Physiographic units of Bangladesh. Description Physiography is the description including form, substance, arrangement and changes of especially, natural features. The term was introduced in geography in 1869 by Huxley for the study or description of 'natural phenomena in general', but later came to mean, 'a description of the surface features of the earth, as bodies of air, water and land', with an emphasis on mode of origin; ie it became synonymous with physical geography, and embraced geology, pedology, meteorology and oceanography. Still later, the term was restricted to the part of physical geography that is involved with the description and origin of landforms. Physiographic region/unit refers to a region of which all parts are similar in terms of physical characteristics and which consequently had a uniform geomorphic history, and whose pattern of topographical features or landforms differs significantly from that of adjacent regions. With about half of its surface below the 10m contour line, Bangladesh is located at the lowermost reaches of three mighty river systems - the Ganges-Padma River System, Brahmaputra-Jamuna River Systems and Surma-Meghna River System. Coinciding with the division of the country based on altitude and relief, the land can be divided into three major categories of physical units: Tertiary hills, Pleistocene uplands and Recent plains (formed in recent epoch). The heavy monsoon rainfall coupled with the low altitude of major parts of the country make floods an annual phenomenon in Bangladesh. Quaternary (began about 2 million years ago and extends to the present) sediments, deposited mainly by the Ganges, Brahmaputra (Jamuna) and Meghna rivers and their numerous distributaries, cover about three-quarters of Bangladesh. The physiography and the drainage pattern of the vast alluvial plains in the central, northern and western regions have gone under considerable alterations in Recent times. The deposition of quaternary sediments was influenced and controlled by structural activities. The eastward shift of the Ganges and Tista as well as the significant westward shift of the Brahmaputra during the last 200 years gives evidence of epeirogenic movements even in recent days. Hillocks and hills are confined to a narrow strip along the southern spur of the Shillong plateau, to the eastern and southern portions of the Sylhet district, and to the Chittagong Hill Tracts (CHT) in the southeast of the country bordering upon the Indian states of Tripura and Mizoram and Myanmar. In the context of physiography, Bangladesh may be classified into three distinct regions (a) floodplains, (b) terraces, and (c) hills each having distinguishing characteristics of its own. The physiography of the country has been divided into 34 physiographic units and subunits have been recognized. These units have been grouped into 30 AEZ regions. The units are as below: Arial Beel (723 sq km) is a large depression lying between the Ganges and the Dhaleshwari south of Dhaka. Heavy clays occupy almost the whole landscape. Despite the proximity to the two major river channels, the deep seasonal flooding is predominantly by accumulated rainwater which is unable to drain into rivers when they are running at high levels. Much of this unit remains wet in the dry season. Lower Atrai Basin (851 sq km) is a small physiographic unit occupies a low-lying area where mixed sediments from the Atrai and Ganges and from the Barind Tract overlie the down-warped southern edge of the Barind Tract. The landscape north of the Atrai is mainly smooth, but floodplain ridges and extensive basins occur south of the river. Heavy clay soils predominate, but loamy soils occur on ridges in the south and the west. Drainage from this unit is blocked when high river levels in the Jamuna obstruct the exit through the Hurasagar. Seasonal flooding was formerly deep and extensive areas in Chalan Beel used to remain wet throughout the year. The construction of polder projects since the 1960s has improved drainage to some extent. However, deep flooding can still occur within polders as well as outside when there is heavy rainfall locally and when flash floods flow down the Atrai or off the adjoining Barind Tract, causing natural or manmade breaches of embankments. Active Brahmaputra-Jamuna Floodplain (3,190 sq km) is a dual name used for the mighty Brahmaputra river, because the Jamuna channel is comparatively new and this course must be clearly distinguished from that of the older Brahmaputra. Before 1787, the Brahmaputra's course swung east to follow the course of the present old Brahmaputra. In that year, apparently, a severe flood had the effect of turning the course southwards along the Jenai and Konai rivers to form the broad, braided Jamuna channel. The change in course seems to have been completed by 1830. This region comprises the belt of unstable alluvial land along the Brahmaputra-Jamuna rivers where land is constantly being formed and eroded by shifting river channels. It has an irregular relief of broad and narrow ridges and depressions. The area is occupied by sandy and silty alluvium, rich in weatherable K minerals that are slightly alkaline in reaction. High Barind Tract (16 sq km) includes the southwestern part of the Barind Tract where the underlying Madhupur Clay had been uplifted and cut into by deep valleys. The soils include puddled silt loam to silty clay loam in the topsoils and porous silt with mottled plastic clay at varying depth. Deep grey terrace soils and grey valley soils are major components of the general soil types of the area. Level Barind Tract (8 sq km) this region is developed over madhupur clay. The landscape is almost level. The predominant soils have a grey, silty, puddled topsoil with ploughpan. Shallow grey terrace soil and deep grey terrace soils are the major components of general soil types of the area. North Eastern Barind Tract(1,079 sq km) three separate sections of the- Barind Tract are surrounded by Tista deposits. These outliners. differ from the main Tract in having deep redbrown soils. The relief is that of an almost level highland, except around Ahshula Bil where it is irregular. It has silty or loamy topsoil and clay loams to clay subsoil. Old Brahmaputra Floodplain (7,230 sq km) In 1787, a remarkable change in the course of the Brahmaputra took place. In that year, the river shifted from a course around the eastern edge to the western side of the Madhupur Tract. This new portion of the Brahmaputra is named Jamuna. The old course (Old Brahmaputra) between Bahadurabad and Bhairab shrank through silting into a small seasonal Channel only two kilometre wide. The old river had already built up fairly high levees on either side over which the present river rarely spills. The Old Brahmaputra floodplain stretching from the southwestern corner of the Garo Hills along the eastern rim of the Madhupur Tract down to the Meghna exhibits a gentle morphology composed of broad ridges and depressions. The latter are usually flooded to a depth of more than one metre, whereas the ridges are subject to shallow flooding only in the monsoon season. Young Brahmaputra and Jamuna Floodplain (5,924 sq km) A dual name is used for the mighty Brahmaputra, because the Jamuna channel is comparatively new and this course must be clearly distinguished from that of the older Brahmaputra. Before 1787, the Brahmaputra's course swung east to follow the course of the present Old Brahmaputra. In that year, apparently, a severe flood had the effect of turning the course southwards along the Jenai and Konai rivers to form the broad, braided Jamuna channel. The change in course seems to have been completed by 1830. Due to the upliftment of the two large Pleistocene blocks of Barind and Madhupur, the zone of subsidence between those turned to a rift valley and became the new course of the Brahmaputra and came to be known as the great Jamuna. Both the left and right banks of the river are included in this sub-region. The Brahmaputra-Jamuna floodplain can again be subdivided into the Bangali-Karatoya floodplain, Jamuna-Dhaleshwari floodplain, and diyaras and chars. The right-bank of the Jamuna - once a part of the Tista floodplain is part of the bigger floodplain. Several distributaries of the Jamuna flow through the left-bank floodplain, of which the Dhaleshwari is by far the largest and sub-classed, namely, the Jamuna-Dhaleshwari floodplain. The southern part of this sub-region was once a part of the Ganges floodplain. Along the Brahmaputra-Jamuna, as along the Ganges, there are many diyaras and chars. In fact, there are more of them along this channel than in any other river in Bangladesh. There is a continuous line of chars from where this river enters Bangladesh to the off-take point of Dhaleshwari river. Both banks are punctuated by a profusion of diyaras. The soil and topography of chars and diyaras vary considerably. Some of the largest ones have point bars and swales. The elevation between the lowest and highest points of these accretions may be as much as 5m. The difference between them and the higher levees on either bank can be up to 6m. Some of the ridges are shallowly flooded but most of the ridges and all the basins of this floodplain region are flooded more than 0.91 m deep for about four months (mid-June to mid-October) during the monsoon. Chittagong Coastal Plain (3,720 sq km) the plain along the coast extends from the Feni river to the mouth of the Matamuhuri delta, a distance of 121 km. It comprises gently sloping piedmont plains near the hills, river floodplains alongside the Feni, Karnafuli, Halda and other rivers, tidal floodplains along the lower courses of these rivers, a small area of a young estuarine floodplain in the north, adjoining sub-regional young Meghna estuarine floodplain, and sandy beach ridges adjoining the coast in the south. Sediments near the hills are mainly silty, locally sandy, with clays more extensive in floodplain basins. The whole of the mainland area is subjected to flash floods. Flooding is mainly shallow and fluctuates in depth with the tide (except where this is prevented by river or coastal embankments). The average daily rise in the tide is about two metres. Some soils on tidal and estuarine floodplains become saline in the dry season. St Martin's Island (8 sq km) this small but distinctive region occupies the whole of st martin's island in the extreme south of the country. The area has very gently undulating old beach ridges and inter-ridge depressions, surrounded by sandy beaches. The soils are developed entirely on old and young coral beach sands. Calcareous alluvium is the only general soil type of the area. General fertility level is low with poor moisture holding capacity. Active Ganges Floodplain (3,334 sq km) this region occupies unstable alluvial land within and adjoining ganges river. It has irregular relief of broad and narrow ridges and depressions interrupted by cut-off channels and active channels. The area has complex mixtures of calcareous sandy, silty and clayey alluvium. Gopalganj-Khulna Beels (2,247 sq km) occupies a number of low-lying areas between the Ganges river Floodplain and the Ganges tidal floodplain. The major two beels of the area are Baghia Beel and Chanda Beel. Thick deposits of peat occupy perennially wet basins, but they are covered by clay around the edges and by calcareous silty sediments alongside the Ganges distributaries crossing the area. This is the largest peat stock basin of Bangladesh. The basins are deeply flooded by clear rainwater during the monsoon. In the basin close to Khulna, the floodwater is somewhat brackish. Subsiding process is still active in this physiographic unit. High Ganges River Floodplain (13,205 sq km) comprises the active floodplain of the Ganges and the adjoining meander floodplain. The latter mainly comprises a smooth landscape of ridges, basins and old channels. The relief is locally irregular alongside the present and former river courses, especially in the west, comprising a rapidly alternating series of linear low ridges and depressions. The Ganges channel is constantly shifting within its active floodplain, eroding and depositing large areas of new char land each flood season, but it is less braided than that of the Brahmaputra-Jamuna. Ganges alluvium is calcareous when deposited, but most basin clays and some older ridge soils have been decalcified and acidified in their upper layers; lime is found only in the Subsoil or substratum of such soils. Clay soils predominate in basins and on the middle parts of most ridges, with loamy soils (and occasionally sands) occurring mainly on ridge crests. Lower Ganges River Floodplain (7,968 sq km) comprises the eastern half of the Ganges river floodplain which is low-lying. The area has a typical meander floodplain landscape of broad ridges and basins. Seasonal flooding is mainly shallow in the west and north, with the highest ridge crests remaining above normal flood levels, but flood depths increase towards the east and the south. Flooding is mainly by accumulated rainwater and the raised groundwater table, except on the active Ganges floodplain and close to distributary channels which cross the meander floodplain. In time of small-scale mapping, Mahananda floodplain in the northwest and some detached areas of the Old Meghna estuarine floodplain in the southeast used to be included within this unit. The Mahananda floodplain comprises all irregular landscapes of mixed Tista and Ganges sediments. The cut-off parts of the Meghna floodplain have a smooth relief and predominantly silty soils, which are deeply flooded (by rainwater) in the monsoon season. The unit covers most of the districts of Rajshahi, Natore, Pabna, entire Kushtia, Rajbari, Faridpur, Meherpur, Chuadanga, Jhenaidaha, Magura, parts of Manikganj, Narayanganj, Munshiganj, Shariatpur, Madaripur, Barisal, Gopalganj, Narail, Khulna, Bagerhat, Satkhira, and most of Jessore. This physiographic unit is almost triangular in shape and bounded by the Ganges tidal floodplain on the south. On its southern end it traps the Gopalganj-Khulna Beels Ganges Tidal Floodplain-Non Saline (00,000 sq km) TThe boundary between this unit and the Ganges floodplain is traditional. The tidal landscape has a low ridge and a basin relief crossed by innumerable tidal rivers and creeks. Local differences in elevation generally are less than 1m compared with 2-3m on the Ganges floodplain. The sediments are mainly non-calcareous clays, but they are silty and slightly calcareous on riverbanks and in a transitional zone in the east adjoining the lower Meghna. This unit covers most of Satkhira, Khulna, Bagerhat, Pirojpur, Barisal, Patuakhali, Bhola and the entire Jhalokati and Barguna districts but excludes the Sundarbans in the southwest. Ganges Tidal Floodplain-Saline (00,000 sq km) The river carries fresh water throughout the year in the northeast and east, but saltwater penetrates increasingly further inland towards the west, mainly in the dry season, but for most or all of the monsoon in the southwest. In the northeast, there is moderately deep flooding in the monsoon season, mainly by rainwater ponded on the land when the Ganges distributaries and the lower Meghna are at high flood levels. Elsewhere, there is mainly shallow flooding at high tides, either throughout the year, or only in the monsoon, except in the extensive areas where tidal flooding is prevented by EMBANKMENTS. Within embankments, there is seasonal flooding with accumulated rainwater. The soils are non-saline throughout the year over substantial areas in the north and the east, but they become saline to varying degrees in the dry season in the southwest. Akhaura Terrace (113 sq km) this small region occupies the eastern border of Brahmanbaria and the southwest corner of Habiganj district. The main soils in the uplands have strong brown clay. The valley soils range from silty clay to clays. Deep red brown terrace soils, grey piedmont soils and acid basin clays are the major components of the general soil types of the area. The general fertility including organic matter status is low. The soils are strongly acidic in reaction. High Hill Ranges (0,000 sq km) comprise an almost parallel ridge running approximately north-south and with summits reaching 300-1000 m. They have very steep slopes - generally >40%, often 100% and are subject to landslide erosion. They are mainly underlain by consolidated shales, siltstones and sandstones. This unit covers most of Chittagong Hill Tracts, some small parts of southern Habiganj, and the south and eastern borders of Maulvi Bazar. All the mountain ranges of the Hill Tracts are almost hogback ridges. They rise steeply, thus looking far more impressive than their height would imply, and extend in long narrow ridges, whose tops are barely 30m wide. Most of the ranges have scarps in the west, with cliffs and waterfalls. These are different from the low rounded foothills to the west. There are extensive stretches of low hills and hillocks in between the ranges. Four ranges, with an elevation of over 300m, strike in N-S direction in the northern part of the region. The western-most, the Phoromani range, reaches 463m at Phoromani, 436m at Rampahar, and 417m at Bhangamura. The next range eastwards is the Dolajeri; its highest peak is Langtrai (429m). On the eastern side of this range are several high waterfalls: two of the highest have falls of 60 and 40m. Further east, across the Maini valley is the Bhuachari range, which rises to 611m at Changpai peak. The eastern-most, within Bangladesh, is the Chipui-Lungsir range (also known as the Barkal range). Its highest peaks, from north to south, are Khantlang (683m), Thangnang (735m), Lungtian (679m), Chipui (480m), Bara Toung (447m), and Barkal (572m). South of the Karnafuli there are seven main mountain ranges within Bangladesh. The Muranja range rises out of the Chunoti hills 5 km east of Harbang, and strikes in a southeasterly direction. Its well-known peaks are Muranja (502m), Nashpo Taung (586m), and Basitaung (664m). East of the Muranja range and also roughly parallel to it are the Tyanbang, Batimain and Politali ranges. The Tyanbang or Chimbuk range rises south of Sangu river and continues into Myanmar. Its main peaks are Lulaing (720m), Thainkhiang (894m), Kro (868m), Rungrang (849m), and Tindu (898m). On a branch of the Lulaingkhal, near Lulaing peak, there is a waterfall of 107m height. The high peaks within Bangladesh are Waibung (808m), Mowdok Tlang (905m), Rang Tlang (958m), Mowdok Mual (1,004m) and Tajingdong, which is the highest peak of Bangladesh, officially called as Bijoy (1,280m). Low Hill Ranges (0,000 sq km) comparatively low hill ranges occur between and outside the high hill ranges. They are mainly formed over unconsolidated sandstone and shale. Their summits generally are <300m above MSL. Most areas are strongly dissected, with short steep slopes, but there are some areas with rolling to early-level relief (eg in the best tea-growing areas of Sylhet region). In the Sylhet region, there are four main hillocks in the northern zone and six hill ranges project into the south of Sylhet district from the Indian state of Tripura. These six ranges, which project into the plains from the south, are, from east to west, Patharia, Harargaj, Rajkandi-Ita, Bhanugach, Tarap and Raghunandan. In the Chittagong region, this unit includes the Sitakunda and Mara Tong ranges and the complex of hills to the south and east of Ramgarh, including the eastern part of the Middle Feni river valley. The Sitakunda range has 32-km long ridge in the middle, which reaches 352m at Sitakunda peak. To the north, the high peaks on this range are Rajbari Tila (274m) and Sajidhala (244m). To the south, there is an abrupt fall and Chittagong city heights are less than 92m. In the Mara Tong range a height of only 113m is reached. Further northeast the hills are higher. The topography is deeply eroded and rounded; the valleys are curved and almost isolated hillocks are common. At the Sitakunda peak, there are several hot springs. There are five broken ranges of hills between Karnafuli river and the southern tip of Bangladesh. South of Bakkhali river the hills reach the sea at Cox's Bazar. Thereafter the main mass of hills goes down the Teknaf peninsula as the Teknaf range. There is a slight break in the west along the Rejukhal valley. In its northern part, the Teknaf range is comparatively low (61 to 91m). From Whykong a high ridge runs south; its main peaks are Baragong (119m), Taunganga (268m), and Nytong (168m). The southwestern end of this range ends at a village called Noakhali where there are a series of impressive cliffs, some 30m in height. The range ends at Teknaf Bazar. South of Gorjania (northeast of Teknaf peninsula) these hills continue into Myanmar. The Rejukhal valley is an important component of this broken-up landscape. Karatoya-Bangali Floodplain (2,577 sq km) This plain was once a part of the Tista floodplain and now through the Bangali distributary of the Jamuna it is part of a bigger floodplain. The relief is that of broad ridges and basins. Along the Brahmaputra Jamuna, as along the Ganges, there arc many Diaras and Chars. In fact there are more of them along this channel than in any other river in Bangladesh. The largest of these are in Rahumari Thana where they form most of the land between the river and the abrupt faulted western end of the Meghalaya Plateau. Lower Meghna River Floodplain (909 sq km) Southward from the junction of the Meghna and Ganges, the sediments on the left bank of the lower Meghna comprise mixed alluvium from the Ganges, Jamuna and Meghna. These deposits are predominantly silty. Close to the riverbank the deposits are slightly calcareous because of the inclusion of Gangetic material. Further inland, the sediments are not calcareous and many have been deposited before the Ganges shifted from the Arial Khan channel into the present Lower Meghna in around 1840 AD. This floodplain area has a slightly irregular ridge and basin relief, but also has large mounds used for settlement and cultivation. Seasonal flooding was formerly moderately deep, fluctuating in depth twice daily with the tides in the south, but flooding is mainly shallow and by rainwater within the area protected and drained by the Chandpur irrigation project. Middle Meghna River Floodplain (1,555 sq km) The main channel of the Meghna upstream from its junction with the Dhaleshwari and Ganges as far as Bhairab Bazar is known as the middle Meghna. The floodplain of this river occupies a low-lying landscape of broad islands and many broad meandering channels which formed part of the Brahmaputra before it abandoned this channel when it changed course into the Jamuna two centuries ago. The Meghna sediments are mainly silty and clays and sandy Brahmaputra sediments occur at the surface on some ridges in the north. Seasonal flooding from the Meghna is mainly deep. Basin sites are submerged early and drain late. Young Meghna Estuarine Floodplain-Non Saline (0,000 sq km) This sub-unit occupies almost the level land within and adjoining the Meghna estuary. It includes both island and mainland areas. New deposition and erosion are constantly taking place on the margins, continuously altering the shape of the land areas. The sediments are deep silts, which are finally stratified and are slightly calcareous. Old Meghna Estuarine Floodplain (7,740 sq km) The landscape in this extensive unit is quite different from that on river and tidal floodplains. The relief is almost level, with little difference in elevation between ridges and basins. Natural rivers and streams are far apart in the southern part and drainage is provided by a network of man-made canals (khal). The sediments are predominantly deep and silty, but a shallow clay layer in some basin centres overlies them. Seasonal flooding is mainly deep, but it is shallow in the southeast. Some basin centres stay wet throughout the dry season. Virtually everywhere, this flooding is by rainwater ponded on the land when external rivers flow at high levels; the exceptions are the narrow floodplains alongside small rivers (such as the GUMTI) which cross the unit from adjoining hill and piedmont areas. Young Meghna Estuarine Floodplain-Saline (0,000 sq km) In many, but not all parts of young Meghna estuarine floodplain, the soil surface becomes saline to varying degrees in the dry season. Seasonal flooding is mainly shallow, but fluctuates tidally, and is caused mainly by rainwater or non-saline river water. Flooding by salt water occurs mainly on the lamed margins and during exceptional high tides during the monsoon; also when storm surges associated with tropical cyclones occur. Madhupur Tract (4,244 sq km) Another Pleistocene upland block in the Bengal Basin is located in the central part of Bangladesh comprising greater Dhaka and Mymensingh districts, between the courses of the Old Brahmaputra and the Jamuna rivers. Towards the south, this physiographic sub-region extends to as far as Dhaka, the capital of the country. Madhupur Tract measures about 4,105 sq km. Comparable to the Barind Tract, the area belongs to a Pleistocene terrace consisting mainly of red coloured and mottled clays. It is characterised by plateau-like hillocks varying in height from 9 to 18.5m, and a dendritic drainage pattern, typical of all Pleistocene terraces in Bangladesh. The valleys, mostly flat, are cultivated. The Madhupur jungle contains Shal trees (Shorea robusta), the hardwood which is second to teak in value. Old Himalayan Piedmont Plain (0,000 sq km) the gently sloping land at the foot of hills formed with colluvial and alluvial sediments deposited by rivers or streams. A portion of the Old Himalayan Piedmont plain stretches into Bangladesh at the northwestern corner of the country. This occupies most of the Dinajpur region. This region is covered by Piedmont sands and gravels which were deposited as alluvial fans of the Mahananda and Karatoya rivers and their distributaries issuing from the Terai area at the foot of the Himalayas. The piedmont deposits may possibly be as old as late Pleistocene or early Holocene, but they are younger than Madhupur clay. The drainage pattern is braided, with broad, smooth, but irregular-shaped ridges crossed by numerous, broad shallow channels which frequently branch out and reconnect. The Tista abandoned this landscape a long time ago, since when the area appears to have uplifted so that the small rivers crossing the plain are now entrenched up to about 6m deep (in the north; less in the south) below the main level of the plain. This plain gently slopes south from about 96m down to 33m above MSL. This physiographic unit underlies most of the northwestern corner of the country, ie, entire Thakurgaon and the major parts of the Panchagarh and Dinajpur districts. Lower Punarbhaba Floodplain (129 sq km) this floodplain, separating the West-Central Barind from the Western Barind begins 25 kilometers south of Dinajpur town, in Indian Dinajpur District. It ends at Rohanpur in Gomastapur Thana where the Purnabhaba river joins the Mahananda river. It is 80 kilometers long and 3 to 8 kilometers broad. Locally the floddplain is known as Duba i.e. swampy. Due to imperfect drainage this floddplain is not so fertile as those to the east of it. Northern and Eastern Piedmont Plains (0,000 sq km) generally sloping piedmont plains border the northern and eastern hills; (similar piedmont plains adjoining the hills in Chittagong region have been included in the Chittagong coastal plain). These plains, which comprise coalesced alluvial fans, mainly have silty or sandy deposits near to the hills, grading into clays in the basin adjoining the neighbouring floodplains. The whole area is subject to flash floods during the rainy season. On the higher parts, flooding is mainly intermittent and shallow; but it is moderately deep or deep in the basin. The sub-region covers most or parts of the upazilas of Nalitabari (Sherpur), Tahirpur, Bishwamvarpur, Dowarabazar, Companiganj (Sylhet), Gowainghat, Madhabpur, Habiganj Sadar, Chunarughat, Sreemangal, Kamalganj and Kulaura. Sylhet Basin (4,573 sq km) a large, gentle depressional feature, is bounded by the Old Brahmaputra floodplain in the west, the Meghalaya Plateau's foothills in the north, Sylhet High Plain in the east and Old Meghna Estuarine floodplain on the south. Its greatest length, both E-W and N-S, is just over 113 km. Numerous lakes (beels), large swamps and haors cover this saucer-shaped area of about 7,250 sq km. The sinking of this large area into its present saucer-shape seems to be intimately connected with the uplift of Madhupur Tract. Local tradition has it that the land sank 9 to 12m in the last 200 years. This area is still undergoing persistent subsidence. In addition to the hills located along the southern spur of the Shillong Massif, a number of hillocks, locally known as TILA, form minor but morphologically distinct, ranges around Sylhet in northeastern Bangladesh. These elevations, as for instance Kailas Tila, Dupi Tila and the tilas at Beanibazar, east of Sylhet, are generally built up of Plio-Pleistocene clastic sediments and reach maximum elevations of about 60m above MSL. It is regularly flooded during the monsoon. Eastern Surma-Kushiyara Floodplain (4,622 sq km) comprises the floodplain of rivers draining from the eastern border towards the Sylhet Basin (Haor Basin). Some small hill and piedmont areas near Sylhet are included within the boundaries. Elsewhere, the relief generally is smooth, comprising broad ridges and basins, but it is locally irregular alongside river channels. The soils are mainly heavy silts on the ridges and clays in the basins. This area is subject to flash floods in the pre-monsoon, monsoon and post-monsoon seasons, so the extent and depth of flooding can vary greatly within a few days. Normal flooding is mainly shallow on the ridges and deep in the basins. Active Tista Floodplain (0,000 sq km) A big sub-region stretches between the Old Himalayan Piedmont Plain in the west and the right bank of the N-S flowing Brahmaputra in the east. An elongated outlier representing the floodplain of the ancient Tista extends up to Sherpur (Bogra district) in the south. Most of the land is shallowly flooded during monsoons. There is a shallow depression along the Ghaghat river, where flooding is of medium depth. The big river courses of Tista, Dharla and Dudhkumar cut through the plain. The active floodplain of these rivers, with their sandbanks and diyaras, is usually less than six kilometres wide. Tista Meander Floodplain (9,468 sq km) this region occupies the major part of the Tista floodplain as well as the floodplain of the Atrai, little Jamuna, Karatoya, Dharla and Dudhkumar rivers. Most areas have broad floodplain ridges and almost level basins. Ganges Tidal Floodplain-Sundarbans (0,000 sq km) South and southwest of the Ganges tidal floodplain, there is a broad belt of land, barely above sea level with an elevation of only 0.91m. This very low land of some 4,827 sq km area contains the Sundarbans forest and the reclaimed estates (cultivated land) - classified as the Sundarbans unit. There are two possible causes for the existence of such a large very low estuarine area - insufficient deposition by the Ganges distributaries or subsidence. The main distributaries of the Ganges never flowed through this region, and the small ones that did last a few centuries at most. The building up of this estuarine area is consequently not complete. On the other hand, it is possible that subsidence has played a major part in depressing this area. There are many evidences of this, such as large ruins in the heart of the swampy estuarine areas such as at Shekertek and Bedkashi, and the presence of human artefacts and tree stumps, buried in the alluvium many feet below the sea level. There is also an isolated part of Sundarbans (Chakaria Sundarbans) at the mouth of the Matamuhuri river near Cox's Bazar. Source Banglapedia; FH Khan, Geology of Bangladesh, University Press Limited, Dhaka, 1991; Haroun Er Rashid, Geography of Bangladesh, University Press Limited, Dhaka, 1991; Klaus-Ulrich Reimann, Geology of Bangladesh, GebrFCder Borntraeger, Berlin-Stuttgart, 1993; Hugh Brammer, The Geography of the Soils of Bangladesh, University Press Limited, Dhaka, 1996. Limitation of Use This GIS data and any other information presented on this web site is strictly limited to non commercial use. Disclaimer Geospatial Bangladesh makes no representation or warranty as to the accuracy, completeness, reliability or suitability of the GIS data and any other information provided in this website for any specific purpose and disclaims any responsibility or liability for any expenses, losses, damages and costs which may be suffered or incurred as a result of or in connection with using or relying on the data or the information provided in this website. Content Type Printable Map Attributes Field Name: unumber Field Type: Integer Field Description: The values in the field defines the physiographic unit number of Bangladesh. Description of Values: The field values has been described in the Description section. Field Name: unit Field Type: String Field Width: 100 Field Description: The values in the field defines the physiographic units of Bangladesh. Description of Values: The field values has been described in the Description section. EPSG:4326