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The Waterscape of the Sundarban, Bangladesh Bushra Nishat

1. Introduction and Context Situated in the lower Gangetic delta the Sundarban is considered to be one of the seven most globally important wetlands of the world (WWF, 2015), and is recognised both as a Natural World Heritage Site and Ramsar listing as a Wetland of International Importance. The Sundarban in its entirety, is arguably the largest block of mangrove ecosystem remaining in the world. The unique ecosystem, also represents a hydrological interface between the freshwater flows of the tributaries and distributaries of Ganga-Brahmaputra riverine system and saline waters of the Bay of Bengal. The complex hydrological systems found in the Sundarbans support diverse faunal assemblages and economically significant fisheries. Besides provisioning services of the mangrove ecosystem, ecological services are in the form of protecting the coast from fury of cyclones, floods, wave action and coastal erosion. While the Sundarbans landscape is celebrated for its ecological attributes, the region is also supports a population of 7.5 million (nearly 5 million in India, about 2.5 million living in the surrounding areas of Sundarban Reserve Forest in Bangladesh) which are directly dependent on the Sundarban for their livelihood and amongst the poorest in the region. Geographically the undivided Sundarbans tract extends approximately 260 km (160 miles) west-east along the Bay of Bengal from the Hugli River estuary in India to the western segment of the Meghna River estuary in Bangladesh and reaches inland for about 80km (50 miles) at its broadest point. The total area of the Sundarbans, including both land and water, is roughly 10,200 s sq. km, about 60 percent of which is in Bangladesh (around 6,000 sq. km) and rest (4200 sq. km) in India (World Bank, 2014). The Sundarbans area experiences subtropical monsoonal climate with an annual rainfall of 1,600–1,800 mm and occasional severe cyclonic storms The estuarine wetlands of the Sundarban are constantly fed by nutrients brought in by the fresh water of the Ganga river system and flushed by the ebb and flow of the tides; their intricate, three-dimensional landscapes promote richly complex interdependencies and thus support a diverse population of plant and animal species, both terrestrial and aquatic. These wetlands also sustain billions of protozoans, cnidarians, barnacles (Amphibalanus spp.), oysters (Crassostrea spp.), lichen and other invertebrates including juvenile fish, crabs, prawns, shrimp, and molluscs, which seek refuge in the shallow inter-tidal reaches. These in turn are food to wading migratory and local birds, pelicans, and the endangered crocodile. Sir David Prain recorded 334 species of plants belonging to 245 genera in the Sundarban mangrove forest and adjoining areas (Prain 1903). As many as 447 species of vertebrate wildlife (amphibians, reptiles, birds and mammals) including the iconic Bengal tiger, Gangetic and Irrawaddy dolphins, and the olive ridley turtles have been also been reported. The shared Sundarban Region represents a strategic opportunity for strategic cooperation and joint actions between Bangladesh and India, resulting in simultaneous poverty reduction and sustainable ecosystem management. Time and again at various fora it has been expressed that the Sundarbans across Bangladesh and India be visualized as a single biogeographical entity and that bilateral research

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cooperation be promoted to inform joint action for sustainable development and conservation of Sundarbans landscape. Recognizing this, the governments of Bangladesh and India signed (non-binding) agreements in September 6, 2011 on a number of issues to pave the way for joint action. These include a Memorandum of Understanding1 that recognizes that, ‘the Sundarban of India and Bangladesh represent a single ecosystem divided between the two countries’ and seeks to facilitate cooperation in the areas of conservation of biodiversity, joint management of resources, livelihood generation for poverty alleviation and development, cataloguing of local flora and fauna and studying the impacts of climate change. So the effort for bilateral cooperation has already begun, but operationalization of this agreement and implementation of joint programs needs to be defined and initiated. Since 2014, the South Asia Water Initiative (SAWI) has initiated multi-stakeholder consultations to facilitate dialogue and confidence building activities for strategic cooperation and joint actions between Bangladesh and India, resulting in simultaneous poverty reduction and sustainable ecosystem management. The overall objective of the Sundarbans Focus Area of is to operationalize joint management of the Sundarbans for sustainable development and to deliver mutual benefits for the Bangladesh and India. The two specific goals in support of this objective are to: (i) enhance bilateral cooperation to support operationalization of the Sundarbans agreements between Bangladesh and India; (ii) enhance technical cooperation between Bangladesh and India to support joint water resources management in the Sundarbans. Any strategic mechanism for supporting collaborative approaches to managing shared ecosystems requires appropriate knowledge and comprehensive understanding of the ecosystem. With this aim, SAWI initiated an inventory of the freshwater Sundarban in both countries. Based on literature review and consultations with local and regional researchers and experts, together with the inventory of the freshwater resources on India this documents synthesises issues and perspectives to describe the hydrology, river sytems, groundwater and water quality as well as on how the Sundarban is greatly dependent on and influenced by its waterscape. Hereinafter, in this document Sundarban refers to the Sundarban Reserve Forest of Bangladesh.

2. The Sundarban Waterscape Around 30 percent of the Sundarban mangrove forest is covered by water, making it the largest mangrove wetlands of the world. The alternating high tide-low tide phenomenon has resulted in a diverse and rich eco-system formed over centuries and one that maintains a dynamic distinction in between alternating tidal regimes. The physiography is dominated by deltaic formations that include numerous rivers and canals associated with surface and subaqueous levees, splays and tidal flats. There are also marginal marshes above mean tide level, tidal sandbars and islands with their networks of tidal channels and subaqueous sandbars. The ecosystem and the landscape of this mangrove forest are shaped by the rivers, creeks and canals of the region and, given the hydrological setting, any alterations in water and water resources will critically affect all aspects of the ecosystem as well as the lives and livelihoods of the people living in its vicinity and the overall growth and development of 1 The MOU between India and Bangladesh on Conservation of the Sundarban signed on September 06, 2011 reads as under (http://mea.gov.in/bilateral-documents.htm?dtl/5141/MOU+between+India+and+Bangladesh+on+Conservation+of+the+Sundarban accessed on 04 March 2015)

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the Sundarban. Hydrologically the Sundarban is located in the south west region of Bangladesh, which is a part of the Ganges delta and is exposed to a number of calamities like cyclones, floods, tidal surges, repeated water logging, and land erosion. The following sections fosusses on the river system, water bodies, groundwater and water quality (mainly salinity) of the Sundarban.

2.1 The River System The main source of fresh water to the Sundarban is mainly the river systems all of which originate in the Ganges basin. These distributaries of the Ganga, traversing mainly south in a dendritic fashion enter into the Sundarban and ultimately fall into the Bay of Bengal. The larger of the channels in the Sundarban are the remains of main channels of the Ganges, which over thousands of years has been continuously changing its position, configuration, shape and position eventually. Historical evidence shows, the main stream which has gradually migrated eastwards, leaving a number of moribund rivers running north to south (Rudra, 2018). Six major river systems which flow through the mangrove forest from its northern boundary. These are the Ichamati-Raimangal, Arpangashia-Malancha, Poshur-Sibsha, Shella and Baleswar river systems distributed from the western side at the India-Bangladesh border to the eastern boundary of the Sundarban. These rivers in turn originate from the Ganges distributeries Mathabhanga, Bhairab, Kapotaksha and Gorai-Madhumati. All the rivers have low flow from February to April (dry season). High flow occurs in July to September (monsoon season). The rivers between the Khulna-Ichamati, Ganges, Gorai-Madhumati and the Bay of Bengal are connected by cross-channels, which are especially numerous in the Sundarban. They are of great importance for inland navigation in the delta. The freshwater flows from the rivers and the tidal ingress result in a gradient of salinity that varies both spatially and temporally. Many rivers and channels in the Sundarban, especially near the sea, carry very little freshwater, but instead act as tidal channels for tides originating in the Bay of Bengal. According to a study, in order to protect the diversity and productivity of the Sundarban, the in-stream flow requirement to push the salinity gradient in the rivers flowing into the Sundarban is minimum 150m3/s in the Gorai and 100m3/s divided equally between the other distributaries feeding into the southwest region of Bangladesh. An additional 245m3/s has been estimated for meeting consumptive demand (WARPO, 2001). 2.1.1 Rivers Upstream of the Sundarban The Ganges River South Asia’s most iconic river, the Ganga (known as Ganges in Bangladesh) orginates in the Himalayas after traversing in a south-easterly direction for about 2,500 kilometers and joins the Brahmaputra before falling into the Bay of Bengal as the Meghna. However, some of the flow joins the Bay of Bengal through its distributaries the Bhagirathi, Mathabhanga and Gorai, all of which arise from the Ganges left bank before it joins the Brahmaputra. The total length of the Ganges from its source to its outfall into the sea is 2,525 kilometers via Hoogly. The area of the total basin is 1,087,300 square kilometres and is spread across China, India, Nepal and Bangladesh (Islam & Gnauck, 2008). Although rainfall in this huge catchment area is the prime contributor to its flow, melting snows from the Himalayas also substantially enhance to the perennial flow of the Ganges. A few kilometers away from the India-

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Bangladesh border, the Ganga gets bifurcated – one takes the name Bhagirati and heads south towards Kolkata, where it assumes the name of Hugli after being joined by the Damodar and the Rupnarayan. The main channel continues to flow southeast towards Bangladesh and enters through Sibganj Upazila of the ChapaiNawabganj district, while continuing its course as the Ganges till it joins the Brahmaputra at Aricha. From its confluence the Ganges changes its name to Padma and travels further down to join the Upper Meghna near Chandpur, where all waters of the three big river systems surge into the colossal Lower Meghna and eventually discharges into the Bay of Bengal.

Figure 1: Major River Systems connected to the Sundarban

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The flow regime in the Ganges is determined to a large extent by monsoon rainfall, snowmelt and abstractions. The annual flow of the Ganges at Hardinge Bridge, before it combines with Brahmaputra is around 350 BCM. Normally in all tributaries peak discharges occur within June to September and low flows in January-May. This is because the June-October period carries monsoon rainfall-runoff from the catchments and also snowmelt-runoff in the tributaries originating in the Himalayas. November-February is the time when snowmelt is almost zero and March-May experiences gradual increase in snowmelt contribution before the onset of the summer. But due to lower base flow in March, April and beginning of May, the period November-February flow volume exceeds that of the March-April flows. The Mathabhanga River The Mathabhanga is one of the distributaries of the Ganga, that originates in the southeast of the river Jalangi, a town in the Murshidabad district of West Bengal. Due to lateral oscillation of the Ganga and its recent Northwards shift of the main flow, Mathabhanga is virtually disconnected from its feeder, except during the peak of monsoon, hence known as Mathabhanga or headless. On its meandering journey in a southern direction, the Mathabhanga forms the international boundary between India and Bangladesh, and then crosses over to Bangladesh through Daulatpur Upazila of the Kushtia district. Keeping its southern run, the river branches out in two opposite directions, its one arm flows towards the east in the name of Kumar or Pangasi, while the other branch turns towards the west retaining its original name and takes a circuitous journey across the Chuadanga district till it crosses over to India. Mathabhanga re-enters India through Gede in the Nadia district of West Bengal, where it gets bifurcated with two different identities. One branch runs towards the west and assumes the name of Churni, connecting important towns of Hanskhali, Ranaghat and Chakdaha, and finally discharges into Hooghly. The other arm runs towards the south in the name of Ichamati and crosses over to Bangladesh via the North 24-Paraganas district. Major distributaries of Mathabhanga include Kumar, Chitra, Nabaganga, Kapotaksho and Bhairab Rivers. The Bhairab River Bhairab, a branch of the Mathabhanga originating at Meherpur, is a trans-boundary river. Right after originating the river acts as the boundary between the two countries at Nadia district of West Bengal and Meherpur district of Bangladesh for almost 2 km and thereafter it meanders southeast through Bangladesh in Jhenidaha and Jessore districts and at Khulna branches out into two flows, Rupsha and Bhairab. Bhairab continues its south-easterly journey and falls into Dartana river. The Rupsha travels southwards through Khulna city and further downstream, changes its name to Pashur and finally drains into the Bay of Bengal. The Kapotaksha River The Kapotaksha is an offshoot of the Bhairab River. Previously, the Kapotaksha River originated from the Mathabhanga river with a great meander. A canal was dug to run the river along a shortened course avoiding the meander and the Kobadak got disconnected from the Mathabhanga. The River Kobadak was once one of the main sources of fresh water in the South-Western region of Bangladesh. Sedimentation combined with upstream low flows caused severe degradation of the river in the last few decades. Local interventions such as construction of polders, bridges and encroachment into the river for cultivation further deteriorated the condition of certain stretches of the river. The river flow became seasonal and tidal. As a result, many of the oxbow lakes (locally known as beels) in the Kapotaksha basin faced severe drainage congestion. The Bangladesh Water Development Board

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maintains a flow from the Ganges by pumping, providing irrigation to its buffer area in the Ganges Kobadak Project (G-K Project). Because of its importance, the river has been rejuvenated and de-silted by capital dredging and manual excavations. This is expected to reduce drainage congestion in the beels and improve flow to the river. The Gorai-Madhumati River The Gorai-Madhumati River is a principal distributary of the Ganges River. The same river has been named as the Gorai in the upper course and Madhumati in the lower course. The Gorai takes off from the Ganges at Talbaria, north of Kushtia town and 19 km downstream from Hardinge Bridge. South of Kushtia its first offshoot, the Kaliganga branches off to join the Kumar near shailkupa. From Magura district it changes its name to Madhumati. The Kumar, the Nabaganga and the Chitra join it through several channels south of Mollahat upazila. There it changes its name to Baleshwar, which in turn changes to Haringhata from the Bogi forest outpost of the sundarbans. As it flows southward a part of this river flows into the Nabaganga which bifurcates into two channels, the Atai River and Majul Khal, and joins with the Bhairab River. A part of the Bhairab River becomes the Rupsa and the Rupsa assumes the name Pashur. The Gorai-Madhumati is one of the longest rivers in Bangladesh and its basin is also very wide and extensive. It flows through Kushtia, Jessore, Faridpur, Khulna, Pirojpur and Barguna districts. Agriculture and irrigation in these areas are very much dependent on the Gorai-Madhumati. The Gorai-Madhumati has a flood discharge of nearly 7,000 m3/s but in winter the flow goes down to 5 m3/s (Banglapedia, 2014). Gorai River normally has 15 % of Ganges River’s annual flow volume. Gorai flows have been decreasing year round due to the reduction in dry season flows in the Ganges and increased sedimentation at the Gorai offtake. This is because the Gorai River, the only spill channel from the Ganges to the southwest region, remains cutoff from the Ganges most of the time during January to March. Minimum dry season flows in the Gorai started to reach zero in 1988 and effectively ceased altogether in 1992, but have been temporarily restored through a major dredging programme. In the present condition, Gorai starts receiving flow from April with the rise of water level in the Ganges.

2.1.2 Rivers within the Sundarban The Raimangal River The Raimangal river rises from the south of Hasnabad in the North 24 Parganas district of West Bengal where the Ichamati joins the Raimangal, running along the Indo-Bangla riverine boundary. In a way, the Raimangal begins where the Ichamati ends. The Ichamati breaks up into several distributaries - the chief of which are: the Raimangal, Bidya, Jhilla and Kalindi - below the Hingalganj town and later, fans out into the wide estuaries in the Sundarban. For some distance, the Raimangal forms the international boundary between the two countries, before it enters Bangaldesh, flowing towards southeast, and then drains into the Bay of Bengal. One distributary of Raimangal, Hariabhanga, also runs parallel to the west side and also outfalls into the Bay of Bengal. The Raimangal forms the west boundary of the SRF.

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The Arpangashia-Malancha Rivers The Malancha receives water from the Jamuna (flows in the south west region of Bangladesh and not from the Brahmaputra) which originates from Kishenganj in India and also the Ishmati through a network of channels. The flows of the Arpangasia originate from the Khulpetua and the Kapotaksho rivers and joins the Malancha River before it falls into Bay of Bengal. This river system flows almost parallel to the Raimangal as it joins the Bay of Bengal. The Pashur-Shibsha Rivers Pashur River of the Sundarbans is connected with the Garai-Madhumati system as well through the river Nabaganga and flows through the middle part of the Mangrove forest thus supplying considerable amount of freshwater into the Sundarbans ecosystem. Shibsa River one of the important rivers of the Sundarban and joins the Pashur south of Mongla port. Hiran Point is on the right-bank of the Pashur-Shibsha system. Inside the Sundarban, the Pashur-Shibsha system receives various rivulets and khals from different directions, enriching its flow. Among them the Barulia, Haria, Bunakhali, Garkhali, Mainas, Taki, Besekhali, Badurgachha, Bhelti, Karua Gamrail, Hadda and Nali Jalla are notable. Since Mongla port, Bangladesh’s second largest seaport is situated on its bank, the Pashur is a busy river, especially near the port. Shella River The Shela river originates south of the Mongla port from the Pashur river in the northern side of the Chandpai range. It is much smaller than the Pashur river. The Shela River, which runs through the sanctuaries for endangered Irrawaddy and Ganges river dolphin and along the northern edge of the Sundarban East Wildlife Sanctuary, is an ecologically important river. The river came under the spotlight since December 9, 2014, when a wrecked tanker released oil into its waters. Any sort of navigation is banned on the Shela river. Baleswar River The Baleswar forms the eastern boundary of the Sundarban and as mentioned before, receives flow from the Gorai-Madhumati system. The Baleshwar River flows south into the Haringhata River, which flows into the Bay of Bengal. The Baleswar is the main source of freshwater to the eastern part of the Sundarban which represents the low saline zone of the forest.

2.1.3 Tidal Water Level in the Rivers The Sundarban is tide dominated delta, the tidal cycle is semi-diurnal2 with minor diurnal inequality. The average tidal amplitude is between 2.5 - 5 m, with the highest amplitudes in July-August and the lowest in December-January (Islam, 2019). The highest tidal range is recorded as 5.12 m at the northern fringe of the forest, and the lowest is recorded in the south, near the bay of 2.75 m. The tidal water reaches up to 120 km inland (Seijger et al, 2019), and as the high water moves in along the estuaries, it is much altered, in path, duration and speed by the shape of the shoreline, inlets and landforms. The Sundarban tides are asymmetrical and the rising tide occupies shorter time in a cycle, inducing faster landward velocity of the tidal current. As a result, sedimentation occurs as the tides move seaward. The tidal range and asymmetry increases landward, suggesting an increasingly higher

2 Consisting of two high tides and two low tides every day.

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rate of sedimentation in the upper part of the estuaries. It should be noted, the Sundarban region receives little or no sediment discharge from the upstream rivers with an exception of the Baleswar. Nonetheless the sediments of the Ganges-Brahaputra rivers reach the remote interior areas of Sundarban through strong tidal currents along the Bengal coast (Rogers and Goodbred, 2014). The maximum net siltation and erosion occur at the end of monsoon. At the end of monsoon, net siltation was found to be 50 mm, while erosion is lower at 19 mm (Islam, 2019). The peak water levels along the entire river system (offtake of Gorai to Passur) vary from 13.65 m+PWD at GoraiRly. Bridge to 9.34 m+PWD at Kamarkhali Transit for the non-tidal BWDB stations. (Table:1).

Table 1: Maximum and minimum water levels (1979-2009) at different stations along the river system

Analysis of maximum and minimum water level data of GoraiRyl Bridge over the period 1945 to 2010 shows a decreasing trend where the range of fluctuation of maximum water level has been observed to near about 2m+PWD and for minimum the value is 4m+PWD (Figure 2).

Type and Source

River name Station Name Maximum Minimum Range

Tidal Water level data of BWDB

Baleswar Pirojpur 3.48 -0.72 4.2 Gorai-Madhumati

Rayanda 3.95 -0.51 4.46

Pashur Mongla 4.2 -1.77 5.97 Shibsha Paikgacha 5.32 -1.92 7.24 Rupsha-Pashur Khulna 3.86 -1.62 5.48 Pashur Chalna 4.75 -2.26 7.01 Shibsha Nalianala

(Hadda) 3.54 -2.63 6.17

Madhumati Bhatiapara 6.72 0.07 6.65 Non-Tidal Water level data of BWDB

Madhumati Kamarkhali 12.51 0.76 11.75 Madhumati Kamarkhali

Transit 9.34 0.8 8.54

Gorai GoraiRly.Bridge 13.65 2.75 10.9

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Figure 2: Trend of maximum and minimum water level

Analysis of maximum water level data of Hiron Point station (BIWTA-101) of both wet season (June to October) and dry season (November to May) shows an upward trend of around 5mm/year (Figure 3). At Hiron Point, the observed water levels are free from any sort of artificial or man-made interventions. Similarly, analysis of tidal water levels of the Rupsa-Pashur River at Khulna for a period of 74 years (1937-2010) indicates that the annual maximum high tidal water levels are increasing at a rate of 18 mm per year and the annual minimum low tidal water levels are decreasing at a rate of 8 mm per year (Mondal et al, 2013). The increasing trends in annual maximum water levels has resulted from the combination of higher rivers beds due siltation, reduction in flood tide propagation areas and a rise in the sea level.

Figure 3: Wet season and dry season yearly average water level of Hiron Point station

2.2 Water and Land Cover Apart from rivers, the Sundarban waterscape includes a large number of surface water bodies including swamps, marshes and even ponds. A comparison of the land and water areas in both parts of the Sundarban over the past three decades provides a clearer understanding of the pattern of land loss in the Sundarban during recent times. A comparison of satellite imagery from the years 1988,

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1998, 2008 and 2015 is shown in Figure 4 and 5. The analysis in Figure 4 shows that the maximum changes of land and water in the Sundarban occurred during 1998–2008. A study of the frequency of cyclonic storms in the northern Bay of Bengal indicated a significant increase in the number of storms during the years 1999–2009 (Danda, 2010). The increase in surface water can be attributed to this frequency of cyclonic storms in the coastal region during this period. The trend of land and water area changes decreased in the later years till 2015.

Figure 4: Land and water changes in the Bangladeshi part of the Sundarban during the years 1988,

1998, 2008 and 2015 (Source: Collected from Centre for Environment and GIS in 2016) There are around 50 manmade ponds inside the Sundarban, which store rainwater for animals. These are often the only source of freshwater especially during the dry season. All villages surrounding the Sundarban will also have several manmade ponds for drinking water, bathing and fishing. Animals frequent these ponds within or near the forest mostly during surise and sunset, thus these ponds often attract tourists and poachers. Upstream of the Sundarban there are numerous oxbow lakes called baors, wetlands called beels and manmade large lakes called dighis. The natural water bodies, which are connected and intesively integrated within the hydrological system of the south west region often retain water and sediment coming from upstream rivers and daily tidal prisms. This retention impacts the sedimentation, creation of river islands or chars and flooding of the Sundarban. The Sundarban ecosystem along the Bay of Bengal has evolved through natural deposition of upstream sediments accompanied by intertidal segregation.

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Figure 5: Comparison of land and water areas of the Sundarban for the years 1988, 1998, 2008 and

2015 (Source: Collected from Centre for Environment and GIS in 2016)

2.3 Groundwater The Sundarban delta is formed largely of alluvial and deltaic sediments of the Ganges, Brahmaputra, and Meghna rivers, and it occupies the greater part of Bengal Delta. There are no boreholes inside the Sundarban, but studies of the surrounding areas show, the surface lithology of the forest is composed of tidal deltaic deposits, deltaic silt deposits and mangrove swamp deposits. The subsurface lithology is characterized by a heterogeneous mixture of sand, silt and clay. The area is underlain predominantly by thick clay and silty-clay aquitards (or aquiclude) up to 300m depth and medium to coarse sandy gravel rich aquifer (shown in Figure 6). The depth of shallow aquifer is considered up to 100m and the depth of deep aquifer ranges from 120 m to 350 m (Dola et al 2008, Adhikary, 2012).

SUNDARBAN YEAR 1988

SUNDARBAN YEAR 1998

SUNDARBAN YEAR 2008

SUNDARBAN YEAR 2015

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Figure 6: Geological Profile of the South West Region (Sundarban shown in the red box). Source: Adhikary, 2012

Groundwater flow velocities are very low due to low hydraulic head gradients in southern Bangladesh. The population of the entire country including the south west region of Bangladesh is dependent on groundwater for irrigation and drinking purposes, resulting in high drawdown. Heavy rainfall and high river discharge during the monsoon replenishes groundwater aquifers through both vertical and lateral recharge, but in the south west zone recharge is usually limited by the impermeable surface mud layer. Within the shallow aquifer sand beds with entrenched silt layers cause groundwater flow to change direction, causing groundwater flow rates to be low (Ayers et al, 2016). Current extraction rates of groundwater are unsustainable, with many studies suggesting extraction exceeds recharge (WARPO, 2014). The lowering of groundwater table is one of the reasons for sea water ingress resulting in salinization (discussed in section 2.2).

2.3 Water quality

2.3.1 Salinity The flat and low lying coastal region of Bangladesh remains one of the most vulnerable areas to salinity intrusion. The overall hydrological balance in the Sundarban has always been achieved through ample inflow from the upstream rivers contributing in supressing the salinity influx from tidal influences. Growth and reproduction of aquatic plants and animals of the Sundarban depend to a great extent on the salinity variation of its waterscape. The ecosystem thus becomes distressed if the waterscape becomes saline, as the ecosystems thrives in brackish environment. The temporal and spatial concentration of salinity shows an increasing trend over the years, taking the form of a slow onset disaster which has serious, complex and long terms affects (Rahman et al, 2017).

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In the Bay of Bengal, salinity increases westward from the mouth of the Meghna estuary as the fresh water discharges from the Meghna and other estuaries flow westward along the coast. During the wet season, salinity in the Sundarban is pushed down to the Bay. At the beginning of the dry season, salinity in the Bay begins to increase from west to east. Thus, the process of salinity intrusion starts with the western rivers. Salinity moves northward along the main estuaries while moving east by the exchange of water through several laterally connecting channels. Salinity continues to increase throughout the dry period until significant local rainfall, or more usually, until the first spills from the Ganges in May or June begin to drive back the salinity front. Analysis of the collected salinity data from various source have shown the typical variation of salinity level in ppt is very critical for the month of April and May. For the selected river system the salinity is reaching the highest level for these months. The Passur River estuary is dominantly pushing the salinity towards the Bay responding the freshwater inflow from the Gorai. The water level data of last two decades clearly mention that the flow of the Gorai in dry season is near about zero except during the dredging work of 1998-2000. The propagation of salinity intrusion has been observed up to Bhatia para and the salinity of the Madhumati river depend mostly on the salinity condition of Bardia. Salinity remains well within 15 ppt3 in the wet period whereas in the dry period salinity increases and exceeds 25 ppt, especially in the south western part of the Sundarban. Typical salinity condition of the studied river system for the month of April and May is shown in Table 2.

Table 2: Observed Salinity Variation of the River Systems of Sundarban (Source: IWM 2012)

Location name River name Typical variation of measured salinity level in ppt

April May

Bhatiapara Gorai 0.41-0.94 0.38-1.16

Bardia Nabaganga 2.86-5.79 3.16-9.19

Arua Atai 4.71-8.57 8.26-12.59

Khulna Rupsa 9.4-12.9 11.7-14.0

It is observed from these maps that salinity is relatively more in the western part of the Sundarban compared to the eastern part on the Bangladeshi side because the dominant freshwater river system Gorai–Modhumati–Passur supplies greater amount of water in this region whereas the Mathabhanga–Kapotaksha–Sibsa river system supplies less fresh water into the mangrove forest. While the eastern part of the mangrove forest exhibits lesser (<5 ppt or 2.45 ds/m) salinity in comparison to other parts of the Sundarban, especially in the monsoon, even then salinity increases in this part significantly (5–15 ppt) in the dry period. High salinity (>15 ppt or 25.56ds/m) is observed all over the coastal side of the Sundarban as this side faces the sea in both the wet and dry seasons.

3 Salinity is usually measured in parts per thousand (ppt) or dS/m. The average ocean salinity is 35 ppt or psu and the average river water salinity is 0.5 ppt or psu.

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Figure 7: Spatial distribution of salinity in the coastal region of Bangladesh (in 2012)4 (Source: Rahman et al 2017)

As discussed in the previous section, the flow of the Ganges River has a remarkable effect on the freshwater flow into the rivers flowing through the Sundarban. Thus it affects the salinity of the surface water in the rivers of the Sundarban. A study conducted by Islam and Gnauck (2011), comparing the water discharge and salinity of the Passur River at Mongla point, portrays such a picture. Figure 8 shows that water discharge until 1974 did not reduce dramatically, although after 1975 there was drastic reduction in water flow which clearly corresponds to the trend of salinity increase from that point on.

Figure 8: Increased salinity in the Sundarban due to the scarcity of the Ganges fresh water

(Source: Islam and Gnauck [2011]) 4 1 ppt = 1.968 dS/m

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Salinization of groundwater in the coastal region resources limits its suitability for human consumption and practical application. The origin of this salinity is the vertical recharge of aquifer with brackish or saline water from numerous rivers and canals or from frequent tidal surges and seawater ingression due to groundwater abstraction in peripheral areas (Dola et al, 2008). The spatial variability of groundwater salinity is high as low flow rate and variable permeability inhibit mixing. Areas where the surface mud layers discontinue, localised recharge from rainwater or surface water create pockets of freshwater in the shallow aquifer. Inland streams and tidal channels are depressions and likely entry points for infiltrating surface water. These are usually dry just in the pre monsson period (April-May) but are filled with freshwater in the wet season. Thus, fresh water is more likely to infiltrate into the subsurface than brackish water, but during dry season the water becomes brackish (Ayers et al, 2016, Rahman et al 2017). Figure 9 presents the groundwater salinity in the coastal region from borehole data in the relevant district. Interpolations in the Sundarban area show that groundwater salinity is relatively high the forest area in Khulna, Satkhira whereas it is slightly lower in Bagerhat and within the Sundarban the salinity ranges vary between 16.64 dS /m and 19.45 dS /m (9.75 ppt and 11.56 ppt). The groundwater salinity is higher in the dry season and similar to surface water salinity is increasing over the years (Rahman et al, 2017.)

Figure 9: Spatial distribution of salinity in the coastal region of Bangladesh in 20125 (Source: Rahman et al 2017)

5 1 ppt = 1.968 dS/m

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Impact of Salinity Increase in the Sundarban The biodiversity of the Sundarban is strongly influenced by the seasonal salinity content and sedimentation dynamics of its waterways, which in turn are determined by seasonally and annually fluctuating freshwater flows and diurnal tides. The Sundarban comprises 64 species of mangroves and associated plants (Das, 2013). If salinity increases, the osmosis process of the saline water plants cannot be fully functional as the absorption of water by the roots of the plants is hindered. Recent studies show that, due to long periods of inundation in saline water, the plants’ respiration rate was reduced resulting in the hindrance of their physiological functions. The growth and survival of mangroves functions well with a salinity range in between 4 to 15 ppt (Mitra and Banerjee, 2010). It has been observed that young 8–10 feet high bain trees located in the Sajne Khali and Prikhali islands are drying up, whereas the older bain trees located in the upper side of the island do not exhibit any kind of abnormality (Das, 2013). There are also studies that show that salinity increase has affected the regular succession patterns and is the thought to be one of the reasons for the top dying of sundari trees (Heritiera fomes), the dominant tree species in Sundarban. The sundari loves to grow in a ground which is inundated for 4-5 days during the spring tide period. Its germination rate decreases with increase of salinity. A recent study reveals that Sundri shows highest seed germination (about 50%) at 0-5 ppt saline condition which decrease with increase salinity level and ceases after 35 ppt (Hussain, et al., 2014). On the other hand, recent studies link fisheries habitat and river salinity has shown that salinity intrusion through the Sundarban estuaries has resulted in significant loss of fish species for the area (Dasgupta et al., 2017).

2.3.2 Various Physico-chemical Parameters Nutrients in water play an important role in the lives of aquatic organisms including fish. Nutrient availability is directly related to the productivity of the aquatic ecosystem. A shortage of nutrients reduces the productivity of the water body whereas an excess of nutrients causes eutrophication and makes the water toxic. Increasing agriculture activities, shrimp farming, increased human settlement and other unscientific changes in land use including industrialisation in upstream areas coupled with reduced fresh water supply from upstream rivers, siltation in the river beds and climate change is causing deterioration of the water quality of the river as well as to the whole mangrove ecosystem. Then again, the mangroves play an important role in regulating water quality, quantity, nutrients and act as a buffer between terrestrial and marine ecosystems. However, very few studies have been done to understand the water quality of intertidal mangrove ecosystems.

The regional variability in land use regulates the spatial variability in nutrints in the Sundarban mangroves and indicates that in the upstream terrestrial, organic matter and mangrove plant litter contribute significant amount of organic matter, whereas the marine influences the nutrient dynamics in the downstream areas. The water quality parameters were acceptable during the monsoon due to dilution, but during dry season the concentrations were higher. A report published by Departement of Environment (DoE), shows that in 2014, recorded levels of Chloride, Total Dissolved Solids (TDS) and Turbidity in the Moyuri, Rupsha, Pashur and Khakshiali Rivers were high. Materials including clay, silt, microscopic inorganic and organic matter, algae, dissolved colored organic compounds, and plankton causes turbidity which impacts aquatic life in the rivers (DoE, 2015). In another study, the western part of the Sundarban, which is characterized by higher salinity throughout the year, showed

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extremely high ammonia levels during monsoon. This is due to agricultural and aquaculture activities of the peripheral areas (Rahman et al, 2013).

2.4 Natural Disasters

The Sundarban, like many mangrove systems of the world, is situated at the front line of many coastal hazards. The fragile ecosystem and communities living in and around the Sundarban region are exposed to tropical cyclones, storm surges, erosion and frequent inundation by high tides causing loss and disruption to lives and livelihood and often irreparable or long-term damage to the hydro-morphological systems of the coastal region. Of these natural disasters, salinity intrusion has already been discussed in previous sections, this section focusses on cyclones and storm surges.

2.4.1 Cyclones The North Indian Ocean accounts for 7 per cent of global cyclones (Gray, 1985) and cyclones occur up to four times more in the Bay of Bengal than in the Arabian Sea (Deo and Ganer, 2014). Cyclones that originate in the Bay of Bengal basin have their landfall in the east coast of India, Bangladesh, and Myanmar making the countries susceptible to medium to severe tropical cyclones almost every year. Tropical cyclones are characterized by a large low-pressure centre and walls of deep thunderstorms that produce strong, converging cyclonic winds and heavy rain. Most damage and loss of life occur through seawater inundation of low-lying coastal regions. A tropical cyclone can have destructive winds having speeds as high as 250 km/h. Tropical cyclones and associated storm surges cause great damage to the ecosystem and impact the life, property and economy of the inhabitants of the Sundarban region in particular the agriculture and fisheries sectors and thus affect the livelihood of the coastal habitants too (World Bank, 2014). Some of the most devastating tropical cyclones in the world have occurred in the Bay of Bengal. An analysis shows that out of fourteen global tropical cyclones associated with the highest fatalities, nine have occurred in the Bay of Bengal (World Bank, 2014). Data on the frequency and magnitude of cyclones before the twentieth century is inadequate and is often more anecdotal with only the most severe events remembered. The first documented severe cyclonic storm in the Sundarban occurred in 1584, when 2,000,000 living creatures were recorded to have perished. Between 1877 and 1995, the 154 cyclones, including 43 severe cyclonic storms, 43 cyclonic storms, and 68 tropical depressions struck the Bangladesh coast (Dasgupta et al, 2011), many of these calamities travelled through the Sundarban region. Although, the annual frequency of tropical cyclones in this region shows a decreasing trend, an increasing trend in frequency can be seen during November and May, these months’ account for maximum number of intense cyclones annually (Singh and Khan, 2001). Moreover, the intensity of these cyclones have also increased and is likely to increase further due to climate change (Dasgupta et al, 2011).

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2.4.2 Cyclone-induced Storm Surges Storm surge are commonly created by cyclones that cause the sea waters to rise abnormally and leads to extreme coastal flooding. Records indicate that the cyclone induced damages are the greatest from the inundation caused by cyclone-induced storm surges (Dasgupta et al, 2011). The extent of coastal flooding and associated damage depends on cyclone intensity, wind speed, and onshore topography. The impacts are further amplified when storm surges coincide with astronomical high tides. For example, the tropical storm Aila made its landfall in the western Sundarban on 25 May 2009 between 1:30 p.m. and 2:30 p.m. India Standard Time. This matched closely with spring high water and caused widespread inundation of the region although the storm was a relatively low-power system with its highest sustained wind speed of 112 km/h (IMD, 2010). Curvature of tropical cyclones in the Bay of Bengal, the wide, shallow continental shelf, high tidal amplitude (4-5 m), nearly sea-level flat coastal land, complex coastline geometry comprising of several inlets, tidal creeks, and river drainage systems coupled with the triangular shape at the head of the Bay of Bengal, which helps to funnel sea water pushed by the wind toward the coast, causes further surge amplification. Thus flooding potential from storm surge is quite high for coastal regions in West Bengal and Bangladesh, especially the Sundarban region (Dasgupta et al 2011; Gayathri et al 2015). The following cyclones are the most devastating natural calamities to hit the Sundarban coast in recent history: Cyclone Sidr hit the south-western coast of Bangladesh on the evening of 15 November 2007. The cyclone intensified to reach peak winds of 215 km/h (135 mph) according to the Indian Meteorological Department (IMD) and destroyed over 450,000 houses across 30 districts in Bangladesh through wind damage, flooding and tidal surge. About a quarter of the World Heritage Site of Sundarban Reserve Forest in Bangladesh was damaged. Severe cyclonic storm Aila combined with storm surge was the worst natural disaster to affect the Sundarban since Cyclone Sidr. The storm was responsible for at least 339 deaths across Bangladesh and India; more than 1 million people were left homeless. Aila became a severe cyclonic storm on 25 May 2009 and made landfall at its peak intensity. The Sundarban was inundated with 6.1 m (20 ft) of water and many tigers were feared to have drowned in Aila’s storm surge along with deer and crocodiles. Post-cyclone impacts include salinization of soil due to salt ingress caused by storm surge and tidal inundation as many embankments were damaged.

3 Natural and Anthropogenic Factors Affecting Water Resources of the Sundarban

The Sundarban is situated in the estuary of the Ganga and many developments upstream impact the fragile mangrove ecosystem and is marked by a sharp transition from dense mangrove forest to a highly cultivated, agriculture and aquaculture landscape. Infrastructure development in and around the Sundarban and even in the upstream areas has impacted the natural dynamics of the Sundarban ecosystem. Over the years, the intricate web of rivers fuelling life through this vast mangrove ecosystem has substantially reduced as these rivers got disconnected due to upstream interferences in both Bangladesh and India. These interventions in the upstream regions hinder flows; as a result the rivers either have dried up or have reduced flows, thus affecting the freshwater flow of the

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Sundarban. The previous section details how the Farakka Barrage impacted the river flow in the Sundarban as it substantially reduced the seasonal discharge of the Gorai system. Other major human interventions include embankments, barrages and dams in India and the polders and irrigation in Bangladesh.

Modern embankments built during 1965 to the 1970s in the upstream areas of Bangladesh have affected river dynamics. In south west region of Bangladesh, more than 129 polders have been constructed in the upstream areas of the Sundarban, encompassing 13,000 km2 of land, or about 44 per cent of the total land area in the south-western region of Bangladesh (EGIS, 2001). While these interventions initially provided flood protection and improved crop production, these embankments restricted tidal flow, disrupting sediment flow into the floodplains. The impacts of upstream polders on the Sundarban can be summarized as follows:

• Polders in the south-western region of Bangladesh reduce the tidal prism just north of the reserve forest, which subsequently encourages sedimentation in the riverbed.

• The so-called tidal pumping process in the polders brings the sediment through the channels to deposition-prone areas.

• Polders thus cause large-scale drainage congestions in the tidal plains, along the northern periphery of the Sundarban.

The third largest city of Bangladesh, Khulna, is located 59 km north of the Sundarban on the banks of the Rupsha, which flows through the Sundarban to reach the Bay of Bengal. Khulna encompassing Mongla was set up as a first subdivision in 1842 and was used as the centre of activities to suppress river pirates in the area and establish law and order on the river routes/canals. Newsprint mills were set up in Khulna, due to its location, in 1959 with gewa wood of the Sundarban as raw materials. However, following the declaration of the Sundarban as a World Heritage Site, the Forest Department reduced the allocation of gewa to the newsprint mills and currently there is a ban on gewa felling. Located around 20 km north of the Sundarban periphery, the Mongla port was established in 1954. Besides cement, petroleum and numerous brick industries, a massive 50,000-ton grain silo has been built along the banks of the Pashur River just upstream of the Sundarban. This causes various kinds of pollution by the ships plying through the Sundarban as well as those anchored at Mongla port (Hussain, 2014). There is also a Mongla Export Processing Zone situated in the region which extends facilities to investors and many industrial plots are being allocated to polluting industries including ship-breaking industries. Over the last few decades, shrimp farming has emerged as an important industry in the coastal region of Bangladesh. However, unplanned and unregulated practices in shrimp aquaculture have led to land degradation. This is mostly due to the nature of shrimp culture which requires letting in saline water into empoldered shrimp beds. The practices of shrimp farming have caused increased water and soil salinity, massive loss of crop production, loss of fruit, loss of indigenous floral species and freshwater crisis for drinking. The absence of national policy and strategy on sustainable shrimp aquaculture has been a fundamental problem of this sector (Kabir and Eva, 2014; Kurien, 2016).

Very recently the Government of Bangladesh has developed plans to establish a 1320-megawatt coal-fired power station at Rampal upazila in Bagerhat district. The proposed project is situated 14 km north of the Sundarban. In January 2016, the Government of Bangladesh awarded the contract for

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construction of the plant to Bharat Heavy Electricals Limited. Governments in both the countries have been criticized for the decision as the protesters and activists believe that the plant will destroy the biodiversity of the forest (The Guardian, 2016). Further to this, more than 150 industrial projects in the peripheral area near to Rampal and Mongla have been given clearance, this unplanned and rapid industrialisation will create pressure on the mangrove forest and due impact mitigation measures need to be developed, implemented and regularly monitored.

4 Proposed Initiatives

To safeguard a forest of this magnificence, to save an ecosystem of this importance requires systematic planning of initiatives which in turn, rely upon comprehensive knowledge on the waterscape as well as landscape of the Sundarbans. As the Sundarbans is shared among the neighbouring Bangladesh and India, initiatives have to be undertaken by both countries, which are to be further supplemented and cemented by joint-initiatives which will act as the backbone of any scheme developed to save the forest ecosystem. Any information gaps found during this concise discussion on the waterscape of the Sundarbans might be overcome by taking several initiatives which can be jointly ventured by both India and Bangladesh. A few of the conceived joint initiatives could be as follows:

• Establishment of monitoring stations at proper locations of the Sundarbans to monitor water level, discharge, tidal flow, salinity and other parameters.

• Utilizing the data obtained from the monitoring stations, maintain the distribution of water flow in the rivers, canals and creeks in the Sundarbans.

• Conduct joint studies to identify measures for the necessity of water diversion from the various polders and embankments.

• Conduct E-flow studies in a more precise way by utilizing the data (water level, discharge, tidal flow, salinity and relevant parameters) collected from monitoring stations on a regular basis.

• Conduct joint studies to identify measures to re-establish connection between various rivers (Mathabhanga-Kopotakha-Sibsa system, Ichamati System, Bidyadhari System, Matla System and Adi-Ganga System, Bhairab, Kumar, Chitra, Nabaganga, Kabadak, Betna etc.) of Bangladesh and India.

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