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Natural Levees and Landform Evolutions in the Bengal Lowland

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Natural Levees and Landform Evolutions in the Bengal Lowland Geographical Review of Japan Vol. 58 (Ser. B), No. 2, 149-164, 1985 Natural Levees and Landform Evolutions in the Bengal Lowland Masatomo UMITSU* This paper analizes the characteristics of natural levees and evolution of landforms in the Bengal Lowland. The natural levees in the lowland are classified into four types, such as broad and obscure, broad and distinct, narrow and continuous, and discotinuous types. Narrow and con tinuous natural levees are further sub-classified into those of meandering, dendritic and irregular patterns. Discontinuous natural levees are also sub-classified into an arc-shaped, mottled pattern and natural levees on the channel bars. Regional distribution of these types of natural levees was considered in relation to the condition of floodings and to the river shiftings reconstructed from various documents and maps. Broad and obscure natural levees were formed according to the shiftings of the Ganges River in the ancient times. Broad and distinct natural levees were formed along the distributaries of the Ganges River by the middle of the eighteenth century. Arc shaped and mottled distributed natural levees were formed by the main flow of the Brahmaputra River and its distributaries by 1830 AD., during the period when the Brahmaputra River had been flowing along the east of the Madhupur Jungle. After the shiftings of the two rivers, natural levees distributed in a dendritic pattern and a meandering pattern have been formed in the deep flooding area and moderate and shallow flooding areas respectively. Natural levees on the channel bars have been formed along the two rivers and small natural levees of irregularly distributed pattern extended in the region under the influence of the tide. on the landform evolution of the region. I. Introduction This study is based on the fieldwork carried out during October, 1983 to January, 1984, Natural levee is one of the most important and indoor works using topographic maps in lanforms in the alluvial lowlands. General scales of 1:50,000 and 1:63,360 (1 mile to 1 features such as distribution, morphology and inch), Irrigation Planning Maps with 5 feet formation of natural levees have been clarified (1.5 meters) contour intervals (1:40,000 scale), by many studies (ALLEN, 1965; RUSSELL, some aerial photos, infrared colored ERTS 1967). However, regional differences in the satellite images in a scale of 1:1 milion and characteristics of natural levees within a single enlarged infrared colored ERTS satellite images alluvial lowland or among several alluvial in a scale of 1:400,000. lowlands have not yet been studied intensive 1y1. The regional characteristics of natural II. Regional setting levees are considered to be deeply related to the formation of alluvial lowlands. Therefore, The Bengal Basin, about 560km long from intensive studies on natural levees with the north to south and 400km wide from east to regional point of view are required for the west, is surrounded by the Tripura and Chit study on formation and landform evolution of tagong Hills in the east, the Rajmahal Hills alluvial lowlands. In this article, the regional in the west, the Sillong Plateau and the characteristics of natural levees are studied on Himalaya Mountains in the north and faces the region of the Bengal Lowland with the the Bay of Bengal (Figure 1). Ganges Delta which is the greatest delta in The Ganges-Padma and Brahmaputra- the world, and some considerations are made Jamna are the main rivers in the basin. They * Department of Geography, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464, Japan. 150 M. UMITSU Figure 1. Map showing the Bengal Basin. 1. Mountains and hills 2. Pleistocene terraces 3. Alluvial fans 4. Older alluvial surface (Tippera surface) 5. Younger alluvial surface 6. Youngest alluvial surface. Contour intervals are three meters. have often changed their courses in the past. are located on the east of the Rajmahal Hills Now they flow from the west and north respec and on the west of the Tripura Hills, and tively, and join at the central part of the interior ones are located in the central and lowland. Further on, the river which is called northwestern parts of the Bengal Basin. They the Padma joins with the Meghna flowing are called the Madhupur Jungle and Barind, from the northeast, and flows into the Bay of respectively. Sediments of the Pleistocene Bengal. terraces in the Bengal Basin are called The landforms of the basin can roughly be Madhupur Clay (ALAM and AURANGZEB, classified into two geomorphological units which 1975; ALAM and KAHN, 1980), which are are Pleistocene terraces and Recent alluvial mainly composed of light brown to reddish lowlands (MORGANand MCZNTIRE, 1959; brown sandy silt and sandy clay. COLEMAN,1909; UMITSU, 1985). The Pleis Recent alluvial lowlands distribute widely tocene terraces are located in the marginal and over the Bengal Basin. Geomorphologic interior parts of the basin. The marginal ones surface of the lowland is subdivided into three Natural levees in the Bengal Lowland 151 different alluvial surfaces which are older, 3. Narrow and continuous type 4. Discontinuous type younger and youngest surfaces (UMITSU, 1985). The older alluvial surface is called 1. Broad and obscure natural levees by MORGAN and MCINTIRE (1959) as Tippera surface which are slightly higher than Broad and obscure natural levees are recog the younger and youngest alluvial surfaces. nized along the Bhagirathi, Bahairab and some It is located to the east of the Meghna River other rivers in the northwestern part of the and characterized by the surface with few Ganges Delta (Figure 3). These natural levees drainage systems. Similar surfaces with few develop tonally from northwest to southeast. drainage systems are seen in the opposite side The width and length of the natural levees are of the Meghna near Naria in Faridpur District more than 20km and 100-150km respectively. and in the region to the south of Calcutta City The surface of the natural levees are about of West Bengal, India. These surfaces are 2-3m higher than the backswamps which can be also considered to be correlated to the Tippera recognized as slightly lower zonal depressions. Surf ace2). Younger alluvial surface is broadly Numerous cut-off lakes and abandoned river distributed in the alluvial lowland of the channels are distributed on the surface of the Bengal Basin. The youngest alluvial surface natural levees. Width of these cut-off lakes is located along the courses of the Ganges and abandoned river channels are 0.5-1.5km, Padma and Brahmaputra-Jamna Rivers. and they are wider than those of the present The alluvial lowlands of the central and rivers flowing in the region. Sediments of the southern Bengal Basin, to the south of the natural levees are composed of brownish light Sillong Plateau and Barind, can roughly be grey sandy silt. divided into the regions of the Ganges Delta, 2. Broad and distinct natural levees Brahmaputra-Jamna Floodplain and Sylhet Basin. The heights of the lowland are mostly In the central area of the Ganges Delta, below 15m. The altitudes of the central Sylhet broad and distinct natural levees develop along Basin and the southern Ganges Delta are the Bhairab, Madhumati and some other rivers. especially low, which are usually below 5m. And the most remarkable ones develop along Landforms of the alluvial lowlands are com the Bhairab River (Figure 4). They extend northwest to southeast from the southeastern posed of slightly higher landform units such as natural levees, point bars, channel bars, end of the broad and obscure natural levee beach ridges3), and relatively lower landform zone. The width of them are 3-5km, and the units such as backswamps, marshes, former lengths are about 50-60km long. Boundaries river channels. There is a considerable dif between the natural levees and backswamps ference in their forms and distributions among are fairly sharp and clear, and the relative the regions. Sediments of the alluvial low heights between them are about 2-3m. Surface lands are mainly composed of non-weathered sediments of the natural levees are composed sand, silt and clay layers, and peat layers can of brownish light grey sandy silt. be seen in several places. 3. Narrow and continuous natural levees III. Characteristics of natural levees a. Natural levees extending in a meander in the Bengal Lowland ing pattern Narrow and continuous natural levees dis Natural levees on the younger and youngest tribute widely in the Sylhet Basin, the northern alluvial surfaces in the central and southern and the eastern parts of the Ganges Delta. Bengal Lowland are studied in this article. In the eastern Sylhet Basin, continuous Various features of natural levees4) can be natural levees bend and extend along the rivers seen in the Bengal Lowland, and they are such as the Surma, Barak and Kusiyara. The classified into the following types (Figure 2). widths of natural levees are about 0.5-1km, 1. Broad and obscure type and the relative heights from the backswamps 2. Broad and distinct type called "haors", are about 1.5-2.5m. Sediments 152 M. UMITSU Figure 2. Typical natural levees in the Bengal Lowland . 1. Broad and obscure natural levees 2, Broad and distinct natural levees 3. Narrow and continuous natural levees (a) Natural levees extending in a meandering pattern (b) Natural levees extending in a dendritic pattern (c) Natural levees extending in an irregular pattern 4. Discontinuous natural levees (a) Natural levees distributing in an arc-shaped pattern (b) Natural levees distributing in a mottled pattern (c) Natural levees on the present and former channel bars. Locations of each figure are shown in Figure 8. 1. Natural levees 2, Back swamps 3. Former river channels 4 .
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