Morphodynamics in the Upper Assam Part of the Brahmaputra River- A Planform GIS Based Study (original) (raw)
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The Brahmaputra is one of the largest tropical rivers of the world and is located in an area of high structural instability as evidenced from the presence of a large number of earthquakes in the Himalayan catchment through which it flows. Syntectonic evidence of changes in the morphodynamics is difficult to identify for the large rivers. Nevertheless, we note that the Brahmaputra River has become astonishingly large in planform in a historical timescale. Reconstruction of planform changes over a period of 90 years in the upper reaches of the Assam valley shows that the 240-km-long channel belt is widening all along its course in the region. From the average width of 9.74 km in 1915, the channel belt has widened to the average width of 14.03 km in 2005 (44% widening), and in certain reaches the average widening is as high as 250%. However, the bank line shift is not symmetric along both banks. Further, the planform characteristics of the Brahmaputra River reveal significant spatial and temporal variability fromupstreamto downstreamreaches, and we attribute this variability to tectonogeomorphic zonation of the river based on subsurface configuration and channel slope. Further, the tributaries joining the northern and southern banks of the Brahmaputra differ remarkably in terms of river dynamics, and this is attributed to the differences in tectonic regimes of theHimalaya in the north and the Naga Patkai hills in the south.
River channel changes of the Subansiri in Assam, India
Geomorphology, 1999
The sequential changes in the position of banklines as a result of bank erosion, as well as various changes in the channel have been studied in the Subansiri river in Assam, India. The study was carried out from available topographic maps of 1920, 1970 and satellite imagery of 1990. The types of changes taking place in the Subansiri river channel as observed from Ž. the above-mentioned maps and imagery are grouped into four categories: 1 alteration of the direction of flow due to neck Ž. Ž. Ž. cutoff , 2 widening of a channel in response to bar development, 3 development and subsequent abandonment of Ž. anabranches, and 4 progressive shifting of meander bends. The river channel has become substantially wider during the period of 1970-1990. There has also been a progressive increase in the number and area of mid-channel bars with a corresponding decrease in the number and area of point bars during 1920-1990. These periodic and progressive changes are responsible for a change in channel pattern of the river. A study made on nature and amount of change in the banklines of Ž. the Subansiri river by dividing the channel 84 km in 1970 into 10 equal segments reveals that the types of change are varied, and in some cases chaotic in nature. Study of the bankline shift of the Subansiri determined separately for the periods 1920-1970, 1970-1990 and 1920-1990 at 20 evenly spaced transverse sections reveals that there has been a substantial westward shift of both the banklines, up to a maximum of 6 km, in the three upstream sections during 1920-1970. However, shifting of banklines in both directions and the unequal amount of shifting in the same direction have caused a marked increase in channel width during 1970-1990. It is found that due to bankline migration, the total amount of area eroded along the entire course under study during 1920-1970 and 1970-1990 are 107.90 and 57.50 km 2 , respectively. The overall sinuosity which was 1.51 in 1920 has decreased to 1.30 in 1970, while the corresponding braiding intensity has increased from 0.79 to 1.32. The river channel seems to make a remarkable change from a meandering pattern in 1920 towards a braided pattern by 1990 as seen from gradual increase in overall braiding intensity.
Bed-Bank Relationship and Flood Characterisation in the Upper Reach of the Brahmaputra Valley, Assam
2021
Brahmaputra valley relief of upper Assam in India bears plenty of evidences to suggest active structural control as well as remarkable unevenness in sediment budgeting. These two major forcings cause highly variable bed-bank relationships along different reaches of the Brahmaputra's channel belt. Under steady average annual precipitation in decadal scale, flood vulnerability can be taken as directly proportional to the decreasing rate of bed-bank elevation difference. Normally, average bed elevation of the highest order river of a given valley reach is the base level of the reach. Bed-bank architecture over the years shows different reach scale possibilities. A big braided river like the Brahmaputra shows alternating narrower 'nodes' and wider 'internodes'. Usually, nodes are deeper and anti-nodes are shallower. Reach scale widening of rivers over time on many occasions is accompanied by shallowing tendency as well. For the upper reach of the Brahmaputra River and the valley, reach scale plano-temporal variability for the period 1915-2015 was monitored. From the confluence of three major rivers, the Siang, the Dibang and the Lohit, up to 230 km downstream, 23 reaches each of 10 km width were chosen. Essentially we have four findings. First, by measuring plano-temporal variability of sandbar/channel areas we could locate reaches having steady rate of aggradation; secondly, two indices for depths and widths were developed which help to identify normalised deeper zones and zones showing normalised widening tendency over three different average widths (1915, 1975 and 2015). This also helps to test the validity of the general assumption whether shallower reaches show a general trend of widening or not for the upper reach of the Brahmaputra valley. Thirdly, by assuming discretisation of the flow into equal width reaches, probable flood inundation areas were identified for incremental jumps of water levels over
European Scientific Journal, 2014
Since ages people and rivers have been closely associated with and a slight change in behavior in any aspect has serious repercussions upon the other. Brahmaputra-the lifeline of about 264 lakh population of Assam, has been intrinsically related with the way of life of the inhabitants within its domain and has both positively and negatively affected them. Bank line migration and consequent channel shift is a common phenomena of the mighty river due to its high braided condition and host of other geological as well as other hydrological factors. The study incorporates the Agyathuri-Sualkuchi stretch of Kamrup district, within 5 kms downstream of the Saraighat bridge (the first bridge over Brahmaputra), where the river shows strong northward migrating tendency. Major changes and displacements have been taking place in the region due to channel shift. Keeping in view all these aspects, the paper tries to examine the extent of area encroached by the river since 1911 to 2005 and thereby analyse the causes responsible for channel shift. The specified objectives have been fulfilled based on the utilization of toposheets and google maps in GIS environment including personal field visit. Necessary maps and diagrams have been prepared for exposition of the problem.
River Bank Line Shift Caused by Brahmaputra in Morigaon District, Assam (1996-2021)
2022
The Brahmaputra River, one of the largest rivers in the world, acts as a backbone for the agro-valley region of Assam. But the chronological shifting of the river, frequent flooding and severe erosion, have adverse effect on riverbank stability. Bank erosion of river Brahmaputra has reached an alarming proportion in many parts of Assam including Morigaon district. The objectives of the study are to examine the status of the bank line and the shifting of the bank line of the Brahmaputra River in the Morigaon district (1996-2021). The status of the bank line and its shifting were carried out from three sets of satellite imageries from Landsat-TM and Landsat-OLI for the year 1996, 2016, and 2021. From Mayong Revenue circle in the west to Laharighat in the east, the river along the bank line has been divided into 43 strips at an equal spacing of approximately 1 km and reference cross-sections have been drawn at the boundary of each strip. The result shows that at least 94 villages got e...
Brahmaputra River, Assam, India: Physiography, basin denudation, and channel aggradation
Water Resources Research, 1985
The Brahmaputra River in Assam, India, characterized by high seasonal variability in flow, sediment transport, and channel configuration, experienced a secular period of aggradation from 1971 to 1979. The suspended load budget indicates an overall aggradation of the ...
Rivers not only represent a vital resource for human activity but they are central to an understanding of geomorphology of most regions. A section of the river Subarnarekha (near Gopiballavpur) is considered for the study on bed and bank morphology associated with patterns of process and channel adjustment. The process-based study of alluvial channel has emphasized an alternative and smaller time and space scale for understanding alluvial channel dynamics. The sand-bed of river tract is complicated by the effects of significantly changing bed forms. However, the gravel bed of the river is lying below the depth of 1 meter from the top of the point bar sand bed which is often not mobile until discharges approach bank-full stage. The bank morphology reveals flood signatures of past events. The depositional activity of over bank flow on the floodplain is very different among the riverine system. The class of extreme events determines channel capacity, affecting channel pattern and more frequent events, although at times widely separated in time, controlled bed load movement and bed form adjustment. Variation of grain size is recorded from the outer bank to inner bank at the meandered section of Subarnarekha. Sand splays are significant feature on the over bank deposits particularly at the section of meandered banks. Bank margin cliffs are found on the older depositional bars at the curvature of the channel. All these modification of channel floor and channel bank are product of bank full discharge in the occurrences of floods. Repeated field survey, sediment grain size analysis, measurement of geomorphic features, satellite image analysis and analysis of discharge data reveal the above features of dynamic alluvial channel.
The purpose of construction of bridge over the river or stream is to ensure and facilitate the smooth communication over the waterways. However, in certain engineering designs of bridge, a significant portion of waterway is occupied by the piers placed on the river bed and approach road build on the river bed. This results the constriction of the natural waterway. When the waterway is constricted because of the construction of bridges, it results detrimental effects on the morphology of the streams. The major consideration of the paper is to study the effect of constriction of the natural waterway on the river bank morphology and river island due to bridge construction. In this study, 4.94 km long Bogibeel Bridge constructed over the river Brahmaputra is taken into consideration to study the morphological changes in its upstream and downstream portion of the river reach due to the constriction of the waterway. With the completion of the construction of the piers and approach road of Bogibeel Bridge, some significant morphological changes have already been observed. The satellite imageries of the area over two decade clearly indicates the changes that took place in the river bank morphology due to the construction of the bridge structure on the river bed. Some significant gradual modification in the river bank as well as on the permanent river island is identifiable with the progress of the bridge construction. The portions of the river bank which recede alarmingly due to changes in the configuration of the river channel have been identified to take up effective and sustainable measures to reduce or stop the current erosion.
Geomorphology, 2015
Rudra (2014, Geomorphology, 227, 87-100) presented a review of geomorphic changes of the Indian part of the Ganga-Brahmaputra delta (GBD) during the last ~240 years, emphasising deterioration of distributary channels, effects of discharge augmentation from the Farakka barrage project into the Bhagirathi-Hugli river and the patterns of coastal erosion. The work contained many inconsistencies, improper understanding, and misrepresentation of facts. The principal of these involved (1) usage of nomenclature like GBD, Bengal basin, and their delineation; (2) discussions made on evolution and current status of the deltaic distributaries; and (3) generation and presentation of data on discharge, tides, and sea level. Besides this, most figures of the work contained positional inaccuracies and/or errors in survey years. We systematically addressed all these issues in the present discussion and offered corrected and up-to-date information wherever applicable.