Geomorphic Threshold and Landslide - Semi-quantitative Approaches for Landslide Assessment and Prediction - page 149

and 102.4 mm respectively which are greater than the estimated threshold rainfall for
initiating slide at Tindharia and Lower Paglajhora. This indicates high possibility of
frequent slide in those places.

5.2.4 Threshold Slope Height to Initiate Slide

(Skempton and Hutchinson 1969 ) in their experience in the development of steep
slope and its evolution through slides in the glacial till of County Durham found a
critical slope height of 45 m at 30

steepness. Terzaghi ( 1962 ) calculated the
critical slope height of the cliff at which failure occurs. In the present study critical
height for slope failure is determined after Culmann ( 1866 ) and Carson ( 1977a , b , c ).

35 °

-

4c
c

sin h: cos u

Critical slope height ð h c Þ ¼ ð 5 : 11 Þ

1 cos ðh uÞ

(Culmann 1866 ; Carson 1977a , b , c )

h c Critical (threshold) slope height
c / Cohesion

u Angle of internal friction

γ Unit weight of materials

θ Slope angle.

Sc
W

Critical slope height ð h c Þ ¼ ð 5 : 12 Þ

(Terzaghi 1962 )

Hc slope height of the cliff at which failure occurs
Sc Compressive strength of the rocks
W Unit weight of the rocks.

5.2.5 Application of the Probability

Distribution Models to Estimate the Temporal
Probability of Landslide Events

Poisson and Binomial

' ' ' '

The frequent occurrence of landslides in the unstable terrain of Skivkhola watershed
and their continuous monitoring through intensive field investigation and in con-

sultation with others research works made by Starkel ( 1972 ), Basu and Sarkar
( 1985 , 1988 ), Basu and Maiti ( 2001 ), Maiti ( 2007a , b ), Ghosh et al. ( 2009 ), Sarkar
( 2011 ) and author himself have provided most reliable earlier landslide frequency