Introduction
This project has been developed in response to the request for proposal on developing solution of an environmental problem.
As the weather gets warmer due to global warming, Singaporeans tend to rely on cooling appliances for their own purposes. This contributes to higher air-conditioning demands, which give rise to the increase in greenhouse gaseous emissions. As a result, a vicious cycle is formed which would further aggravate the issue should there be no actions taken.
The rise in urbanisation along with variations in the man-made global warming effects lead to the increase of the temperature. According to Meteorological Service Singapore (MSS) (n.d.), “Eight of the ten warmest years on record in Singapore have occurred in the 21st century and all the ten warmest years are since 1997.” The surface air temperature has increased at an average of 0.25°C per decade between 1948 and 2017. The graph of the annual surface temperature is shown in Appendix 1.
From the graph, it is clear that the rising of temperature is an issue of concern and is in dire needs of a solution to address the UHI effect in Singapore. The UHI effect will continue to exacerbate the frequency of heat waves. Residents from the vulnerable age groups such as children, elderly, and those with existing medical conditions, are at a particular risk.
Apart from affecting the residents, the rising of temperatures also concerned the authorities in which the government is currently diverting its attention to. According to Parliament Singapore (2019), the Environment and Water Resources Minister Masagos Zulkifli addressed the need for mitigating the UHI effect in Singapore. One of his suggested measures was to integrate open spaces with greenery “to cool our island” through “shade and evapotranspiration.”
With the aim to incorporate greenery in open spaces and reduce surface temperature with evapotranspiration, green roofing would be a viable element. Green roofing is a method of planting vegetation on the roof of a building. The plants of the green roof are able to absorb carbon dioxide, one of the most abundant greenhouse gases, through the process of photosynthesis. As the plants reduce carbon dioxide from the environment, it produces fresh air and improves the quality of air. In addition, as plant experiences transpiration, it releases evaporated water which cools its surrounding environment. This brings down the need for air-conditioning. The previous sentence could be supported by Norquist (2012) that “a green roof would help reduce the air-conditioning demand.” Apart from providing a cooling effect and releasing fresh air, green roofs helps to reflect solar radiation which further reduces the UHI effect.
The pie chart in Appendix 2 represents the number of HDB car-parks that exists in Singapore. Out of the total of 2108 car-parks, 2% of it is represented by “others” which includes basement, covered, and mechanised carpark. Most of the car-parks in Singapore are the surface car-parks (51%) and multi-storey car-parks (47%). Since multi-storey car-parks occupies almost half of the total amount of car-parks in singapore, with the fact that most of the rooftops are usually under-utilised, the implementation of retrofitting green roofs on the vacant spaces would create a considerable impact in reducing the UHI effect.
From the team’s observation, it is noticeable that most rooftop of HDB multi-storey car-parks are empty. This could be supported by Figure 1, which depicts two adjacent MSCP. Retrofitting green roof on the under-utilised space, not only would maximise the space utilisation on the rooftop, more importantly it would be able to reduce the effects of UHI in the estate. The green roofs would also enhance the aesthetical value of the estate and cool the environment with the method of evapotranspiration.
Figure 1. Rooftop of a HDB MSCP to photo taken by Syahiran
Problem Statement
Green roofs should be installed at every open rooftop of HDB multi-storey car park to reduce the temperature around the surrounding vicinity and add aesthetic value too. However, there are still a lot of under-utilised multi-storey car parks, especially at the rooftop. Not only does the roof garden at existing multi-storey car parks help to tackle the UHI effect, it also mitigates the issue of land scarcity. This would create a conducive environment for residents in the estate.
Purpose Statement
The purpose of this report is to propose to the estate management department of Housing Development Board (HDB) to adopt green roof at existing under-utilised multi-storey car parks at the older estate. The implementation of this idea will help to mitigate the UHI effect and further beautify the estate.
Proposed Solution
The team’s proposed solution to mitigate the UHI effect in Singapore is to retrofit green roof at under-utilised top storey of HDB multi-storey car park. Green roof relies mainly on sunlight and water to reduce the surrounding temperature by evapotranspiration and increases the aesthetical value of the estates. There are two types of green roofs that our team would like to propose, namely the extensive and intensive green roof system.
The selection of the different types of green roof to be retrofitted depends on the structural integrity and the budget allocation of each car park.In an interview with Nurul Sharfirin, a former Higher Estate Executive of HDB, different estates or region will have different budget depending on its size and maintenance. Most of the time the Town Council (TC) will be maintaining the MSCPs and not HDB. HDB, TC, and governance, risk management and compliance (GRC) would probably have a joint meeting to discuss on the issues or concerns that may arise from the project.
Extensive Green Roof
Extensive green roof is the lightest type of green roof. It normally weighs between 74 to 122 kg/m2. This greatly reduces the amount of structural change that is required for the car park. The weight of the soil is in proportion to its growing medium depth which ranges from 3 to 5 inches deep.
Drought-resistant plants such as coneflower, lavender and grasses are some of the plants used due to them having shallow root system which requires minimal water. The variety of plants are kept to a minimum to ensure that every plant has similar moisture absorption content. They also require limited fertilisers. During extreme dry season, this design may turn brown if there is not enough irrigation. This design is the easiest to install and limits the access as minimal maintenance is required.
Intensive Green Roof
An intensive green roof can be installed at a better structural integrity of a building as it is much heavier compared to extensive green roof. The soil has a depth of 6 inches deep or more and weighs roughly around 100 kg/m2 when it is dry and 150 kg/m2 when fully soaked. In order to install this design, the weight factor needs to be considered thoroughly as it is dependent on the current strength of the car-park. One of the crucial factor is to consider the irrigation and drainage system as they may overload the roof’s structure.
With a deeper soil medium, this system allows the plantation of the different types of trees or plants of shape and sizes. Normally, the roof will include benches, walkways and shelters for the public to rest or even interact with the natural features.
However, this design requires more maintenance due to the variety of plants. This will lead to an increase of pests and weeds. In addition to that, fertilizers, and amount of water or irrigation needed plays a huge part in maintaining this design. It typically stays more appealing during the dry weather.
Benefits of Proposed Solution
There are many benefits of the proposed solution such as reducing surface temperature, improving surrounding air quality, reducing surrounding noise, managing stormwater, ecological benefits and social benefits.
Reducing surface temperature
The green roof reduces the surface temperature at the estate, provides thermal comfort to the residents and absorbs fewer solar radiations acting as a buffer in between the sun’s radiation and the concrete surface of multi-storey carpark rooftop.
Improving surrounding air quality
The green roof captures harmful fine dust particles from the air and improves the quality of the air.
Reducing surrounding noise
The green roof absorbs the sound waves and reduces the noise level significantly as compared to traditional rooftop caused by the vehicles in the carpark.
Improve stormwater management
The green roof holds the water for a period of time to reduce or delay the flow of water to the drainage system. It also helps evaporate the rainwater, by releasing it back into the atmosphere instead of drains and sewers.
The green roof enhances the aesthetic of the estate and reduces the sunlight glare as it does not reflect it.
The green roof provides relief from the concrete surface of the rooftop. The presence of green has a relaxing psychological effect on the residents. It also substantially enhance residential property value to the estate.
Proposal Evaluation
In this section, the challenges as well as the feasibility of the proposed solutions will be discussed.
The proposed solution of installing extensive and intensive green roofs to mitigate the UHI effect is achievable due to the strategic location of the MSCP which is surrounded by the tall HDB flats. The map of the MSCP is shown under Appendix _.
Concerns of having a Green Roof:
Even though this proposed solution could help to mitigate the UHI effect, there are a few concerns that need to be considered. These primary concerns includes structural limitations, cost, and maintenance.
As seen from the image _ from Appendix _, an average car-park can contain 74 numbers of cars per storey and the average weight of a car is estimated to be 2 tonnes.
Extensive green roof does not pose any weight problem since it is light. However, intensive green roof could alter the structural weight of the car-park due to some of its properties. The planting medium of this design ranges in between 6 to 18 inches deep and subsequently, the weight of the plant increases after a period of time. During the wet season, this design could potentially be fully soaked which further stresses the loading capacity of the car-park. Since the car-park rooftop has a flat surface, planting the green roof would not cause a problem.
Although green roof helps to reduce the energy cost in the surrounding, it is rather costly in installing and maintaining the green roof. The table below shows some of the factors that needs to be considered.
Installation of Green Roof
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Maintenance of Green Roof
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Purchasing of different types of soil and plants
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Pesticide and herbicide
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Purchasing of planting materials such as moisture blanket
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Repairing cost of leakages from water tank or sprinkler
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Equipment
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Replacing of dead plants
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Irrigation and fertilizers
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Inspection of condition of green roof and also stagnant water
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Transportation
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Hiring of green roof professionals
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Maintaining the green roof is a time-consuming and tedious job as it ensures sufficient irrigation and fertilizers for the plants to grow well. In addition, daily inspection is required to check for any types of damages to the plants or leakages from the water tank or sprinklers. Pests and weeds will also affect the growth of the plant. If the green roof is not maintain regularly, it would result in an increase in plant losses and the cost of replacing it would be significantly high. Furthermore, the green roof may turn into an unhygienic place and untidy if the condition is not maintain properly.
Methodology and Procedure
Primary Research
The team conducted an interview with Associate Professor Dr Kum Yung Juan, to understand the challenges and limitations of retrofitting a green roof. In terms of engineering, it was concluded that there did not exist any restriction in retrofitting a green roof in the rooftop of a multi-storey car park.
At this juncture we were clear that in the aspect of retrofitting the green roof on existing multi-storey car park it was possible. We were not sure of the bureaucracy and procedure of HDB. In order to understand how HDB works when involving estate management of buildings and amenities. They conducted an interview with Ms Nurul S, who was a former HDB and gave us the inside on how HDB plans for an estate, for this case how HDB maintains existing older estates multi-storey car parks.
Secondary Research
The main source of our secondary research is from NParks (n.d.). This website provides information on skyrise gardening, planning, design and safety considerations, types of plants to consider, frequently asked questions and case studies of other projects that adopted the green roof concept. Additional sources include online publication, handbooks and websites which supported the contents in this report are Bringing Greenery Skywards (2017), Guidelines on Planting of Trees, Palms and Tall Shrubs on Rooftop (2012), Frese, Z. (2016), Mithraratne, N. (2013) As for figures, they are sourced from Meteorological Service Singapore (n.d.).
Findings
The team surveyed 10 multi storey car parks at different estates during different periods of the day. Cars were seen parked on the rooftop of only 2 out of 10 multi-storey car parks. Of which, less than 5 cars were seen on the 2 rooftops that contained cars. From there it was concluded the multi-storey car park is typically under-utilised. Retrofitting the rooftop of the multi-storey carpark with green roof would be a viable solution since nearly half of the HDB car parks consist of multi-storey car park, which could be seen from the chart in Appendix B. This not only mitigates the UHI effect, it also does not cause further strain on the land scarce problem that Singapore is facing.
Conclusion
The UHI effect is a reciprocal effect of rising surface air temperature. The lack of natural landscape on buildings lead to an increase of the surface air temperature as heat get trapped in between buildings. Since urbanisation is projected to accelerate in the coming years, mitigation measures must be implemented to curb the UHI effect. With the presence of green roof on MSCP, it can significantly reduce the UHI effect in the estate. Not only does the proposed idea of retrofitting a green roof in HDB MSCP reduces noise, improves air quality, manages stormwater, provides ecological and social benefits, it also reduces the surface temperature through evapotranspiration which eventually reduces the UHI effect in the estate. This creates a beneficial meaning to both the community and environment.
