DSD is committed to providing professional sewage treatment and stormwater drainage services to the public so as to protect the quality of Hong Kong waters and protect the citizens against flooding. Since our establishment in 1989, we have been endeavouring to build excellent sewerage and drainage facilities to make Hong Kong a more livable city.

There were drastic climate changes in 2017, with frequent typhoons and rainstorms. The storm surge caused by Super Typhoon Hato which hit Hong Kong in August led to severe flooding in many coastal areas of the city, posing new challenges to our flood prevention works.

The annual total rainfall in 2017 was 2,572 millimetres, slightly higher than the mean annual total rainfall of approximately 2,400 millimetres by approximately 7.2% between 1981 and 2010. In 2017, the Hong Kong Observatory (HKO) issued one Black, four Red and 15 Amber Rainstorm Warnings. The only Black Rainstorm Warning was issued on 24 May, on which day many areas recorded a maximum hourly rainfall of 100 millimetres. Extreme and localised precipitation events were also become more frequent. On 18 July, HKO issued three Amber Rainstorm Warning in total. Tai Po district, however, recorded a rainfall of 135 millimetres within two hours, an amount equalling that of Black Rainstorm Warning. During the year, HKO also issued five tropical cyclone warning signals No. 8 or above, matching the records in 1964 and 1999. Coinciding with the astronomical high tide during the attack of Super Typhoon Hato on August 23, the sea level at Quarry Bay peaked at 3.57 metres above Chart Datum, the second highest record since 1954 and only lower than the record high of 3.96 metres above Chart Datum set by Super Typhoon Wanda in 1962. A maximum sea level of 4.56 metres above Chart Datum was recorded in Tsim Bei Tsui, the highest since records began in 1974.

To minimise the flooding risk arising from the Super Typhoon Hato, DSD stepped up inspections and clearance of major drainage channels and inlets, particularly at flooding blackspots, to ensure that the drains are free from obstruction. Besides, we had identified several low-lying coastal areas that are susceptible to flooding arising from storm surge, such as Tai O, Lei Yue Mun and Nam Wai in Sai Kung. The Government has installed flood prevention facilities such as the construction of flood wall and installation of tidal gate and non-return flap valve. Also, the HKO had established an early alert system for the said locations. When storm surge alerts were issued by the HKO, DSD implemented emergency flood relief measures at relevant locations immediately with an aim to alleviate the flooding impact. Taking an example in Tai O, before the typhoon arrived Hong Kong, DSD deployed over 10 staffs staying in Tai O to install the temporary water-stop boards over the midnight to enhance the flood protection capacity of river wall and to handle emergency situations.

During the severe weather and before the Tropical Cyclone Warning Signal No. 8 is in force, we had activated the Emergency Control Centre and contingency teams were deployed to stand by at the locations prone to flooding, to ensure that inspections and drain clearance would be conducted in time to reduce flooding risk. Around 120 staffs under 30 teams had been mobilised.

Striving to prevent flooding, DSD has designed and constructed drainage facilities with primary reference to international standards, and carries out regular inspections and maintenance works to ensure proper operation of our facilities. In 2017-18, DSD continued to implement various flood prevention projects. Meanwhile, we are reviewing the Drainage Master Plans (DMPs) of various districts in stages, so as to upgrade their flood protection level and tie in with Hong Kong’s future development.

Bringing Down the Number of Flooding Blackspot

We implemented various drainage improvement works in the past year, and after evaluating the effectiveness of each completed drainage improvement works, we removed the flooding blackspot at Tung Tsz Road in Tai Po in early 2018, thus reducing the total number of flooding blackspots in the territory from seven to six. Among the six remaining flooding blackspots, drainage improvement works for two have been commissioned and their effectiveness is being monitored. In order to remove all the flooding blackspots as early as possible, the first stage improvement works for the other four blackspots have been completed while the next stage improvement works are under planning and design. We will closely monitor the drainage condition in these areas during the rainy season.

Overall Flood Prevention Strategy of Hong Kong

Compounded by urbanization, increase in surface runoff and reduction of flood plains, flooding may happen at certain districts in low-lying areas and coastal areas. To address the flooding problem in various places, we have developed a “three-pronged flood prevention strategy”, i.e. stormwater interception, flood storage and drainage improvement, which has been proven effective in mitigating the flood risk arising from heavy rain.

• Stormwater Interception: build drainage tunnels to intercept stormwater from the mid-levels and discharge it directly into the sea or to other channels and drains
• Flood Storage: build storage tanks in the midstream for temporary stormwater storage to relieve the discharge load of the downstream drainage system
• Drainage Improvement: carry out river training works or build new drainage channels and drains to upgrade the capacity of drainage system

To support the sustainable development of Hong Kong and combat climate change, we adopt the “Sponge City” concept of “following the nature with resilience” to facilitate infiltration and reduce surface runoff through flood storage and retention. Part of the stormwater is collected and reused in a bid to optimise water recycling across the city and enhance the flood resilience level of the city.

Operation and Maintenance of Existing Drainage Facilities

A clear drainage system is prerequisite to flood prevention. There are about 2,400 kilometres of underground stormwater drains, 360 kilometres of engineered channels, 21 kilometres of drainage tunnels, and four underground stormwater storage tanks in Hong Kong, all managed by DSD. During the year, we inspected over 2,290 kilometres of drains and rivers. Apart from regular inspections and preventive maintenance works, we conduct regular functional and structural checks and clear blockages before and after the wet season.

Planning, Design and Construction of New Drainage Facilities