Overview
DSD endeavors to improve the drainage systems in Hong Kong continuously for protecting the public against flooding. Although the rainfall in 2008 was more than average, protection against flooding was, in general, effective, particularly in the NT after the substantial completion of a considerable number of drainage improvement works.

Since 1997 DSD has been implementing a $11.8 billion flood prevention programme in the NT, including more than 100 km river training works and 27 village flood pumping schemes. In the urban areas, a $8.8 billion programme of drainage improvement works to raise the flood protection level in the concerned catchment including the construction of Hong Kong West, Tsuen Wan and Lai Chi Kok drainage tunnels which are now progressing in good speed.

Besides continuing with the implementation of planned flood protection projects, DSD is actively reviewing new flood protection needs. The Drainage Master Plan (DMP) Studies for Yuen Long and the Northern NT (NNT) were completed in 1998 and 1999 respectively. The majority of the improvement works and river training works recommended in the Studies have been completed. However, in view of changes in land uses and new developments planned within the areas, DSD commissioned a DMP Review for Yuen Long and North Districts in March 2008. Taking account of the major changes in land uses, new development proposals and town planning, numerical models for evaluating the hydraulic performance of the major rivers/channels are being set up. Further drainage improvement works for major rivers, upstream channels and the local flooding spots would be proposed by the Study which is expected to be finalized in late 2010.

Apart from incorporating green features in drainage works, DSD also makes efforts to beautify existing drainage channels. DSD is planning to improve the appearance of the existing nullahs in Yuen Long Town and enhance their surrounding environments. These nullahs have concrete lining and were constructed more than 40 years ago. DSD has commissioned a consultancy study to examine options and proposals to improve the drainage and environmental conditions. Following public consultation including the concerned green groups and district councillors, the feasibility study and the preliminary design of the improvement of the Yuen Long nullahs have been completed in mid 2009.

In respect of operation and maintenance, DSD continues to carry out preventive measures including inspection, desilting and repair works for drainage systems. In 2008-09, over 1,600 km drains and watercourses were inspected and about $109 million was spent on such maintenance works. DSD always puts efforts in resolving issues related to problematic road gullies, stormwater intakes, drains and watercourses and inspects them on a regular basis especially before and during the rainy season.

Flooding Incidents in 2008
The year 2008 was wetter than usual. The annual rainfall of 3,066 mm was about 29 per cent above the yearly rainfall of 2,382 mm. The number of Amber, Red and Black Rainstorm Warning Signals hoisted were 21, 7 and 2 respectively.

The weather of June 2008 was marked by heavy rain and squally thunderstorms. The monthly total rainfall was 1,346 mm, and at 8 to 9 a.m. of 7 June the hourly rainfall was 145.5 mm, which were the new highest monthly and hourly rainfall records at Hong Kong Observatory respectively.

On 7 June, DSD received more than 600 nos. of flood cases in various locations due to prolonged and heavy rain. The relatively more serious cases include the ones at Wing Lok Street in Sheung Wan, Des Voeux Central in Central, Morrison Hill Road/ Wong Nai Chung Road in Wan Chai and those caused by landslide at North Lantau Highway. The Tai Hang Tung Storage Tank functioned effectively as designed during the heavy rain. Excess runoff which could not be discharged properly via the underground drains into the sea was temporarily stored in this 100,000 m³ storage tank. It had prevented extensive and serious flooding along Nathan Road in Mong Kok area.

Flood Prevention in NT
To resolve the flooding problem and to improve the drainage system in the NT, DSD has been implementing a series of major flood control projects in the NT areas at a total cost estimated to be over $11.8 billion since 1997. The total estimated cost of flood protection works completed in 2008-09 is about 0.7 billion.

Flood Prevention in NNT and NWNT
Low-lying topography, insufficient drainage facilities and rapid urban development in the NNT and the NWNT are the main reasons in the past causing flooding during prolonged heavy rain and tropical cyclones. Since the late nineties, DSD had already constructed a substantial length of about 77 km drainage channels to better collect and convey stormwater; and a number of village flood pumping schemes to protect villages in low-lying areas. The completed 27 village flood pumping schemes have protected 35 low-lying villages from flooding. Now, over 240 hectares of low-lying village area accommodating a population of 30,000 people are now being protected.

NNT
In the past, serious flooding occurred in Lo Wu, Ho Sheung Heung, Yin Kong and Tin Ping Shan. DSD has commissioned a series of projects costing approximately $4.4 billion. Flooding situation in the NNT has been alleviated. Completed major drainage channels include Ng Tung River, Sheung Yue River and Shenzhen River (from Deep Bay to Ping Yuen River section). In September 2008 the construction of approximate 1.8 km Ma Wat River was substantially completed. Flood protection level to Kau Lung Hang and surrounding areas was raised. DSD is now improving the upstream and estuaries of major rivers to enhance the flood protection to upstream areas. In addition, DSD is carrying out a feasibility study on the river training of upstream section of Shenzhen River from Ping Yuen River to Pak Fu Shan in order to cater for the recent development of Liantong/Heung Yuen Wai Boundary Control Point.

NWNT
In the past, most areas in Tin Shui Wai, Yuen Long and Kam Tin in the NWNT were susceptible to flooding. The situation has been greatly improved since the active implementation of drainage improvement works by DSD for the past 20 years. The projects in the NWNT cost approximately $5.0 billion and over 85 per cent of the drainage works have been completed. The latest item of main drainage channel works, namely Yuen Long Bypass Floodway was completed in late 2006. We will continue to carry out drainage improvement works in secondary and tertiary drainage channels at upstream of NWNT basins.

Flood Prevention in Other NT Areas
As some NT locations other than NWNT and NNT still suffer from risk of flooding due to deficiency in drainage system or natural low-lying topography, DSD has on-going and planned drainage projects to improve the drainage systems in other NT areas such as Lam Tsuen River and Tung Tze Road in Tai Po and Ho Chung in Sai Kung to cope with the developments in NT areas.

Flood Prevention in Urban Areas
Due to limited space and dense population in urban areas, DSD has adopted different approaches for drainage improvement works in order to upgrade flood protection capacity. These approaches include the use of stormwater storage, pumping or flow diversion through drainage tunnels to minimize the extent of works that may cause disruptions to traffic or conflicts with underground utilities. In recent years, DSD has reviewed the drainage systems in the urban area of northern Hong Kong Island, West Kowloon, and Tsuen Wan/Kwai Chung and proceeded to construct drainage tunnels to enhance the regional drainage systems.

West Kowloon
Major stormwater drainage works in West Kowloon include Tai Hang Tung flood storage tank, Kai Tak stormwater transfer tunnel and about 42 km stormwater drains. A total cost of $2.0 billion of these works has already been completed in stages since 2004. During the heavy rain on 7 June 2008, the Tai Hang Tung flood storage tank, Kai Tak stormwater transfer tunnel and the associated new drains had worked together and prevented extensive flooding and inconvenience caused to the public in Kolwoon area.

To address the flooding problems in Lai Chi Kok, DSD is constructing the Lai Chi Kok Drainage Tunnel (LCKDT) at a total cost of $1.7 billion. The works commenced in late 2008 for completion in 2012. The LCKDT is 3.7 km long with 4.9 m internal diameter running from south of Chak On Estate via the former Wai Man Tsuen to Victoria Harbour. Six stormwater intakes will be built along the upstream section of the drainage tunnel to intercept stormwater surface runoff on the adjacent hillside.

The tunnel will be constructed by a state-of-the-art slurry shield Tunnel Boring Machine (TBM) to be designed and supplied by a German TBM supplier. Design of the TBM is underway and is scheduled for delivering to the site in early 2010.

Hong Kong Island
Wing Lok Street, Sheung Wan is a low-lying area where flooding may occur in the past under severe rainstorm and high tide condition in Victoria Harbour. To resolve the problem, DSD has completed the construction of Sheung Wan Stormwater Pumping Station in 2008-09. The pumping station was in operation before the 2009 wet season to protect the public in Sheung Wan from flooding. As the lowest ground level in the area is only slightly above the mean high tide level, the flooding situation would become worse with high tide surge because the existing drainage system could not effectively drain the run-off away due to the small difference in ground and sea levels. The Sheung Wan floodwater pumping station scheme protects the low-lying area from being flooded by firstly constructing a diversion chamber at the drainage system downstream of the low-lying area. The penstock inside the diversion chamber will cut off the backflow of seawater from entering into the low-lying area upstream. The stormwater collected from upstream within the low-lying area is conveyed to an underground storage tank which has a storage capacity of 9,000 m³. Six submersible pumps housed within the floodwater pumping station will pump out the stormwater collected in the storage tank to the harbour.

On the other hand, other Northern Hong Kong areas are still subject to flooding risks due to possible rapid overland flows from hillside slope during severe rainstorms. To reduce the overall flood risks in the northern part of Hong Kong Island, DSD has commenced the construction of Hong Kong West Drainage Tunnel (HKWDT).

HKWDT is our major initiative to enhance the flooding protection in Northern Hong Kong Island. Upon project completion, the 11 km long new tunnel will intercept stormwater from the upper catchment at the mid-levels from Tai Hang to Kennedy Town and discharge it to sea near Cyberport, thus reducing the volume of rainwater flowing to the downstream low-lying areas including Sheung Wan, Central, Admiralty and Wan Chai. This tunnel project also includes 34 intakes located in Mid-level and 8 km long connection adits. The design-and-build contract commenced in late 2007 for completion in 2012-13.

The drainage tunnel will be constructed by two TBMs from the east and west sides of the tunnel. The TBMs of 7 m and 8 m diameter have been put into operation in early 2009.

Construction of intakes will proceed in phases. The advance ground investigation and utility identification works are near completion. The intake construction works, comprising intake structure, drop shaft and adit, at Stubbs Road and Smithfield have commenced.