Natural Topography: Location and Characteristics

Natural topography of Bangladesh including the location and characteristics of the main land forms (uplands, hills, flood plains, deltaic plains and coastal plains) are described in detail step by step.

Location

Bangladesh is a transition zone between Southwest and Southeast Asia. Bangal forms the Capstone of an arch formed by the Bay of Bengal, and because of eh Tibetan massif to the north, it is a comparatively narrow land bridge between the subcontinent of India and the subcontinent of Southeast Asia.

More precisely, Bangladesh stretched latitudinal between 20˚34΄ N and 26˚33΄ N, and longitudinally between 88˚41΄ E and 92˚41΄ E. Ii is bounded by India on the west (West Bengal), on the north by Meghalaya and Assam, and on the north-east by Tripura and Mizoram, and it shares a south-eastern border with Myanmar (Burma). The Bay of Bengal is to the south.

Some of the largest rivers in the world flow through the country and form the larges delta in the world.

Natural Topography of Bangladesh

Topography is a configuration of a land surface including its relief and contours, the distribution of mountains and valleys, the patterns of rivers, and all other natural features that produce the landscape.

Although Bangladesh in a small country it has considerable topographic diversity.

There are three distinctive natural features in Bangladesh:

  • A broad alluvial plain subject to frequent flooding
  • A slightly elevated relatively older plain
  • A small hill region drained by fast flowing rivers.

Location and characteristics of the main landforms

Bangladesh can be classified into three distinct topographical regions:

Location and characteristics of Tertiary hills

The hills in Bangladesh were formed at the same time that the Himalayan contains were formed. Therefore, they are called Hills of Tertiary Age. The hilly areas of the South-eastern region of chittagon, the north-eastern hills of sylhet and the highlands in the north and northwest are included in this region.

The Chittagong Hills are the only significant hill system in Bangladesh. They rise steeply to narrow ridges (average 36 m wide), with heights between 600 and 900 m above sea level.

In between the hilly ridges lie the valleys that generally run north to south. West of the Chittagong Hills is a narrow, west coastal plain lying parallel to the shoreline.

The hilly areas of Bangladesh comprise two main kinds of topography:

  • Low hill ranges
  • High hill or mountain ranges.

Law hill ranges

Occur between and outside the high hill ranges. They are mainly formed of unconsolidated sandstone and shale. Their summits are generally less than 300m above sea level (MSL). Most areas are strongly dissected, with short steep slopes.

In the Chittagong region, 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.

High hill or mountain ranges

These comprises an almost parallel ridge running approximately north-south and with summits reaching 300-1000 m. above sea level. Keokaradang (1,230 m) situated in Bandarban is the highest peak of Bangladesh. These hills have steep slopes- generally greater than 40% often 100% and are subject to landslide erosion.

Consolidated shales, siltstones and sandstone mainly underlie them. All the mountain ranges of the Hill Tracts are almost hogback ridges. They rise steeply, and extend in long narrow ridges, whose tops are barely 30 m wide.

The region is characterized by a huge network of trellis and dendritic drainage, consisting of some major rivers draining into the Bay of Bengal. The major rivers are Karnafuli, Sangu, Matamuhuri and Feni.

Uplands / Pleistocene Terrace

The Pleistocene Terrace is a bench-like structure bordering an undersea feature. These types of terraces were formed by the flood of snow-melt water during the Pleistocene epoch about 25,000 years ago. There terraces are slightly elevated form the adjacent active floodplains. The sediments of these terraces are deeply weathere3d and strongly oxidized.

Pleistocene uplands, comprising the Barind Tract, the Madupur Tract and the Tippera Surface, form three individual blocks in Bangladesh. Pleistocene uplands cover an area of about 10% of Bangladesh, with an average elevation of more than 15 m above mean sea level (MSL).

Barind Tract: comprises mid and lower4 western part of Rajshahi division, between the Ganges and Brahmaputra. In the south, the Barind Tract is an older Pleistocene Terrace forming a small plateau with a flat or, in some sectors, a slightly undulating surface. This terrace consists of reddish and yellowish and partiaclly mottled clays and is characterized by a dendritic drainage pattern.

Madhupur Tract: Another Pleistocene upland block in the Bengal Basin, it is located in the central part of Bangladesh comprising greater Dhaka and Mymensing districts, between the courses of the Old Brahmanputra and the Jamauna rivers. Like the Barind Tract it consists mainly of red coloured and mottled clays. The valleys, mostly flat, are cultivated. The Madhupur jungle contains Shal trees harea rousta).

Tippera Surface: The area between the Meghna floodplain in they west and the Tripura hills in the east was uplifted in early recent times. This area of Lalmai terrace consists of red, mottled clay and has a dendritic drainage pattern. The surface is slightly undulating, except the Lalmai hills, with heights ranging from 6 to 50 m above sea level.

Resource Skills Activity

On a map of Bangladesh mark the Pleistocene Terrace.

Recent floodplain

A significant part of Bangladesh (around 90%) is covered by floodplains 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. The floodplains of Bangladesh have been divided into 15 sub-units:

Natural Topography

Natural Topography The Location and Characteristics of the Main Landforms (Uplands, Hills, Flood Plains, Deltaic Plains and Coastal Plains)

Old Himalayan Piedmont Plain: This is the gently sloping land at the foot of hills, formed with alluvial sediments deposited by rivers or streams. A portion of the Old Himalayan Piedmont Plain stretches into Bangladesh at the north-western corner of the country. This occupies most of the Dinajpur region. This region in covered b y Piedmont sands and gravels, which were deposited as alluvial fans of the Mahanada and Karatoy rivers.

Tista Floodplain: This is a big sub-region stretching between the Old HImalayan Piedmont Plain in the west and the right bank of the north-south flowing Brahmaputtra in the east. Most of the land is shallowly flooded during monsoons.

Old Brahmaputra Floodplain: The Old Brahmaputtra floodplain stretches from the south-western corner of the Garo Hills, along the eastern rim of the Madhupur Tract, down to the Meghna. It exhibits a gently morphology composed of Broad ridges and depressions. The latter are usually flooded to a depth of more than one matter, whereas the ridges and depressions. The latter are usually flooded to a depth of more than one meter, whereas the ridges are subject to shallow flooding only in the monsoon season.

Jamuna (Yung Brahmaputra) Floodplain: 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.

Haor Basin: is a lager, gentle, depressional feature, bounded by the Old Brahmaputra floodplain in the west, the Meghalaya Plateau foothills in the north, Sylhet High Plain in the east and the Mehgna estuarine floodplain on the south.

Surma-Kushiyara Floodplain: comprises the floodplain of rivers draining form the eastern border towards the Sylhet Basin (Haor Basin). Some small hill and piedmont areas near Sylhet are included within the baoundaries. Elsewhere, the relief generally is smooth, comprising broad ridges and basins, but it is locally irregular alongside river channels.

Mehgna Floodplain: is divided into four sub-regions:

  • Middle Meghna floodplain: 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.
  • Lower Meghna floodplain: Southward from the junction of the Meghan 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 silted. This floodplain area has a slightly irregular ridge and basin relief, but also has large mounds mused for settlement and cultivation.
  • Old Meghna estuarine floodplain: The landscape in this extensive unit occupies almost the level and 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.
  • Young Meghna estuarine floodplain: 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.

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 each flood season, but it is less braided than that of the Brahmaputra-Jamuna.

Ganges Tidal Floodplain: The tidal landscape has a low ridge and a basin relief crossed by many tidal rivedrs and creeks. Local differences in height are generally less than 1 m compared with 2-3m on the ganges floodplain.

Sundarbans: South and southwest of the Ganges tidal floodplain, there is a broad belt of land, barely above sea level with a height of only 0.91m. This very low land contains the Sundarbans forest and the reclaimed estates (cultivated land).

Lower Atrai Basin:  A small physigraphic unit occupies a low-lying area where mixed sedments form 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. Seasonal flooding was formerly deep, and extensive areas in Chalan beel used to remain wet throughout the year.

Arial Beel: A large depression lying between the Ganges and the Dhaleshwari south of Dhaka. Heavy clays occupy almost the whole landscape. Despite the proximity the two major river channel, the deep seasonal flooding is predominantly by accumulated rainwater that is unable to drain into rivers when they are running at high levels. Much of this area remains wet in the dry season.

Deltaic plains and coastal plains According to the special characteristics of formation, the flood plains can also be classified into two types in Bangladesh. They are:

  • Deltaic plains
  • Coastal plains

Deltaic plains: Geologists predict that the development of the Ganges-Brahmaputra delta, that began some 125 million years ago, is still continuing. The Ganges and Brahmaputra rivers together with a non-Himalayan river, the Meghna, have built one of the largest deltas in the world known as the Ganges-Brahmaputra Delta or the Bengal Delta.

These delta-building activities of the rivers have contributed to the formation of some 60% of the total Bangladesh coastline. As the delta is tide dominated, with strong fluvial influence, the sediments were deposited more on the sea floor rather than redistributed by ocean waves and currents. As a consequence, the depositional plain rose. While the western inactive delta is relatively old, the Meghna deltaic plain is geologically very young.

The delta of Bangladesh are densely populated, with a predominance of agricultural activities due to the high fertility of the soils.

Coastal PlainsRelatively rapid changes in landforms due to corrosion and sedimentation have occurred in the coastal areas. Physical evidence of changing conditions is apparent in eroding riverbanks; areas of new deposition and consequent changes in landforms are also present. Within the last 200 years, the estuary has gone thought changes in shape, channel migration and southward growth of islands. The coastal areas with mangrove plantations are regularly inundated during high tide.

Soil ranges from silty loan to silty clay loam. PH varies between 7.5 and 8.2. Due to environmental factors, the coastal soils are slightly to moderately saline on the surface and highly saline in sub-surface layers.

The saline soils are mainly found tin Khulan, Barisal, Patuakhali, Noakhali nad Chittagong districts of the coastal the offshore lands. The coastal plain zone is the home of the world’s largest mangrove ecosystems, the Dusndarbans mangrove forest.

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