Green Roofs: Increasing Hong Kong’s Resilience

By Anna Zakrisson 20 September, 2022

You've probably heard of green roofs but what about purple roofs? Dr. Zakrisson, VP at GRD, highlights the various benefits of these roofs from tackling heat, flooding & a clear ROI

Meadows act as a natural workhorse providing cooling, stormwater mgmt & increased biodiversity; working with nature is often wiser than working against it
Vulnerable groups esp. at risk due to HK's high building & population density exacerbating urban heat island effect; plus, impermeable asphalt roads prone to flooding
Opportunity to cool & re-establish natural water cycle in HK lies in rooftop greening; ‘Purple-roofs’ free up land + provide benefits of green roof = clear ROI

The Hong Kong Observatory reports that local precipitation levels are expected to rise by 4-6% and mean temperature by 1.4-2.4°C by 2050. This will have severe consequences for the city. Climate adaptation strategies such as green roofs can be implemented to address these challenges.

The Power of Green

Have you ever run barefoot over a cool, lush meadow only to step over to the scorching asphalt on the road next to it? It can really burn your feet! Stones and asphalt heat up while the vegetation remains cool and pleasant…

Meadows are a natural workhorse providing stormwater mgmt & increased biodiversity…

…asphalt comparatively is imperviable

Imagine now that it begins to rain, heavy rain which is easily absorbed into the meadow’s soil layer; the plants suck up parts of the rain shower with their extensive root systems. The rest of the water slowly trickles through the meadow over sticks and stones, reducing the flow rate and minimizing erosion damage. On the contrary, on the imperviable asphalt, the water has nowhere to go and starts running at a fast tempo and into the gutters, gushing down to the lowest laying point where it accumulates in a pool.

Not only is the meadow beautiful, but it is also a natural workhorse and provides a whole range of secondary benefits such as cooling, stormwater management, and increased biodiversity. Working with nature is often wiser than working against it, which is why the introduction of green infrastructure solutions such as green roofs are so critical for our sealed, concrete urban jungles. Let’s bring back nature to our cities and reap the benefits in many areas! But let’s do it smart.

Our Concrete Jungles

Currently, 55% of the world’s population lives in urban areas, and this number is expected to rise to 68% by 2050. Simultaneously, we are faced with dire climatic challenges ranging from increased temperatures but also increased precipitation and flooding.

Urban heat island effect can have serious consequences, especially for HK’s vulnerable groups…

…and then there is flooding

Like the meadow and the asphalt road, there can be stark temperature differences between a city and the surrounding countryside areas. This urban heat build-up is often referred to as the urban heat island (UHI) effect and can have serious consequences. The extremely high building and population density in Hong Kong exacerbate the vulnerability to extreme heat events, especially for already vulnerable groups such as the diseased, the very young and the very old. A mere 1°C increase in mean temperature above 28.2°C is associated with a 1.8% increase in mortality1. Furthermore, the UHIs also exacerbate air pollution such as ground ozone.

However, not only heat is an urban issue, but water too since the natural water cycle has been interrupted through the sealing of the land. Just as with the example of the asphalt road above, cities are highly prone to flooding. In addition, extreme rain events are becoming more frequent, regularly causing the stormwater sewers to overfill, leading to loss of lives and livelihood. Furthermore, the erosion can cause irreparable damage to the landscape and its functionality.

Green Roofs as Solutions

One intuitive solution to tackling urban heat islands, flooding problems, and biodiversity losses is to add back greenery to the city. I agree, but where? Hong Kong is an extremely densely populated area, and land is expensive. There simply isn’t enough space available for the green infrastructures needed. At least not on the ground. Rooftops, however, are often forgotten and neglected areas with plenty of possibilities for lightweight, vegetated solutions.

Opportunity to cool & re-establish natural water cycle in HK lies in rooftop greening

A green roof is a roof covered in vegetation; it can perform the needed functionality for the urban settings, such as cooling and re-establishing the natural water cycle, without taking up space on the ground. There are many different types of green roofs on the market that are suitable for various locations and situations. Some of the green roofs are retention-only types of green roofs and they provide both cooling and biodiversity (apart from the purely aesthetic aspects, of course). Retention describes the volume of water that can be taken up by the plants from the soil and returned to the sky as vapor. This is also the process that cools the plants.

Green Roof Detention – Green Roof ROI

However, sometimes, not all water can be taken up by the plants, and some of it becomes runoff. This often happens if it has been raining several days in a row or if there is an extreme storm event, such as during the monsoon, causing the roof to be fully saturated. This extra water often leaves the roof at high velocities and needs to be slowed down. Hence, for most projects stormwater tanks and cisterns are built to hold the water temporarily and then release it at a predictable and known outflow rate, e.g., 10L/s/ha. This delay is referred to as stormwater detention and is a means of flooding prevention. The delay prevents the sewers from overfilling and can sometimes even meet the ground percolation rates. This can now also be done using green roofs.

New ‘purple-roof’ can free up land by replacing at-grade stormwater solutions + benefits of a green roof = clear ROI

Purple-Roof is a new generation type of engineered lightweight detention plus retention green roof concept suitable for flat or sloped roofs and can replace or reduce at-grade stormwater solutions. Hence valuable land can be freed up while at the same time adding vegetation, cooling, and biodiversity. The freeing up of at-grade space can generate a clear return on investment (ROI) for the project, making it an ecologically and economically sustainable solution. It is an excellent example of working with nature instead of against it.

The outflow from a Purple-Roof compliant system is passive, predictable, and can be modeled. Furthermore, the concept can be adjusted based on the local requirements since the difference in rain patterns vary hugely across the world. The concept is already in use in the USA and Europe.

Let’s green the world!

Further readings

  • Hot, Thirsty, Sweaty & Wet: HK’s Future Down The Drain? – China Water Risk’s Woody Chan & Debra Tan look beyond current tensions and see very real threats to Hong Kong’s future from climate change. Get ready for a hot, thirsty, sweaty & wet future
  • Building Flood Resilience For Hong Kong – HK is the rainiest city in the Pacific Rim and with the threat of climate change, it’s heading for a wetter future. The Drainage Services Department’s senior engineer Patrick Chan shares the city’s strategies to improve flood resilience
  • Hong Kong: A Climate Resilient Sponge City – Hong Kong is prone to tropical cyclones, which will be exacerbated by climate change. Can the city become climate & flood resilient? Hear from Richard Leung from the Drainage Services Department on actions taken so far
  • Sponge City Is Transforming Urban Flood Management – Sponge city was launched to combat urban flood risks yet public has doubts over its effectiveness. What does the Chinese govt do to promote the policy? What role does social media play? Dr. Faith Chan, Dr. Dimple Thadani & Lei Li break it down
  • Designing Resilience – 2 Architectural Students’ Take on Coastal Threats – Shocked by HK’s coastal threat, HKU’s Fergal Tse & Oscar Wong became CWR’s interns to re-design Victoria Harbour. We sit down with them to understand what local youths think about climate change and & their projects with CWR changed their perspective.

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Anna Zakrisson
Author: Anna Zakrisson
Dr. Anna Zakrisson is a biologist researching and consulting for green infrastructure and future urban sustainability. Zakrisson is VP of European Affairs at the independent research institute Green Roof Diagnostics (GRD) and has degrees from world-leading universities such as the University of Cambridge, Max-Planck Institute, and a Ph.D. from Stockholm University. Dr. Zakrisson resides in Berlin, Germany.
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