Science and Culture (ISSN 0036-8156), Vol 68, no 9-12, 2002, p 309-321 Sea Level and associated changes in the Sundarbans Sugata Hazra, Tuhin Ghosh, Rajashree DasGupta and Gautam Sen School of Oceanographic Studies, Jadavpur University, Kolkata 700032, India E-mail: [email protected] Introduction Sundarbans, the only mangrove tiger-land of the globe is presently under threat of severe coastal erosion due to relative sea level rise. The once largest prograding delta which registers the highest species diversity in terms of mangrove and mangrove associate flora and fauna is showing evidences that suggest the rich biodiversity is under threat. Increasingly, this deltaic island system is facing degradation due to natural and anthropogenic changes. Frequent embankment failures, submergence and flooding, beach erosion and siltation at jetties and navigational channels, cyclone and storm surges are all making this area increasingly vulnerable. In addition, alarming growth of population in this ecologically sensitive and fragile niche has posed a major threat for its very existence. Wide scale reclamation, deforestation and unsustainable resource exploitation practices have together produced changes in the physical and biological dynamics of the coastal system. In the present research, we have tried to observe the changes if any, in the physical dimensions of this coastal ecosystem over a period of 10-20 years. For this purpose we have selected components such as i) temperature regime, ii) occurrence of high intensity climatic events like cyclones, iii) shoreline change and iv) sea level change. Simultaneously, biological and social aspects like changes in forest cover and land use patterns, food grain, fish production and population pressure have been studied to understand the vulnerability of the ecosystem with respect to possible climate change. Eventually, a mathematical model developed by the school has been applied to predict future shore line positions of the Sagar island in different sea level rise scenarios. The possible impact of such changes on the other bio-physical and socio economic dimensions of this coastal ecosystem, however, remains to be investigated further. Geological Background The Indian part of Sundarbans lies between 21030’ N and 22040’48” N latitude and 8801’48”E and 89004’48” E longitude. It is delimited in the north by the so called ‘Dampier- Hodges line’ demarcating the northern extension of the intertidal zone marked by mangrove forests of 1830. This line closely corresponds to Kakdwip – Basirhat – Dhaka lineament picked up from satellite imagery (Chakraborty, 1991). In the south, the Sundarbans is bound by the Bay of Bengal. The river Hoogly (in the west) and the river Harinbhanga– Raimangal – Ichamati (in the east) demarcate the western and eastern boundaries respectively. Up to the year 1770, the total area of Sunderbans of India and Bangladesh was estimated to be around 36,000 sq. km., which at present, stands to be 25,000 sq. km. The Indian part consists of 9630 sq. km. and the rest lies within Bangladesh. Out of the 9630 sq. km., 4264 sq. km. of Wetland / Mangrove constitutes reserve forests, which inturn comprises of 2195 sq. km. of Wetland – Mangroves and 2069 sq. km. of tidal river . This means that the reclamed area around 5,366 sq. km. (Figure 1) is used for human settlements in 19 blocks (13 in South 24 Paraganas & 6 in the North 24 Paraganas). Fig. 1 The Sundarban island system is geologically very recent. The Delta outbuilding of Ganga-Brahmaputra system though initiated at the end of Miocene, could have reached the present location of Sundarban delta, not more than 10,000 years back (Pleistocene to Recent). The geological formation covering the island system belongs to the so called 'Bengal Alluvium'. The facies and palaeo environmental maps prepared by Raman & Neogi 1986, show an eastward prograding delta changing southward since the early Miocene, with a position of shoreline change from NE-SW to nearly E-W in the Quaternary. In the Pleistocene- Recent period, depositional trends changed significantly. The discontinuation of the eastward thickening of sedimentary formations is probably due to the reduction in the rate of subsidence of the Bengal Basin floor along the N-S axis, leading to rapid filling up of the basin. The Pleistocene eustatic sea level fall has created widespread terraces and deep erosion of valleys by lowering of base level (Alam, 1996). The sediments brought in by the Ganga-Brahmaputra system during the post-Pleistocene period probably bypassed the deltaic part for a great extent, which contributed to the rapid growth of Bengal deep sea fan (Biswas, 1993). During the recent times the Bengal delta acquired a typical tide dominated lobate form with a tidal range varying between 3.7 to 5 m. The estuarine mouth of Hoogly and it's numerous distributaries like Saptamukhi, Jamira etc. acquired a typical seaward flaring funnel shaped pattern. Due to progressive shallowing of channels, the height of tidal bore became maximum 6.4 m and further inland this increased to 7.17 m (Tide Table of Hoogly river, 1984). The flood and ebb tides have a semi-diurnal nature (12.5 hrs interval) and occur twice daily. Within this cycle floodwater flows for 2-3 hrs duration. At the remaining 8-9 hrs, the estuary is covered by ebb tide flow of lesser velocity. From the very begining, Sundarban with it’s numerous large and small islands remains a subsiding delta, with vertical upbuilding. Fragments of ‘ceriops’ tree obtained during excavation of George’s Dock at Kolkata below sea level (14 m bgl) justify the fact (1st Working Plan, Dept. of Forests, Govt. of West Bengal, 1962). An intricate network of distributaries, channels and tidal creeks dissect the area forming numerous plano-convex islands made up of silt and silty clay. The islands of 3 to 8 m height, are partially/ often completely inundated by water during high tide,. The subsidence in these areas is comparatively more severe than in the open coastal parts of Digha and these are often manifested by mild earth tremors occasionally. Rarely there are instances of sudden subsidence like that during 1897, which created a series of 'Bils' instantaneously at Rangpur & Mymansingh now at Bangladesh; (1st Working Plan, Dept. of Forests, Govt. of West Bengal, 1962). According to Morgan and McIntire (1959), the Bengal Basin, and this part of the deltaic plain is gradually tilting towards east. This has probably caused the main fresh water discharge to shift gradually eastward (through Bangladesh) imposing severe stress on freshwater budget for Hoogly-Matla estuary. According to Milliman (1989) the eastern part of the Sundarban delta is experiencing a higher rate of subsidence due to sediment loading. This subsidence rate is often close to 6 mm/year as measured at some places of Bangladesh. The tide dominated estuarine system exhibits typical flow separation, with downstream freshwater flows along the right bank and upstream saline water tide flowing along the left bank (eastern) of the channel. The Hoogly-Matla estuary however experiences a higher rate of freshwater discharge than its eastern counterpart and therefore is less saline and comparatively well mixed. The salinity gradient increases eastward from Matla river as the erstwhile rivers have now been partially / completely cut from their freshwater source and contain tidal water only, apart from the seasonal rainwater that flows from the islands. Recent Changes in temperature regime over Sundarbans: Surface air temperature anomaly data over the Sundarbans and adjacent parts of the Bay of Bengal (Fig.2) has been analysed to find an increasing trend in the yearly rise in temperature. This finding corroborates with the existing global warming phenomena. Temperature increase is found out to be @ 0.0190C per year (Fig.2). A similar rising trend is also observed for changes in the maximum and minimum temperature record collected from the sand head in the Sundarbans (Fig. 3a & 3b). Fig 2. Surface air temperature anomaly data over 20oN-25oN and 85oE – 90oE reveals an increase of Temperature @ 0.019 oC/Yr., which is correlatable with Sea Level Rise Fig. 3a V ARIATION IN THE M INIM UM TEM PERATURE IN SUNDARBANS (SOUTH 24 PARAGANAS) SINCE 1992 (centre-Sandhead) 25 20 15 y = 0.4107x + 17.476 R2 = 0.3158 10 5 0 1992 1993 1994 1995 1996 1997 1998 years Fig. 3b VARIATION IN THE M AXIM UM TEM PERATURE IN SUNDARBANS (SOUTH 24 PARAGANAS) SINCE 1992 (centre-Sandhead) 33 32.5 32 y = 0.286x + 29.549 31.5 R2 = 0.3459 31 30.5 30 29.5 29 28.5 28 27.5 1992 1993 1994 1995 1996 1997 1998 years Changing pattern of cyclones over the Sundarbans: The analysis of available records of cyclones over the Bay of Bengal adjoining the Sundarbans, exhibits an increasing trend in the degree of their intensity while showing a decrease in the frequency of occurrence (Fig. 4a & 4b). This is a striking phenomena in the perspective of warming trend discussed earlier and has significant bearing on the extent of coastal flooding, erosion and saline water intrusion due to storm surges. The increase in the intensity also implies increase in the precipitation pattern over this part of the Bay of Bengal. Fig. 4a Frequency of depressions, cyclonic storms & severe cyclonic storms in the Bay of Bengal coast (1970-2000) 14 12 y = -0 .1827x + 8.471 R 2 = 0.2837 10 8 6 4 2 0 years No. of Hurricanes in the Bay of Bengal Coast (1990-2000) Fig. 4b 6 5 y = 0.2x + 0.9818 2 4 R = 0.2814 3 2 1 0 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 years Erosion –Accretion pattern in the Sundarban Island system In order to understand the process of natural erosion – accretion of the geomorphic features in a tide dominated deltaic island system; a detailed field study was undertaken in the islands of Hugli-Matla estuary during the period 1996 to 1999.