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Builders deliberating in a tunnel View of two site workers overlooking site at outfall structure View of site worker taking survey measurements at an intake site

Tsuen Wan New Town stormwater tunnel, Hong Kong

To protect Tseun Wan New Town from the effects of devastating rain storms, we have designed and overseen construction of a HK$1 billion flood interception system – a new approach to drainage in Hong Kong.

View of excavation site for intake structure
Tsuen Wan New Town stormwater tunnel
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Tsuen Wan New Town stormwater tunnel 20m diameter shafts were excavated in the island’s hard bedrock using pneumatic picks and excavators.

Country-first flood relief

The solution is a 5.1km long tunnel which will intercept surface water run-off and discharge it into the sea. 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.

The once-in-a-lifetime rainstorm that hit Tsuen Wan New Town, Hong Kong, in June 1997 unleashed a chest-deep torrent that swept up cars and left a trail of destruction. To avoid a repeat, Hong Kong Drainage Services Department commissioned a vast tunnel to intercept rainwater run-off 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.

Challenge

Dense urbanisation made upgrading surface drainage 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 urbanisation has taken place the area of hard surfacing – roads and roofs – has increased. As a result the volume of rainwater run-off has become greater. Flood risk grew because there was no spare drainage capacity.

Solution

The solution is a 5.1km long tunnel designed to intercept surface water run-off and discharge it into the sea. It is sized to cope with the type of rainstorm that occurs on average only once in every 200 years. With a cavernous 6.5m internal diameter it can carry up to 210 cu m of water per second, discharging into the sea via an outfall structure at Yau Kom Tau.

The early design phase focused on optimising the location of the intakes and the tunnel alignment. As much run-off 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 30mm per hour. To limit cost and construction time, the depth of the intake shafts and the total length of the tunnel have been kept to a minimum.

While parts of the tunnel are 200m below surface level, the intakes are only 10m-20m 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 modelling was used to perfect the pitch and width of the ramps. As the first structures of their kind to be built in Hong Kong they were designed from first principles.

View looking down on site for giant ramp under construction inside intake structure
Tsuen Wan New Town stormwater tunnel
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Tsuen Wan New Town stormwater tunnel Reinforced concrete spiral ramp structures were built within the intake shafts. The slope on the spirals is gentle so the water runs down into the tunnel in a smooth, controlled stream. That prevents pockets of air from being forced into the tunnel, which could cause water to back up and overtop the approach channels.

Value and benefits

Our international knowledge-sharing was critical to the design, as these structures are the first of their kind in Hong Kong and the project is one of the first tunnelled flood relief schemes 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.

Tsuen Wan New Town stormwater tunnel
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Tsuen Wan New Town stormwater tunnel Lining installation inside the cavernous 6.5m internal diameter tunnel able to carry up to 210cu m of water per second
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