1. Introduction
Coastal shoreline is a unique and complex feature on Earth’s surface and is identified as one of the 27 distinct characteristics recognized by International Geographic Data Committee (IGDC; Berger 1996). About 60% of the world’s population is concentrated within 100 km of the coast alongside 347 984-km shoreline (Vitousek et al. 1997) covering an area of only 20% of the world’s land surface (Martinez et al. 2007). In addition, nearby seaward resources also serve mankind in numerous ways and forms. The shorelines and their associated coastal areas thus act as enormous resources in terms of economics, habitats, and environments.
Erosion and accretion processes along and across the shoreline are the two most common natural phenomena in coastal areas, and their rates are considered to be the most significant indicators of coastal dynamics (Wood et al. 1990). Shoreline, in general, may be defined as the line of contact between the land and water under normal (i.e., not stormy) conditions (Johnson 1919; Bird 1985). Though shorelines approximate mean high-water (MHW) lines along tidal coasts in Geodetic Survey nautical charts and surveys (Ellis 1978; National Shoreline Assessment System 2022) and National Ocean Service nautical charts and surveys (Coastal Engineering Research Center 1984). Of course, the term coastline is synonymously considered in Coast Survey usage (Shalowitz 1964). And the apparent shoreline denotes the line drawn on a map or chart in lieu of mean high-water line (MHWL) or the mean water level line (MWLL) in areas where it may be obscured by marsh, mangrove, cypress, or any other type of marine vegetation. This line represents the intersection of the appropriate datum on the outer limits of vegetation and appears to the navigator as the shoreline (Ellis 1978).
But in real contexts, the littoral shorelines undergo constant changes through fluvial, marine, and aeolian processes (Camfield and Morang 1996), and hence, its determination in physical aspects invites temporal sense (Boak and Turner 2005). Again, defining the landward limit of high-water tides is also a matter of interpretation and often pervades confusion by itself (Bird 1985).
From the vulnerability point of view, the coasts that are subjected to accretion and seaward progradation are considered to be less vulnerable, as they result in the addition of landmass. Contrarily, the areas with coastal erosion and retrograding landmass are considered as more vulnerable, since they may result in loss of property and lives and important natural habitats such as forests, beaches, dunes, and marshes. Progradational features are characterized when sediment supply cum deposition rate overcomes the accommodation creation (being greater than zero) during normal regression, and retrogradational features are characterized with occurrence of just the opposite phenomenon, within a predetermined spatiotemporal scale (Helland-Hansen and Martinsen 1996). Of course, most often, in any fluvial coastal region, such accretion and erosion processes take place concomitantly, turning the associated landforms into unremittingly dynamic ones.
The coastal zones are thus experiencing significant transformations all over the globe, and the shorelines have been incessantly changing their locational positions and geometry as well. For instance, approximately 80% of the world’s coasts had been flagging with rates ranging from 1 cm yr−1 to 10 m yr−1 (Kermani et al. 2016). Anthropological activities combined with natural forces often aggravate the changes and add risk factors to the coastal resources and communities (Malini and Rao 2004). The coasts of the Indian subcontinent are also facing vigorous transformations. For example, about 34% of the shoreline in the Indian mainland and 61% in West Bengal (WB, an Indian state) are subjected to varying degree of erosion (National Shoreline Assessment System 2022). And as much as 234.25 and 99.05 km2 land losses transpired in mainland India and its WB state, respectively, between 1990 and 2016 (Kankara et al. 2018). Of course, this coastal landform faces more exacerbation when it is associated with deltaic features, since the natural courses of many source rivers are often encumbered by human interventions (Day et al. 1995). For example, the Sundarban Delta (spreading in the estuaries of the river Ganga and its distributaries in Bangladesh and India), where many rivers have been restricted with the construction of embankments and choking of upstream sources, is often battered by cyclonic storm surges, extensive flooding, sea level rise, submergence, etc. Thus, this biome has been turned into one of the most vulnerable deltaic ecosystems in the world (Kotal et al. 2009; Roy 2010; National Shoreline Assessment System 2022). Of course, such vulnerability had exposed long-term threats to the vegetation quantity and quality, in turn affecting the local habitats and ecosystems as a whole.
The rapidly changing platform of the Sundarban Delta has keenly attracted the researchers, scholars, and academicians of diverse fields such as ecology, morphology, physiology, geography, geophysics, economics, and hydraulics and thus has become one of the most comprehensively studied ecosystems in the world. Early reports on coastline changes of the islands in the Indian Sundarban Delta (ISD) include Wilson (1848) and Sherwill (1858). In the later years, a good number of contributions were made on its physical, morphological, and hydrological characteristics since the late nineteenth century or even earlier (Reaks 1919; Bagchi 1944; Bhattacharya 1973; and so on). Of course, Hunter (1875), O’Malley (1914), Ascoli (1921), and Pargiter (1934) vividly accounted for the nature, lives, and various geographical features of the coastal areas all over the Sundarban Delta.
Many reclaimed and inhabited islands of the ISD are reported to have been actively eroded along their coastlines over the decades (Chattopadhyay 1985; Chakrabarti 1991; Paul 1996). Explorations on the change pattern and estimation of shifting of shorelines of the islands, considering few or some of them, were carried out by Rahman et al. (2011), Chatterjee et al. (2015), National Shoreline Assessment System (2022), and many others.
Apart from these, a good number of investigations on shoreline changes have also been carried out on some riverbanks and localized areas. For example, Das and Bhattacharya (1994) detected large shoreline erosion along the Thakuran River, and Ghosh and Mukhopadhyay (2016) accomplished spatiotemporal shoreline movement along with erosion–accretion in the interfluve area between Muriganga and Saptamukhi Rivers using old Indian toposheet (1920–21, 1922–23, 1967–68, and 1968–69), U.S. Army toposheet (1955), Indian Remote-Sensing Satellite P6 Linear Imaging and Self-Scanning Sensor (IRS P6 LISS IV) image of 2014, and so on.
Sagar is the largest inhabited island situated in the estuary of the river Hooghly (outfall of the river Ganga into the Bay of Bengal flowing along the western edge of the ISD). Of course, shoreline changes and resultant erosion–accretion of this Sagar and its adjoining islands have been studied the most, and some of them are Reaks (1919), Bagchi (1944), Kumar et al. (1994), Thakur et al. (2017, 2021), and Halder et al. (2022).
The Sundarban forest (SF, spreading over the Sundarban Delta) is the largest single chunk of tidal halophytic mangrove in the world, and the Indian Sundarban forest (ISF, spreading over the Indian Sundarban Delta) is an important part of it. Changes in shorelines and vegetation cover in this ISD/ISF have also been widely dealt with over the decades. For example, Hazra et al. (2002) highlighted the erosion–accretion along the shorelines and associated land-use changes including the variation in mangrove cover in the western part of the ISD during the period 1989–99; Thomas et al. (2014) estimated 48.37 km2 land loss (more than 28%) along the coastlines in the five sea-facing islands and 81.28 km2 mangrove reduction (out of 1600 km2, i.e., more than 5% depletion) over a period of 37 years. By analyzing historical maps and remote sensing data, Ghosh et al. (2015) categorically identified the total land loss and land gain of 136 and 163 (1968–89), 299 and 270 (1968–2014), 74 and 219 (1989–2001), 263 and 60 (2001–14), and 213 and 159 (1989–2014) km2, respectively, and also estimated the corresponding increase and decrease in mangrove cover of 54 and 475 (1968–89), 235 and 154 (1989–2001), 137 and 220 (2001–14), and 240 and 262 (1989–2014) km2, respectively. They also pragmatically showed mangrove cover depletion of 0.80%, 6.50%, 6.70%, 2.70%, and 3.0% (against 6588 km2 in 1776) per decade, respectively, over the periods 1776–1873, 1873–1968, 1968–89, 1989–2001, and 2001–14. By analyzing satellite data of 1990, 2000, 2010, and 2016 and adopting normalized difference vegetation index (NDVI) approach, Ranjan et al. (2017) prudently investigated mangrove cover changes in the ISD. Mondal et al. (2017) explored the shoreline changes and found that more than one-third of the land and mangrove cover area was depleted as a result of shoreline erosion in Bhagnani, Bulcheri, Dalhousie, and Halliday Islands over the period 1975–2015. Mondal and Saha (2018) worked on the spatiotemporal analysis of mangrove depletion in vulnerable islands of Sundarban World Heritage Sites in India. Thakur et al. (2021) explored the dynamicity of the shorelines in the ISD and showed the outdoing of erosion in the Hooghly estuarine islands during 1975–2017, along with its consequent severe stress on the mangrove vegetation. Likewise, Mondal et al. (2021) detected about 17.5 km2 mangrove cover depletion due to shoreline erosion and anthropogenic activities by analyzing satellite images between 1990 and 2019; Mohanty et al. (2021) claimed that approximately 90 km2 mangrove cover was lost and 50 km2 developed afresh, respectively, due to erosion and accretion of shoreline between 1999 and 2019, and Samanta et al. (2021) analyzed the erosion and accretion of shorelines and detected appearing and disappearing of mangrove cover of 81 km2 within the inhabited parts of the Indian Sundarban Biosphere Reserve (SBR) and 110 km2 in the reserved forest area, respectively. Samanta et al. (2021) also identified the depletion of mangrove of 16.07% (with respect to 303 km2 in 2000) in nine sea-facing islands and 1.39% (against 2074.1 km2 in 2000) in the entire ISD during the period 2000–20.
Of course, most of these works had dealt with few or some rivers or islands, sea-facing or Hooghly estuarine islands in particular, or the ISD as a whole, and seldom studied the trends of erosion–accretion and mangrove depletion along any tract and specific areas or zones. In this context, the present study attempts to assess and determine the trend and rate of shoreline shifting on the horizontal plane, explore the trends along different tracts, and estimate erosional and accretional areas and their corresponding changes in vegetation cover in the ISD. The outcomes of this long time-scale investigation would help the coastal environmentalists, decision-makers, implementers, and likewise professionals in framing and taking up suitably comprehensive and sustainable measures for integrated and effective coastal zone management in this estuarine wetland forest.
2. The study area
The river deltas are considered to be the most significant support systems of major human settlements in the Indian subcontinent, since these ecosystems attribute to momentous gravity in terms of habitats, water resources, agriculture, wetlands, trades, fishing activities, tourism, etc., for example, the Ganga–Brahmaputra Delta (GBD), the largest riverine depositional delta system in the world, covering a subaerial surface area of about 110 000 km2 (Kuehl et al. 2005) homes to more than 150 million people in West Bengal (India) and Bangladesh. This GBD also includes the Sundarban Delta (SD) that covers an area of about 26 000 km2 along the northeast coast of the Bay of Bengal spreading over the Indian state of WB and Bangladesh.
The GBD is formed by the sediment deposition of the two major river systems—Ganga and Brahmaputra (and their tributaries), originating from the Himalayan headwaters. Rivers Ganga and Brahmaputra discharge freshwater at ∼12 000 and ∼18 100 m3 s−1, respectively, on monthly average (Global Runoff Data Centre 2000), while carrying sediment loads at ∼729–744 × 106 tons yr−1 (Singh et al. 2007) and ∼595–672 Mt yr−1 (Darby et al. 2015), respectively. Approximately one-third of the sediment transport gets deposited on the floodplain and drainage systems of the delta (Goodbred and Kuehl 1999), while about 21% is to form and augment the delta fronts (Allison 1998). As a result of such yearly periodic loading [monsoonal (May–November) runoff + sediment] and unloading (during off-monsoon), the entire delta endures a vertical elastic deformation of up to 6 cm (Steckler et al. 2010). The Ganga–Brahmaputra Delta region (gradient only ∼5 × 10−5) is designated as one of the flattest deltas in the world and hence comprises vast lowland floodplains. More than 6000 km2 of the tidally influenced Sundarban Delta lies within 2 m above mean sea level (MSL) with additional 2000 km2 reclining below the MSL and protected by embankments (Brakenridge et al. 2013). And thus, the region is characterized by consequent hazards such as slow drainage, freshwater flooding, and saline water intrusion (Brouwer et al. 2007; Brakenridge et al. 2013). A difference in the channel aspect ratio (depth of water/width of channel) and tidal asymmetry in the mouths also lead to flood dominance in the eastern parts and ebb dominance in the western parts (Barua 1990). The sediment transported by the rivers mostly gets deposited on the subaqueous fan, and only a small portion is carried by waves and tides to the inactive tidal parts of the delta. And this part contributes to overall alleviation at 0–10 mm yr−1 depending on the propinquity to tidal watercourses (Rogers et al. 2013). Thus, the delta-front forests are prograding seaward at 15 m yr−1 as a whole (Michels et al. 1998).
The Sundarban Delta stretches between the Meghna River estuary in the east (Bangladesh) and Hooghly River estuary in the west (WB, India) (∼380 km east–west delta-front) and ∼80 km north–south at its broadest point. The average tidal effect ingresses up to 100 km landward (Kausher et al. 1996). The mean tidal amplitude is maximum (around 2.8 m) at the most east part—with a gradual decrease to the west (minimum around 1.9 m) (Bangladesh Inland Water Transport Authority 1987). The tides (amplitude difference ranges from 2.5 to 5 m) are semidiurnal and approximately synchronous along the delta front.
In terms of the formation of the delta, the entire Sundarban region can be divided into four distinct geographic entities, viz., (from north to south)—inactive (moribund) delta, mature delta, tidally active delta, and subaqueous delta, and major parts of the first three expanses were once reclaimed from Sundarban forest for human needs.
A great natural depression named “Swatch of No Ground” exists near the western part of the delta (∼30 km south to Dublar Char Island, Raimangal River estuary, WB, India) situating between 21° and 21°22′N latitudes in the Bay of Bengal. This submarine canyon, incising the shelf to within 40 km of the shoreline, acts as an active conduit for sediment delivery to the deep sea and helps in trapping sediment migration westward along the subaqueous delta front (Allison 1998). However, tidal currents are perhaps the strongest hydrodynamic influence on the subaerial delta front and subaqueous part in the region.
Sundarban, as a mangrove forest, covers an area of ∼10 000 km2 (Giri et al. 2007) spreading over Bangladesh (∼60%) and India (∼40%) (UNESCO, WHC 2005). Four protected areas (PAs) in it—Sundarban National Park, Sundarban West, Sundarban South, and Sundarban East Wildlife Sanctuaries, were declared as the World Heritage Sites and Global Biosphere Reserve by UNESCO, respectively, in the years 1987 (Giri et al. 2007) and 1989 (Bandyopadhyay 2012) owing to their unique ecosystems.
The shorelines in Sundarban islands undergo continuous deformation resulting from simultaneous erosion and accretion. For instance, in the Meghna estuary region (Bangladesh, most eastern part of Sundarban Delta), net land accretion took place at an average rate of 7.0 km2 yr−1 since 1792 and 4.4 km2 yr−1 since 1840 (Allison 1998). Thus, this estuary had evolved gradual welding of the landward end to the mainland by the seaward progradation of the islands and their subaqueous shoal extensions by up to 50 km over the last 200 years. But contrary to this, the attrition of the shoreline was increasing toward the west (Allison 1998), and erosion came about at a rate of 3.125 km2 yr−1 in the most western part (in Hooghly estuary) during the last 80 years (Bhattacharya 2008; Centre for Science and Environment 2012). The shorelines in the Sundarban Delta, as a whole, underwent net erosion (Allison 1998) by the tune of 57% within the last 200 years (Ganguly et al. 2006).
Of course, the present study is delineated within the western part of the Sundarban Delta that falls under the Indian political periphery. This part of the SD expands in the most northeastern stretch of the 6907.18-km-long Indian mainland coastline (National Shoreline Assessment System 2022) and is known as the ISD. This unique ecosystem is also a part of the Hooghly–Matla estuarine system spreading over ∼9600 km2 and comprising of ∼3% of the world’s mangrove area (Biswas et al. 2007). The ISD ranges between latitudes 21°13′–22°40′N and longitudes 88°05′–89°06′E and is bordered by the River Hooghly (deltaic offshoot of the river Ganga in India) to the west, the Kalindi–Raimangal–Hariabhanga River to the east, Dampier-Hodges Line to the west and north, and the Bay of Bengal to the south. This archipelago also comprises 48 reserve forest islands (nearly 4200 s km2) and 54 inhabited islands (about 5400 km2). Administratively, the ISD stretches over six southeastern blocks of North 24 Parganas and 13 southern blocks of South 24 Parganas districts in the state of WB. But in a practical sense, the study area is constricted within an area of ∼9000 km2 resembling a trapezium, its parallel sides (north–south) being ∼54 km (west) and ∼106 km (east) long, the shortest distance between the parallel sides being ∼111 km (west–east), and the four corners being located at around 21°30′N and 88°05′E, 21°40′N and 89°06′E, 22°26′N and 89°06′E, and 21°56′N and 88°05′E (Fig. 1).
Location map of the study area.
Citation: Journal of Physical Oceanography 54, 8; 10.1175/JPO-D-23-0019.1
This coastal trait is traversed by intricate webs of crisscrossing rivers, rivulets, canals, channels, creeks, swamps, etc., typified with deltas, shoals, dunes, sand spits, beaches, mud flats, etc., characterized by the subduction of landmass, etc., and featured with ∼51% muddy flats and ∼49% marshy coast (Kumar et al. 2006). Five major rivers, viz., Muriganga, Saptamukhi, Thakuran, Matla (the longest and widest), and Gosaba pass through the ISD—all of them being the open-to-sea distributaries of the river Ganga. The river Ganga bifurcated (within the Indian political boundary) into two rivers—the Bhagirathi–Hooghly (India) and the Padma (Bangladesh). A barrage was constructed across the river Ganga near Farakka (hence “Farakka Barrage,” at the upstream of the bifurcating location) to feed the Bhagirathi–Hooghly River through a canal (taking off from an upstream point of the barrage). Thus, the Bhagirathi–Hooghly River is being fed with augmented flow entirely through a feeder canal at the cost of regulated flow into the Padma River (by interception–diversion method), since the commissioning of the barrage in 1975. And as a result, the course, morphology, dynamics, etc. of all the river systems associated with such barrage diversion have undergone significant changes and transformations through the flow regime and sedimentological readjustments. For example, the perennial sources of nonsaline freshwater (originating from the river Padma) of all the rivers and rivulets passing through the ISD have dried up and are being sourced by monsoonal runoff only, after Farakka Barrage put into operation (Seidensticker and Muhammad 1983; Parua 2010; Sinha et al. 1996).
Of course, abundance in mangroves and extensive tidal actions are the two foremost biogeomorphic signatures in the atolls (Chakrabarti 1995). The soils are characterized by clay, silt, and sandy loams (in order of dominance) and an average pH of about 8.0 and are not so apposite for the plants other than the mangroves.
The climate in the ISD is characterized by about 1660-mm annual rainfall of which almost 75% is concentrated over June–September during southwest monsoon. The average temperature varies from 5° to 7°C in winter to as high as 43°C in summer, and humidity remains greater than 70% almost throughout the year. The strong storms in summer seasons sometime give rise to up to 7.5-m instant wave bulge (Seidensticker and Muhammad 1983), which again results in erosion and damages of lands, embankments, sudden saline water ingression, etc.
The shorelines in the ISD are subjected to relentless natural events and forces such as wave actions and resultant nearshore circulations, tidal currents, stormy winds, episodic events, sea level rise, submergence and inundation, erosion, and accretion, as well as various anthropogenic activities such as the construction of various coastal structures such as groins and jetties, mining of beach sand, dredging of tidal entrances and navigational channels, hardening of shorelines with seawalls, beach nourishments, river water regulation by embanking, destruction of mangroves, and other natural buffers. The coastlines are further characterized by various riverine processes such as currents, both vertical and horizontal flow velocities, erosion–accretion, and sediment loads, and perhaps subjected to tectonic subsidence and subduction due to the auto-compaction of the land since it is chiefly composed of soft clay sediments. The shorelines are also exposed to the macrotidal regime of the coastal Bay of Bengal, the spring tide ranges frequently being more than 5 m, and thus, the turbulence is generated simultaneously by wind forces and boundary frictions (at the surface and bottom of the water mass) making the ISD coast a dynamic environment.
Of course, various climatic disasters, triggered by anthropological exploitations, have been imposing serious intimidations to the natural habitats as well as inhabiting people in the ISD, coastal erosion being one of them. Varying degree of erosion and accretion of shoreline in this delta region is, as per Kankara et al. (2018), depicted in Table 1, for instance.
Coast length experiencing different degrees of erosion/accretion rates (m yr−1) during 1990–2016 (high: >5.0, moderate: 5.0–3.0, low: 3.0–0.5, and stable: erosion–accretion up to 0.5). Thus, this distinctive geographic glebe was considered endangered in the International Union for Conservation of Nature (IUCN) Red List of Ecosystems Framework in 2020 (Sievers et al. 2020).
3. Materials and methods
a. Data sources
Remote sensing technique and geographic information system (GIS) technology are the two dominant tools for determining shoreline changes on a temporal scale, and they are rationally employed worldwide (Nayak 2002). The investigation was carried out using multiresolution satellite data of Landsat series, viz., Landsat MSS, TM, ETM+, and OLI/TIRS of U.S. Geological Survey (USGS) acquired from the website (https://earthexplorer.usgs.gov on different dates) (Table 2). To obtain the minimum influence of clouds and the impact of water on the extracted information, relatively cloud-free images were chosen over the dry (winter) seasons. The dates were also selected to avoid wave bulges due to stormy winds and match the timings of almost identical tidal magnitudes. To demonstrate the long-term spatiotemporal changes in shorelines, the extracted satellite images were integrated and compared, and the findings were then purposely articulated in terms of line or area measurements over a specific time period (de Boer and Carr 1969; Carr 1980; Bird 1985; Cooke and Doornkamp 1989; Dugdale 1990). The map of the Indian Sundarban (https://www.researchgate.net/figure/Map-of-Indian-Sundarban-show) was used in preparing the reference map.
Particulars of the satellite dataset. Source: USGS.
b. Methodology
Geometric and radiometric corrections of the satellite data were done using Earth Resources Data Analysis System (ERDAS) IMAGINE 16.6 and ArcGIS software. The shapefile of the Indian Sundarban was prepared by georeferencing the map with Quantum Geographic Information System (QGIS) 3.16 and digitizing the same with ArcGIS 10.8. The layer stacking of different bands was carried out with the dataset in a single image file for each year using ERDAS IMAGINE software. The study area was then clipped from the different dated layer-stacked images using ArcGIS. Standard false color composite (FCC) images were produced from the satellite data by suitably changing the near-infrared (NIR), red and green bands, and shorelines were drawn with the help of QGIS software by thresholding and demarcating the land–water boundaries. The individual vector layer of shoreline for all 5 years’ images was then superimposed successively for extracting the patches (Bandyopadhyay et al. 2004) and to understand the direction of areal change of shorelines. And areas of erosion and accretion were calculated by QGIS software to determine their rates over the study period.
Net shoreline movement in the absolute distance (Jana et al. 2012; Das et al. 2013) and endpoint rate (EPR) (Kankara et al. 2014, 2015, 2018; Selvan et al. 2020) between any two successive shorelines in a temporal sense are the two extensively used statistical methods in delineating spatiotemporal shoreline changes over a long time period. Thus, to determine net shoreline movement and endpoint rate, transects were generated perpendicular to the apparent shorelines of the oldest decade year with the help of QGIS at almost equal spatiointerval against any particular island (but with longer interval for the interior islands). The absolute distance (m) of shoreline movement along the transect and area (km2) of accretion–erosion engendered between the two positions of shoreline were measured with the help of QGIS. The average EPR change was then determined by dividing such a distance by the specific time period (year) elapsed (Fenster et al. 1993; Kankara et al. 2018). Similarly, the average rate of areal erosion–accretion was determined dividing the area by the period of time (year) so elapsed. Thus, the EPR change was expressed in meters per year (m yr−1) and classified into seven groups (Table 3) for long-term estimation as per Kankara et al. (2014, 2015, 2018) and Selvan et al. (2020), and the average rate of areal erosion–accretion was expressed in square kilometers per year. The 276 transects were numbered from west to east for denotation. And the comparative degree of areal change was determined and classified (Table 4) based on the value of the ratio of areal (km2) erosion to accretion over the same time period. However, positive (+) and negative (−) signs were endorsed, respectively, for seaward (accretion) and landward (erosion) shifting of shorelines while presenting the same (both areal and along the transect) in the tables, graphs, etc.
EPR-based shoreline classifications as per Kankara et al. (2014, 2015, 2018) and Selvan et al. (2020).
Classification (by the authors) of the comparative degree of net areal change of shoreline based on the ratio of erosion to accretion.
Because of detecting some notable traits, Sagar and its adjoining islands have together been designated as the “Sagar Group of Islands.” Similarly, north–south flowing (through almost middle) Matla River has been demarcated to divide the ISD explicitly into two lobes—“western” and “eastern,” in terms of shifting of shorelines.
Graticules have graphically been depicted on the maps of the study area with the help of ArcGIS with an interval of 6′ to distinguishably locate and identify the shoreline shifting for the sake of easy visual presentation. The lowest square (with 6′ side) thus generated has been termed as a “block,” and the lowest rectangle with 6′ width elongating throughout the study area (north–south or west–east) has been termed as a “track.” Thus, the entire study area is represented by 11 north–south and 10 east–west tracks, and with 61 blocks.
NDVI approach (the most known and extensively used vegetation index worldwide) has been employed in detecting and determining the variation of spatiotemporal vegetation cover adopting ArcGIS. The threshold limits of NDVI values adopted by Rouse et al. (1974a,b) have been considered in classifying (Table 5) the vegetation cover. Finally, the images for 1980 and 2021 with vegetation cover of different densities have been overlayed to identify and quantify the areal changes of vegetation cover corresponding to such areal shifting of shorelines.
The study emanates from 1980 (taking the year of Farakka Barrage into account), and the 41-yr time span is contemplated sufficiently long, after Boak and Turner (2005), to reconnoitre long-term shoreline changes in the Indian Sundarban Delta.
4. Results
The deltaic Indian Sundarban is very much complex and dynamic. Rates of shoreline shifting and change in the land area thereof were assessed over a period of four decades, viz., 1980–90, 1990–2000, 2000–10, and 2010–21. Corresponding areal change in the vegetation cover of different densities was also estimated. Last, the forces, attributes, and activities that might have caused such changes were dealt with. However, some salient results were thus obtained, and discussion on them are conversed below.
a. Shifting along the transects
Average EPR determines the rate of linear shifting (seaward/landward) of shoreline at a particular location on the horizontal plane. Out of the total 276 transects, the numbers demonstrating seaward, landward, and no shifting, respectively, are found to be 74, 181, and 21 during 1980–90; 98, 157, and 21 during 1990–2000; 135, 119, and 22 during 2000–10; and 115, 149, and 12 during 2010–21 (Fig. 2). Evidently, the numbers indicate the increasing trend of prograding shifting, a decreasing trend of retrograding shifting, and almost no change of no shifting over the consecutive four decadal intervals in 41-yr span. And while the 41-yr span is considered at a stretch (1980–2021), the numbers of transects representing seaward, landward, and no shifting, respectively, are found to be 99 (36%), 166 (60%), and 11 (4%) (Figs. 2 and 3). That is, the number of transects showing retrograding (−) shifting is about 1.68 times that of prograding (+) shifting and that of no shifting is not so significant. Thus, the ISD as a whole is retreating along the transects over the period 1980–2021.
Transects drawn on the apparent shorelines for different years.
Citation: Journal of Physical Oceanography 54, 8; 10.1175/JPO-D-23-0019.1
Endpoint rate of shoreline shifting during 1980–2021 [serial number starting from west, positive (green bars) and negative (red bars) values, respectively, indicate the prograding and retrograding shifts].
Citation: Journal of Physical Oceanography 54, 8; 10.1175/JPO-D-23-0019.1
The numbers of transect showing retrograding shifting, respectively, with high, moderate, and low EPRs are determined to be about 2.45, 3.45, and 1.55 times those demonstrating prograding shifting (Table 3 and Figs. 3 and 4), which also indicates overall retreating physiognomy of the ISD (along the transects) over the period between 1980 and 2021.
Number of transects (excluding 11 transects with zero EPR) based on the classification of the EPR (m yr−1) during 1980–2021.
Citation: Journal of Physical Oceanography 54, 8; 10.1175/JPO-D-23-0019.1
Locations of the transects, in terms of change-contributing factors, have been explored further to see the variation in such receding/proceeding shift. Out of the total (over 1980–2021) 33 sea-facing transects, only four (12%) show positive EPR and the rest 29 (88%) exhibit negative EPR. All four transects representing more than 50 m yr−1 negative EPR are also found to be sea-facing. Thus, the sea-facing islands are dominantly waning along the transects, although the highest positive EPR of 114.63 m yr−1 is detected against a sea-facing island, at the same time (Figs. 2 and 3).
Total numbers of transect showing positive (+) and negative (−) EPRs with values less than 5 m yr−1 (stable to moderate) in the western (west to Matla R.) and eastern (east to Matla R.) lobes are almost comparable (Figs. 2 and 3). However, the ratios of the total numbers of (+)-EPR to (−)-EPR with values 5 m yr−1 or more are found to be about 1:0.61 in the western lobe but 1:0.18 in the eastern lobe (Figs. 2 and 3). Also, the first to ninth highest positive and 1st to 13th (excepting second, fourth, and ninth) highest negative EPRs are distinguished in the western lobe, while the second and fourth highest negative EPRs evolve in the eastern lobe. Thus, the “western lobe” (transect numbers 1–130) of the ISD appears as more dynamic (demonstrating both prograding and retrograding shifts at almost equivalent pace), whereas the “eastern lobe” underwent retrograding dominance in terms of horizontal shoreline shifting along the transects over 1980–2021 (Figs. 2 and 3).
And interestingly, the Sagar Group of Islands culminates some distinct features with regard to shoreline changes in the ISD. For instance, both the two islands undergoing the highest prograding (+114.63 m yr−1) and retrograding (–72.07 m yr−1) EPRs over the period 1980–2021 are identified, respectively, to the northwest and southeast of Sagar Island, the two islands being located in the mouth of the Hooghly and Muriganga (a distributary of the Hooghly) Rivers, respectively (Figs. 2 and 3). Some islands located in the northwest and northeast to this Sagar Island are facing simultaneous seaward and landward shifting. The total number of transects in and around Sagar Island, experiencing retrograding shifting, is found to be 1.88 times that showing prograding shifting. In addition, five number transects (out of total 28 against the Sagar Group of Islands) are found to expose no shifting of shorelines. Again, Sagar Island is found to undergo retrograding shifting along its entire coastline all round, except two locations—one on its northeastern side (+7.89 m yr−1) and another at its north tip (+2.31 m yr−1), where prograding shifting is identified. Thus, the Sagar Group of Islands transpires as the most dynamic chunk in the ISD and Sagar as a retreating island, in terms of horizontal shoreline shifting along the transects drawn.
Such shifting of shorelines along the transects, of course, does not necessarily construe any areal change—neither landward nor seaward. And to determine the areal changes, horizontal erosion/accretion needs to be delved into.
b. Areal changes
Areal erosion and accretion in the ISD due to the shifting of shorelines over 1980–2021 are delineated and depicted in Fig. 5, focusing on some significant islands including the Sagar Group of Islands and a few decisive rivers.
(left) Accreted and (right) eroded areas resulted out of shoreline shifting between 1980 and 2021 (1 → Bhangaduni, 2 → Mayadwip–Dalhousie, 3 → Chulkati, and 4 → Bulcheri Islands).
Citation: Journal of Physical Oceanography 54, 8; 10.1175/JPO-D-23-0019.1
c. Trend
Variation in total areal accretions along the individual north–south track followed roughly two sinusoidal cycles (Fig. 6)—the four consecutive peaks from west to east being on the tracks 88°06′–88°12′E (eastern flank of the Sagar Group of Islands), 88°18′–88°24′E (roughly along the Saptamukhi River), 88°36′–88°42′E (roughly along Matla River), and 89°00′–89°06′E (eastern fringe of the ISD, i.e., roughly along western bank of the Raimangal–Hariabhanga River). But total areal erosions along individual north–south track were found to increase from west to east (Fig. 6) except the track 88°06′–88°12′E (virtually passing over the eastern flank of the Sagar Group of Islands).
Areal accretion, erosion, and net change along north–south tracks (west to east) between 1980 and 2021.
Citation: Journal of Physical Oceanography 54, 8; 10.1175/JPO-D-23-0019.1
A similar variation in total areal accretions along the individual west–east track roughly followed one sinusoidal cycle (Fig. 7)—the three consecutive peaks from north to south being on the tracks 22°30′–22°24′N (the northeast end, where the ISD area is nominal), 21°48′–21°42′N (virtually passing over almost middle of Sagar Island and the rivers in the ISD), and 21°36′–21°30′N (over the islands open to the Bay of Bengal). And that of total erosions along individual west–east track was found exponentially increasing toward the south (i.e., from mainland to open to sea).
Areal accretion, erosion, and net change along west–east tracks (emanating from north to south) between 1980 and 2021.
Citation: Journal of Physical Oceanography 54, 8; 10.1175/JPO-D-23-0019.1
However, the net areal change of accretion–erosion along individual north–south track followed the trend of decreasing accretion to increasing erosion from west to east (i.e., from mainland in India to Sundarban Delta in Bangladesh) and that along the west–east track followed exponentially increasing erosion from north to south (i.e., from Indian mainland to open sea).
Trackwise (both north–south and west–east) total estimated areas in the ISD over 1980–2021, resulted out of erosion–accretion and net change, are also postulated in Figs. 6 and 7 corresponding to Fig. 5. Thus, overall, the ISD lost an estimated net land area of ∼152 s km2 (comprising of blockwise net ∼42 km2 gain and ∼194 km2 loss) over the 41-yr time period.
Shorelines of the islands virtually falling along all the north–south and west–east tracks underwent varying degree of erosion and accretion over the study period. However, to understand and distinguish such degrees of net changes along any trajectory in the ISD, the classification of the tracks based on the ratios of net areal erosion to accretion is depicted in Table 6.
Degree of net areal change along the individual trajectory (ratio of trackwise total erosion to accretion within parentheses) [serial number (Sl. No.) starts from west and north] between 1980 and 2021 (* indicates insignificant in terms of area, km2).
Of course, in the study, some areas/tracks emerge with some idiosyncratic physiognomies in terms of erosion–accretion. For example, the northern part of Sagar and its adjoining islands and islets together (falling on seven blocks lying within 22°00′–21°42′N and 88°00′–88°18′E) contributed more than 39% (∼36 km2) of the total accretional area in the ISD. Single land pieces with the largest (Nayachar Island: ∼15 km2, ∼16%) and second largest (Mousuni Island: ∼8 km2, ∼8%) accreted areas ranging, respectively, between ∼21°54′ to ∼21°50′N and ∼88°04′ to ∼88°07′E and ∼21°46′ to ∼21°44′N and ∼88°11′ to ∼88°13′E are found to situate, respectively, to the north–west and south–east to this northern part of Sagar Island. This sea-facing island itself also demonstrates areal accretion along its northern and northeastern coastlines. The block (21°54′–21°48′N and 88°00′–88°06′E) representing the largest accretional area (∼14 km2, ∼15%) in the ISD is also found to be located to the northwest of this northern part of Sagar Island. Only three most western (in the ISD) north–south tracks ranging from 88°00′ to 88°18′E (and comprising 12 blocks between 22°00′ and 21°30′N) virtually cover “Sagar and all its surrounding islands and islets” (called the Sagar Group of Islands). These three north–south tracks, respectively (west to east), partake the second, first, and third largest accreted areas in the southern lobe of the ISD (22°00′–21°30′N). Mainland India and the Bay of Bengal start, respectively, from north and south to these islands, and the mainstream River Hooghly falls into the sea flowing along the western skirt of them. Thus, the most western three tracks (and hence the Sagar Group of Islands) call for ∼43 km2 areal accretion contributing more than 47% and 61% of total accreted areas in the ISD and southern lobe (22°00′–21°30′N) of the ISD, respectively. And at the same time, this Sagar Group of Islands together contributes more than ∼16% (∼39 km2) of the total eroded area in the ISD. Thus, the Sagar Group of Islands, as a whole, exhibits the largest single chunk of accreted islands vis-a-vis the most dynamic pocket in the ISD.
Likewise chunk of islands exhibiting the second largest accretional area is distinguished in the upper course of the river Matla and south to Gosaba Island. These islands ranging over four blocks (between 22°12′–22°00′N and 88°36′–88°48′E) together contribute ∼11% (∼10 km2) of the total accreted area in the ISD. The two consecutive north–south tracks with the second and fourth largest (from west to east) accreted areas together (88°36′–88°48′E) cover the entire east bank of the Matla River and its adjoining islands contributing ∼31% (∼28 km2) of the total accreted area in the ISD. And they, respectively, possess the sixth and fourth largest positions, together contributing only ∼16% (∼11 km2) of the accreted area, while the southern part of the ISD is taken into account. At the same time, the north–south track 88°36′–88°42′E passing roughly along the Matla River is found responsible for more than ∼9% (∼22 km2) of the total eroded area in the ISD, and one track west to it and four tracks east to it are traced to possess the eroded areas less than this north–south track. Thus, the Matla River represents a typical north–south tract in the ISD.
The most southern parts of the ISD are exposed to the open sea and hence also the most southern virtual track (21°36′–21°30′N), which epitomizes more than 38% (∼94 km2) of the total eroded area in the entire ISD. All the four blocks representing the first (88°48′–88°54′E), second (88°42′–88°48′E), third (88°30′–88°36′E), and fourth (88°06′–88°12′E) largest eroded areas fall on this track also. These four virtual blocks together represent (∼88 km2) more than 36% and 93% of the total eroded area, respectively, in the ISD and most southern west–east track. Four major islands, viz., Bhangaduni, Mayadwip–Dalhousie, Chulkati, and Bulcheri (situated east to west), happen to fall, mostly or entirely, on this most southern west–east track (Fig. 5). And all of them are found to experience severe horizontal erosions (∼34, ∼23, ∼6, ∼17, and ∼15 km2 respectively, from east to west) particularly along the open-to-sea shorelines. These five islands, respectively (from east to west), possess the first (∼34 km2), second (∼23 km2), fifth (∼6 km2), third (∼17 km2), and fourth (∼15 km2) largest eroded areas among all the individual land pieces in the ISD and together lead to disappearing of more than 39% of the total eroded land area in the ISD over the 41-yr span. And more than ∼56% (∼138 km2), ∼69% (∼168 km2), ∼77% (∼189 km2), and ∼89% (∼216 km2) of the total eroded area in the ISD are, respectively, attributed only to two, three, four, and five consecutive most southern west–east tracks together.
The ratio of blockwise total net areal erosion to accretion in the track 88°36′–88°42′E (passing roughly along the Matla River) is found to be the closest to the unity (∼1.71), designating to be the most dynamic north–south track in the study area. And individual trackwise net areal accretion and erosion, respectively, increase and decrease (excepting 88°06′–88°12′E) westward west to this track, while net areal erosion (since no such net areal accretion evolved) increases eastward from this track. Trackwise total net areal accretion and erosion west to it are estimated to be ∼15 and ∼44 km2, respectively, resulting in only ∼29 km2 total net erosion. Contrarily, total net areal accretion and erosion including and east to this track are estimated to be ∼0 and ∼123 km2, respectively, resulting in as much as ∼123 km2 net erosion. Thus, the track stretching roughly over the Matla River is distinguished to be a typical one bifurcating the entire ISD into two protruding lobes—west and east. However, north–south trackwise total areal accretion, erosion, net change, and degree of net change in these two lobes are depicted in Table 7.
Total areal accretion, erosion, and net change (km2; rounded off) and degree of net change (ratio of total erosion to accretion within parentheses) over individual north–south track between 1980 and 2021.
Similarly, the ratio of blockwise total net areal erosion to accretion in the track 22°06′–22°00′N (stretching to the north of the Sagar Group of Islands and passing roughly over the most upstream of the Matla River) is calculated to be the closest to unity (∼0.72), designating to be the most dynamic west–east track in the study area. The shorelines falling over the most northern three west–east tracks (22°30′–22°12′N) are found to be purely erosive but not so significant in terms of the total net eroded area (cumulative total: ∼6 km2). Again, trackwise total net areal accretion and erosion, north to 22°12′N, are estimated to be ∼2 and ∼9 km2, respectively, leading to only ∼7 km2 total net eroded area. But contrary to this, trackwise total net areal accretion and erosion south to this track are estimated to be ∼5 and ∼150 km2, respectively, resulting in as much as ∼145 km2 net erosion. Besides these, the area north to 22°06′N and west to 88°36′E is designated as the mainland and does not belong to the Sundarban Delta in India. Thus, the track 22°06′–22°00′N stretching to the north of the Sagar Group of Islands and passing roughly over the most upstream of the Matla River is found to be a typical bifurcating the entire ISD into two protruding lobes—north and south. However, the west–east trackwise total areal accretion, erosion, net change, and degree of net change in these two lobes are depicted in Table 8.
Total areal accretion, erosion, and net change in square kilometer (rounded off) and degree of net change (ratio of total erosion to accretion within parentheses) over individual west–east track between 1980 and 2021.
d. Variation in vegetation cover
NDVI approach revealed that the area of sparse vegetation cover increased, while the area of moderate vegetation cover decreased considerably and that of dense cover decreased to zero over the period from 1980 to 2021 (Fig. 8 and Table 9). The study also showed that the areas of both moderate and sparse vegetation cover were depleted to large extents resulted out of retrograding shoreline shifting, and such diminution was limited particularly to the sea-facing islands (Fig. 8 and Table 10). Of course, the contribution toward the areal augmentation of moderate as well as sparse vegetation cover due to the prograding shifting of shoreline was not so significant and that did not happen to increase any mangrove forest area.
Areal change in vegetation cover over 41 years.
Citation: Journal of Physical Oceanography 54, 8; 10.1175/JPO-D-23-0019.1
Area (km2) of vegetation cover of different densities in 1980 and 2021.
Areal (km2) change in vegetation cover of different densities over the period 1980–2021.
5. Discussion
Shifting of shorelines in the ISD is a continuous natural process coupled with various anthropological activities. The islands in the northern outreach being situated at more landward and interior tracts as compared to those in the southern parts, their shorelines got less influenced by the working forces and parameters exerted by the sea. Contrarily, the southern islands being located nearer to the Bay of Bengal, their shorelines were more exposed to the open sea and therefore more susceptible to the eroding and depleting forces. Thus, in general, the shorelines in the northern glebe experienced not so significant change in the shape and rate over the study period, while the shorelines in the southern islands underwent considerable changes in their rates and geometry.
Occurrence of significant landward shifting of shorelines against Ghoramara Island, located at the bifurcating point (originating the river Muriganga) of the river Hooghly, might have been caused by the eroding forces of the open sea vis-a-vis augmented flow through the rivers diverted by Farakka Barrage (after putting into operation in 1975). However, the explanation of areal expansion of Nayachar, a sea-facing island located to the northwest of Sagar Island, is remaining uncontended.
Sagar, being a sea-facing island, was retreating along almost all its coastlines but is found to act as a barrier (from the eroding forces) of its eastern and northeastern islands. Thus, Sagar Group of Islands was divulging as the most dynamic (in terms of simultaneous occurrence of both accretion and erosion) chunk in the ISD.
High rate of sediment supply results in rapid aggradation (Mikhailov and Dotsenko 2007) and net accretion in the Hooghly estuary (Allison 1998).
More dynamic appearances of the western glebe of the ISD could also be credited to the augmented perennial flow (and hence enhanced sediment transport) through the Bhagirathi–Hooghly River system flowing along the western edge of the ISD and the changed hydraulic regime resulted out of such augmentation. But such augmented flow through the western edge has taken place at the cost of drying up of the perennial sources (and hence lacking in sediment transport) of all the rivers flowing through the eastern glebe. Such starvation of sediment supply and altered hydraulic regime happened to turn the eastern ISD into eroding dominance. The existence of the Swatch of no Ground submarine canyon nearer to this eastern lobe could be regarded as an added attributer to such retrograding dominance in the eastern ISD.
The track passing roughly along Matla, the widest and largest river in the ISD, represents less landmass and therefore epitomizes lesser areal changes in shorelines. But its northern course and adjoining islands and rivulets are sourced with plenty of sediment supply from their northern hinterlands particularly during the monsoonal runoff. Thus, the islands in and around its most upper course also demonstrated dynamic characteristics in shoreline shifting to a large extent.
About 1.46 km2 dense vegetation was not concentrated in any particular location/island, rather scattered on different locations on the islands ranging east to Matla River (88°42′E) and south to Bidyadhari River (22°01′N).
Depletion of moderate vegetation cover in the sea-faced islands indicates the reduction of mangrove forests, since no other vegetation species could hardly survive in the ecosystems and environs of the most southern ISD. Contrarily, the accreted land was of recent origin and mostly in human-settled areas, and hence claims no such addition of mangrove forests.
The area north to 22°00′N and west to 88°36′E is predominantly mainland (and not islands) and is beyond the scope of the present study area. Again, the north–south track ranging from 88°36′ to 88°42′E and passing roughly along the Matla River can be regarded as the eastern limit of shoreline shifting with dynamic nature. And the west–east track ranging from 22°00′ to 21°54′N can be regarded as the northern limit of Sagar Group of Islands. Thus, the entire ISD can be divided into four virtual lobes (Table 11), each of which has specific physiognomy in terms of horizontal shoreline shifting.
Four virtual lobes based on the horizontal shoreline shifting between 1980 and 2021.
Of course, fragmentation and welding, disappearing and appearing of land pieces were found to occur during the study period, since the islands had been undergoing continuous erosion and accretion processes simultaneously, which again indicates the dynamic feature of this coastal estuary. Nevertheless, net areal changes in shorelines in the entire ISD exhibited erosion of about 152 km2 over the 41-yr period, which is a matter of great concern. Thus, the ISD as a whole was declared as an erosion hotspot region in terms of shoreline shifting (Kankara et al. 2018).
Possible causes of shoreline shifting in the ISD
The shorelines are subjected to continuous changes ensued by various natural forcing parameters such as actions of waves and winds, tidal effects, rip and longshore currents, episodic events, sea level rise due to global warming (sea level rise of 1 mm yr−1 could cause inundation to the tune of 0.5 m yr−1), and several anthropogenic interventions like regulation of natural river courses such as damming, dredging of tidal entrances and navigational channels, mining of beach sand, beach nourishment, hardening of shorelines with seawalls, construction of harbors, and coastal structures such as groins and jetties, destruction of mangroves, and other natural buffers (Kankara et al. 2018). Datta (2010) and Roy (2010) identified 75 strictures that are likely to contribute to coastal changes categorizing as atmospheric, oceanographic, terrestrial, and anthropogenic origins. However, the progression and rate of delta building are mostly contingent on the relative domination of two opposite processes—fluvial accretion and tidal and marine erosion (Reading and Collinson 1996).
Of course, perennial riverine (river Ganga and its distributaries) processes such as current, flow velocity, and sediment load characterized by bio (chiefly fish and plant residues)-tidal processes and colossal variations in transported sediment loads during monsoon and dry spells act as added potencies in the Indian Sundarban Delta. Flood-dominated tidal asymmetry (flood tides: shorter duration; ebb tides: longer duration) of the Hooghly River induces higher landward and lower seaward velocities of the bidirectional tidal currents and thus turns its interior estuary (in western ISD) into sediment sinks (called tidal pumping: Postma 1967), resulting in accretion in both vertical and horizontal planes. Apart from this, to sustain the morphological steady state, any macrotidal (with tidal rage greater than 4 m) resonant estuary calls for its length to be equal to one-fourth of the wavelength of the tide entering into it, where the tidal wavelength critically depends on the mean depth of the estuary (Wright et al. 1973). But the construction of embankments along the shores in the northern and western reclaimed glebe of the ISD restricts the intertidal spill area required for happening of such resonance (Bandyopadhyay 1997; Nandy and Bandyopadhyay 2010), which again leads to in-channel sedimentation (which again enhances the dominance of flood tidal currents) and coastline erosion for stabbing decreased average depth to reinstate equilibrium (Pethick 1994). However, receiving no such sediment loads from the up-country, the coastal edges in some pockets of this interior ISD suffer from sediment starvation, which results in not sufficient replenishing against their eroding land losses.
However, all the sea-facing islands (including in the Hooghly estuary) exhibited retrograding shoreline changes, which may be attributed to the escaping of sediment flow into the deep sea of Bengal fan due to the interception of the Swatch of No Ground submarine canyon (Kuehl et al. 1989, 1997, 2005), following the regional trends of stronger (comparing to the interiors) tidal currents, vigorous wave actions, cyclonic surges, etc. (Allison and Kepple 2001; Ganguly et al. 2006).
The clayey ISD is also subject to enduring of land subsidence at a rate of about 15–50 mm yr−1 caused by the sediment compaction and removal of water, oil, gas, etc., from the underlying strata (Hoque and Alam 1997; Stanley and Hait 2000; Mikhailov and Dotsenko 2007). All the southern islands in the ISD fall within the highly active zone IV of the seismic map of India (Sengupta et al. 2020) and have been getting subsided at a faster pace, as compared to the north, as they are located on the active trough. Thus, subsidence is occurring by tectonic and isostatic adjustments of the crust with some additional compaction of peat deposits and overpressures (Hoque and Alam 1997; Roy 2010). And the abnormal relative sea level rise so evolved (e.g., 5.22 mm yr−1 in Sagar Island: Unnikrishnan and Shankar 2007) might have been agglomerated with low elevation (Wood et al. 1990; Kotal et al. 2009) and a decrease in the sediment discharge due to late Holocene eastward tilt of this deltaic glebe (Curray et al. 1982; Sengupta et al. 2020; Ganguly et al. 2006; Bhattacharya 2008).
The region is also witnessing extensive flooding and storm surge inundation and is considered as one of the most vulnerable deltaic ecosystems in the world. Increased tidal height by cyclonic storms is rendering erosion particularly against the sea-facing islands (Michels et al. 1998; Allison 1998; Gopal and Chauhan 2006; Kotal et al. 2009). More than 33 severe cyclones have come about in the ISD since 1961 with five more than 200 km h−1 velocity, and their occurrence has increased by 26% over the last 120 years (Centre for Science and Environment 2012). Again, the diurnal tidal ranges are further complicated by the frequent occurrence of low air pressure in the eye of any looming cyclone (Michels et al. 1998; Mitra et al. 2010). Such cyclonic landfalls of increased frequency and devastating potential coupled with relative sea level rise could be considered to be the most significant forces for retrograding shifting of shorelines in the sea-facing islands.
An increase in the average temperature by about 6.12% over 27 years (Sengupta and Ravichandran 2001; Mitra et al. 2010) and salinity by about 6 psu decade−1 in the eastern lobe of the ISD is causing the retardation of germination processes and growth of mangroves, and such effects can also be indirectly attributed to increasing areal erosion in the reserve forest areas of the eastern lobe (Chowdhury 1987; Mirza 1998; Sarkar et al. 2002; Mukhopadhyay et al. 2006). But at the same time, clearing of jungles aids in unsettling the balance of dynamic tidal circulation developed by two dissimilar water currents in tidal channels and adjacent mangrove wetlands and thus results in siltation of some channels (Augustinus 1995) over a long period of time.
However, the complex interplay of relative sea level rise, tides (including asymmetry between ebb and flood tides), wave- and wind-induced currents, intensification and magnification of the frequency and pattern of storm bulges, changing hydraulic and sediment load regime due to the diversion of Ganga flow by Farakka Barrage from its eastern distributary (Padma in Bangladesh and Ichamati in India) to western distributary (Bhagirathi–Hooghly in WB, India), off shooting of upstream sources of all the rivers and channels flowing through the eastern lobe, etc., and triggering by various anthropogenic exploitations could combinedly be responsible for such shoreline changes in the ISD.
6. Conclusions
This satellite-data-based study indicates the following about the Indian Sundarban Delta.
Erosion and accretion took place simultaneously in most of the islands and islets of the tidal Indian Sundarban Delta, and that can be accounted for by concurrent landward and seaward positional shifting of the shorelines in the region. And the subaerial delta front experienced an overall net retreat of about 3.70 km2 land surface per year over a 41-yr period.
However, the shorelines and shallow offshore areas in the eastern lobe were undergoing dominantly an eroding phase (average areal accretion: ∼0.50 km2 yr−1, erosion: ∼3.35 km2 yr−1, and net erosion: ∼2.85 km2 yr−1) and those in the western lobe were exhibiting actively dynamic physiognomics (average areal accretion: ∼1.70 km2 yr−1, erosion: ∼2.55 km2 yr−1, and net erosion: ∼0.85 km2 yr−1). And these phenomena may be attributed to the altered hydraulic regime in the entire ISD, sediment starvation in the eastern part, and more sediment-feeding coupled with higher exposure to high currents and waves, all resulted out of diversion of perennial flow from Padma River to Bhagirathi–Hooghly River.
The most southern stretch in the ISD was facing the most eroding phase (average areal erosion: ∼2.20 km2 yr−1; accretion: 0), which is responsible for the islands’ acquaintance with the eroding oceanic forces and parameters in addition to frequent storm bulges. And the Sagar Group of Islands, located in the estuarine mouth of the Hooghly River and its distributary, is demonstrated as the most dynamic area in the entire ISD.
Considerable mangrove forest has already been moved out due to the landward shifting of shorelines. Moreover, prograding aggradation of land area contributed no more to the mangrove vegetation, since a 41-yr span is not sufficient enough to grow up to a potential state for those species of plants.
Such erosion–accretion and associated shifting of shorelines in the ISD are a function of riparian and oceanographic processes, altered sedimentologic and hydraulic regimes, frequent storm swells, and eustatic sea level rise, and possibly by regional subsidence.
Organized and gradual normalization of contributing rivers and riparian hinterlands and the already constructed marginal embankments could be some sort of way out in reducing such erosion problems and improving the depths of navigational channels.
However, the explanation behind the large-scale aggradation of Nayachar (a sea-faced island) and typical dynamic characteristics of the Sagar Group of Islands could not be strongly attended. The findings in the study are, to some extent, supported by various earlier works, viz., Allison (1998) and Ganguly et al. (2006). Vertical erosion/stacking of land mass too occurs while the horizontal shifting of shorelines takes place, and studying such erosion–accretional features on the vertical plane may open new avenues to work with.
Data availability statement.
All the satellite images used in this study are openly available from the USGS Earth Explorer at https://earthexplorer.usgs.gov. The map of the study area was acquisitioned from https://www.researchgate.net/sundarban-study-area. Others are mentioned in section 3a.
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