Drainage Services Department

Tai Hang Tung Stormwater Storage Tank

The Tai Hang Tung Stormwater Storage Scheme (THTSSS) was designed to prevent flooding in Mongkok area caused by heavy rainfall sweeping southward from Lion Rock. The problem was highlighted by the severe flooding in the area in 1997 and 1998. Years of development in Kowloon Tong and Yau Yat Tsuen areas had increased the amount of surface runoff downstream. Although there were three main stormwater drains serving the area, they were no longer adequate for the task.

THTSSS is the first large scale underground stormwater storage scheme in Hong Kong with a construction cost of about HK$290 million. It includes an underground storage tank with a capacity of 100,000 cubic metres and a pumping station. THTSSS is designed to intercept the stormwater from nearby natural hillsides and temporarily store the collected stormwater in an underground storage tank underneath the Tai Hang Tung Recreation Ground. This helps resolve the flooding problem in Mong Kok by substantially reducing the surface runoff discharged into the downstream drainage system in Mong Kok during heavy rainstorms.

THTSSS was commissioned in September 2004 and won the Hong Kong Institution of Engineer’s Innovation Award in 2002 and Merit Award in Skyrise Greenery Award in 2012.

From 6 to 31 January 2018, Drainage Services Department co-organised with the Hong Kong Arts Development Council (HKADC) an art exhibition titled "After the Deluge" and offered the Tai Hang Tung Stormwater Storage Tank (THTSST) as an exhibition venue for the first time. The exhibition attracted overwhelming responses and received over 10,000 visitors at THTSST. The dynamic installation artwork designed by Mr. Kingsley NG, a Hong Kong artist, presented the storage tank from an unseen perspective, bringing the visitors in deep thought about flood prevention works.

After the Deluge Art Exhibition at Tai Hang Tung Stormwater Storage Tank

Artistic Impression of Tai Hang Tung Stormwater Storage Tank

Tai Hang Tung Stormwater Storage Tank Pumping Station

Two duty and one standby centrifugal pumps are installed inside the Tai Hang Tung Stormwater Storage Tank Pumping Station for tank dewatering operation. The pumping capacity of each pump is 0.95 cubic metres per second and dewatering time for full tank takes about 15 hours. The pumping station is remotely monitored by telemetry system at Stonecutters Island Sewage Treatment Works.

The concrete structure of the pumping house is covered by a green blanket comprising vertical greening climbers, such as Parthenocissus himalayana and Ficus pumila, and ground planting, such as Asparagus densiflorus “Sprengeri”, Sansevievia trifasciata var. laurentii and Calliandra haematocephala, to establish a leisure, green environment echoing the Tai Hang Tung Recreation Ground.

Tai Hang Tung Stormwater Storage Tank Pumping Station

Vertical Greening of Tai Hang Tung Stormwater Storage Tank Pumping Station

Vertical Greening of Tai Hang Tung Stormwater Storage Tank Pumping Station

Tai Hang Tung Stormwater Storage Tank

Tai Hang Tung Stormwater Storage Tank

Tai Hang Tung Stormwater Storage Tank (THTST) is the first and largest underground stormwater storage tank in Hong Kong, located underneath the rugby pitch and one of the football pitches in Tai Hang Tung Recreation Ground. The tank is 136 metres long, 130 metres wide and 7.5 metres deep internally, with a storage capacity of 100,000 cubic metres (equivalent to the total size of about 40 standard swimming pools).

Tai Hang Tung Stormwater Storage Scheme (THTSSS) is a complex and sophisticated stormwater detention scheme. The Tai Hang Tung site proved to be the perfect choice because it was located just above the bottleneck and was thus best situated to intercept the heavy flow. THTSSS adopts an overflow weir system to operate. Under normal conditions, the stormwater will flow into the existing drainage system in the downstream Mong Kok area without entering the storage tank. During heavy rainstorms, the stormwater will spill over the overflow weir into storage tank. When rainstorm subsides and the water level in the downstream drains recedes, the stored stormwater will be pumped back to the downstream drainage system for discharge into the sea.

Hence, the effectiveness of the storage scheme will hinge on the arrangement and the hydraulic performance of the overflow weir and inlet/outlet structures of the storage tank. Too early a spill into the storage tank may reduce the storage capacity for a future severe rainstorm. Too late the spill into the storage tank may lead to flooding in Mong Kok roads. Therefore, the height and length of the overflow weir must undergo hydraulic analysis and precise calculation. To add flexibility to the system, a steel mechanism was fitted to the top of the weir to permit height adjustments from 5.4 metres to 5.7 metres.