Overview of Flood Prevention in 2019-20

Facing the challenges brought by extreme weather conditions, such as typhoons and rainstorms, the DSD is dedicated to enhancing the stormwater drainage capacity of Hong Kong. About 2,396 millimetres of annual total rainfall was recorded in 2019, which is similar to the average value of 2,400 millimetres between 1981 and 2010. Frequent rainstorms in 2019 brought flooding risks to Hong Kong, posing challenges to our flood prevention work.

 

Drawing on the past experience in responding to Super Typhoons Hato and Mangkhut, the DSD stepped up precautionary inspections and clearance of major drainage channels and inlets, particularly at flooding blackspots, to ensure that drains were free from obstructions. The DSD has also identified several low-lying coastal areas that are prone to tidal backflow, such as Tai O and Lei Yue Mun, and worked with other government departments to put in place flood prevention measures for these areas, such as installing demountable flood barriers and non-return flap valves, and constructing flood walls. The Government also established an early alert system for the above locations. Once storm surge alerts are issued by the Hong Kong Observatory, the DSD will immediately activate emergency flood relief measures at relevant locations to mitigate the flooding impact.

The DSD has set up the Emergency and Storm Damage Organisation (ESDO) to handle emergencies and flooding incidents. The ESDO operates round-the-clock to coordinate the dissemination of information relating to emergencies and the allocation of resources, as well as liaising with other government emergency units. During heavy rainstorms or just before the Gale or Storm Signal No. 8 is in force, we will activate our Emergency Control Centre and deploy contingency teams to stand by at locations prone to flooding, with a view to conducting inspections and drain clearance in time to reduce flooding risk.

The DSD takes into account international standards in design and construction of drainage system, and carries out regular inspections and maintenance work to ensure proper operation of its facilities. To meet Hong Kong's future development needs, we are reviewing the Drainage Master Plans (DMPs) of various districts in stages to upgrade the flood protection capabilities of the areas concerned, while continuing to strategically raise the general standard of flood protection.

The staff gauge at Lei Yue Mun shows the maximum sea level of past typhoons

 

Overall Flood Prevention Strategy of Hong Kong

With urbanisation, increased surface runoff, reduction of flood plains and the greater likelihood of extreme weather, low-lying or coastal areas are facing increasing threats of flooding. To address the flooding problem at various locations landscape, we have developed, in accordance with different topographical features, a “three-pronged flood prevention strategy”, i.e. stormwater interception, flood storage, and drainage improvement, which has proven to be effective in mitigating the risk of flooding arising from heavy rains.

 

Interception

• Stormwater is intercepted upstream and diverted for direct discharge to the sea or rivers, thereby substantially mitigating the risk of flooding in downstream areas
• Large-scale drainage improvement works in downstream urban areas are avoided to reduce the impacts on traffic and the public
• Four drainage tunnels (i.e. the Kai Tak Transfer Scheme, Hong Kong West, Lai Chi Kok and Tsuen Wan Drainage Tunnels) totalling about 21 kilometres in length are in place

Flood Storage

• During heavy rainstorms, some stormwater in urban areas is diverted to storage tanks for temporary storage to relieve the burden of downstream drainage systems
• Stormwater storage schemes for Tai Hang Tung, Sheung Wan, Happy Valley and On Sau Road are in operation
• Five new stormwater storage schemes as well as the extension of the existing stormwater storage scheme in Tai Hang Tung are under planning to further alleviate the risk of flooding in Kowloon

Drainage Improvement

• Drainage improvement works are carried out to straighten, widen and deepen rivers and to enlarge or construct new underground drains
• To date, over 100 kilometres of river sections have been improved and about 94 kilometres of drains upgraded to date

 

Continual Elimination of Flooding Blackspots

A flooding blackspot at the junction of Morrison Hill Road and Lap Tak Lane in Wan Chai was removed at the beginning of 2020, leaving only five remaining flooding blackspots in Hong Kong. Amongst the drainage improvement works at the remaining flooding blackspots, those at Ting Kok Road in Tai Po have already been completed and their effectiveness is being monitored. Striving to eliminate all flooding blackspots, we have completed the first stage improvement works for the other four blackspots 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.

 

Storm Surge Spots and Overtopping Wave Spots

Having taken into account the records of typhoon passages, the DSD has identified seven Storm Surge Spots which are vulnerable to seawater inundation or infusion due to the rise of sea level caused by storm surge during typhoon, as well as three Overtopping Wave Spots which are vulnerable to flooding caused by waves overtopping the seawall. At present, the DSD actively works with relevant departments to formulate effective measures, including setting up an Early Alert System for Storm Surge Spots and Overtopping Wave Spots. Upon the issuance of storm surge alerts, the DSD will deploy pumping equipment, install demountable flood barriers, provide sandbags to residents, etc., at the locations concerned.

 

Operation and Maintenance of Existing Drainage Facilities

The DSD manages over 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, striving to keep the drainage system free from obstruction. In the year under review, we inspected over 2,320 kilometres of drains and river channels, outperforming the annual total target by about 18 kilometres. Apart from carrying out regular inspections and maintenance work, we also conduct functional and structural checks on facilities periodically and clear blockages before and after the rainy season.

 

Proactive Inspection and Just-in-time Clearance

• Introduce the 'just-in-time clearance' arrangement during daylight hours on weekdays in the rainy season (April to October every year) in 2020 to reduce the risk of flooding when rainstorms occur
• Send staff  to inspect about 200 drain locations throughout the territory which are susceptible to blockage by litter, fallen leaves or the like before the onset of a rainstorm
• Immediately arrange clearance of drainage inlets if blockage is identified

 

Village Flood Protection Schemes

• Embankments are constructed around low-lying villages where flood storage ponds and stormwater pumping stations are built in the villages for temporary storage of rainwater during heavy rainstorms and for subsequent discharge of rainwater by pumping after heavy rainstorms
• 27 Village Flood Pumping Schemes are currently in operation, providing flood protection for 38 low-lying villages

 

Pok Wai Village Flood Pumping Scheme

 

Kiu Tau Wai Flood Pumping Scheme