Himalayas, 1996; Gonzales-Diez et al. 1999; Galadini 2006;

Himalayas,
one of the most active
geodynamic domains of the world,
is highly vulnerable to landslides and associated hazards
(Larsen and Montgomery 2012; Kahlon
et al. 2014). The study of landslides, its triggering mechanism and associated phenomena is an emerging interdisciplinary field of study in the Himalayan
terrain, and a relatively limited study has been accomplished so far on this aspect.  Landslides such as rock-fall and rockslides are most common
natural hazards
in the Kullu valley of the Himachal Himalayas, causing major damages to economy (Varnes 1978; Brabb and Harrod 1989;
Sah and Mazari 1998; Gardner
2002; Kahlon et al. 2014; Banshtu
and Versain 2015). They are of great concern to the public administrators as well as geoscientists.

Landslides caused due to slope failure are dominantly
controlled by interaction of multiple factors of endogenic (i.e. geodynamics & tectonics) and exogeic (e.g. climatic
micro-parameters and site-specific external and internal
slope geometry) geomorphic processes. The significant factors of slope failure are geo-mechanical attributes such as gradient of the slope, nature of the overburden, the
lithological and structural attributes of rock stratum exposed or covered under regolith, and the ground water conditions (Dramis
and Sorriso-Valvo 1994; Agliardi et al. 2001; Agliardi et al. 2012). In geo-dynamically active domain
of the Himalayas, tectonic
forcing has resulted
in a high relief
terrain, rendering it vulnerable to slope failure (Summerfield and Nulton
1994; Hurtrez et al. 1999; Montgomery et al. 2001).
Hence, the tectonic
processes are recognized as the main causative factors for the triggering of gravitational slope failures
coupled with landslide activities (Cruden
and Varnes 1996; Gonzales-Diez et al. 1999; Galadini 2006; Larsen and Montgomery 2012). Long-term denudation and regional rock uplift along thrust planes
lead to topographic up-heaval and the consequent formation of steep slopes. Tectonically induced
rock-deformation creates fracturing and shearing
of the bedrock, rendering the terrain prone to slope failure
(Hutchinson 1995; Cruden and Varnes 1996; Hermanns
et al. 2001; Kellogg
2001). The bedding planes, joints and fault planes may also act as favorable sliding surfaces depending
upon their geometric characteristics (Bommer
and Rodriguez 2002;
Tibaldi et al. 1995; Crosta 1996; Galadini
2006). Geometric relationships between topography (external geometry) and geologic
structure/ discontinuities, geo-mechanical properties of rock units (internal slope geometry), along with ground
water conditions, are the primary
controlling factors
for the slope failures
(Freeze and Cherry 1979; Selby 1993). However,
deformation(s) of the critical
slopes may primarily
get triggered by sudden changes in the pore pressure, natural geodynamics processes (ground movement
and earthquakes), wind driven
vibrations and tree jacking,
lightning (thunder) and some anthropogenic events (machine/vehicular vibrations).

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Physiographically, the Kullu valley constitutes a part of the Beas river catchment area. The river originates from the Pir-Panjal range, near Rohtang crest (4038m) and flows transversally through two parallel ranges of Pir -Panjal and
Dhauladhar (Fig.-1). Kullu valley
hosts the sub-basins of tributaries like River Parvati, Hurla and Sainj. The upper reaches
and high altitude areas are covered with periglacial and glacial
deposits. The terraces, fans and hill slopes have
provided an ideal geo-environment for human activities including agriculture, dense settlements and other civil establishments (Sah and Mazari 2007). The valley is known for (i) a vibrant cultural heritage  attracting
international  tourism,  (ii) major construction activities linked with a 
series of hydroelectric projects (viz. Parvati
valley and Sainj valley Hydel Projects)
and (iii) being a corridor of strategic importance to upper reaches of Himalayas. In September 1995, a
disastrous landslide occurred
at Luggar Bhatti

 

 

near Kullu town in the valley
which killed sixty-five people (Gardner 2002).
Such hazards have been accelerated in the recent past (Sah and Mazari 1998). In the last two decades, no significant study has been carried out on Kullu valley to look into the correlation between the litho-tectonic characters of the region and the geo-hazard events. In the bed rock river domains such as the Kullu valley, the topographic relief
maintains the threshold
angle (maximum slope) by valley incision
(Schmidt and Montgomery, 1995; Burbank
et al., 1996; Hoek and Brown, 1980). However,
the rate of erosion
due to active tectonic
uplift increases nonlinearly with respect
to the slope of topography until a threshold
slope angle is reached (Schmidt and Montgomery, 1995;
Burbank et al, 1996).
Until the topographic slope reaches
the threshold, the slope angles and erosion rates increase
according to the rate of regional uplift. Further,
the threshold slope angle in response
to the tectonics-driven incision is
mainly achieved by slope failures
of the over-steepened river banks (Burbank
et al. 1996; Larsen
and Montgomery 2012).
Moreover the active rate of landslide also generate
more undulating surface. Hence, the quantitative analysis and interpretation of terrain roughness has become increasingly important in characterizing the terrain unevenness dominated by old-landslides (Frankel
and Dolan, 2007; Booth
et al. 2009).

 

The present
study is directed towards the analysis of landslide phenomena in the Kullu-Bhuntar- Manikaran region with a special
emphasis on their relationship with the morpho-tectonic environment of the terrain.
This study calls
for a detailed investigation on the dynamics of slope instabilities leading to high vulnerability and risk of the Kullu valley in terms of landslide hazards. An attempt
has been made to delineate
the relationship of topographic slope with existing deformational structures and spatio-temporal correlation of landslides with seismic
events in the region.

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