flood in Assam
(I need just conclusion of essay?)
Answers
The Brahmaputra river has braided channels in its traverse through the
entire plains of Assam. Due to the heavy sediment load formation of sand
shoal and constant shifting of these channels are very common features of
the river. The river also has a tendency to shift laterally. Besides this
lateral movement the river attacks its banks sporadically through active
erosion and cause loss of land each year through the process of erosion.
The erosion is mainly attributed to extreme sediment charge. The extent of
loss due to erosion depends on the severity of flood. The Brahmaputra
river has a significant role for composing the physiography of the alluvial
plain in and around Hatishal, particularly after the Assam earthquake
1950. In monsoonai season, the high velocity of the water erodes the bank
of the river considerably as the sub-soil of the area mainly composed of
sand and silt.
2. The Hatishal area experiences bank erosion and aggradation phenomena
during the last 90 years, of which erosion in the present time become
more pronounced. Major modification of river bank in the area took place
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after the 1950 earthquake. The position and alignment of the Brahmaputra
thalweg are identified as important factors of bank failure and bank
migration. The scouring is very prominent than the deposition on the
southern bank of the Brahmaputra river in Hatishal area. The total amount
of areas eroded during the period 1915 to 2007 is 917 Ha in the studied
reach and bank erosion ranges from 73 to 2234 metre. It is observed that
there was large variation in amount of area eroded or fills up in different
sections along the studied reach.
3. The sub-soil study reveals that the predominant SP sandy soil which can
not withstand the onslaught of the heavy flood and is therefore susceptible
to bank erosion even at slight increase in velocity of water. The cohesive
soil is found up to nine metre depth below ground level. This soil has
average liquid limit between 43.16 to 46.01 percent indicating low to
medium plasticity of soil. Due to the shrinkage some cracks is develop in
soil and during monsoon the cracks are filled with water and get saturated,
thus increasing the weight of the earth-mass and slides down. On the
other hand during monsoon the upper layer becomes more saturated by
the rainfall and is subjected to soil erosion due to shear failure. At the
investigating area it is also observed that the water content of the soil is
higher at top layers more mainly composed of fine grained soils than at
the bottom layers composed of coarse grained soil. The shear test reveals
that the cohesion of the soil is very less, and the predominant poorly
graded sandy SP soil does not show cohesion as C values is zero. The
top soil with dominant clays, though posses high void ratio and porosity,
are less permeable. Normally in such situation consolidation of the top soil
is not possible and during floods the heavy water pressure in the soil
results extensive erosion through shear failure. When flood receeds, the
pore pressure in the river bank also leads to shear failure in the banks.
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4. The problem of acute erosion by the river Brahmaputra around Hatishal
area started since 1978. In spite of several short terms protection
measures taken from time to time the progressive erosion could not be
arrested in this reach. Anti-erosional measures are taken to guide and
direct the flow of the river and to regulate the river bed. Generally the bank
protection work is done for passing the high flood discharge in the reach
and to reduce bank erosion and to stabilize the river course. The structural
measures are physical measures such as construction of dams and
embankments, modification of roads and bridges, flood proofing, stream
channel modification, drainage improvement, anti-erosion works, raising of
villages and individual houses, construction of high raise platforms. The
non-structural measures are the planning and management for judicious
use of land, agricultural and other resources which can minimise the loss
and secure protection to the flood plain areas. The non-structural
measures of flood management are - the flood forecasting and warning
system, flood plain zonation, preparation of flood risk maps, local
hydrological studies, aforestation, public awareness programme, flood
insurance, research and development and improving information system
for post-flood recovery, etc.
Finally, it may be concluded that the mobile thalwegs of the Brahmaputra river
coupled with the silt and sand strata of its banks are the main causes of the bank
erosion. The soil forming the Brahmaputra bank in and around Hatishal area are
mainly composed of silt and sand with little amount of clay. For this reason this
bank is very susceptible to erosion even through slight increase in water current