Opportunity
Dense urbanization has made upgrading surface drainage in the town of Tsuen Wan difficult. Tsuen Wan sits on the lower slopes of a steep hill, and streams carry rainfall from the upper slopes into the town.
Existing drainage was built in the 1960s and 70s. As urbanization has taken place, the area of hard surfacing and the volume of rainwater runoff have become greater.
In June 1997, a once-in-a-lifetime rainstorm hit Tsuen Wan, unleashing a chest-deep torrent that swept up cars and left a trail of destruction.
Challenge
To avoid a repeat of the disaster, the Hong Kong Drainage Services Department commissioned a vast tunnel to intercept rainwater runoff and carry it underground to the sea. Mott MacDonald produced a drainage master plan for the district and took the flood bypass through design to construction.
The solution is a tunnel 3.2 miles (5.1 kilometers) long. While parts of the tunnel are 656 feet (200 meters) below surface level, the intakes are only 33 to 66 feet (10 to 20 meters) deep. The three intake structures contain giant concrete spiral ramps, which avoid trapping air and dissipate energy to deliver water to the tunnels in a smooth, controlled stream. Physical modeling was used to perfect the pitch and width of the ramps.
The tunnel is sized to cope with the type of rainstorm that occurs on average only once in every 200 years. With a cavernous internal diameter of 21 feet (6.5 meters) it can carry up to 55,000 gallons (210 cubic meters) of water per second, discharging into the sea via an outfall structure at Yau Kom Tau.
Outcome
Our international knowledge-sharing was critical to the design. These structures were the first of their kind in Hong Kong, and the project was one of the first tunneled flood relief projects in the territory.
Our bored tunnel solution avoided the disruption that would have been caused by construction of drainage channels or cut and cover tunnels.
The early design phase focused on optimizing the location of the intakes and the tunnel alignment. As much runoff as possible needed to be intercepted during an extreme rainstorm, but for ecological reasons water had to be kept flowing in the streams during normal conditions. As a result, the tunnel is designed to intercept water only when rainfall exceeds 30 mm per hour. To limit cost and construction time, the depth of the intake shafts and the total length of the tunnel were kept to a minimum.