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Potential assessment and implementation strategy for roof greening in highly urbanized areas: A case study in Shenzhen, China
Cities 95 (2019) 102468
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Potential assessment and implementation strategy for roof greening in highly urbanized areas: A case study in Shenzhen, China
T
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Wuyang Honga,b, Renzhong Guoa,c, , Hao Tangb a
School of Resource and Environment Science, Wuhan University, Wuhan 430079, China Shenzhen Urban Planning and Land Resource Research Center, Shenzhen 518034, China c Research Institute for Smart Cities, Shenzhen University, Shenzhen 518060, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Roof greening Spatial recognition Potential assessment Building Green roof retrofit Shenzhen
Greenery on buildings is an innovative approach used to solve urban and architectural environment problems, also served as a key element of the urban green space system. As a typical highly urbanized area in China, Shenzhen is densely populated and built up, making it short for greenery land, which means promoting roof greening is of great importance. This paper establishes an evaluation method for roof greening constructability by distinguishing existing buildings and planned buildings. Based on the results of a potential assessment, an implementation strategy is derived from two aspects: space and time guidelines. The potential suitable areas for roof greening of existing and planned buildings are 859.76 ha and 1039.64 ha. Based on potential assessment, this paper demarcates 32 key areas for roof greening and clarifies the construction modes of key areas promote the roof greening scale effect. Finally, the paper establishes a strategy of priority order for implementing roof greening according to the type and characteristics of buildings. The particularity of inventory development in highly urbanized areas has been taken into consideration when constructing a method for evaluating roof greening. The space and time guidelines can provide a reference for effectively promoting project implementation, improving sustainable urban development.
1. Introduction
urban biodiversity with additional aesthetical value. It can also mitigate the urban heat island effect, boost energy saving, improve urban air quality and facilitate storm-water management (Ascione, Bianco, Rossi, Turni, & Vanoli, 2013; Cookpatton & Bauerle, 2012; Karteris, Theodoridou, Mallinis, Tsiros, & Karteris, 2016; Rowe, 2011; Speak, Rothwell, Lindley, & Smith, 2013; Williams et al., 2010). Since the 1960s, the European countries, mainly represented by Germany, have begun to study project parameters and technologies for large-scale integrated vegetation in architecture (Besir & Cuce, 2018; Jaffal, Ouldboukhitine, & Belarbi, 2012). Remarkable effects have been shown so far in countries such as Singapore, the United States, Germany, and Japan, as roof greening appears in matters of diversification, scale and legislation. Furthermore, green roof construction in China is smooth and stable (Xiao, Lin, Han, & Zhang, 2014). Metropolitan cities like Beijing, Shanghai, and Shenzhen, have taken the lead in systematic research that mainly focuses on engineering promotion and technical criteria (Deng, Peng, & Qin, 2010; He, Yu, Dong, & Ye, 2016; Wei, 2007). However, this kind of research greatly emphasizes on small-scale and specific cases, with less involvement in urban status analysis, potential assessment, and implementation strategy in an overall
Architecture exists to create places in which people live, and it is the basic unit that constitutes a city. With the acceleration of urbanization processes, urban ecological space has been superseded by increasing numbers of hard facilities such as buildings and roads. The sharp increase in construction area contributes to the intensification of the urban heat island effect, which has a great impact on the natural environment. Currently, the ecological use of building roofs is considered one effective way to resolve urban problems (Mahdiyar, Tabatabaee, Abdullah, & Marto, 2018; Oberndorfer et al., 2007; Saadatian et al., 2013; Stovin, Poë, & Berretta, 2013). Thus, it is necessary to evaluate green roof constructability, explore its building potential, and form an architectural eco-friendly space with an ecological scale effect. Green roofs provide a number of different urban ecosystem services (Grunwald et al., 2017). As the fifth facade of architecture, green transformation of the roofs in a city can effectively produce eco-environment and socioeconomic benefits (Liu, Chen, & Peng, 2014; Ziogou, Michopoulos, Voulgari, & Zachariadis, 2018). Scholars have confirmed the widely known fact that green roofs are able to increase
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Corresponding author at: School of Resource and Environment Science, Wuhan University, Wuhan 430079, China. E-mail address: [email protected] (R. Guo).
https://doi.org/10.1016/j.cities.2019.102468 Received 28 October 2018; Received in revised form 9 September 2019; Accepted 19 September 2019 0264-2751/ © 2019 Elsevier Ltd. All rights reserved.
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applied for differentiating and planning unbuilt and renewal areas. The statutory plan is used for planning land layout and estimating the greening potential of newly constructed buildings. Data such as vegetation resources and key development areas are used to support the space guidelines of greening potential for roofs, and remote sensing imagery helps to analyze the feasibility of roof greening implementations and improve the accuracy of the potential evaluation. This paper processes all geodata on the ArcGIS 10.2 software platform.
perspective. Considering the widespread occurrence of existing buildings, it is worthwhile to conduct an in-depth analysis of green roof feasibility for retrofitting those buildings. In practice, how to construct an operational method to map the potential of green roof is a fundamental and important subject (Grunwald et al., 2017; Mallinis, Karteris, Theodoridou, Tsioukas, & Karteris, 2014). Wilkinson and Reed used Australia’s Melbourne CBD as a research area and selected eight indexes for potential assessment: building location, roof orientation, roof height, roof slope, roof loadbearing capacity, planting, sustainability of construction, and maintenance level (Wilkinson & Reed, 2009). Silva systematically analyzed the a total of 19 widely used indexes to evaluate green roof potential and then noted that roof type, green roof type, number of floors, solar orientation, and building use are the most common indexes (Silva, Flores-Colen, & Antunes, 2017). In terms of planning and management, incentives and policies made by governments and researchers are increasingly becoming a crucial factor. Exploring potential evaluation methods and implementation strategies for roof greening in urban buildings has practical meaning for building an eco-city and regulating management policies. Therefore, it is necessary to realize a technical transformation from qualitative judgement to quantitative evaluation. The focus should be shifted from single buildings to regional units in comprehensive perspectives. Through an analysis of inventory development features and the factors of roof greening constructability, this paper constructs a quantitative model for potential evaluation of roof greening. The potential of retrofitting already-existed buildings and incremental value of planned buildings can be respectively deducted from their characteristics and experiential. Based on this point, the paper demonstrates the key areas and priority order for implementing roof greening from the view of city planning. The results can provide a basis for decision-making to effectively improve the green-roof-related codes, policies and incentives.
2.2. Potential assessment method Shenzhen is a typical highly urbanized area in China, and its space for newly added construction land is limited. Urban renewal in Shenzhen can frequently be seen in the form of transformation of the old towns, villages, and industrial areas. Buildings will be removed once included in the renewal project. Taking timeliness into consideration, building types as the evaluation objects can be subdivided into two types: existing and planned buildings. In this way, roof greening potential of Shenzhen can be divided into two parts: the potential for retrofitting existing buildings and the incremental potential of planned buildings. In this paper, the potential assessment process includes three steps: the spatial recognition of building types, potential assessment and verification. The first step is to divide different building types according to location. The second is to evaluate and combine the transformation potential of existing buildings and the incremental potential of planned buildings. The final step is to conduct an internal inspection to the whole potential existing buildings with the help of satellite imagery data through traditional manual visual interpretation, and verify unsuitable situations such as pitched roofs and roofs for other purposes. The detailed process is shown in Fig. 1. 2.2.1. Spatial identification of building types The urban renewal plan, land use plan and statutory plan are used to identify urban renewal areas and planned unbuilt areas based on the method of spatial analysis in a GIS environment (Fig. 2). Urban renewal area refers to land that has been claimed or included in the urban renewal plan in the last five years, so it can be collected from the urban renewal planning. Planned unbuilt area means the land for planned construction with a building coverage ratio lower than 0.1. Land use planning and legal plans were used to extract the patches planned as construction land, and then the area ratio of the current building within the construction land patches was calculated. So the patches with the ratio less than 0.1 were defined as planned unbuilt areas. After the analysis, the planned unbuilt area turns out to be 6757.12 ha, and the land for the urban renewal unit has an area of 4089.95 ha (Fig. 3). When both are combined with the buildings census data, the buildings outside the range are defined as existing buildings, and roof greening constructability will be analyzed based on their roof features; the remainder are planned buildings that will be carried out on the potential assessment based on the land arrangement.
2. Method 2.1. Study area and available geodata Shenzhen, which has a total land area of 1996 km2, is located on the east bank of the Pearl River Delta, adjoining Hong Kong, in the southern part of China. As a model of China’s reform and opening-up policy, Shenzhen has experienced large-scale migration and high-density population aggregation. The rate of urbanization has reached 100%, putting it at the forefront of world urbanization. Problem of land shortage in Shenzhen has become increasing prominent during the period of Shenzhen’s rapid development. Various urban constructions have continuously occupied the green spaces. The contradictions between social economic development and green construction have intensified. The government has realized that urban greenery cannot only depend on the conventional green space. It must revitalize the gray hard landscape and increase the green space. In recent years, Shenzhen has made great efforts to promote the construction of green roofs, but a large gap in terms of the total amount remains when compared with other cities. Statistics indicate that the green area of the roofs in the whole city is 3,540,000 m2, comprising only 1.67% of the entire construction area. The roofs in the city basically remain empty, and there is clearly large potential for roof greening. All the data used in this research comes from the Planning and Land Resources Administration in Shenzhen, including construction census data, survey data of land use and vegetation resources, the urban annual renewal project, statutory plan, and data on the key development areas, as well as high-resolution aerial imagery (with a spatial resolution of 0.2 m) and satellite imagery (with a spatial resolution of 0.5 m) data in 2016. Among these sources, the building census data are used to extract information of the outlines and attributes of buildings (including construction age, structure, quality, height, floors, area, and so on). The land use survey data and urban annual renewal project are
2.2.2. Establishment of constructability indicators for existing buildings Indicators such as safety, feasibility, and ecological factors must be established to carry out the greening transformation on existing buildings (Cascone, Catania, Gagliano, & Sciuto, 2018; FLLForchungsgesellschaft Landschaftsentwicklung Landschaftsbau, 2008). This paper determines the constructability conditions of roof greening in existing buildings, combining the data and Shenzhen’s actual situation (Table 1). The evaluation system comprises two levels. The first level is an element level that includes building and roof attributes. Whether the attributes of the building including its roof meet the conditions of roof greening should be taken into account; the second level involves indicators, in which each indicator corresponds to the elements from the element level. The eight indicators are construction age, structure, quality, height, floors, available roof area, load capacity, 2
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Fig. 1. Evaluation process of roof greening potential.
small areas of green systems. To reach the scale effect, roof area should be more of 200 m2 (the criteria for municipal facilities can be more flexible). At the same time, highly pitched roofs, arched roofs, and special roofs (such as those built with glass or having movable surfaces) will directly affect implementation and are unfavorable for maintenance so the roof slopes should be less than 15°. Roof greening should be avoided for roofs that cannot meet load capacity. In general, inaccessible roofs (with load capacities below 0.7 kN/m2) are also inappropriate for roof greening. For accessible roofs, the suitable type should take capacity into consideration.
and roof slope. The building census data is used to collect information for these indicators in a bid to see whether a roof is suitable or not. The constructability requirements should be lined with the indicators. (1) Building attributes. In general, old buildings (Completed in the last 20 years) or historical buildings are unsuitable for roof greening. On one hand, these buildings are relics and part of cultural heritage; on the other hand, their load capacities might not support green roof weights, resulting in serious safety concerns. Buildings with reinforced concrete structures may be suitable for roof greening, but roof greening for buildings made with steel or brick may be difficult to implement. When it comes to accessibility and visibility, the building heights should be less than 40 m, and the buildings should have less than 12 floors. Furthermore, water conservation is more difficult with higher buildings, and higher roof temperatures as well as stronger winds are unfavorable for plant growth. (2) Roof attributes. Ecological effects are more difficult to produce with
In order to show the process of indicator analysis intuitively, this paper selects a sample area for illustration (as shown in Fig. 4). The sample area is 77.90 ha, with 272 buildings and an urban renewal unit. The first step is to judge and mark the buildings that meet construction requirements according to the eight indicators mentioned above. The next step is to statistics all the buildings satisfying each index, and then
Fig. 2. Spatial identification process of planned buildings. 3
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Fig. 3. The identification results of the planned unbuilt area and urban renewal area.
interpretation, both the roofs that have been used for other purposes and inclined roofs are regarded as unsuitable situations (as shown in Fig. 5a). Human intervention might also be required in dealing with possible exceptional factors. As shown in Fig. 5b, it is evidently not suitable for roof greening, despite the fact that the roof slope is below 15°. By a thorough examination of each building, 5679 buildings are finalized to be suitable for roof greening.
the evaluation result can be obtained and mapped in this area. 2.2.3. Potential assessment of planned buildings The roof greening potential of planned buildings is evaluated based on experiential deduction. The evaluation process includes three steps: sample selection, proportion calculation, and potential estimation. The first step is sample selection. According to land use type, samples are selected from residential, business, administrative, public service, industrial and storage areas. The second step is proportion calculation. The ratio of area with green roofs to the entire area is calculated based on building types. Then, the average value serves to measure the greening potential. The third and final step is the potential construction estimation. The statutory plan is used to obtain the amount of urban renewal area and planned unconstructed area. Then, the number will be multiplied by the corresponding greening average value according to the type of land. Thus, one can obtain the potential result for new buildings.
3. Results 3.1. Potential assessment results Regarding the potential of the existing buildings, a total of 5679 buildings having an area of 859.76 ha are suitable for roof greening. For the planned buildings, the total amount of urban renewal and planned unconstructed area is 10,847.07 ha. It is estimated that 1039.64 ha of the areas can have green roofs, which include the residential, industrial and commercial areas and areas for public services and facilities. In total, the potential area for green roof construction in Shenzhen is 1899.40 ha. According to the administrative districts (see Fig. 6), the potential objects are mainly located in the Longgang, Bao’an, and Nanshan Districts, and the sum of the potential areas of existing and
2.2.4. Verifications To ensure the accuracy of the evaluation result, verifications need to be carried out on the preliminary outcome (a total of 6592 buildings) by means of high-resolution remote sensing images. Via manual visual
Table 1 Constructability conditions for existing buildings (Shao, Chaosu, & Zeng, 2012; Silva, Flores-Colen, & Coelho, 2015, 2017; Wang, Guoan, Chen, Wang, & Wang, 2016; Wilkinson & Reed, 2009; Wilkinson, Rose, Glenis, & Lamond, 2014). Indicators Building Attributes
Roof Attributes
Age Structure Quality Height Floors Available Area Load Capacity of the Roof Roof Slope
Description
Criteria
Construction completion time Framework of building component Quality level of construction Height of the buildings above the ground Layers above the ground Actual available area for greenery Load capacity transferred from the beam to the wall, column, and base. Angle of the roof’s inclination to the horizontal
Was completed after 1998 Frame, frame shear wall, and tubular structure In good condition Under 40 m Less than 12 floors More than 200 m2 Within the allowable range of the roof load capacity
4
Flat roofs or roofs having angles under 15°
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Fig. 4. Schematic diagram of constructability indicators in sample area.
areas, a city’s key development areas and types, the characteristics of potential objects, the division of key implementation areas, and a priority order for implementation of different types are of great importance. This paper proposes an implementation strategy for roof greening in Shenzhen from two aspects: spatial division and time sequence.
planned buildings exceeds 120 ha. Luohu and Yantian Districts of 78.03 and 52.93 million m2, only 4.11% and 2.79% of them have the potential of roof greening. To show the spatial characteristics of roof greening potential more clearly, this paper divides Shenzhen into a spatial grid of ten-hectare areas and calculates the roof greening potential of each area. The results are shown in Fig. 7, from which it can be seen that the potential objects generally appear in scales. The potential areas greater than zero m2 in the western and central part are basically compact, whereas the foundation for large-scale development in the eastern part is poorer. The distribution of high potential areas is scattered, merely forming at certain scales in local areas such as the southwest part of the Nanshan District and the central parts of the Futian and Guangming Districts.
3.2.1. Spatial strategy: division of key implementation areas The significance of designating key implementations lies in the potential buildings that show spatial aggregation. By constructing green roofs on the buildings, a “face” type of space can be formed. Furthermore, a transition can be made from an individual project to a demonstration area. Combining the spatial distribution of potential objects and taking into account factors such as future development areas, built density and the situation of greenery resources, this paper identifies 32 key areas for roof greening through spatial analysis (as shown in Fig. 8). Key areas should make the best use of opportunities for urban construction and transformation to fully exploit the
3.2. Implementation strategy The purpose of the potential evaluation is to provide a reference for urban roof greening construction. To effectively construct green roof 5
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Fig. 5. Typical cases of unsuitable buildings.
design such as trees, grass, pavilions, pools, rockeries, and wooden chairs can be integrated, combining the functions of green space ecology and leisure. Lawn-style roofs, also known as extensive green roofs, are more easily popularized than garden-style roofs. By placing lawns on roofs, they look like vivid “plant blankets” that have low-cost construction and light loads and are easily maintained. The mixing style combines the two styles; the substrate thicknesses are greater than for the lawn style, they have a wider selection of plants, and simple recreational facilities can be located on them.
characteristics of the districts. Construction patterns of roof greening such as garden-style, lawn-style, and mixing style can be carried out symmetrically to promote the scale and demonstration effect. Systematically carrying out various roof greening construction modes contributes to the scale and demonstration effects. Considering the building types, this paper divides the construction modes into garden style, lawn style, and mixing style. Among these, roofs of the garden style are so-called intensive green roofs; the style takes a roof as a normal surface for a park layout. Usually, the elements of garden
Fig. 6. Areas of potential roof greening in all the districts of Shenzhen (unit: ha). 6
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Fig. 7. Potential for roof greening in grids (ten-hectare per grid, the number represents the area of roof greening potential within a grid).
account factors such as urban key development and renewal areas, thirteen key development areas for garden-style roofs are demarcated in the city. Key Area II: The lawn-style construction mode will be implemented in this area of existing buildings. Vegetation configurations, employing, for example, ground cover plants and low shrubs, are utilized in lawnstyle roof greening, which has the benefits of quick promotion and scale
Key Area I: This area is basically occupied by planned buildings; in this case, the garden-style construction mode will be implemented. Garden-style roofs provide space for sightseeing and leisure activities by allocating multilevel plants, garden roads, and small garden ornaments. Furthermore, they have high building structure and roof load requirements. New and retransformed buildings need to be combined to carry out the integrated design for achieving better results. Taking into
Fig. 8. Key roof greening implementation areas in Shenzhen. 7
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investment priority. For the purpose of reducing their effects, roof greening implementation should be accelerated typically for NIMBY (“not-in-my-backyard”) infrastructures such as waste transfer stations, public toilets, and waste incineration plants. Priority order 2: The second types are the buildings for emerging industries, businesses, and offices, with 3141 buildings that occupy an area of 474.02 ha. Since these buildings are used for running businesses, vegetation can contribute to the improvement of commercial and economic activities. Furthermore, roof greening can also be improved. Taking a commercial complex as an example, the traverse can be used to build small gardens or hanging gardens. The city’s conventional image, represented by white and hard pavement or massive glass curtain walls can be changed in a bid to ameliorate the shopping experience. Priority order 3: The remaining buildings are of the third type. There are 1157 buildings with an area of 214.02 ha. These buildings are mainly residential and are owned by individuals or groups. Public awareness of building green roofs must be aroused at the proper time.
coverage. Furthermore, the urban greenery value can be improved. Considering the spatial distribution of existing buildings and areas with low vegetation coverage, ten key development areas for lawn-style roof greening are demarcated. Key Area III: The planned and existing buildings are relatively balanced in this area, where a mixing style construction mode is implemented. According to the rule of adjusting measures to local conditions, a roof garden can be built with multilayer plantings, and the roof can also be covered with groundcover plants. Nine key development areas for mixing-style roof greening are selected.
3.2.2. Time sequence strategy: formulating an implementation sequence for the different types For planned buildings, top priority will be given to roof greening of buildings located in key areas. Measures such as technical manuals, administrative requirements, government assessments, and encouragement policies can be conducted. Roof greening can be planned, designed and built during the main construction. Roof greening for all the planned buildings must be obligatory. The property developers or owners should assume the respective responsibility; otherwise, the project will not be accepted. Integrating the design with the building construction is an effective way to ensure its implementation. Rigid means such as the approval of urban renewal projects, land selling conditions, guidelines for urban design, and construction regulations should also be included. In addition, special attention should be paid to taking incentive measures. Areas of new construction in which roof greening has been implemented can offset the predetermined area for greening. Roof greening for existing buildings often meets implementation difficulty. This paper proposes the following implementation strategies according to the purposes and property rights of the buildings (as shown in Fig. 9). Priority order 1: The first types are the public or municipal administration buildings, with a total of 1381 buildings that occupy an area of 171.73 ha. The government primarily owns the property rights for these buildings, and the implementation of green roofs will be given
4. Discussion and conclusion 4.1. Discussion (1) The particularity of inventory development in highly urbanized areas must be taken into consideration Reasonable determination of constructability conditions is the premise of a potential assessment of roof greening resources. Currently, despite some dispute about the influence pattern of constructability indicators, the characteristics and attributes of buildings (such as age, structure, height, quality level, and roof slope) are undoubtedly the main principles of potential assessments for roof greening. Given that the region is highly urbanized, urban renewal and demolition in Shenzhen is common and frequent; obviously, merely employing the constructability index for evaluating roof greening potential is insufficient and inaccurate. On that basis, a GIS-based approach for roof
Fig. 9. Priority order of roof greening for existing buildings. 8
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dimensions to the conventional green land surface. Green landscapes can be installed both on urban two-dimensional surfaces and in a building’s three-dimensional elevation, expanding the city’s green space. For highly urbanized regions, employing roof spaces and optimizing roof utilization have become a trend (He, Yu, Ozaki, Dong, & Zheng, 2017; Ng, Chen, Wang, & Yuan, 2012). This paper used a variety of data such as building census data, land use data, urban plan data, and remote sensing images, and it established a method for evaluating roof greening potential that combined the transformed and planned buildings. The buildings suitable for implementing green roofs are identified, space and time guidelines are also proposed based on the potential assessment. Observing the research results, this paper made a proper division of the city’s districts for roof greening in terms of quantity. The key area is designated spatially, and the arrangement and construction requirements are established in priority order. In summary, this paper employs a quantitative model to evaluate the roof greening potential for highly urbanized areas. The results are closely integrated with the planning and management of urban greening and have certain theoretical and practical significance. The proposed approach is an operational method to evaluate the potential of green roof in a city scale, especially a highly urbanized city. Nevertheless, some limitations of the approach that should be clarified including: (1) This paper emphasizes establishing technical constructability indicators, while the feasibility and sustainability of roof greening could be further demonstrated with the help of social research; (2) The methodology depends on the quality of the available geodata, and it may be suitable applied to specific built areas with high needs of green infrastructure in urban areas. Moreover, it can be seen that the government department has not taken the obligatory measures and engaged in specific planning for roof greening for a long time. Effective approaches are absent for the implementation and supervision of roof greening, which remains to be addressed in further research.
Table 2 Statistics on the priority orders of potential buildings in key areas. Type
Key Area I: Garden-style Key Area II: Lawn-style Key Area III: Mixing-style Total
Priority order 1
Priority order 2
Priority order 3
Building number
Building area(ha)
Building number
Building area(ha)
Building number
Building area(ha)
858
329.74
203
52.50
55
16.52
298
64.01
287
49.30
147
20.73
554
123.43
301
54.77
94
18.02
1710
517.18
791
156.58
296
55.26
greening potential assessment is proposed by making a distinction between existing buildings and planned buildings. The number of planned buildings is estimated in accordance with the purposes of land use, and their potential is combined with that of existing buildings; thus, the results better accord with Shenzhen’s development and reality. (2) Roof greening needs to be implemented with a comprehensive plan and policy. Taking roof greening as a part of the urban ecological and green system, integrating a relative plan with an urban comprehensive plan, carrying out scale development through the key demonstration area, and specifying the development goals and priorities are all the means to promote the implementation of roof greening. Firstly, it should accelerate the plan making and implementation for roof greening in the near future. In recent years, regional comprehensive strategies and research have gradually attracted the attention of the government and scholars (Volder & Dvorak, 2014; Weiler & ScholzBarth, 2009; Zhang, Shen, Tam, & Lee, 2012). The research perspective is changing from individual buildings to holistic regions. In comparison, China stays behind when making such plan. Roof greening programs are short of overall plans. To go through the stages from pilot program, promotion to awareness reinforcement, there are still a lot to do. Secondly, select a number of clusters of buildings in key areas as a green roof demonstration project. Multilevel roof greening layout can be developed by creating a “surface” from “points” based on the distribution of key implementation areas and implementation sequence. More specifically, the “points” are green roofs on individual buildings that are considered as basic units. The transformation of green roofs should be carried out according to the rule of adjusting measures to local conditions. The “surface” consists of key building areas with green roofs that are comprehensively established over local areas, thereby creating an urban landscape with high green coverage. The statistic of the spatial overlay analysis shown on the Figs. 8 and 9 indicates that there are 2797 buildings located in the key area. These buildings can benefit from the priority for roof greening in the next period. Among them, priority order 1, 2 and 3 accounts for 1710, 791 and 296 buildings respectively (see in Table 2). Thirdly, the sustainability of the roof greening landscape is to be guaranteed. Roof greening come down to urban buildings and structures and is often restricted by the environment, water and soil conditions. That being the case, it is by nature a fragile artificial ecological landscape. It is of necessity to provide daily maintenance and refined management to revert to the growth of vegetation over the long run, as well as to use scientific data to monitor and evaluate the greening effect.
Acknowledgements This research was supported by the financial aid of the Open Fund of the Key Laboratory of Urban Land Resources Monitoring and Simulation, Ministry of Land and Resources (NO: KF-2015-01-045). We would like to express appreciation to our colleagues for their constructive suggestions and comments. Additionally, we thank the anonymous reviewers and members of the editorial team for their constructive comments and contributions. References Ascione, F., Bianco, N., Rossi, F. D., Turni, G., & Vanoli, G. P. (2013). Green roofs in European climates. Are effective solutions for the energy savings in air-conditioning? Applied Energy, 104, 845–859. Besir, A. B., & Cuce, E. (2018). Green roofs and facades: A comprehensive review. Renewable and Sustainable Energy Reviews, 82, 915–939. Cascone, S., Catania, F., Gagliano, A., & Sciuto, G. (2018). A comprehensive study on green roof performance for retrofitting existing buildings. Building and Environment, 136, 227–239. Cookpatton, S. C., & Bauerle, T. L. (2012). Potential benefits of plant diversity on vegetated roofs: A literature review. Journal of Environmental Management, 106(106), 85–92. Deng, X., Peng, X., & Qin, C. (2010). The review of the functions and characteristics of roof greening, and its current situations and problems in China. Acta Scientiarum Naturalium Universitatis Sunyatseni, 49, 99–101 (in Chinese). FLL-Forchungsgesellschaft Landschaftsentwicklung Landschaftsbau, E. V. (2008). Guidelines for the planning, construction and maintenance of green roofing e green roofing guideline. Germany. Grunwald, L., Heusinger, J., & Weber, S. (2017). A GIS-based mapping methodology of urban green roof ecosystem services applied to a Central European city. Urban Forestry & Urban Greening, 22, 54–63. He, Y., Yu, H., Dong, N., & Ye, H. (2016). Thermal and energy performance assessment of extensive green roof in summer: A case study of a lightweight building in Shanghai. Energy and Buildings, 127, 762–773. He, Y., Yu, H., Ozaki, A., Dong, N., & Zheng, S. (2017). Long-term thermal performance evaluation of green roof system based on two new indexes: A case study in Shanghai area. Building and Environment, 120, 13–28. Jaffal, I., Ouldboukhitine, S. E., & Belarbi, R. (2012). A comprehensive study of the
4.2. Conclusion In recent years, problems in metropolitan cities, such as high population density and shortages of green spaces and land resources, have intensified. Shenzhen is a typical example. Roof greening adds more 9
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Silva, C. M., Flores-Colen, I., & Coelho, A. (2015). Green roofs in Mediterranean areas – Survey and maintenance planning. Building and Environment, 94, 131–143. Speak, A. F., Rothwell, J. J., Lindley, S. J., & Smith, C. L. (2013). Rainwater runoff retention on an aged intensive green roof. The Science of the Total Environment, 461–462(7), 28–38. Stovin, V., Poë, S., & Berretta, C. (2013). A modelling study of long term green roof retention performance. Journal of Environmental Management, 131, 206–215. Volder, A., & Dvorak, B. (2014). Event size, substrate water content and vegetation affect storm water retention efficiency of an un-irrigated extensive green roof system in Central Texas. Sustainable Cities and Society, 10(19), 59–64. Wang, X., Guoan, X. I., Chen, D., Wang, Y., & Wang, J. (2016). Establishment of an evaluation index system for roof greening constructability. Northern Horticulture, (2), 85–88 (in Chinese). Wei, Y. (2007). Studies on the development of green roofs in China——Contrast with Germany, Beijing. Scientia Silvae Sinicae, 43, 95–101 (in Chinese). Weiler, S., & Scholz-Barth, K. (2009). Green roof systems: A guide to the planning, design, and construction of landscapes over structure. John Wiley & Sons. Wilkinson, S. J., & Reed, R. (2009). Green roof retrofit potential in the central business district. Property Management, 27, 284–301. Wilkinson, S. J., Rose, C., Glenis, V., & Lamond, J. (2014). Modelling a green roof retrofit in the Melbourne Central Business District. Flood Recovery Innovation & Response IV, 27, 125–135. Williams, N. S. G., Rayner, J. P., & Raynor, K. J. (2010). Green roofs for a wide brown land: Opportunities and barriers for rooftop greening in Australia. Urban Forestry & Urban Greening, 9, 245–251. Xiao, M., Lin, Y., Han, J., & Zhang, G. (2014). A review of green roof research and development in China. Renewable and Sustainable Energy Reviews, 40, 633–648. Ziogou, I., Michopoulos, A., Voulgari, V., & Zachariadis, T. (2018). Implementation of green roof technology in residential buildings and neighborhoods of Cyprus. Sustainable Cities and Society, 40, 233–243. Zhang, X., Shen, L., Tam, V. W. Y., & Lee, W. W. Y. (2012). Barriers to implement extensive green roof systems: A Hong Kong study. Renewable and Sustainable Energy Reviews, 16, 314–319.
impact of green roofs on building energy performance. Renewable Energy, 43, 157–164. Karteris, M., Theodoridou, I., Mallinis, G., Tsiros, E., & Karteris, A. (2016). Towards a green sustainable strategy for Mediterranean cities: Assessing the benefits of largescale green roofs implementation in Thessaloniki, Northern Greece, using environmental modelling, GIS and very high spatial resolution remote sensing data. Renewable and Sustainable Energy Reviews, 58, 510–525. Liu, W., Chen, W., & Peng, C. (2014). Assessing the effectiveness of green infrastructures on urban flooding reduction: A community scale study. Ecological Modelling, 291, 6–14. Mahdiyar, A., Tabatabaee, S., Abdullah, A., & Marto, A. (2018). Identifying and assessing the critical criteria affecting decision-making for green roof type selection. Sustainable Cities and Society, 39, 772–783. Mallinis, G., Karteris, M., Theodoridou, I., Tsioukas, V., & Karteris, M. (2014). Development of a nationwide approach for large scale estimation of green roof retrofitting areas and roof-top solar energy potential using VHR natural colour orthoimagery and DSM data over Thessaloniki, Greece. Remote Sensing Letters, 5, 548–557. Ng, E., Chen, L., Wang, Y., & Yuan, C. (2012). A study on the cooling effects of greening in a high-density city: An experience from Hong Kong. Building and Environment, 47, 256–271. Oberndorfer, E., Lundholm, J., Bass, B., Coffman, R. R., Doshi, H., Dunnett, N., Gaffin, S., Köhler, M., Liu, K. K. Y., & Rowe, B. (2007). Green roofs as urban ecosystems: Ecological structures, functions, and services. Bioscience, 57, 823–833. Rowe, D. B. (2011). Green roofs as a means of pollution abatement. Environmental Pollution, 159, 2100–2110. Saadatian, O., Sopian, K., Salleh, E., Lim, C. H., Riffat, S., Saadatian, E., & Sulaiman, M. Y. (2013). A review of energy aspects of green roofs. Renewable and Sustainable Energy Reviews, 23, 155–168. Shao, T., Chaosu, L. I., & Zeng, H. (2012). Resource potential assessment of urban roof greening and development strategies: A case study in Futian central district, Shenzhen, China. Acta Ecologica Sinica, 32, 4852–4860 (in Chinese). Silva, C. M., Flores-Colen, I., & Antunes, M. (2017). Step-by-step approach to ranking green roof retrofit potential in urban areas: A case study of Lisbon, Portugal. Urban Forestry & Urban Greening, 25, 120–129.
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Cities journal homepage: www.elsevier.com/locate/cities
Potential assessment and implementation strategy for roof greening in highly urbanized areas: A case study in Shenzhen, China
T
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Wuyang Honga,b, Renzhong Guoa,c, , Hao Tangb a
School of Resource and Environment Science, Wuhan University, Wuhan 430079, China Shenzhen Urban Planning and Land Resource Research Center, Shenzhen 518034, China c Research Institute for Smart Cities, Shenzhen University, Shenzhen 518060, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Roof greening Spatial recognition Potential assessment Building Green roof retrofit Shenzhen
Greenery on buildings is an innovative approach used to solve urban and architectural environment problems, also served as a key element of the urban green space system. As a typical highly urbanized area in China, Shenzhen is densely populated and built up, making it short for greenery land, which means promoting roof greening is of great importance. This paper establishes an evaluation method for roof greening constructability by distinguishing existing buildings and planned buildings. Based on the results of a potential assessment, an implementation strategy is derived from two aspects: space and time guidelines. The potential suitable areas for roof greening of existing and planned buildings are 859.76 ha and 1039.64 ha. Based on potential assessment, this paper demarcates 32 key areas for roof greening and clarifies the construction modes of key areas promote the roof greening scale effect. Finally, the paper establishes a strategy of priority order for implementing roof greening according to the type and characteristics of buildings. The particularity of inventory development in highly urbanized areas has been taken into consideration when constructing a method for evaluating roof greening. The space and time guidelines can provide a reference for effectively promoting project implementation, improving sustainable urban development.
1. Introduction
urban biodiversity with additional aesthetical value. It can also mitigate the urban heat island effect, boost energy saving, improve urban air quality and facilitate storm-water management (Ascione, Bianco, Rossi, Turni, & Vanoli, 2013; Cookpatton & Bauerle, 2012; Karteris, Theodoridou, Mallinis, Tsiros, & Karteris, 2016; Rowe, 2011; Speak, Rothwell, Lindley, & Smith, 2013; Williams et al., 2010). Since the 1960s, the European countries, mainly represented by Germany, have begun to study project parameters and technologies for large-scale integrated vegetation in architecture (Besir & Cuce, 2018; Jaffal, Ouldboukhitine, & Belarbi, 2012). Remarkable effects have been shown so far in countries such as Singapore, the United States, Germany, and Japan, as roof greening appears in matters of diversification, scale and legislation. Furthermore, green roof construction in China is smooth and stable (Xiao, Lin, Han, & Zhang, 2014). Metropolitan cities like Beijing, Shanghai, and Shenzhen, have taken the lead in systematic research that mainly focuses on engineering promotion and technical criteria (Deng, Peng, & Qin, 2010; He, Yu, Dong, & Ye, 2016; Wei, 2007). However, this kind of research greatly emphasizes on small-scale and specific cases, with less involvement in urban status analysis, potential assessment, and implementation strategy in an overall
Architecture exists to create places in which people live, and it is the basic unit that constitutes a city. With the acceleration of urbanization processes, urban ecological space has been superseded by increasing numbers of hard facilities such as buildings and roads. The sharp increase in construction area contributes to the intensification of the urban heat island effect, which has a great impact on the natural environment. Currently, the ecological use of building roofs is considered one effective way to resolve urban problems (Mahdiyar, Tabatabaee, Abdullah, & Marto, 2018; Oberndorfer et al., 2007; Saadatian et al., 2013; Stovin, Poë, & Berretta, 2013). Thus, it is necessary to evaluate green roof constructability, explore its building potential, and form an architectural eco-friendly space with an ecological scale effect. Green roofs provide a number of different urban ecosystem services (Grunwald et al., 2017). As the fifth facade of architecture, green transformation of the roofs in a city can effectively produce eco-environment and socioeconomic benefits (Liu, Chen, & Peng, 2014; Ziogou, Michopoulos, Voulgari, & Zachariadis, 2018). Scholars have confirmed the widely known fact that green roofs are able to increase
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Corresponding author at: School of Resource and Environment Science, Wuhan University, Wuhan 430079, China. E-mail address: [email protected] (R. Guo).
https://doi.org/10.1016/j.cities.2019.102468 Received 28 October 2018; Received in revised form 9 September 2019; Accepted 19 September 2019 0264-2751/ © 2019 Elsevier Ltd. All rights reserved.
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applied for differentiating and planning unbuilt and renewal areas. The statutory plan is used for planning land layout and estimating the greening potential of newly constructed buildings. Data such as vegetation resources and key development areas are used to support the space guidelines of greening potential for roofs, and remote sensing imagery helps to analyze the feasibility of roof greening implementations and improve the accuracy of the potential evaluation. This paper processes all geodata on the ArcGIS 10.2 software platform.
perspective. Considering the widespread occurrence of existing buildings, it is worthwhile to conduct an in-depth analysis of green roof feasibility for retrofitting those buildings. In practice, how to construct an operational method to map the potential of green roof is a fundamental and important subject (Grunwald et al., 2017; Mallinis, Karteris, Theodoridou, Tsioukas, & Karteris, 2014). Wilkinson and Reed used Australia’s Melbourne CBD as a research area and selected eight indexes for potential assessment: building location, roof orientation, roof height, roof slope, roof loadbearing capacity, planting, sustainability of construction, and maintenance level (Wilkinson & Reed, 2009). Silva systematically analyzed the a total of 19 widely used indexes to evaluate green roof potential and then noted that roof type, green roof type, number of floors, solar orientation, and building use are the most common indexes (Silva, Flores-Colen, & Antunes, 2017). In terms of planning and management, incentives and policies made by governments and researchers are increasingly becoming a crucial factor. Exploring potential evaluation methods and implementation strategies for roof greening in urban buildings has practical meaning for building an eco-city and regulating management policies. Therefore, it is necessary to realize a technical transformation from qualitative judgement to quantitative evaluation. The focus should be shifted from single buildings to regional units in comprehensive perspectives. Through an analysis of inventory development features and the factors of roof greening constructability, this paper constructs a quantitative model for potential evaluation of roof greening. The potential of retrofitting already-existed buildings and incremental value of planned buildings can be respectively deducted from their characteristics and experiential. Based on this point, the paper demonstrates the key areas and priority order for implementing roof greening from the view of city planning. The results can provide a basis for decision-making to effectively improve the green-roof-related codes, policies and incentives.
2.2. Potential assessment method Shenzhen is a typical highly urbanized area in China, and its space for newly added construction land is limited. Urban renewal in Shenzhen can frequently be seen in the form of transformation of the old towns, villages, and industrial areas. Buildings will be removed once included in the renewal project. Taking timeliness into consideration, building types as the evaluation objects can be subdivided into two types: existing and planned buildings. In this way, roof greening potential of Shenzhen can be divided into two parts: the potential for retrofitting existing buildings and the incremental potential of planned buildings. In this paper, the potential assessment process includes three steps: the spatial recognition of building types, potential assessment and verification. The first step is to divide different building types according to location. The second is to evaluate and combine the transformation potential of existing buildings and the incremental potential of planned buildings. The final step is to conduct an internal inspection to the whole potential existing buildings with the help of satellite imagery data through traditional manual visual interpretation, and verify unsuitable situations such as pitched roofs and roofs for other purposes. The detailed process is shown in Fig. 1. 2.2.1. Spatial identification of building types The urban renewal plan, land use plan and statutory plan are used to identify urban renewal areas and planned unbuilt areas based on the method of spatial analysis in a GIS environment (Fig. 2). Urban renewal area refers to land that has been claimed or included in the urban renewal plan in the last five years, so it can be collected from the urban renewal planning. Planned unbuilt area means the land for planned construction with a building coverage ratio lower than 0.1. Land use planning and legal plans were used to extract the patches planned as construction land, and then the area ratio of the current building within the construction land patches was calculated. So the patches with the ratio less than 0.1 were defined as planned unbuilt areas. After the analysis, the planned unbuilt area turns out to be 6757.12 ha, and the land for the urban renewal unit has an area of 4089.95 ha (Fig. 3). When both are combined with the buildings census data, the buildings outside the range are defined as existing buildings, and roof greening constructability will be analyzed based on their roof features; the remainder are planned buildings that will be carried out on the potential assessment based on the land arrangement.
2. Method 2.1. Study area and available geodata Shenzhen, which has a total land area of 1996 km2, is located on the east bank of the Pearl River Delta, adjoining Hong Kong, in the southern part of China. As a model of China’s reform and opening-up policy, Shenzhen has experienced large-scale migration and high-density population aggregation. The rate of urbanization has reached 100%, putting it at the forefront of world urbanization. Problem of land shortage in Shenzhen has become increasing prominent during the period of Shenzhen’s rapid development. Various urban constructions have continuously occupied the green spaces. The contradictions between social economic development and green construction have intensified. The government has realized that urban greenery cannot only depend on the conventional green space. It must revitalize the gray hard landscape and increase the green space. In recent years, Shenzhen has made great efforts to promote the construction of green roofs, but a large gap in terms of the total amount remains when compared with other cities. Statistics indicate that the green area of the roofs in the whole city is 3,540,000 m2, comprising only 1.67% of the entire construction area. The roofs in the city basically remain empty, and there is clearly large potential for roof greening. All the data used in this research comes from the Planning and Land Resources Administration in Shenzhen, including construction census data, survey data of land use and vegetation resources, the urban annual renewal project, statutory plan, and data on the key development areas, as well as high-resolution aerial imagery (with a spatial resolution of 0.2 m) and satellite imagery (with a spatial resolution of 0.5 m) data in 2016. Among these sources, the building census data are used to extract information of the outlines and attributes of buildings (including construction age, structure, quality, height, floors, area, and so on). The land use survey data and urban annual renewal project are
2.2.2. Establishment of constructability indicators for existing buildings Indicators such as safety, feasibility, and ecological factors must be established to carry out the greening transformation on existing buildings (Cascone, Catania, Gagliano, & Sciuto, 2018; FLLForchungsgesellschaft Landschaftsentwicklung Landschaftsbau, 2008). This paper determines the constructability conditions of roof greening in existing buildings, combining the data and Shenzhen’s actual situation (Table 1). The evaluation system comprises two levels. The first level is an element level that includes building and roof attributes. Whether the attributes of the building including its roof meet the conditions of roof greening should be taken into account; the second level involves indicators, in which each indicator corresponds to the elements from the element level. The eight indicators are construction age, structure, quality, height, floors, available roof area, load capacity, 2
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Fig. 1. Evaluation process of roof greening potential.
small areas of green systems. To reach the scale effect, roof area should be more of 200 m2 (the criteria for municipal facilities can be more flexible). At the same time, highly pitched roofs, arched roofs, and special roofs (such as those built with glass or having movable surfaces) will directly affect implementation and are unfavorable for maintenance so the roof slopes should be less than 15°. Roof greening should be avoided for roofs that cannot meet load capacity. In general, inaccessible roofs (with load capacities below 0.7 kN/m2) are also inappropriate for roof greening. For accessible roofs, the suitable type should take capacity into consideration.
and roof slope. The building census data is used to collect information for these indicators in a bid to see whether a roof is suitable or not. The constructability requirements should be lined with the indicators. (1) Building attributes. In general, old buildings (Completed in the last 20 years) or historical buildings are unsuitable for roof greening. On one hand, these buildings are relics and part of cultural heritage; on the other hand, their load capacities might not support green roof weights, resulting in serious safety concerns. Buildings with reinforced concrete structures may be suitable for roof greening, but roof greening for buildings made with steel or brick may be difficult to implement. When it comes to accessibility and visibility, the building heights should be less than 40 m, and the buildings should have less than 12 floors. Furthermore, water conservation is more difficult with higher buildings, and higher roof temperatures as well as stronger winds are unfavorable for plant growth. (2) Roof attributes. Ecological effects are more difficult to produce with
In order to show the process of indicator analysis intuitively, this paper selects a sample area for illustration (as shown in Fig. 4). The sample area is 77.90 ha, with 272 buildings and an urban renewal unit. The first step is to judge and mark the buildings that meet construction requirements according to the eight indicators mentioned above. The next step is to statistics all the buildings satisfying each index, and then
Fig. 2. Spatial identification process of planned buildings. 3
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Fig. 3. The identification results of the planned unbuilt area and urban renewal area.
interpretation, both the roofs that have been used for other purposes and inclined roofs are regarded as unsuitable situations (as shown in Fig. 5a). Human intervention might also be required in dealing with possible exceptional factors. As shown in Fig. 5b, it is evidently not suitable for roof greening, despite the fact that the roof slope is below 15°. By a thorough examination of each building, 5679 buildings are finalized to be suitable for roof greening.
the evaluation result can be obtained and mapped in this area. 2.2.3. Potential assessment of planned buildings The roof greening potential of planned buildings is evaluated based on experiential deduction. The evaluation process includes three steps: sample selection, proportion calculation, and potential estimation. The first step is sample selection. According to land use type, samples are selected from residential, business, administrative, public service, industrial and storage areas. The second step is proportion calculation. The ratio of area with green roofs to the entire area is calculated based on building types. Then, the average value serves to measure the greening potential. The third and final step is the potential construction estimation. The statutory plan is used to obtain the amount of urban renewal area and planned unconstructed area. Then, the number will be multiplied by the corresponding greening average value according to the type of land. Thus, one can obtain the potential result for new buildings.
3. Results 3.1. Potential assessment results Regarding the potential of the existing buildings, a total of 5679 buildings having an area of 859.76 ha are suitable for roof greening. For the planned buildings, the total amount of urban renewal and planned unconstructed area is 10,847.07 ha. It is estimated that 1039.64 ha of the areas can have green roofs, which include the residential, industrial and commercial areas and areas for public services and facilities. In total, the potential area for green roof construction in Shenzhen is 1899.40 ha. According to the administrative districts (see Fig. 6), the potential objects are mainly located in the Longgang, Bao’an, and Nanshan Districts, and the sum of the potential areas of existing and
2.2.4. Verifications To ensure the accuracy of the evaluation result, verifications need to be carried out on the preliminary outcome (a total of 6592 buildings) by means of high-resolution remote sensing images. Via manual visual
Table 1 Constructability conditions for existing buildings (Shao, Chaosu, & Zeng, 2012; Silva, Flores-Colen, & Coelho, 2015, 2017; Wang, Guoan, Chen, Wang, & Wang, 2016; Wilkinson & Reed, 2009; Wilkinson, Rose, Glenis, & Lamond, 2014). Indicators Building Attributes
Roof Attributes
Age Structure Quality Height Floors Available Area Load Capacity of the Roof Roof Slope
Description
Criteria
Construction completion time Framework of building component Quality level of construction Height of the buildings above the ground Layers above the ground Actual available area for greenery Load capacity transferred from the beam to the wall, column, and base. Angle of the roof’s inclination to the horizontal
Was completed after 1998 Frame, frame shear wall, and tubular structure In good condition Under 40 m Less than 12 floors More than 200 m2 Within the allowable range of the roof load capacity
4
Flat roofs or roofs having angles under 15°
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Fig. 4. Schematic diagram of constructability indicators in sample area.
areas, a city’s key development areas and types, the characteristics of potential objects, the division of key implementation areas, and a priority order for implementation of different types are of great importance. This paper proposes an implementation strategy for roof greening in Shenzhen from two aspects: spatial division and time sequence.
planned buildings exceeds 120 ha. Luohu and Yantian Districts of 78.03 and 52.93 million m2, only 4.11% and 2.79% of them have the potential of roof greening. To show the spatial characteristics of roof greening potential more clearly, this paper divides Shenzhen into a spatial grid of ten-hectare areas and calculates the roof greening potential of each area. The results are shown in Fig. 7, from which it can be seen that the potential objects generally appear in scales. The potential areas greater than zero m2 in the western and central part are basically compact, whereas the foundation for large-scale development in the eastern part is poorer. The distribution of high potential areas is scattered, merely forming at certain scales in local areas such as the southwest part of the Nanshan District and the central parts of the Futian and Guangming Districts.
3.2.1. Spatial strategy: division of key implementation areas The significance of designating key implementations lies in the potential buildings that show spatial aggregation. By constructing green roofs on the buildings, a “face” type of space can be formed. Furthermore, a transition can be made from an individual project to a demonstration area. Combining the spatial distribution of potential objects and taking into account factors such as future development areas, built density and the situation of greenery resources, this paper identifies 32 key areas for roof greening through spatial analysis (as shown in Fig. 8). Key areas should make the best use of opportunities for urban construction and transformation to fully exploit the
3.2. Implementation strategy The purpose of the potential evaluation is to provide a reference for urban roof greening construction. To effectively construct green roof 5
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Fig. 5. Typical cases of unsuitable buildings.
design such as trees, grass, pavilions, pools, rockeries, and wooden chairs can be integrated, combining the functions of green space ecology and leisure. Lawn-style roofs, also known as extensive green roofs, are more easily popularized than garden-style roofs. By placing lawns on roofs, they look like vivid “plant blankets” that have low-cost construction and light loads and are easily maintained. The mixing style combines the two styles; the substrate thicknesses are greater than for the lawn style, they have a wider selection of plants, and simple recreational facilities can be located on them.
characteristics of the districts. Construction patterns of roof greening such as garden-style, lawn-style, and mixing style can be carried out symmetrically to promote the scale and demonstration effect. Systematically carrying out various roof greening construction modes contributes to the scale and demonstration effects. Considering the building types, this paper divides the construction modes into garden style, lawn style, and mixing style. Among these, roofs of the garden style are so-called intensive green roofs; the style takes a roof as a normal surface for a park layout. Usually, the elements of garden
Fig. 6. Areas of potential roof greening in all the districts of Shenzhen (unit: ha). 6
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Fig. 7. Potential for roof greening in grids (ten-hectare per grid, the number represents the area of roof greening potential within a grid).
account factors such as urban key development and renewal areas, thirteen key development areas for garden-style roofs are demarcated in the city. Key Area II: The lawn-style construction mode will be implemented in this area of existing buildings. Vegetation configurations, employing, for example, ground cover plants and low shrubs, are utilized in lawnstyle roof greening, which has the benefits of quick promotion and scale
Key Area I: This area is basically occupied by planned buildings; in this case, the garden-style construction mode will be implemented. Garden-style roofs provide space for sightseeing and leisure activities by allocating multilevel plants, garden roads, and small garden ornaments. Furthermore, they have high building structure and roof load requirements. New and retransformed buildings need to be combined to carry out the integrated design for achieving better results. Taking into
Fig. 8. Key roof greening implementation areas in Shenzhen. 7
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investment priority. For the purpose of reducing their effects, roof greening implementation should be accelerated typically for NIMBY (“not-in-my-backyard”) infrastructures such as waste transfer stations, public toilets, and waste incineration plants. Priority order 2: The second types are the buildings for emerging industries, businesses, and offices, with 3141 buildings that occupy an area of 474.02 ha. Since these buildings are used for running businesses, vegetation can contribute to the improvement of commercial and economic activities. Furthermore, roof greening can also be improved. Taking a commercial complex as an example, the traverse can be used to build small gardens or hanging gardens. The city’s conventional image, represented by white and hard pavement or massive glass curtain walls can be changed in a bid to ameliorate the shopping experience. Priority order 3: The remaining buildings are of the third type. There are 1157 buildings with an area of 214.02 ha. These buildings are mainly residential and are owned by individuals or groups. Public awareness of building green roofs must be aroused at the proper time.
coverage. Furthermore, the urban greenery value can be improved. Considering the spatial distribution of existing buildings and areas with low vegetation coverage, ten key development areas for lawn-style roof greening are demarcated. Key Area III: The planned and existing buildings are relatively balanced in this area, where a mixing style construction mode is implemented. According to the rule of adjusting measures to local conditions, a roof garden can be built with multilayer plantings, and the roof can also be covered with groundcover plants. Nine key development areas for mixing-style roof greening are selected.
3.2.2. Time sequence strategy: formulating an implementation sequence for the different types For planned buildings, top priority will be given to roof greening of buildings located in key areas. Measures such as technical manuals, administrative requirements, government assessments, and encouragement policies can be conducted. Roof greening can be planned, designed and built during the main construction. Roof greening for all the planned buildings must be obligatory. The property developers or owners should assume the respective responsibility; otherwise, the project will not be accepted. Integrating the design with the building construction is an effective way to ensure its implementation. Rigid means such as the approval of urban renewal projects, land selling conditions, guidelines for urban design, and construction regulations should also be included. In addition, special attention should be paid to taking incentive measures. Areas of new construction in which roof greening has been implemented can offset the predetermined area for greening. Roof greening for existing buildings often meets implementation difficulty. This paper proposes the following implementation strategies according to the purposes and property rights of the buildings (as shown in Fig. 9). Priority order 1: The first types are the public or municipal administration buildings, with a total of 1381 buildings that occupy an area of 171.73 ha. The government primarily owns the property rights for these buildings, and the implementation of green roofs will be given
4. Discussion and conclusion 4.1. Discussion (1) The particularity of inventory development in highly urbanized areas must be taken into consideration Reasonable determination of constructability conditions is the premise of a potential assessment of roof greening resources. Currently, despite some dispute about the influence pattern of constructability indicators, the characteristics and attributes of buildings (such as age, structure, height, quality level, and roof slope) are undoubtedly the main principles of potential assessments for roof greening. Given that the region is highly urbanized, urban renewal and demolition in Shenzhen is common and frequent; obviously, merely employing the constructability index for evaluating roof greening potential is insufficient and inaccurate. On that basis, a GIS-based approach for roof
Fig. 9. Priority order of roof greening for existing buildings. 8
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dimensions to the conventional green land surface. Green landscapes can be installed both on urban two-dimensional surfaces and in a building’s three-dimensional elevation, expanding the city’s green space. For highly urbanized regions, employing roof spaces and optimizing roof utilization have become a trend (He, Yu, Ozaki, Dong, & Zheng, 2017; Ng, Chen, Wang, & Yuan, 2012). This paper used a variety of data such as building census data, land use data, urban plan data, and remote sensing images, and it established a method for evaluating roof greening potential that combined the transformed and planned buildings. The buildings suitable for implementing green roofs are identified, space and time guidelines are also proposed based on the potential assessment. Observing the research results, this paper made a proper division of the city’s districts for roof greening in terms of quantity. The key area is designated spatially, and the arrangement and construction requirements are established in priority order. In summary, this paper employs a quantitative model to evaluate the roof greening potential for highly urbanized areas. The results are closely integrated with the planning and management of urban greening and have certain theoretical and practical significance. The proposed approach is an operational method to evaluate the potential of green roof in a city scale, especially a highly urbanized city. Nevertheless, some limitations of the approach that should be clarified including: (1) This paper emphasizes establishing technical constructability indicators, while the feasibility and sustainability of roof greening could be further demonstrated with the help of social research; (2) The methodology depends on the quality of the available geodata, and it may be suitable applied to specific built areas with high needs of green infrastructure in urban areas. Moreover, it can be seen that the government department has not taken the obligatory measures and engaged in specific planning for roof greening for a long time. Effective approaches are absent for the implementation and supervision of roof greening, which remains to be addressed in further research.
Table 2 Statistics on the priority orders of potential buildings in key areas. Type
Key Area I: Garden-style Key Area II: Lawn-style Key Area III: Mixing-style Total
Priority order 1
Priority order 2
Priority order 3
Building number
Building area(ha)
Building number
Building area(ha)
Building number
Building area(ha)
858
329.74
203
52.50
55
16.52
298
64.01
287
49.30
147
20.73
554
123.43
301
54.77
94
18.02
1710
517.18
791
156.58
296
55.26
greening potential assessment is proposed by making a distinction between existing buildings and planned buildings. The number of planned buildings is estimated in accordance with the purposes of land use, and their potential is combined with that of existing buildings; thus, the results better accord with Shenzhen’s development and reality. (2) Roof greening needs to be implemented with a comprehensive plan and policy. Taking roof greening as a part of the urban ecological and green system, integrating a relative plan with an urban comprehensive plan, carrying out scale development through the key demonstration area, and specifying the development goals and priorities are all the means to promote the implementation of roof greening. Firstly, it should accelerate the plan making and implementation for roof greening in the near future. In recent years, regional comprehensive strategies and research have gradually attracted the attention of the government and scholars (Volder & Dvorak, 2014; Weiler & ScholzBarth, 2009; Zhang, Shen, Tam, & Lee, 2012). The research perspective is changing from individual buildings to holistic regions. In comparison, China stays behind when making such plan. Roof greening programs are short of overall plans. To go through the stages from pilot program, promotion to awareness reinforcement, there are still a lot to do. Secondly, select a number of clusters of buildings in key areas as a green roof demonstration project. Multilevel roof greening layout can be developed by creating a “surface” from “points” based on the distribution of key implementation areas and implementation sequence. More specifically, the “points” are green roofs on individual buildings that are considered as basic units. The transformation of green roofs should be carried out according to the rule of adjusting measures to local conditions. The “surface” consists of key building areas with green roofs that are comprehensively established over local areas, thereby creating an urban landscape with high green coverage. The statistic of the spatial overlay analysis shown on the Figs. 8 and 9 indicates that there are 2797 buildings located in the key area. These buildings can benefit from the priority for roof greening in the next period. Among them, priority order 1, 2 and 3 accounts for 1710, 791 and 296 buildings respectively (see in Table 2). Thirdly, the sustainability of the roof greening landscape is to be guaranteed. Roof greening come down to urban buildings and structures and is often restricted by the environment, water and soil conditions. That being the case, it is by nature a fragile artificial ecological landscape. It is of necessity to provide daily maintenance and refined management to revert to the growth of vegetation over the long run, as well as to use scientific data to monitor and evaluate the greening effect.
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