Continental Shelf part of the sea floor adjoining a landmass over which the maximum depth of sea water is 200m. Continental shelves are regarded as portions of continental masses which are locally submerged. They are very flat, with gradient less than 1:500. Some authors prefer the term “continental terrace”. Shelf widths average about 65 km, varying from only a few tens of meters across in some areas to more than 100 km in others. Most shelves still show the features they acquired when exposed as part of the landmass, such as the scars of old river beds and glaciers formed 15,000 years ago when the sea level stood about 120m lower than it is today. The shallow waters over the continental shelf differ from open-ocean waters. They receive large supplies of dissolved nutrients from coastal rivers. The sediment load is generally high, and the clarity of coastal waters is less than that of open-ocean water. It is inhabited by sea weeds of many kinds, as well as numerous animals: corals, sea anemones, and other coelenterates; many species of burrowing worms and minute colonial rock-encrusting animals and clams, mussels, oysters and other molluscs. There are also sea urchins, starfish, brittle stars, sea cucumbers and sponges as well as bottom dwelling fish.
The width of the continental shelf off the coast of Bangladesh varies considerably. It is less than 100 km off the south coast of Bangladesh, between Hiron Point and the swatch of no ground and more than 250 km off the coast of Cox's Bazar. Seabed evidence suggests that the dominant transport of fine-grained sediment on the continental shelf of Bangladesh is from south and west. sediments are fine seaward and westward with the thickest accumulation of mud near the submarine canyon, Swatch of No Ground. Most of the continental shelf of Bangladesh is covered by silt and clay. 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. Sand waves observed on the continental shelf in this area have considerable relief (3-5m), implying a high-energy environment. Even the shallower part of the 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. This indicates that even under present oceanographic conditions, sediments are being tunnelled to the deeper part of the bay of bengal through the Swatch of No Ground. [Mahmood Alam]