Indian Journal of Geo-Marine Sciences Vol. 40(6), December 2011, pp. 802-812

Trend of sea level change in the Hugli estuary,

Sreetapa Nandy and Sunando Bandyopadhyay* Department of Geography, University of Calcutta, 700019, India * [ E-mail : [email protected] ] Received 22 September 2010 ; revised 23 April 2011

Trends of annual sea level records of four tidal observatories of the Hugli estuary—Sagar, Gangra, Haldia, and —are analysed from the records of Permanent Service for Mean Sea Level (PSMSL). The rates of sea level changes for the stations are found to be –3.82, +0.89, +2.43 and +4.85 mm yr–1 respectively, connoting a significant positive relation between landward distances of the stations and the rates of sea level rise. This seems to be mainly controlled by disequilibrium in the morphological state of the landward-narrowing estuary with some contribution from sediment autocompaction. Sea level trends of the Hugli have no apparent relation with erosion and accretion of its tidal islands.

[Keywords: Sea level change, tide gauge data, estuary, morphological equilibrium, Sundarban, Hugli]

Introduction (Hospital Point), 70 km from the sea (21°34'–22°13'N, An estuary is particularly responsive to changes in 87°46'–88°15'E).5 Tidal currents, however, can be the Sea Level (SL). Besides increasing vulnerabilities detected up to Swarupganj, another 219 km upstream. to storms and coastal erosion, it changes tidal forcing Previously a part of the Sundarban mangrove wetlands, and influences estuarine sedimentation.1 With onset of reclamation of the banks and some of the islands of the the Hologne, the SL rose fast at the beginning, then estuary was initiated in the early 19th century by very slowly from c. 7,000 yr BP to get nearly stabilised placing marginal embankments6,7. 2 at 2,000~3,000 yr BP. According to the Fourth The tidal characteristics and channel evolution of Assessment Report (AR4) of the Intergovernmental the Hugli estuary have been extensively studied.8-10 Panel for Climatic Change (IPCC), the rise in the SL Comparatively, literature on trend of SL variations of –1 picked up again during the 19th century at 1.7 mm yr the estuary is sparse. The only work based on tidal and escalated to 3 mm yr–1 in the final decade of the records of Sagar indicated a rise in SL at 3.14 mm 20th century. Between 1993 and 2003, thermal yr–1.11 This study used tide-gauge data of five expansion of sea water and melting of land ice each discontinuous years within a span of 14 years: 1985, 3 accounted for about half of the rise. On an average, the 1990, 1996, 1997 and 1998. The dataset was not –1 rate of rise was estimated at 1.4~2 mm yr for the last obtained from Permanent Service for Mean Sea Level. century.4 The AR4 unequivocally held the human- This and two other works from the same research induced global warming responsible for this team12-13 linked the erosion of Ghoramara, an island in –1 accelerated increase and suggested a rise of 4 mm yr the Hugli estuary (Fig. 1), to SL rise. The Present study up to 2090s according to one of its future scenarios. consists of the pattern of SL change reflected in the Hugli is the westernmost distributary of the Ganga- tidal records of Permanent Service for Mean Sea Level Brahmaputra delta (GBD). Gradually degenerated pertaining to four stations along the Hugli estuary. Its over the last few centuries, the river’s off-take is not possible causes and effects on evolution of tidal islands naturally connected to the Ganga system anymore. Its of the estuary are also discussed. discharge is now mostly maintained by its right bank tributaries and the 1975 Farakka barrage scheme that Materials and Methods diverts certain amount of water from the Ganga about The SL data for the present study was sourced 426 km upstream of its confluence at Sagar. Based on from Permanent Service for Mean Sea Level morphological constriction, highest tidal range and (PSMSL: www.psmsl.org), world’s archive of mean saltwater intrusion, the landward limit of the macrotidal monthly and mean annual tide-gauge records. Hugli estuary can be fixed at Diamond Harbour The PSMSL SL data are presented in two formats: NANDY & BANDYOPADHYAY: SEA LEVEL CHANGE IN HUGLI ESTUARY 803

Fig. 1Image of the Hugli estuary showing locations of the PSMSL tidal observatories (indicated by red triangles) and major tidal islands. (IRS-P6 LISS-3 image of 28 Feb 2008)

804 INDIAN J. MAR. SCI., VOL. 40, NO. 6, DECEMBER 2011

‘metric’ and ‘revised local reference (RLR)’. The available at www.psmsl.org/train_and_info/geo_signals/ later dataset is released after year-to-year checks are gia/peltier/index.php . performed relative to a common datum and is fit for analysis of long-term changes in SL. The RLR datum Data availability Although some authors maintain that a 20-year record is arbitrarily taken as approximately 7 m below the of mean annual data is adequate for computation of the mean sea level (MSL) to avoid negative values in 24 14-16 rate of sea level change, a 50-year record is considered gauge records. The PSMSL RLR data were the preferable for offsetting all local and short-term variations source of all major works on long-term analysis and like effects of ENSO and tropical storms.2,25,26 Among the projection of global SL from which the assessment data of 27 Indian stations available at the PSMSL (as of reports of IPCC were prepared. The datasets were also 17-21 September 2010), only six stations have 40 or more years used for studies on SL trends around India. of usable RLR records. Most previous studies on the The RLR datasets represent changes in relative SL. At Indian PSMSL data considered all available RLR records a particular station, they reflect all possible local and spanning 20 or more years17–21. There are eight PSMSL regional factors that may cause vertical land movements sites in (Fig. 1) and they all are located by apart from the secular rise in the global SL. These factors the Hugli from its confluence (Sagar and Dhablat) to include glacial isostatic adjustments (GIA), tectonic 208 km upstream (Tribeni). Data from all of these stations movement, subsidence due to sediment compaction or are not usable for time series analysis however. Four groundwater extraction and rise in the water level due to stations can be filtered out that are relatively free from the infilling or constriction of estuaries. Although it is often influence of upstream discharge and possess quality data difficult to tell apart the effect of some or one of these of sufficient duration without unexplained datum shifts or factors on the observed SL change at a given locality, a anomalous values (spikes) (Table 1). All of these few unique examples can be cited in stations like stations—Sagar, Gangra, Haldia and Diamond Harbour— Stockholm, Sweden (falling SL since 1889 due to post- fall within the estuarine part of the Hugli and are spaced glacial rebound), Nezugaseki, Japan (sudden rise in SL in by 17.5 km from each other on an average. Time series 1964 due to earthquake-related subsidence), Fort analyses were performed on these four datasets to bring Phrachula Chomklao of Bangkok, Thailand (accelerated out the mean monthly and annual rates of changes in sea post-1965 rise in SL due to groundwater withdrawal) and level in the estuary. Manila, Philippines (accelerated post-1963 rise in SL due to harbour development and sedimentation)22. Two Results vertical movement models—VM2 and VM4—were Monthly records computed by Richard W. Peltier for the GIA component The PSMSL monthly records indicate that the of land-level change for all PSMSL stations23 and are Hugli estuary has marked intra-annual variation in

Table 1Data status of the PSMSL stations along the Hugli, West Bengal (Data from PSMSL and 2008 IRS-L3 image)

Station Distance Period covered in Available Usable Remarks from sea PSMSL RLR annual data annual data (km) records (in yr) (in yr) Sagar: 0.0 1937–1988 (52 yr) 48 48 Data used in the present study Beguyakhali Sagar: Dhablat 0.0 1882–1885 (4 yr) 4 4 Sample size too small for analysis. Gangra 31.4 1974–2006 (33 yr) 27 27 Data used in the present study Haldia 43.4 1971–2006 (36 yr) 34 33 Data used in the present study Diamond 70.1 1948–2006 (59 yr) 57 57 Data used in the present study Harbour Garden Reach 142.1 1932–2005 (74 yr) 66 0 Data integrity questionable due to unexplained post-1975 datum shift. Influenced by fluvial discharge. Entire dataset flagged by PSMSL. Khidirpur 143.9 1882–1931 (50 yr) 21 0 Data integrity questionable due to unexplained post-1893 datum shift. Influenced by fluvial discharge. Entire dataset flagged by PSMSL. Tribeni 204.8 1962–2006 (46 yr) 44 0 Data integrity questionable due to probable datum shift in mid-1970s. Influenced by fluvial discharge. NANDY & BANDYOPADHYAY: SEA LEVEL CHANGE IN HUGLI ESTUARY 805

local mean water due to the freshets from the Indian major river mouth, is just 117 mm between September summer monsoons. The SL varies by 487 mm and December. Monthly SL fluctuations at between February and August at the mouth of the Visakhapatnam (usable monthly data: 47 yr between estuary (Sagar, usable monthly data: 46 yr). At its 1937 and 2006), situated some 150 km northeast of constricted apex (Diamond Harbour, usable monthly the Godavari delta, is bimodal with the smaller peak data: 55 yr), the range increases to 599 mm. The in June and the higher in October. At 429 mm patterns of monthly fluctuations in sea level in all the between October and March, its seasonal range is tidal observatories of the Hugli estuary closely comparable to the mouth of the Hugli estuary (Fig. 2). correspond to the conventional seasonal divisions of While the unimodal pattern of monthly SL northern coast: pre-monsoon fluctuations separate the four selected tidal stations of (Feb–May), monsoon (Jun–Sep) and post-monsoon the Hugli from rest of India, comparison within these (Oct–Jan). Comparatively, the seasonal range and stations can be carried out as all are subjected to pattern of SL is often different in other Indian tidal similar seasonal conditions. observatories. For example, the SL fluctuates throughout the year in Mumbai (usable monthly data: Annual records 112 yr between 1878 and 2006) and show three The annual sea level trends of the Hugli estuary prominent peaks in April, June and December. Its stations, as estimated from the available PSMSL seasonal range, owing to its location away from any data, are shown in Table 2 and Fig. 3. An anomalous

Fig. 2Average monthly fluctuations of sea level at selected tidal stations (data from PSMSL). Table 2Selected parameters of PSMSL stations of the Hugli estuary (Data from PSMSL, Survey of India tide tables and 2008 IRS-L3 image) Station Distance Estuary Tidal LMWL above SL change due to Observed SL Confidence from sea width range MSLa (m) glacio-isostatic riseb change (mm yr–1 ) limit (95%) (km) (km) (m) (mm yr–1) Sagar 0.00 51.58 4.38 0.18 –0.48 –3.82 ±0.95 –5.73 to –1.90 (Beguyakhali) Gangra 31.37 17.25 4.77 0.34 –0.51 +0.89 ±0.80 –0.75 to 2.53 Haldia 43.36 10.47 4.90 0.41 –0.51 +2.43 ±0.63 1.15 to 3.72 Diamond 70.11 1.88 5.04 0.48 –0.52 +4.85 ±0.42 4.01 to 5.70 Harbour Note: a. LMWL: Local Mean Water Level; MSL: Mean Sea Level. The basic PSML data are available in Revised Local Reference (RLR) datum, which is approximately 7 m below MSL. b. Based on Richard Peltier’s model VM-4 23 806 INDIAN J. MAR. SCI., VOL. 40, NO. 6, DECEMBER 2011

feature of the records is that the Sagar observatory registered decrease in SL—an observation difficult to fit in the general scenario of increasing SL in all nearby stations and continuation of coastal erosion at the vicinity of the observatory for the last 100 years.27 Overall, the estuary’s SL trends denote a positive relationship with the distance from the sea. As one moves northward from the mouth of the estuary at Sagar to its apex at Diamond harbour, the rate of sea level change escalates at 0.12 ± 0.01 mm yr–1 km–1 (Fig. 4). The rate slightly reduces to 0.10 ± 0.01 mm yr–1 km–1 if the data of Sagar is excluded.

Discussion Rates of glacio-isostatic rise along the Hugli estuary have been modelled from 0.48 mm yr–1 at Sagar to 0.52 mm yr–1 at Diamond Harbour (Table 2). These values are not significant either for weakening the SL trend’s positive relationship with the distance from the sea or for explaining the marked differences in the rates of SL change from the widely agreed global secular SL rise rate of 1–2 mm yr–1 at both ends of the estuary. Influence of some regional factor(s), the intensity of which change(s) along the length of the estuary, is obvious here. The possibilities include: (1) rise in the local mean water level due to increasing river discharge and/or sedimentation and/or increasing incidence of tropical cyclones and (2) land subsidence due to sediment compaction and/or groundwater withdrawal. The effects of the first set of factors would accentuate northward because of the progressive squeeze of the estuary area in that direction.

Fluvial discharge Fig. 3Changes in annual sea level at tidal observatories of the Hugli estuary (data from PSMSL). As noted, the terrestrial flow of the Hugli is now chiefly maintained by its west-bank tributaries and the

Fig. 4Relation between landward distance and sea level at tidal observatories of the Hugli estuary (data from PSMSL). NANDY & BANDYOPADHYAY: SEA LEVEL CHANGE IN HUGLI ESTUARY 807

Farakka barrage. The Farakka project was not meant navigational channels free from clogging. Various for improving the draft of the estuary reach of the estimates of average annual terrestrial sediment Hugli. The maximum possible augmentation of 1,134 discharge of the Hugli range from 328 to 616 million cumecs from Farakka would increase the ebb flux at tones.35-37 Understandably, a major portion of the the seaface to two percent only—hardly influencing sediment load is utilised to fill-up the estuary itself. the flow conditions of the estuary.9 The actual For a 15 km reach of the Hugli off Haldia, net contribution of the Farakka, especially during the lean sedimentation of 29.87 × 106 m3 were observed season between January and May, had been between 1993-94 and 1996-97.38 Currently some 15 × considerably lower than this. Principal reasons for this 106 m3 of sediments are required to be dredged are the water-sharing treaties with Bangladesh and annually from the same area by the port authorities to reduction in the discharge of the Ganga itself due to keep its navigational channel open39. upstream diversion of water in its catchment. In India In a morphological steady state, an estuary neither and Nepal, at least 33 major barrages and water becomes a sink nor a source of sediments. It was divergence structures were constructed above Farakka shown that in such condition, length of a resonant 28,29 apart from hundreds of lift irrigation points. macrotidal estuary (λ)—like the Hugli—tends to The discharge of the Hugli at its confluence was equal a quarter of the tidal wavelength (L) generated 6 3 –1 30 estimated by Rao at 493,400 × 10 m yr . Annual in it,40 that is: changes in this record between 1937 and 2006 are hard to find because the data are treated as classified. λ → 0.25 L Indirect evidences like annual rainfall trends in the catchment areas of the Hugli during 1901–2003 Length of the tidal wave—that behaves like a connote mixed signals. While meteorological shallow water wave in an estuary—is determined by subdivisions like Jharkhand and Bihar—chief tidal period (T: a constant in a given locality, 44,640 s), gravitational acceleration (g: another contributors to the west-bank tributaries to the Hugli— –2 registered decreasing precipitation trends, Gangetic constant, 9.81 m sec ) and average depth of the West Bengal recorded increasing tendency.31 Trends of estuary below high water level springs (D: the only the extreme rainfall events during 1901-2000 did not variable) in the following relationship: show any appreciable change in these subdivisions.32 √ Thus, it does not seem likely that the discharge of the L = T (gD)

Hugli could have steadily gone up during the 1937– As seen earlier, the banks of the Hugli estuary, 2006 period so much as to cause the mean annual SL along with some of its tidal islands, has long been trends to rise in its estuary. On the contrary, natural as fully reclaimed from mangrove wetlands. well as human factors like riverwater diversion for Reclamation curbs the area of tidal spill through irrigation during the last few decades have become marginal embankments, takes away shallow intertidal major issues affecting its discharge. Since 1959 seven wetlands from the influence of the estuary and major and a number of minor reservoirs were thereby enhances its average depth. This removes the constructed on the Hugli’s western tributaries of estuary from morphological steady state. It then Mayurakshi, Damodar and Kangsabati. responds by active channel sedimentation and bank Morphological disequilibrium erosion to decrease the average depth and to restore In the Hugli, duration of flood tide is about 3 hours the equilibrium condition.41 As indicated by weighted in a 12.4-hour tidal cycle, with velocity range of 2–3 average of 1,246 soundings depicted in 1992–2002 m s–1 for the flood currents and <1 m s–1 for the ebb hydrographical charts 3011 and 3013 of India’s Naval currents.10 Because of the time-velocity asymmetry in Hydrographic Office (NHO), the mean depth of the tidal propagation, peak flood and ebb discharges Hugli estuary is 8.78 m with respect to high water amount to 2.6×105 and 1.09×105 cumecs respectively level springs. This brings the length of the tidal at the mouth of the main (western) branch of the wavelength to 414,368 m. Thus, the length of the estuary.33 Therefore, the direction of sediment Hugli estuary (70,110 m) at present equals about movement in the Hugli is landward34, making the one-sixth of the tidal wavelength entering into it estuary a major sediment sink that necessitates (λ = 0.17 L), largely explaining the reasons of flood year-round dredging operations to keep its dominance of the Hugli. 808 INDIAN J. MAR. SCI., VOL. 40, NO. 6, DECEMBER 2011

Evolution of tidal basins depends on the interaction 1937 to 2006. At the Ganga delta as well as in the between the tide and basin morphology to a great Indian east coast, SL becomes somewhat higher extent. Morphological disequilibrium of the Hugli during the cyclones of La Niña epochs than during El provides a plausible explanation to the progressively Niño.42 This is mainly because of the augmented higher rates of SL rise towards the constricted apex of freshwater discharge into the estuaries during La the estuary (Fig. 3). The increasingly smaller cross- Niña.43,44 Month-wise records of La Niña and El Niño sectional areas of the Hugli towards the north are epochs are reflected in Southern Oscillation Index probably filling-up at a faster rate than the southern (SOI).45 For the present work, monthly SOI data from sections, causing the tidal levels to rise at a 1937 to 2006 were obtained from the US National progressively higher pace in that direction. This Ocean and Atmospheric Administration.46 These were possibility is supported by a comparison between two compared with all types of cyclonic storms passing consecutive editions of the aforementioned NHO through 100 km of the mouth of the Hugli estuary47 charts of the Hugli. It showed that between 1988– (Table 4). For these 70 years, occurrence of most 1992 and 1992–2002, the average depth of the upper types of cyclonic storms during the La Niña as well as 33% of the estuary north of Sagar island (21º53' N) El Niño epochs bespoke declining tendency and reduced its depth at a faster rate than the lower 67% therefore could not have played a part in the SL rise. (Table 3). Only the severe cyclonic storms of El Niño epochs weakly contradicted the observed pattern. But they Tropical cyclones constituted just 5.8% of the total recorded events of The Hugli estuary is located at the apex of the Bay 346 storms and could not have significantly of Bengal and is prone to storm surges caused by influenced the SL trends. tropical cyclones that take place between May and December. As the long-term records tend to average Subsidence out yearly aberrations, the storm surges can influence Evidences of land subsidence are not uncommon in the SL trends in a significant way only if the Indian Sundarban and are being reported for a long frequency of the storms is increased during the period time.48,49 On a regional scale, general dominance of covered by the PSMSL records of the Hugli, i.e., from silts and sands over clays in the GBD sediments Table 3Bathymetric changes in the Hugli estuary between 1988-92 and 1992-2002 Source Area covered Year of Number of Average Depth Average Average Average Depth (excluding Survey Soundings below CD (m)a Depth below Depth below reduction (m) supratidal islands) MSL (m) HWLS (m)

NHO Chart North of 21º53' N: 1988–1990 844 3.57 6.39 9.19 No. 3013 upper 32.9% of +0.34 estuary area 2000–2002 829 3.23 6.05 8.85 NHO Chart South of 21º53' N: 1988–1992 573 3.19 6.01 8.81 No. 3011 lower 67.1% of +0.06 estuary area 1992–2002 417 3.14 5.96 8.75 Note: a. Chart Datum (CD) of the Hugli is situated 2.82 m and 5.615 m below mean sea level (MSL) and high water level springs (HWLS) respectively (Source: Survey of India tide tables).

Table 4Trend of occurrences of cyclonic storms of La Niña and El Niño epochs passing within 100 km of the mouth of Hugli estuary between 1937 and 2006 (75 years)

La Niña Storms El Niño Storms Number of occurrences Trend of occurrences Number of occurrences Trend of occurrences All types of storms 170 y = –0.0032x + 6.5849 176 y = –0.0028x + 5.685 Depressions (wind speed < 63 130 y = –0.0018x + 3.7931 136 y = –0.0023x + 4.6018 km h–1 ) Cyclonic storms (wind speed 19 y = –0.0008x + 1.6044 20 y = –0.0003x + 0.6078 63–87 km h–1) Severe cyclonic storms (wind 21 y = –0.0003x + 0.6021 20 y = +0.00004x –0.0548 speed > 87 km h–1) Data Source: 46 & 47 NANDY & BANDYOPADHYAY: SEA LEVEL CHANGE IN HUGLI ESTUARY 809

significantly prevents autocompaction.50 Therefore, & mud-content of Holocene sediments at southern the chief cause of the 2–4 mm yr–1 subsidence rate Sagar island, Haldia and Diamond Harbour amount to estimated for the central and coastal GBD was said to 23 m & 89%, >30.5 m & 77% and 26.5 m & 100% be tectonic.51 However, the Holocene sediments of the respectively. Muds in the Holocene strata were Hugli estuary area are largely dominated by clays and markedly softer and less-compacted than their silts (or muds), indicating likelihood of significantly harder (or stiffer) underlying Pleistocene autocompaction-related subsidence. From core counterparts, making them prone to autocompaction. samples, Stanley and Hait observed that the thickness The above three samples did not contain any peat

Fig. 5Evolution of islands in the central Hugli estuary. (Source: 1904-05: Survey Department hydrographic chart; 1922-23 & 1967-68: Survey of India maps 79B/4 & 79C/1, inch & metric editions; 2008: IRS-P6 LISS-3 data of 14 Feb 2008) 810 INDIAN J. MAR. SCI., VOL. 40, NO. 6, DECEMBER 2011

layer. The authors estimated the range of subsidence central part of the estuary during the last few years rate for the Holocene strata of western GBD without close to Ghoramara. Conversely, although the Sagar the occurrence of peats to be 2–5 mm yr–1.52 tidal observatory recorded a fall of SL at 3.82 mm yr–1 Understandably, these rates are averages for the last during 1937–1988, the Beguyakhali mouza (village), 7,500 yr and projecting them to the 70 yr period where the observatory is situated, recorded a loss of between 1937 and 2006 may not be fully justified. 2.19 km2 between 1922-23 and 2001. Sagar island, as They point to the distinct possibility of vertical land a whole, lost 26.14 km2 during the same period.53 The movement in the estuary area that can translate into main reason for post-1988 discontinuance of PSML relative SL rise above the global trend of secular SL records at Sagar is that the tidal observatory itself got rise at Haldia and Diamond Harbour, but cannot destroyed due to coastal erosion and had to be explain the northward increase in the rates of SL relocated. change along the Hugli estuary. They also offer no explanation to the falling trend of SL seen at Sagar. Conclusion The predominant landuse in the region around the The Hugli estuary is acting as a sediment sink and Hugli estuary including its inhabited islands like is largely offsetting the effects of an accelerated SL Sagar and Ghoramara mostly involve mono-cropping rise in terms of land loss. The estuarine islands of rice during the monsoons. Large-scale lifting of emerge, accrete and dissipate due to sediment groundwater for farming or other use is not practised reworking in a high-energy cyclone-dominated in the region. The Haldia urban-industrial area uses macrotidal environment without any obvious relation desalinised water pumped out from the Hugli. None to SL change. The patterns of SL variations along the of the critical or semi-critical blocks of West Bengal Hugli estuary are mainly controlled by disequilibrium with respect to groundwater overexploitation exists at in the morphological state of the estuary and its or close to the Hugli estuary. All these denote that northward squeeze apart from subsidence due to there is little chance of subsidence due to groundwater autocompaction of Holocene sediments. The roles of extraction and it cannot have any obvious influence river discharge and tropical cyclones in regulating the on SL changes of the area. SL trends of the Hugli are not very apparent.

Effects of sea level change on estuarine islands Acknowledgement The effects of SL change on the supratidal Authors thank Prof. Aditya Chattopadhyay, Dept of morphology of the Hugli estuary are not perceptible. Statistics, University of Calcutta, for discussing A recent review showed that as a whole the estuary various aspects of the work. gained 17.63 km2 of supratidal area at the rate of 117 ha yr–1 during 1904-05–1922-32, lost 64.66 km2 at the References rate of 147.04 ha yr–1 during 1922-32–1967-69 and 1 Dyer K R, Responses of estuaries to climate change In Climate Change: Impact on Coastal Habitation, edited by again gained 6.49 km2 at the nominal rate of 19.67 ha –1 53 Eisma D, (CRC Press, Boca Raton) 1995, 85–110 yr between 1967-69 and 2001. The study indicated 2 Pirazzoli P A, Sea Level Changes: The Last 20000 Years, that the accretion-erosion phases of the estuarine (John Wiley and Sons, New York) 1996, pp 211 islands situated on different tidal sand ridges operate 3 Intergovernmental Panel on Climate Change (IPCC), Climate on 50 to 100 year cycles. For example, Ghoramara Change 2007—The Physical Science Basis, (Cambridge University Press, Delhi), 2007, pp 1009 island got detached from Sagar between 1903-04 and 4 Church J A & White N J, A 20th century acceleration in 1904-05 and enlarged to a considerable size in global sea-level rise, Geophysl Res Lett, 33(2006) L01602 1922-23. As Ghoramara entered into a progressively 5 Bandyopadhyay S, Coastal changes in the perspective of eroding phase thereafter, the Nayachar emerged in long term evolution of an estuary: Hugli, West Bengal, India In Quaternary Sea Level Variation, Shoreline Displacement 1948 and gradually became the second largest island and Coastal Environments, edited by Rajamanickam, V & of the estuary. The accretion of Nayachar progressed Tooley, MJ, (New Academic Publishers, New Delhi) 2000, actively during 1971–2006 when the SL at Haldia, pp 103–115 just 2 km to its west, went on increasing by 2.43 mm 6 Ascoli F D, A Revenue History of from 1870 to yr–1 (Table 2 & Fig. 5). Between 1967-68 and 2008, 1920, (Bengal Secretariat Book Depot, Calcutta) 1921, 2 pp 159 Nayachar accreted 30.99 km of supratidal area while 7 Pargiter F E, A Revenue History of the Sundarbans from 2 Ghoramara lost 4.01 km . Three tidal islets—Balari, 1765 to 1870, (Bengal Government Press, Alipur) 1934, Nayachar-South and Shiber Char—emerged in the pp 156 NANDY & BANDYOPADHYAY: SEA LEVEL CHANGE IN HUGLI ESTUARY 811

8 Hiranandani M G & Ghotankar S T, Balari Bar and Regime of 26 Pugh D, Changing Sea Levels: Effects of Tides, Weather and Hooghly Estuary: Technical Memorandum NAV-2, (Central Climate, (Cambridge Univ Press, Cambridge) 2004, pp 265 Water & Power Research Station, Pune) 1961, pp 42. 27 Bandyopadhyay S, Natural environmental hazards and their 9 Roy S C, Hydraulic Investigations on behalf of Hooghly management: A case study of Sagar island, India, Singapore Estuary, (Metteilungen des Franzius, Inst fur Grund-und- J Tropical Geogr, 18(1997) 20–45 Wasserban der Technischen Uni Hannover) 1969, pp 149 28 Mirza M M Q, The water dispersion: environmental 10 Sanyal T & Chatterjee A K, The Hugli estuary: A profile In Port effects and implications—An introduction In The Ganges of Calcutta: 125 Years Commemorative Volume, edited by Water Dispersion: Environmental Effects and Implications Chakraborty S C, (Calcutta Port Trust, Calcutta) 1995, pp 45–54. edited by Mirza, M M Q, (Kluwer Academic Pub, Dordrecht) 11 Hazra S, Ghosh T, Dasgupta R & Sen G, Sea level and 2004, pp 1–11 associated changes in Sundarbans, Sci and Cult, 68(2002) 29 Parua P K, The Ganga: Water Use in the Indian 309–321 Subcontinent, (Springer Science, Dordrecht) 2010, pp 391 12 Hazra S, Ghosh, T, Bakshi, A & Ray, N, Sea level change: 30 Rao K L, India’s Water Wealth: Its Assessment, Uses and Its impact on West Bengal coast, Ind J Geogr Environ, Projections, 2nd edn, (Orient Longman, Calcutta) 1979, 6(2001) 25–37. pp 267 13 Ghosh T, Bhandari,G & Hazra S, Coastal erosion and flooding 31 Guhathakurata P & Rajeevan M, Trends in the rainfall in Ghoramara and adjoining islands of Sundarban deltaic system pattern over India, Int J Climatol, 28(2007) 1453–1469 In Analysis and Practice of Water Resource Engineering for 32 Joshi V R & Rajeevan M, Trends in Precipitation Extremes Disaster Mitigation, Proceedings International Conference on over India National Climate Centre Research Report No Water Related Disaster, Kolkata, (New Age International Pvt 3/2006, (India Meteorological Department, Pune) 2006, Ltd, New Delhi) 2002, pp 38–41 pp 25 14 Woodworth P L, The Permanent Service for Mean Sea Level 33 McDowell D M & O’Connor, B A, Hydraulic Behaviour of and the Global Sea Level Observing System, J Coast Res, Estuaries (Macmillan Press, Cambridge) 1977, pp 292 7(1991) 699–710 34 Ghotankar S T, Tidal propagation in the Hooghly In 15 Woodworth P L & Player R, The Permanent Service for Bhagirathi-Hooghly Basin: Proc Interdisciplinary Symp Mean Sea Level: An update to the 21st century, J Coast Res, edited by Bagchi, K, (Calcutta University, Calcutta) 1972, 19(2003) 287–295 pp 127–132 16 Permanent Service for Mean Sea Level, Description of 35 Wasson R A, A sediment budget for the Ganga–Brahmaputra PSMSL ‘RLR’ and ‘Metric’ Datasets, (2010) catchment, Curr Sci, 84(2003) 1041–1047 www.psmsl.org/data/obtaining/psmsl.hel 36 Bandyopadhyay S & Bandyopadhyay, M K, Retrogradation 17 Emery, K O & Aubrey D G, Tide gauges of India, J Coast of the western Ganga-Brahmaputra delta, India and Res, 5(1989) 489–501 Bangladesh: possible reasons, Nat Geographer, 31(1996) 18 Clarke A J & Liu X, Interannual sea level in the northern and 105–128 eastern Indian Ocean, J Phys Oceanography, 24(1994) 37 Sengupta, R, Murty, C S & Bhattathiri, P M A, The 1224–1235 environmental characteristics of the Hugli estuary In Cost 19 Unnikrishnan A S, Kumar K R, Fernandes S E, Michael G S & Zone Management of West Bengal, edited by Bose, A N, Patwardhan S K, Sea level changes along the Indian coast: Dwivedi, S N, Mukhopadhyay, D, Danda, A K & Observations and projections, Curr Sci, 90(2006) 362–368 Bandyopadhyay, K K, (Sea Explorers’ Institute, Calcutta) 20 Unnikrishnan A S & Shankar D, Are sea-level-rise trends 1989, pp E19–E56 along the coasts of north Indian Ocean coasts consistent with 38 Sanyal, T, Chatterjee, A K & Mandal, G C, Erosion- global estimates? Global Planet Change, 57(2007) 301–307 deposition in Hooghly estuary, Defence Sci J, 50(2000) 21 Unnikrishnan A S, Long term variability in the tide gauge 335–339 records along the coasts of the north Indian Ocean 39 Sivakholundu, K M, Mani, J S, Idichandy, V G, & Kathiroli, S, Commentaries on the Interpretation of Long Sea Level Estuarine channel stability assessment through tidal Records, Permanent Service for Mean Sea Level (2007), asymmetry parameters, J Coast Res, 25(2009) 315–323 www.psmsl.org/products/commentaries/northern_indian_oce 40 Wright, L D, Coleman, J M & Thom, B G, Processes of an.pdf channel development in a high-tide-range environment: 22 Woodworth P L, Measuring long term sea level change, Cambridge Gulf–Ord river delta, western Australia, J Geol, Ostend-GLOSS Course Material, Permanent Service for 81(1973), 15–41 Mean Sea Level, 2006, www.psmsl.org/train_and_info/ 41 Pethick, J, Estuaries and wetlands: Function and form In training/presentations Wetland Management, edited by Falconer, R A & Goodwin, 23 Peltier W R, Global glacial isostasy and the surface of the P (Thomas Telford, London) 1994, pp 75–142 ice-age earth: The ICE-5G (VM2) model and GRACE, Ann 42 Anonymous, India case-study, SAARC Meteorological Rev Earth Planet Sci, 32(2004) 111–149 Research Centre Newsletter, 8(2001). 24 Warrick R & Oerlemans J, Sea level rise In Climate 43 Singh, O P, Khan, T M A, Murty, T S & Rahman, M S, Sea Change—The IPCC Scientific Assessment, edited by level changes along Bangladesh coast in relation to the Houghton, J T, Jenkins, G J & Epharaums, J J, (Cambridge southern oscillation phenomenon, Mar Geodesy, 24(2001) Univ Press, Cambridge) 1990, pp 257–281 65–72 25 Pirazzoli P A, Secular trends of relative sea level (RSL) 44 Singh, O P, ENSO and monsoon induced sea level changes changes indicated by tide gauge records, J Coast Res, Spl and their impacts along the Indian coastline, Indian J Mar Issue 1(1986) 1–26 Sci, 35(2006) 87–02 812 INDIAN J. MAR. SCI., VOL. 40, NO. 6, DECEMBER 2011

45 Ropelewski, C F & Jones, P D, An extension of the Tahiti- 50 Goodbred Jr, S L & Kuehl, S A, Enormous Ganges– Darwin Southern Oscillation Index Month Weather Rev, Brahmaputra sediment load during strengthened Early 115(1987), 2161–2165 Holocene monsoon, Geology, 28(2000) 1083–1086 46 National Ocean and Atmospheric Administration, Southern 51 Goodbred Jr, S L & Kuel, S A, The significance of large Oscillation Index (SOI) monthly data: 1866 to 2006 Earth sediment supply, active tectonism, and eustasy on margin System Research Laboratory, Physical Science Division, sequence development: Late Quaternary stratigraphy and 2009, www.cdc.noaa.gov/gcos_wgsp/Timeseries/Data/soi. evolution of the Ganges–Brahmaputra delta, Sediment Geol, long.data 133 (2000) 227–248

47 India Meteorological Department, Cyclone eAtlas-IMD: 52 Stanley, D J & Hait, A K, Holocene depositional patterns, Electronic Atlas of Tracks of Cyclones and Depressions in neotectonics and Sundarban mangroves in the western the Bay of Bengal and Arabian Sea (1891–2007), ver-1.0 on Ganges–Brahmaputra delta, J Coast Res, 16(2000) 26–39 CD, 2007 53 Bandyopadhyay, S, Mukherjee, D, Bag, S, Pal, D K, Das, R 48 Hunter, W W, A Statistical Account of Bengal, Vol. 1: K & Rudra, K, 20th century evolution of banks and islands Districts of 24 Parganas and Sundarbans, (Trubner and Co, of the Hugli estuary, West Bengal, India: Evidences from London) 1875, pp 404 maps, images and GPS survey In Geomorphology and 49 O’Malley, L S S, Bengal District Gazetteers: 24 Environment, edited by Singh, S, Sharma, H S & De, S K, Parganas, (Bengal Secretariat Book Depot, Calcutta) (ACB Publishers, Kolkata) 2004, 235–263 1914, pp 268