INTEGRATED PLANNING AND DESIGN OF A FLOOD RELIEF PROJECT FOR SHEUNG WAN LOW-LYING AREA Mr. CHAN Kin-kwong, Chief Engineer Drainage Services Department, Hong Kong SAR Government Mr. TAM Kit-fan, Senior Engineer Drainage Services Department, Hong Kong SAR Government Ms. LEUNG Yuen-hei, Engineer Drainage Services Department, Hong Kong SAR Government Abstracts Sheung Wan is one of the earliest settlements in Hong Kong. A low-lying area of about 10 hectares in Sheung Wan, which is an important business and financial district and a reputable shopping place for dried seafood and swallow’s nest, is susceptible to flooding during heavy rainstorms when coincided with high tide. Before construction After completion Figure 1 – Overview of the site of Sheung Wan Stormwater Pumping Station from Shun Tak Centre 1 To relieve the flooding problems in this low-lying area of Sheung Wan, the Drainage Services Department has recently completed some drainage improvement works in 3 phases. Phase 1 includes construction of 660m of intercepting drains to intercept the stormwater runoff from the upland and divert it away to another drainage system. This reduces about 30% of the surface runoff from getting into the Sheung Wan low-lying area. Works started in March 2006 and was completed in June 2008 (construction cost for Phase 1 works is HK$39 million). Phases 2 and 3 include construction of a stormwater pumping station and an underground storage tank of 9,000 cu m capacity for discharging surface run-off collected at the low-lying area to the harbour, and a penstock to stop the back-flow of seawater (construction cost for Phases 2 and 3 is HK$ 200 million). The stormwater pumping station, the underground storage tank and the penstock were completed and started to operate in March 2009 (3 months ahead of the original programme) before the onset of the 2009 rainy season. The commissioning of the stormwater pumping station raises the flood protection level in the low-lying area in Sheung Wan from 1-in-2 years to 1-in-50 years in general. It was difficult to find a site for such sizeable flood relief infrastructures in a developed urban area such as Sheung Wan. A harbour-front site was found to be the only suitable government land available. To minimize the visual impact to the harbour and to reserve more recreation area for enjoyment by the public, the stormwater pumping station was specially designed to reduce its footprint and to provide planning gains. The pumping station building is a single- storey structure and the remaining parts of the pumping station and the underground storage tank are constructed below ground, above which at ground level and in its adjacent area, a 5700 sq m landscaped open space was developed comprising a waterfront promenade, a plaza, a pet garden and a Tai Chi exercise area. The community is now being provided with drainage infrastructures for protecting life and property and a landscaped area for leisure and recreational use. (Figure 1) Traditional thinking is that though civil engineering works are beneficial to the society, it is inevitable that civil engineering works would affect the natural evironment, negate the share use of the land by others and not harmonize with the surroundings. With ground-breaking approach and holistic considerations in the planning and design stages, civil engineering works could be implemented with sustainability, more society-friendly and addressing the society’s functional needs as well as public interest on greening and concern in climate change. The flood relief project for the Sheung Wan low-lying area has demonstrated how such a win-win situation could be achieved by the engineers. This paper will present the background and planning of this urgent and crucial flood relief project in a well-developed and busy commercial and finanical area in Sheung Wan and how the engineering project team developed such innovative, sustainable and integrated engineering solution for the society. Introduction Sheung Wan is one of the earliest settlements in Hong Kong. An area of about 10.3 hectares bounded by Queen’s Road Central, Bonham Strand, Man Wa Lane, Rumsey Street, 2 Connaught Road Central and Queen Street was once a coastal area with the Peak rising to the south. The ground levels in this area are between +2.64mPD to +3.85mPD. As new reclamation along the northern side of this area was formed at a higher level of 4.5mPD in general, this area became a low-lying area. (Figure 2) Figure 2 – Scope of low-lying areas at Sheung Wan (shaded in blue) and alignment of intercepting drains (red line) Most of the infrastructures including the existing drains in the Sheung Wan low-lying area were designed and constructed decades ago to meet the flow requirements and standards at that time. Although the Government has been making local improvements to the drainage systems to cater for developments from time to time, the low-lying area is susceptible to flooding during heavy rainstorms coincided with high tide. As the lowest ground level in the area is only slightly above the mean high tide level, the flooding situation would become worse with high tide surge because the existing drainage system could not effectively drain the run-off away due to the small difference in ground and sea levels. The situation would be the worst during extreme high tides, when the sea level is higher than the ground level in Sheung Wan causing seawater to flow back and overflow from manholes and gratings. From previous flooding reports and historical records, flooding repeatedly occurred in this low-lying area. (Table 1) On 7 June 2008, about 200mm of rainfall were recorded over most parts of the territory, and exceeded 300mm rainfall over the Lantau Island and the urban areas. The Hong Kong Observatory recorded 145.5mm rainfall during the hour from 8:00 a.m. to 9:00 a.m. at Tung Chung, which is the highest clock-hourly rainfall since they have the record. During this historical rainstorm event, flood level up to 1,500mm was observed at the junction of Hillier Street and Bonham Strand, which is the lowest point in the Sheung Wan low-lying area. (Figure 3) 3 Date Rainfall (daily total) Highest Tide Level Max Flood depth 6.7.2001 142mm +3.25mPD 600mm due to typhoon Uter (Signal No. 8) 24.6.2005 233mm, black rainstorm +2.45mPD 600mm warning 16.7.2006 196mm, black rainstorm +1.50mPD 700mm warning 19.4.2008 237mm, black rainstorm +1.85mPD 500mm warning 7.6.2008 302mm, black rainstorm +2.50mPD 1,500mm warning Table 1 – Flood record at Sheung Wan low-lying area (2001 – 2008) Figure 3 – Flooding at Wing Lok Street on 7.6.2008 The Flood Relief Project To relieve the flooding problems in well–developed urban area such as Sheung Wan by conventional expansion or enhancement of existing drainage systems, through increasing the size of drains or by duplications, would have various limitations. Due to the topography of the Sheung Wan low-lying area, which is affected by tidal water back-flow, the flooding problem also cannot be solely resolved by upgrading the downstream drainage system. Drainage Services Department (DSD) has adopted other approaches to overcome the limitations of the conventional approach. The Sheung Wan flood relief project consisited of 3 phases. Construction commenced in March 2006 and was completed in March 2009. Phase 1 includes construction of 660m of intercepting drains along Lok Ku Road, Queen’s Road Central and Gilman’s Bazzar (hereafter referred to as Queen’s Road Central 4 Intercepting Drains) to intercept the stormwater runoff from the upland and divert it away to another drainage system. This reduces about 30% of the surface runoff from getting into the Sheung Wan low-lying area. Works started in March 2006 and were completed in June 2008 (construction cost for Phase 1 works is HK$39 million). Phases 2 and 3 include construction of a stormwater pumping station and an underground storage tank of 9,000 cu m capacity for discharging surface run-off collected at the low-lying area to the harbour, and a diversion chamber with penstock to stop the back-flow of seawater and to divert the upstream flow to the underground storage tank (construction cost for Phases 2 and 3 is HK$ 200 million). The stormwater pumping station, the underground storage tank and the diversion chamber were completed and started to operate in March 2009 (3 months ahead of the original programme) before the onset of the 2009 rainy season. The commissioning of the flood relief scheme raises the flood protection level in the low-lying area in Sheung Wan from 1-in-2 years to 1-in-50 years in general. Queen’s Road Central Intercepting Drains The Queen’s Road Central Intercepting Drains comprise - (a) construction of 530m of stormwater drains ranging from 900mm to 1,500mm in diameter along Lok Ku Road and Queen’s Road Central; and (b) construction of 130m of single cell drainage box culvert of internal size 2,250mm wide and 1,250mm high along Gilman’s Bazaar. The completion of the construction of 660m intercepting drains has brought about gradual improvement to this flood-prone area in Sheung Wan by reducing about 30% of stormwater entering this low-lying area from the upland catchment. Reduction of the runoff to be conveyed to the stormwater pumping station downstream would help to scale down the pumping system required for the stormwater pumping station and hence smaller land take and lower energy consumption. During design stage, trenchless construction method for construction of the intercepting drains was adopted where permitted by site conditions with an aim to minimize disruptions to the public.