Interfan area >> Geomorphology >>  Bank erosion

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Bank erosion is one of the most commonly experienced effects of high flows. Bank erosion is essentially governed by the stream power (tV), where t is shear stress acting on the bank and V is velocity of stream. As the discharge of a stream increases, the depth and mean velocity increases, due to which river banks are subjected to greater erosive action. Empirical relationship between velocity and discharge is given as V = a.Q^b (Richards, 1977), where the values of ‘a’ and ‘b’ would vary not only for different rivers but also for different sections of the same river due to variation in cross-section of channel.
A general observation from this equation is that increase in discharge would result in high stream power of the flow that would cause more bank erosion. In a meandering alluvial stream with a floodplain, the increase in discharge yields high sediment transport capability. The stream erodes sediment from its bed to keep pace with an increasing equilibrium rate, but there may be limits on sediment availability, either because of supply or due to bed-resistance. To maintain the equilibrium rate between sediment and discharge, the flow attacks the banks and hence, river widening occurs because of high discharge in the channel. However, detailed monitoring of alluvial channels shows that initial increase in river stage during a flood produces net filling due to higher sediment load (Leopold & Maddock, 1953). Then, continued rise of the flood peak results bank erosion and net scour to depths below the preflood level. Knighton (1984) proposed that Bank erosion on rising stage is caused due to direct shearing through hydraulic action. However, sometimes erosion occurs during falling stage through liquefaction of floodplain sediments (Baker, 1988). In practice, for bank material of a given cohesiveness, a small stream characterized with lesser discharge would be expected to have a channel of lower width-depth ratio than a large stream.

Most rivers in the north Bihar plains are characterized by rapid and frequent changes in stage and discharge which reduces the resistance to erosion of the bank material. The main erosinal process operating along the river bank is large scale slumping (Fig. 7) which is known to be more closely related to moisture conditions, although oscillations in river stage can influence the degree of bank wetting. It appears that following saturation, the bank material becomes unstable and slides along the curved shear planes in large blocks It does seem that a rise in water table in the channel is conducive to saturation of the bank material and to consequent instability. Detailed investigations are however continuing to develop a better understanding of the bank erosion processes operating in the humid plains of north Bihar.