Indian Journal of Geo-Marine Sciences Vol. 40(1), February 2011, pp. 32-36
Changes of selected hydrological parameters in Hooghly estuary in response to a severe tropical cyclone (Aila) A Mitra, P Halder & K Banerjee1 Department of Marine Science, University of Calcutta, 35, B.C. Road, Kolkata –700 019, India 1 [Email:
[email protected]] Received 01 October 2009; revised 15 February 2010 Effects of Aila, a severe tropical cyclone on three major hydrological parameters namely surface water salinity, pH and DO in the western part of the deltaic Sundarbans region of Indian sub-continent at the apex of Bay of Bengal is ascribed in the present study. It consists the in situ studies before and during Aila at 12 sampling stations in the Hooghly estuarine complex and also extended our studies further (ten days after the storm hit the Gangetic delta) to visualize the situation of the aquatic phase. There were changes in surface water salinity, pH and dissolved oxygen (DO) due to intrusion of saline water from the Bay of Bengal. Due to this severe cyclonic storm, high energy tidal surges increased the average salinity of surface water from 13.64±6.24 ppt to 17.08±8.03 ppt, which is a rise of 25.2%. Average pH changed from 7.99±0.20 to 8.01±0.21 which is an increase of 25.03%. Average DO decreased from 5.24±0.70 ppm to 4.95±0.51 ppm, which reflects a fall of 5.53% in the area of investigation due to intrusion of sea water. Ten days after the Aila incidence was over, the salinity and DO exhibited gradual restoration. Such drastic changes of hydrological parameters due to natural calamities may pose an adverse impact on the ecology of the deltaic complex and requires a systematic planning to combat the ecological effect of the disaster. [Keywords: Aila, severe cyclonic storm (SCS), Hooghly estuarine complex, surface water salinity, pH, dissolved oxygen (DO)]
Introduction Tropical cyclones are major hazards in tropical coastal regions, both in terms of loss of life and economic damage. Many violent severe local storms occur over the Gangetic plain of West Bengal and neighbouring Bangladesh due to Nor’westers. Formations like squall line, wind squall are typically associated with this type of development and many a times, these severe local storms appear as natural hazard1. Such cyclones originate in the Bay of Bengal during the spring (April-May) and fall (OctoberNovember) inter-monsoon2. While the effect of tropical cyclone to the cooling of Sea Surface Temperature (SST) is widely known3,4,5, its effect on the physicochemical characters of near shore water is yet to be documented. Present study consists the changes of physico-chemical variables in the surface layer of Hooghly estuarine complex due to a severe tropical cyclone Aila that passed across the Gangetic delta with a speed of 110 km/hour during 25th May 2009. Materials and Methods The Indian Meteorological Department (IMD) classified cyclone on the basis of sustained wind speed into six major types (Table 1). Considering the
speed of Aila (110 km/hour) on 25th May, 2009 in the Gangetic delta stretch, it can be designated as Severe Cyclonic Storm (SCS). Aila was formed in the central Bay of Bengal as the net output of several factors. Around May 20th 2009, monsoon initiated at Andaman. Under its influence, the southerly surge over the region increased. It resulted in increase in the horizontal pressure gradient and the north-south wind gradient over the region. Hence the lower level horizontal convergence and relative vorticity increased gradually over the southeast Bay of Bengal. This condition triggered the development of the upper air cyclonic circulation extending up to midtropospheric level on 21st May over the southeast Bay of Bengal and associated convective cloud clusters persisted over the region. Under the influence of the cyclonic circulation, a low pressure area developed over the southeast Bay of Bengal on 22nd May morning. It laid over east central and adjoining west central Bay of Bengal on 22nd evening. It concentrated into a depression and lay centered at 1130 hours IST of 23rd near Lat. 16.5º N/ Long 88.0º E about 600 kms south of Sagar Island, the largest island in Indian Sundarbans out of 102 island clusters. The track of the system is shown in Fig. 1.
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Table 1―Tropical Cyclone Classifications (all winds are 10-minute averages) Storm type
Abbreviation Wind Speed (km/hr)
Super cyclone SC >221 Very severe cyclonic storm VSCS 119-221 Severe cyclonic storm SCS 88-118 Cyclonic storm CS 63-87 Cyclonic depression CDP 62 or less Cyclonic disturbance during CD Not specified monsoon Source: Indian Meteorological Department (IMD)
Fig. 1―Track of AILA during 23rd to 26th May, 2009
According to INSAT imageries, a low level circulation developed over South Bay of Bengal on 21st May 2009 at 0830 hrs IST, which developed into a Vortex with center 11.5º N/85.5º E and intensity T1.0 at 1730 hr IST on the same day. It gained intensity of T1.5 corresponding to depression with centre 16.5º N/88.0º E at 1130 hrs IST of 23rd May. It was the shear pattern at the time of cyclogenesis with maximum convection lying to the southwest of the system centre. The INSAT imagery of the system at the stage of depression is shown in Fig. 2. The cyclone retained its intensity for about 15 hours after it hit the landmasses as it was close to the Bay of Bengal. It laid centered over the Gangetic delta for a considerable period of time, ascertaining the availability of moisture. Due to occurrence of Aila there was intrusion of saline water from Bay of Bengal into the Hooghly-Matla estuarine system. The present study was conducted on 18th May, 2009 (before the Aila event, and even before the initiation of monsoon at Andaman) and 27th May, 2009 during Aila. The study was extended further 10 days, 4th June 2009, after the incidence of Aila in 12 different stations in the Hooghly estuary to observe the change in selected hydrological parameters. The entire network of the investigation encompassed the monitoring of hydrological parameters on 18th May,
Fig. 2―(a) Initiation of Aila, (b) Deep depression, (c) Aila with severity
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2009 (pre-Aila phase, designated as Phase A), 27th May (Aila phase, referred to as Phase B) and 4th June (postAila phase, referred to as Phase C) in 12 sampling stations in the Hooghly estuarine system of Gangetic delta (Table 2). A GPS was used to fix the coordinates of sampling sites in the estuarine stretch. For each observational station, triplicate water samples were collected from the surface during two tidal conditions at a distance of 50 meters of each other and analyzed for the selected parameters. The values are thus the mean (of three samples) ± S.D. The surface water salinity was recorded by means of an optical refractometer (Atago, Japan) in the field and cross-checked in laboratory by employing MohrKnudsen method6. The correction factor was found out by titrating the silver nitrate solution against standard seawater (IAPO standard seawater service Charlottenlund, Slot Denmark, chlorinity = 19.376‰). Our method was applied to estimate the salinity of standard seawater procured from NIO and a standard deviation of 0.02% was obtained for salinity. Glass bottles of 125ml were filled to overflow from collected water samples and Winkler titration was performed for the determination of dissolved oxygen. The pH was recorded with a portable pH meter with an accuracy of ± 0.01. Table 2—Location of the sampling stations A Surface Water Monitoring Locations Sl. Sampling Station No. 1 Raichak (Stn.1)
Coordinates
22° 12' 12. 00"N and 88° 07' 42. 09"E 2 Diamond Harbour (Stn.2) 22° 11' 04. 02" N and 88° 10' 50. 52"E 3 Kulpi (Stn.3) 22° 36' 28. 86"N and 88° 23' 28.32"E 4 Balari (Stn.4) 22° 07' 02. 16"N and 88°11'35. 34"E 5 Haldi River mouth (Stn.5) 22° 00' 26. 07"N and 88° 03' 29.64"E 6 Nayachar (Stn.6) 22° 00' 30. 42"N and 88° 03' 32.52"E 7 Khejuri Reserve Forest 21° 54' 51. 66"N and 88° 00' (Stn.7) 56.52"E 8 Ghoramara Island (Stn.8) 21° 56' 15. 24"N and 88° 07' 33. 06"E 9 Harwood point (Stn.9) 21° 56' 15. 24"N and 88° 07' 33. 6"E 10 Harinbari (Stn.10) 21° 46' 54. 12"N and 88° 04' 02. 64"E 11 Chemaguri (Stn.11) 21° 39' 49. 32"N and 88° 09' 11. 88"E 12 Sagar South (Stn.12) 21° 39' 04. 68"N and 88° 01' 47. 28"E
Results and Discussion Tropical cyclones and storms are very common in the Bay of Bengal. They severely affect the eastern coast of India as compared to that of the Arabian Sea. According to Koteswaram7, there were about 346 cyclones that include 133 severe ones in the Bay of Bengal, whereas the Arabian Sea had only 98 cyclones including 55 severe ones between the year’s l891 and l970. Cyclones with tremendous speed hit the coastline and inundate the shores with strong tidal wave, severely damaging the coastal resources. Intrusion of seawater into the upstream riverine zone through estuaries, creeks and inlets has high probability to alter the chemical composition of the aquatic phase, which is a subject of present discussion. Present study recorded the increase of surface water salinity (Fig.3) by 17.00%, 21.52%, 23.44%, 23.92%, 24.07%, 24.49%, 24.76%, 25.02%, 25.32%, 25.71%, 26% and 28.99 % at Raichak (Stn. 1), Diamond Harbour (Stn. 2), Kulpi (Stn. 3), Balari (Stn. 4), Haldi River mouth (Stn. 5), Nayachar (Stn. 6), Khejuri River Forest (Stn. 7), Ghoramara Island (Stn. 8), Harwood point (Stn. 9), Harinbari (Stn. 10), Chemaguri (Stn. 11) and Sagar South (Stn. 12) respectively and this increase is significant at 1% level (Table 3 & 4). Salinity of water can greatly affect organisms surviving in the system. A rise in salinity can cause harmful algal bloom as seen in case of alga, Chattonella marina. These blooms may pose an adverse effect on secondary production especially by killing local fish8. While salinity has been known to have a positive impact on the growth of algal blooms, high salinity has been known to stunt algae growth, which can affect the productivity of the system. The pH value also increased significantly in the study area (Fig.4) due to sudden intrusion of seawater in the estuarine system (e.g., 0.13% increase at Diamond Harbour, 0.12% increase at Haldi River mouth, Khejuri River Forest, Harwood point,
Fig. 3―Salinity in the study area
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Table 3―Variations of hydrological parameters during different phases before and during Aila Stations
Surface Surface Surface pH water salinity water salinity water salinity (Phase A) (Phase A) (Phase B) (Phase C)
pH (Phase B)
pH (Phase C)
D.O (Phase A)
D.O (Phase B)
D.O (Phase C)
Stn. 1 Stn. 2 Stn. 3 Stn. 4 Stn. 5 Stn. 6 Stn. 7 Stn. 8 Stn. 9 Stn. 10 Stn. 11 Stn. 12
3.41± 0.03 4.88 ± 0.02 6.10 ± 0.04 13.17 ± 0.10 12.05 ± 0.07 13.84 ± 0.12 15.55 ± 0.06 14.43 ± 0.08 17.22 ± 0.09 19.33 ± 0.10 20.58 ± 0.16 23.08 ± 0.19
7.65 ± 0.01 7.69 ± 0.02 7.70 ± 0.03 8.00 ± 0.03 8.11 ± 0.04 8.03 ± 0.04 8.11 ± 0.03 8.12 ± 0.03 8.11 ± 0.02 8.15 ± 0.02 8.21± 0.03 8.21 ± 0.03
7.65 ± 0.02 7.68 ± 0.03 7.70 ± 0.02 8.00 ± 0.01 8.10 ± 0.03 8.01 ± 0.01 8.11 ± 0.04 8.11 ± 0.02 8.11 ± 0.02 8.16 ± 0.03 8.21 ± 0.04 8.21 ± 0.03
5.71 ± 0.18 5.43 ± 0.20 6.63 ± 0.22 6.55 ± 0.28 4.80 ± 0.17 4.91 ± 0.10 4.80 ± 0.10 4.75 ± 0.12 4.60 ± 0.08 4.68 ± 0.13 5.05 ± 0.15 5.02 ± 0.12
4.98 ± 0.09 5.00 ± 0.12 6.03 ± 0.14 5.89 ± 0.21 4.74 ± 0.14 4.67 ± 0.10 4.65 ± 0.12 4.43 ± 0.08 4.54 ± 0.09 4.60 ± 0.11 4.99 ± 0.13 4.91 ± 0.11
5.23 ± 0.18 5.21 ± 0.16 6.50 ± 0.24 6.11 ± 0.21 5.31± 0.19 5.12 ± 0.13 5.32 ± 0.17 5.02 ± 0.11 5.99 ± 0.13 5.06 ± 0.12 5.19 ± 0.15 5.27 ± 0.18
3.99 ± 0.05 5.93 ± 0.08 7.53 ± 0.08 16.32 ± 0.14 14.95 ± 0.09 17.23 ± 0.12 19.40 ± 0.16 18.04 ± 0.15 21.58 ± 0.11 24.30 ± 0.14 25.93 ± 0.17 29.77 ± 0.15
3.45 ± 0.03 4.96 ± 0.05 6.18 ± 0.08 13.98 ± 0.15 12.78 ± 0.09 14.01 ± 0.10 15.98 ± 0.14 14.87 ± 0.11 17.96 ± 0.12 20.05 ± 0.14 21.00 ± 0.15 24.67 ± 0.16
7.65 ± 0.02 7.68 ± 0.03 7.70 ± 0.04 8.00 ± 0.04 8.10 ± 0.03 8.00 ± 0.02 8.10 ± 0.02 8.10 ± 0.03 8.10 ± 0.03 8.15 ± 0.04 8.20 ± 0.03 8.20 ± 0.04
Phase A = pre-Aila period (18.05.2009), Phase B = Aila phase (27.05.2009), Phase C = post-Aila phase (04.06.2009); Units of surface water salinity and DO are ppt and ppm respectively Table 4―ANOVA for selected physico-chemical variables during different phases before and after Aila Phase of sampling
F value
P Value
Between Salinity pH DO Salinity pH DO Phase A & 43.61 12.44 15.94 3.84 × 10-5 0.0047 0.0021 Phase B Between 46.45 2.20 24.87 2.89 × 10-5 0.1660 0.0004 Phase B & Phase C (Fcrit = 4.84 at df = 1 and n= 36)
Fig. 4―pH in the study area
Chemaguri and Sagar South, 0.38% increase at Nayachar and 0.25% increase at Ghoramara Island) as confirmed by the ANOVA (Table 4). High pH can affect the benthic community by way of precipitating the heavy metals from the aquatic phase to underlying sediment compartment 9,10,11. The DO level also varied significantly during the pre-Aila and Aila period as shown in Table 4 (Fobs = 15.94 > Fcrit = 4.84). Due to Aila the DO level in the surface water of the study area decreased from an average value of 5.24±0.70 ppm to 4.95±0.51 ppm, which is a fall by 5.53% (Fig. 3 & 5). This low DO during the Aila period (Phase B) may be attributed to
high salinity (because of seawater intrusion), which may result in the mortality of aquatic life or affect their metabolic process and rate. Depletion in dissolved oxygen (and resulting decrease in water quality) can cause major shifts in the kinds of aquatic organisms found in water bodies, and evidences of such shifts, if any, need to be properly documented for such situation originating from tidal surges and intrusion of seawater. Similar in-situ investigation was conducted in the same locations 10 days after the Aila (denoted as Phase C) to evaluate the status of hydrological parameters and observed a recovery trend in all cases. Decrease in aquatic salinity (13.53% at Raichak, 16.36% at Diamond Harbour, 17.93% at Kulpi, 14.34 % at Balari, 14.52% at Haldi River mouth, 18.69% at Nayachar, 17.63% at Khejuri Reserve Forest, 17.57% at Ghoramara Island, 16.77% at Harwood point, 17.49% at Harinbari, 19.01% at Chemaguri, and 17.13% at Sagar South) was observed and which is significant at 1% level as confirmed through ANOVA (Table 3 & 4). This is a clear indication of restoration of the water quality (in terms of salinity) after the Aila incidence. The relatively high salinity in the post-Aila phase (Phase C) in comparison to pre-Aila phase (Phase A) is a normal trend of the present geographical locale as pointed out earlier by several workers12,13,14. Such increase will continue till the end of June, and with the onset of monsoon the salinity value will drop. Restoration of DO is also confirmed through ANOVA (Table 3 & 4). In case of pH, no significant variation was observed between Aila and post-Aila period as revealed from Table 4 (Fobs = 2.2 < Fcrit = 4.84), which may be attributed to buffering
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References
Fig. 5―Do level in the study area
capacity of the seawater and normal rising trend of surface water pH during June in the study area12,13,14. Geographical and geological setting of Indian Sundarbans has made the area extremely vulnerable to cyclones and tidal surges. Dube et al15, created models to simulate the storm surges from past cyclones in the head of the Bay of Bengal (Orissa, West Bengal and Bangladesh) and discussed the formation and effect of each of these storms that were modeled. The extent of storm surges in the Bay of Bengal region and the causes behind extreme sea levels were also analysed by earlier workers16. Main reasons behind the increased vulnerability are: • Coastal waters (shallow bathymetry extending tens of kilometres offshore); • Convergence of the bay; • High astronomical tides; • Thickly populated low-lying islands; • Favourable cyclone tracks impacting perpendicular to coastline; and • Innumerable inlets and river systems. The situation of deltaic Sundarbans coincides with most of these points. Being located at the apex of Bay of Bengal, the islands of the deltaic complex (102 in numbers) are much lower than mean sea level and are criss-crossed by networks of creeks and inlets. The present geographical locale is an understudied area, and there is paucity of scientific studies on the dynamics of physico-chemical characteristics of water induced by cyclones and subsequent tidal surges. Present study may therefore serve as base line information of changes in hydrological parameters due to cyclonic storms and calls for a sound disaster management strategy to combat the ecological damage. Acknowledgement Authors are thankful to the Ministry of Earth Sciences, Govt. of India for providing financial support in undertaking this programme.
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