Of the current black spots, three are considered medium level, which means they affect an area of more than 10 hectares and could cause significant property damage and serious traffic disruption.

Meanwhile, the department is preparing to construct a 60,000 cubic metre rainwater storage tank under Happy Valley racecourse.

Due for completion in 2018, it would help deal with flooding in Wan Chai, including a black spot in Morrison Road, the department said. Rainwater will be stored in the tank during the rainy season and pumped in regular drainage pipes to the sea in the dry season.

An extra one kilometre drainage pipe will also be constructed to divert rainwater faster. The project is expected to cost HK$1 billion and will be discussed in the Legislative Council next week.

Construction of three rainwater tunnels in Tsuen Wan, Lai Chi Kok and Island West was also under way and they were expected to be ready for the rainy season in 2013, Chan said. The tunnels, the first of their kind in the city, are designed to take water flowing from the upper hills directly to the sea to avoid adding pressure to drainage pipes in urban areas.

Chan said the mid-hill tunnels would improve the existing system 'causing less inconvenience to the public and traffic in towns'. The Tsuen Wan tunnel, costing HK$1.3 billion, is 5.1km long and has a diameter of 6.5 metres.

It will pump water at a rate of 60 cubic metres a second. Water will flow into the tunnel from Wo Yi Hop Road, Lo Wai and Tso Kung Tam naturally when there is heavy rain, with no manual operation required.

Chan said it could handle heavy rainstorms, benefiting areas from Tsuen Wan to Kowloon West. 'It may not prevent flooding totally, but if there's a minor flood in the area, the water will disperse quickly,' he said.

Action on drainage

A storage tank beneath Happy Valley racecourse is due to be ready in 2018

Meanwhile, tunnels in Tsuen Wan, Lai Chi Kok and Island West are expected to be ready for the rainy season in: 2013