Bed-Bank Relationship and Flood Characterisation …
197
Apparently, these two hypotheses oppose each other! However, it can be explained
by the fact that the stream power is basically proportional to the product of the slope
and the discharge and the constant of proportionality can be a factor of bank material
consolidation.
For valleys having strong structural controls, slopes show considerable lateral
variability (‘kinks’ in the elevation variability profile). High slope segments witness
increases in the stream power which helps greater bed incision and lesser aggradation
potentiality and hence increased depth and narrower widths (Nodes). On the other
hand, zones of subsidence, showing enhanced aggradation tendency, more numbers
of sandbars and multichannels. Stream power is determined mainly by the discharge
volume—accordingly, wider the channel, higher the stream power and higher the
rate of bed incision (for ‘Internodes’).
Thus, for ‘Nodes’, slopes play the decisive role, whereas for ‘internodes’, relative
discharge volume plays more important role to determine the depth of the thalweg.
Moreover, average stream power at ‘Nodes’ is more than the ‘Internodes’.
Assumption 3 We have further assumed that the continuous flow of the river from
its upstream to downstream end can be replaced by ‘cascade flow’ (discrete) which
essentially means that when the bankfull condition of a given reach is surpassed and
inundation of the banks take place, the turn of the next downstream reach comes
immediately afterwards as if ‘flooding in succession’. However, the delay time is so
small that the process becomes closer to the continuous flow condition. The advantage
of the assumption is that bed-bank relationship of individual reaches can be treated
as principally responsible for inundation of that reach. Impact of inundation in the
upstream reaches on the downstream reaches is neglected.
4 Data and Interpretation
To introduce comparability in the morpho-dynamics of the Brahmaputra River in
terms of the reach scale variability in the depths (for the present situation only) and
the widths (for 1915, 1975 and 2015) two indices were used which are given below.
Depth Index =
Reach depth − Average depth
Maximum depth − Minimum depth
And,
Width Index =
Reach width − Average width
Maximum width − Minimum width
‘+’ve values of both the indices mean higher than the average.
197
Apparently, these two hypotheses oppose each other! However, it can be explained
by the fact that the stream power is basically proportional to the product of the slope
and the discharge and the constant of proportionality can be a factor of bank material
consolidation.
For valleys having strong structural controls, slopes show considerable lateral
variability (‘kinks’ in the elevation variability profile). High slope segments witness
increases in the stream power which helps greater bed incision and lesser aggradation
potentiality and hence increased depth and narrower widths (Nodes). On the other
hand, zones of subsidence, showing enhanced aggradation tendency, more numbers
of sandbars and multichannels. Stream power is determined mainly by the discharge
volume—accordingly, wider the channel, higher the stream power and higher the
rate of bed incision (for ‘Internodes’).
Thus, for ‘Nodes’, slopes play the decisive role, whereas for ‘internodes’, relative
discharge volume plays more important role to determine the depth of the thalweg.
Moreover, average stream power at ‘Nodes’ is more than the ‘Internodes’.
Assumption 3 We have further assumed that the continuous flow of the river from
its upstream to downstream end can be replaced by ‘cascade flow’ (discrete) which
essentially means that when the bankfull condition of a given reach is surpassed and
inundation of the banks take place, the turn of the next downstream reach comes
immediately afterwards as if ‘flooding in succession’. However, the delay time is so
small that the process becomes closer to the continuous flow condition. The advantage
of the assumption is that bed-bank relationship of individual reaches can be treated
as principally responsible for inundation of that reach. Impact of inundation in the
upstream reaches on the downstream reaches is neglected.
4 Data and Interpretation
To introduce comparability in the morpho-dynamics of the Brahmaputra River in
terms of the reach scale variability in the depths (for the present situation only) and
the widths (for 1915, 1975 and 2015) two indices were used which are given below.
Depth Index =
Reach depth − Average depth
Maximum depth − Minimum depth
And,
Width Index =
Reach width − Average width
Maximum width − Minimum width
‘+’ve values of both the indices mean higher than the average.