INTEGRATED PLANNING AND DESIGN OF A FLOOD RELIEF PROJECT FOR LOW-LYING AREA

Mr. CHAN Kin-kwong, Chief Engineer Drainage Services Department, 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, , Man Wa Lane, Rumsey Street,

2 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)

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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. Initially, only 200m of the drains along Queen’s Road Central (out of total 420m) and 80m of the box culvert along Gilman’s Bazaar (out of total 130m) were designed to be constructed by trenchless method. However, during implementation of the project, the situation was reappraised in response to requests from District Council Members and local representatives that more trenchless construction should be considered. After reviewing the construction technicalities of the additional locations, available resources and cost-benefit analysis, the construction method for some road sections was revised from open cut method to trenchless method so that out of the total 660m drains, over two-third of drains (515m) were constructed by trenchless method. The percentage of drains adopting trenchless method was increased from 42% initially to the final 78%. General layout plan for the Queen’s Road Central Intercepting Drains is shown in Figure 2.

5 Sheung Wan Stormwater Pumping Station

The Sheung Wan Stormwater Pumping Station comprises an undergound storage tank, an above-ground pump house, a diversion chamber with penstock and associated connection drainage pipes. (Figure 4) A diversion chamber is constructed at the downstream of the 2,100mm diameter drainage pipe near the outfall to divert the runoff collected from the low-lying area to the underground storage tank. On the other hand, a penstock inside the diversion chamber would disconnect the existing drainage system with the Victoria Harbour and stop the back-flow of sea water to the upstream drainage system.

Figure 4 – General layout plan of Sheung Wan Stormwater Pumping Station and the diversion chamber

During normal condition, the penstock inside the diversion chamber is closed and the runoff collected from the low-lying area is diverted to the stormwater puming station. When the water level inside the underground storage tank reaches the pre-set level, it will trigger the operation of the submersible pumps. According to the hydraulic assessment, four duty submersible pumps each with capacity of 1m3/s are required and an underground storage tank with a volume of 9,000 m3 would be adequate to handle a 50-year design rainstorm. For enhancing the reliabilty of the flood protection, there are two standby pumps with the same capacity as the 4 duty pumps. To minimize the risk of power failure, power supply to the stormwater pumping station is fed from two separate power supply networks. In the event that the underground storage tank is full due to power failure or rainstorm with intensity far exceeding the design value, the penstock inside the diversion chamber will be lifted (by remote control or manually as back-up) to allow the runoff to flow to the harbour directly.

Intergrating Design of the Stormwater Pumping Station

It was difficult to find a site for such sizeable flood relief infrastructure in a developed urban area such as Sheung Wan. A harbour-front site abutting Chung Kong Road was found to

6 be the only suitable government land available. This site falls within an area zoned “Open Space” on the approved Sai Ying Pun & Sheung Wan Outline Zoning Plan (Figure 5). Construction of the proposed stormwater pumping station is regarded as a type of “public utility installation” and is permitted within “Open Space” site subject to the consent of the Town Planning Board. Though the public including the District Council were desirous to seeing the open space to be materialized as early as possible, the Government had no firm programme for its construction at that time. Based on the requirements from the future operation and maintenance parties and comments of other government departments, a preliminary layout for the stormwater pumping station and underground storage tank was produced. However, meetings with Central and Western District Council (C&WDC) and Harbourfront Enhancement Committee (HEC) held during the early design stage revealed that it would be very difficult to seek their support on the site location which would occupy part of an open space site unless it could be shown that the occupation is essential, the footprint is minimized and that the works would bring about social gains and reduce the loss of amenity. With careful planning and innovative approach, and through public engagement, it was considered feasible to meet the public expectation by integrating the stormwater pumping station with the planned open space.

Figure 5 – Excerpt of approved Sai Ying Pun & Sheung Wan Outline Zoning Plan (OZP)

The pump house and underground storage tank was re-orientated to be sited at the south-west corner of the open space site. This would help maintaining the public view of the harbour when approaching from Chung King Road and enhancing the at-grade accessibility to

7 the harbour front from Chung Kong Road. 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 height above ground. The pump house is a single-storey structure of 5.6m high and occupies an area of 448 sq. m (with two levels of basement) accommodating the switch room, transformer room, H.V. switch gear room, deodorization unit and necessary circulation and access, etc. The pressurized pipe, valves and the stormwater pumps at the northern portion of the pump house are all installed below ground and covered by multipart covers. Therefore, that portion could be turned to a column-beam frame structure, which is essential for the maintenance of the underground installations. The ground floor is then used as a Tai Chi exercise area with all E&M equipment housed below ground. This area of 320 sq m would be opened to public and be closed for about 2 weeks a year during annual inspection and maintenance period. The underground storage tank of footprint of 2,331 sq m is constructed below ground, above which at ground level and in its adjacent area, a 5700 sq m open space was developed with 110 trees and over 20,000 shrubs planted and comprising a waterfront promenade, a plaza and a pet garden. (Figure 6) The footprint of the above-ground structure of the pump house accounts for about 7% of the total Open Space area. The open space provision was included in the Sheung Wan Stormwater Pumping Station project in order that the amenity facilities could be provided earlier to the community.

Figure 6 – Layout plan of the pumping station, open space and waterfront promenade

Pumping Station Design

To minimize its visual barrier to the waterfront, its is desirable to reduce the height of the pump house above ground level. The height of the pump house is governed by the clear height required under the ceiling to lift up the pumps onto the ground floor for maintenance.

8 This clear height is the summation of the minimum headroom required under the hoisting crane, the depth of the crane and the hoist, the headroom above the crane and under the ceiling. The overall building height of 5.6m for the pump house is considered the absolute minimum to facilitate operation and maintenance of the E&M equipment.

A smaller footprint of the pump house has inevitably resulted in the inlet and outlet pipeworks to be placed close together and the inflow and outflow would follow excessive bents, drops and rises. Undesirable air entrainment or turbulence might occur during the operation of the stormwater pumps. To verify the hydraulic performance of the underground storage tank and the pumping operation, DSD engaged Hong Kong Polytechnic University for constructing a physical and a numerical model and carrying out testings for predicting the flow pattern inside the underground storage tank and surge analysis of the pumping system. Some modifications to the configurations of the underground storage tank were adopted, based on the results of the physical and numerical modelling tests. A comparison of the original scheme and the revised scheme accepted by C&WDC, HEC and Town Planning Board is summarized in Table 2.

Original Scheme Revised Scheme Difference (%) Footprint of Pump 2,489 2,594 +5% House and Underground Storage Tank (m2) Above ground 624 448 -30% footprint required by Pump House (m2) Building height (m) 6.00 5.65 -6% Table 2 – Comparison of original and revised scheme of the pumping station

Special Features of the Pump House

The design of Sheung Wan stormwater pumping station has taken into account the needs to have a high quality architectural building façade, harmonize with the surrounding open space development and waterfront promenade, minimum building bulk and land take without compromising the functionity and ease of maintenace of the stormwater pumping station. In doing so, the pumping station has adopted a very special design and it is the first DSD pump house integrating culture, greening, communication and education elements into a single structure.

The northern portion of the pumping station where all the utilites and equipments are provided underground is opened to public as a Tai Chi exercise area except during the infrequent maintenance period. This northern portion is sheltered to provide shade for public enjoyment and can be used as the venue for arts and cultural exhibition by the side of the waterfront promenade.

9 The roof of the pump house is developed into a roof garden with skylight and other energy-saving features, such as solar LED lights. (Figure 7) The roof garden is opened to group tours by appointment. Visitors can view the operation of the pumping station through the skylight. The green roof also serves to reduce solar heat entering the inteior of the pump house thereby reducing the energy consumption for the air-conditioning system.

Figure 7 – Green roof of the pumping station and Tai Chi Garden

The special approach to the design of the Sheung Wan stormwater pumping station has prompted DSD to produce Guidelines on Aesthetic Design of Pumping Station Buildings, which are to be followed in the design of all pumping station buildings and above-ground ancillary buildings that are visible to the public.

Landscaping Design

The open space has a total landscape site area of 6,000 m2, with over 2,200m2 of planting area for the planting of ornamental trees, palms, shrubs and groundcovers. The landscape is designed in a modern style in complementing the architectural design outlook of the pumping station.

Recycled materials such as recycled glass-aggregate concrete pavers, benches made of re-cycled plastic, proprietary site furniture (which can be dismantled and re-use), etc., are used to minimize the potential construction waste to be generated.

Overall, over 100 nos. of ornamental trees and palms and more than 20,000 nos. of flowering shrubs and groundcovers are planted within the 2,200 m2 of planting area. In general, the plants are not only selected for their aesthetic values, but also for their ability to withstand strong wind and also being tolerant to salt sprays, being located near the sea.

10 Through the mass planting of ornamental trees and shrubs within the large planting areas, a strong landscape greenery character with vibrant colours has been created to provide colour and viewing interest, and at the same time help screening and buffering the open space and users from noise and air pollutions in the vicinity. (Figure 8)

Figure 8 – Landscaping area of the Open Space

Public Consultation and Engagement

The final scheme of the layout for the Sheung Wan Stormwater Pumping Station and the adjacent open space is achieved through the development of partnership between the Government and the public during the planning, design and construction stages. Since the inception of the project, DSD has conducted more than 10 times of consultation meetings, site visits, exhibitions, etc. with C&WDC, HEC, Town Planning Board and the public to develop a scheme acceptable to the community. The success of the pumping station project is the result of public consultation and engagement of all concerned stakeholders.

Performance of the Stormwater Pumping Station after Commissioning

The Sheung Wan Stormwater Pumping Station was completed and started commissioning in March 2009. The effectiveness of the pumping station performance during the 2009 rainy season was monitored and evaluated based on the following aspects:

z Stormwater Pump Operation and Hydraulic Response

z Silt/Trash collected in Storage Tank and Mechanical Bar Screen Chamber

z Energy Consumption

z Inflow Quantity and Quality of Dry Weather Flow

z Gas Content in Storage Tank

During the wet season in 2009, Hong Kong Observatory has issued 20 amber rainstorm warnings, 2 red rainstorm warnings and typhoon signal No. 8 was hoisted 3 times during the year. The pumping station was shown to capably handle the runoff collected and no flooding was reported in the Sheung Wan low-lying area.

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Referring to the record, only 1 stormwater pump was in operation to cater for most of the rainstorms in year 2009, except on 14-15 September 2009 and 18-19 July 2009, when 2 stormwater pumps were in operation to the most. On 14 and 15 September 2009, Typhoon Koppu brought to Hong Kong more than 200mm rainfall and tide surge reaching +2.86mPD (compared to +2.64m PD, the lowest point of Sheung Wan low-lying area). Two stormwater pumps were triggered with total operation time of 83 minutes at that time (Figure 9). During the passage of Typhoon Molave on 18 and 19 July 2009, red rainstorm warning was in force with 150mm rainfall recorded. Again, two stormwater pumps were triggered with total operation time of 35 minutes during that time (Figure 10).

Figure 9 – Graph showing pump operation and water level inside the underground storage tank during Typhoon Koppu

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Figure 10 – Graph showing pump operation and water level inside the underground storage tank during Typhoon Molave

The average monthly expenditure for operation of the pumping station was found to be $40,000, covering mainly the electricity charge, service charge for telecommunication and cost of consumables for odour control, which is very close to the estimation during the design stage. All in all, the planning and design of the flood relief scheme was found to be successful and the records collected during the post-commissioning was very valuable to the planning and design of future flood allevation project in urban areas.

Conclusion

The works pertaining to the relief of flooding risk in the Sheung Wan low-lying area have called for non-conventional approach, which include construction of underground stormwater storage tank and interecepting drains. Locating potential site for the construction of infrastucture in develped urban area is not an easy task. When the stormwater pumping station and the underground stormwater storage tank would occupy a water-front site earmarked for open space development, extensive public consultation and statutory approvals are necessary. However, it provides a good opportunity for devising a design which would integrate the stormwater pumping station with the adjacent open space. Steps were taken to shorten the overall construction time, by contract strategy for instance, to balance off the longer time required for the public consultation. The design of the Sheung Wan flood relief project commenced in early 2004. The Town Planning Board application for construction of the stormwater pumping station within the Open Space site was approved in December 2005. The construction of the Queen’s Road Intercepting Drains commenced in March 2006 and completed in June 2008. The construction of the pumping station commenced in June 2006 and the pumping station started operation after 33 months in March 2009. The landscaping works were completed in September 2009 and the Open Space was open to public in

13 November 2009. The community is now provided with drainage infrastructures for protecting life and property and a landscaped area for leisure and recreational use.

This project demonstrated how civil engineering works could be implemented sustainably, more society-friendly and addressing the society’s functional needs as well as public interest on greening and concern in climate change.

References [1] Stormwater Drainage Manual – Planning, Design and Management. 3rd Edition. Drainage Services Department, the Government of Hong Kong Special Administrative Region, 2000

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