Landslide disaster perception of the AILA cyclone in the Darjeeling ...
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INTERNATIONAL JOURNAL OF GEOMATICS AND GEOSCIENCES
Volume 3, No 1, 2012
© Copyright by the authors - Licensee IPA- Under Creative Commons license 3.0
Research article ISSN 0976 – 4380
Submitted on May 2012 published on July 2012 13
Landslide disaster perception of the AILA cyclone in the Darjeeling town,
West Bengal, India Bhattacharya Sudip Kumar
©
Assistant Professor, Department of Geography and Applied Geography,
PO- North Bengal University, Dist- Darjeeling, PIN- 734013
skbhatt2002@yahoo.co.in
ABSTRACT
A heavy downpour of 465 mm within 48 hours (which is equivalent to nearly 12 days’
rainfall in the rainiest month i.e. July) was the cause for several destructive landslides
claiming 7 lives and huge property in the Darjeeling town on 26th
and 27th
May, 2009
triggered by the cyclone Aila. The present paper deals with the detailed investigation of two
such most devastating landslides occurred at Frimal Village and Haridas Hatta in the
Darjeeling town. In the Frimal Village, landslide reoccurred five times on a single spot within
a period of 18 hours completely demolishing 22 houses below and three dunny (safety) tanks
within the slide itself. No casualty was recorded except huge loss of property. But in the
Haridas Hatta, landslide occurred only one time with high intensity partially damaging a
house at the lower level and claimed seven lives. It is found that reckless construction of
multi-storied buildings and their pressure, unmanaged building and community outlets,
alteration of the natural setup of the soil by frequent digging for funeral purpose contributed
the physical changes in the slope character. These changes being associated with scanty
vegetation cover on the soil surface and poorly maintained plants of the tea garden incapable
of holding soil on the steep slopes were the causes of such devastations following heavy gush
of rain by the cyclone Aila.
Keywords: Destructive landslides, cyclone Aila, reckless construction, community outlets,
frequent digging, physical changes in slope character.
1. Introduction
On 24th
May, 2009 a cyclonic storm named as Aila occurred in the southern part of west
Bengal, the effect of which was reverberated in the Darjeeling Hills from 25th
to 27th
May,
2009 (Darjeeling Times, 2009). A rainfall amounting 465 mm within 48 hours from 26th
to
27th
May, 2009 ( Figure 1), which usually occurs in 12 days in the rainiest month i.e. in the
month of July, was the triggering factor of several devastating landslides tolling 7 lives &
huge property (Govt. of W.B., 2009) in the Darjeeling town. The mean daily rainfall of the
Darjeeling town from 10 years rainfall data for the month of July being 38.305 mm (see
Figure 2) clearly demonstrates that Aila rainstorm produced 6 days’ rain in one day or 24
hours (which was 232.5 mm).
In the entire Darjeeling hills 40 major landslides occurred claiming 27 lives beyond 2 persons
who are still untraced (Darjeeling Times, 2009; Govt. of W. B., 2009 ). The total damage of
dwelling houses caused by this storm induced landslides was Rs 888.67 million,
approximately 542.989 hectares of agricultural lands have been reported to be damaged. All
major 5 roads connecting Darjeeling to Siliguri and Kalimpong faced 15 to 20 landslides in
each stretch. (Govt. of W. B., 2009). In global perspective this landslide hazard may be
compared with two contemporary landslide events i.e. Nile landslide near the south-central
Landslide disaster perception of the AILA cyclone in the Darjeeling town, West Bengal, India
Bhattacharya Sudip Kumar
International Journal of Geomatics and Geosciences
Volume 3 Issue 1, 2012 14
Washington town of Nile on October 11, 2009, at the Naches River Valley and the typhoon
‘Kiko’ (international codename: Morakot) induced landslide hazard near Manila (Archive for
November, 2009, Abs-cbnnews 2009) which have got much exposure but in vigor these two
events were no more devastating than Aila incidence so far the damage and casualties are
concerned.
The present paper therefore, takes an attempt to investigate two most serve Aila induced
landslides occurred at the Frymal village and Haridas Hatta in the Darjeeling town which can
be considered as typical representatives of all the 40 major landslides occurred by Aila
incidence as the causative factors of slope failure were approximately same all over being
mostly finalized by ‘anthropogenic abuses’ of slope an ubiquitous phenomena over
Darjeeling hills in the present time (Bhattacharya ,1998, 1999, 2002, 2009).
.
Figure 1: Daily rainfall in the month of May 2009
Figure 2: Daily rainfall in the month of July (10 years average).
Landslide disaster perception of the AILA cyclone in the Darjeeling town, West Bengal, India
Bhattacharya Sudip Kumar
International Journal of Geomatics and Geosciences
Volume 3 Issue 1, 2012 15
Figure 3: Location map of the study areas in Darjeeling Hills
2. Study area
The study areas of the Frimal Village and the Haridas Hatta landslides are located within 27°
03’ 10’’- 27° 03’ 16’’ N latitudes & 88° 15’ 25’’- 88° 15’ 29’’ E longitudes and 27° 02’ 56’’-
27° 02’ 59’’ N latitudes & 88° 15’ 41’’- 88° 15’ 45’’ E longitudes respectively (Figure 3) in
the Darjeeling Hills at the extreme north of West Bengal state, India.
Figure: 4: Physiography of the Darjeeling Town
2.1 Geomorphology, geology, climate and vegetation of the study area
Geomorphologically Frimal village and the Haridas Hatta in the Darjeeling town where the
landslides have occurred are situated on the rugged physiographic setup of the Middle hills of
Darjeeling Himalaya having high amplitude of relief and infested by numerous rivers and
streams with very steep valley side slopes. The physiography of the Darjeeling town will be
best understood from transverse cross sections prepared for the Darjeeling town area in the
figure 4.
Geologically the entire Darjeeling hill area is consisted of six types of formations starting
with Alluvium (Pleistocene to subrecent deposite, 1.8 million years - 10000 years), Raised
Terrace of bouldery formation (Pleistocene period, 1.8 million years) and Siwalik (Miocene,
26 million years) successively upward at the foot hills and Damuda (Permian, 280 million
Landslide disaster perception of the AILA cyclone in the Darjeeling town, West Bengal, India
Bhattacharya Sudip Kumar
International Journal of Geomatics and Geosciences
Volume 3 Issue 1, 2012 16
years), Daling Series and Darjeeling Gneiss (Archaean; the Dharwar system, about 3787
million years) on rest of the hilly parts (5a). The Geological set up of the Darjeeling town is
comprised of Darjeeling Gneiss having coarse textured Gneiss with Kyanite and Sillimanite
(Powde and Saha, 1976) of the oldest Dharwarian system (Figure 5b). This rock in
comparison to the other geological formations is far more hard and resistant to denudation
and thus having higher threshold value.
(a)
(b)
Figure 5: Geology (after Powde, and Saha, 1976 ) (a) Darjeeling Hills, (b) Frimal Village
And Haridas Hatta Landslide Spots
The climate of the Darjeeling town is associated with Koppen’s Cwa (Subtropical Humid)
type of climate since data analysis from Table 1 shows that the entire Darjeeling hill region
has an average temperature (18.60 °C) above 10°C in their warmest months, and a coldest
month i.e., January (11.45 °C) has average between −3 °C and 18 °C with dry winter (second
letter w indicates driest winter month with average precipitation [15.54 mm January] less
than one-tenth wettest summer month average precipitation [1215.99 mm July]; one variation
also requires that the driest winter month should have less than 30 mm average precipitation
which is 15.54 mm in January in this area ) and hot summer (third letter a indicates warmest
month average temperature above 22 °C [22.87 0C in this area] with at least 4 months
averaging above 10 °C) (Wikipedia, 2012).
Landslide disaster perception of the AILA cyclone in the Darjeeling town, West Bengal, India
Bhattacharya Sudip Kumar
International Journal of Geomatics and Geosciences
Volume 3 Issue 1, 2012 17
Table 1: Mean monthly rainfall and temperature in the Darjeeling Hills
Months Mean monthly rainfall in mm Mean monthly temperature in 0C
January 15.54 11.45
February 26.33 12.92
March 45.08 17.84
April 206.01 20.51
May 293.31 21.18
Jun 807.72 22.21
July 1215.99 22.37
August 798.10 22.87
September 619.46 22.14
October 183.31 20.61
November 20.33 16.16
December 16.79 12.53
Mean annul rainfall = 4248.00 mm from 25 years’ data
Mean monthly temperature=18.60 0C from 20 years’ data
Figure 6: Forest map of Darjeeling town including Frimal village and Haridas Hatta
landslide area
The forest map of the Darjeeling (Figure 6) clearly shows that the landslide areas of the
Frimal Village and Haridas Hatta are completely devoid of true forest. Vegetation map
revealed that along the NH55 i.e. Hillcart Road there are stretches of forest areas up to
Ghoom. Personal investigation also proves that there are densely scatter tree patches allover
but no true forest cover Ghoom onward is observed.
2.2 General observation
Aila landslide inventory has been prepared (Table2) and compared with the landslide
frequency map (figure 7) which indicates that most of the Aila landslides have occurred in
Landslide disaster perception of the AILA cyclone in the Darjeeling town, West Bengal, India
Bhattacharya Sudip Kumar
International Journal of Geomatics and Geosciences
Volume 3 Issue 1, 2012 18
the low and medium frequency zones and even in landslide free zones. This observation
proves that the medium, low frequency and landslide free zones are being subjected to more
alteration in the natural setup for which more landslides are being added to these areas. So, a
probability of conversion of low and medium frequency zones and even landslide free zones
to higher degree of landslide frequency has been reflected over this region through the
incidence of Aila which clearly denotes that these areas have become more vulnerable and
the addition of rainstorm water is merely an event to unleash the fury.
Table 2: Inventory of the Aila surveyed Landslides
Name Altitude
in metre
Latitude Longitude Shape Loss of
life
Damages
Frimal Village 2050 27° 03.33’N 88°15.32’
E
Semi-
rectangular
- 22 houses (most
are more than
one story) have
been destroyed
fully
Haridash -Hatta 2044 27° 0.68’N 88°15.69’
E
Funnel 7 1 house partially
damaged and
tea garden
severely
damaged
Ghum (Immanual
Church)
2236 27° 00’N 88°15.60’
E
Triangular - Road severely
damaged
Sonada 1960 26° 56.55’N 88°16.78’
E
Rectangular - Many telephone
and electric
posts damaged
and connection
disrupted
Aurangley
Near Tung
1767 26° 56.46’N 88°17.94’
E
Funnel 2 houses
smashed and
many houses
damaged.
Sukhipull 1581 26° 54’N 88°17.15’
E
Funnel
Sirubari 2
(Gourisankar)
1436 26° 52.36’N 88°16.98’
E
Elongated 2 Tea garden and
some houses at
smashed and
damaged
Jungpana
1235 26° 52.93 88°
18.27’
Diamond - Lower level
houses damaged
Below MahanadiI 1187 26° 52.64’N 88°
18.64’E
Elongated - Road severely
damaged
Banduke Gaon 1229 26° 52.90’N 88°
19.26’E
Funnel Houses and
teagarden
damaged
Near Tindharia
Market
792 26° 50.85’N 88°
19.72’E
Funnel Road Damaged
Above Panighata
Road
320 26° 48.46’ N 88°
14.86’ E
Triangular - Road Damaged
Surveyed by the Author
Landslide disaster perception of the AILA cyclone in the Darjeeling town, West Bengal, India
Bhattacharya Sudip Kumar
International Journal of Geomatics and Geosciences
Volume 3 Issue 1, 2012 19
Figure 7: Landslide frequency map of the Darjeeling hills
2.3 Objective
Many landslide studies have been done worldwide by Sarkar S., Kanungo D.P. (2010), Ghosh
Saibal et. al. (2009), Klubertanz G. et. al. (2009), Kitutu M. G. et. Al. (2009), Vidar
Kveldsvik, et. al. (2007), Evans, S. G. et. al. (2006),Seno S. and Thüring M. (2006), Shang,
Yanjun et. al. (2003) focusing on the physical parameters and their critical values that
triggered slope failure. But few of them mentioned how cultural practice induced physical
change in the slope character can be the ultimate agent for which slope failure takes final
shape and gets accelerated to a serious dimension. Therefore, careful investigations have been
made for the Frimal village and Haridas Hatta Landslides to see whether man induced
physical changes in the slope characters play noticeable role over the physical terrain
parameters that are notionally considered to be causative factors of slope failure.
3. Methods and materials
The methods adopted for the landslide study have been associated with detailed analysis of
causative factors related to soil condition, ground cover, anthropogenic stress along with
terrain parameters like slope condition, geological setup etc. Data have been collected both
from secondary sources and field investigation with the help of GPS and survey instruments
and analyzed in a quantitative manner.
4. Findings and discussion
4.1 The Frimal village landslide
During 26th and 27th May, 2009 Darjeeling town received 245.4 and 220.3 mm of rainfall
respectively in different high intensity rainstorm phases by Aila cyclone and resulted a
recurring (5 times) slope failure at Frimal village within a period of 18 hours. This landslide
occurred on southern aspect of the slope behind a series of huge multi storied buildings
Landslide disaster perception of the AILA cyclone in the Darjeeling town, West Bengal, India
Bhattacharya Sudip Kumar
International Journal of Geomatics and Geosciences
Volume 3 Issue 1, 2012 20
(Figure 8 and Photograph 1). Initially the slide area was a graveyard (Photograph 2) at the
rear side of the multi-storied buildings. It was a debris slide which rolled huge amount of
detritus down slope. These detritus struck 22 buildings at the lower level of the slope and
demolished all the buildings completely & partially damaged other houses. There were three
dunny (safety) tanks at the apex of the slide area which were also destroyed.
4.1.1 Morphology of the Frimal village landslide
The potential slide scar has semi rectangular shape being narrowed towards bottom from the
crown (Figure 8). The cross section of the slide scar has been drawn (Figure 9) in order to
estimate quantitatively the physical parameters of morphological features of this scar. The
salient morphological features of this scar are given in the table 3.
Figure 8: Landslide Map of Frimal Village in Darjeeling
Landslide disaster perception of the AILA cyclone in the Darjeeling town, West Bengal, India
Bhattacharya Sudip Kumar
International Journal of Geomatics and Geosciences
Volume 3 Issue 1, 2012 21
4.1.2 Causes of the landslide
Detailed investigation of the Frimal Village landslide bears the evidence of the following
reasons of slope failure which can be grouped into different categories:
(1) (2) (3)
Photographs: (1) Frimal Village Landslide (source: Author), (2) View of Graves behind
Frimal Village Landslide (source: Darjeeling Times, 2009 “b”) (3) Haridas Hatta Landslide
(source: Author)
Figure 9: Cross section of the Frimal Village Landslide
4.1.2.1 Alteration of soil profile and loosening of soil coherence
Detailed analysis of the soil profiles of the Frimal Village landslide area has been done to
understand the causes of slope failure associated with soil properties presented in the Figure
10. As mentioned earlier that the landslide area of the Frimal Village was a graveyard,
digging of graves has made the physical alteration in the soil profile development which is
evident from the physico-chemical change in the soil profile sequence (Figure 10a) when
compared with the soil profile from the neighbouring area where digging has not been done
in Figure 10b. Soil sample collected from the undisturbed soil of landslide scar within
graveyard area shows that with the increase of the profile depth the texture of the soil goes
gradually finer and at 60-90 cm depth soil has become clayey as compared with the soil
profile of the neighbouring site beyond Graveyard where soil texture remains loamy sand all
through the profile. This alteration results from the loosening of the compactness of the soil
aggregates by the process of frequent digging for which pore spaces have been increased and
facilitated percolation of the finer particles towards the bottom layer of the soil. With the
increase of the fineness of the texture the water holding capacity, degree of saturation at field
capacity, volume of expansion have consequently increased. The organic matter of the
bottom layer of the soil has also increased because of percolation. The loosening of
Landslide disaster perception of the AILA cyclone in the Darjeeling town, West Bengal, India
Bhattacharya Sudip Kumar
International Journal of Geomatics and Geosciences
Volume 3 Issue 1, 2012 22
compactness of the soil aggregates also indicates that soil coherence has been effectively
reduced and thereby shearing resistance of the soil in soil profile of graveyard area must be
low in comparison to the surrounding soil area.
Table 3: Morphology of the Frimal Village Landslide
Pre-slide Condition Post-slide condition detected on 29.5.09
1. History:
This area was the back side of
a series of concrete multi-storied
buildings and used as Graveyard.
This area was also used to be
remained wet by community urinal
outlet from the apex building and
road level and by water leakage
from two safety tanks of the above
buildings.
2. Rock Materials:
Gneissic and schistic type of
rocks from Darjeeling Gneiss of
Dharwarian System.
3. Slope:
Steep and uneven slope with
tombstones scattered all around.
4. Vegetation:
i) Several turf layers of grass
in small patches in the middle of
the slope.
ii) Densely scattered
trees along the west side of
the slope.
iii) Low lying bushes
over the crown of the slope
1. Length: 72 m
2. Width: 45 m
3. Depth: of Scar
Maximum: 6.7 m
Minimum: 0.5 m
Average: 3.6 m
4. Shape: Semi-rectangular
5. Total area affected: 2430 m2
6. Total volume displaced: 8748 m
3
7. Process primarily responsible for slide:
Solifluction, continuous wetness of the soil resulted
from the community urinal and defective safety tanks
of the upper level residential buildings in association
with physical alteration of soil from graveyard
digging mechanism led increased shearing stress in
the soil. Heavy gush of rain ultimately created
massacre.
8. Slide scar slope: Concave, Max: 450
Min: 11
0
9. Type of slide: debris slide.
10. Rainfall: 465 mm within 48 hours
12. Spectral feature: (a) Densely scattered trees
on the west side of the scar. (b) Low- lying bushes on
the north-east corner of the slide area. (c) Small
patches of grass in two three areas. (d) huge
multistoried buildings on the crown of the slope. (d)
Several tomb stones on the periphery of the slide
scar. (e) Muddy wet soil covering the whole area (f)
Dump of huge debris at lower level. (g) Shattering of
a guard wall along the lower level metaled road.
As discussed earlier that there is a series of multistoried buildings on the crown of the slope
(Photographs 1) below which three safety tanks, one from the community urinal and two
from the residential buildings were existent (see Figure 8) from where leakage of waste water
was a continuous process for which the entire area remained wet throughout the year.
Analysis of the soil parameters of the Frimal Village Landslide in the Figure 10a exhibits that
various human excrements like uric acid, organic substances etc. have been supplied by the
waste water to the soil for which the bottom layer within 60-90 cm depth there are highest
percentages of dispersing agents like organic matter, acids and very low amount of cohesive
materials like salt, calcium and magnesium (Holmsen 1929, Söderblom 1969, 1974b and
Rosenqvist 1977). Thus high water holding capacity, high degree of saturation, high volume
of expansion being associated with high amount of dispersing agents like organic substances
and acids and low amount of soil cohesive materials like salt, calcium and magnesium have
set the stage ready for massive slope failure by the action of heavy rainstorm which went
against the reduced shearing resistance of the soil. So it can be said unequivocally that the
Landslide disaster perception of the AILA cyclone in the Darjeeling town, West Bengal, India
Bhattacharya Sudip Kumar
International Journal of Geomatics and Geosciences
Volume 3 Issue 1, 2012 23
physical alteration, continuous supply of dispersing agents in the soil and perpetual wetness
worked together as pedological cause for this landslide.
(a)
(b)
Figure 10: Soils (a) at and (b) beyond Frimal Village landslide for comparison
4.1.2.2 Lack of vegetation cover
Landslide map (Figure 8 and Photograph 1) prove that only the western and southwestern
part of this landslide area is covered up by densely scattered trees (not forest) and there has
been mass clearing of vegetation cover over the rest of the portion because of the utilization
of this land by local inhabitants for funeral work. As a consequence most of the area shows
bare soil with scattered grass, also un-vegetated soil surface except few small patches of turf
layers of grass on the middle and low lying bushes at northeast crown of the slope which
Landslide disaster perception of the AILA cyclone in the Darjeeling town, West Bengal, India
Bhattacharya Sudip Kumar
International Journal of Geomatics and Geosciences
Volume 3 Issue 1, 2012 24
played the role of ground cover (see landslide map Figure 8). High overland flow of rain
water of the Aila rainstorm, therefore, ideally found its way to pass over the bare ground and
transported the huge detached materials under the force of gravity.
4.1.2.3 Pressure of the huge buildings on the slope
A row of multi-storied buildings are found along the crown of the landslide affected slope
(Photograph 1) which imposes unbearable pressure on the stability of the ground. The degree
of slope of the landslide area has been calculated to be > 25. Such high degree of slope itself
is severely vulnerable. But in this locality as the rock strata are constructed by the older
Dharwarian Darjeeling Gneissic (coarse grained) types of rocks (Powde, and Saha, 1976)
congenitally this area is more resistant and therefore, bear such extreme pressure due to the
imposition of heavy weight of the concrete building materials. But as the natural setup of the
ground slope has been modified by the anthropogenic misuse like leakage of waste water,
digging of graves, removal of vegetative cover the slope has reached an instability threshold
and the extreme event like Aila rainstorm has facilitated threshold exceedance and resulted
the slope transformation through a series of landslides occurred within a period of 18 hours.
(a)
(b)
Figure 11: Landslide map of Haridas Hatta in Darjeeling: (a) Landslide, (b) Contour
Landslide disaster perception of the AILA cyclone in the Darjeeling town, West Bengal, India
Bhattacharya Sudip Kumar
International Journal of Geomatics and Geosciences
Volume 3 Issue 1, 2012 25
Figure 12: Cross section of the Haridas Hatta Landslide
4.2 Haridas Hatta landslide
This landslide occurred on 26th
May, 2009 at 10-30 a.m. on the southern aspect of the slope
behind a power house and some concrete multistoried buildings (Figure 11 and Photograph 3).
It has been informed by the local inhabitants that cracks were developed over several places
before few hours of the occurrence of the landslide following a mild tremor. This slide had
actually occurred over most of the portion of a tea cultivated field of one of the oldest tea
garden known as Happy Valley except southeastern part where densely settled locality has
been affected. It was a chunk failure of slope uprooting all the tea plants following a heavy
long lasting torrential rain that started from the morning. A deep displacement of materials
took place all of a sudden at 10-30 a.m. hitting and damaging partially one building on the
eastern side at the lower level. This landslide claimed 7 lives. As most of the portion of this
slide area was associated with tea garden this landslide was escaped from further loss of life
and damage of human settlement and property.
Table 4: Morphology of the Haridas Hatta Landslide
Pre-slide Condition Post-slide condition detected on 29.5.09
1. History: Before occurrence of the
landslide the area was a portion of the
tea garden.
2. Rock materials: Gneissic and schistic
type of rocks from Darjeeling
Gneiss of Dharwarian System.
3. Slope: Steep and even slope passes
through garden.
4. Natural: vegetation: Some coniferous
types of shade trees scattered within tea
garden
1. Length: 42.5 m
2. Width: 37.5 m
3. Depth:
Maximum: 3 m
Minimum: 0.5 m
Average: 1.5 m
4. Shape: Funnel
5. Total area affected: 1089.063 m2
6. Total volume displaced: 1633.59
7. Process primarily responsible for
slide: a). ) Heavy load of concretes like Power
house, multistoried buildings at the crown of
the slide area Pressure induced tremor and
cracks magnified the shearing stress of the
slope. b). Random waste water disposal over
the ground of the tea garden led the area wet
throughout the year. This also invoked non-
Landslide disaster perception of the AILA cyclone in the Darjeeling town, West Bengal, India
Bhattacharya Sudip Kumar
International Journal of Geomatics and Geosciences
Volume 3 Issue 1, 2012 26
maintenance of the tea garden over that
portion by the tea garden owners for years
which led old decaying plant roots to hold the
soil without sufficient strength. This resulted
in uprooting of tea plants with deep
displacement of soil in the form of
solifluction. Had there been organized lateral
drainage net the water would have been
passed through the slope and Aila rainstorm
would not have been consequential for slope
failure as seen on the other parts of the tea
garden.
8. Slide scar slope: Concavo-convex
(Fig.13).
Max: 250
Min: 09
0
9. Type of slide: Debris slide.
10. Rainfall: 465 mm within 48 hours
12. Spectral feature: (a) Highly
congested settlement on the east, north-east
and apex of the slide scar. (b) Tea garden with
scattered coniferous shade trees surrounding
the slide scar on south, west and south-west.
.
4.2.1 Morphology of the Haridas Hatta landslide
The potential slide is nearly funnel shape having a concavo-convex slope curvature from
crown towards bottom (Figure 11). The physical parameters of the scar morphology have
been derived from the cross section drawn in the Figure 12. The salient morphological
features of this scar are given in the table 4
4.2.2 Causes of the landslide
Careful investigation revealed the following cause for the Haridas Hatta slope failure:
4.2.2.1 Settlement pressure induced tremor
The crown of the Haridas Hatta Landslide slope was found to be associated with a series of
multistoried buildings with a sudden long gap occupied by a cemented basement and few
high-tension cable towers of an electrical power house (Figure 11a). This gap of land within
series of multistoried buildings is thought to have produced unequal weight on the ground,
thereby lateral pressure differential and disequilibrium in the bedrock. Aila heavy rainstorm
percolating into the fissures and joints of the coarse textured gneissic rocks has accelerated
this disequilibrium in many folds for which lateral cracks following a light earthquake like
tremor was observed by the local people just before few hours of the occurrence of the
landslide.
4.2.2.2 Non-maintenance of happy valley tea garden
Maximum part of the Haridas Hattat landslide scar was covered up by improperly maintained
tea plants of the Happy Valley Garden (one of the oldest garden of the Darjeeling town) since
Landslide disaster perception of the AILA cyclone in the Darjeeling town, West Bengal, India
Bhattacharya Sudip Kumar
International Journal of Geomatics and Geosciences
Volume 3 Issue 1, 2012 27
the waste water disposal outlets of the upper slope buildings occupied this portion of the
garden.
Figure 13: Soil at Haridas Hatta Landslide
Soil analysis (Figure 13) proved that high amount of detergent; sodium etc was supplied to
the soil by outlet water for which this area showed very high amount of Exchangeable
Sodium Percentage (ESP) (17.01%) on the top soil followed by high ESPs in all the horizons
below. This high ESP in the soil is capable enough for breaking down soil aggregates,
dispersal of the clay particle, high erodibility, low porosity and poor tilth (Indorante 2006,
Roesner et.al., 2006). So, in such an environment ultimately tea plant roots were effectively
unable to hold soil properly. High intensity rainstorm of the cyclone Aila, thus, led to a chunk
failure of slope. So, analytical study finally reveals that mild tremor associated cracks,
reduced shear strength of the soil resulted from increased sodium content and with the
inability of the tea plants to hold back soils entangled within the root system are the major
causes of massive landslide in Haridas Hatta.
5. Conclusion
From the detailed analysis of these two landslides in the Darjeeling town following the
incidence of Aila it is apparent that it was not the sudden gush of rain which was the only
triggering factor but man induced causes like digging of slope, unregulated community
outlets, reckless and unplanned high rising building construction and its excessive pressure
on the land and the poor maintenance of present agricultural landuse i.e. tea cultivation made
the slopes fragile for which such devastations took place. Had the man been a little conscious
of his deeds such incidences would have been avoided without losing many precious lives
and sustaining property damage. So it is the high time to think over how far we will let our
hands spread over the nature so that life sustaining processes will not be hampered for the
sake of our own survival or the geo-hazards will remain as a perpetual threat in human
existence.
Landslide disaster perception of the AILA cyclone in the Darjeeling town, West Bengal, India
Bhattacharya Sudip Kumar
International Journal of Geomatics and Geosciences
Volume 3 Issue 1, 2012 28
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International Journal of Geomatics and Geosciences
Volume 3 Issue 1, 2012 29
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