Long Term Improvement Measures

In September 1989, the Government established the Drainage Services Department to take up the overall responsibility of providing an efficient approach to resolving, among others, the flooding problem in Hong Kong. Since then, the department has completed studies on flood prevention strategy, drainage master plan studies (DMP) and drainage studies covering the flood-prone areas of the territory.  To cope with the latest developments in the community and also the changes in the weather pattern and the effects, the department has commenced the review studies for DMPs of different regions in phases.  To mitigate the flooding problem, the department implemented a series of projects to increase the flood prevention level of different districts and has also embarked on the planning, design and construction of further flood protection works.

Details are given below :

Categories of Long Term Improvement Measures
Drainage Master Planning

Rural areas
River training works
Village flood protection scheme

Urban areas
Drainage tunnels
Stormwater storage tank

Categories of Long Term Improvement Measures

The long term improvement measures implemented or being considered by the department involve major capital works of mainly the following types -

1. Expansion and improvement on existing drainage system to increase the flow capacity and facilitate more effective collection of surface runoff;
2. River training works for effective discharge of storm flow;
3. Tunneling works for interception and diversion of storm flow from upland catchment for direct discharge into the sea, thus avoiding the storm flow from overloading the downstream drainage system;
4. Stormwater storage facilities to temporarily retain storm flow from upland catchment in order to attenuate the peak runoff loading for the downstream drainage system;
5. Stormwater pumping scheme to remove directly storm flow at flood prone areas; and
6. Village flood protection scheme comprising a protective bund to stop storm flow from entering into a low-lying village and a stormwater pumping station to pump away storm flow collected within the village.

Upon progressive completion of these extensive long-term improvement measures, the risk of major flooding within the territory have been greatly reduced.

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Drainage Master Planning

Master planning for drainage improvement was carried out on the basis of eight major catchment areas below :
(a) Yuen Long, Kam Tin, Ngau Tam Mei and Tin Shui Wai

(b) Northern Hong Kong Island

(c) Tsuen Wan, Kwai Chung and Tsing Yi

(d) Tuen Mun and Sham Tseng

(e) Northern New Territories

(f) Sha Tin and Tai Po 

(g) Sai Kung, East Kowloon and Southern Lantau

(h) Southern Hong Kong Island

Since1996, the department has completed Drainage Master Plan (DMP) studies for all of the eight catchment areas above, to provide comprehensive solutions to the flooding problems in individual areas.  These studies comprehensively examined the adequacy of the existing drainage systems and recommended short to long term drainage improvement measures to meet the required protection standards and future development needs.  The DMP studies took into account technical constraints, cost effectiveness and environmental consideration in formulating sustainable short term and long term improvement measures.

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Rural Areas

Long term improvement measures for the rural areas mainly involved river training works in particular for the river systems serving the flood-prone basins in the northern part of the New Territories, and also village flood protection schemes to protect low-lying villages.  At the time when the department was established, the two major river networks in North and Northwestern New Territories were most in need of improvement – the Shenzhen River Network and the Yuen Long & Kam Tin Rivers Network – both of which discharged into Deep Bay.  By now, training works for the major river networks have been completed and flooding risk in the region is significantly reduced. There are 35 villages located in low-lying areas.  They are constrained by the natural topography and existing developments, and were susceptible to flooding even after the construction of major drainage channels.  The department has completed 27 village flood protection schemes to protect these villages. 

River training works

The usual capacity of a natural river is roughly sufficient only to meet a biennial flood. In order to increase rivers' capacity to meet the flood under design extreme events, the river needs to be trained by straightening, widening and provision of linings. The flooding risk in most of the flood prone areas has been significantly reduced upon the completion of river training works at Ng Tung River, Sheung Yue River, Shan Pui River, Kam Tin River, Ping Yuen River and Shenzhen River.

Widened Shenzhen River at Liu Pok

Widened Shenzhen River at Lok Ma Chau

Completed river training works at Ng Tung River near Tin Ping Shan, Sheung Shui

Widened Shan Pui River, Yuen Long

 
Widened Kam Tin River, Yuen Long

Yuen Long Bypass Floodway


The completed rehabilitation works at Sheung Yue River, Sheung Shui

Village flood protection schemes

Village flood protection schemes are necessary where the villages are so low-lying that flood water cannot effectively be drained by gravity to the primary drainage network. These schemes involve the construction of bunds around existing villages and pumping of stormwater from within the bunded area to an outside channel during rainstorms. To date, 27 flood protection schemes have been constructed and have proven to work well.

Schematic layout of a village flood protection scheme


Village flood protection scheme in Ma Tin Tsuen, Yuen Long, completed in 2004

Village flood protection scheme at Chuk Yuen Tsuen, Yuen Long, completed in 2003

Village flood protection scheme at Pok Wai, Yuen Long, completed in 2002

Village flood protection scheme at San Tin, Yuen Long, completed in 1999

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Urban Areas

In general, urban areas were equipped with stormwater drainage systems during the urbanization process. In some old towns and aged urbanized areas such as Mong Kok and Sheung Wan, the existing stormwater drainage system were built as a component of the progressive urban extension spanning over the past hundred years. Over the years, various local modifications, improvements and extensions to the systems had been made. Nevertheless, some of the urban systems are still inadequate to meet the current flood protection standards.

Drainage improvement works in the urban areas faces a different kind of construction problems. In Hong Kong, most of the roads are already congested with existing underground utilities such as electric cables, telephone lines, TV cables, gas pipes, water pipes, etc. The usual type of drainage work would unavoidably have to go under the existing layers of underground utilities and thus aggravate disruption to the traffic, and inconvenience to the public.

In view of the serious disruption and numerous constraints associated with drainage works by open trenching in old urban areas such as the lack of space, traffic and utility diversion problems, we have been striving to minimize these works. Apart from the wider application of no-dig technology for installation of stormwater drains, more innovative improvement options including the application of drainage tunnel technology for interception and transfer of stormwater and the provision of underground stormwater storage tanks for temporary retention of stormwater have been adopted.

Drainage tunnels

During heavy rainstorms, fast and huge flows from the hills would run along the paved areas and steep slopes down to the urban areas below, causing flooding hazards.  To resolve these problems, three stormwater drainage tunnel systems are being constructed  to divert excess rain water from upland areas for discharge directly into the sea. By employing this method, less rain water would enter the existing drainage systems in the urban areas and therefore, the flood protection standard of the areas could be improved with much less requirement for the construction of conventional drainage works. 

Conceptual layout of stormwater diversion by drainage tunnel

At present, we are implementing three drainage tunnel projects, namely Hong Kong West Drainage Tunnel, Tsuen Wan Drainage Tunnel and Lai Chi Kok Drainage Tunnel

Location plan of three drainage tunnel projects

Hong Kong West Drainage Tunnel	Tsuen Wan Drainage Tunnel	Lai Chi Kok Drainage Tunnel

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Stormwater storage tank

Flood storage is a common stormwater management approach in controlling the quantity or quality of stormwater runoff. It works on the principle of storing a portion of the surface runoff coming from the upstream and allowing a limited flow to the downstream of the catchment.  In this way, the flow rate is restricted to stay within the capacity of the downstream drainage system.

Stormwater storage approach is often used when the capacity of the downstream drainage network is incapable of meeting the increased peak flow rate arising from development upstream. Urban development usually propagates from the downstream. When the upstream areas are being developed, the downstream areas have already been fully urbanised.  The option of upgrading the drainage network at the downstream areas will cause severe disturbance and is technically difficult due to lack of working space.  The storage approach with construction works to take place at the upstream can be a suitable solution.  The Tai Hang Tung Storage Scheme, an integral part of the West Kowloon drainage improvement scheme, was designed to address the flooding threats in Mong Kok.

Wing Lok Street, situated in a low-lying area affected by tidal back flow, was a flooding blackspot in the Central and Western District in the past.  To provide long-term solutions to the flooding problem, a tide gate was constructed to prevent tidal back flow, and also an underground stormwater storage tank and a pumping station at Chung Kong Road, Sheung Wan, were constructed to provide a new drainage path for stormwater from Wing Lok Street.

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