Bed-Bank Relationship and Flood Characterisation in the Upper Reach of the Brahmaputra Valley, Assam

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 the average bank elevations by 0.5, 1.0, 1.5, 2.0 and 2.5 m assuming absence of embankments. Lastly, assuming complete embankments for both the banks exclude the possibility of immediate flooding but accelerate thereby the river bed construction and practically zero bank construction which can be interpreted in terms of disaster incubation. For uniform aggradation all along the river bed of the study area, reaches having river bed elevation equal or higher than the bank elevation will be more prone towards embankment breaches. Accordingly, for different aggradation thickness values of 0.5, 1.0, 1.5, 2.0 and 2.5 m within the channel belt, the incubation of vulnerability for different reaches of both the banks was identified.