Indian Journal of Geo Marine Sciences Vol. 49 (09), September 2020, pp. 1521-1527

Vulnerability assessment for coast, with respect to inundation hazards

D Chakraborty*,a & P Sahab aDurgapur Government College, Durgapur, West Bengal – 713214, bSchool of Oceanographic Studies, Jadavpur University, 188, S. C. Mallik Road, – 700 032, India *[E-mail: [email protected]]

Received 22 March 2019; revised 14 November 2019

Gangasagar coast of Sagar Island which lies in the Ganga delta is dynamic in nature where coastal processes play a significant role. Analysis of shoreline configuration from the year 1969 to 2009 indicates that shoreline has been retreated. Assessment of vulnerability has been done taking on six geological and physical variables namely geomorphology, sea-level rise rate, coastal slope; shoreline erosion rate, mean wave height and mean tide range. Coastal vulnerable index values vary location wise. In general, CVI values are very high to high in the western sector. In the eastern sector values are moderate to low. In the south western most part, the value is very high where the coast (near Beguakhali) has been substantially eroded which is evident from the observation that older light house now stands in the sea at about 100 m from the coastline.

[Keywords: Coastal slope, Erosion, coast, Shoreline, Vulnerability]

Introduction negligible whereas in clayey sediments this rate will Sagar Island (lat.: 21°37’21” to 21°52’28” N & be over 50 %. long.: 88°02’17” to 88°10’25” E) which lies in the In 2012, INCOIS has published coastal Ganga delta, facing Bay of Bengalis more or less vulnerability atlas of India based on the parameters triangular in shape. The bifurcates near representing geological and physical processes and the northern tip of Sagar Island. The eastern part their study has been done based on satellite data. The flows as Muriganga river and the western part flows maps are in small scale with a limitation to plot all the as the Hooghly river (Fig. 1). The coastline result6. The shoreline and land cover changes in Sagar configuration of the island has been shaped by fluvial Island have been studied by using Indian remote and marine processes. The shape of the island has sensing satellite data7. Gopinath8 has discussed the changed from the year 1851 to 1992 reducing the area critical issues of Sagar Island coast. Vulnerability of this island1. The northernmost tip got separated to index value has also been assessed along the Orissa form a new islet, perhaps due to submarine erosion by coast9. Along the west coast similar study has also strong tidal current in this constricted zone, followed been done in Mangalore10. by landslide2. The coastal tract in the south of the Coastline represents the area where there is an island, known as Gangasagar coast stretches for about interaction between land and the ocean. It is one of 11 km (from 88°02'00" to 88°08'00" E and 21°37'30" the dynamic parts of the Earth surface. Dunes and to 21°39'30" N). It has been recorded from tide gauge beaches are the most constituent features of coastal data of Sagar Island that the relative sea level rise was region. Geological processes like erosion, storm very steep from 1948 to 1963 and it was followed by surges, flooding and threat of level rise are the a drop in sea level up to 1967(ref. 3). It is very possible causes of coastal inundation. This significant that from 1967 to 1985 the sea level was necessitates making an assessment for vulnerability increase around Sagar Island. The subsidence of index. A number of methodologies have been lower delta plain complex is the possible reason for proposed from time to time to make assessment of the relative sea level rise in Sagar Island4. The inundation vulnerability of coastal areas. Some of western part of Sundarban is now undergoing these are the CM (Common Methodology), CVI subsidence and the rate of subsidence is recorded as (Coastal Vulnerability Index) etc. The present study is 1–4 mm/year5. In sandy region, the compaction rate is aimed to get picture on the degree of vulnerability of 1522 INDIAN J GEO-MAR SCI, VOL 49, NO 09, SEPTEMBER 2020

Sagar Island coast. Along the coastal region the (bench mark) from Irrigation Bungalow (SOI physical parameters liketidal amplitude, littoral drift, toposheet no 79 C/2, scale: 1:50,000; surveyed in and longshore current vary from one island to another 1968-1969 and published in 1973), having height of island. Vulnerability index value will also differ from 3.1 m from MSL. one island to another. For this reason the study has not Assessment of coastal vulnerability index has been been done in a regional scale. The Sagar Island coast done by various workers taking into account six has been selected for this study because this coast is geological and physical variables namely the largest island of Ganga delta with a sizeable geomorphology, rate of relative sea-level rise, mean population living along the coast. Study indicates that wave height, shoreline erosion rate, coastal slope, and part of the coast is under erosive phase. mean tide range11-14. The six variables include quantitative and qualitative information. Present work Materials and Methods has also been done by considering vulnerability Field work and topographic survey was done with ranking of these six variables. Six variable parameters help of theodolite and dumpy level to understand the are used in a quantifiable manner. A numerical data is present coastal landforms and relief. The Gangasagar produced by using this method depending on the six beach can be divided into two sectors, the eastern vulnerability index variables of coast (Table 1). This sector and western sector. Eastern sector stretches is a deltaic coast and this CVI equation is suitable for from east of Gangasagar canal to Dhublat and the this coast. Along each profile section of coastline, western sector is from west of the Gangasagar canal separate index value has been generated depending on to Beguakhali. Beach profiling was done along raking of these parameters. The coastal vulnerability 14 lines (profile lines ‘A’ to ‘I’ and ‘1’ to ‘5’) in the index is calculated as square root of product of ranked western sector and along 6 lines (profile lines ‘6’ to variables and after that that product will be divided by ‘11’) in the eastern sector, totalling 20 lines (Fig. 2). total number of variables. The profile lines are aligned perpendicular to the shoreline. An interval of about 500 m between the CVI = √ {(a*b*c*d*e*f ) / 6 } profiles was maintained in the eastern and western part of the coast and about 300 m in the central part. Where, ‘a’ means geomorphology, ‘b’ means coastal The beach profiling was done connecting the R.L. slope, ‘c’ means relative sea-level rise rate, ‘d’

Fig. 1 — Location of the study area CHAKRABORTY & SAHA: VULNERABILITY ASSESSMENT FOR SAGAR ISLAND COAST 1523

Fig. 2 — Profile lines and geomorphic features, Sagar Island coast

Table 1 — Ranges of vulnerability ranking with six variables on the Atlantic coast11 Variables Very low Low Moderate High High (Very) 1 2 3 4 5 Geomorphology Rocky cliffed Medium cliffs, Low cliffs, Glacial drift, Cobble Beaches, Sandy beaches, coasts, Fjords Indented coasts Alluvial plains Estuary, Lagoon Barrier beaches Erosion or > 2.0 1.0 - 2.0 -1.0 – 1.0 -2.0 – -1.0 < -2.0 accretion of Shoreline (m/yr) Mean wave height in meter < 0.55 0.55 - 0.85 0.86 - 1.05 1.06 - 1.25 > 1.25 Coastal slope in % > 1.20 1.20 - 0.90 0.90 - 0.60 0.60 - 0.30 < 0.30 Mean tide range in meter > 6.0 4.0 - 6.0 2.0 - 4.0 1.0 - 2.0 < 1. 0 Relative sea-level change < 1.8 1.8 - 2.5 2.5 - 3.0 3.0 - 3.4 > 3.4 (m/yr) indicates shoreline erosion rate, and ‘e’ and ‘f’ erosion and accretion. This was not used to measure indicate mean tide range and mean wave height, the total shifting of high water line. Temporal change respectively. of relative sea level rise (2001-2008) has been taken15. Degree of coastal slope was calculated in respect of The wave heights were measured during monsoon. rise of elevations from the shoreline on each profile Erosion rate has been computed from the shoreline line as determined by theodolite survey. shifting. The tide data has been taken from tide table Simultaneously, total retreat of high water line was of Sagar (tidal chart of Kolkata Port Trust). This measured in respect of 1969 and 2009. High water approach yields a quantitative as well as relative line data of year 1969 has been taken from SOI measurement of vulnerability. toposheet and has been compared with the geological field data of year 2009. In addition, satellite imageries Results such as 1975 LANDSAT MSS image, 2001 LISS III Geomorphology image and 1990 LANDSAT TM image have been The “coastal geomorphology” is a non-numerical used to get an idea about over all fluctuation pattern variable. This variable has been expressed as the of high water line in between 1969 and 2009 and to relative erodibility of different types of coastal understand the stability of the coast in respect to landforms. According to relative strength of the 1524 INDIAN J GEO-MAR SCI, VOL 49, NO 09, SEPTEMBER 2020

coastal landforms geomorphology has been ranked Mean wave height qualitatively. Geomorphology of this coast has been It is measured along the coastal tract of Gangasagar represented by particle size parameters of sediments. coast is about 1.2 m. It has been measured by using The Gangasagar coast of Sagar Island is semi-arcuate theodolite and staff. in shape along east-west direction. The beach is more or less flat and with gentle slope, varying from 0.42° Mean tide range to 1.51° and width of foreshore ranges between 65 to The entire coast is strongly influenced by high tidal 382 m. The intertidal zone of Gangasagar coast is wave especially during spring tide. The seawater mostly veneered by fine to very fine sands with inundates almost the entire beach during high tide. palaeomud at places. During spring tide of September-October 2008, the In the eastern sector, width of the foreshore varies coastal belt in the Dhublat sector was flooded, from about 65 to 215 m and beach slope is from 0.78° damaging and overtopping the embankment. This is a to 1.51°. A sand bar along with a runnel has been common phenomenon during spring tide. The mean formed at the mouth of the Gangasagar khal. tide range is about 3.1 m. Compared to the western sector, the beach is narrow in the eastern sector. In the easternmost part, near Coastal slope Dhublat, the width is relatively narrow (65 – 68 m). Coastal slope have been calculated along each The eastern sector is mostly covered by sticky clay, profile line in respect to width of foreshore and representing ancient eroded vegetated area. On the corresponding height of high water line (Table 2). eastern bank of the Gangasagar khal, the remnants of Topographic survey reveals that highest relief of the older tree roots on clayey bed indicate that the mouth foreshore is 2.8 m above MSL. Coastal slope ranges of the khal has possibly shifted eastward. At some from 0.42° to 1.51° along Gangasagar coast. In eastern places the clayey zone extends further seaward. There sector, the coastal slope shows higher values. Along are clay balls on the intertidal zone. At places the the profile line ‘9’ in the eastern most sector the value clayey zone is being covered by fine to very fine is maximum as 1.51° (Table 2). In central and western modern sands. The area, south of Gangasagar khal is part, the coastal slope values are relatively lower than experiencing recent sedimentation by silty sand. In Table 2 — Coastal slope along profile lines the eastern sector, sand dunes exist in the backshore. Profile Width of Height of Coastal slope (in The height of this neo-dune varies between about 3 to No. foreshore HWL degree) 11.4 m above MSL. At places the neo-dunes are under (meter) attack from strong tidal waves. Matured sand dune I 116 2.575 1.27 belt lying behind the neo-dune belt is 4.4 to 6.6 m H 174 2.755 0.9 high above MSL and at places are flattened due to G 382 2.825 0.42 anthropogenic activities. F 268 2.445 0.52 In the western sector, width of foreshore varies E 159 1.905 0.68 from about 85 to 382 m and slope of the beach varies D 85 1.395 0.94 from 0.42° to 1.44°. There are sand dunes in the C 172 2.17 0.72 backshore. Behind this neo dune matured sand dunes B 133 2.395 1.03 are present with 4 m height above MSL. Comparing A 195 2.781 0.81 with the eastern sector the relief of matured dune is 1 230 3.005 0.74 low. Further westward there is no sand dune, where 2 250 2.825 0.64 intertidal zone is mostly clayey (sticky). At places 3 285 2.62 0.52 remnants of earlier vegetation exists. At present part 4 95 2.4 1.44 of the clayey zone is being covered by modern 5 125 2.94 1.34 sands. Infact, in the seventies, this area was fairly 6 100 2.64 1.46 dense scrub with scattered trees. As the Sagar Island 7 130 3.78 1.45 coast is mostly veneered by fine to very fine sands 8 215 2.94 0.78 with old clays zone at places, the value of 9 65 2.62 1.51 geomorphology has been taken as 5 as per 10 68 2.15 1.42 USGS report11. 11 160 3.1 1.1 CHAKRABORTY & SAHA: VULNERABILITY ASSESSMENT FOR SAGAR ISLAND COAST 1525

eastern part. Along the profile line ‘I’ in the south the ranges from 3.16 to 7.07 (Table 4). The mean western sector the coastal slopeis 1.27°. value of the index is 5.38; the mode is 6.32 & the values of median and standard deviation are 5.89 and Rate of relative sea-level rise 1.45 respectively. 25th, 50th & 75th percentiles indicate Authors have not done any work on the sea level the values of 4.47, 5.89 and 6.32 respectively. On the change in this domain. Published data on sea level basis of quartile ranges the scores of CVI are rise are taken into consideration while computing classified into four categories namely low, moderate, CVI. Rate of relative sea level rise is 3.14 mm high, and very high-vulnerability. CVI values below (0.00314 m) per year in respect of Sundarban Island 4.47 are recorded as low vulnerability category. In the including Sagar Island has been taken15. western sector along the profile line ‘4’ and in eastern Shoreline erosion rate sector along profile line ‘6’, ‘9’ and ‘10’ the Landward movement of shoreline has been vulnerability index show lower values. Values from measured considering total effective shift of high water line from 1969 to 2009 (Fig. 3). A comparison Table 3 — Rate of shoreline erosion along profile lines of the HWL of 1969 (toposheet) with that of year Profile No. Shoreline erosion rate in meter/year 2009 field data shows that there is landward shift of I 8.05 HWL of the Gangasagar coast. In the western sector H 10.70 the high water line has shifted by about 250 to 500 m G 11.73 and by about 600 to 800 m in the eastern sector. F 2.18 Accordingly, shoreline erosion rate per year has been E 4.9 calculated profile line wise in respect of 1969 vs. D 4.9 2009. Erosion rate varies from place to place. Along C 2.65 the profile lines 8 in the eastern sector the value is B 2.65 maximum such as 15.23 m/year (Table 3). In south A 2.43 western sector along profile lines ‘G’, ‘H’ and ‘I’ are 1 2.43 11.73 m/year, 10.7 m/year and 8.05 m/year 2 1.28 respectively. It has been indicated from the time series 3 1.28 analysis of shoreline that the Sagar Island coastal belt 4 2.35 has experienced a number of events inerosion and 5 5.7 accretion in phases since 1969. 6 5.7 7 4.53 Discussion 8 15.23 On the basis of the six parameters, CVI has been 9 12.05 computed along respective profile lines (Fig. 4). The 10 6.7 calculated index values for Gangasagar coast show 11 8.5

Fig. 3 — Time series fluctuation of high water line, Sagar Island coast 1526 INDIAN J GEO-MAR SCI, VOL 49, NO 09, SEPTEMBER 2020

values. The CVI analysis reveals that 18.47 % of Sagar Island coast has been categorised as very high vulnerable class. The high and moderate vulnerable classes constitute 21.74 % and 33.70 % of the Sagar Island coast respectively. 26.09 % of Sagar Island coast has been marked as low vulnerable class.

Conclusion Vulnerability index values in Gangasagar coastof Sagar Island vary location wise. In general, CVI values are very high to high in the western sector. But in the eastern sector CVI values are moderate to low. In the south western most part, the coastal vulnerability index value is very high to high. Field

observation shows that the south-western corner (near Fig. 4 — Vulnerability pattern of Gangasagar coast of Sagar Island Beguakhali) has faced substantial erosion which is also evident from the field observations that old light Table 4 — Vulnerability index along with beach profile lines house now stands in the sea at about 100 m from the Profile No. CVI Degree of vulnerability present coastline. In western sector of Gangasagar coast the coastal slope percentage shows Western part I 4.47 Moderate comparatively lower value than eastern sector because H 6.32 High coastal regions having low-slope should retreat faster 16 G 7.07 Very high than the regions having steep slope . Along the F 7.07 Very high western part of coast shoreline erosion rate is very E 6.32 High high (10.70 m/year and 11.73 m/year). Interestingly, D 4.47 Moderate the adjoining near coastal seabed off this south C 6.32 High western coast is having relatively steep slope which B 5.47 Moderate have been developed as a result of collapsing of 2 A 6.32 High sediments due to slope instability . The slope 1 6.32 High instability might have been caused along a weak layer 2 7.07 Very high by scouring of seabed by the tidal current close to the 3 7.07 Very high Hooghly river mouth. Regular dredging in the main 4 3.16 Low navigation channel of the Hooghly river may also 5 4.47 Moderate cause instability of slope around the dredged area, Eastern part resulting mass flow of sediments from the adjacent 6 3.16 Low seabed. This segment of the coast shows higher 7 4.47 Moderate value of CVI. 8 6.32 High The effect of neotectonic activity may be an 9 3.16 Low important component to be considered. Subsidence 10 3.16 Low and upliftment of this domain, if any could not be 11 5.47 Moderate considered in the present work as this type of data is not available. A comprehensive study is required to 4.47-5.89 are considered moderate vulnerability. detect any tectonic activity in this part of the coast Along the profile lines ‘I’, ‘D’, ‘B’ and ‘5’ of western and it’s offshore. Along with diagnostic geological sector and along the profile lines ‘7’ and ‘11’ of features, instrumental evidence of seismicity is eastern sector, the vulnerability index show moderate required to be examined with the installation of values. High vulnerability values lie between 5.89 and broadband measurement systems. 6.32. Profile lines ‘H’, ‘E’, ‘C’, ‘A’, ‘1’ and ‘8’ show higher CVI values. The index values above 6.32 have Acknowledgements been classified as very high vulnerability. Profile lines The authors would like to acknowledge B K Saha, ‘G’, ‘F’, ‘2’, ‘3’ show very high vulnerability index Former Senior Deputy Director General of G.S.I. and CHAKRABORTY & SAHA: VULNERABILITY ASSESSMENT FOR SAGAR ISLAND COAST 1527

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