Bed-Bank Relationship and Flood Characterisation …
201
Table 2 Incremental increase in areal extent due to every 0.5 m increase in water level than the
bankfull condition
Increment in water
level (m)
Right bank (increment
in areal extent) (km 2 )
Left bank (increment in
areal extent) (km 2 )
For both banks
(increment in areal
extent) (km 2 )
0.5
1
194.9
295.3
490.1
1.5
198.3
655.2
853.5
2
354.3
1145.0
1499.3
2.5
279.0
530.8
810.0
• In the downstream section, the right bank is more vulnerable to flooding. This is
probably due to the sediment influx from some of the major south bank tributaries
like the Burhi-Dihing, Disang, Dikhau and the Dhansiri which compensate the
sediment influx due to the north bank rivers. Moreover, shifting aggradational
tendency in the river bed reduces the bed-bank elevation difference.
• For 0.5 m to 1.5 m increase in water level head, the areal extent of flood inundation
in the right bank is much higher than the left bank areas (Table 1).
• However, with increase in water level over 1.5 m, the areal extent of inundation
in both banks almost tends to be equal (Table 1).
4.2 Flood Disaster Incubation
Flood incubation is a term used to define the growth of potentiality of flood in a
region. Many anthropogenic as well as natural factors are responsible for intensifying
the flood vulnerability of a region. Embankment construction, a major issue for the
people of Assam is one of the most important attributes to increase the potentiality
of flood vulnerability. Flood occurrence due to embankment breaching is a common
cause of flood in Assam.
Embankments which are built for mitigation of flood increase the flood vulnerability by decreasing the accommodation space for sedimentation by the river, thus
decreasing bed-bank depth which in turn greatly influences the flood vulnerability
of the region (see Fig. 2). For a constant annual discharge of water, the river water
overtops in the subsequent years thus flowing above the bank. This overtopping
water on entering the pore spaces of the embankments decreases the stability of the
embankments and hence acts as an incubator of flood.
By plotting average elevation of the bed with increase in unit amount of sediment
from present-day elevation of the river bed versus distance and the present-day reach
scale elevation of both the banks (Fig. 6), following observations were made:
• For 0.5 m to 1 m increase in river bed due to sediment deposition, the water level
overtops left bank at a particular reach.
201
Table 2 Incremental increase in areal extent due to every 0.5 m increase in water level than the
bankfull condition
Increment in water
level (m)
Right bank (increment
in areal extent) (km 2 )
Left bank (increment in
areal extent) (km 2 )
For both banks
(increment in areal
extent) (km 2 )
0.5
1
194.9
295.3
490.1
1.5
198.3
655.2
853.5
2
354.3
1145.0
1499.3
2.5
279.0
530.8
810.0
• In the downstream section, the right bank is more vulnerable to flooding. This is
probably due to the sediment influx from some of the major south bank tributaries
like the Burhi-Dihing, Disang, Dikhau and the Dhansiri which compensate the
sediment influx due to the north bank rivers. Moreover, shifting aggradational
tendency in the river bed reduces the bed-bank elevation difference.
• For 0.5 m to 1.5 m increase in water level head, the areal extent of flood inundation
in the right bank is much higher than the left bank areas (Table 1).
• However, with increase in water level over 1.5 m, the areal extent of inundation
in both banks almost tends to be equal (Table 1).
4.2 Flood Disaster Incubation
Flood incubation is a term used to define the growth of potentiality of flood in a
region. Many anthropogenic as well as natural factors are responsible for intensifying
the flood vulnerability of a region. Embankment construction, a major issue for the
people of Assam is one of the most important attributes to increase the potentiality
of flood vulnerability. Flood occurrence due to embankment breaching is a common
cause of flood in Assam.
Embankments which are built for mitigation of flood increase the flood vulnerability by decreasing the accommodation space for sedimentation by the river, thus
decreasing bed-bank depth which in turn greatly influences the flood vulnerability
of the region (see Fig. 2). For a constant annual discharge of water, the river water
overtops in the subsequent years thus flowing above the bank. This overtopping
water on entering the pore spaces of the embankments decreases the stability of the
embankments and hence acts as an incubator of flood.
By plotting average elevation of the bed with increase in unit amount of sediment
from present-day elevation of the river bed versus distance and the present-day reach
scale elevation of both the banks (Fig. 6), following observations were made:
• For 0.5 m to 1 m increase in river bed due to sediment deposition, the water level
overtops left bank at a particular reach.