Abstract
The rainwater system is an important part of the urban infrastructure as well as a key hub for maintaining the dynamic operation of the city and a clear indicator of the level of urban development. With the rapid development of urbanization, the hardened area of roads and residential areas has increased, and the construction of rainwater systems is so far insufficient, causing the urban waterlogging and water pollution problems to become increasingly serious. Accordingly, combined with the “sponge city” construction concept of the six-character policy of “seepage, retention, storage, use, purification, and drainage,” we propose to adopt measures for the local conditions and to reasonably select sponge city engineering measures to increase rainwater utilization, effectively reduce rainwater runoff, and alleviate the city waterlogging and water pollution problems. We used the analytic hierarchy process (AHP) to evaluate the effect of a sponge city “pocket park” rainwater system in Chaohu City before and after the transformation. The results showed that the pocket park after the renovation was well controlled, the waterlogging was basically eliminated, the water quality pollution was clearly improved, and the ecological environment was significantly improved.
1. Research Background
With the rapid economic development of the city’s urbanized area increasing year by year, the green area has been decreasing, resulting in the city park feature not being fully functional, and urban infrastructure has increasingly exposed shortcomings that have occurred in the “city to see the sea.” Improper maintenance and management of the urban stormwater systems [1], a lower return period, insufficient drainage zoning [2], improper pipe connections, and other issues resulted in a waste of resources, rain, and the serious phenomenon of urban waterlogging. This issue is not conducive to urban economic development and also seriously affects the standard of living. The sponge urban philosophy is based on building a new pocket park, which can not only effectively alleviate the problem of urban stormwater systems but also lay a foundation for the construction of an environmentally friendly city. A pocket park in Chaohu City, for example, can be related to the transformation of its rainwater sponge system.
2. Overview of Concepts and Features
2.1. Sponge City Concepts
The term “sponge city” indicates that the city has the function of a sponge, which can respond flexibly to natural disasters and adapt to environmental changes [3]. The functions of a sponge city are mainly embodied in the six aspects of “seepage, retention, storage, purification, use, and drainage” [4]. The core of the construction is to build aquatic ecological infrastructure across different scales, combining multiple specific technologies [5], and promote coordinated development of urban and ecological environment.
2.2. Evaluation Criteria for Sponge City Construction
The sponge city concept is implemented based on three main aspects: protecting the existing ecological system in the city, promoting urban ecological restoration and restoration, and maintaining low impact during exploitation. Evaluation of construction typically includes the construction hardening rate, the total annual runoff control rate, the rainwater utilization rate, and the pollutants (SS) control rate. In general, sponge city divides the construction area into 5 categories based on the annual runoff control rate in the implementation standards: category 1 is 85%–90%, category 2 is 80%–85%, category 3 is 75%–85%, and category 4 is 70%–85%, and category 5 is 60%–85%. The overall control rate of pollutants is 40%–60%.
2.3. Pocket Park (Vest-Pocket Park or Minipark) Concepts
A “pocket park” indicates a smaller urban open space, generally with a patchy scattered nature or hidden in the urban fabric [6]. Typically, pocket parks are parks formed by further adding leisure and entertainment facilities on the basis of planting greenery on smaller plots. Pocket park construction areas are generally one to three times the area of housing construction. With general domestic rural housing construction areas of 400 m2, the provisions in the “park design specifications GB51192-2016” [7], and the district resident population of 10,000, the area of the inner park is not less than 10,000 m2. Therefore, the construction area of pocket parks is generally 400–10,000 m2.
2.4. Pocket Park Features
The location of a pocket park is less limited and can be developed from any open space or forgotten space in a city. Compared with traditional parks, pocket parks have characteristics including a small area, built in high-density urban areas, few but not single functions, patchy discrete distribution, ecological nature, and a high frequency of use. Through analysis, pocket parks can be divided into parks with different functions according to different properties. According to the location of the pocket park, it can be summarized as a park with ecological, social, landscape, psychological, and filling functions [8]. If divided according to the layout form and use function of the pocket park, the park can be classified as a recreational, decorative, transportation, and comprehensive park [9]. The scope of pocket parks is shown in Figure 1.
2.5. State-of-the-Art Pocket Park Construction
As a structure that effectively utilizes the abandoned sites in the city, the pocket park not only makes full use of the urban space but also increases the urban green spaces, forming a unique urban landscape structure. Many experts have conducted research on pocket parks. Shen Lingzhi explained that combining pocket parks with old city landscapes helps to create more urban land resources; Zhang Haopeng sorted out the principles of pocket park construction in the community, which is adopted in several construction cases. Yu Guojiang highlighted the importance of pocket parks in protecting urban ruins. However, problems still exist in the construction of pocket park, including unreasonable choices of the pocket park location, uniformed construction layout, not fully realized ecological functions, and shortage of internal circulation system of the park. Currently, the pocket park is not conducive to the urban water environment and limits its ecological functions. Therefore, it is important to adopt a reasonable and effective construction plan that transforms the construction method of pocket parks. It would help to address the urban water environment problems and have important significance in promoting the construction of environmentally friendly cities.
3. Related Research and Case Analysis
3.1. Related Research
The concept of a sponge city can be reflected in the construction of residential areas, campuses, parks, roads, and so on, and the concept of a sponge city can be fully reflected by adopting various sponge transformation measures. As a small urban public open space, a pocket park not only provides people with comfortable leisure and entertainment places but also adds many green elements to the city, promoting the construction of an environmentally friendly city. At present, many scholars have conducted research on this aspect. For example, Li and others analyzed that the combination of pocket parks and sponge cities can greatly help strengthen urban rainwater control [10]. Zheng et al. and others proposed that the concept of sponge cities and pocket park construction has an important role in regulating the regional microclimate [11]. Tan et al. used the concept of the city as the basis for sponge pocket park sites to solve the lack of urban green space, inadequate drainage facilities, and other issues [12].
3.2. Case Analysis
With the continuous promotion of the sponge city concept, there have been many specific cases of integrating the sponge city concept into the reconstruction and construction of pocket park at home and abroad. A specific case analysis of the sponge renovation and construction of pocket park is shown in Table 1.
4. Sponge Transformation of the Rainwater System in a Pocket Park: Taking a Pocket Park in Chaohu City as an Example
4.1. Overview of the Pocket Park before the Renovation
The studied pocket park is located in Chaohu City, Anhui Province. The city has four distinct seasons, sufficient sunlight, and a mild climate. The annual average rainfall is 1124 mm, and the rainfall is abundant, which meets the requirements for sponge transformation of pocket park.
This pocket park is located next to the old residential area, with a construction area of 5644 m2. Its functions are mainly sports, entertainment, and ecology; the green area in the park is approximately 50% of the construction area about 2822 m2. There are also a circular ecological lake and an arc-shaped wisteria corridor and two Chinese-style background walls; the ecological lake area is approximately 25% of the construction area, about 1411 m2. There is also a 2 m wide circular plastic runway inside the park to provide sports conditions for the surrounding residents.
4.2. Operation Status and Existing Problems of the Pocket Park Rainwater System before Transformation
4.2.1. The Original Rainwater System Operation of the Pocket Park
The underlying surface of the park that receives rainwater is mainly embodied in three aspects: road paving, green space, and water bodies. The operation process of the rainwater system is primarily that the runoff formed by rainfall is discharged into the nearby rainwater outlet and then enters the municipal rainwater pipeline; the rainwater that falls on the green space is infiltrated by the soil, and the excess runoff flows into the ecological lake; and the rainwater that falls on the ecological lake that does not exceed the net municipal level is accepted as consumptive rain.
4.2.2. Problems in the Original Rainwater System of the Pocket Park
Due to the short construction time of the pocket park, the internal rainwater system facilities are relatively incomplete and there are obvious problems in the operation. This is mainly reflected in the lack of timely drainage when the rainfall is large, and water accumulation forms easily; the utilization rate of rainwater is low, the phenomenon of rainwater waste is serious, and the overall layout of greening is single.
4.3. Sponge Renovation Measures for the Rainwater System of the Pocket Park
4.3.1. Comparison of the Advantages and Disadvantages of Sponge City Concept Transformation Measures
The sponge city concept emphasizes the three aspects of “natural accumulation, natural infiltration, and natural purification” [13] to reduce runoff, reduce rainwater pollution, and improve the circulation function of the rainwater system. Practice showed that various transformation measures based on the sponge city concept have certain advantages and disadvantages in different actual projects. The advantages and disadvantages of various sponge transformation measures are shown in Table 2.
4.3.2. Basis for Sponge Transformation Measures
Analyzing the pros and cons of various sponge renovation measures, we summarize that all renovation measures have applicable scenarios. We analyze the selection of sponge renovation measures of different construction types here. Select different building measures, such as the transformation of sponge shown in Table 3.
4.3.3. Retrofit of the Rainwater System in the Pocket Park
According to the existing renovation conditions of the pocket park, through the analysis and comparison of various renovation measures, several measures were selected, including water-permeable pavement ground, grass ditches, sunken green space, rainwater gardens, a rainwater circulation system, and a rainwater storage system. This transformation was performed, which organically combined several transformation measures.(1)Pave the ground with water-permeable materials. The road in the park is composed of pedestrian roads and plastic runways, combined with the different functions of the road, to achieve the water permeability transformation. They chose to use a permeable cement concrete material to pave the sidewalk and to use permeable asphalt concrete to modify the original plastic runway [14]. The reconstructed road can not only speed up the absorption of rainwater on the ground but also enhance the aesthetic effect of the pocket park. Considering that the rainwater is heavy and urgent during heavy rains and that the permeable ground cannot absorb all the rainwater in time, the permeable pavement ground can be set with a certain slope during implementation, and the excessive rainwater can be drained to the surrounding green space in time.(2)Plant grass ditch. The grass ditch is a landscape surface drainage ditch that integrates rainwater transmission function with the reduction of suspended solid particles and organic pollutants [15]. During the renovation design, the green belt and green space in the park were added with grass-planting ditches, so that rainwater runoff from roads and greening will slowly flow through the grass-planting ditches to achieve the purpose of removing suspended solid particles and organic pollutants and effectively control the nonpoint source pollution in the park.(3)Change the sunken green space. The green space in the park was transformed into a sunken slope to form a sunken green space. Through the infiltration and storage functions of the green space, the phenomenon of rainwater runoff was reduced. To reduce the degree of rainwater pollution, a certain amount of gravel can be added to the reconstructed sunken green space to effectively intercept the larger solid impurities in the rainwater.(4)Add a rain garden. Combined with the topographical features and planning layout of the pocket park, a rainwater garden was added at the end of the sunken green space, and plants were planted in the rainwater garden to achieve the purpose of storing and purifying rainwater.(5)Add a rainwater circulation system. Sprinkler irrigation water supply pipes and underground backwater filtering blind pipes were added in the pocket park [16], connecting the backwater filtering blind pipes to the catch basin located at the lowest part of the park, and the corresponding operating facilities were installed to form a complete rainwater circulation system to ensure the dynamic flow of rainwater.(6)Add terminal storage facilities. Rainwater storage tanks were set up at the end of the rainwater pipe network in the park to achieve the purpose of reasonable control of rainwater drainage and utilization. Through the calculation of the total annual diameter of the pocket park and related requirements, a reasonable rainwater regulation and storage mode was selected from the independent internal regulation and storage mode, the regional coordination regulation and storage mode, and the external system regulation and storage mode.
4.3.4. Reconstruction Design of the Pocket Park Rainwater System
(1) Design Calculation of the Permeable Pavement. The pocket park is located in Chaohu City, part of which belongs to Hefei City. Therefore, the rainstorm intensity formula refers to the Hefei City standard. The rainstorm intensity formula is [17]where i is the formula of rainstorm intensity, mm/min = 0.6 mm/h = 0.006 L/(s·hm2); t is the rainfall duration, 45 min; and P is the return period, 2 a. Substituting the data, this is 0.838 mm/min.
The thickness of the permeable ground base layer H1 [18] is as follows:where k is the soil-based gun and permeability coefficient [18], 10−4, and n is the aquifer porosity [18], 20%. Substituting the data, this is 18.59 cm. As the base layer thickness of sidewalks and motor vehicles is 15–20 cm, the calculated H1 meets the requirements.
The thickness of the permeable ground surface H2 is as follows.
As the reconstructed roads are all water-permeable concrete, the surface layer range is 30–40 mm, so we chose 40 mm.
(2) Design and Calculation of the Planting Ditch. The design rainfall runoff Qj is as follows:where Φ is the comprehensive runoff coefficient of the catchment area [19], 0.4–0.5; choose 0.4; B is actual catchment area, hm2. Substituting the data, this is 0.024 m3/s.
The rainwater runoff transmission capacity of grass ditch Qc is as follows:where A is the cross-sectional area of grass ditch, according to relevant data [20]; the value is 1.04 m2; R is the hydraulic radius of the cross-section, m; ƞ is the longitudinal slope [20], less than or equal to 4%; χ is the wet cycle of the cross-section [20], 4 m; and ψ is the drag coefficient [20], 2%–10%, and we chose 8%. Substituting the data, this is 0.92 m3/s. As Qc is much greater than Qj, the grass-planting ditch met the design specifications.
(3) Calculation of the Sunken Green Space. The critical sinking depth Δh0 of the sinking green space is as follows:where C is the green space runoff coefficient [21], 0.15; Y is the green space rainfall, 45 mm; γ is the park soil infiltration rate, 2 × 10−5 m/s; and f is the green area ratio, 50%. Substituting the data, this is −0.02 m. Since the calculation result is less than 0, this indicates that the original green space in the park is in a good position, can play a role without further sinking, and meets the relevant requirements. The calculation is 0.1 m. To test the reliability of the calculation, the following formula is used for the calculation [22]:where t1 is the time when the storm runoff and green space infiltration at the beginning of rainfall are equal, min; t2 is the time when the storm runoff and green land infiltration are equal in middle and late rainfall, min. The calculated value of the depth Δh0 of the sinking green underground depression met the requirements.
The rainwater infiltration rate S [19] is as follows:where J is the hydraulic slope, assuming vertical infiltration of rainwater, J = 1; F1 is the green area, m2; and T is the infiltration time, 60 min [19]. Substituting the data, this is 203.18 m3.
The difference in the water storage of the sunken green space ΔU [19] is as follows:where Δh0 is the critical sinking depth of the sunken green space, m. Substituting the data, this is 282.2 m3.
The rainwater infiltration rate N [19] is as follows:where S is the rainwater infiltration capacity, m3; ΔU is the sunken green space water storage difference, m3; C is the green space runoff coefficient; and F2 is the green space service runoff area, m2. Substituting the data, this is 355%, and 355% is much greater than 100%. Then, the sinking green space in the pocket park can meet the rainwater collection capacity.
(4) Rainwater Garden Calculation. The rainwater garden surface area D [23] is as follows:where E is the rainwater garden catchment area, m2; ƞmax is the maximum runoff coefficient before rainwater garden construction, based on the asphalt pavement runoff coefficient as the standard 0.9 [21]; Ht is the maximum rainfall within t hours of the return period, taken 2 hours from Chaohu City, and the maximum rainfall is 200 mm; б is the permeability coefficient of the rainwater garden, generally 0.3; and ζ is the time for complete rainwater infiltration in the rainwater garden, 2 h. Substituting the data, this is 423.3 m2.
(5) Calculation of the Water Demand for the Pocket Park. According to the relevant norms and standards, the greenery in the park is maintained at the first level [24]. The quota standards are shown in Table 4.
The average daily water consumption for greening is Q1:where L1 is the greening and sprinkling quota, m3/(m2·a); X is the sprinkling days, 365 d. Substituting the data, this is 3.866 m3.
The average daily water consumption of roads is Q2:where F3 is the road area in the pocket park, m2; ω is the annual sprinkling times, times/year; and L2 is the road sprinkling quota, L/(m2·times). Substituting the data, this is 0.174 m3.
The average daily water consumption of the rainwater treatment system is 5% of the average daily water consumption of greening and roads [24], which is Q3:where Q1 is the average daily water consumption for greening, m3; Q2 is the average daily water consumption for roads, m3. Substituting the data, this is 0.202 m3.
The average daily water consumption not encountered was 10% of the average daily water consumption of greening, roads, and rainwater treatment systems [24], which is Q4:where Q3 is the average self-consumption water consumption of rainwater treatment system, m3. Substituting the data, this is 0.425 m3.
Then, the average daily rainfall demand of the pocket park is Qr:where Q4 is the average daily water consumption not met, m3. Substituting the data, this is 4.667 m3.
The daily variation coefficient is 1.5, and the maximum daily rainfall demand is Qmax:where e is the daily variation coefficient, 1.5; Qr is the average daily rainfall demand of the pocket park, m3. Substituting the data, this is 7.001 m3.
(5) Recyclable Rainwater in the Pocket Park. Combined with the related codes [21], the rainwater runoff coefficient is shown in Table 5.
Rainwater daily runoff Wd (the rain-recyclable coefficient is taken as 0.7) [24] is as follows:where W1 is the greening daily runoff, m3; W2 is the road daily runoff, m3; θ is the rainfall recyclable coefficient, 0.7; λ1 is the Greenland rainwater runoff coefficient; λ2 is the road rainwater runoff coefficient; ha1 is the maximum Greenland rainfall, mm; ha2 is the maximum rainfall on the road, mm; F1 is the green area, m2; and F3 is the road area in the pocket park, hm2. Substituting the data, W1 is 13.334 m3, W2 is 40.002 m3, and Wd is 53.336 m3.
The initial rainfall runoff must be discarded. Taking the initial rainwater discarded height as 2 mm, the rainfall discarded flow Wq iswhere ν is the height of the initial rainwater discharge, mm. Substituting the data, this is 28.220 m3.
The available rainwater W iswhere Wd is the rainwater daily runoff, m3; Wq is the rainfall discarded rainfall, m3. Substituting the data, this is 25.116 m3.
After calculating W > Qmax, the reconstruction of the pocket park meets the requirements.
(6) Calculation of the Pocket Park Runoff Pollutant Control. The pollutants in the pocket park use suspended solids (SS) as the pollutant control index for the pocket park runoff. The standard for the removal rate of suspended solids (SS) for each part is shown in Table 6.
Then, the average removal rate, β, of suspended solids (SS) after taking low-impact development measures iswhere F is the total area of the park, m2. Substituting the data, this is 52.5%.
Annual runoff control rate G is as follows:where Z is the total annual runoff control rate, 85%; β is the average removal rate of suspended solids (SS) after measures are taken. Substituting the data, this is 44.625%.
We calculated that the annual total control rate of SS was greater than 40%, and the annual total control rate of SS after sponge transformation was 44.625% > 40% [22]; therefore, the pollutant control rate requirement is met.
5. Reconstruction Effect and Effect Evaluation of the Pocket Park
5.1. Analysis of the Effect of Sponge Renovation in the Pocket Park
After the pocket park underwent a series of sponge renovation measures, the phenomenon of water accumulation and waterlogging in the park was significantly reduced, the utilization rate of rainwater was greatly improved, the runoff pollution was effectively controlled, and the sense of the park landscape experience was further enhanced. The comparative analysis of the pocket park sponge before and after transformation is shown in Table 7.
5.2. Using Analytic Hierarchy Process (AHP) to Evaluate the Effect of Pocket Park Sponge Renovation
Sponge cities will encounter various challenges in the construction process. The sponge city indicators and decision-making evaluation models constructed by the fuzzy multicriteria method are conducive to discovering problems and providing a basis for solving problems [25]; the construction effect evaluation of sponge cities can be used; AHP is conducive to the study of problems at different levels. AHP refers to a decision-making method that decomposes complex target decision-making problems into the target level, criterion level, and plan level and, on this basis, carries out quantitative and qualitative analyses [26]. AHP is divided into the establishment of a hierarchical model, judgment matrix, single-level sorting, and consistency check. The total level sorting and consistency check several processes and, by model and calculation, finalize the lowest compared to the highest level of importance. We used AHP to evaluate and analyze the effect of pocket park reconstruction. The hierarchical structure system of the transformation effect of the pocket park rainwater system is shown in Figure 2.
Five relevant experts were selected to rank the effects of various transformations of the pocket park rainwater system, and the effects of various measures were selected according to the expert’s ranking results. The criterion of index scale is shown in Table 8.
The index scale of the criterion level is shown in Table 9.
The index scale of the social effect plan is shown in Table 10.
The index scale table of environmental effect plan is shown in Table 11.
The index scale of the rainwater control effect plan is shown in Table 12.
The index scale of the economic control effect plan is shown in Table 13.
The analysis software Yaahp of the AHP was used to calculate and analyze the data of various indicators and to obtain the relative weight of each indicator of the pocket park rainwater system. The relative weight values of various indicators are shown in Table 14.
By attributing weights in the comparative analysis of the programs, we show the stormwater control effect maximum weight value and prove that, after a series of measures to sponge the pocket park, the rain system was significantly improved, reaching the initial transformation of the sponge purpose. At present, the relevant construction evaluation of sponge cities is mainly focused on technology but lacks its multidimensional evaluation of society and economy. In response to this phenomenon, mathematically related network models can be used to construct an evaluation system, such as neural network models [27] and n-Prism network models [28], to optimize the reasonable layout of sponge cities.
6. Conclusions
Through the renovation of the pocket park, we further demonstrated that the pocket park, as a leisure and entertainment place for residents, not only provides a comfortable resting environment but also promotes the development of a good urban environment. Incorporating the concept of a sponge city into the construction of a pocket park not only improves the function of rainwater system and alleviates water pollution but also accelerates the pace of building an environmentally friendly city.
With the improvement of the quality of life of the urban residents, people’s requirements for the surrounding living environment also increase. Concept-built pocket parks will be the mainstream direction of urban infrastructure construction. At present, although there have been many cases of building pocket parks based on the sponge city concept, there are still many shortcomings in the construction or renovation process. Therefore, when incorporating the sponge city concept into pocket park construction, we must pay attention to the principles of ecological priority and local conditions [29]. To achieve the purpose of protecting and developing the environment simultaneously, we provided a solid foundation for the construction of sponge cities.
Data Availability
All the data used to support the findings of this study are included within the article. Any other data are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This paper was supported by the National Science and Technology Major Water Project (2014ZX07303-003-09) and Anhui Province Quality Engineering Project (2018sjjd072).