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Baleshwar Singh et al. / International Journal of Engineering Science and Technology (IJEST)

INFLUENCE OF LANDFORM AND GEOMORPHIC PROCESS ON TOPOGRAPHIC EVOLUTION OF A

RIVER

BALESHWAR SINGH Associate Professor Department of Civil Engineering Indian Institute of Technology Guwahati – 781039, [email protected]

R. K. Formerly Research Scholar Department of Civil Engineering Indian Institute of Technology Guwahati Guwahati – 781039, India [email protected]

ABSTRACT: island is a fluvial landform developed by the Brahmaputra system in India. In the last few decades, the island has lost a considerable part of its total geographical area due to severe caused by the and its . The island has also been witnessing gradual morphological changes, particularly, after the devastating 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 result from the long-term cumulative action of the flow, erosion and depositional processes. In lower-energy fluvial systems, cumulative changes in patterns and 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 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.

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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 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

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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 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 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 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 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 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- 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 called Subansiri emanating from the Himalayan ranges. After , 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 Average Average Annual Rivers Length Area Annual 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: 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 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

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minimum flow observed in that year. The 100-year 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

82 High Flood Level 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. km. as against its earlier extent of 1246.00 sq. km. during 1950 (Revenue record of Assam). Investigations carried by the National Remote Sensing Agency, Ahmedabad, with the help of topographic maps of Survey of India indicated that during the period from 1969 to 1994, the island has lost a large tract of its landmass extending more that 50 sq. km. Out of this, the eroded area due to Subansiri river is 3.91 sq. km (0.42% of total area) as against 46.36 sq. km (5.43% of total area) eroded by the Brahmaputra river. Figure 3 shows the extent of bank erosion in Majuli island. Analysis of multi-date satellite data supplemented by ground observations indicated that the island is facing primarily three classes of bank erosional response, as highlighted in Table 2 [ISRO and Brahmaputra Board (1996)]. The general trend that emerges from the analysis also indicated that by and large, the island suffers moderate to considerable erosion due to the Brahmaputra river affecting areas towards the eastern and southern parts of the island.

Table 2. Classes of bank erosional response of the Brahmaputra river at Majuli Class Type of erosional response Criteria Location

A Major erosional changes Area loss more Kathnigaon, Ujanigaon, Sounal affecting a large area than 10 sq. km Kacharigaon

B Moderate to considerable Area loss between Lachangaon, Kumargaon, erosion affecting certain areas 1 and 10 sq. km Batiamarigaon, Khoraparagaon, along bank line Kumarbari area

Area loss less than Salmaragaon, Bechamara, Mirigaon, C Minimum erosion with response 1 sq. km Chitadarchapari, Borbori, only at isolated sites Kaibartagaon

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Fig. 3. Channel changes of Brahmaputra river indicating bank erosion of Majuli Island

3.3. Nature and Type of Sediment Coleman (1969) studied the channel process and sedimentation in Brahmaputra and reported catastrophic sediment by Brahmaputra and some of its tributaries. The Brahmaputra river carries a phenomenal sediment load of 597 million tones every year. The river is second only to the Yellow river (Hwang Ho) in China in terms of the sediment amount transported per unit discharge area, viz. 1,128 metric tons/km2. The average annual sediment carried by the Brahmaputra river as well as the tributaries adjoining the Majuli island are already shown in Table 1. Qualitative analyses carried out on the modern sediments of Brahmaputra river at reaches on and immediate upstream of Majuli island have shown that the sediments in the area are variable. The textural classifications of the sediments are mostly silty sand type with very little only sandy type of sediment. The identification of heavy minerals with their mineralogical composition indicates that the provenance is of metamorphic (mostly pelitic) and reworked sedimentary rocks as well as some acid igneous rocks. This complex assemblage also indicates uprooting of new horizons and tectonics in the source area as well as a recent nature of sediments. The study of roundness of heavy minerals indicates their immaturity due to short-distance transportation. The clay minerals of the sediments include illite, kaolinite and chlorite. They are detrital in nature and this indicates their derivation from acid igneous and metamorphic rocks. (mostly schistose). The presence of kaolinite indicates acidic condition in the environment and adequate drainage with heavy rainfall in the source area. Kaolinite may have been derived from pre-existing sedimentary rocks. The detrital nature of the clay minerals indicates fluviatile environment. The sediments contain a good amount of heavy minerals which consist of tourmaline, rutile, zircon, epidote, kyanite, silimanite, staurolite, micas (biotite and chlorite), hornblende, zoisite and opaque minerals. Tourmaline is the most dominant of the heavy minerals. The coarser fraction of the sediments are found to contain a higher percentage of tourmaline, rutile and opaque minerals, while the finer fractions show a higher percentage of epidote, zircon and garnet. The remaining minerals do not show such significance with respect to the grain size

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of the sediments. Quartz is the most dominant light mineral. Their sphericity and roundness indicate immaturity of sediments. A study of the light minerals also shows very low feldspar content of the sediments.

3.4. Seismic Activity The entire Brahmaputra Basin is marked by numerous active faults because of which occurrence of earthquake in this region is very frequent. Some of the recent major earthquakes of this region are the Great Indian Earthquake - 1897 (8.7 magnitude), Earthquake - 1930 (7.0), Assam Earthquake - 1950 (8.6) and Indo- Burma Earthquake - 1988 (7.0). Earthquake activity has a great impact on the regime of the Brahmaputra river, particularly on the morphology of channels and . Since the river drains through geologically younger, easily erodable formation in the upper reaches of its basin, triggering of an earthquake causes an inflow of an enormous quantity of sediments into the river system, thereby upsetting the regime of the river. The earthquake of 1950 caused devastation on the upper reaches of the Brahmaputra basin. The river silted up to 2.5 to 3.0 m near and downstream [Goswami (1985)]. The effect of the 1950 earthquake on the highest and lowest water levels of the Brahmaputra river at Dibrugarh is shown in Figure 4. A sudden change in the lowest water level (LWL) is an indication of the rise in the bed level of the river due to the deposition of sediments. However, the river pushes the sediments downstream, year after year, in an attempt to get back to its previous regime. This results in the scouring of already aggreded bed and subsequent deposition of sediments in low-energy reaches. Majuli is also one of such reaches where the change in the energy condition of the flow results in dramatic changes of channels and other fluvial landforms.

Fig. 4. Highest and lowest flood levels of Brahmaputra river at Dibrugarh before and after the 1950 earthquake [After Goswami (1998)]

4. Properties of Soils of Majuli Island In general, the floodplain form and channel conditions can be regarded as a part of a temporal sequence related through transfer and storage of sediments from reach to reach. So, geomorphological explanations are incomplete unless they involve some understanding of the properties of the materials by which the present Majuli island has been formed. Soil resource mapping and taxonomic classification carried by the Natural Bureau of Soil Survey and Land Use Planning (NBSSLUP) indicated that the soils of floodplains of

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Brahmaputra lack in profile development and are deep to very deep, grey to mottled grey, imperfectly drained to well-drained, sandy to silty loam with coarse and/or fine stratification, slightly acidic to neutral with low cation exchange capacity but moderate to high base saturation. Using image interpretation and laboratory analysis, NBSSLUP classified soils of Majuli island as Aeric / Mollic Fluvaquents, Typic Haplaquepts and Aquic Udifluvents, as listed in Table 3 [Chamuah et al. (1998)]. Table 3. Taxonomic classification of soils of Majuli Island Taxonomic name Description of soils Coarse loamy, Very deep, well-drained, coarse loamy soils occurring on very gently Aeric Fluvaquents sloping floodplain, having loamy surface with moderate erosion flood hazard.

Fine loamy, Associated with very deep, moderately well-drained fine loamy soils Typic Haplaquents occurring on level to nearly level floodplain with slight erosion and moderate flood hazard.

Coarse silty, Deep, well-drained, coarse silty soils occurring on river of active Mollic Fluvaquents floodplain having loamy surface with very severe flood hazard.

Associated with moderately shallow, well-drained coarse loamy soils Coarse loamy, with severe flood hazard. Aquic Udifluvents

Sub-soil investigations carried out at different locations of Majuli island by the River Research Station extending up to a depth of 30 m indicate that the island is mostly underlain by grey coloured, fine to medium size, poorly graded sand (SP-type) covered by light grey coloured silt mixed with clay and/or fine sand (CI and ML-type) of varying thickness ranging from 1.5 to about 12 m. Pockets of soil rich in inorganic clay content (CH-type and CI-type) are also found in areas like Dakhinpat located on the southwestern part of the island bordering Brahmaputra river where the depth of clay rich horizon extends even beyond 15 m from the ground. Tables 4 to 7 show the distribution as well as physical, index and engineering properties of different types of soils of Majuli island, as obtained from the River Research Station of the Flood Control Department. The values within the parentheses indicate averages. CI-type of soil found as topsoil is moderately plastic where as CH-type is highly plastic in nature having moderate cohesion of 0.31 kg/cm2 and low angle of internal friction equal to 7°. The underlying soil is loosely packed and rich in sand fraction, having high angle of internal friction (26.4°) with no cohesion making it highly vulnerable to erosion by the action of running water.

Table 4. Distribution of different types of soils over Majuli Island Description Classification Place of Depth within which it is formed of soil as per IS code occurrence Light grey Bessamara 2.0 m to 10.0 m coloured silt with CI Kumargaon 0 to 12.0 m and 16.0 to 18.0 m clay Salmara 0 to 14.0 m 0 to 10.0 m Kamalabari satra 0 to 1.5 m Grey coloured silt mixed with fine ML Auniati satra 0 to 5.5 m sand and little or no clay Grey coloured, Bessamara 0 to 2.0 m and below 10.0 m fine to medium SP Kumargaon 12.0 to 16.0 m and below 18.0 m size, poorly Salmara Below 14.0 m graded sand Auniati satra Below 1.5 m Kamalabari satra Below 1.5 m Grey coloured clayey silt mixed CH Dakhinpat satra Below 10.0 m with sand

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Table 5. Physical properties of soils of Majuli Island Type of Specific Grain size distributi soil gravity on Clay (%) Silt (%) Sand (%) Gravel (%)

CI 2.59 to 2.67 4 to 22 65 to 83 10 to 28 - (2.62) (6.7) (73.3) (20.0) ML 2.68 1.0 27.0 72.0 - SP 2.61 to 2.70 0 to 3 2 to 17.5 82.5 to 98 - (2.68) (1.67) (15.34) (86.08) CH 2.64 22 68 10 -

Table 6. Index properties of soils of Majuli Island Type of soil Liquid limit Plastic limit Plasticity index WL (%) WP (%) IP (%) CL 50.8 to 37.0 23.1 to 18.9 23.7 to 19.0 (41.7) (19.8) (21.0) ML Non-plastic Non-plastic Non-plastic

SP Non-plastic Non-plastic Non-plastic CH 72.6 33.8 38.2

Table 7. Engineering properties of soils of Majuli Island Type of soil Natural Cohesion C Angle of Coeff. of void ratio (kg/cm2) internal friction permeability (Degree) (cm/sec) CI 0.73 to 0.78 0.25 to 0.28 6.5 to 8° (0.044 x 10-4) (0.74) (26.67) (7°) ML 0.52 to 0.57 0.25 to 0.32 9° to 10° - (0.54) (0.27) (9.3°) SP 0.47 to 0.57 0 24° to 28° (17.0 x 10-4) (0.49) (26.4°) CH 0.75 to 0.78 0.28 to 0.34 6° to 8° - (0.77) (0.31) (7°)

5. Formation and Evolution of the Island The floodplain landform assemblage has a potentially complex sedimentological background. Geological history indicates that the landform of Majuli island is depositional in origin and results from the long-term cumulative action of flow, erosion and depositional processes of Brahmaputra river and its tributaries. Efforts have been made to understand the paleoenvironments of Majuli island and link it with the present architecture. As per history of the period 1662-63, the Mughal emperor Aurengzeb conducted a land survey in Assam, where Majuli has been stated to be consisting of 13 numbers of small islands between the confluence of Dhansiri and Dihing rivers with Brahmaputra river. Before 1735, Majuli was a small island in the north-eastern position of the present Majuli land, and it was known as Kherkatia Majuli (See Figure 5). At that time, the Brahmaputra river was known as Luhit and it was flowing in north of Majuli. Dihing, Disang, Dikhow and Dhansiri rivers were flowing in the southern side of Majuli. As a consequence of earthquakes in the year 1691 and 1696, the changed its course and joined in the upstream of Luhit. This new Dihing was known as Noa-Dihing. Subsequently a catastrophic flood occurred in the Debang river and devastated the entire area in 1735. After this event, the Dibang river along

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with Luhit and Disang rivers abandoned their original courses and followed the abandoned route of Dihing through the southern side of Majuli creating the present landform of Majuli.

Fig. 5. A sketch of Majuli Island before 1735 A.D. (Source: Brahmaputra Board)

6. Conclusions Majuli is a part of alluvial floodplain of the Brahmaputra river and its present landform is the outcome of geomorphic processes, like fluvial actions of Brahmaputra, Subansiri, Desang, Dikhow and Dihang rivers acting in tandem with the tectonic disturbances on the upper reaches of these rivers. Studies on the regimes of these rivers as well as material properties of the present landform indicate that the landform of the island is associated with the temporal sequence of transfer and depositional process of sediments of the Brahmaputra river and its tributaries particularly the Subansiri river. Material property indicates that the landform is purely depositional in origin and susceptible to heavy erosion due to fluvial action. Tectonic activity in the neighbouring areas, particularly in the upper reaches of the main Brahmaputra river, have significant control over the regime of the river, thereby inducing changes in the present landform. Although, with the present set of database available, it is difficult to establish the exact mode of topographic evolution of the landform of Majuli island, present investigations confirm that the landform is changing. The process of change is continuous and slow in comparison to rapid and drastic changes that may be induced by catastrophic floods or a major seismic event. In this context, a catastrophic flood event with the potential to inflict drastic and rapid change of landform of Majuli will be an unlikely event. However, the North-eastern region of India, being highly seismic, a major seismic event has the potential to induce drastic changes in the landform of Majuli island.

Acknowledgments The authors wish to thank Mr. Bharat Chandra , Former Executive Engineer, Hydraulic Research Division, River Research Station, Flood Control Department, , for his help in the procurement of the necessary research material.

References

[1] Chamuah, G. S., Sen, T. K., Baruah, U., Kumar, K. S. A. and Das, A. K. (1998): Soil resources map of Assam and its application in . NBSLLUP Soil Survey Report, pp. 21-40. [2] Coleman, J. M. (1969): Brahmaputra river: Channel processes and sedimentation. Sedimentary Geology, 3, pp.129-239. [3] Goswami, D. C. (1985): Brahmaputra river, Assam, India: Physiography, basin denudation, and channel . Water Resources Research, 21, pp. 959-978.

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[4] Goswami, D. C. (1998): Fluvial regime and flood hydrology of the Brahmaputra basin, Assam. Memoir Geological Society of India, 41, pp. 53-75. [5] ISRO and Brahmaputra Board (1996): Bank erosion of Majuli island, Assam. Unpublished Report. [6] Krishnan, M. S. (1982): Geology of India and Burma. CBS Publishers & Distributors, New Delhi, India. [7] Lewin, J. (1978): Floodplain geomorphology. Progress in Physical Geography, 2, pp. 408-437. [8] Nanson, G. C. and Croke, J. C. (1992): A genetic classification of floodplains. Geomorphology, 2, pp. 459-486. [9] Wolman, M.G. and Leopold, L.B. (1957): River floodplains: Some observations on their formation. U.S. Geological Survey, Professional Paper 282-C, pp. 87-109.

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