- Email: [email protected]
Landslide hazard evaluation and zonation mapping in mountainous terrain
Engineering Geology, 32 (1992) 269-277 Elsevier Science Publishers B.V., Amsterdam
269
Landslide hazard evaluation and zonation mapping in mountainous terrain R. A n b a l a g a n Department of Earth Sciences, University of Roorkee, India (Received October 23,1990; revised version accepted January 14,1992)
ABSTRACT Anbalagan, R., 1992. Landslide hazard evaluation and zonation mapping in mountainous terrain. Eng. Geol., 32: 269-277. Landslide hazard zonation (LHZ) maps are of great help to planners and field engineers for selecting suitable locations to implement development schemes in mountainous terrain, as well as, for adopting appropriate mitigation measures in unstable hazard-prone areas. A new quantitative approach has been evolved, based on major causative factors of slope instability. A case study of landslide hazard zonation in the Himalaya, adopting a landslide- hazard evaluation factor (LHEF) rating scheme, has been presented.
Introduction The planning, design and execution of developmental schemes, such as road and building construction, are often carried out too quickly due to financial, time and other constraints. As a result many projects may not incorporate adequate details of geological and geotechnical considerations, causing instability of hill slopes and a manifold increase in the incidence of landslides. This demonstrates the necessity of preparing multipurpose terrain evaluation maps, based on the geo-environment of the mountainous terrain and using them as the basis for planning future development schemes. A landslide hazard zonation (LHZ) m a p devides the land surface into zones of varying degrees of stability, based on an estimated significance of causative factors in inducing instability. The L H Z maps are useful for the following purposes: (a) The L H Z maps identify and delineate unstable hazard-prone areas, so that environmental
Correspondence to: R. Anbalagan, Department of Earth Sciences, University of Roorkee, India. 0013-7952/92/$05.00
regeneration programmes can be initiated adopting suitable mitigation measures. (b) These maps help planners to choose favourable locations for siting development schemes, such as building and road constructions. Even if the hazardous areas can not be avoided altogether, their recognition in the initial stages of planning may help to adopt suitable precautionary measures. The methodology of preparation of these maps s h o u l d be systematic, practicable and, as far as possible, simple so that the practicing engineers, geologists and planners may understand and use them effectively. Hence, a new quantitative approach for L H Z mapping has been developed, based on a numerical rating scheme called landslide hazard evaluation factor ( L H E F ) rating scheme. This technique may be effectively used during the preliminary stages of geotechnical investigations when a cheap and rapid hazard assessment technique is needed.
Landslide hazard evaluation factor (LHEF) rating scheme The L H E F rating scheme is based on an empirical approach which combines past experience
© 1992 - - Elsevier Science Publishers B.V. All rights reserved.
270
R. ANBALAGAN
gained from the study of causative factors and their impact on landslides with conditions anticipated in the area of study. Similar approaches have been adopted in the well-known rock mass classifications such as the R M R system and Q system (Barton et al., 1974; Bieniawski, 1979). The L H E F rating scheme is a numerical system which is based on major inherent causative factors of slope instability such as geology, slope morphometry, relative relief, land use and land cover and groundwater conditions. Factors like rainfall and seismicity are not included for the purpose of LHZ mapping. The maximum LHEF ratings for different categories are determined on the basis of their estimated significance in causing instability (Table I). The number 10 indicates the maximum value of the total estimated hazard (TEHD). Initially, the topography of the area to be covered by LHZ mapping is studied carefully and the hill slopes are divided into a number of facets generally delimited by ridges, spurs, gullies and rivers. A detailed L H E F rating scheme, showing ratings for a variety of subcategories for individual causative factors as given in Table 2, is discussed below.
Geology The geological map provides information on the lithological and structural setting of the area. The lithological and structural maps may also be prepared separately for better representation. TABLE 1 Proposed maximum LHEF rating for different contributory factors for macro-zonation Contributory factor
Lithology Relationship of structural discontinuities with slope Slope morphometry Relative relief Land use and land cover Groundwater conditions Total
Maximum LHEF rating 2.0 2.0 2.0 1.0 2.0 1.0 10.0
Lithology The erodibility or the response of rocks to the processes of weathering and erosion has been the main criteria in awarding the ratings for subcategories of lithology. E.g., rocks like quartzite, limestone and igneous rocks are generally hard, massive and resistant to erosion, forming steep slopes. In comparison, terrigenous sedimentary rocks are vulnerable to erosion and form more easily landslides. Phyllites and schists are characterised by flaky minerals which weather quickly and promote instability. Accordingly, the L H E F ratings have been awarded. A correction factor concerning the status of weathering of rocks has also been incorporated. In the case of soil, genesis and age are the main considerations in awarding the ratings. Older alluvium is generally well compacted and has a high shearing resistance. Recent materials such as slide debris are loose and have low shearing resistance.
Structure Structure includes primary and secondary discontinuities in the rocks such as bedding, joints, foliations, faults and thrusts. The disposition of structural discontinuities in relation to slope inclination and direction has a great influence on the stability of slopes. In this connection, the following three types of relations are considered important: (1) The extent of parallelism between the directions of the discontinuity, or the line of intersection of two discontinuities and the slope. (2) The steepness of the dip of the discontinuity, or the plunge of the line of intersection of two discontinuities. (3) The difference in the dip of the discontinuity, or the plunge of the line of intersection of the two discontinuities to the inclination of the slope. The more the discontinuity or the line of intersection of two discontinuities tends to be parallel to the slope, the greater the risk of failure. When the dip of the discontinuity or plunge of the line of intersection of two discontinuities increases, the probability of failure also increases, because the angle of friction for the discontinuity surfaces may be reached. Moreover, till the dip of the discontinu-
LANDSLIDE H A Z A R D EVALUATION A N D ZONATION MAPPING IN M O U N T A I N O U S TERRAIN
ity plane or the plunge of the line of intersection of the two discontinuities does not exceed the inclination of the slope, the failure potential remains high. Accordingly, the LHEF ratings have been assigned for various stability conditions, broadly on the basis of the approach indicated by Romana (1985). In the case of soil, the inferred depth of the soil cover has been used for awarding the ratings. Slope morphometry
Slope morphometry maps define slope categories on the basis of the frequency of occurrence of particular angles of slope. The distribution of the slope categories is dependent on the geomorphological history of the area; the angle of slope of each unit is a reflection of a series of localised processes and controls, which has been imposed on the facet. The slope morphometry map has been prepared by dividing the larger topographical map into smaller units. The contour lines have the same standard spacing, i.e., the same number of contour lines per km of horizontal distance. The chosen categories are six in number, representing the slopes of escarpment/cliff (> 45°), steep slope (35°-45°), moderately steep slope (25°-35°), gentle slope (15°-25 °) and very gentle slope (< 15°).
271
prone to mass wasting processes. Forest cover, in general, smothers the action of climatic agents on the slopes and protects them from the effects of weathering and erosion. A well-spread root system increases the shearing resistance of slope material. Agriculture, in general, is practised on low to very low slopes, though moderately steep slopes are not spared at places. However, the agricultural lands represent areas of repeated water charging for cultivation purposes and as such may be considered stable. Based on criteria of intensity of vegetation cover, the ratings have been awarded. Groundwater conditions
Because groundwater in hilly terrain is generally channeled along structural discontinuities of rocks, it does not have a uniform flow pattern. The evaluation of observations of the behavior of groundwater on hill slopes is not possible over large areas. Therefore, in order to make a quick appraisal, the nature of surface indications of the behavior of groundwater will provide valuable information on the stability of hill slopes for hazard mapping purposes. Surface indications of water such as damp, wet, dripping and flowing are used for rating purposes. The observations taken after the monsoon, provide probably the worst groundwater conditions possible.
Relative relief
The relative relief map represents the local relief of maximum height between the ridge top and the valley floor within an individual facet. This shows the major breaks in the slopes of the study area. Three categories of slopes of relative relief have been chosen for hazard evaluation purposes, namely low (< 100 m ), medium (101-300 m) and high (> 300 m). Land use and land cover
Land cover is an indirect indication of the stability of hill slopes. Barren and sparsely vegetated areas show faster erosion and greater instability as compared to reserve or protected forests, which are thickly vegetated and generally less
Methodology for landslide hazard zonation (LHZ) mapping The LHZ mapping technique is a macrozonation approach showing the probabilities of landslide hazards. The LHZ maps are generally prepared on 1:25,000 to 1:50,000 scales. The LHZ mapping comprises mainly two components: desk study and field investigations. The desk study consists of preparation of prefield maps showing the status of causative factors in the study area with the help of aerial photographs, satellite imageries, topographic maps and geological maps. The prefield maps, i.e., lithological map, structural map, slope morphometry map, relative relief map, rock outcrop and soil cover map, land use and land cover map, and hydrogeological map are
272
R. ANBALAGAN
TABLE 2 Landslide hazard evaluation factor (LHEF) rating scheme Description factor
LITHOLOGY Rock type
Category
Rating
Remarks
0.2 0.3 0.4
(a) Highly weathered - - rock discoloured, joints open with weathering products, rock fabric altered to a large extent; correction factorC~ (b) Moderately weathered - - rock discoloured with fresh rock patches, weathering more around joint planes, but rock in-tact in nature; correction factor C2 (c) Slightly weathered - - rock slightly discoloured along joint planes, which m a y be moderately tight to open, in-tact rock; correction factor C 3
Type-1 Quartzite and limestone Granite and Gabbro Gneiss
Correction .factor ,for weather&g
Type-H Well-cemented terrigenous sedimentary rocks, dominantly sandstone with minor beds of claystone Poorly cemented terrigenous sedimentary rocks, dominantly sandstone with minor clay shale beds
1.0
1.2 1.3
The correction Factor for weathering should be multiplied with the fresh rock rating to get the corrected rating For rock type 1
1.8
For rock type I1
1.3
Type-Ill Slate and phyllite Schist Shale with interbedded clayey and nonclayey rocks Highly weathered shale, phyllite and schist
Soil type
Older well-compacted fluvial fill material (alluvial) Clayey soil with naturally formed surface (eluvial) Sandy soil with naturally formed surface (alluvial) Debris comprising mostly rock pieces mixed with clayey/sandy soil (colluvial) Older well compacted Younger loose material
Cl=4, C2=3, C3=2 C 1 = 1.5, C2= 1.25, C 3= 1.0 2.0 0.8 1.0 ot.¢~ 1.4
1.2 2.0
Parallelism between the slope and the discontinuity
STRUCTURE
(~j/c<,- ~).
Relationship of Structural Discontinuity with slope Relationship of parallelism between the slope and the discontinuity* Planar (cq - cq) Wedge (cq - ~q)
Relationship of dip of discontinuity* and inclination of slope Planar (Bj-Bs) Wedge (B~-B~)
~5" V *~j/*¢i
I >30 ° II 21°-30 ° II 11°-20 ° IV 6°-10 ° IV < 5 °
0.20 0.25 0.30 0.40 0.50
cq = d i p direction of joint ~t~ = direction of line of intersection of two discontinuities ~ = direction of slope inclination
I II II IV IV
0.3 0.5 0.7 0.8 1.0
*Discontinuity refers to the planar discontinuity or the line of intersection of two planar discontinities, whichever, is important concerning instability
> 10° 0°-10 ° 0° 0°-( - 10°) ( - 10°)
Bj = d i p of joint B i = plunge of line of intersection of two discontinuities B~ = inclination of slope Category I = very favourable II = favourable III = fair I V = unfavourable V = very unfavourable
273
LANDSLIDE HAZARD EVALUATION AND ZONATION MAPPING IN MOUNTAINOUS TERRAIN
TABLE 2 (continued)
Description factor
Category
Rating Remarks
Dip o f discontinuiO,*
I
0.20
Planar Wedge
11 16'~-25 ' I1 2ff'-35" IV 36~'-45 '~ IV > 45' <5m 6-10 m
Bj B~
Depth of soil cover
< 15 °
0.25 0.30 0.40 0.50 0.65 0.85 1.30 2.0
11-15 m
16 2 0 m >20 m
~ / / ~ o %
~
~.~,.~ ~J~.~
%
1.20
SLOPE MORPHOMETRY
Escarpment/cliff Steep slope Moderately steep slope Gentle slope Very gentle slope
> 45 ° 36 ° 45" 26°-35 '~ 16" 25" < 15~'
2.0 1.7 1.2 0.8 0.5
Relationship of dip of discontinuity and the inclination of s l o p e (flj/fli
fls).
15"
RELATIVE RELIEF
Low medium High
-
< 100 m 101-300 m > 300 m
0.3 0.6 1.0
LAND USE AND LAND C O V E R Agricultural land/populated fiat land Thickly vegetated forest area Moderately vegetated area Sparsely vegetated area with lesser ground cover Barren land
0.65 0.80 1.2 1.5 2.0
GROUND-WATER CONDITIONS Flowing Dripping Wet Damp Dry
1.0 0.8 0.5 0.2 0.0
prepared. The information collected from the desk study helps to plan and execute the field investigations systematically. During the field study, more detailed lithological and structural maps are prepared. The details of other maps prepared during the desk study can be verified in the field and modified wherever necessary. The field studies are carried out to collect the required data facet-wise for estimating the total hazards of the facets. The general procedure of the LHZ mapping technique is outlined in the form of flow a chart (Fig. 1).
./'
Dip of discontinuity (flJflj). Number of contour lines over one cm length (I : 50,000)
Slope angle
>25 19-25 13 18 8-12 <7
>45" 36°-45 ° 26"-35 '~ 16"-25" <15 '~
Calculation of total estimated hazard (TEHD) and hazard zonation mapping The total estimated hazard (TEHD) indicates the net probability of instability and is calculated facet-wise, because adjoining facets may have entirely different stability conditions. The TEHD of an individual facet is obtained by adding the ratings of the individual causative factors obtained from the L H E F rating scheme. Total estimated hazard (TEHD) = R~tings of
274
R. ANBALAGAN
DESKSTUDY)
t
J, I AQUISITIONOF] TOPOGRAPHIC MAPS 1:50.000
FIELD STUDY~ i
AQUISITIONOF I AERIAL PHOTOGRAPHS AND SATELLITE IHAG
I REGIONAL AOUtSmO O NFJ GEO- I LOGICAL MAP t
1
ERIES 1:50.000
t
K)ENTIFICATION OF FACTORS FOR HAZARD EVALUATION
[
t
PRE-FIELD GEO LOGICAL MAP ~ 1:50.000
I
LITHOLOGICAL AND ;TRUCTURAL I MAP 1:50.000
1
SLOPEMORPHOMETRICMAP RELATIVE RELIEF MAP
I ASSIGNMENTCF"LANDHAZARDEVALUA-I i TION FACTCR (LHEF) RATINGFOR DIFFE~NT CATEC~RIES
ROCK OUTCROP AND SOL COVER MAP LANIOUSE AND LAND COVER MAP HYDROGEOLOGICAL MAP
t I
CALCULATION OFTOTAL
ESTIMATED HAZARD (TEHD] PREPARATION Oil= LAND HAZARD
ZONATION(LHZ HAP
]
J
Fig. 1. General procedure for LHZ mapping.
(lithoiogy + structure + slope morphometry + relative relief + land use and land cover + groundwater conditions) On the basis of TEHD, five categories of landslide hazard zones have been identified (Table 3), namely, very low hazard (VLH), low hazard (LH), moderate hazard (MH), high hazard (HH) and very high hazard (VHH). Landslide hazard zonation of the KathgodamNainital area
The LHZ map of the Kathgodam (Lat. 29°13': Long. 79034' )-Nainital (Lat. 29°25': Long. 79°28 ') (Fig. 2) area in Kumaun Himalaya has been prepared using the LHEF rating scheme to study the stability environment. For that purpose, the slope facet map (Fig. 3), geological map (Fig. 4) and TABLE 3 Landslide hazard zonation on the basis of total estimated hazard (TEHD) Zone
TEHD value
Description of zone
I
<3.5
II III IV V
3.5-5.0 5.1-6.0 6.1-7.5 >7.5
Very Low Hazard (VLH) zone Low Hazard (LH) zone Moderate Hazard (MH) zone High Hazard (HH) zone Very High Hazard (VHH)zone
Fig. 2. Location map of Kathgodam-Nainital area.
LANDSLIDE HAZARD EVALUATION ~,ND ZONATION MAPPING IN MOUNTAINOUS TERRAIN
275
OI FL D[ BF AP LI LC KF IN BI NAGTHAT FORMATION SIWALIK FORMATION LAKE BEDDING I JOINT FAULT Fig. 3. Slope facet m a p o f K a t h g o d a m - N a i n i t a l area.
Fig. 4. G e o l o g i c a l map.
other terrain evaluation maps [such as slope morphometry map (Fig. 5), relative relief map (Fig. 6) and land use and land cover map (Fig. 7)] have been prepared, covering all the slope facets of the area. The groundwater condition remained generally dry though wet to flowing conditions have been observed at some places. These conditions have been incorporated in the calculation of the TEHD of the facets.
belt to the south. Towards the north, a thick succession of predominantly Palaeozoic sedimentary rocks is thrust over Lower Siwalik rocks, the Main Boundary Thrust (MBT) marking the contact. The Paleozoic sedimentary rocks are divided into the Nagthat, Blaini and Krol Formations (Valdiya et al., 1984). The Nagthat Formation comprises white and purple quartzarenites interbedded with minor grey and green slates and phyllites. These are found to be associated with Bhimtal volcanics comprising altered diabase, amphibolite or chlorite schist. The Blaini Formation consists of red shales and paraconglomerates interbedded with thin limestones beds. The InfraKrols Formation, succeeding the underlying Blainis without any angular discordance, comprises black carbonaceous slates and shales. The Upper Krol Formation consists of massive limestone and dolomites with minor bands of red and grey slates,the Lower and Middle Krols are com-
Geology of the area The autochthonous Lower Siwalik rocks constituting the Outer Himalaya are exposed in the southern parts of the area. They comprise greyish brown and brownish yellow, fine- to mediumgrained, micaceous, thick sandstone beds with subordinate siltstone and claystone beds. The Lower Siwalik rocks, exposed for 19 km along the road, are bounded by gravelly fans of the piedmont
276
R. ANBALAGAN
Fig. 5. Slope morphometry map. Fig. 6. Relative relief map.
posed of calcareous slates with bands of dolomitic limestone. A narrow wedge of granite and quartzporphyry (the Amritpur granite) on the eastern part is thrust over the Siwalik rocks along the Salari thrust (Fig. 4). The disposition of structural discontinuities including bedding and joints have been observed and marked.
These landslides may be correlated to the neotectonically active fault plane (Valdia et al., 1984) off° setting the MBT and subsequent mass wasting along the fault scarp. The area to the east and southeast of Nainital lake fall under H H zones with a pocket of V H H zone.
L H Z map of Kathgodam-Nainital area
Conclusions
The LHZ map of the Kathgodam-Nainital area (Fig. 8) indicates that the area has slopes of VLH to VHH. The southern part of the area has generally LH to MH slopes with narrow stretches of H H slopes, trending E - W along the V-shaped gorges.In the middle altitudes, the Jeolikote valley shows LH slopes, generally bounded on either sides by MH slopes. Few narrow strips of active landslides are present along the Balia river course close to its confluence with the Kuria stream as well as the Nalena stream and fall under the V H H zone.
The planning of development schemes, including road construction in hilly terrain, should take into consideration existing instabilities of slopes, so that schemes may be executed with minimum disturbance to the environmental balance of the area. Hence, a new quantitative approach based on a numerical rating scheme, the landslide hazard evaluation factor (LHEF) rating scheme, has been evolved. This scheme incorporates major inherent causative factors of slope instability, and adopts a simple practical and effective approach. It may
L A N D S L I D E H A Z A R D E V A L U A T I O N A N D Z O N A T I O N M A P P I N G IN M O U N T A I N O U S T E R R A I N
277
Fig. 7. Land use and land cover map. Fig. 8. Landslide hazard zonation map.
have wider applications among planners, geologists and engineers for route location and other mountain development programmes. The VLH and LH zones are generally safer for development schemes. The M H zones may contain some local vulnerable zones of instabilities. In the case of H H and V H H zones, detailed appraisals on l:1000 to 1:2000 scales should be carried out, to evaluate the nature of the instabilities, in order to come to appropriate mitigation measures to protect the geoenvironmental stability of the area.
Acknowledgements The author is grateful to Prof. Bhawani Singh, Department of Civil Engineering, University of Roorkee, for his encouragement and critical review of this work. The valuable suggestions by Prof. R.
Chander in the preparation of the manuscript are thankfully acknowledged.
References Barton, N., Lien, R. and Lunde, J., 1974. Engineering classification of rock masses for the design of tunnel support. Rock Mech., 6(4): 189-236. Bieniawski, Z.T., 1979. Tunnel design by rock mass classifications. Pennsylvania State Univ., USA, Tech. Rep., GL-7919. Romana, M., 1985. New adjustment ratings for application of Bieniawski classification to slopes. Int. Symp. Role of Rock Mechanics, Zacatecas, pp. 49-53. Valdia, K.S., Joshi, D.D., Sanwal, R. and Tandon, S.K., 1984. Geomorphic development across the main boundary thrust, an example from the Nainital hills in Kumaun Himalaya. J. Geol. Soc.Ind., 25(12):761-774.
269
Landslide hazard evaluation and zonation mapping in mountainous terrain R. A n b a l a g a n Department of Earth Sciences, University of Roorkee, India (Received October 23,1990; revised version accepted January 14,1992)
ABSTRACT Anbalagan, R., 1992. Landslide hazard evaluation and zonation mapping in mountainous terrain. Eng. Geol., 32: 269-277. Landslide hazard zonation (LHZ) maps are of great help to planners and field engineers for selecting suitable locations to implement development schemes in mountainous terrain, as well as, for adopting appropriate mitigation measures in unstable hazard-prone areas. A new quantitative approach has been evolved, based on major causative factors of slope instability. A case study of landslide hazard zonation in the Himalaya, adopting a landslide- hazard evaluation factor (LHEF) rating scheme, has been presented.
Introduction The planning, design and execution of developmental schemes, such as road and building construction, are often carried out too quickly due to financial, time and other constraints. As a result many projects may not incorporate adequate details of geological and geotechnical considerations, causing instability of hill slopes and a manifold increase in the incidence of landslides. This demonstrates the necessity of preparing multipurpose terrain evaluation maps, based on the geo-environment of the mountainous terrain and using them as the basis for planning future development schemes. A landslide hazard zonation (LHZ) m a p devides the land surface into zones of varying degrees of stability, based on an estimated significance of causative factors in inducing instability. The L H Z maps are useful for the following purposes: (a) The L H Z maps identify and delineate unstable hazard-prone areas, so that environmental
Correspondence to: R. Anbalagan, Department of Earth Sciences, University of Roorkee, India. 0013-7952/92/$05.00
regeneration programmes can be initiated adopting suitable mitigation measures. (b) These maps help planners to choose favourable locations for siting development schemes, such as building and road constructions. Even if the hazardous areas can not be avoided altogether, their recognition in the initial stages of planning may help to adopt suitable precautionary measures. The methodology of preparation of these maps s h o u l d be systematic, practicable and, as far as possible, simple so that the practicing engineers, geologists and planners may understand and use them effectively. Hence, a new quantitative approach for L H Z mapping has been developed, based on a numerical rating scheme called landslide hazard evaluation factor ( L H E F ) rating scheme. This technique may be effectively used during the preliminary stages of geotechnical investigations when a cheap and rapid hazard assessment technique is needed.
Landslide hazard evaluation factor (LHEF) rating scheme The L H E F rating scheme is based on an empirical approach which combines past experience
© 1992 - - Elsevier Science Publishers B.V. All rights reserved.
270
R. ANBALAGAN
gained from the study of causative factors and their impact on landslides with conditions anticipated in the area of study. Similar approaches have been adopted in the well-known rock mass classifications such as the R M R system and Q system (Barton et al., 1974; Bieniawski, 1979). The L H E F rating scheme is a numerical system which is based on major inherent causative factors of slope instability such as geology, slope morphometry, relative relief, land use and land cover and groundwater conditions. Factors like rainfall and seismicity are not included for the purpose of LHZ mapping. The maximum LHEF ratings for different categories are determined on the basis of their estimated significance in causing instability (Table I). The number 10 indicates the maximum value of the total estimated hazard (TEHD). Initially, the topography of the area to be covered by LHZ mapping is studied carefully and the hill slopes are divided into a number of facets generally delimited by ridges, spurs, gullies and rivers. A detailed L H E F rating scheme, showing ratings for a variety of subcategories for individual causative factors as given in Table 2, is discussed below.
Geology The geological map provides information on the lithological and structural setting of the area. The lithological and structural maps may also be prepared separately for better representation. TABLE 1 Proposed maximum LHEF rating for different contributory factors for macro-zonation Contributory factor
Lithology Relationship of structural discontinuities with slope Slope morphometry Relative relief Land use and land cover Groundwater conditions Total
Maximum LHEF rating 2.0 2.0 2.0 1.0 2.0 1.0 10.0
Lithology The erodibility or the response of rocks to the processes of weathering and erosion has been the main criteria in awarding the ratings for subcategories of lithology. E.g., rocks like quartzite, limestone and igneous rocks are generally hard, massive and resistant to erosion, forming steep slopes. In comparison, terrigenous sedimentary rocks are vulnerable to erosion and form more easily landslides. Phyllites and schists are characterised by flaky minerals which weather quickly and promote instability. Accordingly, the L H E F ratings have been awarded. A correction factor concerning the status of weathering of rocks has also been incorporated. In the case of soil, genesis and age are the main considerations in awarding the ratings. Older alluvium is generally well compacted and has a high shearing resistance. Recent materials such as slide debris are loose and have low shearing resistance.
Structure Structure includes primary and secondary discontinuities in the rocks such as bedding, joints, foliations, faults and thrusts. The disposition of structural discontinuities in relation to slope inclination and direction has a great influence on the stability of slopes. In this connection, the following three types of relations are considered important: (1) The extent of parallelism between the directions of the discontinuity, or the line of intersection of two discontinuities and the slope. (2) The steepness of the dip of the discontinuity, or the plunge of the line of intersection of two discontinuities. (3) The difference in the dip of the discontinuity, or the plunge of the line of intersection of the two discontinuities to the inclination of the slope. The more the discontinuity or the line of intersection of two discontinuities tends to be parallel to the slope, the greater the risk of failure. When the dip of the discontinuity or plunge of the line of intersection of two discontinuities increases, the probability of failure also increases, because the angle of friction for the discontinuity surfaces may be reached. Moreover, till the dip of the discontinu-
LANDSLIDE H A Z A R D EVALUATION A N D ZONATION MAPPING IN M O U N T A I N O U S TERRAIN
ity plane or the plunge of the line of intersection of the two discontinuities does not exceed the inclination of the slope, the failure potential remains high. Accordingly, the LHEF ratings have been assigned for various stability conditions, broadly on the basis of the approach indicated by Romana (1985). In the case of soil, the inferred depth of the soil cover has been used for awarding the ratings. Slope morphometry
Slope morphometry maps define slope categories on the basis of the frequency of occurrence of particular angles of slope. The distribution of the slope categories is dependent on the geomorphological history of the area; the angle of slope of each unit is a reflection of a series of localised processes and controls, which has been imposed on the facet. The slope morphometry map has been prepared by dividing the larger topographical map into smaller units. The contour lines have the same standard spacing, i.e., the same number of contour lines per km of horizontal distance. The chosen categories are six in number, representing the slopes of escarpment/cliff (> 45°), steep slope (35°-45°), moderately steep slope (25°-35°), gentle slope (15°-25 °) and very gentle slope (< 15°).
271
prone to mass wasting processes. Forest cover, in general, smothers the action of climatic agents on the slopes and protects them from the effects of weathering and erosion. A well-spread root system increases the shearing resistance of slope material. Agriculture, in general, is practised on low to very low slopes, though moderately steep slopes are not spared at places. However, the agricultural lands represent areas of repeated water charging for cultivation purposes and as such may be considered stable. Based on criteria of intensity of vegetation cover, the ratings have been awarded. Groundwater conditions
Because groundwater in hilly terrain is generally channeled along structural discontinuities of rocks, it does not have a uniform flow pattern. The evaluation of observations of the behavior of groundwater on hill slopes is not possible over large areas. Therefore, in order to make a quick appraisal, the nature of surface indications of the behavior of groundwater will provide valuable information on the stability of hill slopes for hazard mapping purposes. Surface indications of water such as damp, wet, dripping and flowing are used for rating purposes. The observations taken after the monsoon, provide probably the worst groundwater conditions possible.
Relative relief
The relative relief map represents the local relief of maximum height between the ridge top and the valley floor within an individual facet. This shows the major breaks in the slopes of the study area. Three categories of slopes of relative relief have been chosen for hazard evaluation purposes, namely low (< 100 m ), medium (101-300 m) and high (> 300 m). Land use and land cover
Land cover is an indirect indication of the stability of hill slopes. Barren and sparsely vegetated areas show faster erosion and greater instability as compared to reserve or protected forests, which are thickly vegetated and generally less
Methodology for landslide hazard zonation (LHZ) mapping The LHZ mapping technique is a macrozonation approach showing the probabilities of landslide hazards. The LHZ maps are generally prepared on 1:25,000 to 1:50,000 scales. The LHZ mapping comprises mainly two components: desk study and field investigations. The desk study consists of preparation of prefield maps showing the status of causative factors in the study area with the help of aerial photographs, satellite imageries, topographic maps and geological maps. The prefield maps, i.e., lithological map, structural map, slope morphometry map, relative relief map, rock outcrop and soil cover map, land use and land cover map, and hydrogeological map are
272
R. ANBALAGAN
TABLE 2 Landslide hazard evaluation factor (LHEF) rating scheme Description factor
LITHOLOGY Rock type
Category
Rating
Remarks
0.2 0.3 0.4
(a) Highly weathered - - rock discoloured, joints open with weathering products, rock fabric altered to a large extent; correction factorC~ (b) Moderately weathered - - rock discoloured with fresh rock patches, weathering more around joint planes, but rock in-tact in nature; correction factor C2 (c) Slightly weathered - - rock slightly discoloured along joint planes, which m a y be moderately tight to open, in-tact rock; correction factor C 3
Type-1 Quartzite and limestone Granite and Gabbro Gneiss
Correction .factor ,for weather&g
Type-H Well-cemented terrigenous sedimentary rocks, dominantly sandstone with minor beds of claystone Poorly cemented terrigenous sedimentary rocks, dominantly sandstone with minor clay shale beds
1.0
1.2 1.3
The correction Factor for weathering should be multiplied with the fresh rock rating to get the corrected rating For rock type 1
1.8
For rock type I1
1.3
Type-Ill Slate and phyllite Schist Shale with interbedded clayey and nonclayey rocks Highly weathered shale, phyllite and schist
Soil type
Older well-compacted fluvial fill material (alluvial) Clayey soil with naturally formed surface (eluvial) Sandy soil with naturally formed surface (alluvial) Debris comprising mostly rock pieces mixed with clayey/sandy soil (colluvial) Older well compacted Younger loose material
Cl=4, C2=3, C3=2 C 1 = 1.5, C2= 1.25, C 3= 1.0 2.0 0.8 1.0 ot.¢~ 1.4
1.2 2.0
Parallelism between the slope and the discontinuity
STRUCTURE
(~j/c<,- ~).
Relationship of Structural Discontinuity with slope Relationship of parallelism between the slope and the discontinuity* Planar (cq - cq) Wedge (cq - ~q)
Relationship of dip of discontinuity* and inclination of slope Planar (Bj-Bs) Wedge (B~-B~)
~5" V *~j/*¢i
I >30 ° II 21°-30 ° II 11°-20 ° IV 6°-10 ° IV < 5 °
0.20 0.25 0.30 0.40 0.50
cq = d i p direction of joint ~t~ = direction of line of intersection of two discontinuities ~ = direction of slope inclination
I II II IV IV
0.3 0.5 0.7 0.8 1.0
*Discontinuity refers to the planar discontinuity or the line of intersection of two planar discontinities, whichever, is important concerning instability
> 10° 0°-10 ° 0° 0°-( - 10°) ( - 10°)
Bj = d i p of joint B i = plunge of line of intersection of two discontinuities B~ = inclination of slope Category I = very favourable II = favourable III = fair I V = unfavourable V = very unfavourable
273
LANDSLIDE HAZARD EVALUATION AND ZONATION MAPPING IN MOUNTAINOUS TERRAIN
TABLE 2 (continued)
Description factor
Category
Rating Remarks
Dip o f discontinuiO,*
I
0.20
Planar Wedge
11 16'~-25 ' I1 2ff'-35" IV 36~'-45 '~ IV > 45' <5m 6-10 m
Bj B~
Depth of soil cover
< 15 °
0.25 0.30 0.40 0.50 0.65 0.85 1.30 2.0
11-15 m
16 2 0 m >20 m
~ / / ~ o %
~
~.~,.~ ~J~.~
%
1.20
SLOPE MORPHOMETRY
Escarpment/cliff Steep slope Moderately steep slope Gentle slope Very gentle slope
> 45 ° 36 ° 45" 26°-35 '~ 16" 25" < 15~'
2.0 1.7 1.2 0.8 0.5
Relationship of dip of discontinuity and the inclination of s l o p e (flj/fli
fls).
15"
RELATIVE RELIEF
Low medium High
-
< 100 m 101-300 m > 300 m
0.3 0.6 1.0
LAND USE AND LAND C O V E R Agricultural land/populated fiat land Thickly vegetated forest area Moderately vegetated area Sparsely vegetated area with lesser ground cover Barren land
0.65 0.80 1.2 1.5 2.0
GROUND-WATER CONDITIONS Flowing Dripping Wet Damp Dry
1.0 0.8 0.5 0.2 0.0
prepared. The information collected from the desk study helps to plan and execute the field investigations systematically. During the field study, more detailed lithological and structural maps are prepared. The details of other maps prepared during the desk study can be verified in the field and modified wherever necessary. The field studies are carried out to collect the required data facet-wise for estimating the total hazards of the facets. The general procedure of the LHZ mapping technique is outlined in the form of flow a chart (Fig. 1).
./'
Dip of discontinuity (flJflj). Number of contour lines over one cm length (I : 50,000)
Slope angle
>25 19-25 13 18 8-12 <7
>45" 36°-45 ° 26"-35 '~ 16"-25" <15 '~
Calculation of total estimated hazard (TEHD) and hazard zonation mapping The total estimated hazard (TEHD) indicates the net probability of instability and is calculated facet-wise, because adjoining facets may have entirely different stability conditions. The TEHD of an individual facet is obtained by adding the ratings of the individual causative factors obtained from the L H E F rating scheme. Total estimated hazard (TEHD) = R~tings of
274
R. ANBALAGAN
DESKSTUDY)
t
J, I AQUISITIONOF] TOPOGRAPHIC MAPS 1:50.000
FIELD STUDY~ i
AQUISITIONOF I AERIAL PHOTOGRAPHS AND SATELLITE IHAG
I REGIONAL AOUtSmO O NFJ GEO- I LOGICAL MAP t
1
ERIES 1:50.000
t
K)ENTIFICATION OF FACTORS FOR HAZARD EVALUATION
[
t
PRE-FIELD GEO LOGICAL MAP ~ 1:50.000
I
LITHOLOGICAL AND ;TRUCTURAL I MAP 1:50.000
1
SLOPEMORPHOMETRICMAP RELATIVE RELIEF MAP
I ASSIGNMENTCF"LANDHAZARDEVALUA-I i TION FACTCR (LHEF) RATINGFOR DIFFE~NT CATEC~RIES
ROCK OUTCROP AND SOL COVER MAP LANIOUSE AND LAND COVER MAP HYDROGEOLOGICAL MAP
t I
CALCULATION OFTOTAL
ESTIMATED HAZARD (TEHD] PREPARATION Oil= LAND HAZARD
ZONATION(LHZ HAP
]
J
Fig. 1. General procedure for LHZ mapping.
(lithoiogy + structure + slope morphometry + relative relief + land use and land cover + groundwater conditions) On the basis of TEHD, five categories of landslide hazard zones have been identified (Table 3), namely, very low hazard (VLH), low hazard (LH), moderate hazard (MH), high hazard (HH) and very high hazard (VHH). Landslide hazard zonation of the KathgodamNainital area
The LHZ map of the Kathgodam (Lat. 29°13': Long. 79034' )-Nainital (Lat. 29°25': Long. 79°28 ') (Fig. 2) area in Kumaun Himalaya has been prepared using the LHEF rating scheme to study the stability environment. For that purpose, the slope facet map (Fig. 3), geological map (Fig. 4) and TABLE 3 Landslide hazard zonation on the basis of total estimated hazard (TEHD) Zone
TEHD value
Description of zone
I
<3.5
II III IV V
3.5-5.0 5.1-6.0 6.1-7.5 >7.5
Very Low Hazard (VLH) zone Low Hazard (LH) zone Moderate Hazard (MH) zone High Hazard (HH) zone Very High Hazard (VHH)zone
Fig. 2. Location map of Kathgodam-Nainital area.
LANDSLIDE HAZARD EVALUATION ~,ND ZONATION MAPPING IN MOUNTAINOUS TERRAIN
275
OI FL D[ BF AP LI LC KF IN BI NAGTHAT FORMATION SIWALIK FORMATION LAKE BEDDING I JOINT FAULT Fig. 3. Slope facet m a p o f K a t h g o d a m - N a i n i t a l area.
Fig. 4. G e o l o g i c a l map.
other terrain evaluation maps [such as slope morphometry map (Fig. 5), relative relief map (Fig. 6) and land use and land cover map (Fig. 7)] have been prepared, covering all the slope facets of the area. The groundwater condition remained generally dry though wet to flowing conditions have been observed at some places. These conditions have been incorporated in the calculation of the TEHD of the facets.
belt to the south. Towards the north, a thick succession of predominantly Palaeozoic sedimentary rocks is thrust over Lower Siwalik rocks, the Main Boundary Thrust (MBT) marking the contact. The Paleozoic sedimentary rocks are divided into the Nagthat, Blaini and Krol Formations (Valdiya et al., 1984). The Nagthat Formation comprises white and purple quartzarenites interbedded with minor grey and green slates and phyllites. These are found to be associated with Bhimtal volcanics comprising altered diabase, amphibolite or chlorite schist. The Blaini Formation consists of red shales and paraconglomerates interbedded with thin limestones beds. The InfraKrols Formation, succeeding the underlying Blainis without any angular discordance, comprises black carbonaceous slates and shales. The Upper Krol Formation consists of massive limestone and dolomites with minor bands of red and grey slates,the Lower and Middle Krols are com-
Geology of the area The autochthonous Lower Siwalik rocks constituting the Outer Himalaya are exposed in the southern parts of the area. They comprise greyish brown and brownish yellow, fine- to mediumgrained, micaceous, thick sandstone beds with subordinate siltstone and claystone beds. The Lower Siwalik rocks, exposed for 19 km along the road, are bounded by gravelly fans of the piedmont
276
R. ANBALAGAN
Fig. 5. Slope morphometry map. Fig. 6. Relative relief map.
posed of calcareous slates with bands of dolomitic limestone. A narrow wedge of granite and quartzporphyry (the Amritpur granite) on the eastern part is thrust over the Siwalik rocks along the Salari thrust (Fig. 4). The disposition of structural discontinuities including bedding and joints have been observed and marked.
These landslides may be correlated to the neotectonically active fault plane (Valdia et al., 1984) off° setting the MBT and subsequent mass wasting along the fault scarp. The area to the east and southeast of Nainital lake fall under H H zones with a pocket of V H H zone.
L H Z map of Kathgodam-Nainital area
Conclusions
The LHZ map of the Kathgodam-Nainital area (Fig. 8) indicates that the area has slopes of VLH to VHH. The southern part of the area has generally LH to MH slopes with narrow stretches of H H slopes, trending E - W along the V-shaped gorges.In the middle altitudes, the Jeolikote valley shows LH slopes, generally bounded on either sides by MH slopes. Few narrow strips of active landslides are present along the Balia river course close to its confluence with the Kuria stream as well as the Nalena stream and fall under the V H H zone.
The planning of development schemes, including road construction in hilly terrain, should take into consideration existing instabilities of slopes, so that schemes may be executed with minimum disturbance to the environmental balance of the area. Hence, a new quantitative approach based on a numerical rating scheme, the landslide hazard evaluation factor (LHEF) rating scheme, has been evolved. This scheme incorporates major inherent causative factors of slope instability, and adopts a simple practical and effective approach. It may
L A N D S L I D E H A Z A R D E V A L U A T I O N A N D Z O N A T I O N M A P P I N G IN M O U N T A I N O U S T E R R A I N
277
Fig. 7. Land use and land cover map. Fig. 8. Landslide hazard zonation map.
have wider applications among planners, geologists and engineers for route location and other mountain development programmes. The VLH and LH zones are generally safer for development schemes. The M H zones may contain some local vulnerable zones of instabilities. In the case of H H and V H H zones, detailed appraisals on l:1000 to 1:2000 scales should be carried out, to evaluate the nature of the instabilities, in order to come to appropriate mitigation measures to protect the geoenvironmental stability of the area.
Acknowledgements The author is grateful to Prof. Bhawani Singh, Department of Civil Engineering, University of Roorkee, for his encouragement and critical review of this work. The valuable suggestions by Prof. R.
Chander in the preparation of the manuscript are thankfully acknowledged.
References Barton, N., Lien, R. and Lunde, J., 1974. Engineering classification of rock masses for the design of tunnel support. Rock Mech., 6(4): 189-236. Bieniawski, Z.T., 1979. Tunnel design by rock mass classifications. Pennsylvania State Univ., USA, Tech. Rep., GL-7919. Romana, M., 1985. New adjustment ratings for application of Bieniawski classification to slopes. Int. Symp. Role of Rock Mechanics, Zacatecas, pp. 49-53. Valdia, K.S., Joshi, D.D., Sanwal, R. and Tandon, S.K., 1984. Geomorphic development across the main boundary thrust, an example from the Nainital hills in Kumaun Himalaya. J. Geol. Soc.Ind., 25(12):761-774.