Introduction - Semi-quantitative Approaches for Landslide Assessment and Prediction - page 45

Generally the location of the landslide and its behavior is governed by the
hydrology of the sub-catchment in which it is located rather by the characteristics of
the catchment as a whole. Without a thorough mapping of the sub-catchment and
without assigning the weighting accordingly, the match between the inferred hazard
rating and the observed hazard rating will remain elusive (Bhandari 1987 ). Disaster
Management Support Group suggested a working scale for a slope instability
analysis is determined by the requirements of the user for whom the survey is
executed. Planners and engineers use the following examples of scales:

National scale (<1:1,000,000) provides a general inventory of problem areas for
an entire country, which can be used to inform national policy makers and the
general public.

￿

Regional scale (1:100,000 1:500,000) is used in the early phases of regional

development projects to evaluate possible constraints, due to instability, in the
development of large engineering projects and regional development plans.

￿ -

Medium scale (1:25,000 1:50,000) is used for the determination of hazard

zones in areas affected by large engineering structures, roads and urbanization
plans.

￿ -

1:15,000) is used at the level of site investigations prior to
the design phase of engineering works.
There are some basic principles proposed by Varnes ( 1984 ), Carrara et al.
( 1991 ), Hutchinson and Chabdler (1991), Hutchinson (1995), Turner and Schuster
(1996), Guzzetti et al. ( 1999a , b , c , 2003 ) behind the landslide susceptibility
zonation, though there are some conflicting views among experts. These are:

￿ Large scale (1:5,000 -

1. Firstly, the main controlling factors for initiating landslide phenomena should be
identi ed and mapped. So, slope failure can be recognized, classi ed and

field or through remote sensing, chiefly stereoscopeic aerial
photographs (Rib and Liang 1978; Varnes 1978 ; Hansen 1984; Hutchinson
1988; Cruden and Varnes 1996; Dikau et al. 1996; Guzzetti et al. 2003 ).
2. According to Varnes ( 1984 ), Carrara et al. ( 1991 ), Hutchinson (1995), for
landslides,

mapped in the

. Under this assumption,
landslide in future is likely to occur under the same geologic, geomorphic and
hydrologic conditions as those that led to landslide in the past. That is why
Varnes ( 1984 ), Carrara et al. ( 1991 ), ( 1995 ), Hutchinson (1995), Guzzetti et al.
( 1999a , b , c , 2003 ) expressed that the understanding of the past failure is
essential for the assessment of landslide hazard.
3. Researchers like Crozier ( 1986 ), Hutchinson (1988), Dietrich et al. (1995),
Guzzetti et al. ( 2003 ) assured that conditions that initiate landslides, or which
are directly or indirectly linked to slope failures, can be collected and used to
build predictive models of landslide occurrence, because landslides are con-
trolled by mathematical laws that can be determined empirically, statistically or
in deterministic fashion.
4. Landslide occurrence can be inferred from the heuristic investigations, com-
puted through the analysis of environmental

“ the past and present are keys to the future ”

information or inferred from