Baleshwar Singh et al. / International Journal of Engineering Science and Technology (IJEST) INFLUENCE OF LANDFORM AND GEOMORPHIC PROCESS ON TOPOGRAPHIC EVOLUTION OF A RIVER ISLAND BALESHWAR SINGH Associate Professor Department of Civil Engineering Indian Institute of Technology Guwahati Guwahati – 781039, India [email protected] R. K. GOSWAMI Formerly Research Scholar Department of Civil Engineering Indian Institute of Technology Guwahati Guwahati – 781039, India [email protected] ABSTRACT: Majuli island is a fluvial landform developed by the Brahmaputra river system in India. In the last few decades, the island has lost a considerable part of its total geographical area due to severe erosion caused by the Brahmaputra river and its tributaries. The island has also been witnessing gradual morphological changes, particularly, after the devastating Assam earthquake of 1950. In this paper, some of the important features of landform and geomorphic process are examined in relation to the topographic evolution of the island. An attempt has also been made to understand the genesis, properties and behaviour of soils of Majuli island in order to integrate the topographic evolution with the flow process of Brahmaputra river. Keywords: Geomorphic process; landform, river island; topographic evolution. 1. Introduction Floodplains result from the long-term cumulative action of the flow, erosion and depositional processes. In lower-energy fluvial systems, cumulative changes in channel patterns and floodplain morphology over a time scale of decades are often great enough to require their incorporation into engineering design. Geomorphological studies of floodplains have traditionally focussed on the formation and evolution of landforms [Lewin (1978); Nanson and Croke (1992)]. As such, most of the attention has been directed to fluvial morphology and more particularly to the interactions between channel migration and floodplain construction/destruction [Wolman and Leopold (1957)]. Majuli, the world's largest inhabited river island, is a classic example of landform developed in such a lower energy fluvial system due to the passage of the Brahmaputra river system through the state of Assam in India. The island has a rich cultural heritage created by the great Vaishnavite movement started by the saint Shankardeva in the 16th century. The island has a conglomeration of a large number of socio-religious institutions called Satras (monasteries) that are regarded as the centres of excellence for Indian classical dance, art and literature. However, severe erosion and flood inundation by the Brahmaputra river and its tributaries are changing the landform morphology of the island. The problem has become more acute after the great Assam earthquake of 1950. Since then, the island has lost more than 370 sq. km. of its total geographical area. As such, there is an urgent need to protect and restore the floodplains of the island for which a thorough understanding of geomorphic processes as well as the nature and behaviour of soil materials that shaped the landform is essential. The present study is an attempt to examine some of the important features of the landform and geomorphic process in the context of formation and evolution of Majuli island of Brahmaputra river in India. ISSN : 0975-5462 Vol. 3 No. 7 July 2011 5562 Baleshwar Singh et al. / International Journal of Engineering Science and Technology (IJEST) 2. Environmental Setting of the Island Majuli island forms a part of hyporheic zone of the Brahmaputra river basin. The island is bounded by the Subansiri river on the northwest, the Kherkatia suti (a spill channel) on the northeast and the main Brahmaputra river on the south and southwest. The island extends for a length of about 80 km from east to west and for about 10 to 15 km width along the north to south direction with a total area of about 875 sq. km. [Figures 1(a) and 1(b)]. Fig. 1(a). A view of the entire Brahmaputra basin Fig. 1(b). A view of the Majuli Island within the Brahmaputra basin ISSN : 0975-5462 Vol. 3 No. 7 July 2011 5563 Baleshwar Singh et al. / International Journal of Engineering Science and Technology (IJEST) Geologically, the island is a part of the great alluvial tract of Brahmaputra river, which is by nature a geo- synclinal basin formed concomitantly with the elevation of the Himalayas to its north. The island, along with the floodplains of Brahmaputra river in its adjoining areas is formed by alluvial deposits in the form of older alluvium, newer alluvium and recent deltaic deposits of the Pleistocene age [Krishnan (1982)]. Moreover, the area is surrounded by a very complex geological setting of very young and unweathered sedimentary formations covering the entire Brahmaputra basin. On the northern side, the basin is flanked by the Sub-Himalayan ranges consisting mainly of tertiary sandstone, and is marked by the presence of many raised, relatively young terraces. On the eastern and southern sides, the Brahmaputra valley borders with the Naga-Patkai ranges consisting of tertiary formations riddled with numerous active faults. Geological surveys aided by drilling for oil in this part of the valley have shown that under the recent deposits, there are thousands of metres of tertiary sediments which overlie the Archaean Basement complex. Being an active floodplain, the island is marked by an array of alluvial features including natural leaves, crevasses, splay deposits, point bars, channel bars etc. The main channel of Brahmaputra on the southern side is characterised by rapid aggredation, dramatic channel shifts and excessive bank line recession. The island and the valley as a whole is seismically very active. The seismic activity in the region has a great impact on the fluvo-sedimentary regime of the Brahmaputra river and its tributaries. The climate of the island and the entire Brahmaputra basin lie within monsoon rainfall regime receiving annual rainfall to the tune of 2,153 mm. The temperature varies from 28° to 33°C and relative humidity varies from 54% to 86%. 3. Regimes of the Rivers around Majuli Island Knowledge of the flow regimes of the rivers around Majuli island is essential to understand the complex assemblage of the island. Majuli is situated between the Brahmaputra river reach starting from chainage 483 km and ending at chainage 568 km from the Indo-China border. In this reach, the river flows from northeast towards southwest direction with a gradient of about 1:5600. The island, at its beginning at Tekeliphuta on the northeastern border, bifurcates the Brahmaputra river diverting the main river to flow pass the southern boundary of the island. The river in this portion has braided channels with sand bars widening the river with width varying from 5 km to 14 km. Around this portion, the river is joined by three south-bank tributaries, namely Disang, Dikhow and Jhanji emanating from the Naga-patkai range. The other branch of Brahmaputra river at Tekeliphuta flows through the northern boundary of the island by the name of Kherkatia-suti, and joins with another major north-bank tributary called Subansiri emanating from the Himalayan ranges. After confluence, these tributaries take a new name called Luit river which flows southwestward to outfall again at Brahmaputra river at the western end of Majuli island. The salient features of these rivers are presented in Table 1. Table 1. Salient features of Brahmaputra river and its tributaries around Majuli Island Catchment Discharge Average Average Annual Rivers Length Area Annual Sediment Load Max. Min. Yield (km) (km2) (m3/s) (m3/s) (MCM) Brahmaputra 2108† 5,80,000 51,384 1119.77 4,49,300 400 million ton†† Subansiri 375* 28,200 14,433 129.84 52,705 2042.07 Ha. m Jhanji 108* 1,349 --- --- 783 --- Disang 230* 3,950 1,069 3.37 4,992 76.008 Ha. m Dikhow 236* 4,022 1,378 5.44 4,230 65.812 Ha. m †Up to Majuli island; *Up to confluence with Brahmaputra river; ††at Pandu Source: Flood Control Department, Brahmaputra Board 3.1. Flood Inundation A considerable part of Majuli island is inundated by spill waters of Brahmaputra river and Subansiri during monsoon causing disastrous floods in its lowlands. The high flood level of the Brahmaputra river at Kamalabari ferry ghat situated on the central part of the island is shown in Figure 2. The highest flood discharge recorded in the Brahmaputra at Bessamara (Southeast of Majuli) is 51,384 m3/s in the year 1987, which is 23 times the ISSN : 0975-5462 Vol. 3 No. 7 July 2011 5564 Baleshwar Singh et al. / International Journal of Engineering Science and Technology (IJEST) minimum flow observed in that year. The 100-year return period flood has a magnitude of around 47,131 m3/s with corresponding High Flood Level of 87.83 m at Bessamara gauge discharge site of the river. The magnitude and frequency of bank-full discharge is considered to be of great geomorphic significance in the development of channel geometry and formation of flood. The bank-full discharge (corresponding to danger level, a level or stage at which the river just tops over the banks) has a magnitude of approximately 20,000 m3/s. On the flood frequency curve, the recurrence interval of this discharge is 1.01 years. 90 88 86 Meters 84 High Flood Level 82 Danger level : 84,90M 80 1970 1975 1980 1985 1990 1995 2000 Years Fig. 2. High flood level at Kamalabari ghat 3.2. River Bank Erosion In addition to recurring floods, the island is facing a heavy loss of landmass due to processes like channel bank erosion and change/migration of channel. The erosion, in particular, is more severe on its southern side along the Brahmaputra river. According to the census of 1971, the geographical area of the island was 924.60 sq.