Environmental Geology

, Volume 48, Issue 8, pp 1058–1067

Rapid erosion of the coast of Sagar island, West Bengal - India

Authors

    • Department of Marine Geology and GeophysicsCochin University of Science and Technology
  • P.  Seralathan
    • Department of Marine Geology and GeophysicsCochin University of Science and Technology
Original Article

DOI: 10.1007/s00254-005-0044-9

Cite this article as:
Gopinath, G. & Seralathan, P. Environ Geol (2005) 48: 1058. doi:10.1007/s00254-005-0044-9

Abstract

The coastal zone of the Sagar island has been studied. The island has been subjected to erosion by natural processes and to a little extent by anthropogenic activities over a long period. Major landforms identified in the coastal area of the Sagar island are the mud flats/salt marshes, sandy beaches/dunes and mangroves. The foreshore sediments are characterized by silty, slightly sandy mud, slightly silty sand and silty sand. Samples 500 m inland from high waterline are silty slightly sandy mud, and by clayey slightly sandy mud. The extent of coastline changes are made by comparing the topographic maps of 1967 and satellite imageries of 1996, 1998 and 1999. Between 1967 and 1999 about 29.8 km2 of the island has been eroded and the accreted area is only 6.03 km2. Between 1996 and 1998 the area underwent erosion of 13.64 km2 while accretion was 0.48 km2. From 1998 to 1999, 3.26 km2 additional area was eroded with meager accretion. Erosion from 1997 to 1999 was estimated at 0.74 km2 /year; however, from 1996 to 1999, the erosion rate was calculated as 5.47 km2/year. The areas severely affected by erosion are the northeastern, southwestern and southeastern faces of the island. As a consequence of coastal erosion, the mud flats/salt marshes, sandy beaches/dunes and mangroves have been eroded considerably. Deposition is experienced mainly on the western and southern part of the island. The island is built primarily by silt and clay, which can more easily be eroded by the waves, tides and cyclonic activities than a sandy coast. Historic sea level rises accompanied by land subsidence lead to differing rates of erosion at several pockets, thus periodically establishing new erosion planes.

Keywords

Coastal erosionCoastal landformsShoreline changeSatellite dataSagar islandIndia

Introduction

Coastal environment issues are highly complex and due to settlement, waste disposal, aquaculture, fishing and recreation. As land becomes more and more crowded and terrestrial resources are used up, greater attention must be paid to the development of a coastal zone. Continuous physical interaction amongst the land, sea and atmosphere makes the coast a dynamic zone. The Sagar island coast is tide-dominated and is characterized by tidal creeks, mud flats/salt marshes, mangroves and sandy beaches/dunes. These landforms are subjected to natural processes (cyclones, waves and tides) and anthropogenic activities. Tidal amplitudes up to 6 m during extreme high tide are very common. Sometimes these effects will be much greater during cyclones, which frequently occur in this part of the coast.

Earlier studies without the use of satellite imageries (Paul and Bandyopadhyay 1987; Bandyopadhyay 2000; Baksi et al. 2001; Ghosh et al. 2001) indicate that the island has been subjected to erosion by various processes. This investigation applied the use of remote sensing data for delineation of coastal landforms, shoreline changes and coastal vegetation pattern.

Regional Setting

Sagar Island in India (21° 37′ 21′′ to 21° 52′ 28′′ N lat. and 88° 2′ 17′′ to 88°10′ 25′′ E long) is the largest island of the Sundarbans deltaic complex. The Hoogly river is to the north and west, the Muriganga distributary in the east and the Bay of Bengal in the south (Fig. 1). The north-south length of the island is 30 km. It has a maximum width of 12 km. The average elevation of the island is 6.5 m above the mean sea level (Mukherjee 1983). Fluvial, marine, tidal and aeolian processes are the chief agents actively shaping the narrow coastal belt.
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Fig. 1

Location map of Sagar Island with sampling stations

Materials and methods

Multidated (Dec. 1996, Dec.1998 and Oct.1999) satellite data of Indian Remote Sensing Satellite -IC Linear Imaging Self-scanning Sensor (LISS) III (both digital and geo-coded FCC) of the present Sagar island were obtained from National Remote Sensing Agency, Hyderabad. Shoreline changes of Sagar island were compared using the topographic map of 1967 and digital data of 1996, 1998 and1999. Geocoded data were used in the field to obtain ground control points and digital data were processed in the laboratory. Analysis and interpretation of satellite data was done by digital image processing using ERDAS Imaging. Satellite images were rectified with ground control points (GCPs) and the topographic sheet of Survey of India (SOI), 79C/1 and 79C/2. The rectified images were classified on the basis of ground truth observations. Area covered by different classes of images was estimated by grid analysis multiplied by spatial resolution factors. Accuracy test was also performed with ERDAS Imaging software to confirm the procedure.

The Satellite data procured were of similar tidal level exposures. The tidal heights were attained from the Survey of India Tide Table and co-ordinated with satellite data to minimize tidal differences. For calculation purposes, band 4 image was extracted from rectified images for each data set. The images were classified into water and land zones. By subtracting one image from the other overtime (1996–1999, 1998–1999 and 1996–1998), the differences reveal the erosion/accretion patterns. Arial extent of erosion and accretion was calculated. The 1967 topographic map in raster format was correlated with images of 1999 and used to estimate the arial extent of erosion and accretion. Distribution of the landforms (mud flats/salt marshes, sandy beaches/dunes) and vegetation patterns (mangrove and casuarinas) identified from satellite imageries were crosschecked during the field survey conducted in 2000. Extent of change in vegetation and landforms were demarcated by classification of satellite imageries from 1996, 1998 and 1999. During the field survey, 36 surface sediment samples were collected, 18 from the shore and the remaining at a distance of 500 m inland from high waterline. Samples were subjected to textural analysis (sieving and pipette methods) for finding the sand, silt and clay percentages. The ternary diagram of Pejrup (1988) and Flemming (2000) were used respectively to demarcate the variability of hydrodynamic conditions and sand--silt--clay ratios. Demographic data as per the 1991 and 2001 census were used in this study. Tide gauge were obtained from Permanent Service for Mean Sea Level (PMSL). Evolutionary trends of the Sagar island from 1881 to 1920 were obtained from Central Water Power Research Station (CWPRS), Pune.

RESULTS

Landforms

Silt and clay sized particles deposit on the periphery of the island and are carried interiorly by creeks during high tides. Major mudflats exposed during low tide are found in the vicinity of st. 4, sts. 6--8, st. 16 and st. 17. Near st. 17, the beach is muddy (Fig. 2.). At station 6, a well-developed mudflat of over 150 m wide is beyond the sandy beach (Fig. 2). Low tidal flats are composed mostly of silts and clays, while higher mudflats consist of silty sand and hard, muddy patches. Marshes and mangroves thrive in the mudflats. These plants facilitate deposition of fine-grained fluvial sediments. Near sts. 6 and 16, the entire mudflat is covered with extensive salt marshes/mangrove forest. The total area covered by mud flats/salt marshes during 1996, 1998 and 1999 was estimated as 7.8 7.75 and 7.72 km2 respectively (Table 1).
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Fig. 2

Variation of sedimentary environments across the coast at different stations

Table 1

Description of landuse/landcover classes from supervised classification of LISS III data

Classes

Area in 1996 (km2)

Area in 1998 (km2)

Area in 1999 (km2)

Mangroves

0.9

2.1

1.3

Sandy Beaches/dunes

2.3

1.2

1.2

Mudflats/salt marshes

7.8

7.75

7.72

Casuarinas

6.5

7.4

7.2

Sandy beaches with sand content ranging from 80 to 90% are located on the southern, south western and southeastern parts of Sagar island. Major beaches are at st. 4 and south of st. 6. Sand dunes of different heights (1--2.5 m) are behind st. 4 (Fig. 2). The dune tops are stabilized by halophytic plants (Plate 1). Between sts. 4 and 8, vast sand dunes of moderate elevations of 1--1.5 m are categorized as parabolic dunes (Paul and Bandyopadhyay 1987). Between the dune ridges, mangroves are well developed and the substratum is very muddy. In general, wherever considerable sandy beaches are developed, aeolian activities are prominent and large-scale sand dunes have developed behind the beaches. Combined areal extent of sandy beaches/dunes for the years 1996, 1998 and 1999 respectively were 2.3, 1.2 and 1.2 km2 (Table 1).

Coastal vegetation

Along the coast, the dominant mangrove species are Avicenia marina, A. alba, A. officinalis and Exoeccaria agallocha. Casuarinas are a major vegetative cover (Table 1). The areal extent of mangroves in 1996 was 0.9 km2 and was increased to 2.1 km2 during 1998 by an artificial plantation near Krishnanagar (st. 8) and on the southwestern part of the island. However, in 1999, the mangrove forest declined to 1.3 km2, possibly due to human interference and coastal erosion. The southeastern and southwestern coasts of the island are areas where beach erosion is severe and mangrove vegetation is also critically affected. Salt marshes are mostly in mud flats. Unlike the mangroves, the distribution of Casuarinas is very significant (Table 1).

Shoreline changes

The southwestern part of the Sagar island near Beguakhali has been subjected to continuous large-scale erosion by wave activities. A comparison of the topographic map of 1967 and the satellite imagery of 1999 shows that most of the northeastern, southeastern and southwestern parts of the island were subjected to severe erosion, resulting a land loss of 29.8 km2 over the three decades. Between 1996 and 1998 the land loss due to erosion was13.64 km2, while between 1996 and 1999 the total loss of land was 16.90 km2, a rate of erosion of about 5.63 km2/year (excluding deposition). The rate of erosion from 1967 to1999 was 0.74 km2/year.

Some deposition was recorded on the western (sts. 8, 9 and 10) and southern side of the island (Fig. 3). Between 1996 and 1998 land surface gained 0.48 km2. The accretion recorded during 1998 – 1999 was 0.08 km2. The total accretion between 1967 and 1999 was 6.03 km2 (Table 2). However the island as a whole has been eroding since 1881, with a major change during 1914 (Fig. 4).
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Fig. 3

Shoreline changes of Sagar Island (1967--1999)

Table 2

Shoreline changes of Sagar island

Class

Area in 1967–1999 (km2)

Area in 1996–1998 (km2)

Area in 1996–1999 (km2)

Area in 1998–1999 (km2)

Erosion

29.8

13.64

16.90

3.26

Accretion

6.03

0.48

0.56

0.08

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Fig. 4

Evolution of Sagar Island from 1881 to 1999 (1881 to 1920, CWPRS; 1967 topomap and 1996, 1998 and 1999, are band 3 images of IIRS IC LISS III data)

Sediment characteristics

Percentages of sand, silt and clay were calculated using triangular diagram (Flemming 2000). In general, sand content varies from 2 to 90% (Fig. 5a). Along the foreshore, high content of sand (>74%) is recorded at stations 1, 2, 3, 4, 6 and 18, while considerable silt content (>50%) is found at stations 7, 9, 10, 11, 13, 15 and 16. High content of clay (59.3%) is found only at st. 5. In general, high percentage of sand in the foreshore are recorded on the western part of the island while the eastern margin is characterized by high silt and lesser amount of clay. Sediment samples collected 500 m inland from high water line show high content of silt (40.5--72.8%), except at stations 1, 4 and 8 (8.3--28%).

The ternary diagram (Fig. 5a) indicates that most of foreshore samples are characterized by silty, slightly sandy mud (sts. 9, 10, 11, 12, 13 and 15), followed by slightly silty sand (sts. 1 and 6), silty sand (sts. 3 and 17), silty sandy mud, clayey silt etc. Samples 500 m inland from high waterline are characterized by silty slightly sandy mud (sts. 3, 5, 6, 7, 12, 16 and 17), followed by clayey, slightly sandy mud (sts. 13, 14 and 15). Pejrup (1988) has proposed a scheme of ternary diagram (Fig. 5b) characterizing hydrodynamic condition in which foreshore sediments are generally deposited under high-energy conditions as compared to sediments 500 m inland from high water line. Sediments from stations 1, 2, 3, 4, 6and 18 (foreshore) and stations 1 and 4 (500 m inland from high water line) are deposited under high-energy condition.
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Fig. 5a

Ternary diagram of sand/silt/clay ratios illustrating textural trends from the foreshore (filled circles) and 500 m inland from high water line (filled rectangle) of the Sagar island (After Flemming 2000) b. Ternary diagram of sand/silt/clay ratios illustrating textural trends from the foreshore (filled circle) and 500 m inland from high water line (filled rectangle) of the Sagar island (After perjrup 2000)

Discussion

The coastal zone of Sagar island consists primarily of mudflats/salt marshes/mangroves and sandy beaches/ dunes. Sedimentological studies indicate that coastal sediments are primarily muddy except the sandy beach. Sand content is greater on the western coast, while silt predominates on the eastern side of the island. Studies carried out by Colemann (1969) indicate that the sediment load of the Ganges--Bhramaputra consists more than 70% as silt and sand content is just 10%. Study carried out by Allison et al. (2003), also confirms these findings. According to Allison et al. (2003) the growth of the lower delta plain of the Ganges--Bhramaputra system progressed from west to east in successive sub-phases such as G1, G2, G3 and G4. G1 sub-delta represents the earliest phase of the delta formation of the Ganges and evolved during the period between 5,000 and 2,500 years BP

Sagar island which forms a part of G1 sub delta would have formed during the said period by the union of several small islands such as Dublat, Beghukhali, Govacahacks, Shikarpur, Harinbari, Tower island etc. These islands are separated by a network of tidally influenced creeks such as Chemagari Khal, Ganga Sagar Khal, Satbaki Khal and Muriganga Khal. The 1902 map of Sagar island also revels the existence of these creeks (Fig. 4). The Chemagari Khal cut across the island in a north--south direction over a distance of 10 km length.

Allison et al. (2003) indicated that the western delta plain complex (G1, G2 and G3 phases) have reached maturity and are longer prograding as water discharges in the western distributaries such as Hoogly, Gorai and Arial Khan have decreased during the historical time. Sagar island shows some morphological changes in its evolutionary history since 1881 particularly at the northern tip of the island. Up to 1881, the Ghoramara and Khasimara were part of the Sagar island (Das 2000). However, by 1914, the northern tip of the island was cut off from the rest of the island and subsequently Ghoramara, Khasimara, Lohachara and Supaibhanga were formed (Fig. 4). Allison (1998) has reported that the western Sunderban (G1 sub-delta) has retreated 3--4 km in historical time. Several pockets of the coast of Sagar island are also eroding. A comparison of the topographic map of 1967 and satellite imagery of 1999 shows that most of the northeastern, southeastern and southwestern faces of the island are subjected to severe erosion (Fig. 3). From 1967 to 1999, about 29.8 km2 were lost. Between 1996 and 1998 the land loss due to erosion was 13.64 km2, while between 1996 and 1999 the total loss of land was 16.9 km2. The accretion of land between 1967 and 1999 has been estimated as 6.03 km2. The rate of erosion for the three year period from 1996 to 1999 is nearly 5.47 km2.

There are major causes for this alarming rate of erosion are several. Cliff erosion mainly by fluvial action is prominent on the northeastern part of the island as the river course meanders (Fig. 1) The height of the river bank is about 6 m, and decoupling of top and toe is common (Plate 2). Unlike the eastern front, which is very steep, the western and southern coasts of the Sagar island are rather gentle and vast areas of beach are exposed during low tide. These coasts are affected by waves, winds and tidal activities and at times, cyclones. The southwestern part of the island has been subjected to continuous large-scale erosion by wave activities. The old light house, which is now 100 m offshore of the island (Fig. 1), stands testimony to the retreating nature of the shoreline. Sea wall destruction and beach ridge erosion (Plate 3) are also prominent between sts. 17 and 18. The sea walls are now totally collapsed. Tropical cyclones are one of the most destructive natural disasters affecting Sagar island. Cyclones bring strong wind, heavy rainfall and flooding, resulting in severe beach erosion. The decadal frequency of storms from 1891 to 1961 as per the record of Indian Metrological Department (1964) indicate that a maximum of 56 cyclones have occurred from 1921--1930, while a minimum of 32 have been reported for 1951–1960. The steady decrease in size of Sagar island may partly be related to shore erosion by storms. Emery and Aubrey (1989) have reported that land loss due to cyclone activity is a common phenomenon in the northern Bay of Bengal region. This coastal region is considered tide-dominated coast as per the nomenclature of Davies (1964). Regular tidal rise of up to 6 m are very common (Mukarjee, 1983). During monsoon, the combined effect of tidal rise and cyclonic effects are severe. The break up of northern tip of the island between 1902 and 1914 may have been triggered by cyclonic activities. Paul and Bandhopadaya (1987) opined that the island is subjected to erosion on all sides except the south. Bandhopadaya (2000) has stated that since 1860 nearly 71 km2, which is equal to one fourth of the island area, has been eroded. Ghosh et al, (2001) have stated that between 1989 and 1995 the land loss through erosion was 3.88 km2.

The continuous decline in the size of the island might also be influenced by steady rise in sea levels accompanied by subsidence of the lower delta plain (Pirazzoli 1998). Relative sea level rise is a progressive hazard in coastal areas and can increase the frequency of storm surges, flooding, water logging and flood water and inundation of low lying land (Chen, 1997). Tide gauge data of Sagar island indicate that from 1948 to 1963, the sea level rise was very steep, followed by a drop in sea level up to 1967 (Fig. 6a). However, there has been a steady rise in sea levels in subsequent years. From 1967 to 1985, the increase in sea level around Sagar island was very significant (Fig. 6a). Baksi et al. (2001) have reported the sea level rise for the Sagar island is 2.6 mm/year, possibly due to subsidence of the lower delta plain complex. Allison, et al (2003), have stated that the western Sundarban is undergoing subsidence at the rate of 1--4 mm/year. The rate of compaction will be over 50% in clayey sediments but in sandy region, it is negligible. Subsidence increases with the steady withdrawal of groundwater from the island for domestic and industrial use. The population of the island in 1864 was 1,466 persons i.e., 5.2 persons/km2. However, as per the1991 census, the population was 1,49,222, while the 2001 census indicates that the population was 1,85,301, which is 914.6 persons/km2. Human impacts on the island have been considerable as the people migrated to the Coastal Regulation Zone (CRZ). Utilizing the mangrove resources, aquaculture activities and clay mining in the CRZ, which is common in the eastern part of the island, have all contributed to changing land use.
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Fig. 6

Sea level changes in mm for a Sgar island and b Kolkatta

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Plate 1

Stabilized dunes at Beguakalhi

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Plate 2

Cliff (mud) erosion by decoupling of top and toe on the northeastern side of the island

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Plate 3

Exposed muddy beach, seawall collapse and erosion of sand ridges at station 17

Over time, rises in sea level lead to alteration of the hydrodynamic conditions of the estuarine region; the tidal amplitude will be larger and siltation will increase in the upper reaches of the estuary. The steady rise in sea levels in the Kolkatta region (Fig. 6b) supports this statement. Reduction in sediment supply in the lower delta plain through Hoogly in recent time (Allison et al. 2003) accompanied by rise in sea levels will ultimately increase the depth of the estuary and the sea around the island. Waves can then directly attack the coast. Therefore, the combined effects of decreased river discharge in historical times, the steady rise in sea levels and faster subsidence of the delta plain, will establish new erosion planes and lead to the steady decline in the size of the Sagar island.

Conclusion

Major geomorphological features found along the coast of the Sagar island are mudflats/salt marshes and sandy beaches/dunes. There is considerable reduction in the areal extent of sandy beaches/dunes and mudflats/salt marshes in recent years. The major coastal vegetations, namely mangroves and casuarinas, are under threat. From 1967 to 1999 the net erosion of the island was 29.8 km2, while accretion was 6.03 km2. The northeastern, southeastern and southwestern faces of the island are subjected to severe erosion. Deposition is found only in limited parts of the coast. Possible major causes for the overall coastal erosion are the reduction in fluvial supply for a long period, wave, tidal and cyclonic activities, sea level rise and land subsidence, and lastly human interference. A proper coastal zone management plan to balance the fragile ecosystem is urgently needed.

Remedial measures

To protect the coast from erosion, regular coastal protection measures like strengthening of mangroves/marshes/casuarinas by artificial plantation are needed. Water supplies should be diverted into Hoogly estuary, to enhance sedimentation in lower reaches by flushing the sediments already deposited in the upstream and balance tidal effects. Educating local people to the need for protecting the coast by conservation of coastal vegetation and banning of clay mining should be a Top priorities.

Acknowledgement

The authors are grateful to Dr. P.S. Roy, Dean and Faculty of Forestry and Ecology Division and Dr. D. Mitra and Dr. A.K. Mishra, Marine Science Division, Indian Institute of Remote Sensing, Dehradun, for their unending support during the study.

Copyright information

© Springer-Verlag 2005