Rainfall-landslide relationship for
Hong Kong
Proc. Instn
Civ. Engrs
Geotech. Engng,
1995, 113, Apr.,
117—118
J. N. Kay, BE, MS, PhD, FJE(Aust), FHKIE, and T. Chen, BSc, MSc, Mphil
Ground Board
Following experiences elsewhere, early
Hong Kong researchers anticipated that the
maximum landslide activity would be
associated with an accumulative wetting
process and build-up of pore-water pressures in the ground over a period of several
days. More recently, attention has been
focused on rainfall levels occurring during
the preceding one hour as a preferable
indicator. However, compilation of data in a
way that includes both the hourly and daily
rainfalls together with the intensity of
landslide activity demonstrates that this
combination of time periods may be
superior as an indicator of landslide
activity.
logical and meteorological environments to
those of Hong Kong that indicate rainfalls over
relatively long periods to be significant for
subsequent landslides. For example, Peck
(1967)~ determined that creep movements at
Moanaloa, Hawaii, compared best with ten-day
antecedant rainfall, i.e. the cumulative rainfall
over the ten days immediately preceding the
creep measured for that day. A similar
approach was taken for Hong Kong by Lumb
(1975)~ as shown in Fig. 1, but as more
detailed information emerged some doubt was
thrown on the value of this indicator for Hong
Kong by Brand et al. (1984).~ Brand
demonstrated that the one-hour rainfall associated with the landslide activity was a far more
relevant indicator.
Introduction
The combination of steep slopes and highintensity rainfall leads to major landslide problems in Hong Kong. Careful attention is
required in construction planning in order to
minimize the potential risk of damage to life
and property, and commendable contributions
have been made by the Geotechnical Engineering Office (GEO), formerly the Geotechnical
Control Office, of the Hong Kong Government
in addressing design and maintenance procedures and ensuring a significant decrease in
reported damage levels. However, the problem
is an ongoing one and an important specific
measure adopted by the GEO has been the
establishment of formal data gathering procedures. These have included the installation
of automatic rainfall data acquisition systems
and formalized procedures for gathering data
on landslide events from the government
departments most closely involved. Details of
these have been reviewed most recently by
Premchitt et al.’ Since 1984, data have been
summarized and reported in annual
publications.2 The data are extremely valuable
for various aspects of ongoing research. Of
specific interest to the public is its value in
relation to the Hong Kong Landslide Warning
System. Consideration of the data from these
reports by the Authors in terms of daily and
hourly rainfall levels has led to a consistent
relationship for probability levels of landslide
activity, and it suggests a possible basis for
improvement of criteria for the present
warning procedures.
Alternative plot details
3. It may be noted from Fig. 1 that a
reasonable, though imperfect, correlation of
data was obtained by Lumb.4 However, any
form of the correlation is likely to be good,
provided the time periods are reasonable,
simply on the basis that a high correlation
would be expected among hourly, daily and 15
day rainfall levels. On the other hand, the
probability of a landslide occurring is closely
associated with the maximum pore-water
pressure generated in the ground, and this will
depend on the level of rainfall intensity as well
as the time period for that intensity. No doubt,
different relative combinations will be critical
for different slopes; higher pore-water
pressures may be produced in some cases
where a longer period at a lower maximum
intensity exists, and in others where a shorter
period at higher maximum intensity exists. It is
doubtful that this matter can be approached in
a fundamental way. The best method is
probably to continue to examine alternative
ways of plotting the measured data.
4. Figure 2 shows a plot of daily rainfall
against hourly rainfall, with different symbols
indicating different levels of landslide activity
at the time of the event in a similar form to that
of Lumb.4 A major difference, however, is that
in addition to the rainfall events for various
levels of landslide activity, all cases are shown
when rainfall exceeded 50 mm/day, including
those cases when no landslide activity
occurred. The advantage of this approach is
that the relative frequencies of landslide
occurrence for ranges of rainfall states can be
assessed. This provides some approximate
quantitative guidance to the probability levels
of landslide activity for the various corn-
Earlier correlations
2. Important contributions have been made
in other parts of the world in different geo-
Geotechnical
Engineerin~
Advisory Panel
Paper 10729
Written discussion
closes 15 June 1995
J. N. Kay,
Senior Lecturer,
University of
Hong Kong
T. Chen,
Research Assistant,
University of
Hong Kong
117
Fig. 1. Rainfall—
landslide relationship
after Lumb (1975)4.
binations of hourly and daily rainfall. It may be
observed from Fig. 2 that it is possible to
establish a simple combination of zones within
which valuable probability assessments may be
made. Significantly, the results are such that
the probability levels implied are highly
consistent from zone to zone, and this provides
some level of confidence in the results. For
each of the zones shown in Fig. 2, the
frequency of landslide occurrences in relation
to total events was calculated and these values
were used as a guide to selection of the
probability ranges shown in Table 1.
substituting the constant s from Table 2 into
Equation (1). For example, the equation for the
boundary between Zones 3 and 4 is given by
Equations for zone boundaries
5. The zone boundaries shown on Fig. 2
are given by the simple relationship
where h is the hourly rainfall and d is the daily
rainfall. The slope coefficients s are given in
Table 2. The equations of each of the zone
boundaries shown in Fig. 2 may be obtained by
Conclusions
6. An initial proposal by Lumb4 to relate
shorter- and longer-term rainfall occurrences to
landslide activity for Hong Kong established a
valuable basis for analysis of the problem.
However, the respective time periods chosen
were not optimum. Extensive data compiled by
the GEO since the time of Lumb’s work now
permit some improvement in the relevant criteria. It
is clear that it is useful to consider both hourly
and daily rainfall levels in order to optimize
prediction capabilities. A somewhat more
quantitative approach to the likelihood of landslide implications for various rainfall conditions is now possible.
References
Fig. 2. Rainfall—
landslide relationship
based on 1984—1992
CEO data
1. PI~aMcInn J. et al. Rain-induced landslides in
Hong Kong, 1972—1992. Asia Engr, I. Hong
Kong Jnstn Engrs, 1994, June, 43—51.
2. GE0TECHNICAL ENGINEERING OFFICE.
Hong Kong rainfall and landslides. GEO
reports Nos 1—6, 14, 20, 35. Government
Publications Centre, Hong Kong, 1984—1992.
3. PECK R. B. Stability of natural slopes.!. Soil
Mech. Fdns Div. Am. Soc. Civ. Engrs, 1967,
9S, No. SM4, July, 403—418.
4. LuMB P. Slope failures in Hong Kong. Quart.
J. Engng Ceo 1., 1975, 8, 31—65.
5. BRAND E. W. et al. Relationship between
rainfall and landslides in Hong Kong. Proc. 4th
Int.. Symp. Landslides, Toronto, 1984. Vol. 1,
377—384.
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