ارزیابی مبتنی بر RS-GIS پویایی رودخانه برهماپوترا در هند RS-GIS Based Assessment of River Dynamics of Brahmaputra River in India

۱٫ Introduction The river Brahmaputra has been the lifeline of northeastern India since ages. This mighty river runs for 2880 kms through China, India and Bangladesh. Any alluvial river of such magnitude has problems of sediment erosion-deposition attached with it; the Brahmaputra is no exception. The problems of flood, erosion and drainage congestion in the Brahmaputra basin are gigantic. The Brahmaputra River is characterized by its exceedingly large flow, enormous volume of sediment load, continuous changes in channel morphology, rapid bed aggradations and bank line recession and erosion. The river has braided channel in most of its course in the alluvial plains of Assam. The lateral changes in channels cause severe erosion along the banks leading to a considerable loss of good fertile land each year. Bank oscillation also causes shifting of outfalls of its tributaries bringing newer areas under waters. Thousands of hectares of agricultural land is suffering from severe erosion continuously in the Brahmaputra basin covering parts of states like Assam, Arunachal Pradesh, Meghalaya, Nagaland and Manipur. Stream-bank erosion and its effects on channel evolution are essential geomorphic research problems with relevance to many scientific and engineering fields. Remote sensing satellite data having ability to provide comprehensive, synoptic view of fairly large area at regular interval with quick turn-around time integrated with Geographical Information System (GIS) techniques makes it appropriate and ideal for studying and monitoring river erosion and its bank line shifting. Various studies in this regard have been carried out for some major rivers all over the world [1-6]. Several investigators have used remotely sensed data for ascertaining channel changes of Brahmaputra River and its tributaries. NRSA has done the river migration study of the Brahmaputra using airborne scanner survey and has carried out repetitive survey to monitor changes in landuse, river channels and banks to provide a base for estimating the response of the rivers to flood events [7]. Bardhan [8] studied the channel behavior of the Barak River using satellite imagery and other data to identify the river stretches, if any, which remained reasonably stable during the period 1910-1988. SAC (Space Application Centre), Ahmedabad and Brahmaputra Board jointly took up a study to access the extent of river erosion in Majuli island in order to identify and delineate the areas of the island which have undergone changes along the bank line due to dynamic be havior of the river [9]. Naik [10] studied the erosion at Kaziranga National Park using remote sensing data. Goswami [11] carried out a study on river channel changes of the Subansiri (northern tributary of Brahmaputra River) in Assam, India. Mani [12] studied the erosion in Majuli Island using remote sensing data. Bhakal [13] have quantified the extent of bank erosion in Brahmaputra River near Agyathuri in Assam, India over a period of thirty years (1973-2003) using remote sensing data integrated with GIS. Kotoky [14] studied selected reach of Brahamputra (from Panidihing Reserve Forest to Holoukonda Bil of the Brahmaputra River) with two sets of Survey of India toposheets (1914 and 1975) and a set of IRS satellite images (1998, IRS-1B, LISS II B/W geocoded data. Sarma [15] studied the nature of bank line migration of Burhi Dihing River (southern tributary of Brahmaputra River) course during a period of time from 1934 to 2004. Das and Saraf [16] made a study in respect to a trend in river course changes of Brahmaputra River and influence of various surrounding geotectonic features for varying period between 1970-2002 for different sections of the river using Landsat MSS, TM and ETM images. However, a comprehensive study of the bank erosion and channel migration of the entire Brahmaputra in India with most recent satellite data has not yet been reported in the literature. The present paper briefly describes a study of the Brahmaputra river—its entire course in Assam from upstream of Dibrugarh up to the town Dhubri near Bangladesh border for a stretch of around 620 kms for a period of 18 years (1990-2008) using an integrated approach of Remote Sensing and GIS. The satellite data has provided the information on the channel configuration of the river system on repetitive basis revealing much needed data on the changes in river morphology, erosion/deposition pattern and its influence on the land, stable and unstable reaches of the river banks, changes in the main channel of the Brahmaputra River, etc. It is envisaged that the conjunctive use of the information generated out of this study with other ground based data will contribute substantially in more meaningful approach towards planning and execution of means and measures to combat recurring floods and erosion. ۲٫ Study Area The Brahmaputra River, termed a moving ocean, is an antecedent snowfed river which flows across the rising young Himalayan Range. Geologically, the Brahmaputra is the youngest of the major rivers of the world. It originnates at an altitude of 5300 m about 63 km south-east of the Mansarovar lake in Tibet. The river is known as Tsangpo in Tibet. Flowing eastward for 1625 km over the Tibetan plateau, the Tsangpo enters a deep narrow gorge at Pe (3500 m) and continues southward across the east-west trending ranges of the Himalayas, viz. the Greater Himalayas, Middle Himalayas and sub-Himalayas. After crossing the Indo-China border near Pasighat the river is called as the Siang or the Dihang. Two major rivers namely the Dibang and the Lohit join the Dihang at a short distance upstream of Kobo to form the river Brahmaputra. The river flows westward through Assam for about 700 km distance from Dhola until downstream of the town Dhubri, where it abruptly turns south and enters Bangladesh. The gradient of the Brahmaputra River is as steep as 4.3 to 16.8 m/km in the gorge section upstream of Pasighat, but near Guwahati it is as flat as 0.1 m/km. In the course of its 2880 km journey, the Brahmaputra receives as many as twenty two major tributaries in Tibet, thirty three in India and three in Bangladesh. The northern and southern tributaries differ considerably in their hydro-geomorphologic characteristics owing to different geological, physiographic and climatic conditions. Due to the colliding Eurasian (Chinese) and Indian tectonic plates, the Brahmaputra valley and its adjoining hill ranges are seismically very unstable. For the present study, a reach of 620 km on the main stem of Brahmaputra River, i.e., its entire course in Assam from upstream of Dibrugarh up to the town Dhubri near Bangladesh border has been considered. The geographical bounds of the area are 25˚۴۵′ to 28˚N latitude and 90˚ to 96˚۳۰’E longitude. Figure 1 shows the study reach of the river.