Indian Journal of Marine Sciences Vol. 43(7), July 2014, pp.

Impact on of Chilika due to changes in the inlet system

B. Gopikrishna*, J. Sinha & M.D. Kudale

Central Water & Power Research Station, Pune.

*[Email:[email protected]]

Received 15 August 2013; revised 22 October 2013

In Chilka lake a new inlet was formed due to natural coastal processes at Gabakunda in August 2008 at 1 km north of the existing inlet at Sipakuda. Present study deals with 2-D mathematical model (MIKE21) studies with two inlets to assess the hydrodynamic changes and salinity variations due to the formation of new inlet at Chilika lake. Predicted tide, monthly average inflows of the rivers and observed salinity are used for the model studies with a constant width of inlets throughout the simulation period. Newly formed inlet has a dominant role in contributing tidal exchanges over Sipakuda inlet. Decrease in tidal prism with two inlets is observed in comparison with that of single inlet system at Sipakuda, which leads to decrease in salinity of about 10 ppt in all the sectors of the lake. Lake salinity is improved by preserving tidal exchange by maintaining the cross sectional area and depth of the inlets.

[Keywords: hydrodynamics, salinity, tidal exchange]

Introduction August, 2008 a new inlet had formed naturally at Chilika is a unique natural wealth of Gabakunda which is located at one kilometre , being one of the largest distance from the present inlet of Sipakuda. Panda in the world, is situated between latitude et al 5 studied the impact of salinity with multiple 190 28’- 190 54’ N and longitude 850 06’- 850 36’ E inlets considering the period from July to August, on the Orissa coast. This lake is the assemblage of 2008 covering the date of formation of Gabakunda marine, brackish and fresh water eco-systems with inlet and annual mean salinity variation was studied estuarine characters1. The pear-shaped lagoon is covering the period from December, 2008 to about 64.5 km long and its width varies from 18 km November, 2009. The impact of this formation was to 5 km2. The lagoon referred here as a lake is studied on lagoon environment comparing it with connected to through narrow cut the salinity of 2006 with single inlet at Sipakuda. made on September 23, 2000 at Sipakuda. The The present study aims at analyzing the proper functioning of Lake depends hydrodynamic changes and salinity variations due mainly on the configuration of inlets in their size, to the formation of new inlet at Gabakunda for the number and locations for the enough tidal exchange non- period from November, 2009 to May, between the lake and the . The single inlet 2010. system of the lake restored the salinity to the desired levels. The de- of the lead channel followed by opening of the artificial inlet at Sipakuda not only rejuvenated the lagoon ecosystem but also immensely benefited the fisher folk whose average annual income increased significantly (Chilika.com). This hydrological intervention is considered to be the most successful implementation in the history of Chilika Lake. Later, several studies have been carried out on behaviour of the single inlet system of Chilika lake on circulations and salinity structure3-5. The littoral drift along the East Coast and inflow from the drainage basins of rivers contribute sediment to the lake over the years that cause the inlet to shift gradually towards north and reduce the size of the inlet. During the high tide of solar eclipse day in Fig. 1: Two inlets at Chilika lake INDIAN J. MAR. SCI., VOL. 43, NO. 7, JULY 2014

Materials and Methods Later, this inlet was analysed for its stability The present scenario of the lake system (Fig. 1) through Bruun analysis8. Per Bruun9 has chosen indicates that it is connected to the Bay of Bengal the parameter Ω/Mtotal for describing the stability by two narrow openings through which the tidal criteria where Ω is the tidal prism obtained through 3 exchange takes place and is separated by a sand bar the inlet (27.5 Million m ) and Mtotal is the net of about 60 km in length. One inlet is located at littoral drift in the area (1.2 Million m3). Based on village Sipakuda, an artificial cut made in the year this ratio the inlet is classified as a typical bar 2000 by 80 m wide channel over the bypassing inlet. Model studies for variation of 200 m wide sand spit and the other at village salinity in the lake upon progressive shifting of Gabakunda naturally formed due to coastal existing Lake Inlet (CWPRS Technical Report No processes in August, 2008. The water-spread area 4705 of 2010) from 0 km to 15 km indicated that of the Chilika Lagoon varies from as high as 1196 the decrease in salinity levels at the rate of 5 ppt for km2 during the monsoon to as low as 815 km2 every 4 km shift in Northern sector and 4 ppt in all during summer1,3-7. The lagoon is divided into four other sectors. Dube et al6 studied the variation of sectors based on its physical and dynamic sector wise salinity considering the two characteristics. The northern sector receives representative months of two regular floodwaters from the tributaries of rivers (SWM, NEM) for pre cut -post cut scenarios at viz. Bhargavi, Daya, Makara, Luna and shows Sipakuda. seasonal salinity variation. The southern sector is relatively smaller and does not show much seasonal Present model set up variation in salinity. The central sector has features 2-D finite difference unsteady hydrodynamic intermediate between the features of the other cum Advection-Dispersion model of MIKE21 sectors. The area falls in littoral zone, a large developed by DHI, Denmark has been set up volume of littoral transport causes instability and covering the entire lake and sea portion up to 25 m migration of the openings towards North. The past contour (MSL). Grid intervals of 33 m in both X studies of CWPRS indicate the littoral quantity of and Y directions have been used to accommodate a the order of 1.2 Million m3 towards North. minimum of 5 grids at the minimum widths of both the inlets (Fig. 2). In order to meet the stability criteria, a time step of 10 s was adopted in the model. The lake system with two existing inlets with one kilometre distance between them is considered for modelling as shown Fig. 2. The observed salinity for the month of October, 2009 is used as initial condition in the model (Fig. 3). The salinity from the sea side is taken as 33 ppt corresponding to constant temperature of 250C and the salinity of the river inflows is taken as zero. The sinusoidal tide was given as boundary condition at the seaside throughout the period of simulation. The simulation runs of coupled HD and AD modules were done from the month of November, 2009 to May, 2010. The widths of inlets corresponding to October, 2009 and average depths of the two inlets supplied by Chilika Development Authority were adopted in the model and are assumed to be constant throughout the simulation

Fig. 2: Bathymetry of Chilika Lake with two inlets periods.

Past studies with single inlet at Sipakuda The studies pertaining to pre cut-post cut scenarios of single inlet lake system with 750 m width for examining the improvement in tidal flux and salinity variations in the lake were carried out. The hydrological intervention significantly improved the Lake Ecosystem and . GOPIKRISHNA et al.: IMPACT ON SALINITY OF CHILIKA LAKE

a single inlet at Sipakuda of width 750 m in 2005 (CWPRS Technical report no 4308 of 2006). An attempt has been made to compare the predicted tidal ranges with the observed data at various locations. At Magarmukh the predicted tidal range is of the order of 0.20 m as against observed range 0.21 m. Similarly at Satpara and Gabakunda the tidal ranges were of the order of 0.4 m and 0.95 m respectively as against observed tidal range of 0.4 m and 1.1 m. The variation in velocity at

Fig. 3: Salinity observed in October, 2009 for Chilika lake Magarmukh and Satpara are of the order of 0.2 m/s. The discharges through Gabakunda are simulated to The model was calibrated for water levels and be more than that of Sipakuda inlet and are the velocities at different locations inside the lake by order of 550 m3/s during flooding and 450 m3/s altering the calibrating parameters chezy’s constant during ebbing. The model results of the two inlets and eddy viscosity. Similarly, an attempt has been are shown in Table 1. made to compare the sector averaged salinity with the observed salinity measurements inside the lake. The dispersion coefficients proportional to currents in both the directions are adjusted for calibrating the salinity. The comparison of salinity for April and May 2010 show good agreement. The large variation in initial months of simulation period (November, 2009) is due to the average inflows of northern and western rivers that have been used for the study. However, the initial salinity of November, 2009 depends on river inflows of October, 2009. The river inflows were given as isolated sources in the model at appropriate places. Time varying wind forcing was adopted in the model with wind friction type varying with wind speed.

Results Hydrodynamics The model results at the Sipakuda inlet indicated that the flow enters through this inlet (flood) with a peak velocity of 0.6 m/s with varying cross Fig. 4: A typical flow field around Chilika inlets sectional area between 720 m2 - 870 m2 during low and high water. The total exchange of tidal water Salinity during influx and efflux is worked out to be 8.2 Salinity variation inside the lake depends on Million m3 in one complete tidal cycle. Similarly at many factors like position and size of the inlet, Gabakunda inlet the flow enters (flood) with a number of inlets, the distance between the inlets; magnitude of 0.6 m/s with tidal flux section varying tide at sea, magnitude and direction of wind between 780 m2 - 1060 m2 during low water and prevailing over the area and net evapo- high water. The exchange of tidal water during precipitation. The observed salinity measurements influx and efflux is worked out to be 11.7 Million are point observations and monthly averages. m3 and 17.3 Million m3 respectively in one tidal Keeping in view the above parameters continuously cycle at Gabakunda based on the existing varying during each month, an attempt has been configuration of the two tidal inlets in October made to compare the sector wise salinity in the 2009. A typical flow phenomenon of flood and ebb lake. at both the inlets is shown in Fig. 4. The The salinity distributions at the end of every contribution of the tidal prism through both the month from November to May for non-monsoon inlets for spring tide is of the order of 19 Million m3 have been simulated. The depth averaged salinity whereas the same was 27.5 Million m3 with that of simulations at the end of November and April have INDIAN J. MAR. SCI., VOL. 43, NO. 7, JULY 2014

been compared with the observed salinity Table 2: Sector wise averaged observed and computed measurements and are shown in Table 2. The Salinity computed salinity distributions at the end of every month are shown in Fig. 5A and Fig. 5B in order Sector wise from November, 2009 to April, 2010. The averaged Computed Salinity differences in observed and computed salinity are Observed (ppt) more in the month of November, 2009 in all the Salinity (ppt) four sectors except at Southern Sector where the Nov April May Nov April May comparison is in good agreement since the sector is Northern 0.2 13.7 15 4 14 16 not directly influenced by river inflows. The model Sector simulated salinity of other months April and May Central 1.6 13.7 16 8 16 18 are in good agreement with observed values except Sector in the control section (which is the area between Southern 6.9 10.9 17 6 10 12 Satpara and Magarmukh). This large variation in Sector salinity at control section may be due to Control 1.6 33 33 6 16 18 configurations adopted at the inlet openings that are Section assumed constant throughout the simulation period. At the end of monsoon period (November) the The salinity variation starts building up until the salinity variation was 0 ppt - 8 ppt in the entire lake start of monsoon period. The model results indicate with the maximum salinity computed in the Central that the salinity at the end of April is varying Sector and minimum at the Northern Sector. In the between 16 ppt-28 ppt with the maximum at the absence of relevant data on inflows of the rivers Central Sector and minimum at the Northern proper simulation of salinity for November cannot Sector. The salinity at the extreme end of Southern be obtained from the model. Sector is simulated to be of the order of 10 ppt in Table: 1 Hydrodynamic analysis of Chilika Lake the month of April against the observed value of 10.9 ppt in 2010. Variation Discharge

of c/s Location Discussion Area Flood Ebb 3 3 It is observed from model simulations that the (m2) (m /s) (m /s) naturally formed inlet at Gabakunda has dominant Sipakuda role in the tidal exchange as compared to the inlet 720-870 150 70 Inlet at Sipakuda and hence Gabakunda inlet contributes salinity significantly to the lake with a flood Gabakunda 3 780-1060 550 450 discharge of the order of 550 m /s. The major part Inlet of the ebb discharge from the lake is through Right of Gabakunda, ebb discharge through Sipakuda being Gabakunda 800-1200 200 140 insignificant. This aspect, however, needs to be Inlet confirmed through field observations by measuring the actual cross section of the two Inlets. The model Left of 1050- results for exchange of water in ebb phase of tide Sipakuda 500 450 1800 are shown in Table 1. The efflux through Sipakuda Inlet inlet is 0.7 Million m3 as against 17.3 Million m3 through Gabakunda inlet. The tidal prism with the Exchange of water Tidal 3 (Million m3) prism existing configuration of two inlets is 19 Million m Phase of as against 27.5 Million m3 simulated for the single tide Million artificially opened inlet at Sipakuda (CWPRS Sipakuda Gabakunda m3 Technical report No 4308 of 2006). The simulations cover the period from November, 2009 Flood 7.5 11.7 19.2 to May, 2010 (Fig. 5 & Fig. 6). The model Ebb 0.7 17.3 18.0 simulation of salinity in the lake with the existing configuration of two inlets indicated a reduction in Total 8.2 29.0 overall salinity as compared to that with the single inlet 10.

246 GOPIKRISHNA et al.: IMPACT ON SALINITY OF CHILIKA LAKE

Fig. 5A: Simulated Salinity of Chilika Lake at the end of November, 2009 to end of January, 2010 Fig. 5B: Simulated Salinity of Chilika Lake at the end of February, 2010 to end of April, 2010 The dilution of the salinity usually continues till the end of November beyond the monsoon season. Similarly in the Central Sector of the lake the In November the salinity variation in the lake is values decreased from 26 ppt to 16 ppt and observed to vary between 8 ppt to 2 ppt. The Southern Sector from 20 ppt to below 10 ppt (Fig. maximum salinity is in the Central Sector and the 6). This aspect has been verified with the similar minimum at the end of Northern Sector. study 5 of salinity variation of 2009. Panda et al 5 The salinity simulated at the Southern Sector of compared the variation in salinity of annual mean the lake is 6 ppt (Fig. 5). This part of the lake is observations of the single inlet system (2006) with considered usually the trapped region because of that of double inlet system (2009). It was indicated less tidal exchange and absence of river discharge. that there is significant reduction in salinity in In May, 2010 the salinity in Northern, Central and Central and Southern Sectors. The effect was not Southern sectors is of the order of 16 ppt, 18 ppt, seen in Northern Sector and Outer channel. The and 12 ppt respectively (Fig. 6). The impact of model results of 2010 indicated that the impact was salinity with the opening of the new inlet at seen in all the sectors including the Northern sector Gabakunda on the lake has been examined by and control section. Thus the model simulations comparing the simulations of May 2010 with the indicate that with the formation of the new inlet, the previous simulations of May 2005 (Fig. 6) with tidal exchange is not sufficient to restore the single inlet at Sipakuda 11. salinity in the lake due to the continuous loss in The simulations for the two months indicate that widths and depths every month. there is much decrease in salinity values in all the sectors by about 10 ppt which may be attributed to Conclusions decrease in tidal exchanges with the opening of the The hydrodynamics and salinity regime of the new inlet. In the Northern sector the salinity Chilika Lake are influenced by the configuration of decreased much below 24 ppt and is less than 14 tidal inlets and river inflows. The formation of the ppt for the month of May. new tidal inlet near the existing one has significantly reduced the tidal exchange of the lake. INDIAN J. MAR. SCI., VOL. 43, NO. 7, JULY 2014

lagoon environment: A numerical model study. Estuarine, Coastal and Shelf Science, 2013. 116(0):29-40. 6. Dube A., Jayaraman, G., Rao, A.D., and Mohanty, P.K., Numerical Simulation of Salinity Structure in Chilika Lake, in Monitoring and Modelling and Coastal Environments, P. Mohanty, Editor. 2008, Springer Netherlands.136-150. 7. Pattnaik, Hydrological intervention for restoration of chilika lagoon. chilika news letter, 2001-02:3-5. 8. CWPRS Technical Report - 4341., Study of behaviour of inlet of Chilika Lake and methods of stabilisation. 2006: Orissa. 9. Bruun P., Mehta, A.J., and Jonsson, I.G., Stability of tidal inlets: Theory and engineering. 1978: Elsevier Scientific Publishing Company Amsterdam, The Netherlands. 10. CWPRS Tech Report-4308., Mathematical model studies for salinity variation due to freshwater flow from Naraj Barrage in Chilika Lake. 2006: Orissa. 11. CWPRS Tech Report -4705., Mathematical model studies for examining impact on salinity variation in Chilika lake due to progressive shifting of lake inlet. Fig. 6: Simulated Salinity of Chilika Lake at the end of May, 2010(4705). 2010 and May, 2005

The comparison of model simulations with the observed salinity data shows overall decrease in salinity of the order of 10 ppt by the end of May, 2010 in all sectors of Chilika Lake with two inlets configurations. Further model simulations need to be carried out with recent data on inlet cross sections, recent bathymetry of the lake, currents, tides, wind and river discharges for more realistic results.

Acknowledgements Authors are thankful to Dr I.D.Gupta, Director, Central Water and Power Research Station, Pune for his continuous encouragement and kind consent for publishing the paper. The authors are also thankful to Chilika Development Authority, for supplying the relevant data for carrying out the study. References 1. Pattnaik A.K., Opening of a new mouth-A step to restore the ecosystem of Chilika lake - A Ramsar site of India : Proc. Workshop on Environmental Management & Wiseuse Brackish water lakes, lake and Bays, in 9th International Conference on Construction. & Management of lakes. 2001.14-18. 2. Chilika Report., Habitat evaluation of Chilika lake with special reference to birds as Bio indicators. 2001-2005. 3. Jayaraman G., Rao, A.D., Dube, A., and Mohanty, P.K., Numerical Simulation of Circulation and Salinity Structure in Chilika Lagoon. Journal of Coastal Research, 2007. 23(4):861-877. 4. Mohanty P.K. and Panda, B.U.S., Circulation and mixing processes in Chilika lagoon. Indian Journal of marine sciences, 2009. 38(2): 205-214. 5. Panda U.S., Mohanty P. K., and Samal R. N., Impact of tidal inlet and its geomorphological changes on