Green Roofs in Urban Areas
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This entry is adapted from the peer-reviewed paper 10.3390/land12101831

The presence of green roofs in urban areas provides various ecosystem services that help mitigate climate change. They play an essential role in sustainable drainage systems, contribute to air quality and carbon sequestration, mitigate urban heat island, support biodiversity, and create green spaces supporting public well-being. Bus stops provide good opportunities for installing green roofs. 

blue-green cities ecosystem services green infrastructure cost benefit analysis green roof

1. Introduction

The installation of green roofs (GRs) in cities contributes to the development of a healthy blue-green infrastructure network [1], which is indispensable for sustainable urban development [2][3]. GRs are partially or fully vegetated roofs that extend the conventional roof by a waterproof membrane (with the possible addition of a root barrier), a drainage layer, a filter layer, a lightweight substrate, and vegetation (Figure 1) [4].
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Figure 1. Structure of a typical green roof.
The fundamental division of GRs according to substrate depth and plant species recognises extensive (EGRs) and intensive green roofs (IGRs). EGRs are specified by shallow mineral-based substrate supporting low plants such as Sedum spp., moss, wildflowers, or grass, which make them well-suited for bus shelter applications. Bus shelters provide unused roof spaces that can be transformed into additional green urban infrastructure. This research gives a brief overview of the benefits provided by EGRs. It presents a case study comparing the benefits and costs of their installation on bus shelters in Edinburgh.

2. Benefits of Green Roofs in the Urban Context

GRs provide a number of environmental and social benefits. Substrate and vegetation are the essential and the most understood structural layers of GRs regarding the benefit provided.
Substrate captures rainwater, reduces the runoff volume, and delays its peak. Warmer and wetter weather with increased heat islands and frequent heavy rainfalls is predicted for the east of Scotland. Between 1981–2010, the frequency of heavy rainfalls increased by 5% compared to the years 1961–1990 [5]. GRs applied on roofs in urban areas have an essential role in Sustainable Drainage Systems (SuDS), being one of the leading stormwater management solutions [1][4][6][7]. GRs in urban settings can simultaneously achieve three main aims of SuDS—reduce the quantity of runoff and delay its peak, improve runoff quality, and provide good amenity values [6][8][9][10][11].
Vegetation is also essential in erosion prevention and evapotranspiration, providing a greater surface and influencing initial substrate moisture [12][13][14]. The vegetation layer mitigates urban heat island (UHI) through increased evapotranspiration, increased albedo, and reduced direct sensible heat emissions [15][16]. Roads are among the hottest areas in cities due to their low albedo [17][18][19]. It has been concluded that GRs might be more efficient when their height is lower than 10 m [20], making city bus stops an ideal location.
The green roof hydrological cycle is prosperous for both stormwater management and evapotranspiration’s cooling effects [21]. According to various authors, the water content in soil is the strongest factor for evapotranspiration [22][23][24][25][26]. Feng et al. [27] found that increased water content in the substrate from 30% to 60% reduced heat stored in GR by 24%. This agreed with studies by Coutts et al. [28] and Wong et al. [20], showing limited evapotranspiration rates and cooling effects of GRs after a dry period. Water content in leaves may also cause higher reflectance [29].
In addition, the vegetation layer improves air quality by direct pollutant sequestration and lowering air temperature, which results in a reduction in air pollution creation, thus preventing ozone formation [4]. Plants provide a surface area for the wet and dry deposition of pollutants [30][31]. Particles that are not captured by leaves can be absorbed by cuticle digestion, stomata penetration, or deposited in soil and absorbed by roots [32][33][34]. Gaseous pollutants are sequestered by vegetation through their stomata [34]. The roof type and planted vegetation also control the sequestration of CO2 [35]. Generally, GRs can reduce CO2 through direct sequestration or indirect shading effect and evapotranspiration, lowering ambient air temperature and electricity demand [36].
One of the main reasons for GR installation in urban areas is the creation of new habitats and biodiversity support. With the spread of GRs in the 20th century, more profound research focused on their design, rainwater retention, and insulation benefits. However, there is still a lack of studies targeting GRs’ importance in biodiversity, habitat support, and the long-term monitoring of fauna and flora behaviour and establishment [37][38]. Together with the substrate layer, the vegetation layer creates new habitats for lost urban biodiversity and supports pollinators [1], which are threatened by the expansion of urban conglomerates [7]. Furthermore, it can ease the movement of wildlife and deliver measurable biodiversity net gain (BNG) [39]. This is especially vital in the context of new policies requiring 10% of BNG in any new urban planning project in the UK [40].
In addition to all of the environmental benefits GRs deliver, improving previously unused roofs makes people happy and more inclined to care about nature [41]. GRs have an important role in adding aesthetics to highly built-up areas and increasing the value of surrounding properties by 9% [42]. Furthermore, therapeutic benefits were measured from flower terpenes, colours, and sound variations that GRs provide [39][43]. One of the main barriers to implementing GRs is the lack of public knowledge [44]. However, when implemented, GRs in urban areas might be a great opportunity to raise awareness of their benefits and encourage further implementation [45].

3. Summary of the Case Study

Different cities have adopted the law of implementing at least partially vegetated roofs on new buildings [46]. Utrecht was the first city to implement green bus shelters, installing 316 of them. Different cities in the Netherlands (Haarlem, Gouda, Apeldoorn, Woerden, Wageningen), Leipzig in Germany, and Helsingborg and Malmö in Sweden are following this trend. In the UK, Leicester has installed 30 GRs on bus shelters; Milton Keynes, Manchester, Newcastle, Cardiff, Oxford, and Brighton have installed GRs on some of the bus shelters as well (Figure 2) [47]. The majority of these projects are funded by grants and funds with no additional costs to the city councils [48].
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Figure 2. Green bus stops across cities: (a) Utrecht [49], (b) Leicester [48], (c) Manchester [50], (d) Milton Keynes (Ch. Bridgman, personal communication, 21 January 2022).
Despite the global expansion of GRs integration into urban architecture, there is a lack of studies based on solid scientific analysis of the costs and benefits involved [1], which was identified as the main knowledge gap. This research’s objective is to provide a comparison of the costs and benefits of EGRs application in the City of Edinburgh with suggestions for its green future as part of the City Plan 2030. The results of the research show that the application of EGRs on bus shelters in Edinburgh would have the highest benefit value in education, amenity value and lowering air pollution, and may outweigh the associated costs. This will help the city transition to net-zero carbon emissions by 2030 and contribute to the creation of thriving green spaces as part of Edinburgh’s 2050 city vision [51]. Moreover, it will benefit initiatives such as Cleaner Air for Scotland—The Road to a Healthier Future [52] and bring opportunities for increasing BNG in new and existing infrastructure.

References

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