Lokesh K Sinha, R S Paul and SD Mehta
Slope failure processes are the common sites in the hilly terrain of Himalayas. These are one major natural hazards which not only results in the loss of life and property but also cau economic burden on the society. Hence, there is a necessity for better methods of landslide h evaluation and its zonation.
An area of about 100 sq. Ian. Around Dadahu bounded by latitude 30 33’5″N to 30 38~277 longitude 77 23’48’T to 77 3 0″E (Fig. 1) was undertaken as a pilot area for the detailed stu landslide hazard zonation. nation.
The investigated area, forming a part of the major Krol belt of Himachal, has been subjected to repeated phases of tectonic movements which has resulted in a very complex geology. Dadahu area, where the poly-phased deformation has been reported (Sinha, 1992), shows an intricate but interesting geology. The rocks of the investigated area are highly disturbed due to repeated folding faulting and thrusting. The stratagraphy of the area is shown in the following table:
|Pleistocene to Miocene||Siwalik Fm.||Upper Siwalik
|Precambrian||Krol Fm.||Krol C+D+E
The identification of different litho-units (Fig.2) was carried out on the basis of aerial photographs (1:30,000 scale) and Satellite Image of IRS LISS-1.
The Basantpur Formation, comprising conglomerate (at base), shale, slate and limestone is lying tectonically over the Subathus along the Krol Thrust and runs parallel to the Jalal river (WNW-ESE direction). These rocks are forming the base of the SianDhar syncline. -The calcareous and shaly horizon of this formation shows beautiful drag folds puckers.
The rocks of the Chandpur Formation are :
- A massive limestone, white grey in colour (microcrystalline) and occurs at the base ( to Bansa Member of Auden, 1937).
- Quartzitic sandstones occurring more prominently in the southern flank of the main Nigali Dhar syncline, north of the Giri and stratigraphically above the Bansa member.
- Black, slaty, finely laminated blackish shales and phyllites interbedded with silt from the bulk of the deposit.
The Blaini Formation undoubtedly marks a major unconformity in this part as it is lying over the Basantpur and Chandpur Formations leaving the Nagthats. The lithostratigraphy of the Blaini Formation of the Krol belt is :
- boulder beds at the base,
- carbonaceous shale, slate, grey limestone, varved argillite, minor grey and pale quartzite in the middle and overlain by
- flesh coloured and purple dolomitic limestone, interbedded with red shale in the upper part.
The outcrops of Blaini along the Jagar ka Khala are of particular interest because only here one can find the famous Blaini boulders of considerablesize . They are found randomly enclosed in a finely bedded silty matrix of slates. The siz from few centimeters to over half a meter in length.
The Krol Formation forming the core of the SianDhar syncline in the study area are divided into three namely Lower (Krol A) comprising of intercalations of thin bedded limestones and calcareous shales, Middle (Krol B) comprising of purple green calcareous slates/shales, and Upper (Krol C+D+E) comprising of thick bedded massive limestones, sometimes showing micro-crystalline nature.
The Subathu/Dagshai/Kasauli formation in the area is sandwiched between the two major thrusts of Himalaya namely Main Boundary Thrust and Krol Thrust. They are mainly dark brown to purplish splintery shales, thinly bedded greenish white, fine grained sandstones and occasional beds of carbonaceous shales faulted at places.
The Siwalik Group in the present text are described into three namely
- Lower Siwalik consisting of sub-greywacke with fine to medium grained clastics containing calcareous cement disseminated throughout the rockmass and interbedded with well developed clays of maroon and chocolate colours,
- The Middle Siwaliks are a gradation frc greywacke to arkose interbedded with thinner clays and calcareous matter occurring as segregation.
The sandstones are less sorted, coarse grained and soft. The sequence generally starts with a lower alteration of sandy clays and sandstones which gradually merge with massive, soft sandstones above and 3) the Upper Siwalik consist of very coarse clastics. They generally start with a pebble horizon at the base and goes upto conglomeratic horizon. In the basal part the beds alternating with conglomerates are soft sandstones, whereas in the upper part they are generally dull red clays with some sandstones.
The investigated area forming a part of the SianDhar Syncline is structurally very complicated and has undergone repeated phases of tectonic movements during which the litho-units have suffered foldmg fiaultmg and thrustmg. The main structural features which are encountered in the area are:
- Main Boundary Thrust :
In the investigated area this has brought the rocks of Lower Tertiaries which are now justaposed with the Siwaliks. At number of places this thrust has been displaced by younger transverse faults roughly trending in a NE-SW direction. In the field it was observed that the areas nearby this thrust are showing intense crushing and slickensides. Sometimes the faulted breccia are also found along this thrust.
- Krol Thrust :
This has brought the rocks of Basantpur Formation over the Subathus. Drag folds and puckers are intensely developed in the calcareous and shaley horizon of Basantpur Formation. The middle limbs of the puckers are frequently sheared locally producing a coarse strain slip cleavage and the rocks often show effects of cataclasis. Though the beds on the opposite sides of this junction are generally parallel, truncation of strips of Subathu against the trace of this junction near Sataun (not in the model area) is quite remarkable.
- Giri Thrust :
It is named after the Giri River (Pilgrim and West, 1928) and separated the Simla Group from the Krol Formation. In the present area the Giri river follows the Giri Thrust except near Dadahu village where this river changes its course due to a transverse fault. Intense crushing, mylonitization and the course of the Giri river are the main criteria for recognizing this thrust in the field.
- Transverse Faults:
A number of transverse faults oblique or sometimes perpendicular to the thrust trending NE-SW has displaced mainly Krol Thrust and Main Boundary Thrust at several places.
Most of the streams, south of Giri river, are following these transverse faults. One of the most important fault winch has changed the course of the Giri river is passing through Dadahu village and named as Dadahu Fault. In addition to these major faults, a number of E-W trending faults parallel to the axes of major synclines have also affected the formations. This E-W trending fault system is more or less contemporaneous with the folding and must have preceded the overthrusting by the pre-Siwaliks Whereas, the faults which are trending NE-SW direction are of recent age. According to our opinion the major landslides are occurring due to these faults as quite good number of landslides are coming in the vicinity of these faults (eg. Jagar ka Khala).
Landslide Hazard Zonation
A natural hazard means the probability of occurrence within a specified period of time and within a given area of a potentially damaging phenomenon. Though hazard is a process and it is very difficult to map a process which has not yet occurred. However, hazard mapping may be defined as “the identification of those sites where there is a likelihood of hazardous events rather than hazard affected sites”.
Hazard mapping is stated to be undertaken with respect to 4 key properties, magnitude, location, frequency and time. Under the present study the main emphasis is given on the location of landslides (Spatial).
Landslide Hazard Zonation may be defined as a technique of classifying an area into zones of relative degrees of potential hazards by ranking of various causative factors operative in a given area, based on their influence in initiation of landslides. It is therefore, the first task to identify various terrain factors which govern the stability of slope. Under the present study an attempt has been made to prepare a landslide hazard zonation map based on the synthesization of data acquired from various geo-environmental thematic maps.
From the exhaustive literature survey and the field checks, following geo-environmental factors are found which are playing a significant role in causing slope instability problems ‘in the area
- Slope Aspect
- Slope Morphometry
- Dip Slope Relation
- Rockmass Strength
- Ridge/Crest Line
- Relative Relief
A GIS software was used for integrating different thematic maps and assigning their combined effect. These thematic maps were quantified by giving them a relative score. In this process first the different thematic maps which were prepared are digitized and then the whole area is divided into 10,000 pixels (100m x 100m) while carrying out the rasterization. The cross match of each parameter was carried out with the existing landslide map (Fig.3) and finally the score for each class of the theme was calculated using the formula:
Z = Xn/Yn X/Y
where, Z Score of the class
Xn Area occupied by landslides in a particular class
Yn Area occupied by that class
X Total area of the landslides
Y =Total area
(for the pilot area X=9.99 sq. km. and Y=100.00 sq. km.)
Integration of grid-cells overlay is utilized for the present study (deGraff and Ronesburg,1980). Here all the controlling parameters are combined by giving equal weightage for all the themes and the final map is reclassified into five zones i.e. very low, low, high and very high hazardous zones.
Here, the high and very high hazardous zones are covering approximately 60%, the moderate is covering 37% and the low and very low hazardous zones are covering