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Floodplain


Floodplain relatively smooth valley floors adjacent to and formed by alleviating rivers which are subject to overflow. In the context of physiography, Bangladesh may be classified into three distinct regions, viz (a) Floodplain, (b) Terrace, and (c) Hill areas, each having distinguishing characteristics of its own. A significant part of Bangladesh is covered by floodplain formed by different rivers of the country. It is a very important type of landscape in the country in the context of agriculture and culture. Most of the fertile cultivable lands belong to this physiographic region and the culture of the country is very much influenced by the landscape. Floodplains of Bangladesh have been divided into 18 sub-units:

(i) Old Himalayan Piedmont Plain; (ii) Tista Floodplain; (iii) Old Brahmaputra Floodplain; (iv) Jamuna (Young Brahmaputra) Floodplain; (v) Haor Basin; (vi) Surma-Kushiyara Floodplain; (vii) Meghna Floodplain: (a) Middle Meghna Floodplain, (b) Lower Meghna Floodplain, (c) Old Meghna Estuarine Floodplain, and (d) Young Meghna Estuarine Floodplain; (viii) Ganges River Floodplain; (ix) Ganges Tidal Floodplain; (x) the sundarbans; (xi) Lower Atrai Basin; (xii) Arial Beel; (xiii) Gopalganj-Khulna Peat Basin; (xiv) Chittagong Coastal Plain; and (xv) Northern and Eastern Piedmont Plain.

Old Himalayan Piedmont Plain comprises gently sloping land at the foothills with colluvial and alluvial sediments derived from the hills deposited by rivers or streams. A portion of the Old Himalayan Piedmont Plain stretches into Bangladesh at the northwestern corner of the country, which occupies the whole of Thakurgaon, and major parts of Panchagarh and Dinajpur districts. 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 the madhupur clay. The drainage pattern is that of a braided river, with broad, smooth, but irregular ridges crossed by numerous broad, shallow channels which frequently branch out and are again reconnected. The tista abandoned this landscape a long time ago, since when the area appears to have been uplifted so that 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 (mean sea level).

Tista Floodplain a big sub-region stretching 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 upazila of Bogra district in the south. Most of the land is shallowly flooded during the monsoon. There is a shallow depression along the ghaghat river, where flooding is of medium depth. The big river courses of the 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.

Old Brahmaputra Floodplain A remarkable change in the course of the Brahmaputra took place in 1787. 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 the jamuna. The old course (old brahmaputra) between Bahadurabad and Bhairab shrank through silting into a small seasonal channel only two kilometres broad. The old river had already built up fair 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 river 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.

Jamuna (Young Brahmaputra) Floodplain an alternative 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 one. 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 southward 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 the Barind and Madhupur, the zone of subsidence between them was turned in to a rift valley and became the new course of the Brahmaputra as the great Jamuna. Both the left and right banks of the river are included in this sub-region. The Brahmaputra-Jamuna floodplain again could be subdivided into the Bangali-Karatoya floodplain, the Jamuna-Dhaleshwari floodplain, and diyaras and chars.

The right bank of the Jamuna was once a part of the Tista floodplain, and now through the Bangali distributary of the Jamuna is a 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; this floodplain is sub-classed as 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 the Dhaleshwari. 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 the 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 0.91m deep for about four months (mid-June to mid-October) during the monsoon.

Haor Basin a large, gentle depressional feature, is bounded by the Old Brahmaputra floodplain in the west, the Shillong Plateau's foothills in the north and by the Sylhet high plain in the east. Its greatest length, both E-W and N-S, is just over 113 km. Numerous lakes (beels) and large swamps (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 upliftment of the 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. It is regularly flooded during the monsoon.

Surma-Kushiyara Floodplain comprises the floodplain of the rivers draining from the eastern border towards the Sylhet Basin (Haor Basin). Some small hill and piedmont areas near the Sylhet hills, too small to map separately, are included within its 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, with flood depths tending to the Haor Basin. The basin centres (haors) stay wet in the dry season.

Middle Meghna Floodplain The main channel of the Meghna upstream from its junction with the Dhaleshwari and Ganges rivers to 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 clayey and only thinly bury the former Brahmaputra char deposits, 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.

Lower Meghna Floodplain Southward from the junction of the Meghna and Ganges rivers, the sediments on the left bank of the lower Meghna comprise mixed alluvium from the Ganges, Jamuna and Meghna rivers. 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 its present lower Meghna channel around 1840. This floodplain area has a very slightly irregular ridge and basin relief, but large area mounds are 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.

Old Meghna Estuarine Floodplain 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, drainage is provided by a network of man-made canals (khals). 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 are flowing 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 This sub-region occupies almost 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 slightly calcareous. In many, but not in all parts, the soil surface becomes saline to varying degrees in the dry season. Seasonal flooding is mainly shallow, but fluctuates tidally, mainly by rainwater or non-saline river water. Flooding by salt water occurs mainly on the land margins and during exceptional high tides in the monsoon; also when storm surges associated with tropical cyclones occur.

Ganges River Floodplain 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 in 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. 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 occurs mainly because of accumulated rainwater and the raised groundwater table, except on the active Ganges floodplain and close to distributary channels which cross the meander floodplain.

Because of the small scale of the map, the Mahananda floodplain in the northwest and some detached areas of the Old Meghna Estuarine Floodplain in the southeast have been 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 Rajshahi, Natore, Pabna; the whole of Kushtia, Rajbari, Faridpur, Meherpur, Chuadanga, Jhenaidaha, Magura; parts of Manikganj, Narayanganj, Munshiganj, Shariatpur, Madaripur, Barisal, Gopalganj, Narail, Khulna, Bagerhat, Satkhira; and most of Jessore districts. This physiographic unit is almost triangular in shape and bounded by the Ganges tidal floodplain on the south. This unit on its southern end traps the Gopalganj-Khulna Beels.

Ganges Tidal Floodplain The boundary between this unit and the Ganges river floodplain is traditional. The tidal landscape has a low ridge and 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 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 whole of the Jhalokati and Barguna districts, but excludes Khulna Sundarbans in the southwest.

The rivers carry fresh water throughout the year in the northeast and east, but saltwater penetrates increasingly further inland towards the west, mainly in the dry season. In the northeast, there is moderately deep flooding in the monsoon, mainly by rainwater ponded on the land when Ganges distributaries and the lower Meghna are at high flood levels. Elsewhere, there is mainly shallow flooding at high tide, 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 east, but they become saline to varying degrees in the dry season in the southwest.

Sundarbans South 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 Sundarbans reclaimed estates (cultivated land) - classified as 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 the 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 is much evidence of this, such as large ruins in the heart of the swampy estuarine areas, eg at Shekertek and Bedkashi, and the presence of human artefacts and tree stumps, buried in the alluvium many feet below sea level. There is also an isolated part of the Sundarbans (Chakaria Sundarbans) at the mouth of the Matamuhuri river near Cox's Bazar.

Lower Atrai Basin a small physiographic unit that occupies a low-lying area where mixed sediments from the atrai and Ganges rivers and from the barind tract overlie the down-warped southern edge of the Barind Tract. The landscape north of the Atrai river is mainly smooth, but floodplain ridges and extensive basins occur to the south of the river. Heavy clay soils are predominant, but loamy soils occur on ridges in the south and west. Drainage from this unit is blocked when high river levels in the Jamuna burden the exit through the hurasagar channel. 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 man-made breaches of embankments.

Arial Beel a large depression lying between the Ganges and Dhaleshwari rivers in the south of Dhaka region. 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 though the dry season.

Gopalganj-Khulna Peat Basin occupies a number of low-lying areas between the Ganges floodplain and the Ganges tidal floodplain. The two major beels of the area are Baghia and Chanda. Thick deposits of peat occupy perennially wet basins, but they are covered by clay around the edges and by calcareous silty sediments alongside 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. Subsidence is still going on in this physiographic unit area.

Chittagong Coastal Plain The plain along the coast extends from the feni river to 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 young estuarine floodplain in the north, adjoining the sub-region 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.

Northern and Eastern Piedmont Plains include the 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 the hills, grading into the basin adjoining 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 Nalitabari, Tahirpur, Bishwamvarpur, Dowarabazar, Companiganj (Sylhet), Gowainghat, Madhabpur, Habiganj Sadar, Chunarughat, Sreemangal, Kamalganj and Kulaura upazilas of the country. [Masud Hasan Chowdhury]