The Flooding Incidents in 2001
Hong Kong is on the common track of tropical cyclones
in Southeast Asia and can experience very severe rainstorms at times.
The average annual rainfall is about 2,200 mm, one of the highest
among the cities in the Pacific Rim. Last year we experienced a
total annual rainfall of 3,092 mm, and in June alone, the monthly
rainfall marked a new record of as high as 1,084 mm. The Red Rainstorm
Warning Signal was hoisted 9 times and the Black Rainstorm Warning
Signal once in the year. Significant flooding occurred in the NT
and Tsuen Wan and more details are given in the Feature Articles.
These flooding incidents illustrated that flooding
can happen in the natural flood plains and low-lying areas of the
northern part of the NT as well as in the urban areas during heavy
rainstorms. In order to upgrade the flood protection standard to
the affected areas, drainage improvement measures are necessary.
Usually, collected stormwater runoff is conveyed through natural
or man-made drainage networks for discharge to the sea for effective
alleviation of flooding.
However, under some special circumstances, other
methods such as flood storage, flood pumping or drainage tunnels
are still required. These long-term improvement measures are often
coupled simultaneously with other smaller mitigation measures to
tackle local problems. Of course, regular maintenance is always
required to ensure proper functioning of the drainage system.
The trained Shenzhen River
(left photo) as compared with the narrow and sinuous original
river at the same location (right photo) |
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Delivery of Our Projects
The Government is now constructing a series of
major drainage improvement works to tackle the flooding problems,
with active construction contracts costing about $3.2 billion for
the Northern and Northwestern NT and about $3.3 billion for West
Kowloon. A further $10 billion of drainage improvement projects
in the NT, West Kowloon and other flood prone areas such as East
Kowloon, Tsuen Wan, Northern Hong Kong Island, is under planning
or design. These major flood control projects are listed in Appendix
A, whereas locations of the projects are shown in Appendix
B.
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Rainfall (mm) in major Pacific Rim cities |
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Flood Prevention Projects in the NT
Flood prevention projects in the NT can broadly
be classified into two main types, namely drainage channels to collect
and convey floodwater to the sea and village flood pumping schemes
to protect the low-lying villages in flood plain areas.
While DSD is assigned to implement the majority
of these projects, the Territory Development Department (TDD) also
takes part in the implementation of drainage improvement works in
connection with new town development projects. In 2001/02, about
$0.9 billion of flood prevention projects have been substantially
completed. These include a village flood pumping scheme at Pok Wai
in Yuen Long and 15 km of new drainage channels to improve Ho Pui
River in Kam Tin and River Indus, River Beas and River Sutlej in
the North District. Work is also at an advanced stage in the construction
of Middle River Indus, Ngau Tam Mei River, Upper Kam Tin River and
the village flood pumping scheme at Chuk Yuen Tsuen and Ha San Wai.
Apart from the on-going construction works, new
works contracts were awarded last year for the construction of the
Stage III works under the Shenzhen River Regulation Project, tributary
channels at Upper Kam Tin River and Pok Wai, urban drainage improvement
works in Yuen Long and Tin Shui Wai, and two village flood pumping
schemes at Ma Tin Tsuen and Shui Pin Wai. Moreover, we commenced
last year the detailed design for the drainage improvement works
recommended under the two Drainage Master Plan Studies covering
Sha Tin and Tai Po as well as Tuen Mun and Sham Tseng.
With the completion of drainage channels over recent
years, the flooding situation in the NT has been significantly improved,
particularly in Lo Wu, Ho Sheung Heung and Yin Kong in the Northern
NT and Tin Shui Wai, Yuen Long and Kam Tin in the Northwestern NT.
In the past, floodplains in the NT were often susceptible to a flood
depth of 1 m to 1.5 m as overflow took place alongside the old river
sections which were narrow and sinuous. Nowadays, tens of thousands
of residents in these flood prone areas have been relieved of flooding
risks. The flooding situations in the Northern and Northwestern
NT will be further improved upon the completion of the new drainage
channels in Tin Ping Shan and Ngau Tam Mei in 2003; and the completion
of the Shenzhen River Regulation Project Stage III as well as the
other new drainage channels in Ta Kwu Ling, Yuen Long and San Tin
in 2005.
As regards low-lying villages in the flood plain
areas, we have already completed 20 village flood pumping schemes
to remove the flood risks at 27 low-lying villages. Over 130 hectares
of low-lying villages areas accommodating a population of 20,000
people are now being protected. We will complete 7 more village
flood pumping schemes before end 2005 to extend the protection to
another 12 low-lying villages with a population of 7,000 people.
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River training works in progress at Upper River
Indus, near Tin Ping Shan Tsuen |
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![](images/02/2-4.jpg) |
Recently completed drainage
channel of River Sutlej, near Fanling |
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Recently completed drainage
channel of Upper River Beas, near Hang Tau Tai Po |
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Flood Prevention Projects in Urban Areas
In urban areas, apart from the traditional method
of laying underground drainpipes, DSD has introduced a new approach
of stormwater storage and flow transfer to minimize the traffic
disruption due to trench excavations. Within the urban area in West
Kowloon, the construction of about $3.3 billion of drainage improvement
works is in good progress. These include 43 km of stormwater drains,
a 100,000 m3 flood storage tank in Tai Hang Tung and a 1.5 km-long
drainage tunnel for transferring stormwater from Kowloon Tong to
the Kai Tak Nullah. All these works are scheduled to be completed
progressively from 2004 to 2007 to raise the flood protection for
a total population of 230,000 people in West Kowloon. The most significant
improvement was seen in Mong Kok which was flooded in 1997 and 1998.
In addition, the planning of 14 km of stormwater drains in Sham
Shui Po district including Lai chi Kok and Cheung Sha Wan areas
and a 4.2 km long drainage tunnel in Lai Chi Kok, amounting to an
estimated total construction cost of $1.4 billion, are underway.
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Construction of stormwater drains
at junction of Canton Road and Bute Street, Mong Kok |
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Construction of stormwater drains
at junction of Tong Mei Road and Lai Chi Kok Road, Mong Kok |
For the flood prone urban areas in the Northern
Hong Kong Island, Tsuen Wan, Kwai Chung and East Kowloon, drainage
improvement projects costing about $3.7 billion have been planned.
In order to minimize the disturbance to the local residents and
road traffic due to trench excavations in the Northern Hong Kong
Island and Tsuen Wan, we would adopt the use of stormwater drainage
tunnels to divert the flood flow from the existing drainage systems
which are under capacity.
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Construction of the underground flood storage
tank in progress at Tai Hang Tung Playground, Shek Kip Mei |
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Proposed stormwater drainage tunnels under planning |
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Tai Hang Tung Underground Flood Storage Tank |
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Operations and Maintenance of the Drainage System
In 2001, the stormwater drainage system maintained
by DSD has increased to comprise 160 km of engineered drainage channels,
2,250 km of stormwater drains and culverts and 20 village flood
pumping schemes. To ensure the proper operation and functioning
of the drainage system, we continue to implement a preventive maintenance
programme which includes regular inspection, desilting and repair
of the drainage system, particularly at locations categorized as
flooding blackspots.
In the past year, 1,620 km of drains, engineered
channels, culverts and watercourses were inspected and 450 km of
them were cleansed with 102,890 m3 of silt removed at a cost of
about $124 million. We have also maintained a close connection with
the Food and Environmental Hygiene Department and Highways Department
to ensure that roadside gullies and drainage catchpits are in good
working condition.
Since 1994, DSD has identified more than a hundred
flooding blackspots. All of the blackspots are under close surveillance
and improvement measures have been taken to eradicate the flood
risk. Up to now, more than 37 flooding blackspots have been eliminated.
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Before works |
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After works |
Local drainage improvement works at Shek Wu
Wai, San Tin |
At times of heavy rainfall and severe weather conditions,
DSD is also committed to dealing with emergency situations. These
include monitoring the operation of all major flood control facilities,
conducting immediate investigations of flooding complaints, and
carrying out urgent remedial actions. Arrangements have also been
made to inspect all flooding blackspots to ensure that any blockages
or damage to drains and stormwater inlets are rectified in a timely
manner.
We have also carried out local drainage improvement
works to provide immediate relief of flooding problems. For example,
while the major drainage improvement works in West Kowloon are still
underway, we have recently completed local improvement works at
Nelson Street - a well-known flooding blackspot mainly because of
the inadequate capacity of the existing stormwater drainage system.
With the construction of bypass drains and new roadside gullies
as short-term measures, the flood risk has been alleviated at this
location which is renowned as one of the most densely populated
zones and a major shopping area. Local drainage improvement works
have also been completed at Fat Yip Lane, near Belvedere Garden
in Tsuen Wan where several serious flooding incidents took place
in June 2001 as boulders were washed into the drainage system.
The operations and maintenance works carried out
in the past years on flood prevention are shown in Appendix
C.
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Flood wall constructed as
local drainage improvement works alongside the open channel
above Fat Yip Lane, near Belvedere Garden, Tsuen Wan |
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Significant Floods in 2001
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Chief Executive Mr. TUNG Chee-hwa and senior
government officals in the meeting after the flooding incidents |
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Flooding at Tin Ping Shan, Sheung Shui |
The year 2001 was wetter than usual. In June 2001,
the total rainfall of 1084 mm set a new record for the month and
the torrential rainfall in the period from 7.6.2001 to 11.6.2001
resulted in significant flooding in the NT and Tsuen Wan. Significant
flooding incidents also occurred in Tsuen Wan due to the rainfall
on 12.6.2001, 23.6.2001 and 27.6.2001.
Areas affected were mainly low-lying flood prone
areas alongside existing river channels. Some were in the vicinity
of construction sites of government drainage projects under the
management of DSD and TDD. These areas included Tin Ping Shan and
Shek Wu San Tsuen in Sheung Shui, Wai Tsai Tsuen and Yau Tam Mei
Tsuen in Ngau Tam Mei, Tsat Sing Kong and Tai Kong Po in Kam Tin
and Pok Wai in San Tin.
The HKSAR Government was deeply concerned about
the flooding incidents and a Task Force led by the Secretary for
Works was set up to review the flood prevention implementation programme
and to identify ways to further enhance the improvement of the drainage
systems. The Task Force also appointed the Director of Drainage
Services to conduct an independent investigation on the causes of
the flooding and to assess whether or not the construction works
on site had aggravated the flooding incidents.
The investigation report was completed in September
2001. It concluded that low-lying topography, inadequate drainage
capacities of existing river channels, prolonged heavy rainstorms
and high tide had been the principal causes of the flooding in the
Northern NT. Heavy rainfall with the rising tide would have caused
severe flooding even if no construction activities were taking place.
Since the flooding incidents in June 2001, DSD
and TDD had critically reviewed the implementation programme of
the respective drainage projects already under construction and
stepped up the site supervision to ensure smooth flow of river channels
and stream courses, particularly under inclement weather. Both departments
have strived to expedite and complete the critical sections in the
relevant projects to provide early relief to the flooding before
the 2002 wet season. As an interim measure, the contractors were
instructed to strengthen their standby resources to deal with emergency
situations arising from inclement weather.
Upon completion in 2003 of the various river training
works in Sheung Shui, Kam Tin and Ngau Tam Mei, the corresponding
drainage systems will be significantly improved and the concerned
areas, although still low-lying, will no longer be flood prone.
In Pok Wai, the village flood pumping scheme has already been completed
to provide the required flood protection level. In Kau Lung Hang,
the planning of a flood prevention programme is now underway to
improve the existing public drainage system. At Belvedere Garden,
the flood risk due to boulders being washed into the drainage system
has been reduced in the interim by means of local drainage improvement
works while long-term improvements to further reduce the risk are
being planned.
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Collapse of temporary pipe bridge at Tsat Sing Kong, Kam Tin |
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Flooding at Tin Ping Shan, Sheung Shui |
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A car was washed into Ma Wat River near Kau Lung Hang, Tai Po. Both the driver and the passenger had escaped safely |
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Flooding at Ngau Tam Mei |
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Flooding at Belvedere Garden, Tsuen Wan
During the heavy rainfalls in June 2001, large
quantities of muddy water flowed down along a natural stream course
at the upland catchment near Belvedere Garden, Tsuen Wan. Large
amount of boulders and debris were washed down from the stream course
and blocked the drainage intake at Fat Yip Lane. The majority of
the flow overshot the stream course as well as the drainage intake
and flooded the section of Castle Peak Road opposite Belvedere Garden.
As the floodwater found the way down Lai Chi Road, a section of
Hoi On Road was also flooded.
After the flooding incidents, the department has
immediately carried out local drainage improvement works as interim
measures to alleviate the drainage problem. These works included
the clearance of loose boulders and debris from the stream course,
shotcreting of the eroded stream course between Tuen Mun Highway
and Fat Yip Lane, installation of additional bar screens at the
drainage intake, construction of a solid parapet wall alongside
the stream course as well as the cascade immediately upstream of
the drainage intake, and construction of flow interceptors at Fat
Yip Lane, Castle Peak Road, Lai Chi Road and Hoi On Road. The interim
measures aimed at preventing the wash down of the boulders and debris
from the eroded stream course, stabilizing the eroded stream course,
confining the flow within banks, trapping the debris at the upstream
to prevent the blockage of the intake at Fat Yip Lane, and enhancing
the interception of the surface runoff.
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Flooding at Fat Yip Lane and Castle Peak Road, Tsuen Wan |
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Flow interceptor at Castle Peak Road as one of the interim improvement measures |
After completion of the local drainage improvement
works, no flooding incidents were recorded in the vicinity during
the subsequent heavy rains in July and August 2001. The interim
measures proved to be effective. Furthermore, the department has
also employed a consultant to investigate and design the long-term
drainage improvement measures. The improvement works will include
the provision of additional drains and boulder traps to enhance
the robustness of the existing system. Completion of these works
would further reduce the flooding risks at Belvedere Garden. In
the meantime, regular inspection and monitoring of the performance
of the drainage intake and the public drainage system are being
conducted to ensure they are functioning properly.
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Overflow from the natural stream at upstream of Fat Yip Lane, Tsuen Wan |
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Flooding at junction of Castle Peak Road and Lai Chi Road, Tsuen Wan |
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Completion of Pok Wai Village Flood Pumping
Scheme
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Pok Wai village flood pumping station and floodwater storage pond |
For the low-lying villages in Northwestern NT where
floodwater cannot be effectively drained by gravity to the adjacent
river channels, village flood pumping schemes are necessary. Pok
Wai in the low-lying flood plains of Yuen Long was one of such villages
prone to flooding during typhoons and heavy rainstorms. Frequent
flooding incidents in the past had caused economic losses, disruption
to transport and social activities, and created a threat to life
to the local people. The scheme for Pok Wai village, which covers
an area of about 5 hectares, forms part of the Government's overall
flood control programme for the Northwestern NT.
In April 2002, the village flood pumping scheme
for Pok Wai was completed and put into operation. The flood protection
level for the areas in Pok Wai Village has been raised from the
previous less than 1 in 2 years to presently 1 in 200 years.
The scheme protects the village at Pok Wai from
being flooded by firstly forming a flood protection embankment and
walls around the village. The embankment and walls will prevent
external floodwater from entering into the village. Secondly, the
scheme provides an internal stormwater drainage system to collect
rainwater within the village and to convey the water to a floodwater
from storage pond. Screw pumps are provided to pump out the water
collected in the pond to the existing streams outside the embankment
when the water level inside the floodwater storage pond reaches
preset levels. In addition, the project includes widening, deepening
and straightening of the existing stream in order to increase its
capacity to convey the floodwater.
Construction of the scheme commenced through the
implementation of a major civil engineering contract in October
1999. The essential works of the scheme were completed and put into
operation in April 2002. With a cost approaching $90 million, it
represents another step forward in the HKSAR Government's plan to
solve the flooding problem in the Northwestern NT.
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Flooding at Pok Wai during a historic flooding event before the completion of the village flood pumping scheme |
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Pok Wai Village Flood Pumping Scheme |
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