Flood and Bank Erosion in Downstream of Panchanoi A Case Study of Dagapur and Bandijot, Sukna, Darjeeling

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22 févr. 2014 (il y a 3 années et 4 mois)

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Flood and Bank Erosion in Downstream

of

Panchanoi



A Case Study of
D
agapur and Bandijot
,
Sukna, Darjeeling





GUPTA

SUBHADIP
*



*Lecturer in Geography, PCMM,
Bonhugly, Kolkata


e
-
mail: sg.birati@gmail.com



Abstract


Flood is nothing but one of the important cumulative atmospheric hazards which are
caused due to cumulative effects of weather events and it is found as the
acute

environme
ntal hazard of north
Bengal. Almost

the
yearly

occurrence of flood as well
as river bank erosion makes

a real measurable condition for the inhabitants of the tea
estate.

Here in this paper the flood

and bank erosion problem
of
Panch
a
noi

river

has
been
studied by primary observation of field geomorphology. A micro level study has
been carried out for getting the geomorphic status of the

downstream

basin and a point
to point observation, analysis and interpretation relating to the problem of flood and
ba
nk erosion.



A
cknowledgement


The following concerns are acknowledged for their contribution in preparing
the
p
aper
,

t
he

organization of Natmo
, GSI,

and Survey

of India for supply
necessary
maps, database for whole field work.


Objective




The present problem has been taken into consideration primarily to study the nature
and characteristics flood to identify the pattern of bank erosion as major quarry of this
problem studied area.

T
he studied area, Bandijot and Dagapur Nadipar
are such
villages which are nearer to Siliguri township area, situated at the bank of R Panch
a
noi
downstream (tributary of R Mahananda).Almost the yearly occurrence of

flood as
well as river bank erosion make

a real measurable condition for the inhabitants of the
tea estate
.



Magnitude of the

H
azard



A considerable number of people become homeless
.

S
ome of them lose their only crop
practicing land or their vegetable garden due to the enormous bank erosion. Not only
that a number of cattle even villagers died in last flood occurred on October, 2007.
Now the Dagapur tea estate factory, the only source o
f income for the slum villagers,
will be severely affected in future flood occurrence, if no serious measures are taken
immediately
.


Probable

C
auses


The present study is given emphasis on

the

probable causes of the
flood in this region
.
T
he heavy downpou
r in a very sh
ort span of time, the high rate
of sedimentation in the
river bed and mostly the human interference play the major role for the execution of
such a

devastating flood
. Not only that the increasing tendency of deforestation at the
sight of rive
r bank of R Panch
a
noi, has been
accelerated the rate of bank erosion. It
passively increases the rate of surface run off
which also accelerates the rate of soil erosion
and passively the amount of bed load.

The
geomorphological

characteristics are very much
responsible for t
he said floods in regular
occurr
ence
.

The study across
-
valley profiles of
the panch
a
noi are also revealing some
important facts for such determination. From
north to south a number cross profiles are
drawn to

determine the valley as well as
channel configuration status of the panch
a
noi
from one bank to other at its downstre
am
section. From those profiles, the status of
erosion, transportation, and deposition of the
channel can be represented

an
d

the

Shifting

o
f
wetted section in the valley
is

observed. Deeps
are scattered and differentiated by moderately
highs and offer the channel secti
on diversified in
configuration. The

River Panch
a
noi,
from the Himalayan hilly terrain, got its course on the wide tract of
terai plain and has
a wide valley cross profile
.

Due to sudden decrease of velocity and seasonal variation
of discharge the river panch
a
noi has its pattern purely different from high altitude
course. Due

to wide valley
profile;

low velocity and less discha
rge the huge amount of
load with big size boulders, pebbles, and rock
fragments.

For the studied
span, thalweg

has bee
n recorded
. Loose sediment and rock fragments in different segments in the
channel bed are supported by annual renewal of sediment load es
pecially during the
rainy season; which

is significantly affecting the overall channel configuration of the
river panch
a
noi.

The movement of channel as well as its adjustment to the prevailing
topography and relief condition is another s
ignificant indicato
r for this

recurring

north
bengal

flood.

The

panch
a
noi in the downstream

recorded ideal meander and complex
pattern through

micro level observation of the
channel.

A number

of highs and lows
registered by pools and riffles are recorded in the channel denoting the channel depth
variation of river panch
a
noi.

Channel highs are denoting sedimentation or siltation in
the channel carried by the river during the rainy season
.
Chann
el highs are registered
as different depositional featu
res in the channel, such as mid
channel bar, channel bar,
point bar etc. of different sizes and shape. Channel deposition constitutes the
depositional features of the channel. This is slightly higher t
han the deepest part of the
channel, but during the rainy season these sectors are submerged into water an
d
became the active sector for bank erosion system.

The altitude of this part usually
ranging from 0.15m to 0.30m. Coarse to moderately

coarse sand, fine sand, silt,

medium size rock fragments,

small pebbles constitute this part. This part is elongated
in shape and has an ov
al shape crest,
diverging the water into braided channel.

Shallow and thin
sand deposition ranging from 0.05m to 0
.10
m.
in heig
ht. This
segment of R Panch
a
noi
is badly affected by man. Sand, pebbles, coarse and fine grain
sand are collected from the channel bed for
construction purpose. So
some abnormal
lows in the channel are created by anthropogenic
involvement.

Shallow

water track
constitute the part of channel where a small amount of turbid water with silt and clays
are flowed. Deep water channel usually maintain the perennial flow all throughout the
year and during the rainy season this part play an active role for th
e

development of
channel and changing channel configuration.


Suggestion

and

Mitigation




Some

measures

should be taken

to prevent the future
damages.

S
uitable

river training
measures

should be taken to

save the inhabitants. Though a
number of measures
have

already been taken by the government, such as
-

embankment has made, revetment
concept has also been introduced. But still the situation is not under control A huge
damage has done at the bottom of the pillar of

Toy train Panch
a
noi
River Bridge

due
to
high rate of scouring mainly in rainy season or at the time of high water level. So it
is so important to make a

artificial

diversion

or a flood wall

in the channel bed

to get
down the velocity
.

The wire
-

crating spur will be

a

useful measure to reduce the

flow
velocity
At the same time it is a
lso
to be said that the anthropogenic effects on

Panch
a
noi

river course should be reduced

to allow it to flow freely in its own course,
otherwise it is quite impossible to make success by adopting a scientific river training
management system. A mass awareness

should be made to reduce the tendency of
deforestation, agricultural practices in ri
ver bed etc.













Refference




Hundey, A & Banerjee, S., 1967, Geology & Mineral resources of West Bengal,
Mem. Geol.



Leopold, L.B., Wolman, M.G. & Miller, J., 1964, Fluvial processes in
Geomorphology, Freeman, Sanfrancisco



Mukhopadya, S.C., 19
82, The Tista Basin; A study in fluvial
Geomorphology,K.P. Bagchi and Co., Kol



Marrisawa, M., 1968, Streams their dynamics and morphology., Mc.graw
-
Hill
Book Co., Newyork



Wadia, D.N. (1975), Geology of India, Tata Mc.graw
-
Hill, New Delhi



Wooldridge, S.W.,
Morgan, R.S., 1959, An Outline of Geomorphology; The
physical basis of Geography, Longmans., London