long-term island area alterations in the indian and bangladeshi … · 12,371 km2 and 250 (1967),...
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Long-term Island Area Alterations in the Indian and Bangladeshi Sundarban: An
Assessment Using Cartographic and Remote Sensing Sources
Sunando Bandyopadhyay
Department of Geography, University of Calcutta, Kolkata 700019, India. Email: [email protected]
Nabendu Sekhar Kar
Department of Geography, University of Calcutta, Kolkata 700019, India. Email: [email protected]
Susmita Dasgupta
Development Research Group, World Bank, Washington, DC 20433, USA. Email: [email protected]
Dipanwita Mukherjee
Department of Geography, University of Burdwan, Burdwan 713104, India. Email: [email protected]
December 2018
Funding for this research has been provided by the South Asia Water Initiative
administered by the World Bank.
The findings, interpretations, and conclusions expressed in this paper are entirely those
of the authors. They do not necessarily represent the views of the International Bank for
Reconstruction and Development/World Bank and its affiliated organizations or those of
the Executive Directors of the World Bank or the governments they represent.
mailto:[email protected]://mail.google.com/mail/u/0/?view=cm&fs=1&[email protected]
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LONG-TERM ISLAND AREA ALTERATIONS IN INDIAN AND BANGLADESHI SUNDARBAN: AN ASSESSMENT
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Abstract
The seaboard areas of the western Ganga–Brahmaputra delta (GBD) harbours the largest
unfragmented mangrove forest of the world, the Sundarban. The region is shared by India and
Bangladesh. Cartographic and remote sensing materials of 1904–24 (Survey of India
toposheets), 1967 (Corona images), 2000–02 (IRS-1D LISS-3+Pan fused scenes), and 2015–
16 (Resourcesat-2 LISS-4 fmx scenes) are digitally evaluated for this area (12,164 km2) for
bringing out area transformations of ~250 forested and reclaimed islands. To facilitate analysis,
the islands are sorted into three groups based of their distance from the northern and southern
edges of the study region. The outcome show that while its interior channels, mainly in the
west, are getting clogged, the bay-proximal islands are decreasing in area at a fast pace. Total
island area and island count values detected by the study are: 12,618 km2 and 254 (1904–24),
12,371 km2 and 250 (1967), 12,227 km2 and 243 (2000–02), 12,164 km2 and 250 (2015–16).
The rate of area reduction kept a near-constant value of –4.45 km2/yr all through this interval
and is reflected both in the India- and Bangladesh-administered portions of the Sundarban. The
loss of the bay-facing islands of the region can mainly be attributed to non-availability of fluvial
and marine sediments due to degeneration of deltaic distributaries and shelf bypassing through
a submarine canyon. The reason for land gain in the northern Sundarban can be related to flood
tide dominance due to loss of morphological equilibria in the reclaimed estuaries. It is difficult
to link the effects of relative sea level rise and vertical accretion of island surfaces to the
alterations in island sizes due to conflicting and overlapping signals. The detected trends of area
change are likely to continue in the foreseeable future; therefore, they need to be integrated into
planning for the region.
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LONG-TERM ISLAND AREA ALTERATIONS IN INDIAN AND BANGLADESHI SUNDARBAN: AN ASSESSMENT
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Introduction
Contributed by a combined catchment area of 1.6 × 106 km2, the Ganga–Brahmaputra delta
(GBD) of the northern Bay of Bengal is the world’s largest in terms of land area (about 120,000
km2) as well as yearly release of sediments into the sea (about one billion tons per annum). The
Ganga–Padma–Lower Meghna river diagonally divides the GBD into two parts. The
southwestern portion, along with the southern coastline of the delta between the Hugli and the
Baleswar estuaries, is primarily contributed by the distributaries of the Ganga system—active
or dissipated. Its 200-km littoral stretch constitutes about 47% of the GBD coastline and
harbours the largest unfragmented mangrove forest of the world—the Sundarban. Delta-
building bio-tidal processes, at the southern frontier of the GBD, was responsible for phased
accretion of all sea-front islands of the Sundarban c. 5–2 kyr before present (Allison et al, 2003),
from what probably was a number of disconnected incipient subtidal and intertidal shoals to ots
present configuration. The region is shared by India and Bangladesh, with its international
border running through the rivers of Ichhamati, Kalindi, Raimangal, and Harinbhanga (Fig. 1).
The Sundarban is exposed to a unique set of natural environmental hazards – storm inundation,
saline intrusion, sea level rise, coastal erosion, and channel sedimentation – that are set to
aggravate in a warming world. Among these, the problems of erosion of the tidal islands and
sedimentation of their intervening creeks did not receive the attention they deserve. In this
context, the present work attempts to document the long-term island area changes through
coastline shifts that took place in this region since the early 19th century using maps and images.
Sundarban: Reclamation and present status
Although the frontiers of the Sundarban mangroves started to move southward for expansion
of rice farms from the 13th century (Eaton, 1990), forests still occupied some 16,500 km2 of
tidal seaface of the GBD between the Hugli and the Meghna estuaries about 240 years ago
(Rennell, 1779). Rennell reflected in 1781 that ‘this tract ... is so completely enveloped in
woods, and infested with tygers [sic], that if any attempts have ever been made to clear it (as is
reported) they have hitherto miscarried’. He noted that ‘sand and mud banks ... extend twenty
miles off some of the islands’ of the delta and imagined that ‘some future generation will
probably see these banks rise above water, and succeeding ones possess and cultivate them!’
(Rennell, 1788:259, 266).
Under the British colonial government, reclamation of the Sundarban was initiated in 1770. It
transformed into an institutionalised effort from 1783 (Pargiter, 1934) manifesting
administrative policies that viewed the wetlands mostly as wastelands (Richard and Flint,
1990). By 1831, reclaimable portion of the region between the Hugli and the Pussur was divided
into 236 compartments or ‘lots’ south of the Dampier–Hodges (D–H) Line, surveyed in 1829-
30 to demarcate the contemporary frontier of the wetlands (Pargiter, 1934). The scheme was
later extended up to the Baleswar by adding 22 more lots. This formed the basis of all
subsequent reclamation efforts (Ascoli, 1921). At that time, the area of Sundarban forests was
11,610 km2.
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LONG-TERM ISLAND AREA ALTERATIONS IN INDIAN AND BANGLADESHI SUNDARBAN: AN ASSESSMENT
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Among the three original districts constituting Sundarban, reclamation of the eastern Bakarganj
was nearly complete by 1910 (Jack, 1918). Situated between the Baleswar and the Meghna, the
deltaic distributaries of this part had lower tidal range than the west and received freshwater
from upcountry sources. This helped to reduced salinity and replenishment of fertile sediments
that sustained agriculture. In contrast, reclamation went on up to the start of this century in the
western 24 Parganas somewhat sporadically. Occupying the abandoned GBD between the
Hugli and the Raimangal, here the salinity as well as the tidal range were higher than the east,
requiring higher embankments to arrest tidal spill. Between these two regions, the wetlands of
Khulna, bounded by the Raimangal and the Baleswar, mostly remained intact. Consideration
of commercial significance of the forest produces enforced Government protection in varying
degrees since 1875 in Khulna and adjacent parts of 24-Parganas (Ascoli, 1921). As the
contemporary Survey of India (SoI) maps indicate, extent of mangroves in Khulna did not
reduce appreciably after the 1920s.
Currently, the littoral tract between the Hugli and the Baleswar rivers is known by the name of
Sundarban. The Indian Sundarban Biosphere Reserve is demarcated by 19 community
development blocks south of the D–H Line and incorporates both reclaimed and non-reclaimed
regions. In Bangladesh, Sundarban denotes only the mangrove wetlands. A 10-km and a
proposed 20-km buffer from the forest boundary delineate the Ecologically Critical Area and
the Sundarban Impact Zone (SIZ), respectively (Fig. 1). 20 upazilas (sub-districts) of
Bangladesh get wholly or partly included within the reach of the proposed SIZ. Area of the
non-reclaimed Sundarban wetlands is 6,262 km2 at present, of which India administers 28%,
and Bangladesh, 62%.
F.E. Pargiter detailed the procedure of reclamation of Sundarban in The Calcutta Review of
July, 1889. The first task was to embank the low-lying forested islands. This meant that a line
was drawn through the forest along the banks of the streams surrounding the area to be
reclaimed and an earthen embankment was constructed along the line to prevent tidal
inundation. Strong dams were constructed across the mouth of small streams in order to keep
salt water out. This work was followed by felling of the forests, digging of tanks, and
construction of huts; systematic cultivation could then begin. Apart from depriving the
embanked area from sediment accretion, the process exposed the settlers of the reclaimed
Sundarban a number of natural hazards enlisted earlier. In Bangladesh, extensive forest-front
areas are now protected large-scale polder dykes since 1960s (Bari Talukdar, 1993). In India,
the marginal bunds have only recently been started to be reinforced.
Previous works
Among the works that studied coastline shifts of the Sundarban region from comparative
cartographic / remote sensing materials or field studies, notable are: Chakrabarti (1995),
Bandyopadhyay and Bandyopadhyay (1996), Bandyopadhyay (1997), Allison (1998), Hazra et
al. (2002), Bandyopadhyay et al. (2004), Ganguly et al. (2006), Rahman et al. (2011), Rahman
(2012), Sarwar and Woodroffe (2013), Raha et al. (2014), Chakrabarti and Nag (2015), Ghosh
et al. (2015), Quader et al. (2017), and Ahmed et al. (2018). Although these works detected a
prevailing eroding trend of the delta front, none of these, however, estimated alterations in
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LONG-TERM ISLAND AREA ALTERATIONS IN INDIAN AND BANGLADESHI SUNDARBAN: AN ASSESSMENT
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island-wise data of the area of the interiors of Sundarban. These studies mostly remained
confined either to Indian or to Bangladeshi, close to the southern coast of the GBD. The present
research fills-up this gap in information.
Methodology
Maps and satellite data
Finding out the century-scale rates of area change of the islands constituting the Sundarban is
the chief aim of this work. To achieve this, four datasets with resolution of ~5 m or finer are
selected. These are as follows.
Survey of India (SoI) 1:63.360 toposheets of 1904–24 (38 maps),
Corona KH4A space photographs of 1967 (9 images),
IRS-1D LISS-3+Pan merged False Colour Composite (FCC) of 2000–02 (15 images), and
Resourcesat-2 LISS-4 fmx FCC of 2015-16 (11 images).
To facilitate orthorectification, a couple of additional mosaics were prepared:
A map of 16 SoI 1:50,000 toposheets,
A FCC of four Landsat-8 OLI panmerged scenes.
Table 1 states the details of the above six datasets.
Table 1: Cartographic and remote sensing scenes of Sundarban used for data generation and
orthorectification
Map / Image 1, Scale /
Resolution
Year of Survey /
data Acquisition
[central-value within
brackets]
Interval from
Preceding
database
Area covered by each
map / satellite scene
38 SoI Topographic
Sheets: Zone 79B, C, F,
G, J
1:63,630
(1 inch 1 mile)
1904–24 [1914] — 1515:
26 km 28 km
16 SoI Topographic
Sheets: Zone 79B, C, F,
G 2
1:50,000
(1 cm 0.5 km)
1959–81, 14 sheets
surveyed in 1967–69
— 2 1515:
26 km 28 km
09 Corona KH4A
Images: DS1038-
2102DA-183 to -191
~2 m 1967 Interval from
1914: 53 yr
70 km 17 km (each
divided scene)
04 IRS-1D LISS-3
Images: Paths 108, 109,
110; Rows 55, 56, 57
23.5 m 2000–02 [2001] Interval from
1967: 34 yr 140 km 140 km
15 IRS-1D Pan Images:
Paths 109, 110; Rows 55,
56, 57; Quadrates A, B,
C, D
5.6 m 70 km 70 km
04 Landsat-8 OLI
Images: Paths 137, 138;
Rows 44, 45 2
15 m
(panmerged)
2015 — 2 170 km 183 km
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LONG-TERM ISLAND AREA ALTERATIONS IN INDIAN AND BANGLADESHI SUNDARBAN: AN ASSESSMENT
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11 Resourcesat-2 LISS-
4 fmx Images: Paths
109, 110; Rows 56, 57;
Quadrates A, B, C, D
5 m 2015–16 [2015.5] Interval from
2001: 15 yr 70 km 70 km
1. Acronyms: fmx: full-swath multispectral, IRS: Indian Remote Sensing Satellite, LISS: Linear Imaging Self Scanner, OLI:
Operational Land Imager, Pan: Panchromatic, SoI: Survey of India
2. Mosaic only utilised for orthocorrection
Orthorectification and mosaicing
The orthorectification and mosaicing of the satellite scenes of all datasets are carried out using
Geomatica (ver. 2015) software on Universal Transverse Mercator projection (zone 45Q) and
World Geodetic System–1984 ellipsoid. The 2015 OLI scenes are used as the reference for
orthocorrection of all satellite data. The individual satellite scenes constituting a mosaic are
colour matched where necessary to achieve a visually pleasing greyscale image (Corona) or
standard FCC (OLI, LISS-3+Pan and LISS-4), with infra-red, red, and green bands / panmerged
bands of satellite data shown in red, green, and blue colours, respectively.
All Survey of India toposheets are individually corrected using their graticules on spherical
coordinates (degree-minute-second) on Everest ellipsoid. Next, they are mosaiced, and
reprojected to Universal Transverse Mercator projection (zone 45Q) and World Geodetic
System–1984 (WGS–84) ellipsoid to match rest of the data layers.
Island area demarcation
Northward extent of the present study area boundary is determined using the northernmost
definable islands in the 1904–24 topographical sheets of Sundarban (Table 1). This means that
an active channel separate the upper islands from the mainland of the north; beyond these, no
definable island exists. Most of the channels used for delineating the study area remain active
in 2015–16, the acquisition year of the latest dataset used here.
The coastline of a given island is delineated by manual digitisation of the spring High Water
Level (HWL). A coastline is normally defined as the landward limit of seawater during the high
spring tides in fair-weather conditions. (Bird, 1985:5–8). Determining these lines from maps is
straightforward, because an HWL is already plotted in a map. Most satellite scenes used in this
study represented high-water environment, and did not show intertidal features seaward of the
HWL. Moreover, the HWL extraction here is done manually here on the basis of clear
understanding of the coastal features of reclaimed and non-reclaimed areas of Sundarban and
different types and resolutions of satellite images that represent them.
Island ID, group, and centroid generation
In this work, the individual islands in the maps and images are all identified by a numbered
prefix of letters representing administrative divisions of either community development blocks
(Indian reclaimed Sundarban), or upazilas (Bangladeshi reclaimed Sundarban), or forest blocks
(Indian non-reclaimed Sundarban), or forest ranges (Bangladeshi non-reclaimed Sundarban).
The identity of a given islet is kept same in all the four mapping / imaging years, if not it gets
separated into one or more entities or merges with another island.
For simplifying data analysis, the islands are classified according to their locations from the
southern seaface and northern limit of the study area. Two 12-km buffers from the 1904-24
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LONG-TERM ISLAND AREA ALTERATIONS IN INDIAN AND BANGLADESHI SUNDARBAN: AN ASSESSMENT
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northern and southern bounds of the study area are generated and use for designating the islands
crossing the northern and southern buffers and northern islands and southern islands
respectively. The islands not transecting the buffers are labelled as central (Fig. 11). Two large
and elongated islands situated at the eastern and western ends of Sundarban—Sagar and
Bhandaria—intersected both the buffers and were kept out of this analysis.
In addition, centroids for the islands are plotted for the four mapping/imaging years for getting
an idea on how island-wise land loss and land gain changed in relation to their northward
position from the seaface and westward location from 88.04°E, which is the western limit of
the study area at the southwestern Sagar island.
To estimate erosion and accretion figures from the island vectors of two consecutive
mapping/imaging years, spatial analysis principles of overlay, and intersection are used. A set
of vectors belonging, for example, to 1967 and 2000–02, are overlaid to generate a third vector
with superposed areas. In this vector, the different segments are designated either to erosion or
to accretion or to no-change. All these vector-related analysis are performed in ArcGIS
ver.10.3.1 (centroid creation) and Geomatica v.2015 (buffer formation, spatial analyses).
Statistical tasks are done in MS Excel ver.2016.
Outcome
The summary of the data generated by this work are displayed in nine maps (Fig. 2–9, 11) and
12 diagrams (Fig. 10, 12–22). The evaluation was done on the basis of the northern, central,
and southern island groups, stated in the methods section. Table 2 shows country-specific data
of changes in total island numbers, total area, and rate of total area alterations.
The summary table and diagrams connote the following trends for the region.
Almost the entire coast of the Sundarban facing the Bay of Bengal retrogrades for the
last~100 years, regardless its status as non-reclaimed or settled portions. The Indian
Sundarban coastline bounded by the Saptamukhi and the Gosaba channels (Fig. 3–6)
registers highest rate of erosion at 40 m/yr. The rates of shoreline retreat slows on either
sides of this tract, and becomes negligible at the eastern edge of Sundarban.
Table 2: Summary of island number and island area data: Indian and Bangladeshi Sundarban
Zone Specifics Mid-value of Mapping / Imaging Years
1914 1967 2001 2016
Sundarban
Region
(Indian and
Bangladeshi)
Island count 254 250 243 250
Supratidal area (km2) 12,617.8 12,370.5 12,227.2 12,164.0
Area Index: base = 1914 100.0 98.0 96.9 96.4
Alteration rate since preceding
mapping / surveying year (km2/yr)
–4.665 –4.216 –4.358
Western
Sundarban
Island count 136 132 126 134
Supratidal area (km2) 4,508.2 4,345.9 4,271.2 4,235.3
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LONG-TERM ISLAND AREA ALTERATIONS IN INDIAN AND BANGLADESHI SUNDARBAN: AN ASSESSMENT
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(Indian) Area Index: base = 1914 100.0 96.4 94.7 93.9
Alteration rate since preceding
mapping / surveying year
(km2/yr)
–3.061 –2.197 –2.481
Eastern
Sundarban
(Bangladeshi)
Island count 118 118 117 116
Supratidal area (km2) 8,109.6 8,024.6 7,956.0 7,928.8
Area Index: base = 1914 100.0 99.0 98.1 97.8
Alteration rate since preceding
mapping / surveying year
(km2/yr)
–1.603 –2.019 –1.878
It is possible to group the trends of area loss and gain of the individual islands into six classes:
loss (linear), loss (non-linear), gain followed by loss, gain (non-linear), gain (non-linear),
and loss followed by gain, as depicted in Fig. 9 and 10.
In the islands, the land lost (5.68% of total) is close to three times of the land gained (1.82%),
which denotes a general erosional trend. In the northern islands, gain (4.31%) is seen to score
over loss (3.03%). Conversely, both central and southern islands show greater loss (4.74%
& 17.85%, respectively) than gain (1.46% & 1.65%, respectively) (Fig. 12, 17). In the study
region, more islands are eroding (68% of total count) than accreting (32%), suggesting an
overall erosive inclination. In the north, the growing islands (52% of total count) closely
match the count of eroding islands (48%). More of the central islands are affected by erosion
(76%) compared to accretion (24%). Similar trends are detected in the southern islands,
where, 78% of islets recorded erosion while the rest (22%) eroded (Fig. 13). Overall, the
trends of area alterations appear similar and linear in the whole of Sundarban region as well
as Indian ad Bangladeshi sectors (Fig. 16).
From 1967 to 2000–02, the central and southern isles show fast and progressive erosion;
while the northern islands recorded massive land gain. However, the overall scenario is that
the supratidal area of the studied region show unidirectional decline, with areas of 78% of
the islets reducing and just 22% increasing (Fig. 14/15).
In the entire area and across the periods, minor gaining trend is seen eastward, with very low
significance level. Similarly insignificant trends are recorded for the islands of north and
central area. For the northern Sundarban, slightly accreting trend of high significance is
detected landward. The coast-proximal islands of the south show significant area gain
eastwards (Fig. 19). Insignificant trends are brought out for the northern and central islands,
while the coastal islands show significant gain in area northwards (Fig. 20).
Many tidal channels of the interior areas are getting clogged, mostly in the reclaimed western
areas and partly in the eastern reclaimed stretches. This caused certain trend reversals in the
northern islands (Fig. 21, 22). Pearson’s correlation coefficient (r-value) derived through
processing of island area change data pertaining to the four mapping/imaging years indicate
that significant land gain takes place northwards.
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LONG-TERM ISLAND AREA ALTERATIONS IN INDIAN AND BANGLADESHI SUNDARBAN: AN ASSESSMENT
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Reasons for change
How can the observed trends be explained in the light of existing knowledge of regional
sedimentation? The main reasons seems to be delta abandonment, shelf bypassing of sediments,
regional tidal asymmetry, and relative sea level rise.
Delta abandonment and shelf bypassing of sediments
Advancement and retreat of a deltaic coast rest on the relative supremacy of the erosional wave
and tidal forces and the depositional fluvial inputs. With most of the Ganga’s distributaries
feeding the Sundarban becoming degenerated, the region is now fluvially abandoned. Although
some 109 t of sediments are annually routed through the Meghna estuary into the continental
shelf off the GBD (Milliman and Syvitski, 1992; Goodbred, 2003), a large part of it is captured
by the Swatch of No Ground submarine canyon, and routed into the deep sea Bengal fan as it
moves westward (Kuehl et al., 1989, 1997, 2005) (Fig. 23). This lack of sediment supply results
in continuous land loss in the coastal islands, aggravated by the periodic effects of tropical
cyclones (Fig. 24), and monsoonal waves.
Time–velocity asymmetry of tides
Wright et al. (1973) showed that in a morphological equilibrium condition, length of a resonant
macrotidal estuary (), that has a tidal range of more than 4 m, tends to equal a quarter of the
tidal wavelength (L) generated in it, that is:
→ 0.25 L
Length of the tidal wave, behaving as a shallow water wave in an estuary, is determined by tidal
period (T: a constant in a given locality), gravitational acceleration (g: another constant, 9.81
m sec–2) and average depth of the estuary (D: the only variable) in the following relationship:
L = T(gD)
Reclamation restricts the area of tidal spill through marginal embankments, takes shallow
intertidal wetlands away from the realm of the estuary and thereby increases its average depth
(Pethick, 1984, 1994) (Fig. 25). This removes the estuary from morphological steady state. For
example, both of the 1988-92 and 1992-2002 hydrographical charts of India’s Naval
Hydrographic Office indicate that the length of the westernmost estuary of reclaimed
Sundarban, the Hugli, equals about one-sixth (=0.17 L) of the tidal wavelength entering into
it (Bandyopadhyay, 2000; Nandy and Bandyopadhyay, 2010).
The macrotidal estuaries of the Sundarban responds to this by setting up time-velocity
asymmetry in tidal cycles. The flood tides span for shorter duration in a cycle, and have higher
landward velocity than the longer ebb. This causes active channel sedimentation and, in selected
reclaimed stretches, erosion of the embankments to decrease the average depth and to restore
the equilibrium condition (Bandyopadhyay et al., 2014). Moreover, the dissimilarity of water
current in tidal channels and in adjacent mangrove wetlands leads to dynamic tidal circulation,
which maintains the channel depth. Clearing of mangroves disrupts this balance and also results
in siltation of the channel (Augustinus, 1995:353). All these put forward a plausible explanation
for the accretion seen in the reclaimed northern islands of the Sundarban region.
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LONG-TERM ISLAND AREA ALTERATIONS IN INDIAN AND BANGLADESHI SUNDARBAN: AN ASSESSMENT
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Relative sea level rise
The relative sea change data of tidal stations of the coastal GBD, available from
www.psmsl.org, around the Sundarban region show a range of 1.19–6.25 mm/yr and centres on
3–4 mm/yr (Fig. 26). The effect of this on island size changes in Sundarban remain somewhat
inclusive because of known vertical accretion rates from the region. Despite lateral land loss all
around, these rates have been estimated to be 0–11 mm/yr (Allison and Kepple, 2001) and 10
9 mm/yr (Rogers et al., 2013) from the no-reclaimed surfaces of Khulna Sundarban.
Concluding notes
The outcome of this study indicate that with present overall rate of area alterations that clearly
depict higher land loss than land gain, Sundarban is set to get reduced by 3.12% of its current
size (380 km2) at 4.57 km2/yr by 2010. In erosion, the Indian and Bangladeshi parts will lose
1.87% and 1.25% of area, respectively. Most of this is estimated to take place in the south and
eight seafront islands, seven of them in India, will get fully eroded within the next few decades
(Table 3).
A sizable extent of mangroves protects the settled areas of Bangladesh, defending them from
the intimidations of coastal retreat and other sea-front hazards associated with landfalls of
tropical cyclones. In the western section of the Indian Sundarban, the mangroves are obliterated
right up to the southern coastlines, exposing the resident population to all adverse eventualities.
Future planning for the region must accept the change that have been introduced into the system
by the humans and choose a moderate course between the requirements of the nature and the
needs of the people.
Table 3: Islands projected to erode completely within 21st century.
Island
Name
Island ID
(see Fig. 2)
Forest Block /
Range
Projected year of
erosion
Haliday Hd Haliday, India 2030
Bhangaduni Md-04 Mayadwip, India 2067
Kankramari
West
NK 10 ms Namkhana, India 2061
Kankramari
South
NK 11 ms Namkhana, India 2061
Ghoramara Sg 02 Sagar, India 2036
Chhota Haldi
(b) Fragment
Ch-03 Chhota Haldi, India 2100
Bulchery Cl-04 Chulkati, India 2100
Sutarkhali Khl 25 Khulna, Bangladesh 2045
Acknowledgments
This research was conducted under the South Asia Water Initiative – Sundarbans Targeted
Environmental Studies. We thank Karabi Das, Pritam Kumar Santra, and Abhijit Das for their
help with data processing.
The results, analyses, and conclusions presented in this article are entirely those of the authors.
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LONG-TERM ISLAND AREA ALTERATIONS IN INDIAN AND BANGLADESHI SUNDARBAN: AN ASSESSMENT
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They do not necessarily stand for the views of the International Bank for Reconstruction and
Development / World Bank and its affiliated organizations, or those of the Executive Directors
of the World Bank or the governments represented by them.
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FIGURES
Figure 1: Composite map of the Sundarban region. Selected populated places and boundaries are shown in the map. Comparison between 1904–24 topographical maps and 2015–16 satellite images brings out the extent of erosion along nearly the entire seaface of the delta and accretion in the interior parts, chiefly in the west. Source: See Table 1. D–H Line extracted from 1:253,440 Atlas of India map # 121 & 122 of c. 1860 (modified from Bandyopadhyay, in press).
-
Figure 2. Tidal islands of the Sundarban region identified by centroids and unique identification code of mapping year 1904–24. Outlines of the islands in 2015–16 are represented in ochre. Island centroids of 1904–24, 1967, 2000–02 and 2015-16 are shown in red, green, grey, and blue, respectively. Source: See Table 1.
Figure 3: Lost and gained island area between 1904–24 and 1967 (53 yr), shown in red and green, respectively. Source: See Table 1.
-
LONG-TERM ISLAND AREA ALTERATIONS IN INDIAN AND BANGLADESHI SUNDARBAN: AN ASSESSMENT
16
Figure 4: Lost and gained island area between 1967 and 2000–02 (34 yr), shown in in red and green, respectively.
Source: See Table 1.
Figure 5 Lost and gained island area between 2000–02 and 2015-16 (14 yr), shown in in red and green,
respectively. Source: See Table 1.
-
LONG-TERM ISLAND AREA ALTERATIONS IN INDIAN AND BANGLADESHI SUNDARBAN: AN ASSESSMENT
17
Figure 6: Lost and gained island area between 1904–24 and 2015-16 (102 yr), shown in in red and green,
respectively. Source: See Table 1.
Figure 7: High Water Level polygons of southern seafront islands of Sundarban indicate continuous land loss
throughout the mapping / imaging years. Source: See Table 1.
-
LONG-TERM ISLAND AREA ALTERATIONS IN INDIAN AND BANGLADESHI SUNDARBAN: AN ASSESSMENT
18
Figure 8: Sedimentation and emergence of new islands in the northern areas of Sundarban within the estuaries of
Saptamukhi (left) and Thakuran (right). Source: See Table 1.
Figure 9: Types of supratidal area alterations in the islands of Sundarban during the four survey / imaging years
used in the study. Figure 10 provides examples of representative islands. See Fig. 2 for ID of islands. Source: See
Table 1.
-
LONG-TERM ISLAND AREA ALTERATIONS IN INDIAN AND BANGLADESHI SUNDARBAN: AN ASSESSMENT
19
Figure 10: Types of supratidal area alterations in islands of Sundarban during the four survey / imaging years
shown by type-islands. The islands can be located using their identity code in Fig. 2. Source: See Table 1.
-
LONG-TERM ISLAND AREA ALTERATIONS IN INDIAN AND BANGLADESHI SUNDARBAN: AN ASSESSMENT
20
Figure 11: Classification of the study area islands into northern (represented in yellow), central (represented in
ochre) and southern (represented in green) groups on the basis of two 12-km buffers drawn from the northern and
southern (coastal) boundaries of the study area, shown here in red and cyan lines, respectively. All diagrams in
Fig. 12–15, and 17–22 are based on this division. Sagar island (Sg-03) of India and Bhandaria island (U-08) of
Bangladesh are excluded from all of the three groups because of their north–south extension. Shown in light grey,
these two islands occupy the western and eastern extremities of Sundarban, respectively.
-
LONG-TERM ISLAND AREA ALTERATIONS IN INDIAN AND BANGLADESHI SUNDARBAN: AN ASSESSMENT
21
Figure 12: Change in supratidal area of the Sundarban islands between 1904-24 and 2015-16, the first and
last year of mapping/imaging. (A) In entire Sundarban, area lost by erosion (5.68% of total area) is near about three
times of the area gained by accretional processes (1.82% of total area) which signifies an overall erosional trend.
(B) In case of northern island group, accretion (4.31% of total area) is greater than erosion (3.03% of total area).
(C) & (D) Both the central and southern group of islands are experiencing greater erosion (4.74% & 17.85% of total
area, respectively) than accretion (1.46% & 1.65% of total area, respectively). Sagar Island (Sg-03) of India and
Bhandaria island (U-08) of Bangladesh are excluded from the spatial groups (B–C). Please see text and Fig. 11 for
details. Source: See Table 1.
227.7
-713.1-750
-625
-500
-375
-250
-125
0
125
250
Are
a (k
m2)
(A) Entire Sundarban
122.5
-86.1
-750
-625
-500
-375
-250
-125
0
125
250
Are
a (k
m2 )
(B) Northern Islands
71.9
-233.3
-750
-625
-500
-375
-250
-125
0
125
250
Are
a (k
m2 )
(C) Central Islands
33.2
-359.4
-750
-625
-500
-375
-250
-125
0
125
250A
rea
(km
2 )
(D) Southern Islands
-
LONG-TERM ISLAND AREA ALTERATIONS IN INDIAN AND BANGLADESHI SUNDARBAN: AN ASSESSMENT
22
Figure 13: Islands with positive and negative change in area between 1904-24 and 2015-16, the first and last
year of mapping/imaging. (A) In entire Sundarban, majority of islands faced erosion (68% of total count) over
accretion (32% of total count) which indicates an overall erosive inclination. (B) In case of northern island group,
accreting (52% of total count) islands nearly matches the numbers of islands facing erosion (48% of total count).
(C) In central part, most islands are affected by erosion (76% of total count) than accretion (24% of total count) (D)
Trends similar to central group is seen in southern part. Here, 78% of the total islands are eroding whereas only
22% of islands are accreting. Sagar Island (Sg-03) of India and Bhandaria (U-08) island of Bangladesh are excluded
from (B), (C), and (D). Source: See Table 1.
64
137
0
20
40
60
80
100
120
140
160
Positive changecount
Negative changecount
Isla
nd
Co
un
t(A) Entire Sundarban
32 30
0
20
40
60
80
100
120
140
160
Positive changecount
Negativechange count
Isla
nd
Co
un
t
(B) Northern Islands
22
70
0
20
40
60
80
100
120
140
160
Positive changecount
Negative changecount
Isla
nd
Co
un
t
(C) Central Islands
10
35
0
20
40
60
80
100
120
140
160
Positive changecount
Negativechange count
Isla
nd
Co
un
t
(D) Southern Islands
-
LONG-TERM ISLAND AREA ALTERATIONS IN INDIAN AND BANGLADESHI SUNDARBAN: AN ASSESSMENT
23
Figure 14: Changes in supratidal area through the years of mapping/imaging. Progressive and rapid loss of land
area is observed in case of central (C) and southern islands (D); though the northern island group (B) experienced
massive accretion between 1967 and 2001. The overall trend is that the land area of the entire Sundarban (A) is
progressively declining. Here, 78% of the total islands are eroding whereas only 22% of islands are accreting.
Sagar island (Sg-03) of India and Bhandaria (U-08) island of Bangladesh are excluded from (B), (C), and (D), as
shown in Fig. 11. Source: See Table 1.
12,618
12,371
12,227 12,164
11,900
12,000
12,100
12,200
12,300
12,400
12,500
12,600
12,700
Are
a (k
m2)
(A) Entire Sundarban
2,843 2,858
4,751 4,776
0
1,000
2,000
3,000
4,000
5,000
6,000
Are
a (k
m2)
(B) Northern Islands
4,918
4,822
4,6834,662
4,500
4,550
4,600
4,650
4,700
4,750
4,800
4,850
4,900
4,950
Are
a (k
m2)
(C) Central Islands
2,014
1,875
1,819
1,764
1,600
1,650
1,700
1,750
1,800
1,850
1,900
1,950
2,000
2,050
Are
a (k
m2)
(D) Southern Islands
-
LONG-TERM ISLAND AREA ALTERATIONS IN INDIAN AND BANGLADESHI SUNDARBAN: AN ASSESSMENT
24
0
Figure 15: Time series change in area through the years of mapping/imaging. Linear rapid loss of land area is
observed in case of central (C) and southern islands (D) though northern island group (B) experienced massive
accretion between 1967 and 2001 and connoted positive trends. The overall trend is that the land area of the entire
Sundarban (A) is progressively reducing. Sagar island (Sg-03) of India and Bhandaria island (U-08) of Bangladesh
are excluded from (B), (C), and (D). See Fig. 11. Source: See Table 1.
12,618
12,371
12,227
12,164
y = -4.4652x + 21161R² = 0.9994
12,100
12,200
12,300
12,400
12,500
12,600
1900 1950 2000
Are
a (k
m2)
Year
Entire Sundarban
2,843
2,858
4,7514,776
y = 21.277x - 38205R² = 0.7596
0
1,000
2,000
3,000
4,000
5,000
6,000
1900 1950 2000
Are
a (k
m2)
Year
Northern Islands
4,918
4,822
4,683
4,662
y = -2.6295x + 9963.4R² = 0.9665
4,600
4,650
4,700
4,750
4,800
4,850
4,900
4,950
1900 1950 2000
Are
a (k
m2)
Year
Central Islands
2,014
1,875
1,819
1,764
y = -2.3528x + 6513.6R² = 0.9895
1,750
1,800
1,850
1,900
1,950
2,000
2,050
1900 1950 2000
Are
a (k
m2)
Year
Southern Islands
-
LONG-TERM ISLAND AREA ALTERATIONS IN INDIAN AND BANGLADESHI SUNDARBAN: AN ASSESSMENT
25
Figure 16: Country-wise changes in supratidal area through the years of mapping/imaging. If the changes in
net supratidal area of the Sundarban region is viewed in its entirety (A), or as western Indian (B) or eastern
Bangladeshi (C) sectors, the trends of changes appear noticeably similar and linear, irrespective of their absolute
values. Source: See Table 1.
-
LONG-TERM ISLAND AREA ALTERATIONS IN INDIAN AND BANGLADESHI SUNDARBAN: AN ASSESSMENT
26
Figure 17: Individual island-wise change in area between 1904-24 and 2015-16, the first and last year of
mapping/imaging for the entire Sundarban (A), and northern (B), central (C), and southern (D) groups of islands.
The diagrams show that the islands of the southern group are most affected by coastal erosion. Sagar island (Sg-
03) of India and Bhandaria island (U-08) of Bangladesh are excluded from (B), (C), and (D) because of their north–
south extension. See Fig. 11. Source: See Table 1.
-50
-40
-30
-20
-10
0
10
20
30
40
0 100 200
Are
a (
km2)
Island Count
(A) Entire Sundarban
-50
-40
-30
-20
-10
0
10
20
30
40
0 50
Are
a (
km2)
Island Count
(B) Northern Islands
-50
-40
-30
-20
-10
0
10
20
30
40
0 50 100
Are
a (
km2)
Island Count
(C) Central Islands
-50
-40
-30
-20
-10
0
10
20
30
40
0 20 40
Are
a (
km2)
Island Count
(D) Southern Islands
-
LONG-TERM ISLAND AREA ALTERATIONS IN INDIAN AND BANGLADESHI SUNDARBAN: AN ASSESSMENT
27
Figure 18: Distribution pattern of the Pearson’s correlation coefficient (r-value) generated by analysing the
time series trends of area change of individual islands considering all mapping/imaging years for the entire
Sundarban (A), and northern (B), central (C), and southern (D) groups of islands. The diagrams show that most of
the islands of Sundarban are eroding at a rapid rate having r-values very close to (–1). Some of the islands,
especially in the northern group, are also accreting at a high rate. Sagar island (Sg-03) of India and Bhandaria
island (U-08) of Bangladesh are excluded from (B), (C), and (D). See Fig. 11. Source: See Table 1.
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
0 100 200
r va
lue
Island Count
(A) Entire Sundarban
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
0 20 40 60
r va
lue
Island Count
(B) Northern Islands
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
0 50 100
r va
lue
Island Count
(C) Central Islands
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
0 20 40
r va
lue
Island Count
(D) Coastal Islands
-
LONG-TERM ISLAND AREA ALTERATIONS IN INDIAN AND BANGLADESHI SUNDARBAN: AN ASSESSMENT
28
Figure 19: Relationship between easting and area change between 1904-24 and 2015-16. In case of the entire
Sundarban (A), very slight increasing trend (accretion) is seen towards east though the significance level is low.
Very insignificant trends are also observed for northern (B) and central (C) island groups. However, the coastal
islands show significant increase in area towards east. Sagar island (Sg-03) of India and Bhandaria island (U-08)
of Bangladesh are excluded from (B), (C), and (D). See Fig. 11.
y = 1.392x - 126.39R² = 0.0075P = 0.21542F = 1.4858
-50
-40
-30
-20
-10
0
10
20
30
40
88 88.5 89 89.5 90
Are
a (k
m2)
Eastings
(A) Entire Sundarban
y = -1.3674x + 122.2R² = 0.0114P = 0.40624F = 0.69289
-50
-40
-30
-20
-10
0
10
20
30
40
88 89 90
Are
a (k
m2)
Eastings
(B) Northern islands
y = -0.5205x + 44.646R² = 0.0021P = 0.67584F = 0.19011
-50
-40
-30
-20
-10
0
10
20
30
40
88 89 90
Are
a (k
m2)
Eastings
(C) Central Islands
y = 8.1369x - 731.92R² = 0.1589P = 0.00620F = 8.12232
-50
-40
-30
-20
-10
0
10
20
30
40
88 88.5 89 89.5 90
Are
a (k
m2)
Eastings
(D) Southern Islands
-
LONG-TERM ISLAND AREA ALTERATIONS IN INDIAN AND BANGLADESHI SUNDARBAN: AN ASSESSMENT
29
Figure 20: Relationship between northward distance from shoreline and change in area between 1904-24
and 2015-16, the first and last years of mapping/imaging. In case of the entire Sundarban, (A) only slightly
increasing trend (accretion) is seen northwards but this trend is of emphatic significance. Insignificant trends are
detected for northern (B) and central (C) island groups, but the coastal islands are showing significant increase in
area towards north. Sagar island (Sg-03) of India and Bhandaria island (U-08) of Bangladesh are excluded from
(B), (C), and (D) because of their north–south extension. See Fig. 12. Source: See Table 1.
y = 0.0696x - 5.1977R² = 0.0683000P = 0.00000001F = 14.5830200
-50
-40
-30
-20
-10
0
10
20
30
40
0 50 100
Are
a (k
m2)
Distance from shoreline (km)
(A) Entire Sundarban
y = -0.0282x + 2.4153R² = 0.0152P = 0.24326F = 0.92351
-50
-40
-30
-20
-10
0
10
20
30
40
0 50 100A
rea
(km
2)
Distance from shoreline (km)
(B) Northern Islands
y = -0.0557x + 0.363R² = 0.0384P = 0.76255F = 3.59847
-50
-40
-30
-20
-10
0
10
20
30
40
0 25 50 75
Are
a (k
m2)
Distance from shoreline (km)
(C) Central Islands
y = 0.4351x - 11.685R² = 0.0621P = 0.00032F = 2.84700
-50
-40
-30
-20
-10
0
10
20
30
40
0 10 20 30
Are
a (k
m2)
Distance from shoreline (km)
(D) Southern Islands
-
LONG-TERM ISLAND AREA ALTERATIONS IN INDIAN AND BANGLADESHI SUNDARBAN: AN ASSESSMENT
30
Figure 21: Relationship between island easting and rate of change, i.e. Pearson’s correlation coefficient (r-
value) generated by analysing the time series trends of area change of individual islands considering all map/image
years. No significant trend is observed for the entire Sundarban (A). Increasing trend (accretion) is seen for the
southern islands (D) and the trend is of high significance. Central islands are also having significant trends and
show accretion towards the east. Interestingly, the northern islands (B) are showing significant opposite trend.
y = 0.0016x - 0.5147R² = 8E-07
P = 0.96334F = 0.00015
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
88 89 90
r va
lue
Eastings
(A) Entire Sundarban
y = -0.336x + 29.904R² = 0.0376P = 0.13066F = 2.34564
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
88 89 90
r va
lue
Eastings
(B) Northern Islands
y = 0.2389x - 21.836R² = 0.0151P = 0.23162F = 1.38007
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
88 89 90
r va
lue
Eastings
(C) Central Islands
y = 0.5171x - 46.645R² = 0.1273P = 0.01492F = 6.27140
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
88 88.5 89 89.5 90
r va
lue
Eastings
(D) Southern Islands
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LONG-TERM ISLAND AREA ALTERATIONS IN INDIAN AND BANGLADESHI SUNDARBAN: AN ASSESSMENT
31
Figure 22: Relationship between northward distance from shoreline and rate of area change in Pearson’s
correlation coefficient (r-value) generated by analysing the time series trends of area change of individual islands
considering all mapping/imaging years. Significant accretion is observed northwards for the entire Sundarban (A).
Increasing trend (accretion) is seen for the southern islands (D) and the trend is of high significance. Notably, the
northern islands (B) are showing significant trend in opposite direction. Central islands (C) are also showing erosion
towards north but the trend is insignificant. Sagar island (Sg-03) of India and Bhandaria island (U-08) of Bangladesh
are excluded from (B), (C), and (D). See Fig. 11. Source: See Table 1.
y = 0.0036x - 0.5217R² = 0.0153
P = 0.0000004F = 3.10018
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
0 50 100
r va
lue
Distance from shoreline (km)
(A) Entire Sundarban
y = -0.008x + 0.5354R² = 0.0661P = 0.05212F = 4.24409
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
0 50 100
r va
lue
Distance from shoreline (km)
(B) Northern Islands
y = -0.0034x - 0.4086R² = 0.0048P = 0.53613F = 0.430034
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
0 50 100
r va
lue
Distance from shoreline (km)
(C) Central Islands
y = 0.0036x - 0.5217R² = 0.0153
P = 0.000008F = 9.52981
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
0 50 100
r va
lue
Distance from shoreline (km)
(D) Southern Islands
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LONG-TERM ISLAND AREA ALTERATIONS IN INDIAN AND BANGLADESHI SUNDARBAN: AN ASSESSMENT
32
Figure 23: The Swatch of No Ground submarine canyon in the continental shelf of the GBD that intercepts the outpoured sediments of the Ganga–Brahmaputra system from replenishing the fluvially abandoned Sundarban region. Red arrows indicate main pathways of sediments received from riverine sources. Green arrows denote possible direction of net movement of reworked sediments that flood-dominated tidal currents carry into the interiors of Sundarban, aiding northern and vertical accretion. Source: isobaths from National Hydrographic Office chart No. 31. (From Bandyopadhyay, in press)
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LONG-TERM ISLAND AREA ALTERATIONS IN INDIAN AND BANGLADESHI SUNDARBAN: AN ASSESSMENT
33
Figure 24: Tropical cyclones landfalling in the Sundarban region between 88 and 90 E. Storm-driven surges and
waves work as powerful forcings that cause coastal erosion in the exposed seafront islands of the region.
Depression: a storm with 10-min average wind speed of 31–61 km h–1; Cyclonic Storm: 62–88 km h–1; Severe
Cyclonic Storm: >89 km h–1. Source:
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LONG-TERM ISLAND AREA ALTERATIONS IN INDIAN AND BANGLADESHI SUNDARBAN: AN ASSESSMENT
34
Figure 25: Diagrams explaining consequences of deforesting and reclaiming macrotidal Sundarban estuaries.
A: Intertidal areas are not reclaimed—estuary is in morphological equilibrium. B: Intertidal areas are reclaimed—
mean depth is increased and the estuary is removed from morphological equilibrium. Feedback includes in-channel
siltation and erosion of the embankments, requiring year-round maintenance. (after Bandyopadhyay, 2000).
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LONG-TERM ISLAND AREA ALTERATIONS IN INDIAN AND BANGLADESHI SUNDARBAN: AN ASSESSMENT
35
Figure 26: Available RMSL trends (in mm yr–1) in the coastal Ganga–Brahmaputra delta at or close to the
Sundarban region. Data of Khepupara and Sagar are not represented by bars due to their anomalous values. Blue
and magenta dotted lines denote the limits of the Sundarban Biosphere Reserve in India and the proposed
Sundarban Impact Zone in Bangladesh, respectively; black line is the international boundary. Source: Permanent
Service for Mean Sea Level (PSMSL: www.psmsl.org), retrieved on 30 May 2016; data for Mongla from Orford and
Pethick (2013), originally sourced from Mongla Port Authority; data for Rayenda and Amtali from Sarwar (2013),
originally sourced from Bangladesh Water Development Board. (From Bandyopadhyay, in press)