Landslide hazard zonation (LHZ) mapping on meso-scale for...

486 J SCI IND RES VOL 67 JULY 2008

*Author for correspondence

E-mail: [email protected]

Journal of Scientific & Industrial Research

Vol. 67, July 2008, pp. 486-497

Landslide hazard zonation (LHZ) mapping on meso-scale for systematic town

planning in mountainous terrain

R Anbalagan*, D Chakraborty and A Kohli

Department of Earth Sciences, Indian Institute of Technology Roorkee, Roorkee

Received 27 July 2007; revised 29 February 2008; accepted 09 May 2008

Planning and execution of development schemes in Himalayan terrain is always challenging because of inbuilt

fragile nature of mountain ecosystem. Landslide hazard zonation (LHZ) mapping on meso-scale (1: 5000 –10,000) may guide

planners to choose suitable locations for urbanization and expansion in hills. In present work, scope of regional scale LHZ

mapping technique has been increased to accommodate more detailed aspects of inherent causative factors responsible for

slope instability. This technique also incorporates effects of external causative factors such as seismicity and rainfall as correction

ratings. This technique has been effectively applied to prepare a LHZ map on meso-scale in Nainital area. It will be useful for

town planners to plan civil constructions in relatively safe zones. In addition, environmentally unstable slopes can be given

adequate attention by planning suitable control measures.

Keywords: Hazard classes, LHZ mapping, Meso-scale, Nainital, Planning in hilly terrains

Introduction

The fast rate of construction practice often

overlooks adverse geological features that are inherently

present in a mountain ecosystem. Lack of proper

geological and geotechnical investigations in planning

stage has adversely affected existing geo-environmental

condition in the Himalaya leading to increased

incidences of hill slope instability. So there is an urgent

requirement for a landslide hazard evaluation technique,

which shall be adopted in the planning stage so that major

problems related to slope instability (SI) can be avoided

during implementation of these projects or even after

their completion. In this context Landslide Hazard

Zonation (LHZ) mapping on meso-scale is one such

hazard evaluation technique, which may find application

for systematic town planning and expansion of urban

settlements in hilly terrains.

LHZ Mapping Technique

LHZ mapping on meso-scale is an empirical

approach, which demarcates hill slopes into zones of

varying degree of stability on the basis of their relative

hazards. Mapping scale is 1:5,000 – 1:10,000. The

smallest unit of study while carrying out meso-scale LHZ

mapping is a slope facet, which is that part of hill slope

having similar slope characteristics (same amount of

slope inclination and direction with a maximum variation

of ± 20° for both) and is usually bordered by natural

features (ridges, spurs, gullies, depressions, streams and

rivers). This technique accounts for inherent causative

factors (whose effect in inducing instability can be

studied or assessed on slope) responsible for SI and

accordingly rates them depending on their influence in

inducing instability. These include geology of slope

material comprising: a) lithology; b) structure; c) slope

morphometry; d) relative relief; e) land use and land

cover; and f) hydrogeological conditions. Hazard

probability of a facet usually depends on combined effect

of all inherent parameters, which can vary from facet to

facet1,2. However, this approach also incorporates effect

of external parameters (whose effect cannot be assessed/

studied on the slope facet) like seismicity and rainfall,

as separate correction parameters.

This map provides useful information to town

planners in following ways: i) It indicates nature of

instability of hill slopes that can be suitably accounted

in site selection; ii) It indicates potentially unstable

slopes which require further detailed studies following

analytical techniques to assess their status of stability;

ANBALAGAN et al: LANDSLIDE HAZARD ZONATION MAP ON MESO SCALE IN NAINITAL AREA 487

iii) If potentially unstable slopes become unavoidable

during planning stage, suitable precautionary measures

can be taken up in potentially unstable slopes before

starting excavation; iv) If unstable zones are located

close to important engineering structures, urbanized

colonies or other important installations, it may be

essential to monitor slope to understand time dependent

deformation behavior of the slope. Accordingly,

instrumental slope monitoring can be carried out on

these slopes; and v) The data available on meso-scale

can be used as an input data for risk assessment.

Landslide Hazard Evaluation Factor (LHEF) Rating Scheme

LHEF rating scheme, which also follows an

empirical approach, takes into consideration individual

and net effect of all inherent causative factors

responsible for SI (Table 1). Inherent factors3 are used

for preparation of LHZ mapping on macro-zonation

approach. Maximum value of ratings for individual

parameter is awarded keeping in mind its estimated

significance in causing SI and also to represent overall

field conditions. External parameters (seismicity and

rainfall) are also incorporated in LHEF rating scheme,

apart from six inherent parameters. Various causative

factors and their corresponding LHEF rating values are

as follows:

Lithology

a) Rock Slopes

Lithology or rock type is an important factor in

controlling slopes stability, and hence maximum LHEF

rating of 2 is given. It controls the nature of weathering

and erosion for a rock slope and this point is taken care

of while awarding the ratings. Under this parameter, rocks

are broadly classified into three categories (Table 2).

Type-I rocks consist of crystalline rocks (igneous and

Table 1 — Maximum LHEF rating for different causative factors

Causative Factor Maximum LHEF rating

Inherent factors Geology

1 Lithology 2.0

2 Structure 2.0

3 Slope morphometry 2.0

4 Relative relief 1.0

5 Land use and land cover 2.0

6 Hydrogeological conditions 1.0

Total LHEF Rating 10.0

Correction due to external factors – a) Seismicity & b) Rainfall 1.0

(to be added separately to the total of LHEF)

Corrected LHEF Rating 11.0

Table 2 — Ratings for rock types

Category Rock types Ratings

Type-I Basalt, quartzite and massive limestone & dolomite 0.2

Granite, gabbro and dolerite 0.3

Granite gneiss and metavolcanics 0.4

Type-II Well-cemented terrigenous sedimentary rocks (dominantly sandstone) with 1.0

minor beds of clay stone and Gneissic rocks

Poorly-cemented terrigenous sedimentary rocks (dominantly sandstone) with 1.3

intercalations of clay or shale beds

Type-III Well foliated gneiss 1.0

Shale, slate, phyllite and other argillaceous rocks like

siltstone, mudstone and claystone 1.2

Schistose rocks 1.4

Shale with inter-bedded clayey rocks (siltstone, mudstone, etc.) 1.8

Weathered shale and other argillaceous rocks, phyllite and schistose rocks 2.0


metamorphic) along with massive calcareous rocks,

which suffer less erosion resulting in steep slopes. Type-

II rocks mainly comprised of well and poorly cemented

terrigenous sedimentary rocks. Type-III category consists

of soft argillaceous rocks, their low grade metamorphic

equivalents and well foliated gneissic rocks. Soft rocks

(claystone, siltstone, mudstone, shale, schist, phyllite and

other such rocks) erode faster and are easily weathered

close to surface. Moreover, phyllite, schist and gneissic

rocks have well developed foliation plane along which

act as weak plane for sliding to take place. In LHEF

rating scheme, weathering of fresh rocks is also included

as a correction factor (Table 3), which is to be multiplied

with corresponding rating of Type-I and Type-II rocks.

Type-III rocks usually have an inbuilt higher rating, for

which there is as such no requirement to multiply with

correction factor. But depending on weathering

condition, rating can be suitably modified to represent

the field condition. Maximum value for Type III can be

increased to 2 as the worst possible condition.

b) Soil Slopes

Some hill slopes may be composed of mainly

loose soil and debris overburden, where genesis and

Table 3 — Correction factors for weathering

Weathering Description Rating

condition Rock type-I Rock type-II

Completely Rock totally decomposed/ disintegrated to C1 = 4.0 C

1 = 1.5

weathered soil, no or minor existence of initial rock

structure (Correction factor C1)

Highly weathered Rock totally discolored, discontinuity C2 = 3.5 C

2 = 1.35

planes show weathering products, rock

structure altered heavily with minor soil

formation near surface (Correction factor

C2)

Moderately Rock prominently discolored with remnant C3 =3.0 C

3 =1.25

weathered isolated patches of fresh rock, weathering

and alteration prominent along discontinuity

planes, considerable alteration of rock

structure (Correction factor C3)

Slightly Rock partially discolored along C4 =2.5 C

4 = 1.15

weathered discontinuity planes indicating weakening

of rock mass, rock structure is slightly

altered (Correction factor C4)

Faintly weathered Rock slightly discolored along discontinuity C5 =2.0 C

5 =1.0

planes which may be moderately tight to

open in nature, intact rock structure with or

without minor surface staining (Correction

factor C5)

Table 4 — Ratings for soil types

Description Rating

Older well compacted fluvial fill material (alluvial) 0.8

Clayey soil with naturally formed surface 1.0

Sandy soil with naturally formed surface (alluvial) 1.4

Debris comprising mostly of rock pieces Older well compacted 1.2

mixed with clayey or sandy soil Younger loose material 2.0


relative age are considered as the main criteria, while

awarding ratings (Table 4). Older alluvial soil is

generally well compacted and characterized by high shear

strength and also resistant to weathering. On the other

hand, younger colluvial soil is loose or incompact in nature,

soil for which they generally have low shear strength

parameters.

Structure

a) Rock Slopes

Stability of hill slopes consisting of in situ rocks

is largely dependent on relationship between slope

orientation and attitude of dominant discontinuities.

Structures include both primary and secondary

discontinuities like bedding, foliation, schistosity, joints,

shear zones and other such features. In this connection,

failure modes taken into account are namely plane,

wedge and toppling. For individual failure modes,

following three types of conditions (Table 5) exist

between slope and the most unfavorable discontinuity

plane or wedge:

i) Parallelism between slope and discontinuity —

Extent of parallelism between inclination direction

of slope and dip of discontinuity plane or the plunge

of line of intersection of two such planes is

considered here. With increasing parallelism, the

chance of failure increases. In LHEF rating scheme,

maximum rating given for this condition is 0⋅50

(Table 6).

ii) Relation between inclination of slope and

amount of dip of discontinuity/ plunge of wedge

line — The differences in dip amount of slope and

discontinuity plane or plunge of line of intersection

of two such planes are taken into consideration. If

slope is steeper than discontinuity surface or line of

intersection of planes (day lighting condition), the

slopes become vulnerable to plane or wedge failure

modes. For toppling failure, dip of unfavorable

discontinuity is added to inclination amount of

slope. Most unfavorable situation appears when the

added value exceeds 160°. Maximum rating for all

these cases are given as 1⋅00 (Table 7).

iii) Amount of dip of discontinuity/ plunge of wedge

line — With increasing amount of dip of discontinuity

plane or the amount of plunge of line of intersection

of two such planes, the material may cross angle of

friction of rock mass constituting the slope leading

to its instability. Steeper the slope angle more is the

chance of slope failure. Maximum rating for this

Table 5 — LHEF rating for relationship between structure and slope

Condition Rating Total rating of all conditions

1 Parallelism between slope and discontinuity 0.5

2 Relationship between slope inclination and

dip/ plunge of discontinuity 1.0 2.0

3 Dip of discontinuity/ plunge of wedge line 0.5

Table 6— Ratings for relationship of parallelism between slope and discontinuity

Category Difference in angle of parallelism Rating Slope condition

1 Plane: |(αj - α

s)| 2 Wedge: |(α

i - α

s)|

3 Topple: |(αj - α

s) ± 180°| or |(α

j - α

s)|

I > 30° 0.20 Very favorable

II 21 - 30° 0.25 Favorable

III 11 - 20° 0.30 Fair

IV 6 - 10° 0.40 Unfavorable

V ≤5° 0.50 Very unfavorable

Where αj = Dip direction of discontinuity, α

i = Direction of plunge of the line of intersection of

two discontinuity surfaces and αs = Direction of slope inclination

Note: For slopes falling in category I in Table 6, the ratings for structure as awarded in Tables 7 and 8 will not be applicable and hence a

rating of zero may be awarded


relation, as awarded in the rating scheme, is 0.50

(Table 8).

b) Soil Slopes

In case of slope facets consisting of overburden

soil and debris material, assumed depth of overburden is

considered for awarding ratings (Table 9), as the mode

of failure changes with increasing depth of overburden.

Slope Morphometry

Slope morphometry maps for meso-zonation

purpose are prepared after getting slope angle from

sections drawn through slope facet along the direction

of inclination incorporating highest and lowest contours

passing through it. For meso-zonation purpose, an

average slope angle for the whole facet is judiciously

selected. If there is a significant variation (>20°) along

slope profile, it is better to study that slope, making a

separate facet. Finally, all slope angles are categorized

into six different classes, with a maximum rating of 2.0

(Table 10).

Relative Relief

Relative relief represents maximum height of

a facet, from bottom (valley floor) to top (ridge/ spur)

along slope direction. Relief of a facet can simply be

calculated by counting difference between elevations at

bottom most point of a facet to top most point of the

same, along slope direction. For meso-zonation purpose,

five classes of relief are considered. Maximum rating

under this parameter is 1.0 (Table 11).

Table 7—Ratings for relationship between amount of dip/ plunge of discontinuity and that of slope inclination

Cate- Difference in angles Rating Sum of angles Rating Slope condition

gory 1 Plane (βj - β

s) & 3 Topple

2 Wedge (βi - β

s) (β

j + β

s)

I > 10° 0.30 <110° 0.30 Very favorable

II 0 - 10° 0.50 111 - 130° 0.50 Favorable

III 0° 0.70 131 - 140° 0.70 Fair

IV 0 – (-10°) 0.80 141 - 160° 0.90 Unfavorable

V > -10° 1.00 >160° 1.00 Very unfavorable

Where βj = Dip amount of discontinuity, β

i = Amount of plunge of line of intersection of two discontinuity surfaces

and βs = Amount of slope inclination

Table 8 — Ratings for amount of dip of discontinuity

Category Dip amount Rating Dip amount Rating Slope condition

1 Plane (βj) 3 Topple (β

j)

2 Wedge (βi)

I < 15° 0.20 < 50° 0.20 Very favorable

II 16 - 25° 0.25 51 - 60° 0.30 Favorable

III 26 - 35° 0.30 61 - 70° 0.40 Fair

IV 36 - 45° 0.40 71 - 80° 0.45 Unfavorable

V > 45° 0.50 > 80° 0.50 Very unfavorable

Where βj = Dip amount of discontinuity and β

i = Amount of plunge of line of intersection of two discontinuity

surfaces

Table 9 — Ratings for depth of soil cover

Depth of soil, m Rating Probable mode of failure

< 5 0.65 Dominantly Talus

6 – 10 0.85 Talus & sometimes Circular

11 – 15 1.30 Circular & sometimes Talus

16 – 20 1.50 Dominantly Circular

> 20 2.00 Dominantly Circular

Note: When depth of soil cover is <5m, then put rating of 1 if the

slope angle is more than 35°.


Land Use and Land Cover

Land use and land cover pattern is one of the

important parameters governing slope stability.

Vegetation has major role to resist slope movements,

particularly for failures with shallow rupture surfaces.

A well spread network of root system increases shearing

resistance of slope material due to natural anchoring of

slope materials, particularly for soil slopes. Moreover, a

thick vegetation or grass cover reduces action of

weathering and erosion, hence adds to stability of the

slopes. On the other hand, barren or sparsely vegetated

slopes are usually exposed to weathering and erosion,

thus rendering it vulnerable to failure. Additional water

for agriculture purpose recharges slopes in agricultural

fields, apart from receiving natural precipitation. Similarly,

a populated land on a very gentle slope (slope angle ≤15°)

under normal circumstances is least expected to suffer

from SI, which is also induced because of anthropogenic

activities, like urbanization, particularly on higher slope

angles (>30°). It not only removes vegetation cover but

also adds to the natural weight of the slope as surcharge

due to weight of civil structures.

In a hill slope with higher slope angle, buildings

are usually located by constructing local cut slopes and

flat terraces. With this concept, urbanization is broadly

classified into three categories: i) A sparsely urbanized

slope is one where construction terraces are located far

apart (more than 15m of horizontal spacing) providing

a considerable distance between two terraces along the

slope; ii) A moderately urbanized slope is characterized

by comparatively closer location of construction terraces

but leaving an optimal horizontal spacing of 5-15m

between individual terraces; and iii) In a heavily

urbanized slope construction terraces are located very

close to each other (≤5m horizontal spacing) in such a

way that successive terraces almost touch each other at

places. With increasing urbanization, water due to

domestic usage may be released on the slope surface

wherever the drainage measure is inadequate. This water

may get added up to the subsurface water and may

develop pore water pressure, leading to SI. Similarly,

barren land affected by anthropogenic activities is also

most vulnerable to landslides. The maximum rating for

this parameter is 2.0 (Table 12).

Hydrogeological Conditions

Presence of water generally decreases shear

strength of slope forming material and thereby increasing

the probability of slope failure. Since it is difficult to

assess subsurface flow of groundwater quantitatively for

entire facet, visual estimation of field condition have

been considered as an alternative measure to award the

ratings. For better representative groundwater condition

assessment, it is advisable to take field data after

monsoon. Maximum rating for this parameter is 1.0. The

qualitative hydrogeological conditions of facets are rated

as follows: flowing, 1.0; dripping, 0.8; wet, 0.5; damp,

0.2 and dry, 0.

Correction Parameter Incorporating Effects of External Factors

External factors like seismicity and rainfall may

initiate slope movements and are accordingly called as

triggering factors. Seismically, India is divided into four

major seismic zones where ‘Zone-II’ represents an area

of minimum seismic intensity while ‘Zone-V’ indicates

maximum intensity of seismicity. Intensity of ground

shaking increases proportionately from ‘Zone-II’ to

‘Zone-V’. So a slope, which is critically stable under

Table 10— Ratings for slope morphometry

Slope type Slope angle Probable type of failure Rating

Escarpment / Cliff > 65° Falls & topples 2.0

Very steep slope 46- 65° Falls & topples 1.8

Steep slope 36 - 45° Slides 1.6

Moderately steep slope 26 - 35° Slides 1.3

Gentle slope 16 – 25° Slides with creep movement 0.8

Very gentle slope < 15° Slides with creep movement 0.5

Table 11— Ratings for Relative relief

Relief classes Relative relief (m) Rating

Very low < 50 0.3

Low 50 - 100 0.4

Medium 101 – 200 0.6

High 201 - 300 0.9

Very high > 300 1.0


existing slope conditions, may become unstable if it falls

in higher seismic zones and may result landslide

phenomenon. Similarly, zones of high annual precipitation

are also problematic as there are always chances of

sudden pore water pressure built up in slopes following

a heavy spell of rain and this may also induce slope

instability. Ratings for these two factors shall be given

separately and shall be added to the total estimated

hazard (TEHD) values as a correction parameter

(Table 13).

Calculation Of Total Estimated Hazard (TEHD) From LHEF

Ratings

TEHD is calculated by adding LHEF ratings

obtained for individual inherent parameters and later

applying suitable corrections for external parameters

(rainfall and seismicity). Depending upon the location

of study area, correction parameters may vary widely

(Table 13). The final TEHD value indicates overall

condition of instability and shall be calculated facet wise

by adding all values of inherent and external parameters

as

Total Estimated Hazard (TEHD) = Ratings for (lithology

+ structure + slope morphometry + relative relief +

land use and land cover + hydrogeological conditions)

+ Correction for external parameters (seismicity and

rainfall)

LHZ map on meso-scale of an area is prepared

from corrected TEHD values of the facets. On the basis

of range of TEHD values, all slope facets in an area can

be categorized into five classes of relative hazard zones

(Table 14). A LHZ map on meso-scale will show spatial

distribution of these hazard zones and accordingly help

town planners to select relatively safe areas for future

development.

For town planning and construction purposes,

slope facets, which fall, in VLH and LH zones are

Table 12 — Ratings for land use and land cover types

Land use & land cover types Rating

Agricultural land or populated flat land ( < 15°) 0.65

Thickly vegetated forest area 0.80

Moderately vegetated area 1.20

Sparsely vegetated area with thin grass cover 1.50

Sparsely urbanized 1.20

Moderately urbanized 1.50

Heavily urbanized With proper surface and/ or subsurface

drainage measures – no wet patches on slope 1.60

Inadequate drainage – wet patches observed

on slope 1.70

Barren land 1.80

Barren land with slope excavation (cut slopes for road construction, mining 2.00

activities, etc)

Table 13 — Ratings for external factors

Seismic zone Rating Average annual rainfall of the area Rating

II 0.2 < 50 cm 0.2

III 0.3 51 – 100 cm 0.3

IV 0.4 101 – 150 cm 0.4

V 0.5 > 150 cm or history of cloud burst 0.5

Table 14 — Landslide hazard zones based on corrected Total

Estimated Hazard

Hazard Range of Description of zone

zone corrected TEHD

value

I TEHD < 3.5 Very low hazard (VLH) zone

II 3.5 ≤TEHD < 5.0 Low hazard (LH) zone

III 5.0 ≤TEHD ≤ 6.5 Moderate hazard (MH) zone

IV 6.5 < TEHD ≤ 8.0 High hazard (HH) zone

V TEHD > 8.0 Very high hazard (VHH) zone


suitable. Slopes falling in HH and VHH are unfavorable

and may be avoided as far as possible. In comparison to

VHH and HH facets, slopes falling in MH class are

considered relatively safer for construction practice, but

may contain local areas of instability. If constructions

are to be carried out in HH and VHH slopes, then suitable

control measures should be taken up prior to construction.

Preferably, these facets shall be studied in detail on 1:1000

- 1:2000 scale incorporating analytical and observational

techniques in order to understand status of stability and

to plan suitable control measures. Similar approach can

be adopted for MH slopes to identify the unstable pockets

and accordingly reduce the probability of hazard.

Landslide Hazard Zonation Mapping of Nainital

on Meso-Scale – A Case Study

Nainital (Lat: N29°22„ - N29°24„ and Long:

E79°26„ - E79°28„) is located in Kumaun Lesser

Himalaya. The study area is approx. 8 km2 and falls in

Survey of India Toposheet No. 53 O/7 (1:50,000).

Nainital lake is located within a saucer shaped

depression, bounded by hills from all sides. The lake is

fairly elliptical (eye) in shape with a maximum length

of approx. 1 km along NW-SE direction and maximum

width of about 500 m. Highly urbanized Sher-ka-danda

hills are located to E and NE of Nainital lake. It merges

with Naina Hills in the north. The lake is bounded by

Ayarpatha-Deopatha hills in W and SW directions. It is

surrounded by Lesser Himalayan hills from all sides,

except in S-SE direction, where from Balia ravine, the

only outlet of lake, emerges and passes through

Kailakhan area, before descending into the plains to meet

Gola River near Kathgodam. Nainital is facing hill slope

instability problems for over a long period. Earliest

reported incidence of landslides dates back to 18th

century. Since then, the landslide problems were being

reported intermittently causing damages to civil structures.

In view of this, a LHZ map of Nainital has been prepared

on meso-scale (1:5,000) to study landslide hazard

probability of town area surrounding the lake.

Geology of Study Area

Geologically, the area is represented by rocks

of Infra-Krol, Krol & Tal Formations of Proterozoic

age4,5. Dominant rock types include grey slates and

phyllites (Lower Krol Formation), calcareous slates

(Middle Krol Formation), massive limestones and

dolomites with minor slates (Upper Krol Formation) and

quartzites with intercalated sandstones of Tal Formation

(Fig. 2). Sher-ka-danda hills located on the easterly

direction to lake are made up of slates and phyllites of

Lower Krol Formation. These slopes often show

manifestation of creep movements such as tension

cracks, tilting of trees and other such features.

Meso-scale LHZ Map of Nainital

For preparation of LHZ map of Nainital town

on meso-scale, initially a slope facet map (Fig. 1) of

Fig 1 — Slope facet map of study area


Fig 2 — Geological map of study area

Fig 3 — Slope morphometry map of study area


lake catchment was prepared from SOI toposheet

(1:5,000). Altogether, 29 slope facets were identified for

the purpose of LHZ mapping. Slope morphometry and

relative relief maps were also prepared from toposheet.

The condition of all inherent parameters was actually

examined on slope facets in order to assign suitable

LHEF ratings and to prepare various thematic maps

(Figs 2 - 6). As the study area falls in seismic zone IV

with an average annual precipitation of the order of 100-

150 cm, adequate correction ratings for external

parameters were also added. Thus, LHEF ratings

obtained for individual facets were added up to get the

facet wise corrected TEHD value. On the basis of these

values, various hazard classes were determined (Fig. 7).

Hazard Classes

Meso-scale LHZ map (Fig. 7) indicates that hill

slopes on E and NE side of the lake forming part of

Fig 4 — Relative relief map of study area

Fig 5 — Land use & land cover map of study area


Sher-ka-danda hill and Naina hill fall dominantly on HH

and VHH zones. Slopes of Ayarpatha-Deopatha hills,

bordering SW flanks of the lake, fall dominantly in MH

zone with small pockets of HH and VHH zones. The

concentration of urbanization on Sher-ka-danda hill is

generally of high order, particularly in the lower levels

close to lake. Similarly, high urbanization is also seen in

foothill region of Naina hill. The constructions in these

areas are steadily growing without taking into consideration

existing instabilities. This may add to natural unstable

Fig 6 — Map of hydro-geological condition in study area

Fig 7 — Landslide hazard zonation (LHZ) map of study area


conditions and may aggravate overall stability of these

hill slopes. This fact has been validated by detailed

stability analysis of HH and VHH slopes taking into

account the mode of failure, shear strength parameters

and slope geometry. Analysis indicates stable slope

condition when dry, but becomes unstable under water

saturation. Hence, it is time to control unplanned

constructions on these hill slopes, which may help to

protect existing geo-environmental balance of Nainital.

Conclusions

The Himalaya represents one of the most fragile

mountain ecosystems of the world, where systematic

planning is a must for successful implementation of

developmental schemes. LHZ mapping on meso-scale

(1:5,000-10,000) may guide town planners to identify

relatively safe areas for future constructions and town

expansion. Meso-scale LHZ mapping is an empirical

approach, which takes into, account both inherent and

external parameters responsible for slope instability.

Stable zones like VLH and LH are considered safe for

civil constructions. Hill slopes falling in MH class are

also safe for construction practice, but may contain local

pockets of instability, which should be suitably accounted

during constructions. For slopes falling in HH and VHH

classes, it is always advisable to avoid constructions. If

unavoidable, detailed study on 1:1000-2000 scale shall

be done to evaluate the status of stability of these slopes.

Suitable control measures shall be identified before taking

up constructions in order to minimize geo-environmental

hazards. As a case study, a meso-scale LHZ map of

Nainital town was prepared, which indicates about 40%

falling in HH class and about 25% falling in VHH class.

The conditions of these potentially unstable hill slopes

were also validated by detailed stability analysis and shall

be suitably accounted during civil constructions.

References1 Anbalagan R, Singh B, Chakraborty D & Kohli A, A Filed

Manual For Landslide Investigation (DST, Govt. of India, New

Delhi) 2007, 153p.

2 Anbalagan R, Landslide hazard evaluation and zonation

mapping in mountainous terrain, Engineering Geology, 32

(1992), 269-277.

3 BIS 14496, Preparation of Landslide hazard Zonation Maps

in Mountainous terrains – Guidelines, Part 2 Mcro-zonation

(BIS, New Delhi) 1998.

4 Valdiya K S, Geology and Natural Environment of Nainital

Hills, Kuamun Himalaya (Gyanodaya Prakashan, Nainital)

1988, 155p.

5 Valdiya K S, Geology of Kumaun Lesser Himalaya (Wadia

Institute of Himalayan Geology, Dehradun) 1980, 291p.

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