by siltation, choking of the mouth connecting the lagoon to the sea, eutrophication, weed infestation, salinity changes, and a decrease in fishery resources. The lagoon was added to the list of Ramsar sites in danger (the Montreux DOI:10.2112/04-0225R.1 received 17 May 2004; accepted in revision 8 April 2005.
record in 1993). Following some years of innovative and exemplary remedial efforts by the Indian government, the ecological character of the lagoon was restored, and it was removed from the Montreux record on 11 November 2002. In the next section, a brief description of the analysis area is presented. This is followed by the mathematical equations pertaining to our study domain, which are solved using a finite difference scheme with a staggered C-grid. Field observations and data required to validate the model are given in the following section. Finally, the results, based on several numerical experiments, are analyzed, and important conclusions are presented. The overall objective is to understand the dynamics of the lagoon corresponding to (i) the southwest monsoon (SWM) and the northeast monsoon (NEM) seasons and, (ii) pre- and post–new-mouth-opening conditions, which will add to our initiative to improve the lagoon and its ecosystem.
DESCRIPTION OF STUDY AREA The water-spread area of the Chilika Lagoon varies from 1165 to 906 km2 during the monsoon and summer respec-
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Figure 1. Map of Chilika Lagoon showing different sectors.
tively (SIDDIQI and RAO, 1995). A significant part of the freshwater and silt input to the lagoon comes from the Mahanadi river and its distributaries (MOHANTY et al., 1996). Direct rainfall on the lagoon surface also makes a significant contribution to the freshwater input to Chilika. There is an overall increase in depth of about 0.5–1 m because of the SWM. The lagoon area experiences mean wind speeds ranging from 2 m/s in winter to 3–5 m/s in summer. The coastal areas experience higher average wind speeds of up to 7 m/s (CHANDRAMOHAN, PATTNAIK, and JENA, 1998; NAYAK et al., 1998). The seasonal mode of variation in Chilika is dominant over the interannual mode (PAL and MOHANTY, 2002). Based on its physical and dynamic characteristics, the lagoon is divided into four sectors (Figure 1). The northern sector receives discharge of the floodwaters from the rivers. The southern sector is relatively smaller and does not show much seasonal variation in salinity. The central sector has features intermediate between the features of the other sectors. The
lagoon is separated from the Bay of Bengal by a sand bar 60 km in length. The eastern sector of the lagoon comprises of a 24-km narrow and curved channel that runs parallel to the coast to join the Bay of Bengal near Arakhakuda. Tidal effects are important in this area. This inlet was the only connection of Chilika Lagoon with the Bay of Bengal until September 2000. The width of this inlet is about 1.5 km. High tides near the inlet mouth drive in salt water through the channel into the lagoon. A distinct salinity gradient exists along the lagoon because of the influx of fresh water during monsoons and the inflow of seawater through the outer channel. The position of the inlet controls the current and salinity patterns in the lagoon and its size determines the amount of water and salt exchanged with the Bay of Bengal. Because of choking of the outer channel and northward shifting of the inlet mouth (CHANDRAMOHAN and NAYAK, 1994) exchange of water and sediment between the lagoon and the sea was affected. To preserve the rich biodiversity of
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Figure 2. Representation of the boundary by orthogonal stair steps.
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Chilika, the Indian government, through the Chilika Development Authority (CDA), initiated many conservation measures, and the opening of the inlet mouth near Sipakuda on 23 September 2000 is considered one of the major accomplishments (CHILIKA DEVELOPMENT AUTHORITY, year). The new inlet mouth is also situated in the northeastern end of the lagoon, and the distance between the old and the new inlet mouths is approximately 17 km along the coastline (Figure 1). Opening of the new mouth has significantly changed the lagoon environment (CHILKA, 2001), and therefore it is essential to compare the pre- and post–mouth-opening conditions, particularly the changes in circulation patterns and salinity structures. The conclusions, based on a systematic
study of the dynamics, salinity, and ecology, should help in analyzing whether the significant improvement found in the productivity of the lagoon, is, indeed, sustainable.
MATHEMATICAL FORMULATION We use a system of rectangular Cartesian coordinates in which the origin O is within the equilibrium level of the sea surface, Ox points toward the east, Oy points toward the north and Oz is directed vertically upwards (Figure 2). Let (u, v, w) be the Reynolds averaged components of velocity in the directions of x, y, and z respectively; f (5 5 3 1025), the Coriolis parameter; g (5 9.81 ms22), the acceleration caused
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Table 1. Summary of hydrographic parameters in Chilika Lagoon. Values in parentheses indicate ranges. Northern
Central
Southern
0.95 (0.3–1.6) 0.85 (0.4–1.3)
2.2 (1.6–2.8) 6.9 (4.6–9.2)
2.4 (1.5–3.3) 9.6 (8.0–11.2)
2.0 (1.0–3.1) 66.9 (42–88) 5.6 (0.4–12.7)
2.5 (1.0–3.6) 57 (0–98) 18.0 (12.7–20.2)
July (pre–mouth-opening) Depth (m) Salinity (ppt) July (post–mouth-opening) Depth (m) Surface water current (cm/s) Salinity (ppt)
1.7 (1.1–3.4) 65.2 (33–117) 0.9 (0.4–4.4)
by gravity, r (5 1.025 3 103 kg•m23), the density of the water supposed to be homogeneous and incompressible; t, time, S, salinity; and Kx and Ky the kinematic eddy diffusivities in the x and y directions respectively. (FS, GS) and (FB, GB) are the x and y components of the surface wind and bottom stress respectively. z 5 z(x, y, t) gives the displaced position of the free surface and the position of the sea floor is given by z 5 2h(x, y). The ratio of the fractional change in density for a unit change in salinity is given by b (5 6.5 3 1024). The lagoon is shallow (2.5 m average depth) and there are considerable horizontal gradients of hydrographic parameters. Hence, it is reasonable to assume that the wavelength is large compared to the depth and to use the hydrostatic approximation for vertical momentum equation. Further, the shallow water approximations lead to a two-dimensional depth-averaged model. We give below the basic equations describing the dynamics and salinity distribution of the shallow water body:
]z ]u˜ ]v˜ 1 1 50 ]t ]x ]y
(1)
]u˜ ] ] 1 (u˜ u¯ ) 1 (u˜ v¯ ) 2 fv˜ ]t ]x ]y
[
]
(2)
]v˜ ] ] 1 (v˜ u¯ ) 1 (v˜ v¯ ) 1 fu˜ ]t ]x ]y
[
5
[
] [
]
] ]S¯ ] ]S¯ Kx (z 1 h) 1 Ky (z 1 h) ]x ]x ]y ]y
(u¯ , v¯ ) 5 S¯ 5
1 (z 1 h) 1 (z 1 h)
E E
z
(u, v) dz and
2h z
S dz
(6)
2h
Difference in the atmospheric pressure at the water surface can give rise to water level variations. However, in the analysis area being modeled here, the forcing attributable to the barometric pressure gradient is insignificant and is therefore not included in the governing equations. A parameterization of the bottom stress is made by the conventional quadratic law
]
(3)
]p 5 2rg (Hydrostatic Pressure Approximation) ]z
(4)
where cf 5 6.25 3 1023 is an empirical bottom friction coefficient. Surface shear stress caused by wind is usually computed using a bulk aerodynamic formula
where CD is the drag coefficient, ra is the density of air, and (ua, va) are the wind velocity components in the x and y directions, respectively. Values of ra and CD are taken as 1.176 kg•m23 and 1.125 3 1023, respectively. The real boundary of Chilika Lagoon is approximated as
Table 2. Summary of hydrographic parameters in Chilika Lagoon. Values in parentheses indicate ranges. Northern
Central
Southern
0.8 (0.2–1.4) 3.75 (0.65–6.85)
1.7 (0.7–2.6) 8.46 (7.84–9.08)
2.1 (1.2–3.0) 8.95 (8.39–9.42)
0.6 (0.6–0.7) 3.7 (0.00–25.0) 5.2 (4.9–5.5)
1.7 (0.7–3.0) 1.3 (0.00–5.4) 5.7 (4.4–7.7)
2.2 (1.9–2.4) 4.3 (2.0–7.7) 7.4 (6.8–7.8)
January (pre–mouth-opening)
January (post–mouth-opening) Depth (m) Surface water current (cm/s) Salinity (ppt)
(7)
FS 5 CDra ua (ua2 1 va2 )1/ 2 and GS 5 CDra va (ua2 1 va2 )1/ 2 (8)
]z 1 ]S¯ 1 1 b(z 1 h) 1 [GS 2 GB ] ]y 2 ]y r
Depth (m) Salinity (ppt)
(5)
where u˜ 5 (z 1 h)u and v¯ 5 (z 1 h)v˜ are new prognostic variables, (z 1 h) gives the total depth of the water column, and overbars denote depth-averaged values given by
FB 5 rcf u¯ (u¯ 2 1 v¯ 2 )1/ 2 and GB 5 rcf v¯ (u¯ 2 1 v¯ 2 )1/ 2
]z 1 ]S¯ 1 5 2g(z 1 h) 1 b(z 1 h) 1 [FS 2 FB ] ]x 2 ]x r
5 2g(z 1 h)
¯ ¯ ¯ ][(z 1 h)S] ](u˜ S) ](v˜ S) 1 1 ]t ]x ]y
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Figure 3. Bottom topography (m) for Chilika Lagoon
closely as possible by a stair-step boundary (Figure 2). The number of grid points in the x and y directions are 81 and 68 respectively, and hence Dx 5 Dy 5 approximately 0.75 km. With this grid specification, it was found that computational stability could be maintained with a time step of 60 seconds. The coastal boundary in the model is taken to be a vertical/ horizontal sidewall through which there is no flux of water except for the two open channels connecting the lagoon to the sea.
specified for the salinity equation. Along with the normal transport, the diffusive flux of salinity at the lateral boundaries must vanish, that is,
]S 50 ]n
where n is the unit normal to the lateral boundary. At the tidal inlets, a sinusoidal tide is prescribed and is given by
z(t) 5 a cos
Boundary and Initial Conditions The motion in the lagoon is generated from an initial state of rest, i.e., z 5 u 5 v 5 0 at t 5 0. It is also assumed that initially the lagoon has only fresh water, so that S 5 0. Theoretically, the only boundary condition needed in the vertically integrated system is that the normal transport vanish at the coast, i.e., u cos a 1 v sin a 5 0 for all t $ 0
(9)
where a denotes the inclination of the outward-directed normal to the x axis. It then follows that u 5 0 along the ydirected boundaries and v 5 0 along the x-directed boundaries. In addition to these boundary conditions for the hydrodynamic model, appropriate boundary conditions have to be
(10)
1T2 2pt
(11)
where a is the tidal amplitude and T is the time period of the M2 tide, which is equal to 12.4 hours approximately. The two tidal inlets of the lagoon are very close to each other (17 km apart); therefore, the tides at both the tidal openings are considered to be in phase and having the same amplitude. The freshwater flux is provided at a vertical sidewall opening in the solid boundary in the northernmost part of Chilika (Figure 2). The freshwater flux at this opening is provided in terms of the velocity of fresh water entering into the Chilika basin from the rivers; this is calculated by
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u(x, y) 5
q h(x, y¯ ) 3 L
(12)
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Figure 4. (a) Circulation caused only by wind forcing in July. (b) Flood tidal current (old inlet). (c): Ebb tidal current (old inlet). (d) Circulation caused by freshwater influx in July.
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Figure 4. (e) Flood current as a result of July wind, tide, and freshwater influx. (f) Ebb current as a result of July wind, tide, and freshwater influx. (g): Mean circulation as a result of July wind, tide, and freshwater influx.
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Figure 5. (a) Flood current as a result of July wind, tide, and freshwater influx (both inlets). (b) Ebb current as a result of July wind, tide, and freshwater influx (both inlets). (c) Mean circulation as a result of July wind, tide, and freshwater influx (both inlets).
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Figure 6. Horizontal salinity in ppt as a result of July wind, tide and freshwater influx (old inlet).
where q is the amount of fresh water in cubic meters entering into the Chilika basin obtained by observations made during the SWM and L is the breadth of the inlet. For the solution of Equations (1–5), subject to the boundary conditions, a finite difference scheme with a staggered C-grid is used (MESINGER and ARAKAWA, 1976).
OBSERVATIONS Observations on hydrographic parameters such as depth, salinity, current, and tidal variation were carried out during SWM and NEM, covering about 33 to 37 stations in the body of the lagoon after the opening of the new mouth. Sporadic information is available on current distribution and its seasonal variation in Chilika Lagoon in the pre–mouth-opening period. RAMNADHAN, REDDY, and MURTY (1964) observed the seasonal variation of current in the lagoon during 1960– 61, i.e., long before the opening of the new inlet mouth in September 2000. For the present study, observations before the opening of the new tidal inlet were obtained from MOHANTY, PAL, and MISHRA (2001). Methodology for observation and analysis of most of the hydrographic parameters was adopted from PARSONS et al. (1984). The observations are given in Tables 1 and 2 with more details in the following:
Depth Depth of water in the lagoon shows wide spatial and temporal variability. Because of freshwater influx and heavy precipitation during SWM, the water levels in the lagoon are higher in SWM as compared to NEM. The model bathymetry for NEM (Figure 3) is generated based on our spot observations made during January 2002. The corresponding bathymetry for SWM is generated by increasing the depth by 1 m. The northern sector is observed as the shallowest region in Chilika Lagoon, with depths ranging from a low of 0.2 m near the northernmost regions to about 1.5 m toward the central sector (1.5–2.5 m). The southern sector, at 2.5–3.5 m, is deeper than both the central and northern sectors. The channel, however, is the deepest region in Chilika Lagoon at 1–4.5 m.
Surface Water Current Water current in the lagoon was measured using a digital EMCON current meter in the post–mouth-opening conditions. During NEM, magnitudes of current velocity are much lower in all sectors as compared to the SWM. Maximum velocity was observed in the channel (tidal effect) and near the northern sector (river water flux). Strengths of flood (from
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Figure 7. Horizontal salinity in ppt as a result of July wind, tide, and freshwater influx (both inlets).
sea to lagoon) and ebb (from lagoon to sea) currents were examined over a diurnal cycle during the NEM season near the new inlet mouth. It was observed that the peak flood current was 50 cm/s, whereas the peak current during ebb was 75 cm/s. Tidal variations were examined at three important points: the Sipakuda, Satpara, and Magarmukh (Figure 1). During January, the maximum tidal variation was observed as 0.76 m near Sipakuda; tidal velocity gradually declined toward Satpara (0.37 m) and Magarmukh (0.25 m). These values were slightly lower during July.
Salinity Salinity variation in the lagoon is significant, ranging from almost fresh water (0.2 ppt) to slightly less than saline water Table 3. Comparison of observed and computed salinity values for July.
Salinity (ppt)
Pre–mouthopening (observed)
Pre–mouthopening (simulated)
Post–mouthopening (observed)
Post–mouthopening (simulated)
Northern Central Southern
(0.4–1.3) (4.6–9.2) (8.0–11.2)
(1–4) (4–10) (4–6)
(0.4–4.4) (0.4–12.7) (12.7–20.2)
(0.5–6) (1–14) (6–10)
(22 ppt) in the pre–mouth-opening condition and from 0.4 to 33 ppt in the post–mouth-opening condition during summer. This increase in salinity because of the new inlet is reduced during SWM because of the influx of fresh water into the lagoon during that period. Differences between the observed salinity values at the surface and bottom are negligible (PANDA et al., 1989). The observed salinity concentration in the outer channel is high, but in the northern sector, which receives fresh water from Mahanadi river system, the salinity is almost zero during SWM. The salinity gradient is lower in the southern and central sectors and increases gradually in the direction of the outer channel (JNANENDRA and ADHIKARY, 2003). Besides spatial variability, the temporal variability of salinity associated with seasonal cycling of freshwater and tidal input is also significant. The impact of the new inlet mouth on salinity variation is very significant in the central sector and the channel.
RESULTS AND DISCUSSION The two-dimensional depth-averaged model is used to obtain information on the response of Chilika waters to (i) wind forcing and (ii) tidal effects. The model is forced by the cli-
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matological mean winds representative of the months of July and January (chosen as the representative months for SWM and NEM respectively). During July the direction is almost SW (2008) with a uniform speed of 6.5 m/s and for January the direction of the wind is almost NE (408) with a uniform speed of 2 m/s over the entire lagoon area (HASTENRATH and LAMB, 1979). At the open sea boundary, a sinusoidal tide of amplitude a 5 0.56 m and a salinity of 33 ppt are considered (NAYAK et al., 1998). The values of Kx and Ky, the eddy diffusivities for salinity in the x and y directions, have been calculated from the observed current velocities available at different locations in the lagoon. In the model, an average value of 600 m2 s21 has been used for both Kx and Ky. In addition, fresh water influx from the rivers draining into the northeast region of Chilika Basin is also included in the model for SWM. Based on our observations, the total inflow during the SWM (q) was calculated to be about 3300 million m3. Computational steady state was achieved in 25 tidal cycles and the results discussed below were obtained after steadystate solution was reached. In the following section, the results are discussed separately for July (SWM) and January (NEM), corresponding to (i) pre–mouth-opening and (ii) post–mouth-opening conditions. Comparisons are later made and conclusions drawn in the next section for (i) the two different seasons and (ii) preand post–mouth-opening conditions.
Southwest Monsoon Circulation (Pre–Mouth-Opening) Circulation in Chilika is affected by three main factors during the SWM: wind, tide, and freshwater influx. Numerical experiments were carried out with these forcing factors individually and collectively in order to analyze their effect on the circulation. Also, results were analyzed separately for circulation corresponding to flood, ebb, and mean, where the mean was taken over a tidal period after the steady state was achieved. Wind Effect. Figure 4a shows the wind-driven circulation in the lagoon, which is found to be aligned with the wind direction. Eddy formation is observed in the central and northern sectors. These eddies rotate in a clockwise direction and are connected to each other. Numerical experiments performed with uniform depth failed to show this eddy formation. Hence, it can be concluded that the eddies are formed because of the depth gradient in these regions (Figure 3), implying that bottom topography plays an important role in determining the circulation within the main body of the lagoon. The maximum current velocity is near Satpara Island (10 cm/s); current velocity is much lower in the other sectors (1–3 cm/s in the northern sector, 3–6 cm/s in the central sector, and 2–4 cm/s in the southern sector). There is almost no circulation in the channel as we move toward the inlet. A reason for this could be the shallowness of the channel near the inlet, which can obstruct the flow in the channel. Pure Tidal Effect. Figures 4b and 4c show the flood and the ebb currents in Chilika respectively. The peak flood current velocity is around 45 cm/s, whereas the peak ebb current velocity is about 65 cm/s, showing that the ebb current is stron-
ger than the flood current. The maximum velocity in both cases is seen only in the channel region; in the rest of the lagoon the velocity in response to tidal forcing is low. In order to distinguish between the circulations in the channel and the main body of the lagoon, the channel has been separately plotted in Figures 4b and 4c and is depicted by 1 and 2. The current velocity is low in the northern sector and in parts of central sector adjoining the northern sector during both flood tide (1 cm/s) and ebb tide (1–5 cm/s). Regions of the central sector adjoining the channel show a higher velocity range (5– 15 cm/s). Freshwater Effect. The freshwater flux is provided in the model in the form of the velocity of the fresh water entering into the lagoon. This velocity is calculated from the freshwater flux q (5 3300 million m3) observed during SWM (Equation [12]). Figure 4d shows the response exclusively to freshwater influx. The effect of fresh water is felt mainly in the northern and the central sectors of the lagoon. The maximum current velocity is 50 cm/s very near the freshwater inlet. This velocity decreases from its maximum value as we proceed further into the northern sector and has a range of 10–15 cm/s. In the main body of the lagoon, the flow is seen to be northeasterly; this is caused by the forcing from the fresh water entering from the northern sector. Combined Effects of Wind, Tide, and Fresh Water. Figures 4e–4g show the trends corresponding to flood, ebb, and mean circulation respectively. The mean profile suggests ebb current is stronger than flood current—a result confirmed by flood and ebb profiles (peak flood current velocity is 45 cm/s and peak ebb current velocity is 55 cm/s). The currents are almost uniform (1–5 cm/s) in the main body of the lagoon. The northern sector, which has freshwater flux (10–45 cm/s), and the channel, which is significantly influenced by the tides (10–15 cm/s), show distinct circulation characteristics. In the northern sector, the flow is being driven toward the central sector, which allows the mixing of fresh water with the saline water present there. In the mean profile, the maximum velocity in the channel region near the inlet length is 15 cm/s.
Circulation (Post–Mouth-Opening) In this experiment, the new tidal inlet is included in the model and the simulations are depicted in Figures 5a–5c for flood, ebb, and tidal mean, respectively. The tides at both tidal openings are considered to be in phase because of the short distance of 17 km between the two inlets. All the experiments done for the pre–mouth-opening are done here. The results are found to be qualitatively very similar; hence, we cite below only the quantitative changes in circulation caused by the presence of an additional mouth. The peak flood current velocity is 95 cm/s, observed near the new inlet mouth, as against 45 cm/s for the condition of one inlet mouth. Likewise, the peak ebb current velocity is 110 cm/s as against 55 cm/s for the condition of one inlet mouth. Proceeding toward the channel from the central sector, there is an increase in velocity of 10–30 cm/s because of the tidal influence. The northern sector has almost uniform velocity (5–10 cm/s) except at the place where the rivers drain into the Chilika Basin (10–45 cm/s). In the southern sector,
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Figure 8. (a) Circulation caused only by wind forcing in January. (b): Flood tidal current (old inlet). (c) Ebb tidal current (old inlet). (d) Flood current as a result of January wind and tide (old inlet).
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Figure 8. (e) Ebb current as a result of January wind and tide (old inlet). (f) Mean circulation as a result of January wind and tide (old inlet).
the velocity is higher, ranging from 5 to 10 cm/s. Thus, there is an overall increase in current intensity because of the new inlet. The mean circulation profile is almost the same as with one inlet, showing that the residual circulation in the lagoon is not affected qualitatively due to the opening of the new inlet mouth.
Salinity Structure (Pre–Mouth-Opening) To simulate the salinity structure, numerical experiments were carried out for different forcing factors. All discussions in the following are based on the mean salinity structure, because flood and ebb salinity structures resemble the mean. This can be attributed to the narrowness of the channel, which obstructs the flow from the sea into the main body of the lagoon. Figure 6 corresponds to the mean salinity taken over a tidal period after steady state solution was achieved. It confirms that the region of maximum salinity is the channel, and shows that there is a steady decrease in salinity from a maximum of 33 ppt near the inlet to 10 ppt where the channel ends and the central sector begins, where the salinity distribution takes on a plumelike structure (6–8 ppt). Further into the central sector, the salinity distribution is almost uniform (4–6 ppt). The northern and southern sectors show low salinity values (1–4 ppt).
Salinity Structure (Post–Mouth-Opening) Figure 7 shows the mean salinity distribution after newmouth-opening. The salinity values show an overall increase because of the new inlet mouth. The outer channel shows a salinity range of 10 to 33 ppt. A large plume dominates the central sector and has higher salinity (8–10 ppt) as compared to the one-inlet plume (6–8 ppt). In the central sector salinity ranges from 8 to10 ppt (as against 4–6 ppt for one inlet). The northern sector also shows an increase in the salinity levels (2–8 ppt) as compared to the single-inlet case (1–4 ppt). In general, the new tidal inlet affects salinity values throughout the main body of the lagoon except in the northernmost and southernmost regions. Simulated salinity values in different sectors compare well with the observed values in the corresponding sectors (Table 3).
Northeast Monsoon Circulation (Pre–Mouth-Opening) Circulation in Chilika is affected by only two forcing factors during the NEM: wind and tidal forcing. Compared to SWM, there is (i) no appreciable freshwater influx, (ii) lesser depth, and (iii) a difference in magnitude and direction of wind. Wind Effect. Figure 8a shows the wind-driven circulation in the lagoon, which is found to follow the wind direction. Maximum current velocity is about 2.5 cm/s, which is low as
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Figure 9. (a) Flood current as a result of January wind and tide (both inlets). (b) Ebb current as a result of January wind and tide (both inlets). (c) Mean circulation as a result of January wind and tide (both inlets).
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Figure 10. Horizontal salinity in ppt as a result of January wind and tide (old inlet).
compared to that of SWM. Two eddies, which are connected to each other, are formed in the central and southern sectors, but the direction of rotation is counterclockwise, which can be attributed to the change in the wind direction. Wind effect is again seen to be almost negligible in the channel; there is almost no circulation in this region. Pure Tidal Effect. Figures 8b and 8c show the flood and ebb currents caused by pure tidal forcing. The peak flood current velocity is 45 cm/s and the peak ebb current velocity is 50 cm/s, showing that ebb current is stronger than flood current even during NEM. In almost the entire main body of the lagoon, velocity is very low and uniform (1 cm/s) for both flood and ebb tides, showing that tidal effects are negligible in the main water body. Combined Effect of Wind and Tide. Figures 8d and 8e depict the flood and ebb currents in the lagoon. Not many qualitative changes are observed here as compared to the pure tidal case, except for the velocity in the main body, which has increased up to 9 cm/s as against an earlier uniform value of 1 cm/s; this can be attributed to the inclusion of wind effects. The mean circulation profile (Figure 8f), which is ebb, shows a maximum velocity of 10 cm/s observed in the channel.
Circulation (Post–Mouth-Opening) Figures 9a and 9b, depicting the flood and ebb currents in the presence of an additional inlet, show the peak flood and ebb current velocities to be 81 cm/s and 95 cm/s respectively (as against 45 cm/s and 50 cm/s for a single inlet). The fact that ebb current is stronger than flood current is also made apparent by the mean profile (Figure 9c), which is ebb. The channel shows a velocity range of 10 cm/s near the central sector to 50 cm/s near the new inlet mouth. Velocity in the main body of the lagoon is the same as the pre–mouth-opening case. Also, the residual circulation remains unaffected by the opening of the new inlet mouth.
Salinity Structure (Pre–Mouth-Opening) To simulate the salinity structure, numerical experiments were carried out considering the pure tidal effect and a combination of wind and tidal effects. All the results discussed here are based on the mean salinity structure. Salinity values show a change from their values during SWM because of a lack of freshwater influx during NEM. Figure 10 depicts the mean salinity structure during NEM. Maximum salinity (14–33 ppt) is observed in the channel.
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Figure 11. Horizontal salinity in ppt as a result of January wind and tide (both inlets).
Salinity values show a steady decrease from their high values as we proceed toward the central sector, where the salinity distribution takes on a plumelike structure (10–12 ppt). The central sector has a range of 12 ppt near the channel to 6 ppt near the northern and southern sectors. The northern sector has a salinity range of 6 ppt (near the central sector) to 1 ppt. The southern sector is the zone with the lowest salinity (1–4 ppt).
Salinity Structure (Post–Mouth-Opening) Figure 11, depicting the mean salinity structure, shows that maximum salinity is observed in the channel (30–33 ppt). At Magarmukh, salinity decreases to 26 ppt (as against 16 ppt observed with one inlet). The progress of salinity into the central sector is in the form of plumes that show a rapid variation in salinity, with values ranging from 26 to 16 ppt. Large salinity plumes dominate the central sector. The northern sector shows an increased salinity range (2–14 ppt), which is also caused by a lack of freshwater influx. Among all the other sectors the southern sector shows the lowest salinity (10–2 ppt). In general, the way salinity progresses from the channel into the main body of the lagoon remains the same as in the previous case, except that salinity values
show an overall increase throughout the lagoon area because of the increased tidal influx.
CONCLUSIONS A two-dimensional numerical model is developed to simulate circulation and salinity structure in Chilika Lagoon. First, ebb current is always found to be stronger than flood current irrespective of the seasons. The channel is the deepest region of the lagoon (1–4 m) e central sector is shallower (1–2 m) compared to the channel. Experiments performed with varying depths proved that it is the gradient toward the channel that facilitates ebb current in Chilika. Second, two large eddies are formed in the central and northern sectors because of the depth gradient in these regions. These two results, validated by observations, are confirmed to be caused by the depth gradient because the numerical experiment with uniform depth failed to show these results. We may conclude that, in addition to wind and tides, bottom topography plays an important role in determining circulation patterns in the lagoon. It is also seen, from the numerical experiments with wind forcing alone, that wind forcing has an effect only in the main body of the lagoon. Experiments with tidal forcing alone show
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that tides have an effect only in the channel. Though the second opening has helped in increasing the tidal influx and hence the salinity, its influence is still not felt far into the interior of the lagoon because of the constriction of flow area between the lagoon and the channel area near Magarmukh. The simulated results have been validated against the limited observations and found to be in fairly good agreement (Table 3). The government of India has made restoration and conservation of Chilika Lagoon a high priority, and this task has been assigned to the Chilika Development Authority. It is expected that there will be more sophistication in realistic field estimations and monitoring observations in the future. More accurate ground data, complemented with Indian remote sensing satellite data, hold a lot of promise for added interest in the improvement of the Chilika model with better validation. An important extension of our present work that is under way is (i) to incorporate freshwater influx more accurately by including the tributaries on the western bank of Chilika, (ii) to include the recent connection of the southern sector with the Rushikulya estuary, and (iii) to formulate a physicobiological model by introducing the velocity profiles in the ecological model. Seasonal studies based on such a model will help in understanding if the significant improvement in the biological productivity of the lagoon post–mouth-opening is sustainable.
ACKNOWLEDGEMENTS Part of the work reported in this paper was carried out under a sponsored project funded by the Department of Ocean Development, Government of India. The authors are thankful to the referees for suggestions that have helped in improving the contents of the paper.
LITERATURE CITED CHANDRAMOHAN, P. and NAYAK, B.U., 1994. A study for the improvement of the Chilka lake tidal inlet, East Coasts of India. Journal of Coastal Research, 10, 909–918. CHANDRAMOHAN, P.; PATTNAIK, A.K., and JENA, B.K., 1998. Sediment Dynamics at Chilika Outer Channel. Proceedings of Chilika Development Authority, pp. 22–30. CHILIKA DEVELOPMENT AUTHORITY. Official Home Page. http:// www.chilika.com (accessed). CHILKA, 2001. CHILKA—A New Lease of Life. Bhubaneshwar, Orissa, India: Chilika Development Authority, 13p. HASTENRATH, S. and LAMB, P.J., 1979. Climatic Atlas of the Indian Ocean, Part 1: Surface Climate and Atmospheric Circulation. The University of Wisconsin Press, viii–xi. JNANENDRA, R. and ADHINKARY, S.P., 2003. Growth response of selected micro-algae of Chilika Lake to different salinity. Seaweed Research and Utilization, 25(1 and 2), 127–130. MESINGER and ARAKAWA, 1976. Numerical Methods Used in Atmospheric Models. GARP Publication, Series 17, Volume 1, 1–64. MOHANTY, P.K.; DASH, S.K.; MISHRA, P.K., and MURTY, A.S., 1996. Heat and momentum fluxes over Chilika: a tropical lagoon. Indian Journal of Marine Sciences, 25, 184–188. MOHANTY, P.K.; PAL, S.R., and MISHRA, P.K., 2001. Monitoring ecological conditions of a coastal lagoon using IRS data: a case study in Chilika, east coast of India. Journal of Coastal Research, 34, 459–469. NAYAK, B.U.; GHOSH, L.K.; ROY, S.K., and KANKARA, S. 1998. A study on hydrodynamics and salinity in the Chilika Lagoon. Proceedings of Chilika Development Authority, 31–47. PAL, S.R. and MOHANTY, P.K., 2002. Use of IRS-1B data for change detection in water quality and vegetation of Chilika Lagoon, east coast of India. International Journal of Remote Sensing, 23(6), 1027–1042. PANDA, D.; TRIPATHY, S.K.; PATNAIK, D.K.; CHOUDHURY, S.B.; GOUDA, R., and PANIGRAHY, R.C., 1989. Distribution of nutrients in Chilka Lake, east coast of India. Indian Journal of Marine Sciences, 18, 286–288. PARSON, T.R.; MAITA, Y., and LALLI, C.M., 1984. A Manual of Chemical and Biological Methods for Seawater Analysis. New York, New York: Pergamon Press, 173p. RAMNADHAM, R, REDDY, M.P.M. and MURTY, A.V.S., 1964. Limnology of the Chilika lake. Journal of Marine Biological Association India, 6(2), 183–201. SIDDIQI, S.Z. and RAMA RAO, K.V., 1995. Limnology Of Chilka Lake. In Fauna Of Chilka Lake, Wetland Ecosystem, Series 1. Edited by the director. Calcutta, India: Zoological Survey of India, 11–136.
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