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General Statement
Channel avulsion is one of the main fluvial
processes in the area. Apart from the major fan rivers such as Kosi and Gandak
which are known for their frequent migration, the interfan rivers of the north
Bihar plains have also recorded rapid and frequent avulsions. We select the
Baghmati river as an example of hyperavulsive river in the interfan area.
Avulsion
history – a case study of the Baghmati river
The course of the Baghmati River in north Bihar Plains has been very dynamic
through historic times. The midstream anabranching reach of the Baghmati
River has been particularly unstable due to frequent avulsions. Recent work
on the Baghmati river (Jain & Sinha, 2003, in press) has provided new
insights into the long-term evolution of the hyperavulsive river system.
This is among the most complete data sets available for the avulsion history
of any river in the world and, additionally, represents a style of
hyperavulsive river evolution that has not been documented in detail
elsewhere. This is especially valuable in view of our current limited
knowledge of many aspects of the globally important Gangetic Plains.
Window-wise reconstruction of channel
configuration
for the Baghmati river
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to get more information
(maps, satellite images, field photos)
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Summary of
avulsion history
(Jain & Sinha in press (1))
Avulsion mechanism
The causative factors of channel instability of the Baghmati are manifold.
Firstly, the frequent overbank spilling, very high wash load, and rapid
aggradation of the Baghmati channel are some of the hydrological factors
which make the channel prone to avulsion. Extreme hydrological variation and
sedimentological readjustments are the prime factors influencing channel
migration in the region. A zone of excessive deposition in a particular
reach causes significant changes in channel configuration thereby triggering
channel movement, helped by local slope conditions. Further, the plains of
north Bihar area are neotectonically active as evidenced by two major
earthquakes in recent past (1934 and 1988) and several sub-surface faults.
In the Baghmati plains, a number of fluvial anomalies such as U-shaped
meander cutoffs, straight channels, rectangular drainage, compressed
meanders, offset of river courses etc. are observed, which prove beyond
doubt that neotectonic factors cause significant changes in channel
morphology and affect channel-floodplain slope conditions. It is likely
therefore that such tectonic tilting is another important triggering
mechanism for the avulsion events.
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