EVALUATION OF CHANGES IN CHANNEL CONFIGURATION OF THE BRAHMAPUTRA RIVER FROM KOBO TO TEZPUR USING SATELLITE DATA

Abstract

The study of channel configuration, is concerned. with the structure and form. of rivers, which include channel geometry, bed form and profile characteristics. Evaluation of changes in channel configuration with time and space, is a logical first step towards understanding the behaviour of the channel, which enables the watershed managers, planners and designers to take comprehensive decisions as regards the location, type and extent of the river training measures for effective and balanced utilization of this versatile resource for the benefit of the mankind. Commensurate with the above aim, quantification of the channel configuration in terms of the various morphometric parameters is inevitable. The Brahmaputra river is one of the mightiest rivers in the world and it is the largest river in the Indian sub continent if compared with respect to length and discharge. The 2906 km long 'river traverses a length of 1625 km in Tibet(China), 918 km,in NE India and the rest 363 km in Bangladesh before outfalling into the Bay of Bengal. In this study an assessment of the changes in channel configuration in the upper reach of the Brahmaputra river within Assam (from Kobo to Tezpur) having a length of nearly 320 km has been attempted on a regional basis for a period of 15 years (1973 to 1987-'88)using Landsat hardcopy data at a scale of 1:250,000 assisted by a few ancillary ground data. For delineating the spatial object regions, within, along and in the vicinity of the two bank lines, visual interpretation technique has been adopted. The main analysis carried out on this braided stretch of the river consists of the following :- co'fita. 1. Estimation of bankline migration over the entire reach and identification of erosion prone areas, along with the overall migratory trend in terms of shift of the centre line with respect to time. 2. Changes in overall width along closely spaced cross-sections and changes of planform areas between each pair of standard cross-sections with time and space in order to have an idea of areal loss of fertile Land. 3. Identification of nodal points which have not undergone appreciable changes with time, as these points are important for consideration of bridge sites. 4. Evaluation of braiding intensities to obtain information regarding status of braiding for each 'cross-section pair zone' to have a quantitative idea about the zones posing problems to navigation. 5. Computation of cross-sectional areas, average depth, maximum depth and bed profile slope from section to section and their temporal variations in terms of aggradations and degradations. All these parameters have their utilities in various riverine forecasting models and structural design of river regulation works. 6. Estimation of form-ratio values at each cross-section, as higher value of form ratio reflect the braided planform pattern, shallowness, multithreaded channel water ways and lower hydraulic efficiency at the cross-section concerned. 7. Profile concavity for the entire reach within Assam i.e ranging from Indo-Bangladesh border to Kobo using the average bed level of 1971, 19B1and 1988 to have a qualitative idea on the variation of water volume content and flow velocity along the river length, which helps to decide appropriate locations of diversion canals and water using industries. B. Preliminary study on the possibilty of transformation of the present braided pattern to meandering taking into account three controlling parameters viz. dominant discharge, average slope and median grain size of the bed material. 9. An Erosion study with identification of apparent causes has been incorporated far the reach between Phuloguri and Moriahola. In the Moriahola region the river has been observed to have created a real threat to the Kaziranga National (Reserved) Forest and in Majuli, the world's largest riverine island, the river has engulfed extensive fertile land causing immense miseries to the inhabitants.