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CHAPTER 2

STUDY AREA: THE SUNDARBANS MANGROVE AND

THE HOOGHLY ESTUARY

2.1 INTRODUCTION

Mangroves in India account for about 5 percent of the World's

mangrove vegetation and are spread over an area of about 6,740 km2 along

the coastal States / Union Territories of the country (Untawale 1987). The

largest delta, Ganges-Brahmaputra delta (80,000 km2) formed from sediments

deposited by three major rivers, the Ganges, Brahmaputra and Meghna,

converges in the Bengal Basin. The largest mangrove forest of the world, the

Sundarbans occupies 10,200 km2 area of the delta, out of which ~59% area

falls under Bangladesh. The Indian Sundarbans mangrove forest in the

estuarine phase of the River Ganges covers an area of 9630 km2, out of which

4264 km2 is classified as Reserve Forest.

The Sundarbans is covered with thick mangroves, which are

subdivided into forest and aquatic sub-ecosystems (1,781 km2). In the year of

1985, the Indian Sundarbans was inscribed as 'World Heritage Site' by

UNESCO. The Government of India declared the Sundarbans region,

including the protected area of the Sundarbans Tiger Reserve as Biosphere

Reserve in 1989, and it received the recognition of UNESCO, under its Man

& Biosphere (MAB) Programme, in November 2001. The Sundarbans

Reserve Forest has been nominated for recognition as a Ramsar Site

(A Wetland of International Importance). It is the only mangrove forest in the

45

world having among its denizens, the famous Royal Bengal Tiger (Panthera

tigris). In 1885, the area of the Sundarbans forest was about 20,000 km2,

which has now shrunk to only 9,600 km2, out of which < 4,200 km

2 is

mangrove forest (Bose 2004).

2.2 GEOGRAPHICAL LOCATION AND ECO-GEOGRAPHY

The Indian Sundarbans is restricted within the latitudes of 21°31' N

and 22°30' N and longitude between 88°10' E and 89°51' E, spread over the

districts of 24 Parganas (South) and southern parts of 24 Parganas (North)

(Figure 2.1). West Bengal coast stretches to about 950 kms, out of which 260

kms are occupied by the Sundarbans mangrove forest (Agricultural Research

data book 2002).

The Indian Sundarbans falls within the geographical limits of the

Dampier and Hodges line (imaginary line drawn in 1831) in the northwest,

the river Hooghly in the west, the river Icchamati - Kalindi - Raimangal -

Harinbari in the east (Figure 2.1), which is also the International boundary

between India and Bangladesh and the Bay of Bengal in the south (Gurmeet

Singh 2009).

The eco-geography of this area is totally dependent on the tidal

effect. The tidal action of the sea inundates the whole of the Sundarbans to

varying depths. Due to tidal action, the silts carried down to the sea are

pushed back to the channels where they are deposited. The bed of the channel

thus gets steadily raised, ultimately blocking the flow of water and gradually

evolving into a small island (Bhattacharya 1989).

46

Figure 2.1 Study area map of the Sundarbans mangrove and the

Hooghly estuary

2.3 GEOMORPHOLOGICAL SETTINGS AND GEOLOGY

The tract of the Sundarbans, falls under the Ganges delta and is

highly dynamic; and the delta formation here is very rapid (Morgan and

McIntire 1959). The tectonic activity that tilts the delta towards the east has

47

also brought changes in the geomorphology and biota of the Sundarbans

(Junk et al 2006). Several river channels criss-cross the Sundarbans delta and

bring with them tons of sediments from terrestrial sources. They also play a

major role in eroding and accreting nature of this deltaic estuary. Though the

surface horizon is of uneven thickness and width, but the morphological

nature is more or less uniform, i.e., these are alluvial plains of lower Bengal

formed by the deposition of sediments. The entire process is assisted by tides

from the sea side.

Geomorphologically, the Sundarbans can be classified as a coastal

plain estuary. The major landforms observed in this coastal zone are creeks,

mangrove swamps, mudflats, salt flats and dune complexes. These tidal

mudflats provide a suitable micro environment for colonization of the

mangrove species, which if allowed grow into dense and tall mangrove plant

communities, providing valuable coastal protection (Selvam 2003). The lower

delta plain is sensitive to change in the balance between hydrodynamics and

riverine input and to change in relative sea level. A variety of dune landscape

types provide protection of hinterland, water storage, biotic resources and

recreation. However, coastal dunes of the western islands are now being

engulfed by the encroaching sea waves due to cyclonic activities.

The alluvial-filled basin of the Hooghly River, i.e., the estuarine

stretch of the Ganges, is entrenched into the deltaic deposits of the combined

Ganges - Brahmaputra River. The depocentre is known as the Bengal Basin

and is flanked by Pleistocene terraces along its border and successively by

older rocks away from the basin border (Sarkar et al 2004). The rock bodies

are susceptible to erosion as favored by both climate and tectonic factors. This

leads to sediment compaction with simultaneous subsidence of the basin

(Goodbred Jr. et al 2003).

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2.4 SOIL TYPE

The sediments of the Sundarbans mangrove forests are different

from the inland soils as they are frequently inundated with saline water. The

“Active Delta” bears the soil types of silty clay loams, sandy clays and loams,

soil with organic and peaty deposits, swampy and marshy soil also called as

‘mangrove soil’ (Mandal and Ghosh 1989; Banerjee 1998). Alluvial soils

along the coast, and especially in the Sundarbans area, show a white coating

of sodium chloride, as they are impregnated with salts by tidal estuaries. The

soil pH averages 8.0 (Christensen 1984).

2.5 HYDROCLIMATIC CONDITION

2.5.1 Seasons and Precipitation

Indian climate is a manifestation of the seasonal change in the

position of the Inter-Tropical Convergence Zone (ITCZ) from about 10°S

over the north of Indian Ocean in January to about 25°N, over Asia in July.

The seasonal climate of the Sundarbans can be conveniently categorized into

pre monsoon (February to May), monsoon (June to September) and post

monsoon (October to January). The total annual rainfall is about 1500 - 2000

mm (Gopal and Chauhan 2006). Most of the rainfall (about 74% of the total)

occurs during the Southwest monsoon period (June - September). Some

precipitation is received in the latter half of the hot season and in October.

Cyclonic storms occur sometimes in April but frequently associated with

monsoon rainfall in July - August.

2.5.2 Temperature and Relative Humidity

The Sundarbans experiences subtropical humid climate. The

average maximum temperature is 34°C and average minimum temperature is

49

13.7°C. During pre monsoon the maximum temperature varies between 26°C

to 40°C. The climate is pleasant during the winter and temperature rarely goes

below 10°C. Relative humidity is over 80% from June - September and over

75% from October - May (Sen and Naskar 2003).

2.5.3 Wind

Two dominant prevailing winds during Southwest and Northeast

monsoons lead to the Southwesterly and the Northeasterly wind (Sen and

Naskar 2003). The normal maximum wind velocity is 16.7 - 50 kms hr-1

(April - June) and minimum wind velocity is 10.7 - 11.8 km hr-1

(December -

February).

2.5.4 Tidal Waves and Tidal Amplitude

The mean elevation of the Indian Sundarbans above sea level is

about 3.30 m and the mean highest high water (MHHW) and mean lowest

high water (MLHW) levels are 5.94 and 0.94 m, respectively (Untawale

1987). The average rise and fall of tide is high in the western rivers than in the

eastern. There is a great natural depression called “Swatch of No Ground” in

the Bay of Bengal, south of the Raimangal estuary between 21° to 21°22´

latitude. The surrounding water depth is around 20 m, but changes almost

suddenly to 500 m in depth (Bose 2004).

The Sundarbans experiences two flood tides (inflow) and two ebb

tides (outflow) within 24 hours, which are macro tidal in nature and tidal

range varies from 3 - 5 m rising up to 8 m in normal spring tide (Mandal and

Ghosh 1989; Banerjee 1998). Mean current velocities are between 117 and

108 cm s−1

during low and high tide. Due to gentle slope of the coast and

large tidal amplitude, tidal water penetrates at an average distance of 110 kms

50

inland from the shoreline and in some areas; the effect of the tides is felt over

300 kms inland (Choudhuri and Choudhury 1994).

2.5.5 Cyclones

The Bay of Bengal is prone to cyclones and storms. On an average

3 - 4 cyclones may occur in a year. Maximum wind velocity during cyclones

ranges from 80 - 140 km h-1

. Cyclones initiate large scale littoral drift, severe

coastal erosion and underwater accumulation of eroded material apart from

causing heavy loss of life and property. From the year 1891 to 1989, there

have been about 442 severe and small-scale cyclones (CPCB 1999), strong

wind or tidal surges partly or fully devastating the Sundarbans ecosystem

again and again, and the human settlement suffered the most.

2.6 RIVERINE SYSTEM

The entire Sundarbans area is intersected by an intricate network of

interconnecting waterways, of which the larger channel is about 1.6 km or

more in width and run in a north-south direction and is influenced by tides

from the Bay of Bengal (WCMC 1992). The River Hooghly (Ganges) in the

extreme west is the only river carrying freshwater from upstream of lower

Gangetic delta in the Indian Sundarbans. While most of the other estuarine

rivers (Muriganga or Baratala River, Saptamukhi River, Thakuran River,

Matla River, Goasaba River and Raimangal / Harinbari / Herobhanga River),

have lost their upstream connections with the Ganges since 12th

century due to

the neo-tectonic movement of the river Ganges towards east (Morgan and

McIntire 1959) and massive siltation (Gopal and Chauhan 2006; Mitra et al

2009). Thus, the delta-building process has nearly ceased in the west, but has

accelerated in the eastern part. Besides these major rivers and distributaries,

several other minor but important rivers also exist namely Bidya, Raimangal,

51

Ichhamati, Jhilla, Gomor and Panchamukhani. There are numerous islands in

the Sundarbans; important ones from west to east are Sagar, Fraserganj,

Lothian, Bulcherry and Halliday Islands (Banerjee 1998).

2.7 BIODIVERSITY

2.7.1 Flora

The Sundarbans mangrove has the highest number of mangrove and

swamp flora (Naskar and Guha-Bakshi 1987; Naskar and Mandal 1999). The

name Sundarbans derives its origin from dominance of the tree species

Heritiera fomes, locally known as 'Sundari' because of its elegance (Jain and

Sastry 1983). Other characteristic mangrove species include Rhizophora sp.

(Garjan), Bruguiera gymnorhiza (Kankara), Ceriops sp. (Goran), and

Avicennia officinalis (Baen). Heritiera fomes are scattered over areas of

higher elevation, along with Sonneratia apetala (Keora), Excoecaria

agallocha (Gengwa), Carapa obovata (Dhundul) and Phoenix paludosa

(Hental). Nypha fruticans (Golpata palm) is relatively uncommon but occurs

on wet mud-banks along the creeks. Avicennia and Ceriops are the

commonest trees, occupying extensive areas but only growing up to 2 m

(Chaudhari and Chaudhary 1994). Sand dunes are partially covered with

spear-grass, behind which are creepers and shrubs or trees, such as Tamarix

troupii (Jhao), Erythrina variegata (Palita) and Aegiceras corniculatus

(Kulsi).

2.7.2 Fauna

The Sundarbans is a home to a variety of faunal species. The total

of 1586 species has been recorded, which includes 481 vertebrate species,

1 Hemi-Chordate, 1104 Invertebrate and 106 Protozoans. The Sundarbans

mangrove forest is a unique habitat of euryhaline / brackish water algae,

52

shell-fish and fin-fish species, prawns, crustaceans, estuarine crabs, ghost

shrimps, mollusks, nematodes, annelids, animals like various types of jungle

cat, the deer, rhesus monkey, wild pig, otter, water monitor, various snakes,

including python, estuarine crocodiles, sharks, dolphins, large variety of local

birds and migratory pelicans (Banerjee 1998). The biodiversity is, however,

under threat owing to man - mangrove conflict and the increasing salinity

(Sen and Naskar 2003). The Sundarbans mangrove, the only mangrove chunk

of the planet serving as the home of Royal Bengal Tiger (Panthera tigris

tigris), according to the 2011 tiger census, about 270 Bengal tigers left over in

this forest (www.bforest.gov.bd/highlights.php).

2.8 THREATS TO THE SUNDARBANS MANGROVE

ECOSYSTEM

2.8.1 Evolutionary History and Hydromorphology

The Sundarbans mangrove is in crisis for a number of reasons,

ranging from the evolutionary history and paleoclimate to the current

biophysical and anthropogenic factors, including global climate change. In the

Sundarbans the interplay of hydrology, geomorphology and chemistry is most

pronounced. The Sundarbans mangrove ecosystem is the region of transition

between the fresh water of the Ganges distributaries system and the saline

water of the Bay of Bengal. The river system brings a huge sediment loads

which is deposited at the delta. Increased siltation in the riverine flow is the

major problem in the Sundarbans mangrove area.

2.8.2 Climate Change and Sea-Level-Rise (SLR)

Climate change components that affect mangroves include changes

in sea-level, precipitation, temperature, atmospheric CO2 concentration, ocean

circulation and high-frequency events - like cyclones. According to Hazra

53

et al (2010), future sea-level-rise will make around 10 Sundarbans islands

(Sagar, Ghoramara, Jambudip, Namkhana, Mousuni, Dakshin Surendranagar,

Dhanchi, Dalhousi, Bulchery and Bhangaduani) vulnerable, which constitutes

68% of the lost land to the sea. There is a rising trend of sea-surface

temperature (SST) at a rate of 0.0453°C y-1

and which almost double the rate

observed (0.019°C y-1

) during the period between 1985 and 2000 (Hazra et al

2010). It is also evident from different studies that the monsoonal rainfall has

significantly increased at the rate of 0.0041 mm y-1

, along with SST.

According to a study, severe cyclonic storms over Bay of Bengal registered

26% increase over the last 120 years (Gurmeet Singh 2009). These all events

have been created an alarming situation (coastal erosion, decreased growth of

mangroves, biodiversity loss and shift of water carbonate chemistry) for the

Sundarbans in the perspective of climate change. Rate of coastal erosion in

the Indian Sundarbans has been measured about 5.5 km2 y

-1 during 2001-2009

(Hazra et al 2010), and eventually it is dominant in the south-western edges of

individual islands.

2.8.3 Population

Approximately, 2.5 million people lived in small villages

surrounding the Sundarbans in 1981 which by 1991 had increased to 3 million

(UNESCO 2002). About 95% of the population here depends on agriculture,

practicing rain fed mono-cropping. At times when there are lean agricultural

benefits, these people exploit mangrove forest wealth by engaging in fishing,

collection of wild prawn seeds, honey, timber and Non Timber Forest

Products (NTFPs) by risking their life with man eating tigers and crocodiles.

Land reclamation by locals for agriculture and human settlement destroying

the forest area, construction of series of irrigation and drainage canals over

centuries interfering in the natural gradients, setting up of fisheries in the

rivers, canals, creeks and estuaries, raising embankments along the major

54

river systems against an insurgence of saline water, etc. deteriorates this

sensitive mangrove ecosystem (Banerjee 1998; Gopal and Chauhan 2006).

The gradual extinction of forest has given rise to less resistance of the land

from the ravages of cyclonic storm and soil erosion. During the last two

centuries, more than 5000 km2 of the mangrove forests in the Indian part of

Sundarbans has been reclaimed (Naskar 2004). Actually at present, only the

eastern part of river Matla now exhibits the true core area of the mangrove

forest. Apart from the waste generated by the local inhabitants of the

Sundarbans, untreated domestic waste of 397 tons per day reaches the

Sundarbans area from Kolkata metropolis, which is about 130 kms away.

Gradual decrease in water quality is also causing an environmental threat to

the system. The Sundarbans also suffers from environmental degradation due

to enhanced tourism and port activities, operation of excessive mechanized

boats, along with increasing agricultural and aquacultural practices

(Bhattacharya 2008).

2.9 THE HOOGHLY ESTUARINE SYSTEM

The Hooghly estuary is one of the most important estuaries of India

and the foremost along the east coast. The Ganges in terms of freshwater

discharge is the seventh largest on a global scale. The Hooghly estuary, the

first deltaic offshoot of the Ganges is located along Latitude 21°40' N and

Longitude 87°47' E (Figure 2.1). It is also known as the Hooghly - Matla

estuarine system lies in the Indian state of West Bengal. The Hooghly River

having a length of 260 kms; after traversing a considerable distance inland,

passes through the city of Kolkata, which has a riverine port, located on its

banks. Further down, the estuary passes through Diamond Harbour from

where it fans out to about 4 kms into a funnel-shaped water body which

spreads out to more than 20 kms before merging into the Bay of Bengal. It

serves as a navigable waterway to Haldia Port and also Kolkata port

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(Estuaries of India 2002). The offshore region of the estuary extends to a

distance of 50 miles south of the Sagar Island and merges to the Bengal fan.

The Bengal Fan is the largest submarine fan in the world, with a length of

about 3000 kms, a width of about 1000 kms and a maximum thickness of

16.5 km (Curray et al 2003). It has been formed as a direct result of the India -

Asia's collision and uplift of the Himalayas and the Tibetan Plateau.

2.9.1 Hydrological Conditions

The Hooghly is a globally significant estuary and drains a

catchment of 6 x 104

km2. The depth of the water column varies from

10 - 15 m, with an average 6 m. High values of surface runoff generally have

been recorded between July and September, and its’ values are found to be in

the order of 21.15 km3 in September and 18.7 km

3 in July (Mukhopadhyay et

al 2006). Fresh water discharge of the Hooghly river, varies between 2952 -

11,897 m3 s

-1 during the southwest monsoon and between 900 - 1500 m

3 s

-1 in

the non-monsoonal months (Biswas et al 2007). Normally, the fresh water

discharges are regulated from the Farakka Barrage to maintain water levels at

Kolkata (Biswas 1985).

2.9.2 Tidal Dynamics

The Hooghly estuary is a well-mixed estuary because of shallow

depth (average 6 m) and intense tidal mixing (Sadhuram et al 2005). This

estuary is vertically homogenous throughout the year (Gole and Vaidyaraman

1969). The estuary gets semi-diurnal tides with a maximum range of 5.5 m at

spring and minimum 1.8 m at neap. The tidal limit extends up to 296 kms

upstream and normally remains well-mixed. The flood current varies between

0.5 and 2.0 m s-1

. A combination of tidal forces, relatively shallow depth and

geomorphology of the estuary produces relatively large tides, and it is

56

classified as a “hyper-synchronous macrotidal estuary” according to the

widely accepted estuarine classification scheme of Dyer (1973).

The geology, climate, geomorphology, soil types of the Hooghly

estuary are similar to the adjacent Sundarbans mangrove, which is already

described in the previous sections.

2.10 POLLUTION SCENARIO IN THE HOOGHLY ESTUARY

Over the past decade, many estuarine systems have been affected

by the serious environmental impacts. Although the Hooghly estuary has

large tidal amplitude, considerable mixing, dilution and flushing, a gradual

accumulation of toxic wastes and progressive reduction of the freshwater flow

over a period of years, has resulted in the deterioration of aquatic

environment. The City of Joy (Kolkata) supports a wide spectrum of

industries along with a teeming human population, the entire range of which

largely uses the Hooghly estuary as a waste disposal ‘sink’. According to

several surveys, in a stretch of 92 kms revealed 40 major industries (Jute

mills, Brick Kiln, Pulp and papers, Textile, Distilleries, Tannery, Rubber,

Thermal Power Plants, etc.) which discharge their wastewater into the estuary

either directly or through specially-built drains or through municipal drainage

canals (Figure 2.2). This estuary is under stress due to the construction of the

Farakka Barrage, port activities of Haldia and Kolkata, dredging and

discharge of sewage from Haldia town and Kolkata metropolis.

The estuary lies adjacent to the Sundarbans mangrove, but is

largely separated from it by urbanization and development along its banks.

The exception is the lower part of the estuary which is associated with

mangrove patches on the outskirts of the Sundarbans. This is significant in

terms of this study because it allows a direct comparison between two

57

environments broadly impacted by identical freshwater and marine end

members but with significantly different levels of human impact.

Figure 2.2 Map showing different industries around the Hooghly

estuarine system and the Sundarbans mangrove ecosystem

(Source: Gurmeet Singh 2009)

2.11 IMPORTANCE OF THE STUDY AREA

Globally, the Sundarbans mangrove and the Hooghly estuarine

systems are of great importance because former one is not only a unique

forested region but also home to over four and a half million human

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population, and thus there is a host of development issues, which need to be

resolved apart from the ecological considerations. The Sundarbans is an

example of an endangered ecological system that is highly populated and both

fragile and economically valuable. The Sundarbans protects the metropolis

Kolkata from the frequent gales originating from the Bay of Bengal.

Anthropogenic inputs as well as the changing natural conditions (change in

riverine flow, increase in salinity, sea-level rise, etc.) have led to a dire need

of sustainable management of the ecosystem. The Hooghly estuary is highly

dynamic in terms of suspended load, erosion and deposition resulting in

problems of safe and assured navigation throughout the year. Despite a

number of contemporary studies focusing on pollution and other ecological

aspects, there has been a complete lack of study on the role of buffer zones,

which helps in maintaining the water quality and prevents siltation. In view of

the present scenario, buffer zones may play an important role in governing the

nutrient biogeochemistry in the Sundarbans mangrove.

Ecosystem resilience provides insurance to societies through

ecological stability, in terms of sustaining a flow of ecological goods and

services (Costanza et al 2000). Knowledge of these factors and functional

groups are important in predicting mangrove resilience and ecological

stability. The objective of this study was to have a holistic approach in order

to understand these ecosystems with all its potentials for conserving it as it

was gifted to us to benefit from it, preserve it and pass on to future

generations.