Long-term Island Area Alterations in the Indian and Bangladeshi … · 12,371 km2 and 250 (1967), 12,227 km2 and 243 (2000–02), 12,164 km2 and 250 (2015–16). The rate of area

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]

LONG-TERM ISLAND AREA ALTERATIONS IN INDIAN AND BANGLADESHI SUNDARBAN: AN ASSESSMENT

2

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.


3

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.


4

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


5

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


6

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


7

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



8

(Indian) Area Index: base = 1914 100.0 96.4 94.7 93.9


mapping / surveying year

(km2/yr)

–3.061 –2.197 –2.481

Eastern

Sundarban

(Bangladeshi)

Island count 118 118 117 116


Area Index: base = 1914 100.0 99.0 98.1 97.8


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.


9

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.


10

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.


11

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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41.

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.


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.


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.


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.


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.


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.


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


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


Negativechange count

Isla

nd

Co

un

t


22

70

0

20

40

60

80

100

120

140

160


Negative changecount

Isla

nd

Co

un

t

(C) Central Islands

10

35

0

20

40

60

80

100

120

140

160


Negativechange count

Isla

nd

Co

un

t



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)


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)


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)



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


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.


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


-50

-40

-30

-20

-10

0

10

20

30

40

0 50

Are

a (

km2)

Island Count


-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



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


-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


-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


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


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



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)


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)



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)


(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)




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


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


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



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



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



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


(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




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)


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:


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).


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)

Documents

Long-term Island Area Alterations in the Indian and Bangladeshi … · 12,371 km2 and 250 (1967), 12,227 km2 and 243 (2000–02), 12,164 km2 and 250 (2015–16). The rate of area