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Blue-green drainage infrastructure - Rain gardens

The Greening, Landscape and Tree Management Section (GLTMS) of the Development Bureau (DEVB) steered the development of the first publicly accessible rain garden in 2017 by transforming an existing traffic island at the junction of Wylie Road and Princess Margaret Road. This was made possible through the collaborative efforts of the Civil Engineering and Development Department, the Drainage Services Department (DSD) and the Leisure and Cultural Services Department. In 2020, DSD completed two additional pilot rain garden projects on existing roadside in Chai Wan and Tai Po as well as another one within its facility in Shatin.

 

As one approach to the blue-green drainage infrastructure, a rain garden is a designed basin with various drainage layers of multiple particle sizes. Suitable species of vegetation are planted in the rain garden to collect rainwater. This provides opportunity to harvest, cleanse and infiltrate the collected rainwater from impervious urban areas including driveways, walkways and parking lots, before it is discharged to a traditional drainage system or natural water bodies. As a result, pollution loading, runoff rate and volumes are substantially reduced.

 

In addition to key considerations such as sunken topography, soil composition and plant selection, a crucial factor is site location. The performance of rain gardens, particularly those to be built in the city areas, is affected by the existing environment imposing site constraints. Existence of underground utilities plus location and level of the existing downstream underground drainage system may limit the available depth of the basin and soil, thereby posing key challenges for constructing rain gardens in urban areas.

 

Catchment size and water quality are also important design considerations. Experience of DSD in implementing its pilot rain garden projects confirms that variations in design are warranted in light of site-specific conditions. For example, perforated pipes are installed in the rain garden in Chai Wan, but not in the Tai Po rain garden, which is instead designed to infiltrate runoff into the ground.

 

For landscape design in Hong Kong, apart from the ecological value, attention must be given to selecting plant species that effectively remove pollutants, tolerate wet and dry conditions, require low maintenance and look aesthetically pleasing. For the rain gardens in Tai Po and Shatin, a wet zone planted with Cyperus involucratus (風車草), Phragmites australis (蘆葦) and Eleocharis dulcis (馬蹄) is sandwiched between a dry zone with planting of Alternanthera dentate f. rubiginosa (紅龍莧), Rhaphiolepis indica (石斑木) and Cuphea hyssopifolia (細葉萼距花) on the two sides of the gardens.

 

While DSD is reviewing the performance of the above pilot projects so as to optimise future rain garden designs, GLTMS sees opportunities for a wider application of blue-green infrastructure concept in all projects, including the abovementioned small rain gardens in urban context, river revitalisation, and open space design. Construction professionals are encouraged to incorporate the blue-green infrastructure elements into projects in making our city landscape more sustainable and resilient to extreme weather conditions.

 

This article is contributed by the Development Bureau of the HKSAR Government


District Cooling System at Kai Tak Development reaches two significant milestones

The District Cooling System (DCS) at the Kai Tak Development is a large-scale energy efficient infrastructure that is the first of its kind in Hong Kong. Construction works commenced in 2011 and the DCS has been completed and commissioned by phases. After ten years of construction, the ring circuit of the chilled water pipework has recently been completed. With the ring circuit in place, chilled water to user buildings may be supplied from different directions depending upon the situation, thereby enhancing the reliability of the DCS. The Electrical and Mechanical Services Department (EMSD) will continue to provide excellent and reliable district cooling service to users.

 

In light of an increase in the development density of the Kai Tak Development as announced in the 2017 Policy Address, EMSD has commenced an additional DCS project with the support of the Environment Bureau. On top of the cooling capacity of 284 megawatt (MW) of refrigeration provided by the existing North and South DCS Plants, the additional DCS will provide further cooling capacity of 178 MW. Upon full utilisation, the additional DCS will save an estimated 53 million kilowatt-hour of electricity per year, corresponding to annual reduction of approximately 37 000 tonnes of carbon dioxide emissions.

 

The above marks two significant milestones in the development of the DCS at the Kai Tak Development. EMSD, together with the consultants and the contractors, jointly organised two ceremonies on 19 April 2021 - Works Commencement Ceremony for the Additional District Cooling System and Chilled Water Pipework Ring Circuit Completion Ceremony. Both ceremonies promoted the DCS as an infrastructure for energy efficiency and sustainable development. At the ceremonies, the Director of EMSD, Mr Eric Pang, welcomed as Guests of Honour the Secretary for the Environment, Mr Wong Kam-sing; the President of The Hong Kong Institution of Engineers, Ir Prof P L Yuen; the Commissioner for Sports, Mr Yeung Tak-keung; and student representative, Miss Chiu Ching-yan, from SKH Holy Cross Primary School. Mr Pang also expressed gratitude to Ir Dr Hon Lo Wai-kwok for his congratulatory video messages at the ceremonies.

 

This article is contributed by the Electrical and Mechanical Services Department of the HKSAR Government

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