Physiographic regions in Bangladesh
Bangladesh is situated in the Bay of Bengal in close proximity to India, Bhutan, Burma and Nepal . The majority of the country consists of large alluvial river plains formed by the Brahmaputra, Ganges and Meghna rivers. Bangladesh covers an area of 143,998 km2 and can be generally categorised as having a humid tropical climate with the south eastern corner having a tropical rainforest climate. The Brahmaputra River is the largest sand-bedded braided river in the world in terms of catchment area, discharge and sediment load. Its catchment area covers 666 000 km2, incorporating areas of Tibet, Bhutan, China, India and Bangladesh (figure). The catchment area receives intense rainfall during the summer monsoon which together with snowmelt in the Himalayas contributes to a mean peak annual discharge of 65,500 m3/s.
The Ganges and Brahmaputra Rivers
The Ganges and Brahmaputra Rivers combined have formed one of the largest deltas in the world, comprising approximately 105 640 km2. The Bengal Basin, into which this delta has prograded, is bordered on the west and northwest by Lower Jurassic volcanics and on the east by Eocene sandstones and limestones. The southern boundary is the Bay of Bengal. The Ganges River originates near the Tibet/India border, and then flows southeast across India to combine with the Brahmaputra in the country of Bangladesh. The Brahmaputra River has its source in Tibet along the northern slope of the Himalayas, and flows across Assam into Bangladesh. The drainage basin, approximately 1.6 million km2 in area, is geologically young, with large volumes of unconsolidated sediment available for transport (Morgan and McIntire, 1957, 1959).
The Ganges is primarily a meandering river, while the Brahmaputra is primarily a braided channel (Fig.). Their combined discharge into the Bay of Bengal is approximately 82 000 m3/sec during flood. Sediment load is extremely high, with suspended sediment load during flood stage reaching as high as 13 million tons per day (Coleman, 1969).
In the northwest part of the image, the morphology of the delta plain is dominated by the abandoned channel scars of former river courses. These abandoned courses display evidence of both migratory braided and meander channel scars. These surfaces form the bulk of the rice-and jute- farming areas of Bangladesh.
The Bengal coast is mesotidal, with an average tidal range of 2 m. Wave energy modifying the coast is relatively low because of the extremely low offshore gradients that front the active delta. The mosaic covers the eastern active deltaic part of the Ganges/Brahmaputra Rivers and illustrates those landforms that are characteristic of a rapidly migrating channel system partially modified by tidal processes. In the southeastern part of Bangladesh, the delta surface elevation becomes low enough to be inundated by tidal waters, and much of this area displays tidal plain morphology.
Two of the world's largest rivers, the Brahmaputra and the Ganges confluence in Bangladesh, these together with numerous other rivers interlace Bangladesh with over 24000 km of river channels. During the annual monsoon the catchment areas for the Brahmaputra and Ganges, 90% of which lie outside of Bangladesh, receive very intense rainfall. Despite the extensive river system, consisting of approximately 230 significant rivers, the rainfall is not effectively drained through Bangladesh and severe overland flooding usually occurs.
Spate (1954) was the first author to delineate physiographic regions in Bangladesh, he outlined five physiographic regions in the Bengal basin, three of which fell in Bangladesh. Johnson (1957) took the regions outlined by Spate (1954) and further redefined five physiographic regions in Bangladesh, with twelve sub divisions. Johnson (1957) excluded the mountains of the Sylhet and Chittagong regions. Johnson's physiographic map contained several errors (Rashid, 1991), in particular in the exact delineation of the Barind Tract. However, Johnson (1957) was the first to recognise the significance of the clay plain, now known as the Tippera Surface. Morgan and McIntire (1959) went on to further sub-divided the Barind Tract and outlined the piedmont nature of the alluvial plains to the north. Rashid (1991) refined the previous definitions based on topographic features, drainage patterns, soil associations, morphologies and land use. Rashid (1991) identified 24 physiographic regions in Bangladesh (figure ).
(1) Himalayan Piedmont Plains:
The Himalayan Piedmont Plains are the alluvial cones of the rivers originating in the Terai region of the Himalayan foothills. The region is bounded by the Mahananda River in the west and the Dinajpur-Karatoa River in the east. The rivers in this region are entrenched in recent alluvial deposits of fine sand and silts with gradients of approximately 0.00091. The alluvial deposits in the south of the Himalayan Peidmont Plains overlay Pleistocene clays of the Barind Tract.
(2) Tista Floodplain:
The Tista Floodplain covers a large area from the high sandy levees of the Dinajpur-Karatoa River to the right bank of the Brahmaputra River. To the South, the Tista floodplain reaches down to Bogra along the course of the ancient Tista River. The floodplain is cut across by the Tista, Dharla and Dudkumar rivers.
(3) Barind Tract:
The Barind Tract forms one of the many terraces within the Bengal Basin of Pleistocence age. Three rivers have cut valleys into the Barind Tract and separate it into four parts. The Barind Tract is characterised by its relatively high elevation, entrenched dedritic stream pattern and scarcity of vegetation.
(4) Little Jamuna Floodplain:
The little Jamuna is a former path of the Tista River. Its valley is very narrow in the region of Dinajpur but widens south of Hili. The recent alluvial deposits of sandy-silt contrast with the clay deposits of the Barind Tract. The valley terminates in south Naogaon Upazila.
(5) Middle Atrai Floodplain:
The Middle Atrai floodplain is a 81 km long valley with the Barind tract rising on both sides. It stretches from Chirirbandar to Mahadebpur. The lower areas of the Middle Atrai floodplain are subject to flash flooding. The Atrai River is entrenched into the clay deposits of the Barind tract, whilst its floodplain consists of sandy material.
(6) Lower Purnabhaba Valley:
The Lower Purnabhaba valley is 81 km in length, beginning south of Dinajpur town in India and finishing where the Purnabhaba and the Mahananda rivers confluence. The valley is on average 3 to 8 km in width with the higher Barind tract on either side giving the valley an entrenched appearance. The valley has very poor drainage and is known locally as duba (swampy).
(7) Lower Atrai Basin:
The Lower Atrai Basin has an approximate area of 3120 km2. The entire basin is inundated during the rainy season with a depth of water of between 0.6 to 3.7 m. The western part of the basin is aggrading with silt from the Barind tract.
(8) Lower Mahananda Floodplain:
The Lower Mahananda Floodplain has an area of 402 km2 and lies between the Barind and the Ganges floodplain. The Mahananda River, which is slightly entrenched, forms the western boundary of Bangladesh along the Piedmont plain in Dinajpur district. It crosses the border of Bangladesh in Gomastapur Upazila before confluencing with the Ganges south of Chapai-Nawabganj town.
(9) Ganges Floodplain:
Parts of the Ganges floodplain to the south of the Ganges River are considered by Rashid (1991) to be part of the delta. The north Ganges floodplain is an elevated area that stretches from Premtali in Godagari Upazila to Shujanagar Upazila. The southern most portion of the north Ganges floodplain forms a levee that in places follows the course of the Ganges River, the land is characterised by saucer-shaped basins, old river levees and point bars. As the levee has built up this area has become progressively more arid.
(10) Brahmaputra-Jamuna Floodplain: The right bank floodplain of the Jamuna River was once a part of the Tista floodplain (region 2). On average the right bank floodplain is flooded for 3-4 months of each year, during the peak of the annual monsoon. The most notable distributary of the Jamuna on the right bank is the Bengali River. The Jamuna-Dhaleswari floodplain forms the left bank of the Jamuna. Several distributaries of the Jamuna flow through this region, of which the Dhaleswari is the most significant. The southern part of this region was once part of the Ganges floodplain.
(11) Old Brahmaputra Floodplain:
The Brahmaputra River underwent an avulsion around 1790, and adopted a southern course along the Jamuna River. The old course between Bahadurabad and Bhairab is now known as the Old Brahmaputra and is significantly smaller. The Brahmaputra had built up large levees along the floodplain, which the new river rarely tops. In the north of the Old Brahmaputra floodplain there is a long depression running parallel to the Meghalaya Plateau, to the south the floodplain is level.
(12) Susang Hills and Piedmont:
The Susang hills extend for 161 km from Jamalpur district to Sunamganj district, they include the foothills of the Meghalaya Plateau. Entrenched mountain streams cut through this region depositing sand. The rocks in this region are mostly sandstones and shales of Eocene age, with nummulitic limestone and white clay also prominent. The Piedmont plains further south cover the majority of Nalitabari, Haluaghat and Kalmakanda Upazila. The plains have a low gradient and are generally only mildly flooded during the monsoon season, but are prone to flash flooding.
(13) Madhupur Tract:
The Madhupur Tract is a large Pleistocene inlier, with an area of approximately 2558 km2. This elevated region is tilted towards the south east. The northern area of this region is dominated by its plateau like hills between which run narrow meandering valleys. The central tract area is again characterised by plateau like hills, but with slender tops and deep circular valleys. The southern part of the Madhupur Tract is very flat, with noticeable gradients due to entrenched streams. The flatness of the southern area has been reinforced by artificial levelling in rice fields and around Dhaka.
(14) Haor Basin:
The Haor basin region is characterised by its large number of lakes. It stretches from the Mahadeo and Mogra rivers to the plain of central Sylhet. The Haor basin covers an area of approximately 4505 km2. The region appears to be sinking at a rate of approximately 2cm per year (Rashid, 1991), and has sunk 9 - 12 m over the last several hundred years (Morgan and McIntire, 1959).
(15) Sylhet High Plains
The Sylhet high plains is an elevated region that separates parts of the Haor Basin. This region is characterised by entrenched streams, lakes and water filled depressions that drain out in the early winter. The average elevation of the region is 9m above mean sea level.
(16) Sylhet Hills:
The foothills of the Megahalaya Plateau fall within Bangladesh in the northern part of the Sylhet district. The Megahalaya foothills are composed of sandstones and limestones, sandy shales and pebble beds of the Pliocene. In addition to the Megahalaya foothills there are four groups small hills in northern Sylhet known as the Tila ranges. These Tilas are composed of Pleistocene clays and sands over coarse ferruginous sandstones and Miocene shales.
(17) Meghna Floodplain:
Most of the Meghna Floodplain was built up by the Brahmaputra River when it followed its old course. The Meghna River is still filling in most of the depressions left by the Brahmaputra River.
(18) Tippera Surface: The Tippera Surface is a distinctive physiographic area to the southeast of Dhaka. It is comprised of three sub-regions, the eastern Piedmont strip, the low floodplain and the high floodplain. The eastern Piedmont strip consists of Pleistocene sediments overlain by sandy clays from the Tripura Hills in India. The long low floodplain of the Tippera Surface stretches from Nabinagar to Maijdi in the south and like the high floodplain is almost level. Both the low and high floodplains contain extensive man-made raised areas to protect against flooding.
(19) Moribund Delta:
The Moribund delta is characterised by heavily sediment laden entrenched rivers with low discharge capacities, an abundance of cut-off meanders and large areas of plains high above normal flood levels. The main distributary of the Ganges in this area is the Gorai.
(20) Central Delta Basins:
The Central Delta Basins are an area of approximately 1930 km2 at the junction of the Moribund, Immature and Mature deltas. This area is low-lying, probably due to the absence of active distributaries and hence rapid deposition in this region of the delta coupled with steady subsidence (Rashid, 1991).
(21) Immature Delta:
The Immature Delta lies to the south of the Moribund Delta and covers an area of land of approximately 4800 km2. The maximum elevation of the Immature delta is 0.91 m above sea level, which compares with an elevation of 3m for the southern edge of the Moribund delta. There are two possible explanations for the presence of such a large area of low elevation; insufficient deposition by the Ganges' distributaries which over the last 200 years have tended to follow course more to the east, or subsidence. Archaeological evidence exists to support the theory of large scale subsidence in this region, many artifacts have been found buried in the alluvium well below sea level.
(22) Mature Delta:
According to Rashid(1991), the Mature Delta is composed of four floodplains, the Old Ganges floodplain, the Podda-Madhumati floodplain, the non-saline tidal floodplain and the saline tidal floodplain. The old Ganges floodplain lies to the north of the current course of the Podda River, and receives flood water from both the Podda and Jamuna rivers. The Podda-Madhumati floodplain has a wide range of land levels from higher land in the north-west, which is generally only slightly flooded, to lower land in the south west which experiences more severe flooding. This part of the delta was built up by the Madhumati floodplain when it was the main channel of the Ganges River. When the Ganges River shifted its course this process stopped. The non-saline tidal floodplain is very similar in many respects to the Podda-Madhumati floodplain but with the added affects of tidal currents. The non-saline floodplain is shallowly flooded every monsoon season, with the degree of flooding varying with the tidal conditions. In the saline tidal floodplain region of the Mature delta, the tidal effects are much stronger and deposition is continuing at the mouth of the major rivers.
(23) Active Delta: The Active Delta lies at the mouth of the Meghna River and consists of four large islands; Bhola, Ramgati, Hatiya and Shondip. All of these islands are undergoing relatively fast rates of growth, with new chars continually being attached. The huge amount of sediment carried by the Meghna River shallows the estuary for a considerable distance into the Bay of Bengal.
(24) Chittagong Sub-Region: The Chittagong region of Bangladesh forms a very distinct area that is different in many respects to the rest of the country. It lies south of the Feni River containing many lakes, islands, mountain ranges and forests. It can be further sub-divided into 10 physiographic areas characterising the coastal plains, islands and deltas, the central valley, the western hills and the mountain ranges to the east.