192
S. K. Lahiri et al.
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.
Keywords Geomorphic relief · Flood vulnerability · Brahmaputra · Nodes ·
Internodes · Aggradation
1 Introduction
Big rivers show considerable degree of reach scale variability in the bed-bank relationship from source to sink. Thus, rivers even during peak flood season seldom
flood all the banks. Mountain fed rivers, acting as conduits between the high elevation source zones and the sinks in seas or oceans, run across reaches having variable
slopes. Moreover, tributaries of different size join the rivers at different places. Slope,
discharge and the load (both bedload and the suspended load) together play more
significant components to determine the stream power as well as the channel patterns
[1] of a river at a given reach. There are reaches where slopes are the principal factor,
and there are reaches where the discharge can play the main role to determine the
stream power [2]. Thus, the response of a river for equal slopes near the source zone
where it is rich mainly in bedload and before approaching the sink where it is usually
rich in suspended load needs different explanations.
2 Brahmaputra
Brahmaputra, having highly variable thalweg [3], the seventh-largest river in the
world [4] and after being joined by the Ganges, the highest sediment carrier [5, 6],
is also one of the worst flood causing rivers.
In recent times, the Brahmaputra came to headlines at least due to six major
reasons. Firstly, a trans-boundary international issue relating the construction of big
dams in the Chinese part; secondly, sudden influx of black clayey materials since the
last quarter of the year 2017 through the Siang causing a prolonged change in the
colour of the river water and drastic reduction in the fish population (the causes of
which are yet to be ascertained and there might be a connection with the unreported
dam failures in the upstream side); thirdly, changing characters of the floods in terms
of frequency, suddenness, intensity and the magnitude of losses. The average annual
S. K. Lahiri et al.
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.
Keywords Geomorphic relief · Flood vulnerability · Brahmaputra · Nodes ·
Internodes · Aggradation
1 Introduction
Big rivers show considerable degree of reach scale variability in the bed-bank relationship from source to sink. Thus, rivers even during peak flood season seldom
flood all the banks. Mountain fed rivers, acting as conduits between the high elevation source zones and the sinks in seas or oceans, run across reaches having variable
slopes. Moreover, tributaries of different size join the rivers at different places. Slope,
discharge and the load (both bedload and the suspended load) together play more
significant components to determine the stream power as well as the channel patterns
[1] of a river at a given reach. There are reaches where slopes are the principal factor,
and there are reaches where the discharge can play the main role to determine the
stream power [2]. Thus, the response of a river for equal slopes near the source zone
where it is rich mainly in bedload and before approaching the sink where it is usually
rich in suspended load needs different explanations.
2 Brahmaputra
Brahmaputra, having highly variable thalweg [3], the seventh-largest river in the
world [4] and after being joined by the Ganges, the highest sediment carrier [5, 6],
is also one of the worst flood causing rivers.
In recent times, the Brahmaputra came to headlines at least due to six major
reasons. Firstly, a trans-boundary international issue relating the construction of big
dams in the Chinese part; secondly, sudden influx of black clayey materials since the
last quarter of the year 2017 through the Siang causing a prolonged change in the
colour of the river water and drastic reduction in the fish population (the causes of
which are yet to be ascertained and there might be a connection with the unreported
dam failures in the upstream side); thirdly, changing characters of the floods in terms
of frequency, suddenness, intensity and the magnitude of losses. The average annual