Tidal variations in the Sundarbans Estuarine System, India

Situated in the eastern coastal state of West Bengal, the Sundarbans Estuarine System (SES) is India’s largest monsoonal, macro-tidal delta-front estuarine system. It comprises the southernmost part of the Indian portion of the Ganga–Brahmaputra delta bordering the Bay of Bengal. The Sundarbans Estuarine Programme (SEP), conducted during 18–21 March 2011 (the Equinoctial Spring Phase), was the first comprehensive observational programme undertaken for the systematic monitoring of the tides within the SES. The 30 observation stations, spread over more than 3600 km², covered the seven inner estuaries of the SES (the Saptamukhi, Thakuran, Matla, Bidya, Gomdi, Harinbhanga, and Raimangal) and represented a wide range of estuarine and environmental conditions. At all stations, tidal water levels (every 15 minutes), salinity, water and air temperatures (hourly) were measured over the six tidal cycles. We report the observed spatio-temporal variations of the tidal water level. The predominantly semi-diurnal tides were observed to amplify northwards along each estuary, with the highest amplification observed at Canning, situated about 98 km north of the seaface on the Matla. The first definite sign of decay of the tide was observed only at Sahebkhali on the Raimangal, 108 km north of the seaface. The degree and rates of amplification of the tide over the various estuarine stretches were not uniform and followed a complex pattern. A least-squares harmonic analysis of the data performed with eight constituent bands showed that the amplitude of the semi-diurnal band was an order of magnitude higher than that of the other bands and it doubled from mouth to head. The diurnal band showed no such amplification, but the amplitude of the 6-hourly and 4-hourly bands increased headward by a factor of over 4. Tide curves for several stations displayed a tendency for the formation of double peaks at both high water (HW) and low water (LW). One reason for these double-peaks was the HW/LW stands of the tide observed at these stations. During a stand, the water level changes imperceptibly around high tide and low tide. The existence of a stand at most locations is a key new finding of the SEP. We present an objective criterion for identifying if a stand occurs at a station and show that the water level changed imperceptibly over durations ranging from 30 minutes to 2 hours during the tidal stands in the SES. The tidal duration asymmetry observed at all stations was modified by the stand. Flow-dominant asymmetry was observed at most locations, with ebb-dominant asymmetry being observed at a few locations over some tidal cycles. The tidal asymmetry and stand have implications for human activity in the Sundarbans. The longer persistence of the high water level around high tide implies that a storm surge is more likely to coincide with the high tide, leading to a greater chance of destruction. Since the stands are associated with an amplification of the 4-hourly and 6-hourly constituents, storm surges that have a similar period are also likely to amplify more during their passage through the SES.

Appendices

Appendix 1

1.1 A1.1 The Hoogly

Logistic considerations prevented us from undertaking tidal observations on river Hoogly during the SEP. Nevertheless, for the interested reader, we give here a brief description of this most well-known and best studied of the SES estuaries, gleaned from the references cited in this section, particularly, Chugh (1961), Gole and Vaidyaraman (1967) and Parua (2010) and other articles and websites, too numerous to mention. The various rivers and places mentioned in the course of this description can all be located in the schematic (figure A1) showing the course of the Bhagirathi–Hoogly.

Figure A1

A schematic representation of the Bhagirathi–Hoogly river system. Longitudes (latitudes) along the horizontal (vertical) axis are in °E (°N).

River Hoogly (21.517°–23.333°N, 87.75°–88.875°E) originates as the combined stream of the rivers Bhagirathi, Jalangi, and Mathabhanga-Churni near Nabadwip (23.42°N, 88.37°E). This river, approximately 350 km long, enters the Bay of Bengal at Sagar Island (21.56°–21.88°N, 88.08°–88.16°E) through an almost 25 km wide, funnel-shaped mouth. The semi-diurnal tides entering the Hoogly propagate up to Nabadwip, nearly 290 km north of the seaface. The tidal range at Sagar (21.65°N, 88.05°E) varies between 4.24 and 5.19 m and increases up to 5.3–6.1 m at Diamond Harbour (22.192°N–88.185°E). The tidal wave begins to decay after this, ranging between 3.88 and 4.78 m at Garden Reach within the Calcutta Port area, and to 0.16–0.35 m at Samudragarh (23.37°N, 88.256°E), about 250 km north of Sagar. During the summer monsoon, additional freshwater discharges enter the Hoogly from the rivers Damodar, Rupnarayan, Haldi, and Rasulpur at their outfalls located at different points in the last stretch between Kolkata (Calcutta) and Sagar. During the lean period, however, navigability of the river depends heavily on the periodic release of Ganga waters through the barrage at Farakka (24.8°N, 87.9°E).

Till the 15th century, the Bhagirathi–Hoogly was the course followed by the Ganga flowing into the Bay of Bengal. Owing to geological reasons, the Ganga gradually shifted eastward, so that by the 16th century, the Bhagirathi–Hoogly became the first deltaic distributary of the river. Although not significantly observed till the late 19th century, the effects of the subsequent reduction in the Gangetic flow through the Bhagirathi–Hoogly was a gradual silting up of the Bhagirathi at its offtake point near Farakka and a loss of navigability of the Hoogly.

The Bhagirathi–Hoogly had always been an important shipping route used for trade between India and the rest of the world. It was, however, the phenomenal development in the 18th century of the Port and Metropolis of Calcutta (22.548°N, 88.301°E), that further increased the importance of the river, making it a well-known name globally. Situated on the east bank of the Hoogly about 145 km north of its mouth at Sagar Island, Calcutta (now known by its original name, Kolkata), the erstwhile capital of British India (1772–1912) and the second largest city in the British Empire, is at present the most densely populated city in India. The Port of Calcutta, the then most important international port serving a vast Indian hinterland and the Asian colonies of the British Empire (Bayly, 1985; Ray, 1990; Tan, 2007), is still one of the major ports of India, but with a much reduced hinterland consisting of northeast India, parts of north and east India, and the countries of Bhutan and Nepal.

From the very beginning of its establishment in the late 18th century as a port of call for deep draft vessels, the Port of Calcutta has suffered constantly from the problem of loss of navigability due to the reasons mentioned above. The consequent reduction of the cubic capacity of the channel, formation of sandbars, particularly in the southward stretch from Garden Reach to Sagar, and the shoaling of various stretches, further aggravated the problem. Meandering over nearly 46% of its length, the geometry of the Hoogly estuarine channel is made more complicated by the existence of as many as 10 shifting sandbars that exist in the 70 km stretch between the Port of Calcutta and Diamond Harbour lying to its south. Constant changes are observed in the tidal axes, bank-line migrations, and erosion in various sections of the river. These changes and associated sediment load movements make the Hoogly a very dynamic estuary (Gole and Vaidyaraman, 1967; Parua, 2010).

This dynamism has also been the main reason for the regular monitoring of the hydrological and morphological characteristics of the river over a long time. The Port Authorities (the Commissioners for the Port of Calcutta in British India and the Calcutta Port Trust in Independent India) have maintained tide-gauge stations (see figure A1) at Sagar/Dublat (21.633°N, 88.133°E) from 1881–1886, and again from 1944 onwards, Diamond Harbour (1875–1886, then 1947 onwards) and at Garden Reach/Kidderpore (22.533°N, 88.317°E), lying within the Calcutta Port area, from 1881–1932 and 1932 onwards. Automatic tide gauges for continuous monitoring of tidal levels now function at Tribeni (22.938°N, 88.4°E), Garden Reach, Diamond Harbour, and Haldia. Semaphores for displaying the rises of tide are operational at the sand bars at Akra (22.5°N, 88.25°E), Moyapur (22.417°N, 88.117°E), Hoogly Point (22.217°N, 88.083°E), Balari (22.067°N, 88.150°E), Gangra (21.950°N, 88.017°E) and Sagar. The tide-gauge data are supplied to the Survey of India, the agency responsible for predicting and preparing tide tables for the six Standard Ports on the Hoogly, namely, Sagar, Gangra, Haldia, Diamond Harbour, Moyapur, and Garden Reach.

The earliest tidal observations in the Hoogly were recorded during 1805–1828 at Calcutta by Kyd (1829). This was for the first time that 24-hour tidal curves for a full year, 1823–1824, the highest high water and lowest low water levels for each year during 1806–1828, and the high and low water levels at each lunar phase during 1806–1807 and 1825–1826 were documented.

Oag (1939) was perhaps the first to identify the three distinct seasonal patterns in the tides in the Hoogly. During the summer (pre-monsoon) ‘dry period’ of March–June, flood-tide effects dominate over the ebb tide. During the monsoon or ‘wet period’, July–October, freshwater discharges in the upstream areas of the Ganga and ebb-tide effects dominate. Owing to the banking of the freshwater in the Ganga during the summer monsoon, the dominance of the ebb tide continues even during the post-monsoon months (November–February).

During 1853–1968, hydrological studies on the Hoogly were conducted by as many as 16 Expert Committees set up by the Government of India, with the primary objective of studying and improving the rapidly deteriorating navigability of the river and the port. The findings of these committees (summarized in Parua, 2010) inevitably led to their recommendation of increasing the upland supply of freshwater to the Bhagirathi–Hoogly during the lean seasons by diverting at least 40,000 cusecs of Gangetic freshwater through a suitably constructed barrage across the Ganga. The recommendations were finally implemented with the beginning of the construction of the barrage at Farakka in 1960 and its commissioning in 1975. The first diversions of freshwater from the Ganga following the recommended manner, during the 1976 lean season, had the immediate effect of completely flushing out the Hoogly (Nandy et al., 1983; Sinha et al., 1996) and considerably improving the overall state of the river.

Thus, the Hoogly provides an outstanding example of the resuscitation of an estuary from its moribund state by the application of modern engineering technology firmly based on the scientific understanding of river dynamics.

Chugh (1961) made a thorough study of the tidal characteristics of the Hoogly based on the long tide-gauge records for 1881–1959 from Sagar, Diamond Harbour, and Garden Reach. The study revealed that tidal parameters such as the lunitidal intervals (the diurnal inequality), mean high and low water levels, mean tidal levels, and tidal ranges and the mean river levels all went through seasonal changes during this period. The various tidal planes were found to have changed differently, indicating changes in the tidal regimes and reflecting changes in the conditions of the river bed. Annual variations of the mean sea levels at Sagar and Diamond Harbour were found to be insignificant in comparison to the observed annual variations of the mean river levels at Garden Reach during this period. This low variability was attributed to the variations in depth and other conditions in the river.

Monthly mean high water levels at Garden Reach were found to have changed considerably since 1882. The Highest High Water level recorded here was 8.03 m (in August 1890) and 8.07 m (in October 1959). The Lowest Low Water level of 0.54 m observed by Kyd (1805–1828), remained the minimum level till 1919, after which it decreased to 0.5 m as observed in 1935, 1936, and 1949. A steady increase in the Mean, Spring and Neap Tidal ranges at Garden Reach was observed during the years 1882, 1949–1953 and 1953–1959. Since it could be confirmed that no changes in the mean sea levels occurred during this period, this increase could be attributed to the widening and deepening of the mouth of the river, which were undertaken to improve its navigability.

During the freshet season (the summer monsoon ‘wet period’), the tidal limit moved down south of Swarupganj (23.448°N, 88.428°E) to Kalna (23.22°N, 88.37°E). It was also found that if the strength of the freshets were adequate enough, Calcutta experienced continuous ebb during neap tides. Up to Garden Reach, lower tidal ranges occurred at the beginning of the freshet months May–June, followed by a sharp increase. However, tidal ranges remained low at Bansberia (23.034°N, 88.378°E), 50 km north of Calcutta, during the entire freshet season. The lowest tidal ranges in the Hoogly occurred during the maximum flood years: 0.48 m in 1823, 0.99 m during 1910 and 1938, 2.07 m in 1956, and 1.59 m in 1959. These were also the years of unprecedented floods, during which the highest Low Water levels of 6.1, 3.27, 3.51, 3.27 and 3.68 m, respectively, were observed.

The Hoogly is also one of the few rivers in the world that experiences bore tides (Chugh, 1961; Gole and Vaidyaraman, 1967; Pugh, 1987; Chanson, 2001; Parua, 2010), as a result of the combination of the large tidal ranges and the low (around 6.5 m) channel depths that occur specially during the lean-periods. Appearing first about 40 km south of Calcutta around Royapur (22.291°N, 88.304°E), soon after low water is reached there, the tidal wave increases rapidly in height and travels northwards up to Konnagar (22.7°N, 88.35°E), approximately 14.5 km north of Calcutta, as a 4–5 m high wall of water with a typical speed of about 9 m/s (Chugh, 1961). The time taken by the bore to reach the maximum height (‘the break of the bore’) is typically, 32 minutes from the time of low water.

Before the construction of the barrage at Farakka, the incidence of bore tides in the Hoogly increased (table A1) and they were also observed to occur throughout the year. This increase in the frequency of occurrence of bores was a result of the progressive reduction in the supply of freshwater in the upstream reaches of the river, leading to continuously decreasing channel depths during the lean periods, and the narrowing of the channel in some sections. By the end of 1976, the effect of freshwater discharges from Farakka during the dry period began to manifest itself and the number of tidal bores decreased rapidly.

Table A1 Tidal bores observed on the Hoogly during 1935–2000 (after Parua, 2010).

Changes in the salinity of the estuary follow the seasonal tidal pattern, with increased salinity observed during the pre-monsoon dry period and decreased salinity during the monsoonal wet period (Oag, 1939; Gole and Vaidyaraman, 1967). The Central Inland Fisheries Research Institute, in their biological observational studies (Rao, 1969; Nandy et al., 1983; Sinha et al., 1996; Das and Samanta, 2006), have followed the practice of dividing the entire tidal stretch of the Hoogly into three zones based on the existing salinities: An Upper Freshwater Zone, a Middle or Estuarine Zone (salinity range: trace values to 12.68 psu), and a Lower or Marine Zone (salinity range: 4.32–29.77 psu). The extent of each of these zones varies seasonally and annually, depending on the strength of the freshets received in the upstream parts of the river.

Increased salinity in the Hoogly, again a result of reduced upstream freshwater, became a grave concern during the late 1950s and throughout the 1960s (Parua, 2010). The freshwater from the river has been the only source of drinking water supplied to Calcutta and adjoining areas and has been used heavily by the industries and thermal power generation plants lining both banks of the river. The dry-period salinity levels, monitored regularly since 1865, at the intake point at the Water Works at Palta (22.783°N, 88.370°E), 24 km north of Calcutta, rose steadily from ~0.05 psu in 1900 to 0.19–0.65 psu during 1915–1957 and 1.038–1.5 psu during 1960–1975, far above the potable limit of 0.5–1.0 psu prescribed by the World Health Organization. The corrosion and scaling of boilers and other machinery by the highly saline water affected the smooth functioning of the industries, power generation units and even in the running of the railways all throughout this period. Impacts were also considerable on the agricultural sector. Post-Farakka dry-period salinity values at Palta showed a rapid decrease from 1.5–0.2 psu during 1975–1980 to 0.2–0.167 psu during 1980–1985. Values during 1990–2000 (0.2–0.236 psu), however, have again started to show an upward trend.

A comparative study of ecological changes in the Hoogly during pre- and post-Farakka periods (Nandy et al., 1983; Sinha et al., 1996) documented the immediate effect of the freshwater released on the salinity observed in the different stretches of the Hoogly. In general, the salinity decreased over the entire estuary. The 54 km stretch of the Hoogly between Kalna and Barrackpore (22.76°N, 88.37°E; approximately 23 km north of Calcutta), identified during 1960–1961 as the freshwater zone, was extended downstream during 1975–1976 and 1976–1977 by a further 30 km up to Uluberia (22.47°N, 88.11°E). The true estuarine zone was shifted 40 km down south to Geonkhali (22.2°N, 88.05°E), near the confluence of the rivers Hoogly and the Rupnarayan (figure A1), and the marine zone was pushed out further south to Kakdwip (21.85°N, 88.2°E) and into the Bay of Bengal.

Observational studies on the chemical and biological characteristics of the Hoogly (Roy, 1949a; Roy, 1955; Dutta et al., 1954; Bose, 1956; Shetty et al., 1961; Basu and Ghosh, 1970; Saha et al., 1971; Mukhopadhyay et al., 2002; Sadhuram et al., 2005; Mukherjee et al., 2006; Manna et al., 2010) have mostly accounted for tidal influences by taking and comparing the observations during high and low tide periods. Remotely sensed data have been used for studying erosion/accretion at Sagar, the largest island in the Hoogly (Jayappa et al., 2006; Dinesh Kumar et al., 2007). Pollution of the coastal waters of the Sundarbans has also been investigated, again as localized studies (Ganguly et al., 2006, 2007; Mukherjee et al., 2010).

1.2 A1.2 The Saptamukhi and Matla

As mentioned in their publication CS(AR) 21/96-97 (Jha et al., 1999), an extensive hydrological survey of the southern stretch of the river Saptamukhi was carried out during 1961–1963 by the National Institute of Hydrology, Roorkee. The flood volume of the combined Saptamukhi East and West Gullies at the seaface was estimated to be of the order of 43750, 500, and 200 cumecs during the Spring, Mean, and Neap phases, respectively, with the SEG carrying the major portion of the inflow from the Bay. At the tip of the Bara Rakshaskhali Island (figures 2 and 3), the mean flow was estimated to be of the order of 30 cumecs, about 6% of the flood flow at the mouth. Following this, no such surveys have been undertaken to the best of our knowledge.

Salinities at Port Canning on the Matla have been measured by several researchers. Variations between 10.72 and 30.41 psu during 1962–1963 and 1965–1966 were reported by Rao (1969).

It is known that the twice-daily mixing of the estuarine waters by the tides causes changes in its sediment load, salinity, nutrient, and pollutant content, all of which have an impact on the biodiversity, biological, and chemical characteristics of the estuaries. These latter characteristics are by far, the most studied aspects of a few of the inner estuaries of the SES (Guha Bakshi et al., 1999; Naskar and Mandal, 1999; Naskar, 2003; Mukherjee and Sen, 2008; Mukherjee et al., 2008; Mandal et al., 2009; Manna et al., 2010).

Appendix 2

2.1 A2.1 The Visual Tide Staff

The Visual Tide Staff (VTS) or Tide Pole (figure A2a, h and i) was used to measure tidal water levels because of the simplicity involved. This instrument, and other observational procedures used in the SEP, exactly follow those used in a similar study of the Mandovi–Zuari Estuaries (Shetye et al., 1995, 1996, 2007; Sundar and Shetye, 2005).

Figure A2

The Sundarbans present a remarkably challenging environment for making observations. The high tidal range and the soft, clayey mud make it difficult even to erect a tide pole. These photographs present a glimpse of the conditions under which the SEP was conducted. (a) The Visual Tide Staff (VTS) or tide pole at Kaikhali (M14; figure 4). (b) At low tide, the soft, clayey mud of the Sundarbans is exposed. This photograph is from Ramganga (S6; figure 3) and shows the boat lying on the exposed mudbank at low tide. The VTS can be seen at the extreme right. (c) The two pipes broke at the socket joint at a few stations. This photograph is from Melmelia (GH21; figure 4), where the observers managed to repair the pole and resume observations. (d) Reading the water level at night was made tough by the distance of the pole from the boat. This distance was necessitated by the need to ensure that the zero of the pole was not exposed during the lowest low water level. (e) Observations at Dhanchi. Note the three guy wires attached to the top of the pole. These guy wires were anchored to keep the pole vertical. (f) The omnipresent mud disappears during high tide, with even the mangroves barely standing above the high water level. This photograph is from R. Barchara near Mahendranagar (S5; figure 3). (g) Even jetties barely survive submergence during high tide. This photograph shows an abandoned jetty at Pakhiralay (G18; figure 4). (h) A VTS at the Sabuj Sangha building in Nandakumarpur. Note the iron socket, used to join the two GI pipes, around the 140 cm mark. (i) The VTS deployed at Dhulibhasani. Note the two black marks near the bottom of the part of the VTS visible in this photograph. The upper one is the socket and the lower one is an iron clamp used to hammer the VTS into the mud.

Qualitative information regarding tidal levels and the duration of high and low water at particular locations were collected from local residents during the reconnaissance trips. Heights of jetties, wherever they existed, were noted along with the levels of saline depositions on the mangroves covering the estuarine banks; the latter observations were made whenever convenient. This information allowed us to estimate approximately the expected levels of high/low waters and tidal ranges at the selected observation stations.

In order to accommodate the high tidal ranges (>7 m) observed at some of the locations, the VTS had to be at least 7–8 m above the estuarine low water levels. Everywhere in the Sundarbans, and specifically in the banks of the estuaries or any other water channel, the soil is soft clayey mud (figure A2b, c and i). To install the long and narrow (2 or 3 inches in diameter) tide pole in a vertical position and make the installation strong enough to withstand the impact of the tidal waves and high winds, a good length of the pole had to be sunk deep into the mud. Construction of a concrete or brick base had to be ruled out, particularly at the forward locations because the heaviness of the construction would make it sink into the mud.

The tide poles used for observations in the SEP were fabricated at the School of Oceanographic Studies, Jadavpur University. Galvanized iron (GI) pipes, with graduation marks painted in black and red on a white background (figure A2a and h), were used. To avoid inaccuracies in painting in the graduation marks, a basic stencil was used. Since in this part of the world, GI pipes are manufactured in units of 6 m lengths, two such units, joined by an iron socket, were used to fabricate each of the 12 m long tide poles (figure A2c, h and i). Optimizing between the readability of the scale, particularly during night (figure A2d), and the weight of the 12 m long pipe, the pipe diameters had to be restricted between 2 and 3 inches. To ensure that the tide poles remained vertical and firmly fixed to the banks, three iron guy wires were attached to the top of the pole and tied to the banks through suitable anchors (figure A2e, h).

The exposure of the mud (figure A2b, c and e) during low tide is one phase of the tidal cycle in the Sundarbans. The other phase of the tidal cycle submerges the mangrove roots and masks the omnipresent mud (figure A2f) and almost submerges jetties (figure A2g), leaving, only a small part of the 12 m long pole visible (figure A2g) during high tide.

2.2 A2.2 Missed observations

Most observation teams recorded details of the ambient atmospheric and estuarine conditions, reasons for missed observations, and difficulties faced in taking measurements. These proved to be very helpful while interpreting the data.

The most common reasons for the missed observations (listed in table 4), were the following.

• 1. The breaking of the upper section of the tide pole from the socket joint due to the combined impact of wind and tide. Repairing of the joint was possible in stations close to local markets. An example is shown in figure A2(c). For stations not having this facility, the upper length was tied tightly with the lower portion and the guy wires suitably adjusted for making the shortened pole vertical. For R28 (Shitalia), this resulted in the shortening of the tide-pole scale to such an extent that both High Water and Low Water levels could no be read.

After refixing the tide pole in this manner, the total length of the lower pole and the time of resumption of the readings were recorded in the observation sheets and Water Levels read off according to the graduation marks in the upper pole. During the data analysis, this length was subtracted from the reported water levels from this time onwards.

At M4 (Bonnie Camp), after re-fixing the pole and proceeding with the measurements in the manner described, the upper pole broke again and all guy wires snapped. Observations here were finally abandoned after 56 hours (see table 2). Fortunately, at most of the stations experiencing this problem, tide poles functioned properly after repair, thus leaving only one stretch of missed observations.

• 2. Slanting of the tide pole from its vertical position. This problem was corrected by the observers every time it occurred by means of a plumbline, but resulted in some missed observations.

• 3. The water level receding during low-water events, leaving the tide-pole ‘zero’ exposed (under-ranging events). Tide poles were installed during the lowest low water period on the 17th afternoon. Information regarding tidal ranges, particularly in the remote and seaward stations were sparse, however, and as the Spring Tidal Ranges continued to increase, stations like Indrapur, Dhanchi (figure A2e), and Shibganj faced this problem and missed the lowest low water observations.