The existing drainage systems in Tsuen Wan and Kwai Chung were constructed over 30 years ago, and they don't have the spare capacity to handle the additional stormwater arising from the increasing urbanization of the areas, and flooding is experienced during severe rainstorms. For example, in 1997, several rainstorms caused flooding in many areas of the Tsuen Wan and Kwai Chung districts.
In view of the above, the DSD commisioned a “Stormwater Drainage Master Plan (DMP) Study in Tsuen Wan, Kwai Chung and Tsing Yi”,which was completed in July 1999. It concluded that the drainage systems in Tsuen Wan and Kwai Chung could not meet current flood protection standards. The Study recommended the construction of the Tsuen Wan Drainage Tunnel (TWDT) as the vital link in the overall drainage improvement strategy for these districts.
Selection of tunnel option
Tsuen Wan and Kwai Chung are two well developed districts with busy traffic flow and congested underground utilities. The use of conventional open-cut method to improve the urban drainage pipes newtork would have a significant impact on traffic and the use of pipe-jacking methods would encounter serious underground utility problems. In order to minimize disturbance to residents, the adoption of tunnel in the upstream rural area was therefore considered to be the best option.
Scope of TWDT
TWDT project comprises the following :
(1) a tunnel of about 5.1km long with internal diameter of 6.5m;
(2) three intakes at Wo Yi Hop, Lo Wai and Tso Kung Tam;
(3) an outfall at Yau Kom Tau; and
(4) associated landscaping and civil works.
The tunnel will be constructed by a double-shield tunnel boring machine (TBM) with a precast concrete lining placed in sequence with the excavation, in order to prevent any ground water loss and excessive settlement in adjacent areas. The intake structures in Lo Wai and Tso Kung Tam will be constructed concurrently with the TBM advance. Connecting adit-tunnel will be constructed to link the main tunnel with the intakes. Upon completion of the tunnel, cascade structures at Yau Kom Tau and Wo Yi Hop will be constructed to complete the system.
Extensive landscaping works, including planting of more than 1,700 trees and 9,000 shrubs, will form part of the project, to improve the environment of the intake and outfall areas.
The completion of the TWDT will see the flood protection standard in Tsuen Wan and Kwai Chung raised to accommodate a rainstorm return period of 1 in 50 years.
The construction of the TWDT will also help to create more than 200 job opportunities.
Cost and programme
The contract sum for TWDT is HK$1,123 million, and construction commenced in December 2007 with completion anticipated in late 2012.