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.
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.