Bed-Bank Relationship and Flood Characterisation …
199
Explanation: While considering the depth index, the channel belt included newly
evolved relict island the Dibru-Saikhoa. This was essentially due to the course correction of the Lohit River which helped to increase by jump the width of the Brahmaputra
channel belt. Thus the reach scale width index is showing much higher value.
In Box 2, deeper reaches are also associated with increasing width index.
Explanation: This is probably due to the massive erosion of the banks constituted
of loose bank materials. Some of the shallow subsurface loose sand units help rapid
‘toe cutting’. Massive lateral erosion helped to redistribute the sediments in the
downstream but did not cause effective shallowing tendency at the place of erosion
itself.
Box 3 represents a transition zone where some of the higher depth reaches (in the
upstream side) are wider and some are narrower (in the downstream side).
Explanation: The river beyond these reaches is going back to its normal logic of
aggradation
For Box 4, the reaches of the river show the standard opposite characteristics—
‘Deeper the reach narrower it is’.
Box 5 representing downstream end of the Majuli Island, shows a mixed behaviour
which is most probably due to anthropogenic intervention of various kinds of river
management schemes operational in this area since a long time.
The anomalous situations arising due to high degree of variability in the
sandbar/channel ratio can influence the bed-bank relation which is shown in Fig. 4c
by a second-order approximation of depth variation as a function of variation in width
during the year 2015.
4.1 Flood Characterisation
The flood characterisation done is mainly based on the areal extent of inundation
due to flooding with respect to an unit increase of head in the water level in full bank
condition. Each of the reaches selected every 10 km interval was further subdivided
into five sub-reaches and every incremental rise was identified in the DEM of the
Google Earth and the final equal interval contours of probable inundation were plotted
in the ArcGIS platform.
Using the DEM data areal extent of flooding is delineated (Fig. 5) and simple
volumetric calculations are being done. To keep the calculation simple the presence
of ridge and swale geomorphological features of the flood plain are not considered.
Areal extent of the inundation areas had been calculated as shown in Table 1.
From the data thus obtained and calculated assuming no difference in elevation
between both the banks, following observations are made (Table 2).
• A 0.5 m increase in the water level head can cause a great areal extent of flooding
than its preceding increase ranging between the minimum 490 to maximum
1500 km
2 .
199
Explanation: While considering the depth index, the channel belt included newly
evolved relict island the Dibru-Saikhoa. This was essentially due to the course correction of the Lohit River which helped to increase by jump the width of the Brahmaputra
channel belt. Thus the reach scale width index is showing much higher value.
In Box 2, deeper reaches are also associated with increasing width index.
Explanation: This is probably due to the massive erosion of the banks constituted
of loose bank materials. Some of the shallow subsurface loose sand units help rapid
‘toe cutting’. Massive lateral erosion helped to redistribute the sediments in the
downstream but did not cause effective shallowing tendency at the place of erosion
itself.
Box 3 represents a transition zone where some of the higher depth reaches (in the
upstream side) are wider and some are narrower (in the downstream side).
Explanation: The river beyond these reaches is going back to its normal logic of
aggradation
For Box 4, the reaches of the river show the standard opposite characteristics—
‘Deeper the reach narrower it is’.
Box 5 representing downstream end of the Majuli Island, shows a mixed behaviour
which is most probably due to anthropogenic intervention of various kinds of river
management schemes operational in this area since a long time.
The anomalous situations arising due to high degree of variability in the
sandbar/channel ratio can influence the bed-bank relation which is shown in Fig. 4c
by a second-order approximation of depth variation as a function of variation in width
during the year 2015.
4.1 Flood Characterisation
The flood characterisation done is mainly based on the areal extent of inundation
due to flooding with respect to an unit increase of head in the water level in full bank
condition. Each of the reaches selected every 10 km interval was further subdivided
into five sub-reaches and every incremental rise was identified in the DEM of the
Google Earth and the final equal interval contours of probable inundation were plotted
in the ArcGIS platform.
Using the DEM data areal extent of flooding is delineated (Fig. 5) and simple
volumetric calculations are being done. To keep the calculation simple the presence
of ridge and swale geomorphological features of the flood plain are not considered.
Areal extent of the inundation areas had been calculated as shown in Table 1.
From the data thus obtained and calculated assuming no difference in elevation
between both the banks, following observations are made (Table 2).
• A 0.5 m increase in the water level head can cause a great areal extent of flooding
than its preceding increase ranging between the minimum 490 to maximum
1500 km
2 .