Introduction - Semi-quantitative Approaches for Landslide Assessment and Prediction - page 37

The hydrological factors mainly runoff depends on the upslope contributing
factor. The calculation of contributing area was made considering multiple flow
direction where the cumulative flow at a point (surrogated by the upslope area)
was distributed among more than one neighbouring down slope pixel (Borga
et al. 1998 ). The topographic irregularities at concave and convex slope are
responsible for the convergence and divergence of flow and thus necessitate the
implication of topographic index by Quinn et al. ( 1991 ). Another concept of
speci

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c contributing area (total contributing area divided by the contour length)
was computed by distributing flow from a pixel among its entire lower elevation
neighbour pixel (Borga et al. 1998 ). Fraction of Flow (Fi) i ) allocated to each
lower neighbour was calculated using Eq. 1.5 (Quinn et al. 1991 ).

S i L i
R

F i ¼ ð 1 : 5 Þ

S j L j

where the summation is for the entire lower neighbour; S is the directional slope,
and L is an effective contour length that acts as the weighting factor. The value
of L used here is 10 m of the pixel size of the cardinal neighbour and 14.14 m of
the pixel diagonal for diagonal neighbour.

The rain is the important factor for triggering slide by introducing lubrication
and increasing weight of wet soil in saturation condition. Once the stability
threshold is crossed, the slope remains unstable at greater rainfall rates. Thus it
is possible to determine the minimum steady state rainfall predicted to cause
instability, called critical rainfall (r cr ) (Borga et al. 1998 ).

￿

bp s
ap W ½

tan h r cr ¼ Tsin h 1

tan u

ð 1 : 6 Þ

The amount of surface runoff from certain rain was calculated using USDA SCS
Curve Number Technique ( 1972 ) and Ministry Agriculture, Government of
India (1972).

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Angle of repose is considered to be the important stability factor (Van Burkalow
1945 ; Bloom 1991 ). The cohesion and angle of internal friction were measured
by tri-axial compression test following Mohr stress Diagram following (Brudsen
1979 ; Jumikis 1967; Carson 1977 ; Borga et al. 1998 ). Geo-technical parameters
of the collected soil and rock samples from different locations were measured at
Geotechnical Laboratory, G.S.I., Kolkata through tri-axial soil testing mecha-
nism. Besides, Keen Box Method and Sieve Method were used to measure bulk
density and texture. To assess the geotechnical parameters of the soil in the
Geological Survey of India (GSI) Laboratory (East Kolkata, Salt Lake) Indian
Standard IS: 2131 (1981) were applied. This is the Indian Standard (First
Revision), adopted by the Indian Standard Institution on 24th December, 1981,
after the draft

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finalized by the Soil Engineering and Rock Mechanics Sectional
Committee approved by the Building Division Council. Finally, using all the