Introduction - Semi-quantitative Approaches for Landslide Assessment and Prediction - page 41

precipitation and rainfall duration for the landslide susceptibility analysis. Some
researchers e.g. Siddle and Swanston (1982), Keefer et al. (1987) concluded that
short-term rainfall intensity is the most important determinant whereas others e.g.
Endo (1969) and Glade ( 1998 ) found a correlation of long-term precipitation with
landslide phenomena. Slope stability over the hill slope is also governed by the rate
of water movement into and through the regolith and the water holding capacity.
These two important landslide governing parameters are influenced by the structure,
density and orientation of fractures and intensities in bedrock and other substrata
that underlie the soil pro

le. On micro-scale, the rate of water movement in hill
slope soils is best understood by the hydraulic conductivity (K) and the sub-surface
flux of water per unit hydraulic gradient. Clayey soils and compact silty soils with
very small interstitial pores have much lower values of K than coarse textured soils.
Siddle et al. (1985) put forwarded that the hydraulic conductivity of a con

ning
layer underlying unstable landforms regulates long-term drainage and thus controls
the moisture content of the overlying soil mantle. Hardenbicker and Grunert (2001)
and Siddle and Ochiai (2006) studied pore water pressure induced slope failure on
steep slope with high porosity in moderately deep soils. In ltration into the soil

increases pore-water pressure and make the slope materials unstable which is
controlled by soil physical properties (porosity, hydraulic conductivity, pore size
distribution, and preferential flow networks), vegetation cover, cultural practices,
freezing phenomena and macro and micro topography. Horton (1993) analyzed all
these properties and concluded that there exists an indirect relationship between the
rate of water in

ltration and slope instability. The preferential flow of water both
within the soil and with the underlying bedrock was studied by Tsukamoto et al.
(1982), Siddle et al. (2000a, 2001), Montgomery et al. (1997), and Siddle and
Chigira (2004). Anderson and Burt (1978), Pierson (1980b), Tsukamoto et al.
(1982, 2000) analyzed pore water pressure and revealed that the development of
perched water table within the regolith is responsible for the initiation or acceler-
ation of landslide.
After analyzing all the geologic, geomorphic, hydrologic, engineering, chemical
and mineralogical factors of slope instability, ten landslide triggering factors have
been taken into account in the present work such as slope angle, slope aspect, slope
curvature, lithology, drainage, lineaments, upslope contributing area, land use and
land cover, road contributing area, and settlement density. Besides, relative relief,
ruggedness index, constant of channel maintenance, and drainage confluence were
also studied to understand the nature of slope instability.
The landslide mechanism and triggering process are key problems acknowl-
edged by the Ersmann ( 1979 ), Sassa ( 1988 ), Huang ( 2004 ). The study of the
processes and the triggering mechanism of landslide from the view point of rainfall
and hydro-dynamics is attacting more importance in the present day. The mecha-
nism of landslide is accomplished by sliding plane which results from loss of
cohesion and friction before the formation of a boundary during a heavy rainfall. It
is very much dif cult to identify the potential landslide sites through investigation.

But there is a close relationship with rainfall where the landslide can be forecasted
and analyzed through statistical analysis of the rainfall data. The landslide is also