These underground drain pipes make up the whole stormwater drainage system of Hong Kong, which totals more than 2,800 kilometers. That is longer than the distance from Hong Kong to Beijing.

With an average annual rainfall of about 2400mm, Hong Kong has one of the highest rainfalls in the Pacific Rim. During exceptionally heavy rainfall, flooding may occur in low-lying areas and natural flood plains in the rural areas of the northern part of the New Territories.

Hong Kong also suffers from severe flooding during typhoon season, which is usually from April to October. In 1995, there were about 90 flooding black spots in Hong Kong, with serious ones larger than 100 hectares, the size of 14 soccer fields, resulting in substantial socio-economic losses.

The Drainage Services Department (DSD) was established by the government in September 1989 to provide effective approaches to deal with flooding issues. To protect the city from flooding, they built and kept improving the network of tunnels and dikes, hidden just below the streets.

However, drainage improvement in urban areas faces another type of construction problem.

Since most of the roads in Hong Kong are densely packed with utilities, such as cables, telephone lines, and gas pipes, traditional drainage works with excavation techniques inevitably require construction under existing underground utilities, thus affecting traffic and causing inconvenience to the public.

Therefore, the DSD tries to reduce such work. In addition to the broader use of trenchless excavation techniques to lay drainage pipes, they have adopted more innovative improvement options, including the use of stormwater drainage tunnels to intercept and convey stormwater and the construction of underground flood storage tanks for the temporary storage of stormwater.

Those large tunnels can intercept rainwater midway and directly discharge it to the sea, reducing the upstream flow to the downstream urban runoff.

The Hong Kong Island West drainage tunnel, the longest and largest one with a length of 11 kilometers from Tai Hang to Cyberport, can drain away nearly 7,500 cubic meters — equivalent to three standard swimming pools of rainwater in a minute.

The construction of flood storage tanks underneath Happy Valley Stadium began in 2012 and was opened in March 2017. The first and second phases of the underground flood storage tank with a total capacity of 60,000 cubic meters is equivalent to 24 standard swimming pools.

After its operation, the flood discharge capacity of Happy Valley and the neighboring Wan Chai area was increased to withstand a 50-year rainstorm, protecting the lives and property of the people in the area.

In order to increase the capacity of the river to cope with the expected extreme conditions, the rivers need to be straightened, widened, deepened, and laminated.

After the completion of the river management projects in Shenzhen River, Wutong River, Shuangye River, Shanbei River, Kam Tin River, and Plain River, the risk of flooding in most flood-prone areas has been greatly reduced.

The Yuen Long Drainage Bypass was specially built to divert stormwater from Yuen Long Town Center. In addition to expanding the capacity of the river, river training projects will also grow many plants on both sides of the river, and even build artificial wetlands to provide a living environment for wildlife.

Since the rainy season in Hong Kong runs from April to October, it is not easy to complete the desilting, maintenance, and repair work for the drainage facilities within such a tight time frame.

DSD Secretary for Development Wong Wai Lum said every year they remove up to 500 tons of sand and silt from the four drainage tunnels and four flood storage ponds, which is equivalent to the weight of about 34 double-decker buses.

Since 1995, the Drainage Services Department has eliminated a total of 127 flooding black spots. Now, only four flooding black spots remain, one of which is Chatham Road South. Its flood control project is expected to start in the third quarter of this year in hopes that this flooding black spot can eventually be removed from the list.

The large scale of flood prevention facilities now includes over 2,400 km of underground stormwater drains, over 360 km of man-made watercourses, four underground flood storage ponds with a total capacity of over 180,000 m³ (equivalent to 72 standard swimming pools), and four stormwater drainage tunnels with a total length of about 21 km.

With these large-scale long-term structural measures completed one after another, Hong Kong’s flooding black spots will be eliminated someday. These major underground projects will also help Hong Kong cope with the increased and more devastating natural disasters that climate change brings in the future.