The long-term improvement measures implemented or being considered by the department involve major capital works of mainly the following types -
- Expansion and improvement on existing drainage system to increase the flow capacity and facilitate more effective collection of surface runoff;
- River training works for effective discharge of storm flow;
- 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;
- Stormwater storage facilities to temporarily retain storm flow from upland catchment in order to attenuate the peak runoff loading on the downstream drainage system;
- Stormwater pumping scheme to pump and discharge storm flow at flood prone areas directly to the sea
- 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 risks of major flooding within the territory have been greatly reduced.
Long-term improvement measures for the rural areas mainly involved river training works in particular for the rivers, 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 for many years and flooding risk in the region is significantly reduced. There are 35 villages located in low-lying areas over the territory. They are constrained by the natural topography and existing developments, and were susceptible to flooding even after the construction of major drainage channels. To this end, we have also completed 27 village flood protection schemes to protect these villages.
The usual capacity of a natural river is roughly sufficient only to meet a biennial flood. In order to increase river capacity to meet the flood under design extreme events, the river needs to be trained by straightening, widening, deepening and provision of linings. The flooding risks in most of the flood prone areas have been significantly reduced upon the completion of river training works at Shenzhen River, Ng Tung River, Sheung Yue River, Shan Pui River, Kam Tin River and Ping Yuen River.
|Widened Shenzhen River at Liu Pok||Widened Shenzhen River at Lok Ma Chau|
Completed river training works at Ng Tung River , 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 are necessary where the villages are so low-lying that stormwater cannot effectively be drained by gravity to the primary drainage network. The scheme involves the construction of bund around existing village 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
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 development 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 drainage 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 have already been congested with existing underground utilities such as electric cables, telephone lines, TV cables, gas pipes and water pipes. The conventional type of drainage construction works involving road opening would unavoidably has to go underneath the existing 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 laying stormwater drains, more innovative improvement options including the application of drainage tunnel for interception and transfer of stormwater and the provision of underground stormwater storage scheme for temporary retention of stormwater have been adopted.
During heavy rainstorms, fast and huge flow 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, stormwater drainage tunnel is an effective option to divert rain water from upland areas for discharge direct to the sea or river. By employing this method, less rain water would enter the existing drainage systems in the downstream urban areas. Therefore, the flooding risk of the downstream urban areas could be reduced without restoring to extensive drainage upgrading works that would cause traffic impact and public disturbance.
Conceptual layout of stormwater diversion by drainage tunnel
Currently, we are operating 4 drainage tunnels namely Kai Tak Transfer Scheme, Hong Kong West Drainage Tunnel, Lai Chi Kok Drainage Tunnel and Tsuen Wan Drainage Tunnel.
Location plan of the drainage tunnels
Tunnel Boring Machine for Hong Kong West Drainage Tunnel
Tunnel Boring Machine for Tsuen Wan Drainage Tunnel
Tunnel Boring Machine for Lai Chi Kok Drainage Tunnel
Stormwater storage is a common stormwater management approach in controlling the quantity of stormwater runoff. It works on the principle of temporarily storing a portion of the surface runoff coming from the upstream and allowing a limited flow to the downstream catchment. In this way, the flow rate is controlled within the capacity of the downstream drainage system, thus relieving the burden of downstream drainage system.
Stormwater storage approach is often used when the capacity of the downstream drainage network is incapable to cope with the increased peak flow rate arising from the development at upstream areas. Urban development usually propagates from the downstream. When the upstream areas are being developed and the downstream areas have already been fully urbanized, the conventional method of upgrading the drainage network at the downstream areas will cause severe disturbance to the traffic and public, not to mention congested underground utilities that would cause great constraint to drainage construction. The Tai Hang Tung Stormwater Storage Scheme and Happy Valley Underground Stormwater Storage Scheme, which are the integral part of the West Kowloon drainage improvement scheme and Happy Valley drainage improvement scheme respectively, were constructed to address the flooding threats in Mong Kok and low-lying area of Happy Valley and Wan Chai.
|Schematic layout of Tai Hang Tung stormwater storage tank||Inside view of Tai Hang Tung stormwater storage tank|
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.
|Schematic layout of Sheung Wan Stormwater Storage Scheme||Sheung Wan Stormwater Storage Scheme|