Bed-Bank Relationship and Flood Characterisation …
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flood toll of human lives increased from 41 (during 1953–1995) to 63 (during 2001–
2017) in Assam. In 2017, the number of deaths was 158. Most of these deaths were due
to embankment breaches; fourthly, unprecedented magnitude of bank-line migration;
the fifth factor was related to a grand design of river linking project of exporting
surplus water from the northern India to the water starved southern provinces and the
sixth factor was related to a massive dredging project of the Brahmaputra channel belt
apparently aimed at improving the navigability of the river as a part of an ambitious
planning to expand the length of waterways along with the roadways as a part of
raising a robust infrastructure of communications. A common factor that runs under
all these issues is the hydro-sedimentary budgeting, its spatio-temporal variability
annually and over different cycles. To refresh a few information and general statistics,
the Brahmaputra River is approximately 3848 km in length and runs through Tibet
(autonomous region of China) in between the great Himalayan ranges in the south
and the Kailash range to its north for nearly 1650 km in west to east direction. The
river is also known as the Tsangpo (purifier) in China. It enters India (Arunachal
Pradesh) after taking a southward direction near the Namcha Barwa mountain range
in the Upper Siang district. One school [7–11] likes to call the Brahmaputra as Dihang
or Siang River in the Arunachal Pradesh which flows along the north-south direction
through Upper Siang and east Siang districts covering a total of 400 km of Arunachal
Pradesh before it enters Assam. The river runs for 650 km from east to west through
Assam and acts as a valley divider before entering Bangladesh (rechristened as the
Jamuna) and ultimately drains into the Bay of Bengal. The other school [12–14] puts
the name Brahmaputra after three major rivers, the Siang, the Dibang and the Lohit
confluence (at a place called Kobo as could be seen in the topographic map prepared
during 1912–1926 having the scale 1inch = 4miles). Present study covers only the
upper reach of the Brahmaputra River and the adjacent banks, a 230 km long stretch
(Fig. 1).
3 Bed-Bank Relationship
Flood characterisation essentially means spatio-temporal variability study in the
bed-bank relationship of a river system which due to changing hydro-sedimentary
budgeting makes different reaches of a valley susceptible to inundation due to the
‘excess water’ over the ‘bankfull condition’. A simple model of flooding is first to
visualise an inclined trough having an inlet and outlet at the opposite ends, sediment
loaded water flowing through it. When the rate of influx (water or/and sediment) is
more than the outflow, height of the water column keeps on increasing and a stage
comes when the spill-over happens. Now, the question is wherefrom the spill-over
begins first? For the analogy taken up in the present discussion, the obvious answer
will be the downstream side. However, for a river valley, when the flood prone areas
are mapped, a considerable degree of arbitrariness is observed (Fig. 2). For example,
(i) both the banks of a river may not be reaching ‘bankfull condition’ at the same point
193
flood toll of human lives increased from 41 (during 1953–1995) to 63 (during 2001–
2017) in Assam. In 2017, the number of deaths was 158. Most of these deaths were due
to embankment breaches; fourthly, unprecedented magnitude of bank-line migration;
the fifth factor was related to a grand design of river linking project of exporting
surplus water from the northern India to the water starved southern provinces and the
sixth factor was related to a massive dredging project of the Brahmaputra channel belt
apparently aimed at improving the navigability of the river as a part of an ambitious
planning to expand the length of waterways along with the roadways as a part of
raising a robust infrastructure of communications. A common factor that runs under
all these issues is the hydro-sedimentary budgeting, its spatio-temporal variability
annually and over different cycles. To refresh a few information and general statistics,
the Brahmaputra River is approximately 3848 km in length and runs through Tibet
(autonomous region of China) in between the great Himalayan ranges in the south
and the Kailash range to its north for nearly 1650 km in west to east direction. The
river is also known as the Tsangpo (purifier) in China. It enters India (Arunachal
Pradesh) after taking a southward direction near the Namcha Barwa mountain range
in the Upper Siang district. One school [7–11] likes to call the Brahmaputra as Dihang
or Siang River in the Arunachal Pradesh which flows along the north-south direction
through Upper Siang and east Siang districts covering a total of 400 km of Arunachal
Pradesh before it enters Assam. The river runs for 650 km from east to west through
Assam and acts as a valley divider before entering Bangladesh (rechristened as the
Jamuna) and ultimately drains into the Bay of Bengal. The other school [12–14] puts
the name Brahmaputra after three major rivers, the Siang, the Dibang and the Lohit
confluence (at a place called Kobo as could be seen in the topographic map prepared
during 1912–1926 having the scale 1inch = 4miles). Present study covers only the
upper reach of the Brahmaputra River and the adjacent banks, a 230 km long stretch
(Fig. 1).
3 Bed-Bank Relationship
Flood characterisation essentially means spatio-temporal variability study in the
bed-bank relationship of a river system which due to changing hydro-sedimentary
budgeting makes different reaches of a valley susceptible to inundation due to the
‘excess water’ over the ‘bankfull condition’. A simple model of flooding is first to
visualise an inclined trough having an inlet and outlet at the opposite ends, sediment
loaded water flowing through it. When the rate of influx (water or/and sediment) is
more than the outflow, height of the water column keeps on increasing and a stage
comes when the spill-over happens. Now, the question is wherefrom the spill-over
begins first? For the analogy taken up in the present discussion, the obvious answer
will be the downstream side. However, for a river valley, when the flood prone areas
are mapped, a considerable degree of arbitrariness is observed (Fig. 2). For example,
(i) both the banks of a river may not be reaching ‘bankfull condition’ at the same point