landslide disaster perception of the aila cyclone in the darjeeling

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

[email protected]

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).





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





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).





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





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





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





(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





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





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





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





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





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-





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





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.





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