Rainfall-landslide relationship for
Hong Kong
Proc. Instn
Civ. Engrs
Geotech. Engng,
1996, 119, Oct.,
242—243
J.N. Kay and T Chen
Russeli J. Maharaj, Kyoto University,
Japan The authors present information about a
subject of importance to geotechnical
practitioners. However, in their analysis, they
have considered only an alternative method of
plotting rainfall data, while geotechnical,
geological, geomorphological, hydrological,
land-use and landslide data are not presented
nor discussed.
8. In the discusser’s opinion, finding alternative methods of plotting and analysing
rainfall data may indeed be useful, as the
authors attempt to show. However, other
information about ground conditions must be
analysed in order properly to characterize
rainfall-induced failures. The authors state that
the possibility of landslides occurring depends
on the generation of pore-water pressure,
which depends on rainfall intensity, as well as
the time period of the intensity. This is only
one of the factors which determines pore-water
pressure generation on slopes, in response to
rainfall. For example, soil types, texture,
plasticity, permeability, structure, density,
slope form and angle, soil and relict bedrock
discontinuities also affect the generation of
pore-water pressure. Localized positive porewater pressure within a slope during lowintensity rainfall may also cause many failures.
9. In addition, the failure of slopes depends
on a number of factors, not only rainfall.
Indeed, rainfall is a main trigger to failure.
However, during a single rainstorm, all slopes
may not fail. Why? Where failures occur, they
may be immediate or delayed by a few days or
weeks. Failures may be of different types on
different or similar slopes. They may be
residual state or first-time landslides and
secondary or progressive failures. What was
the nature of the failures mapped in their study
and what were the prefailure ground conditions
on different slopes of the same and different
materials? What are the effects of seepage
erosion, weathering grades, slope form and
angle, geological and geotechnical properties
of materials on slope instability in response to
rainfall? Are the ground conditions uniform for
the area investigated? The publications
referenced by the authors show that there are
variable ground conditions on natural and manmade slopes in Hong Kong.
10. Therefore, how should one examine the
relationship between landslides and rainfall?
Surely, generalizations should not be made nor
uniform ground conditions and site response
assumed. Of course, if conditions are identical
on all slopes, then generalizations are applicable. When one speaks about rainfall-induced
landslides, what types of landslides is one
speaking about, natural or cut-slope failures,
shallow or deep failures, embankment failures,
first-time or residual state landslides?
11. In light of these points, I feel that the
author’s omission of pertinent data about
ground conditions and their influence on slope
instability in response to rainfall, leaves much
to the speculation of the reader and also
generalizes the relationship between landslide
frequency and rainfall, which may be quite
misleading. Although some indication of the
probability of landslide occurrence may be
obtained from their analysis, their analysis
does not characterize the nature of rainfallinduced landslides for all of Hong Kong.
Authors’ reply
The interest in the paper by Maharaj is
appreciated. However, there appears to be
some misunderstanding of the purpose
intended for relationships between landslides
and rainfall of the type represented by Fig. 2
and Table 1 of the paper. As mentioned in the
introduction to the paper, Hong Kong has a
Landslide Warning System that uses the
current weather information as its basis.
Warnings given are not usually directed at
particular locations except in so far as they
apply to those locations where there are steep
slopes. As yet, no capability appears to exist in
weather warning procedures to predict specific
locations within areas of the size of Hong
Kong where peak rainfalls are likely to occur.
Therefore, any associated landslide warning
has to be relatively general in its nature. By far
the best source of information on what would
be the outcome to an expected rainfall occurrence comes from the statistical evidence from
rainfall and landslides gathered in recent years.
In this work there is no distinction between
manmade slopes and natural slopes or between
materials originating from predominantly
colluvial, volcanic or igneous origins. It is
likely that the potential for failure among these
different conditions and others will vary but, at
the present time, no basis exists for
differentiating, and for the purpose of this
particular application, there is a question as to
whether such a distinction would be valuable.
13. To envisage that the graphs might
apply to other locations outside of Hong Kong
would be most optimistic as the ranges of the
values of the various components (as listed by
Paper 10729
Paper published:
Proc. Instn Civ.
Engrs, Geotech.
Engng, 1995,
113, Apr., 117—118
211
Maharaj) that control slope stability are
unlikely to be duplicated. There is no
suggestion that the results might be applicable
elsewhere.
14. As to how the results might be
applied to specific sites in Hong Kong, the
authors have also made no recommendation.
Extreme levels of rainfall are critical for slope
performance and others have made proposals
for construction of flow nets to determine pore
pressure distributions based on return period
precipitation and infiltration rates for Hong
Kong conditions. It has been suggested that
these might take advantage of such methods as
electric analogue, finite difference and finite
element techniques. However, for practical
purposes, the levels of uncertainty associated
with the required assumptions for these
methods place serious limitations on their
value. In spite of the continuing emergence of
‘improved’ slope stability computer programs,
these methods are unable to account for poor
levels of prediction of (among other things) the
critical pore-water pressure conditions. It is
doubtful, therefore, whether any rational
method has yet been found for reasonably
interpreting the data of the type under
discussion for inclusion in slope stability
analysis in Hong Kong.
15. Maharaj has suggested extension of
the statistical analysis type of approach to
include a wide range of variables as a means
for improving design and analysis methods.
The data presently available are not sufficient
for this purpose. As it happens, recent tragic
events in Hong Kong have led to the
availability of large amounts of government
funding for improvement in slope engineering
procedures. Some allocation of these funds to
research that would include an emphasis on
statistical analyses to improve assessment of
slopes would be extremely valuable.
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