Underground stormwater storage scheme points way to future flood prevention
By Angela TAM
As the world warms, extreme weather events have become more frequent around the world, with flooding, landslides, drought and high summer temperatures afflicting millions. Between May and July this summer, Hong Kong has been lashed with several severe rainstorms that resulted in heavy flooding in some areas. The situation has made the completion of the Happy Valley Underground Stormwater Storage Scheme (HVUSSS) all the important, both for the prevention of flooding in Happy Valley and downstream areas and as a reference for the development of similar schemes to protect other areas of Hong Kong.
Begun in 2011, HVUSSS involves the construction of an underground stormwater storage tank under five football pitches at the Happy Valley recreation ground inside the Happy Valley Racecourse. Black & Veatch was engaged by the Drainage Services Department (DSD) as the preliminary design consultant and Chun Wo Construction & Engineering Co Ltd was the project's main contractor.
According to DSD, the location of the storage tank was driven by its three-pronged approach towards flood protection covering, respectively, the upstream, midstream and downstream areas of the northern part of Hong Kong Island.
Under the approach, runoff from the uphill areas is intercepted by the Hong Kong West Drainage Tunnel, which was completed in 2012, diverting the stormwater for direct discharge to the sea near Cyberport. HVUSSS is located in the midstream and serves to temporarily store excessive stormwater during peak flows via a side weir on a major box culvert, to reduce the risk of flooding further downstream. This allows DSD to raise the overall flood protection level of the district without extensive works which would be highly disruptive in the urban setting. Instead, only small-scale works are required for local drainage improvement.
According to DSD, the infield area of the racecourse was once used as a flood pond a long time ago so it made sense to locate a flood prevention structure there. What's more, the area is occupied by sports pitches, which means its underground space is not cluttered with the foundation of buildings, thus making it ideal for the construction of a stormwater storage tank.
The stormwater storage tank, which has a capacity to hold 60,000 cu m of stormwater, is located underneath five of the 11 sports pitches in an L shape. The construction of the storage tank was then split into two phases so that some of the pitches would remain available to users while the rest were temporarily closed to facilitate the construction works.
The original box culvert ran along Wong Nai Chung Road. To make it work with the storage tank, the original box culvert was diverted and a new 650 m long twin cell box culvert was constructed down the middle of the recreation ground. Fifteen 'intelligent' movable weirs were installed between the box culvert and the storage tank to control the flow of stormwater into the latter. The movable weirs are described as 'intelligent' because they are operated automatically by a supervisory control and data acquisition (SCADA) system that tracks the tidal level as well as water levels inside the tank, upstream and downstream through sensors in real time. This allows the weirs to be opened to let in overflowing stormwater from the box culvert at the optimal time and for the weir position to be adjusted as the water levels change.
Once a heavy rain episode is over, the weirs will be lowered further to allow up to one-third of the stored stormwater to drain back into the box culvert under gravity, thus reducing the need for pumping and achieving a more energy-saving design. The remaining two-thirds is pumped away using pumps with a peak capacity of 1.5 cu m/sec.
Having the real-time SCADA-controlled weirs that can be opened or closed and positioned depending on the actual tidal and water levels prevents premature or belated filling of the tank, thereby maximising the effectiveness of the flood storage capacity and allowing a smaller design tank volume.
"The Tai Hang Tung underground stormwater storage tank (see HKE, Sep 2004, pp 8-9) has fixed weirs and the inflow of stormwater is not controlled, so the tank volume has to be larger," explained DSD engineer Ir C L Leung.
As a result of adopting movable weirs, the required tank volume was reduced by 25%, from 80,000 cu m to 60,000 cu m. This has translated into substantial time and cost savings. Considerable capital cost was saved as there was much less excavation and disposal to be carried out. There is also a reduction in operating cost as the shallower tank facilitates discharge of the stored stormwater by gravity, thereby saving an estimated 35% of pumping energy and the associated electricity cost.
According to DSD, it has studied the feasibility of reusing the rainwater collected in drainage tunnels and storage tanks but concluded that it would not be cost-effective to do so as any plans for large-scaled reuse would require the construction of additional water transfer tunnels/pipe works as well as pumping or storage facilities while the frequency of their use is basically limited to that of intense rain episodes.
"Nevertheless, a pilot trial is being undertaken under HVUSSS to look into the quality of stormwater collected in the storage tank and the required treatment to acceptable standards for non-potable use," DSD stated.
At the same time, it has proceeded with a water harvesting system under HVUSSS that collects underground water from the subsoil drains, excess irrigation water and rainwater from the sports pitches. The collected water goes through an on-site treatment unit, after which it is distributed through the recreation ground for irrigating the sports pitches, watering plants and toilet flushing.
The subsoil drainage system is innovative not only because it conserves water, but also because it removed the need to build expensive foundations to tie down the storage tank. To prevent flotation of the storage tank under Happy Valley's high water table, a conventional design would require an estimated 533 piles to be driven to bedrock to anchor the structure. Using the sub-soil drainage system, the water harvesting system can take up to a design maximum of 220,000 cu m of water per year while reducing water pressure build-up underground. It is part of the 'sponge city' concept adopted for the project, which also saw the use of porous paving blocks to reduce surface runoff.
NEC Option C
Innovation was applied not only to the design of the HVUSSS, but also the contract form adopted for the project.
The New Engineering Contract (NEC) was introduced in the UK in the 1990s and recommended for adoption in Hong Kong under the Government's "Construct for Excellence" report in 2001. The department first tried NEC in 2009, on a relatively small project involving improvement works on the Fuk Man Road nullah in Sai Kung. The project was completed six months ahead of schedule and registered cost savings. Courageously, DSD then decided to adopt NEC for a major project, namely HVUSSS. It adopted NEC Option C, for target cost contracts; and introduced additional clauses covering proactive sub-contractor management.
"Clause 10.1 emphasises mutual trust and cooperation; there is more equitable risk sharing under NEC," explained DSD senior engineer Ir Ellen N S Cheng. "Under the 'pain gain share' mechanism gains are shared equally while 'pain share' is capped at 110% of cost. It provides a real incentive to find innovative solutions that save cost and time."
The approach was so successful that the project was commissioned a year ahead of schedule with latest estimated savings amounting to HK$60 million, or 5% of the construction cost.
Open book account under the contract form facilitates budgetary control and transparency. It also facilitates the timely settlement of disputes with anything involving compensation promptly assessed and resolved. The project also benefited from the additional clauses covering sub-contractor management. For the client, the establishment of a direct line of communication with sub-contractors means issues were identified and resolved more quickly. For example, if a design element clashed with a sub-contractor's work method, the sub-contractor could discuss it with both the main contractor and DSD directly rather than go through the traditional chain of command. As a result there was better understanding of the challenges facing all parties and decisions could be made more quickly.
"One thing that the sub-contractors really appreciated was the 'pay when paid' clause we introduced. It means we would only pay the main contractor when they had paid the sub-contractors, with documentary proofs. They no longer had to worry about delayed payment," Ir Cheng said.
The partnering approach adopted for this project extends well beyond the contracting parties. Those identified early on as stakeholders by DSD included the Hong Kong Jockey Club, the Leisure & Cultural Services Department (LCSD), which is responsible for managing the public facilities in the recreation ground; the Wanchai District Council, and users of the affected recreational facilities.
"The Jockey Club was initially concerned about the impact of the project on their race programme, but we made sure we cleared the site as required before 5:00pm every Wednesday so they could prepare for the evening race in Happy Valley. There were heavy rain episodes that might have affected the evening race but after HVUSSS was commissioned we were able to reassure them on those occasions that the racecourse was fine. The two sides didn't really know each other to begin with but we eventually became good friends," Ir Leung said.
In response to the Wanchai District Council's request, LCSD entrusted the construction of a new toilet and changing room block in the recreational ground to DSD, which worked with an architect to come up with a landscaped roof structure that is both functional and green. For users of the football pitches, a 'ball pickup' hotline was set up to help footballers locate balls that had fallen into the project site. The hotline helped both sides. On one hand, footballers could get their balls back quickly; on the other, the contractor could prevent the players from climbing into the site to retrieve their balls, which could pose safety issues.
"It's not always easy to find the balls quickly so we actually had some new balls in reserve to hand over to the players so they can get on with their games without having to wait around for a long time," Ir Cheng said. "The players are happy because sometimes they end up going home with two balls, one they already have and one we've given them."
Even the above-ground structures of HVUSSS play a role in the recreational function of the recreational ground. For example, the fan room, which has been installed with solar panels that generates 6,130 kWh of electricity a year, features a cantilever over wooden decking where fitness enthusiasts have taken to using for diverse exercises. The sloping top of the pump house has been landscaped and incorporated into the jogging path with lighting that doubles as seats.
HVUSSS won several awards as a result of these innovations, including the Highly Commended NEC Large Project of the Year at NEC Awards 2015 and being the winner of the Contract Innovation Through Additional Clauses at NEC Awards 2017.
"The additional clause we introduced provides an effective communication platform, which promotes the spirit of collaboration among members of the project team and facilitates the effective use of the early warning mechanism for early resolution of project risks," said DSD chief engineer (drainage projects) Ir Kan Hon-shing.
Alongside the successful delivery of HVUSSS, DSD is planning to build similar stormwater storage facilities in other parts of Hong Kong. "Co-locating stormwater storage facilities under open spaces such as sports pitches is a good option for long-term flood relief in the congested urban areas of Hong Kong. Hence we are considering implementing similar schemes in DSD's on-going and upcoming drainage master plan review studies," Ir Kan said.
DSD’s three-pronged approach towards flood protection. Images: DSD
The HVUSSS storage tank
Tidal and water levels are monitored by four sensors
The movable weirs when closed (left) and when opened (right)
Flooding in Happy Valley in 2008, before HVUSSS was commissioned
The fan room (bottom right) provides a covered wooden deck for fitness enthusiasts while the landscaped top of the pump house (left) includes a section of the jogging path and lighting that doubles as seats