Geo-spatial Variability of Physiographic Parameters and Landslide Potentiality - Semi-quantitative Approaches for Landslide Assessment and Prediction - page 74

instability. Darjiling Himalaya is situated in the southern flank of Himalayan
Mountain Range, which receive frequent orographic rainfall during rainy period.
Most of the destructive landslides occur due to high intensity rainfall and it is
observed that the south facing slopes of Darjiling Himalaya are dominated by
higher amount of rainfall and large number of landslides. In Shivkhola Watershed,
concentration of drainage is one of the major causes of landslide. Slope charac-
terized by high positive (slope convexity) and high negative (slope concavity)
curvature value allows more concentration of water and drainage which on the other
hand decreases mineralogical bonding of soil and invite slope instability. The major
landslide prone areas in the Shivkhola Watershed are registered by such feature.
The occurrences of landslide phenomena are also aggravated because of the exis-
tence of high intensity lineaments at slope convex and slope concave section.
Lineaments permit seepage that helps to entrain the particles from vertical section
of the soil and introduce slope instability. It is to be concluded that the presence of
fragile lithological composition (>60 % area) and the continuous development of
drainage network over it, has made the Shivkhola Watershed more vulnerable to
landslides.

2.3 The Soil

Of cially the soil of the Darjiling Himalaya is divided into two main groups (a) the

brown forest soil and (b) the terai soil. The first group of soil is found in the moun-

tainous region whereas the second group is found in the lower elevated zone. The
shivkhola watershed is falling under the brown forest soil as it covers hilly area of the
Kurseong Division. The inherent fertility of this brown forest soil is very high and has
free drainage through their pro

les and also have very rich in well-distributed humus.
The percentage of organic matter in the top horizon increases with altitude but
gradually decreases down the pro les. The principal source of the organic matter

present in the top layer of the soil are leaves, stems, branches, roots, barks, fruits,
seeds, animals and micro-organism. The maturity of this forest soil depends on the
decomposition of the organic matter into humus. Accordingly, the decomposition is
slower at higher altitude due to climatic factors, but the decomposition is quicker at
lower altitude. Decomposition at higher altitude generally occurs only because of the
presence of fungi while at lower elevation it happens by climate, bacteria and animals.
In the Shivkhola watershed the major types of humus dominated soil found in the
forest soils are

. The former is a porous, loose, crumble and friable
mass that develops under deciduous species. The

' Mull ' and ' Mor '

is dominated by fungi and
mosses and associated to coniferous forest and also display a high degree of saturation
of A horizon and little accumulation of sesquioxide in the B horizon (DarjilingDistrict
Gazetteers by LSS O

' Mor '

'

Malley 1999 ) (Table 2.8 ). Sarkar ( 1987a , b ) introduced pedo-
geomorphic parameters and its impact on soil loss and landslip.