Hong Kong, located in a humid subtropical region with a marine subtropical monsoon climate, is on the common track of tropical typhoons and has one of the highest rainfall in the Asia Pacific region with an average annual rainfall of about 2,400mm. As global climate change continues, better adaptation, particularly in urban areas where impervious surfaces are more prevalent, is needed to reduce the peak load in stormwater management.
WSP collaborated with Ronald Lu & Partners to help tackle these challenges to enhance Hong Kong’s flood and climate resilience. We provided water environment consultancy services for Architectural Services Department (ArchSD) of Hong Kong on the use of sponge city concept and technologies for Hong Kong government buildings.
City acting like a sponge
Sponge city is an urban construction model for flood management first proposed in China. Similar concepts and practices in other parts of the world include low impact development (LID), water-sensitive urban design (WSUD) and sustainable urban drainage systems (SUDS).
As the term suggests, the city will act like a sponge during rainy days where rainwater could be absorbed, stored, filtered and cleaned, and be used as needed to enhance ecological function and reduce flooding in the city. When effectively implemented, it would reduce the need for largescale artificial drainage channel works.
Learning from international cases
To better understand how sponge city concept could benefit Hong Kong, we looked into eight successful cases from mainland China, Hong Kong and overseas in terms of hydrologic regime, demographics, government structure, and geographic and political systems. The cases were most relevant to Hong Kong in terms of its tropical climate, high water-table and being a coastal territory. They also used different techniques and strategies to implement sponge concepts.
Through investigations, we identified the key opportunities and barriers such as the reasons and importance of integration between urban stormwater management, and urban planning and design. We analysed the data following the same framework and compared the results qualitatively and quantitatively.
Hydrological analysis through LID
We considered hydrological factors in modelling simulation and sponge city concept design, comparing the hydrological characteristics of post-development and pre-development through tracking the runoff volume, peak runoff, runoff duration and non-point source water quality.
We also applied Storm Water Management Model (SWMM) software to simulate LID control types represented by a combination of vertical layers for verification and optimisation. The LID controls include bio-retention cell, rain garden, green roof, infiltration trench, permeable pavement, rain barrel, rooftop disconnection and vegetated swale.
Achieving international sustainability standards
The adoption of sponge city concept in buildings means incorporating requirements from pertinent local and international green building certifications for design and planning, construction and maintenance of relevant features. It also contributes to achieving more sustainable architecture, using natural processes and materials, discharging less polluted water, avoiding waterlogging, preserving and supporting biodiversity and so on.
We discovered that sustainable rainwater management in buildings is promoted by the following green building labelling schemes: BEAM Plus, LEED (v4 and v4.1), Sustainable Sites Initiative (SITES) and RELi, which are credit-based rating systems to evaluate features against various categories.
Designing for urban areas
Through this study, we provided the authority a conceptual idea of how sponge city concept can be integrated in the urban context on design, construction and maintenance, which could be further developed, enhanced and adapted to sites, projects and budgets.
The planning and design stage involve the calculation of rainwater catchments and the formulation of a combination of the different techniques. The optimal combination of sponge city concept features depends on the coordination of landscape architects, urban planners, regulators, engineers and hydrologists to assess site conditions, rain patterns, agree objectives and maintenance duties.
Maintenance, on the other hand, requires adaptive response to the environmental changes and human activities. Based on construction, track and record the operation and maintenance of various features, designers will be required to review the effectiveness of those techniques and lengthen the lifespan of the sponge city features.