Indian Journal of Geo-Marine Sciences Vol. 40(6), December 2011, pp. 802-812 Trend of sea level change in the Hugli estuary, India Sreetapa Nandy and Sunando Bandyopadhyay* Department of Geography, University of Calcutta, Kolkata 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 Diamond Harbour—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. 1Image 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 West Bengal (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 1Data 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.