Vulnerability of Cochin Backwaters to Meteorological Disturbances with Special Reference to Tidal Propagation Antony Joseph 1, K. K. Balachandran 2, Prakash Mehra 1, R. G. Prabhudesai 1, Nitin Dabolkar 1, Vijay Kumar 1, C. Revichandran 2, and Yogesh Agarvadekar 1 1 National Institute of Oceanography, Dona Paula, Goa, India 2 National Institute of Oceanography, Regional Centre, Dr. Salim Ali Road, Cochin, India. E-mail:joseph@nio.org Abstract Features of water level oscillation in the Cochin backwaters are presented in relation to its vulnerability to meteorological disturbances. A steep surge formed after a sharp atmospheric depression ( 8 mb) followed by storm ( 20 m/s) and incessant rains ( 665 mm spanning over a fortnight) lead to a significant rise in estuarine water levels ( 50 cm). Delay of 3 days between the depression and flooding is attributed to the run off time from catchment areas. Sensitiveness of this estuary to meteorological disturbances has significant implications on its ecological and environmental health. Spectral analysis of tides revealed some of the special features of a complex shallow estuary. As the tide propagates into the interior estuary, it undergoes progressive attenuation (by up to 73%) and weakening of semi-diurnal character. But, the fortnightly tidal constituent (M sf ) unusually gets amplified with its crest (trough) coinciding with spring (neap) tides. Influence of M sf constituent increases 10-fold towards the upper reaches. Non-existence of the S 1 constituent, manifested throughout the estuary, is a peculiar feature. Progressive weakening of the tides semi-diurnal character is due to the presence of OO 1 constituent, which, unlike other diurnal constituents, amplify with increasing distance from the entrance. Influence of OO 1 constituent increases 3-fold towards the upper reaches. Propagation delay of low (high) tide phase at the farthest location from the inlet is 5.6 (4.6) h. Dominance of M sf constituent in the upper reaches is expected to render a fortnightly character to the flushing process, and in turn, the biological productivity of this region. 1. Introduction The Cochin backwaters, extending from Azhikode in the north to Alappuzha (formerly Alleppey) in the south, is a semi-enclosed water body ( 256 km 2 in area) of irregular topography and interspersed by numerous islets and shoals of varied sizes and shapes (Fig. 1). It is the largest and the most complex estuarine system
on the west coast of India. This lagoon is a unique environmental habitat on account of its sheltered nature and capacity in providing nursery ground for a wide variety of brackish water species of plants, shell and fin-fishes. Seawater enters into the estuary system primarily via two openings (inlets) at Azhikode and Cochin, which are 25 km apart, in which the former is shallower than the latter. The Azhikode- and Cochin- inlets respectively are 250m and 450m wide. The latter forms the entrance to the Cochin harbor. The depth of the estuary varies considerably from place to place (10 to 12 m around Cochin harbor channel down to <2 m at the southern deltaic portion). The other regions of the estuary are in the depth range 2-7 m. Six rivers empty themselves into the estuary, discharging large quantities of fresh water during the southwest and the northeast monsoonal seasons. Mixing with seawater through tidal exchange together with considerable amount of fresh water influx from these rivers gives this backwater system the characteristic of a typical Fig.1 Map showing Vembanad Lake brackish water estuary. and Lakshadweep archipelago. Fig.1 Map showing Vembanad Lake and Lakshadweep archipelago. Most of the past studies on tidal propagation in this estuary (Srinivas et al., 2003; and Srinivas and Kumar, 2006) were based on measurements of very short duration from limited locations in the vicinity of the inlet. Lack of synoptic measurements at closely spaced time-intervals and in relation to meteorological forcing has posed severe restrictions in understanding the features of water level oscillations and their vulnerability to meteorological disturbances. From the present study, we identify and explain some of the unique and hitherto unknown features of the water level oscillations in this estuary. In this paper we describe the observed peculiarities of the tidal and non-tidal oscillations showing the vulnerability of the estuary to an episodic meteorological event that fortuitously occurred during the April-June 2006
synoptic measurements. Measurement schemes adopted and the observed thermohaline features are described elsewhere by Joseph et al., (2007). 2. Episodic meteorological event While the time-series measurements were in progress, the region witnessed an episodic surface meteorological event involving heavy rainfall in Fig. 2. 60-day measurements (April-June 2006), depicting an episodic surface meteorological event involving heavy rainfall, intense wind, and barometric pressure drop. association with a depression and strong winds (Fig. 2). The 2006 southwest monsoon rainfall over the west coast of India was the second largest during the past five decades. The heaviest rainfall (103 mm) occurred on Julian day 140 (20 May). Again, the region witnessed a depression (8 mb) for a period of 4 days during Julian days 147-151 (27-31 May), resulting in very heavy rainfall on Julian day 150. The severe depression (for 90 minutes) was recorded on 30 May. The sharp depression was accompanied by very strong winds (25.6 m/s) and heavy rain (93 mm). It can be seen that during this short period, westerly cross-shore winds was particularly strong.
There were other two isolated days of persistent winds [Julian day 125 (5 May) and 137 (17 May)] during which gust (average wind) were 15.5 (13.4) m/s and 17.5 (12.1) m/s respectively. These events inflicted immediate influence on atmospheric temperature and relative humidity as well. 3. Characteristics of water level oscillations and their variability during episodic meteorological event Surge developed in the estuary following an atmospheric depression which induced strong winds, incessant rain and river run off. Progressive attenuation of tidal amplitude and increasing dominance of fortnightly tidal periodicity with increasing distance from the estuary s entrance are readily apparent. A closer analysis of the various tidal constituents provides more information on these intricate aspects. 4. Discussion The characteristics of water level oscillations in a shallow estuarine water body are expected to vary under strong winds and river runoff. The major aspects addressed in the present study, based on a 60-day observation (10 April 10 June 2006) are (1) surge driven by an episodic meteorological event, and (2) tidal characteristics including attenuation, propagation delay, progressive weakening of semi-diurnal characteristic, and increasing modulation by a relatively large signal of fortnightly periodicity as the tide propagates from the entrance to the upper reaches of the estuary. 4.1 Surge The episodic changes in the meteorology of May 2006 provided a rare opportunity to capture the response of this shallow water body during such events. The usually tide-dominated water level oscillations in the estuary were considerably obliterated by the gushing fresh water flow and intense wind following the atmospheric depression and heavy rains. This is better explained as follows. Daily-mean water levels were computed from the measured data sampled at 10- minutes interval. This procedure averaged out all the diurnal, semi-diurnal, and other short period tidal components while retaining signals having periodicity more than a day. Fig. 3 indicates that water level oscillations in the Cochin backwaters are controlled by a myriad of meteorological and hydrological factors in addition to the well-known astronomical forcing. Julian day 125 (5 May) witnessed a relatively strong northeasterly wind ( 14 m/s) which supported a set-down (water level depression). Although there was no rain during this time, the residual still indicated a set-down. This suggests that the Cochin backwater body is highly sensitive to water level
variability even under moderate wind forcing. A sharp drop in barometric pressure and strong westerly cross-shore winds leading to heavy rains initiated sharp increase in setup throughout the estuary around Julian day 150 (30 May). The surge was particularly severe towards the upper reaches (Fig. 4). The largest surge observed at station #9 could be the result of constriction of the lake in this region. Lesser surge observed at the southernmost region (station #10) relative to that at station #9 could be due to geometric widening of the southern lake together with the opening of the temporary spillway at Alleppey as a local flood control measure. The observed delay of 3 days from the development of a depression to its ultimate impact in the estuary with a peak set-up around Julian day 153 is because of the fact that the precipitation over the Western Ghats (the catchments areas) leading to fresh water influx takes approximately 3 days to traverse across 100-125 km before entraining into the estuary. Daily-mean water level (cm) 120 100 80 60 40 20 1 3 5 6 7 8 9 10 0 100 110 120 130 140 150 160 Julian days - 2006 Surge (cm) 60 50 40 30 20 10 0 1 2 3 4 5 6 7 8 9 10 Station Nos. Fig.3 Variation in daily-mean water levels at 8 stations from the mouth to the upper reaches of the Cochin backwaters, indicating meteorologically induced surge. Influence of wind is particularly evident. Fig. 4 Growth of surge with increasing distance from the mouth. 4.2 Tidal characteristics Spectral analysis of tides provides some of the interesting features of the estuarine system. In addition to major astronomical constituents, the water levels are influenced, to a lesser extent, also by compound tides and over-tides. Propagation delay, attenuation of tidal amplitude and weakening of the semi-diurnal characteristics are the general features but, the progressive amplification of the fortnightly tidal constituent in the south estuary is curious and invites further investigation. The observed features are discussed below. 4.3 Attenuation of tidal amplitude A cursory look at the pattern of tidal oscillations at various locations in the estuary reveals progressively diminishing amplitudes towards the upper reaches. To examine this feature, water levels during a spring tide (average of 6 ranges) were plotted for all the stations (Fig. 5), and it is seen that tidal range suffers progressive attenuation (by up to 73%) from the inlet towards upper reaches. The observed
Fig. 5 Attenuation of tidal range with increasing distance from the entrance. 4.5 Tidal propagation delay attenuation of tidal amplitude is natural and results from the peculiar bathymetry of this estuary, wherein it opens to the sea through the two narrow inlets, resembling a blown up balloon whose neck represents the inlets. As the tide enters the estuary through the two inlets and propagates head-wards, the volume transport diffuses over a larger area, thereby lowering the water level following momentum conservation. Because of the shallowness of the estuary, delay in tidal propagation is anticipated. Our measurements reveal that low tide (LT) phase suffers a larger propagation delay than the high tide (HT) phase (Fig. 6). Trough is retarded more than the crest as a result of the difference in depth at high and low water. Water depth at LT phase is smaller than that at HT phase and, therefore, the LT phase propagates at a relatively lower speed. The more or less similar propagation delays for stations 3, 5 and 6 suggest that tide reaches these locations simultaneously, because of their uniform axial distance from the inlet. Propagation is further delayed between stations 6 and 7 ( 2.75 h for LT and 1.50 h for HT). But, among stations 7, 8, and 9, the propagation delay is marginal only during LT. This suggests that while low tides reach these locations simultaneously, the high tide reaches at different times. At the southernmost station (10), tide Fig. 6 Tidal propagation delay with increasing distance from the entrance. 4.5 Progressive weakening of semi-diurnal characteristics reaches after a delay of 0.75 h relative to station (9). Thus, the approximate tidal propagation delays in LT and HT at station No. 10 relative to the inlet are 5.6 h and 4.6 h respectively. Significant decay of semi-diurnal characteristics of tides towards the upper reaches of the estuary is clearly seen (Fig. 7). Examination of the prominent diurnal tidal constituents (Fig. 8) reveals that they are responsible for the longitudinal
transformation mentioned above. Unlike the other diurnal constituents that attenuate with distance, the OO 1 constituent shows amplification from station No. 5 onwards to the south. The influence of OO 1 constituent is further evidenced by the ratio of OO 1 to the corresponding tidal amplitude for all the stations, which increases by a factor of 3 towards the southernmost location. 4. 6 Progressive dominance of fortnightly tidal constituent Tidal analysis reveals an interesting feature of progressive modulation of tidal pattern with increasing distance from the entrance (Fig. 9), and the reason for this is attributable to the presence of M sf (fortnightly) constituent (Fig. 10), in the midst of other attenuating astronomical constituents. This feature is more conspicuous between stations 5 and 10. The influence of M sf constituent is more clearly seen from the ratio of M sf to the corresponding tidal amplitude for all the stations, which increases by a factor of 10 towards the upper reaches of the estuary (Fig. 11). Fig. 7 Decay of semi-diurnal characteristics of tides (both spring and neap) as it propagates from mouth towards the upper reaches of the Cochin backwater system. Amplitude (cm) 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 0 1 2 3 4 5 6 7 8 9 10 Station No. Fig. 8 Prominent diurnal constituents in the measured tides from various locations in the Cochin backwater system. In the present study, this is a hitherto unknown, but a prominent feature of the Cochin backwaters, which might be playing significant role in the ecological functions. Q1 J1 M1 OO1
The progressive amplification of fortnightly constituent towards upstream indicates a kind of waviness leading to sluggish movement of water, thereby increasing the flushing time. The Cochin backwater body is known to receive voluminous amount of industrial and domestic sewage from various sources (Balachandran, et al., 2005, 2006; Madhu et al., 2007). Therefore, the observed special feature of tidal propagation (M sf amplification) is likely to influence the health of the water body by modulating the flushing rates at fortnightly periodicity. Fig. 9 Progressive fortnightly modulation of the tides with increasing distance from the entrance. Amplitude (cm) 25 20 15 10 5 s2 N2 M2 O1 K1 MSF 0 0 1 2 3 4 5 6 7 8 9 10 Station No. Fig. 10 Prominent constituents in the measured tides at various locations in the Cochin backwaters indicating the amplification of the fortnightly constituent (M sf ) from station No. 5 to the upper reaches of the estuary. 1 Ratio (MSF:Tide) 0.8 0.6 0.4 0.2 0 0 1 2 3 4 5 6 7 8 9 10 Station No Fig. 11 Ratio of M sf amplitude to the corresponding tidal amplitude for various locateions in the Cochin backwaters.
5. Conclusions Water level oscillations in the Cochin backwaters with special reference to their vulnerability to meteorological disturbances were examined. Although field observations were conducted during generally fair weather conditions, occurrence of a fortuitous episodic meteorological event provided a rare opportunity to examine their responses on this vulnerable estuary. Whereas mixed predominantly semi-diurnal tides is the most important factor influencing the renewal of waters by flushing of pollutants, the tidal cyclicity is superseded by surge during sudden atmospheric and hydrological forcings. Fresh water influxes from rivers play an important role in the water level variability in the estuary. The southern part of the estuary indicating M sf amplification is vulnerable and sensitive to environmental pollution. The net movement of water in the estuary has strong implications on dispersion of pollutants, which in turn, have a direct effect on the water quality and estuarine ecology. The present study brings out several features of water level oscillation in the Cochin backwater system, and these can be considered as baseline information for future studies of this estuary. Acknowledgements The authors thank Dr. S. R. Shetye, Director, National Institute of Oceanography, Goa, for the personal interest and initiative for the measurements reported here. We are also thankful to Dr. B. R. Subramanian, Director, ICMAM-PD, Ministry of Earth Sciences, Chennai and Dr. C. T. Achuthankutty, the then Scientist-in-Charge, NIO, Cochin, for encouragement and support. We are grateful to the Marine Surveyor s Office, Cochin Port Trust for providing tide data at the Cochin inlet. We thank M/s. EMCON, Kochi and Dr. T. K. Sivadas for supplying uninterrupted and high quality data on water level oscillation (7 locations) using their instruments. We also thank all the team members of the above project for their contribution in data collection. The authors are grateful to Mr. S.G. Akerkar for the assistance rendered for the preparation of figures References Balachandran, K. K., Laluraj, C. M., Nair, M., Joseph, T., Sheeba, P., Venugopal, P., (2005), Heavy metal deposition in a flow-restricted, tropical estuary, Estuarine, Coastal and Shelf Science, 65, 361-370. Balachandran, K. K., Laluraj, C. M., Martin, G. D., Srinivas, K., Venugopal, P. (2006), Environmental analysis of heavy metal deposition in a flow-restricted tropical estuary and its adjacent shelf, Environmental Forensics, 7, 345-351. Joseph., A., K. K. Balachandran, P. Mehra, R. G. Prabhudesai, N. Dabolkar, V. Kumar, C. Revichandran, and Y. Agarvadekar (2007), Influence of episodic
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