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Bay of Bengal


Bay of Bengal a northern extended arm of the indian ocean, is located between latitudes 5°N and 22°N and longitudes 80°E and 100°E. It is bounded in the west by the east coasts of Sri Lanka and India, on the north by the deltaic region of the Ganges-Brahmaputra-Meghna river system, and on the east by the Myanmar peninsula extended up to the Andaman-Nicobar ridges. The southern boundary of the Bay is approximately along the line drawn from Dondra Head in the south of Sri Lanka to the north tip of Sumatra. The Bay occupies an area of about 2.2 million sq km and the average depth is 2,600m with a maximum depth of 5,258m. Bangladesh is situated at the head of the Bay of Bengal.

Bottom topography characterised by a broad U-shaped basin with its south opening to the Indian Ocean. A thick uniform abyssal plain occupies almost the entire Bay of Bengal gently sloping southward at an angle of 8°-10°. In many places underwater valleys dissect this plain mass.

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Continental Shelf the width of the continental shelf off the coast of Bangladesh varies considerably. It is less than 100 km off the south coast between Hiron Point and the swatch of no ground and more than 250 km off the coast of cox's bazar. sediments are fine seaward and westward with the thickest accumulation of mud near the submarine canyon, the Swatch of no Ground. The shallow part (less than 20m) of the continental shelf off the coast of chittagong and teknaf is covered by sand and the intertidal areas show well-developed sandy beaches. The shallower part of southern continental shelf off the coast of the sundarbans, patuakhali and noakhali is covered by silt and clay; and extensive muddy tidal flats are developed along the shoreline. Some of the shoals and sand ridges present on this part of the continental shelf show an elongation pattern pointed towards the Swatch of no Ground.

Swatch of no Ground also known as Ganga Trough. Swatch of no Ground has a comparatively flat floor 5 to 7 km wide and walls of about 12° inclination. At the edge of the shelf, depths in the trough are about 1,200m. The Swatch of no Ground has a seaward continuation for almost 2,000 km down the Bay of Bengal in the form of fan valleys with levees. The sandbars and ridges near the mouth of the ganges-brahmaputra delta pointing toward the Swatch of no Ground showing sediments are tunnelled through this trough into the deeper part of the Bay of Bengal. The Swatch of no Ground is feeding the Bengal Deep Sea Fan by turbidity currents.

Sunda Trench' also known as Java Trench. Running parallel along the west side of the arc of the Nicobar and Andaman islands it is extended northward up to 10°N into the Bay and joins the eastern limit of the Himalayan range. It originated tectonically at the junction of the Indian and Myanmar plates.

Ninety East Ridge major feature of the Indian Ocean which runs in a north-south direction approximately along the longitude 90°E. It lies at the immediate outboard of the Sunda Trench between the Bengal Fan and the Nicobar Fa n. The Ninety East Ridge has existed since early in the formation of the Bay of Bengal. The ridge represents the trace of a hot spot formed during the northward flight of India and its associated oceanic lithosphere of the Bay of Bengal.

Eighty-five Ridge' a ridge along 85°E longitude. More than 5 km thick sediments have been deposited on either sides of the ridge. The main turbidity current channel of the subaerial drainage pattern lies immediately east of the buried ridge.

Bengal Deep Sea Fan the world's largest submarine fan, also known as Bengal Fan. Together with its eastern lobe, the Nicobar fan, it covers an area of 3106 sq km. It is 2,800 to 3,000 km long, 830 to 1,430 km wide and more than 16 km thick beneath the northern Bay of Bengal. Sediments are tunnelled to the fan via a delta-front trough, the Swatch of no Ground. It can be divided into three parts: upper fan, middle fan and lower fan. Rapid terrigenous sedimentation on an incipient Bengal fan began in the Eocene age (58 to 37 million years ago) as a response to the first intraplate collision and continued to the present, building the world's largest submarine fan. [Mahmood Alam]

Geographical characteristics Hydrological conditions surface hydrology of the Bay of Bengal is basically determined by the monsoon winds and to some extent by the hydrological characteristics of the open part of the Indian Ocean. Fresh water from the rivers largely influences the coastal northern part of the Bay. The rivers of Bangladesh discharge the vast amount of 1,222 million cubic metres of fresh water (excluding evaporation, deep percolation losses and evapotranspiration) into the Bay. The temperature, salinity and density of the water of the southern part of the Bay of Bengal is, almost the same as in the open part of the ocean. In the coastal region of the Bay and in the northeastern part of the Andaman Sea where a significant influence of river water is present, the temperature and salinity are seen to be different from the open part of the Bay. The waves and ripples entering from the southern part of the Bay provide the energy for mixing the water and consequently bring uniformity in its chemical and physical properties. Tidal action is also very great in the shallow coastal zones.

Temperature the mean annual temperature of the surface water is about 28°C. The maximum temperature is observed in May (30°C) and the minimum (25°C) in January-February. But the annual variation in temperature is not great, about 2°C in the south and 5°C in the north.

Salinity the surface salinity in the open part of the Bay oscillates from 32% to 34.5% (parts per thousand, ie grams per kilogram of sea water) and in the coastal region varies from 10% to 25%. But at the river mouths, the surface salinity decreases to 5% or even less. The coastal water is significantly diluted throughout the year, although the river water is greatly reduced during winter. Along the coast of the Ganges-Brahmaputra Delta, salinity decreases to 1% during summer and increases up to 15% to 20% in winter. Salinity gradually increases from the coast towards the open part of the Bay. The surface salinity at the mouths of some large rivers like the ganges, brahmaputra, Irrawaddy and some Indian rivers like the Krishna, Godavari, Cauvery and Mahanadi varies widely from one day to another, especially in summer. Salinity of water also changes vertically. The influence of the fresh water is experienced up to depths of 200-300m. From the surface, the salinity gradually increases downward and at about 200-300m it reaches 35% and at about 500m the salinity is more than 35.10%, but at 1,000m it decreases slightly and attains 34.95%. With further increase of depth salinity decreases and at 4,500m it is close to 34.7%.

Tides the semi-diurnal type of tides, ie two high and two low tides during the period of 24 hours and 52 minutes. The highest tide is seen where the influence of bottom relief and the configuration of the coast are prominent, ie in shallow water and in the Bay and estuary. The average height of tidal waves at the coast of Sri Lanka is 0.7m and in the deltaic coast of the Ganges it is 4.71m. In the Bay of Bengal tidal currents specially develop in the mouths of the rivers, like the Hooghly and the meghna.

Colour and water transparency the colour of the water in the open part of the Bay is dark blue which gradually changes to light blue to greenish towards the coast. Transparency is great, 40-50m in some places. In the central part of the Bay of Bengal, the anticyclone circulation is generated and in the centre of this lies the zone of convergence. This region is characterised as a rule by high transparency of water. Regions of low transparency and turbid water are available in the limited area of the pre-deltaic part of the rivers Ganges and Brahmaputra.

Sea Level due to the influence of water density and wind the seasonal changes of the sea level in the Bay are remarkable and one of the highest in the world. The range at Khidirpur is 166 cm, at Kolkata 130 cm and at Chittagong 118 cm. But towards the southwestern coast at Madras and Vishakhapatnam' [Vishakhapatnam] the range is small compared to the northern and northeastern coasts of the Bay. The lowest variation of sea level at the southeastern coast of India is due to its geographical location at the edge of a comparatively deep sea.

Ocean Current surface circulation is found to be generally clockwise from January to July and counter-clockwise from August to December, in accordance with the reversible monsoon wind systems. The flow is not constant and depends on the strength and duration of the winds. The effects of a strong wind blowing for a few consecutive days are reflected in the rate of flow. Currents to the northeast generally persist longer and flow at greater speed because of the stronger southwest monsoons. An important vertical circulation in the Bay of Bengal is up-welling. In this process, sub-surface water is brought toward the surface, and conversely a downward displacement is called down-welling or sinking.

Up-welling and down-welling are seasonal, being created by monsoon winds that blow from the southwest during the summer, then reverse direction and come from the northeast during the winter. The persistence of the monsoon, especially from the southwest and the orientation of the coasts cause up-welling to occur along most of the east coast of India. That is why in the east coast of India the up-welling takes place in summer and down welling in winter, and in the eastern part of the Bay of Bengal and in the Myanmar coast, up-welling occurs in winter and the down-welling in summer. However, the duration and intensity of vertical movement of water on both sides of the Bay of Bengal is not as great as on the Somali or North and South American coasts. But it does have a profound effect on the food economy of the sea through its influence on chemical properties and biological populations. [Subash Chandra Das]

Biological characteristics the occurrence of marine species - both plants and animals - has largely been controlled by the physico-chemical properties of ocean water. Water discharges from the surrounding river catchments carry huge influx of sediments full of nutrients to the Bay, particularly along the near shore region. This has turned the Bay into a fertile marine fishing ground of the region. The near-shore up-welling zone not only has a high yield of nutrients, but also is a high primary production area for the phytoplankton and related zooplankton zones.

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Fishing the hydrological conditions of the Bay of Bengal is favourable for a variety of shrimps and fishes. Although fishes remain scattered in the Bay in some places they get concentrated and constitute important fishing grounds. Four fishing grounds have been identified so far. They are south patches, south of south patches, middle ground and Swatch of no Ground.

South patches located at 91.30°E to 92.10°E and 20.55°S to 21.52°S, having a total area of 3,662 sq km. Depth ranging from 10m to 100m, but 90% of the total area is less than 40m deep. Bottom sediment is sandy or slightly muddy sand. Nearest distance of the ground from Chittagong and Cox's Bazar is 40 km and 10 km respectively. Salinity in surface water ranges from 26% to 32% and 30% to 35% in bottom water. Water temperature varies between 20° and 28°C.

South of south patches located at 91.30°E to 92.20°E and 20.15°S 20.50°S, having an area of 2,538 sq km. The nearest boundary of this area is 5 km from Teknaf. Depth ranges from 10m to 100m. Within this ground 75% of the area is more than 40m deep. Bottom is sandy or muddy sand. Surface salinity ranges from 18% to 34% and bottom water salinity from 28% to 38%. Water temperature ranges between 22°C and 30°C.

Middle ground located at 90.20°E to 91.30°E and 20°25'S to 21.20°S, having a total area of about 4,600 sq km. The nearest distance from Cox's Bazar is about 65 km. The depth of 70% of the total area is more than 40m. Bottom sediment is soft mud or muddy sand. Surface salinity ranges from 22% to 34% and bottom salinity 28% to 35%. Water temperature is between 26'C and 28'C.

Swatch of no Ground located at 89.35°E to 90.10°E and 20.55°S to 21.55°S, about 30 km away from Dublarchar and 40 km from Sunarchar. Total area is about 3,800 sq km, of which 70% is more than 40m deep. Overall depth of the area ranges from 10m to 100m. Bottom sediment consists of muddy sand. Surface salinity is 28% to 34%, while the bottom salinity is 30% to 35%. Water temperature falls within 24°C to 30°C.

All these fishing grounds are potential reserves for fish and shrimp. Most of the known commercial species of shrimps and fishes are harvested from these areas by trawlers or mechanized fishing boats. Commercially important shrimp and fish species include tiger shrimp, karuma shrimp, cat fish, Bombay duck, snapper, flounder, Indian salmon, crocker, seabream, jawfish, mullet, pomfret, ribbon fish, anchovy, hilsa, oil sardine, tuna, mackerel and skipjack. [Hossain Zamal]

Pollution marine environment of Bangladesh is directly or indirectly becoming polluted due to addition of so many polluting agents. To protect water pollution and control of mine drainage, large scale drilling and production of natural gas from the Sangu offshore gasfield, discovery of Kutubdia offshore gasfield and exploration of beach sand placer deposits from the offshore islands of Chittagong and Cox's Bazar districts must follow standard regulations both from mine safety and environmental protection point of view. Sangu gasfield is located about 50 km southwest of Chittagong City and stands at a depth of 10m water in the Bay of Bengal. Kutubdia gasfield is about 92 km southwest of chittagong port.

The coastal environment of Bangladesh is contaminated by oil tanker traffic, harbour operations and effluents discharged from petroleum processing. sewage disposal by the karnafuli and pasur rivers into the Bay of Bengal contains higher concentration of Biochemical Oxygen Demand (BOD) and foecal coliform. Moreover, the coastal environment of Bangladesh has also been affected by industrial effluents, agricultural residues, some other human activities like deforestation and irrational expansion of coastal shrimp farming, etc resulting in ecological degradation. Overfishing and dumping of discarded fishes in these areas are becoming a threat to the resourcefulness of the fishing grounds. Only proper management of the existing grounds and finding of new fishing grounds can ensure the continuous harvest of fisheries resources. [Sifatul Quader Chowdhury]