Flood Prevention

Overview
Most parts of Hong Kong have a reasonable degree of flood protection. However, serious flooding had occurred on the natural flood plains and low-lying areas of the northern part of the NT. Flooding is also a recurring problem in the older urban areas, like West Kowloon, because of the inadequacy of the drainage system constructed long ago.



To mitigate the problem, DSD has been implementing a massive $10 billion flood prevention programme of river training works and village flood pumping schemes in the NT, and a $3.7 billion programme of urban drainage improvement works in West Kowloon so as to raise the flood protection standards substantially. Since 1997, about 55km of rivers in the northern parts of NT have been trained with 24 village flood pumping schemes in operation, bringing relief to the flooding situation in NNT and NWNT. Since 1995, we have removed 82 flooding black spots in both the urban and rural areas.



Flooding Incidents in 2004/05
In 2004, the annual rainfall depth was around 1,739 mm, lower than the average of 2,214 mm by about 21%. The Amber Rainstorm Warning Signal was hoisted 16 times. Red and Black Rainstorm Warning Signals were only hoisted in the morning on 8.5.2004 - all attributable to the same rainfall event, which mainly affected the western part of the Kowloon Peninsula. Flooding was reported at low-lying areas in Mong Kok and Sham Shui Po where the relevant flood prevention works under the West Kowloon Drainage Improvement Project had not yet been completed. There was no major flooding event in other areas.



Nevertheless, the flooding problem at West Kowloon has been progressively improved since the substantial completion of the Stages 1 and 2 works in 2003 and 2004 respectively. In particular, the completion of the Tai Hang Tung Storage Scheme and Kai Tak Transfer Scheme in end 2004 has denoted a significant achievement of the department in alleviating the flooding problem of West Kowloon.

Progress made in 2004-05
Flood prevention projects with a total project cost of about $2 billion have been substantially completed, signifying the overall completion of a village flood pumping scheme at Wang Chau, 2.3 km of new drainage channels in Ngau Tam Mei, 23 km of stormwater drains in West Kowloon, a 100,000 m³ flood storage tank at Tai Hang Tung and 1.42 km of drainage tunnel from Waterloo Road to Kai Tak Nullah.



To further improve the flooding situation, the Government is actively implementing a series of major drainage improvement works that aim at providing Hong Kong with a high degree of flood protection. Construction works costing about $2.2 billion and $0.7 billion are being carried out in the NT and West Kowloon respectively. A further $6.3 billion of drainage improvement projects is also under planning or design for the NT, West Kowloon and other flood prone areas such as East Kowloon, Tsuen Wan, Northern Hong Kong Island. All major flood control projects are listed in Appendix A, whereas locations of the projects are shown in Appendix B.

Reduce Flooding Black Spots
Since 1994, DSD has identified more than a hundred flooding black spots. All of the black spots are under close surveillance and improvement measures have been taken to eradicate or reduce the flood risk. Last year, we have successfully removed 10 flooding black spots.




Flood Prevention in the Urban Areas
Flood prevention projects in the urban areas mainly include improvements of the stormwater drainage networks in order that stormwater would be collected and conveyed efficiently. Under some special circumstances, innovative methods such as flood storage, flood pumping or flow diversion are used to minimize the extent of works that may cause disturbances to traffic or conflict with underground utilities.

In West Kowloon, works under Stages 1 and 2 of the West Kowloon Drainage Improvement Project were completed in June 2003 and December 2004 respectively. These included about 33 km of stormwater drains in Yau Ma Tei, Mong Kok, Kowloon Tong, Sham Shui Po and Lai Chi Kok, as well as an underground flood storage tank in Tai Hang Tung and a stormwater diversion tunnel from Kowloon Tong to Kai Tak Nullah. Under Stage 3, the construction of about 12 km of stormwater drains is in progress for completion in 2007. Plan is also in hand to implement a stormwater transfer scheme at Lai Chi Kok, as this area is susceptible to flood risk due to surface runoff from the hinterland and the potential overflow from the Kowloon group of reservoirs.



For flood prone areas in the Northern Hong Kong Island and East Kowloon, drainage improvement projects costing about $2.6 billion are being planned. In order to minimize the disturbance to local residents and road traffic caused by trench excavation, stormwater drainage tunnels will be constructed to divert stormwater away from mid-level so as to reduce the loading on the existing urban drainage systems which are under capacity.

Flood Prevention in 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 villages in low-lying areas.

With the completion of several drainage channels in recent years, the flooding situation in the NT has been significantly improved, particularly in Lo Wu, Ho Sheung Heung, Yin Kong and Tin Ping Shan 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 flooding to 1 to 1.5 m depth as overflow took place along the old river sections which were narrow and sinuous. Nowadays, tens of thousands of people in these flood prone areas have been relieved of flooding risks.



As regards low-lying villages in the flood plain areas, we have already completed 24 village flood pumping schemes to remove the flood risks at 31 low-lying villages. Over 240 hectares of low-lying villages areas accommodating a population of 30,000 people are now being protected.

In the Northern NT, the Stage 3 works of the Shenzhen River Regulation Project and the Rehabilitation of River Ganges were in good progress. With the completion of these two drainage improvement projects in 2006, the flooding situation at Lo Shue Ling, the last regional flooding black spot in the Northern NT, will be significantly improved.



In the Northwestern NT, construction works of the Yuen Long Bypass Floodway and San Tin Eastern Main Drainage Channel, as well as three village flood pumping schemes at Tai Kiu, Shui Pin Tsuen, and Mai Po Lo Wai/Mai Po San Tsuen were progressing well.

Plan is also in hand to improve the stormwater drainage systems in Tuen Mun, Sha Tin, Tai Po, Tsuen Wan, Sai Kung and Lantau Island.

Better Preparation for the Rainy Days with New System
DSD has developed a Flood Monitoring and Reporting System (FMRS) to monitor the flooding situation and to collect the hydrological data for the calibration of computer drainage models. Under the system, more than 60 automated gauging stations were installed at rivers and channels throughout the territory. The gauging stations provide real-time round-the-clock surveillance of water levels and to collect hydrological data including rainfall depths and tide levels. Most of these stations are equipped with ultrasonic probes to measure water depths in rivers with great precision, and transmit the real-time data immediately to DSD control center. Duty officers at the control center could analyze the flooding situation based on the real-time trends of hydrometric information, video images and operation status of the gauging stations, and issue any necessary flood warnings to relevant emergency services such as Police and Fire Services Department and the drainage maintenance crews to mitigate the flood risk.

Through collaboration with the University of Hong Kong, DSD improved the FMRS in 2002 by installing the Mobile and Online Surveillance and Incident Controller II system for gathering and transmission of hydrometric information based on wireless telemetry technology. This system, which is a forerunner in the Asian-Pacific region, adopts the advanced mobile communication technology called General Package Radio Services to compress and transmit various data collected from the gauging station to DSD control center in a speed 8 times faster than common GSM mobile phones. To make use of renewable energy and to reduce the installation and operation costs, many of the stations have been powered by solar battery cells. Moreover, DSD has installed night vision digital network cameras at some flooding black spots to provide real-time high resolution images of the critical drainage locations. Due to the application of these innovative technologies, the average failure rate of the gauging stations during adverse weather conditions was significantly reduced from 17.4% in 2001 to 4.6% in 2004.

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Working with University to Solve Urban Flooding
The existing drains in the northern part of Hong Kong Island from Tai Hang to Kennedy Town were generally built decades ago. Areas like Connaught Road West, Des Voeux Road West, Queensway, Hennessy Road, Morrison Hill Road, Happy Valley etc are susceptible to flooding due to rapid developments and changes in land use over the years. Substantial drainage improvement in the areas by re-building or enlarging the existing drainage system will inevitably require extensive excavation in busy roads and hence cause tremendous nuisance to the public.

The design concept of the drainage tunnel is to intercept stormwater by 36 water intakes located in the mid levels from Tai Hang to Kennedy Town. Stormwater from the upper catchment intercepted by the intakes will be directed into a deep tunnel and discharged at the western side of Hong Kong Island. The lower catchment areas can thus be better protected from flooding under heavy rainstorms.

Most of the proposed water intakes are located in congested build-up areas. To ensure that the intakes can effectively collect huge volumes of high speed flows during heavy rains, we engaged a consultant to prepare the preliminary design for the vortex water intake structures and the University of Hong Kong to carry out physical hydraulic model test for verification purpose.



Flooding in Ngau Tam Mei Basin Becoming History
Ngau Tam Mei area is a low-lying areas in the NT and has long been a well-known flooding black spot. Serious flooding has occurred almost every year. The area has been classified as a major flooding black spot (the most serious rank) according to DSD's classification. Rapid development over the last 20 years has reduced the flood storage capacity of the original flood plain and also increased the volume of surface runoff that exacerbate the flooding problem.

To tackle the flooding problem, a new channel was designed to divert the flood water from the upstream area of Ngau Tam Mei to the newly constructed Kam Tin Channel directly. The channel is designed so that it can accommodate the flow generated from a 200-year return period storm event.

During low flow condition the water inside the channel was comparatively small. The tidal water will back-flow from Kam Tin Channel to the upstream area and thereby creating nuisance problem. As such, an inflatable dam together with a low flow pumping station is provided at the downstream end of the channel as a tidal barrier.

Construction of the channel started in late 1999 and completed in March 2005. The flooding problem at Ngau Tam Mei areas is now resolved.

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