| Most
landslides in Hong Kong are caused by heavy rainfall. By using
a combination of real-time rainfall data and rainfall forecasts
from the HKO, and based on GEO's study on the landslip/rainfall
relationship, the Government is able to identify instance
when landslide danger is high and to issue the Landslip Warning
through the media. The purpose of the Landslip Warning is
to alert the public to reduce their exposure to possible danger
from landslides. The issuing of the Landslip Warning also
triggers an emergency system within government departments,
that mobilizes staff and other resources to deal with landslide
incidents.
The Landslip
Warning is issued when it is predicted that numerous landslides
will occur. Isolated landslides may occur from time to time
when the Landslip Warning is not in force.
Each year,
on average, approximately three to four hundred landslides
are reported to the GEO and the Landslip Warning is issued
about three times. Most landslides are small, but occasionally
large ones occur. Landslides, particularly when large, can
cause casualties, damage property and block roads.
The Landslip
Warning draws the public's attention to the landslide danger.
It is different from the Amber, Red and Black Rainstorm Warnings
that are issued by the HKO to alert the public to the occurrence
of heavy rain.
Considerations for Issuing the Landslip Warning
The GEO operates an extensive network of automatic raingauges
providing real-time rainfall data to the Landslip Warning
System. This network, which dates back to 1984, was upgraded
in 1999 to 86 number of raingauges to improve its coverage.
In 2005, 65 GEO raingauge units were upgraded to wireless
transmission using General Packet Radio Services (GPRS)
technology. This enhances the reliability of the system,
provides more flexibility in locating raingauges and lowers
the recurrent costs on maintenance and data transmission.
The data capture, control and processing system receives
data from the 86 GEO raingauges and from an additional 24
automatic raingauges operated by the HKO. The raingauge
system utilises modern, advanced data logging and processing
equipment and software.
Decisions
as to whether to issue or cancel the Landslip Warning are
made jointly by the Director of the HKO and the Head of
the GEO. The Landslip Warning will be issued if the 24-hour
rainfall is expected to be heavy enough to cause numerous
landslides. The criterion for the issue of the Landslip
Warning is related to the size of the area receiving heavy
rainfall, the rainfall intensity and the number of slopes
in the affected area. The latest weather information available
including short-term rainfall forecast is also considered.
In order
to issue timely warnings to the public, the HKO keeps a
continuous watch on the weather in and around Hong Kong.
In addition to data from raingauges, the Observatory uses
radars to monitor continuously the movement and development
of rain-bearing clouds. High-resolution meteorological satellite
images also provide information on cloud patterns around
Hong Kong.
Neither
weather forecasting nor landslide prediction can be regarded
as exact sciences. There will inevitably be occasions when
the Landslip Warning is raised and not many landslides occur.
Equally, if heavy rain develops suddenly and unexpectedly,
landslides can occur before the Landslip Warning is issued.
The criteria
for the issue and cancellation of the Landslip Warning are
reviewed regularly to take account of the gradual improvement
in slope safety.
When the Landslip Warning is in Force
When
the Landslip Warning is issued, local radio and television
stations are notified and are requested to broadcast the Warning
to the public at regular intervals, together with advice on
the precautions that should be taken. The Hong Kong Slope
Safety Website ( http://hkss.cedd.gov.hk
) also publishes the warning message and the number of landslide
incidents reported to the GEO at regular intervals.
When the Landslip
Warning is in force, the public should keep away from slopes
and watch out for signs of landslide danger. The Home Affairs
Department opens temporary shelters for the public including
anyone whose dwelling is endangered by unstable slopes or
boulders. Pedestrians should avoid walking or standing close
to steep slopes, especially where landslip warning signs are
erected. When the landslide situation is becoming serious,
members of the public are advised to cancel non-essential
appointments and to stay at home or in a safe shelter. Motorists
are advised to avoid driving in hilly areas or on roads with
landslip warning signs.
For an update
on the situation with respect to weather warnings, members
of the public can use the HKO Dial-a-weather service at 187
8200. Weather information is also available on the HKO's Internet
Homepage ( http://www.weather.gov.hk ).
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