Bed-Bank Relationship and Flood
Characterisation in the Upper Reach
of the Brahmaputra Valley, Assam
Siddhartha Kumar Lahiri, Angkuran Sarma, and Robert James Wasson
Abstract 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
S. K. Lahiri (B) · A. Sarma
Department of Applied Geology, Dibrugarh University, Dibrugarh 786004, Assam, India
e-mail: siddharthalahiri@dibru.ac.in
R. J. Wasson
ANU, Canberra, Australia
JCU, Singapore
UKM, Bangi, Malaysia
© Springer Nature Singapore Pte Ltd. 2021
C. Bhuiyan et al. (eds.), Water Security and Sustainability,
Lecture Notes in Civil Engineering 115,
https://doi.org/10.1007/978-981-15-9805-0_16
191
Characterisation in the Upper Reach
of the Brahmaputra Valley, Assam
Siddhartha Kumar Lahiri, Angkuran Sarma, and Robert James Wasson
Abstract 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
S. K. Lahiri (B) · A. Sarma
Department of Applied Geology, Dibrugarh University, Dibrugarh 786004, Assam, India
e-mail: siddharthalahiri@dibru.ac.in
R. J. Wasson
ANU, Canberra, Australia
JCU, Singapore
UKM, Bangi, Malaysia
© Springer Nature Singapore Pte Ltd. 2021
C. Bhuiyan et al. (eds.), Water Security and Sustainability,
Lecture Notes in Civil Engineering 115,
https://doi.org/10.1007/978-981-15-9805-0_16
191