Chapter 7 Causes of Flood 7.1 Causes of Flood 7.1.1 Natural Causes Flood is such a natural phenomenon that is uncontrollable and, to some extent, unpredictable. Flood basically a natural fluvial event especially in the late mature and old stage of the cycle of erosion. From such statement it needs not to be interpreted that no flood events are experienced by the youth or early mature stage of the fluvial cycle. In most of the periods the upper part of this basin remains flood free. In lower part of Mayurakshi basin including eastern part of Birbhum and entire Murshidabad district, there is a well bred geo-physical environment to invite and nourish the flood devastations. Moribund deltaic morphology, poor drainage condition, bifurcation of river Mayurakshi, huge deposition in the river bed, large number of braids, highly winding channel, lack of forest coverage, long standing siltation within the wetland command area, effect of external drainage morphology, poor infiltration capacity of the soil, highly concentrative rainfall etc. collectively responsible for intensive flood situation. As soil remains already saturated with moisture before peak monsoon, any additional input of heavy rain is enough to create a devastating flood (Bandyopadhyay, 1992). Key Words: Natural Flood Vectors, Anthropogenic Flood Vectors, Impact of Dam, Barrage and Embankment on Flood
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Chapter 7
Causes of Flood
7.1 Causes of Flood
7.1.1 Natural Causes
Flood is such a natural phenomenon that is uncontrollable and, to some extent, unpredictable.
Flood basically a natural fluvial event especially in the late mature and old stage of the cycle of
erosion. From such statement it needs not to be interpreted that no flood events are experienced
by the youth or early mature stage of the fluvial cycle. In most of the periods the upper part of
this basin remains flood free. In lower part of Mayurakshi basin including eastern part of
Birbhum and entire Murshidabad district, there is a well bred geo-physical environment to invite
and nourish the flood devastations. Moribund deltaic morphology, poor drainage condition,
bifurcation of river Mayurakshi, huge deposition in the river bed, large number of braids, highly
winding channel, lack of forest coverage, long standing siltation within the wetland command
area, effect of external drainage morphology, poor infiltration capacity of the soil, highly
concentrative rainfall etc. collectively responsible for intensive flood situation. As soil remains
already saturated with moisture before peak monsoon, any additional input of heavy rain is
enough to create a devastating flood (Bandyopadhyay, 1992).
Key Words: Natural Flood Vectors, Anthropogenic Flood Vectors, Impact of
Dam, Barrage and Embankment on Flood
Chapter -7, Causes of Flood
126
7.1.1.1 Huge Rainfall within Very Short Range of Time
The most important meteorological and climatic factor, inducing run off processes on earth is
precipitation (rain and snow), propelled by large scale (macro and synoptic scale), small scale
(meso and storm scale) precipitation processes (Hirschboeck et al., 2000).
Out of total rainfall almost 85% rainfall occurred within 3 to 4 monsoon months of the
year. About 60 to 70% rainfall was happened within very short period of time e.g. 3-4
days during 1956, 1970, 1973, 1978, 1999, 2000, and 2007.
Arrhythmic short range outburst interleaved by long lean period is recent trend of Indian
monsoon. For example during 23rd
to 25th Sept., 2007 amount of rainfall was 174.8 mm.
recorded by Kandi Meteorological Station (at Lower catchment) and large amount of rainfall in
the Chottanagpur area and the Rarh region of West Bengal (Upper catchment) were mainly
responsible for flood.
About 872.4 mm. or more rain within 3 days (19th
to 21st Sept. 2000) was happened only in the
lower segment of the basin area which is highly responsible for acute flooding in the lower and
middle catchments ever seen by the present dwellers. About 358.2 mm. rainfalls between 9th
to
11th July of 2006 and 435.4 mm. between 9
th to 13
th July of 2007 had beaten the region with
black and blue in shape of flood surge.
In figure 7.1 rainfall pattern during two devastating flood years 1978 and 2000 have been shown
in relation to average rainfall. Very high concentration is noticed in September which is far
greater than average rainfall and massive flood were recorded on that period.
Chapter ‐7, Causes of Flood
127
Figure 7.1
7.1.1.2 Poor Drainage Condition
It has been observed that lower part of Mayurakshi river basin has been suffering under poor
drainage. Large number of rivers like Kuya, Banki, Hizuli, etc. have concentrated in Hizole
wetland and influxed with Dwarka Babla river within very short range of space. So, in general
situation, Mayurakshi itself becomes brimful with its own huge volume of water as well as it
doesn’t have any further ability to carry out any extra water debouched by the tributaries like
Kuya. Average gradient of the confluence region is 1: 400 which is further reasonably low. Due
to such poor physiographic slope swift water movement is beyond expectation.
Banki, Hizuli, Mor, Kyua, Beli etc. river are mixing with Mayurakshi near to confluence area.
Cross sectional area of the tributaries are 563.65 sq.m. more than master Mayurakshi river (table
7.1). The slope of the confluence streams in this region is almost same but cross sectional
differences create difference in flow volume. If cross sectional area of some other micro streams
those exist in this region is calculated the surplus cross sectional area will be 476.13 sq. m.
greater than master stream. Therefore, the water of the tributaries is not freely influxes into the
main river and flood situation is getting appeared.
0
100
200
300
400
500
600
700
J F M A M J J A S O N D
Rai
nfal
l in
mm
.
Month
Rainfall Pattern in Flood Years
19782000Average
IndexRainfall distribution
Chapter -7, Causes of Flood
128
Table 7.1: Comparative Cross Sectional Area of Master Mayurakshi and its Confluence
Stream Tributaries
Inceptor river Area (sq.m.)
W x D
Tributary
rivers
Area (sq.m.)
W x D Remarks
Mor or
Mayurakshi
22.5x3.89
=87.52
Banki 17×2.77 = 47.09
(563.65-87.52) =
476.13 sq. m.
Extra cross sectional
area of the
tributaries than
master Mor
Hizuli 13×1.83 = 23.79
Dwarka 39.5×7.78 = 307.31
Kuya 35×3.78 = 132.30
Beli 15.5×3.43= 53.16
87.52
563.65 High flood
possibility
Moreover, large number of river segments has narrowly spaced and interwoven manner in the
confluence zone of Mayurakshi river in such a fashion that no one could find their paths other
than disappear to the extensive Hizole wetland area.
7.1.1.3 Effect of Bhagirathi and Uttrason
Mighty Bhagirathi River (master stream of Mayurakshi river) is flowing 2 to 4 km. eastern off
course of Dwarka Babla river (after mixing with Dwarka, the channel known as Dwarka Babla
river) like a Yazoo pattern of flow. Uttarasan river connects Dwarka-Babla river with Bhagirathi
river, which flowing few km. south east of Dwarka confluence point. This pattern of river
network influences the flood intensity in different ways (Pal, 2010).
i. There is very little relief variation (1 m. to 1.5 m.) between Dwarka-Babla river and
Bhagirathi river as well as very meager differences of water level. So, during monsoon
period as the elevation of water level in Bhagirathi rises up, huge water carried by the
tributary (Dwarka-Babla) does not get free access to meet with master river. Similarly,
Chapter -7, Causes of Flood
129
swelled water level in Dwarka-Babla river does not facilitates Mayurakshi river to pour
huge volume of water and therefore inundate the lower catchments.
ii. Uttarasan is an excellent bypass route of river Dwarka-Babla river which bypasses huge
water directly to Bhagirathi River (fig. 7.2). Due to little relief differences between
source to mouth of Uttarasan river, during monsoon period excess water of river
Bhagirathi or released water from Farakka reversely enters into the river Uttarasan like
back thrust, which hampers the normal bypassing system from Dwrka-Babla river to the
Bhagirathi river. Strong eddying and helical flow in Uttarasan river during monsoon are
the empirical evidences of such flow character. Moreover, this connecting channel today
has become so chocked that its carrying capacity has lost. However, the external effects
to some extent directly responsible for such dreadful, long durated flood.
Chapter -7, Causes of Flood
130
Source: Toposheet of SOI. Fig.7.2
7.1.1.4 Huge Siltation and Sand Deposition in the Wetland and River Bed
Human induced land use change, incremental losses of wetland, urbanization etc. have greatly
augmented runoff and associated risk of flooding (Bandhopadhyay and Mallik, 2005,
Rasmussen, 1994). Lower portion of this basin in Murshidabad is a natural storage unit but
steady deposition has forced it to lose its retention capacity.
Similarly, the rivers like Mayurakshi, Kuya, Bakreswar have lost their carrying capacities and
competencies in substantial degree. Due to deposition in lower portion and loss of carrying
capacity of the rivers are greatly responsible for flood severity. Some places of Mayurakshi river
Chapter -7, Causes of Flood
131
course, point bar and sand bar deposition are so active that river flow often choked. Sand clogged
rejected or kana channel near Narsingpur (Site no.7) clearly makes this argument stronger.
Plate 7.1 at Narsingpur, Birbhum Plate 7.2 at Boipur, Birbhum
Plate 7.3 at Satpolsa, Birbhum
7.1.1.4 Channel Shape
The magnitude, flood frequency, duration of flood flows etc. depend on the valley morphology
(Wohl, 2000). Areal morphmetric analysis reveals that the cross sectional areas of the river
between upper, lower middle and near confluence of the channel with variety of width. Its not a
normal character of a channel as river flows towards its maturity the width generally widening.
But here the abnormality has noticed that at middle part of the basin the channel width is so wide
(at Narsingpur 470.6 m., at Sainthia 519.7 m, at Harishchandrapur 424.9 m) but near confluence
just narrow down (at Nalghosa 16 m), (vide figure 3.16) which is also an important cause for
flood.
Meanders and more sinuosity raised the probability of flood. The sinuosity index of this basin is
4.145 which is quite high as we know that sinuosity index more than 1.5 means meandering river
course. So, here we found a positive relation between flood and meandering.
Chapter -7, Causes of Flood
132
Plate 7.4 wide river valey at Harishchandrapur Plate 7.5 narrow river valey at Nalghosa
7.1.1.6 Bottleneck Channel Shape and Channel Sinuosity
Generally the width of the channel gradually increases downstream but the situation is reverse in
case of Mayurakshi river. Noteable to mention that the thalweg of Mayurakshi river has tendency
to shift right ward, that’s why the width of the right bank from thalweg line is lesser than left
bank width (fig. 10). At the middle catchment near Sainthia, Boipur the channel width is 519.9
m. and 437 m and cross sectional area is 2012.013 sq. m. and 1520.76 sq.m. respectively. But at
the confluence area the width and cross sectional area is 15.5m. and 51.615 sq.m. respectively
(table 7.2, fig. 3.16 ). Due to such bottle neck like cross sectional pattern, the huge amount of
water coming from upstream do not getting enough space to drain out during full fledged
monsoon. Therefore, water certainly spreading outward and devastatingly inundates the entire
area.
Table 7.2: Cross-sectional pattern and potential discharge ability along Mayurakshi
downstream.
Do
wn
stre
am
Cross Section Sites Width Depth Cross sectional
area(sq.m.)
Santhia 519.9 m. 3.87 m. 2012.013
Boipur 437 m. 3.48 m. 1520.76
Near Saspara 51.2 m. 2.13 m. 109.056
Confluence 15.5 m. 3.33 m. 51.615
Source: Field Measurement
Chapter -7, Causes of Flood
133
7.1.1.7 Sand Terrace or Sand Shoulder
Thick sand made terrace like features developed at the bank foot is called sand terrace or sand
shoulder. From Sainthia to Boipur there is discontinuous 17 km. lengthy thick sand terrace in
two foots of the River banks. Its witdh is about 5 to 12 m. and thickness is 1.3 m. So, only
within the stretch of Sainthia and Boipur the volume sand in the sand terrace above channel bed
is about 1,87,850 cubic m. sand. These voluminous sand depositions resist the normal flow
character of this River and make the drainage poorer.
Plate 7.6 Sand terrace in Boipur Plate 7.7 Extended sand shoulder (width: 28m.), at Ektala
7.1.2 Anthropogenic Causes
Human induced regulation of flows has a history of about 8000 years, but its extent increased
spectacularly over the last 100 years as well as river behaviour also changed due to intrusive
human activities (Dunbar and Acreman, 2001, Sengupta, 2000). Similarly, modern flood is the
combination of natural system and human interaction system (Kates, 1971). Population pressure
is gradually increasing and to keep parity with the needs of human being, there has been a
parallel attempt to rule the river flow and to force the rivers surrender to the feet of human
civilization. For fulfilling the dream, lofty embankments have built up alongside the rivers or
dam or barrage across the rivers, or sometimes diversion of the river course or canal. Through
dam, barrage or river lift irrigation human beings have attempted to produce substantial volume
of crops. Multipurpose river valley projects like DVC, (Damodar Valley Corporation), Sutlej
project in India have been developed to gain an integrated package of profit in diversified
Chapter -7, Causes of Flood
134
ground. Such apparently beneficial activities today in many cases has been yielding a series of
consequent curse like boomerang effect. Few relevant aspects here have been pointed out briefly.
7.1.2.1 Lofty Embankment Construction
Present day’s river is mostly guided by the arrogant signature of the technological advancement
of the engineers e.g. embankment. In lower part of Mayurakshi and its major tributaries are
regulated by river embankments and most of the embankments are 4.26 m. to >6.40 m. in height.
Almost every 2-3 years interval, elevation of the embankment is getting rise in parity with rising
flood height. During 2007 centre pool of the Mayurakshi and Dwarka-Babla embankments have
been raised up almost about 0.914 m. (vide Plate 7.8 & 7.9).
Plate 7.8 at Sundarpur, Murshidabad Plate 7.9 at Sanspara, Murshidabad
Table 7.3 Breach of embankment
Year No. of breaching point
1991 1
1999 3
2000 5
Source: Directorate of Irrigation and Waterways Department, West Bengal-2008
Noteworthy, (a) Constructional material of the river embankment are very fragile e.g. loose sand
or sandy soil. Huge sands have been employed to erect the embankment along the river. So, how
much venturesome it may be is really perceptible to any common people (Ahmad et al., 2001,
Kale, 1998) (b) Base width of the embankment should be far wider than the apex but in reality
EMBANKMENT
EMBANKMENT
Chapter -7, Causes of Flood
135
there is no significant difference base and apex width therefore vulnerable for collapsing. (c)
Moreover, soil used for embankment, is just collected from the base of the embankments, as a
result deep scours have been developed at the off base of the embankment. These scours contain
water like marshy land almost all through the years and this situation is weakening the basement
of the embankment increase the threats of vulnerability for easy breaching. It again creates
potential hazards for sand splay in the embankment breaching points. (d) Due to embankment as
the geo materials of the river is not getting escape to spread outside the channel, continuous
accretion of sand in the river bed raises the bed level and reduces the carrying capacity of the
channel. Embanked channel even sometimes is not able to retain the water. Therefore by
embankment shifting far from the channel and increaseing the width of embanked channel the
holding capacity of swelling water during monsoon period has increased (see fig. 7.3). If
embankment is the ultimatum its base should be much wider than apex to resist huge water
pressure.
Figure 7.3 shows the cross section pattern across the river Mayurakshi at Sanspara (near Kandi,
Murshidabad). In this cross section old embankment remains as discarded one, because after
breaching of the embankment river has extended its floodplain, so, new embankments are being
constructed with much more height at some distance away from the river.
Fig. 7.3
Chapter -7, Causes of Flood
136
7.1.2.2 Presence of Zamindari Gher Bundh
Before independence Zamindars had built up several zamindari gher bundhs to save their
agricultural farmyard from flood invasion. Few significant gher bundh in the Hizole beel area
and surrounding area are 16 like Bhageer gher, Piprikuri gher, Tharkuramichak, Hazar bigha