Computational Hydraulics Aspects of the Drainage System Planning for Happy Valley Flood Protection Scheme, Hong Kong

Computational Hydraulics Aspects of the Drainage System Planning for Happy Valley Flood Protection Scheme, Hong Kong K.W. Mak*, Jimmy Poon**, Kelvin Lau***, Eric Lo****, Kevin Chan***** * Assistant Director, Drainage Services Department, the Government of the Hong Kong Special Administrative Region (E-mail: [email protected]) **Senior Engineer, Drainage Services Department, the Government of the Hong Kong Special Administrative Region (E-mail: [email protected]) ** Associate Vice President & Project Director, Black & Veatch, 25/F, Millennium City 6, 392 Kwun Tong Road, Kowloon, Hong Kong (E-mail: [email protected]) **** Technical Director, Black & Veatch, 25/F, Millennium City 6, 392 Kwun Tong Road, Kowloon, Hong Kong (E-mail: [email protected]) ***** Engineer, Black & Veatch, 25/F, Millennium City 6, 392 Kwun Tong Road, Kowloon, Hong Kong (Email: [email protected]) Abstract: Happy Valley is a low-lying urban area located in the hinterland of the Wan Chai District and is surrounded by a hilly terrain. Serious flooding occurred in the Happy Valley catchment, in particular the Happy Valley Recreation Ground (HVRG), Hong Kong Jockey Club (HKJC) racecourse and nearby streets, during heavy rainstorms in 2000, 2006 and 2008. To address the flooding problem, the Drainage Services Department of the Government of the Hong Kong Special Administrative Region proposed a Happy Valley Underground Stormwater Storage Scheme (HVUSSS) project at the HVRG. The project would raise the flood protection level of the drainage system in Happy Valley to cater for rainstorms with a return period of 1 in 50 years, and in turn significantly reduce the risk of flooding in Happy Valley and areas in the vicinity. The HVUSSS comprises several major components including an inlet structure, a twin cell diversion box culvert, an automatic movable overflow side weir system, an underground storage tank and a pump house. In the planning and design stages, particular attention has been given to minimising both the volume and depth of the underground storage tank to achieve a more economical and sustainable design for construction and operation in long-term. Without adopting the traditional fixed crest weirs, automatic movable crest weirs with real-time monitoring of upstream and downstream water levels have been designed. The adjustable crest weirs use real-time water levels as the basis for controlling the timing and amount of stormwater flowing from the twin cell diversion box culverts to the storage tank so as to avoid early filling of the storage tank which in turn resulted in reducing the required volume of the storage tank by 30%. To further enhance sustainability by saving energy consumption and recurrent costs of discharging water from the storage tank, a shallow tank has been designed to allow most of the stored water to be discharged by gravity via the automatic movable crest weirs from the tank to the box culvert after rainstorm events. Another advantage of minimising the depth of the storage tank is to facilitate the adoption of lower pressure pumps, with a lower capacity, to pump the remaining stored water out of the tank thus helping to save energy. This paper discusses the design and computational hydraulics aspects of the planning for Happy Valley Flood Protection Scheme in Hong Kong. In particular, the adoption of the latest technique of 3-dimensional Computational Fluid Dynamics in the hydraulic analysis to optimize the design will be elaborated. Keywords: 3-dimensional Computational Fluid Dynamics; inlet structure; underground storage tank; overflow side weir Introduction Happy Valley is one of the urban areas with a long development history in Hong Kong. Over the past few decades, Happy Valley has undergone significant urbanization and changes in land use from green land to developed areas. The low-lying and hilly-surrounded topographical characteristics, together with the city development, has induced major increases in overland runoff. The existing drainage system is no longer fully capable of protecting the region from flooding in which Lap Tak Lane, Page 1 Morrison Hill Road and Wong Nai Chung Road in Happy Valley have all experienced serious flooding during severe rainstorms and high tides in years 2000, 2006 and 2008. The maximum flood depth exceeded 1m and the flood extent was as much as 30 hectares during the rainstorm in 2008 (Figure 1 refers). (a) (b) (c) Figure 111: Serious flooding on 7 June 2008: (a) a Happy Valley Recreation Ground; (b) at Junction between Morrison Hill Road and Queen’s Road East; (c) at Wong Nai Chung Road. The Government of the Hong Kong Special Administrative Region (HKSARG) has commissioned two major flood relief projects in the northern part of Hong Kong Island, namely the Hong Kong West Drainage Tunnel (HKWDT) and Lower Catchment Drainage Improvement Works (LCDI), to relieve the flooding hazards of the Happy Valley catchment. The HKWDT serves to intercept stormwater runoff from an upstream catchment area of approximately 140 hectares, while the LCDI at the downstream catchment involves conventional drainage improvement methods to mitigate the flooding problem in the low-lying areas including Wan Chai and Causeway Bay (Figure 2 refers). However, flooding hazards cannot be fully removed by these two projects as the existing pipes in the drainage system in the mid-stream and downstream of Happy Valley catchment are rather flat in gradient and susceptible to tidal influence. Hydraulic assessments show that under a 50-year return period storm event, the infield area of the Happy Valley Recreation Ground (HVRG) and the roads in the vicinity are still exposed to a high risk of flooding, which affects about 70,000 residents and 2,000 shops (Figure 3 refers). Therefore, further improvement works are required in the mid-stream catchment of Happy Valley to enhance flood protection to an acceptable level in order to safeguard the continuous and sustainable development of the area. LCDI at Wan Chai and After Completion of the HKW DT and Causeway Bay LCDI Mid-level area of Happy Valley Catchment HVRG HKWDT Figure 2: Happy Valley Sub-catchment Plan and Locations of Figure 3: Flood Extent under 50-yr Return Period Rainfall Event HKWDT and LCDI after completion of HKWDT and LCDI Page 2 HKSARG therefore proposed to implement a stormwater storage scheme to temporarily store the discharges from the major stormwater box culverts/drains so as to alleviate the loading on the existing downstream drainage system during heavy rainstorms. When the rainstorm is over, the stored water in the underground storage tank would then be discharged to the sea via the existing drainage system. The main components of this Happy Valley Underground Stormwater Storage Scheme comprise an inlet structure with a stilling basin for intercepting the discharges from the major box culverts/drain, a diversion twin-cell box culvert conveying intercepted discharges to the underground storage tank and downstream existing drainage system, an automatic moveable overflow side weir penstock system, and an underground storage tank and associated pump house. Happy Valley Underground Stormwater Storage Scheme (HVUSSS) To meet the flood protection standard of a 1 in 50-year return period, an underground storage tank with a capacity of 60,000m 3 would be needed. Of the limited land available in the Happy Valley area, HVRG was found to be the most suitable site for locating the underground stormwater storage tank (Figure 2 and 5 refer). HVRG is an open area with 11 sports pitches currently managed by the Leisure and Cultural Services Department of HKSARG and situated close to the existing trunk drainage network of this catchment. It is surrounded by the race track of Happy Valley Racecourse of the Hong Kong Jockey Club (HKJC). HKJC Race Underground Storage Tank Course Overflow Weir Pump House Diversion Twin -cell Box Culvert Inlet Structure Crescent Garden Figure 4: Major Components of the HVUSSS Figure 5: Photomontage of the HVUSSS Runoff from the upstream catchment will be converged to the inlet structure at Crescent Garden before discharging further downstream. The existing box culvert will be modified and the runoff will be conveyed through the newly-constructed approximately 630m long diversion twin-cell box culvert crossing underneath the HKJC Race Course and HVRG. The inlet structure will also function as a stilling basin to stabilize the flow before entering the diversion twin-cell box culvert (Figure 4 refers). A stabilized flow is crucial for avoiding excessive runoff flowing into the storage tank via the overflow weir system. During the days of low flow, stormwater collected at the inlet structure will be conveyed along the diversion twin-cell box culvert to the downstream drainage network without entering the underground Page 3 storage tank. During heavy rainstorm events, the excess runoff in the diversion box culvert will enter the storage tank via the “movable” overflow weir system. The crest levels will be adjusted automatically based on the real-time monitoring data of flow depths in the drainage network at the upstream and downstream of the storage tank, water levels in the storage tank, and tide levels in Victoria Harbour which thus allow filling of the storage tank at the most optimal time. Subsequently, emptying the tank by means of gravity draining firstly and then pumping will be adopted to discharge the stored water from the tank after a rainstorm event. The underground storage tank beneath HVRG will have a capacity of 60,000m 3 with a plan area of about 20,000m 2. The associated pumping station with design pumping rate of 1.5m 3/s will be constructed for pumping the remaining stored water from the underground storage tank. After completion of the HVUSSS, it is expected that the risk of flooding in Happy Valley catchment will be substantially lowered. The amenity facilities on the HVRG will be restored for public use after completion of the HVUSSS.

Load more