1. Introduction
The escalating impact of global climate change has exacerbated the frequency and intensity of extreme storms, precipitating a more acute urban flood [
1,
2,
3,
4]. Concurrently, rapid urbanization has accelerated the expansion of impermeable surfaces within urban areas, thereby disrupting natural hydrological processes [
5,
6]. This alteration provides an obstacle to the natural infiltration of rainwater, leading to an upsurge in surface runoff and imposing formidable challenges on existing urban drainage systems [
2,
7,
8,
9]. In light of the ongoing repercussions of climate change and urbanization, conventional stormwater infrastructure (i.e., gray infrastructure, such as pipes, tanks, wastewater treatment plants, etc.) has been inadequate in mitigating urban flooding and curbing pollutant loads, thereby imperiling ecosystem health and human well-being [
5,
10,
11]. Consequently, it is important to develop appropriate policies to respond to these disasters, which are aimed at mitigating the dual threats emanating from climate change and urbanization, ensuring the resilience of urban environments against uncertain hazards [
12,
13].
To systematically address the issue of urban flooding, the Chinese Government introduced the concept of the Sponge City initiative in 2013 [
6,
14,
15,
16]. This strategic approach is grounded in the principles of “source reduction, process control, and system management”. The overarching objectives include averting or mitigating urban flooding, curtailing the adverse impacts of urbanization on natural hydrological processes, and enhancing the operational efficiency of urban water bodies [
3,
17,
18]. Sponge city construction mainly relies on green infrastructure (GI), which refers to an intricately interconnected network of green spaces for safeguarding the values and functions of natural ecosystems and conferring tangible benefits to humanity [
4,
19,
20]. Widely used urban GI in sponge cities includes permeable pavements, rain gardens, green roofs, and so on, each contributing significantly to reinforcing the sustainability and resilience of urban areas [
9,
12,
13,
21,
22]. This holistic approach not only addresses the immediate challenges of stormwater management but also engenders a diverse array of natural ecosystem benefits by curbing and treating stormwater at its sources [
6,
10,
23,
24].
The configuration and planning of GI constitute a pivotal aspect within the ambit of sponge city construction [
3,
25,
26]. The malleability and assortment of GI components, coupled with spatial heterogeneity, display a profound influence on the quantitative analysis of regional characteristics and the judicious siting of GI [
27,
28,
29]. Consequently, the benefits derived from GI implementation exhibit a high degree of localization [
30]. The strategic application of suitability analysis on the installation layout and design of urban GI proves instrumental in discerning optimal GI construction sites and sizes, thereby refining spatial arrangements [
31,
32]. The utilization of Geographic Information Systems (GIS) enables the identification of priority areas for strategically placing and leveraging GI [
30,
33]. This analytical approach assumes paramount importance in enabling urban planners to make informed decisions regarding GI, fostering sustainable urban development [
34,
35]. However, existing research with limited exploration into the evaluation of GI’s suitability [
31] is often confined to small-scale analyses and sparse investigations at the broader city or community scales [
1].
In addition, urban environments are complex, and urban resilience is subject to multifaceted influences [
36]. Multi-functional, interconnected GI systems stand out as an effective strategy for augmenting urban resilience to stormwater [
37,
38,
39,
40,
41]. The concepts and connotations of stormwater resilience are not uniform and fixed and mostly change with practice and application targets [
42]. In this study, we define urban stormwater resilience as the capacity of a city or urban resilient system to withstand and absorb disruptions amid stormwater, adapt while preserving its foundational structure, and achieve recovery and transformation, which needs to take into account the dynamics of stormwater or inundation [
36,
43,
44]. Urban stormwater resilience has gained policy endorsement and widespread application in diverse national management frameworks [
36,
45]. There is a relative scarcity of analyses concerning urban sub-systems (e.g., green and gray infrastructure systems) during stormwater events and comparisons of resilience enhancements under optimal configuration alternatives [
45]. Hence, the establishment of a comprehensive stormwater resilience evaluation system becomes essential, serving as a metric to gauge urban resilience and aiding planners in comprehending and fortifying urban resilience [
46].
The existing urban resilience evaluation indicator system is mainly from the perspective of social, economic, ecological, infrastructure, and other perspectives [
47,
48,
49], and few of them are constructed from the intrinsic features of the resilient system, including absorption, robustness, self-recovery, redundancy, connectivity, etc. For example, in the development of resilience indicators, Mugume et al. [
50] used a theoretical system performance curve to simulate the process of flooding events in evaluating urban stormwater resilience and proposed three evaluation indicators, namely response, robustness, and restorability. In addition, the thinking around Complex Adaptive Systems (CASs) was incorporated into resilience assessment and emphasized the utility of the absorption, adaptation, and recovery capacities [
51]. The fundamental principle of CAS lies in the concept of adapting to building complexity, signifying the sponge city’s ability to preserve its original functionality in the face of perturbations and swiftly revert to its prior state or attain alternative scenario stable states [
52]. Recent contributions by Tansar et al. [
40] created a resilience evaluation system by defining three indicators (e.g., flood volume and flood duration of flood nodes, and sensitivity of flood nodes) and comprehensively evaluated the maximum percentage of each type of GI in each sub-catchment of the land use analysis. Nevertheless, the existing research also exhibits a lack of uniformity in sponge city stormwater resilience evaluation indicators. Scientifically establishing a resilience evaluation framework for sponge cities becomes a paramount concern, and a well-constructed system is instrumental in translating the resilience theory into effective practice [
45].
Moreover, in the current evaluation of urban stormwater resilience, researchers have made different selections of how many years the rainfall return period is. Some researchers, such as Rabori and Ghazavi [
53], only simulated an extreme design return period or a specific design return period when using the 50 a return period to evaluate the performance of urban drainage systems in arid urban areas. Zhang et al. [
54] selected the resilience results of 20 a when indicators showed significant changes to evaluate urban stormwater resilience. In reality, various rainfall intensities may occur in a year, and when evaluating the implementation of an alternative scenario, the performance of the alternative under multiple return periods should be comprehensively considered. Although few scholars have also considered multiple return periods [
55], the resilience evaluation method still lacks consideration of the comprehensive performance of the alternative under various return periods for a whole year.
Therefore, establishing a unified stormwater resilience evaluation framework that captures the dynamic changes in rainwater and waterlogging is crucial. It necessitates quantitative analysis to understand the comprehensive impact of the implemented alternative across different rainfall return periods. In essence, this study endeavors to evaluate the suitability of GI installations through the utilization of ArcGIS (v10.8, Redland CA). The stormwater management model (SWMM) was chosen for the urban stormwater simulation due to its widespread adoption in stormwater management planning, research, and design [
56]. Employing CAS, we introduced an innovative system that focused on the dynamics of stormwater/waterlogging for the creation of an urban stormwater resilience evaluation system tailored to the sponge city. In contrast to previous studies, this system aims to employ quantitative methods to unify the different performances of alternatives under various rainfall return periods rather than only simulating an extreme design return period or a specific design return period, thereby serving as a valuable tool for evaluating the effectiveness of implementing GI alternatives. This study not only offers a novel comprehensive system for urban stormwater resilience but also endeavors to explore the role of GI in enhancing urban stormwater resilience. This study furnishes a scientific reference for city managers or planners, facilitating a deeper understanding of urban stormwater resilience and aiding in informed planning decisions. Such insights are pivotal for advancing the sustainability of urban stormwater management.
4. Discussion
4.1. GI Suitability Analysis and Spatial Layouts
Numerous studies have evaluated the suitability of GI based on diverse criteria, such as ecosystem service contributions, ecological connectivity, and social and natural factors [
62,
82]. In the context of sponge city construction, which heavily relies on water ecological infrastructure techniques [
83], our approach extends the evaluation to encompass engineering conditions. This includes the integration of specific indicators like roof type, architectural design life, and the building structure for green roofs. The incorporation of these indicators contributes to the generation of more scientific and practical layouts for GI installation within the urban environment.
In the analysis of the GI installation layout, it was evident that permeable pavements exhibit the least suitability, encompassing a mere 2.01% of the study area. This finding aligns with similar observations in the studies of other scholars; for instance, Chuang et al. [
1] reported a comparable 2% coverage using the water footprint method. This was due to the limited size of the area consisting of sidewalks and parking lots where permeable pavement could be placed [
35]. Moreover, the predominant suitability areas for GI were concentrated in the western and northern parts of the study area. This distribution is presumed to be influenced by the southern and eastern areas of Fengxi New City being predominantly cultivated land.
4.2. Selection and Improvement of Indicators for Urban Stormwater Resilience
This paper presents an innovative urban stormwater resilience evaluation system designated for sponge cities, evaluating the advantages of GI implementation to form flooding resilience. We improved the theoretical system performance curve proposed by Mugume et al. [
50] by integrating CAS thinking with absorptivity, restorability, and robustness as indicators of the urban stormwater resilience evaluation system. This adaptive system approach allows for a full understanding of how waterlogging dynamically emerges and how the various sub-systems in the city dynamically change amid stormwater.
As mentioned earlier, few existing urban stormwater resilience evaluation indicator systems have been constructed from the perspective of the intrinsic features of the resilient system. In the selection of indicator quantification, we utilized the ratio of the difference between rainfall and runoff volume for each sub-catchment to the total rainfall volume to express absorptivity, which is one of the innovations of our study. This metric effectively captures the system’s intrinsic capability to autonomously absorb perturbations, showcasing the extent to which it minimizes the impact of such disturbances [
84]. It also takes into account the ability of GI to support urban water sources, the security of urban water supply (stormwater reuse), and the enhancement of urban sustainability and adaptability. Robustness was quantified by the ratio of the remaining conduit capacity to total conduit capacity, emphasizing its role in reducing and delaying runoff by enhancing stormwater infiltration and storage capacity [
77]. This indicator provides an accurate indication of the volume and water storage capacity of the entire urban conduit system in response to external shocks, reflecting how well it maintains its function and avoids complete functional collapse [
85], demonstrating the engineering resilience of urban conduits in response to stormwater flooding. Restorability, represented by the overflow time of junctions, embodies the dynamic nature of repairs and serves as an apt measure to reflect the coordination efficiency among GIs [
84]. This indicator also reflects the resilience of the system as a whole in the occurrence of flooding. This evaluation system can provide a better understanding of the system’s internal structure and operating mechanisms.
Three key elements (conduits, junctions, and sub-catchments) were selected for the calculation of these three resilience indicators, aiming to provide comprehensive coverage that is consistent with the different phases of the simulation curve of the stormwater process. They are all directly accessible from the SWMM simulation results, ensuring a scientifically sound and reasonable evaluation with reduced uncertainty.
As previously highlighted, a single design return period is not sufficient to represent the actual capability of the alternatives, but a combination of simulated values for various design return periods can improve the overall evaluation. Therefore, the performance of different alternatives with various return periods (2, 5, 10, 20, and 50 a) was considered in the evaluation system, and the entropy method was used to determine the significance of each design return period. Higher weights were assigned to the design return periods with high variability, which were then multiplied with the simulation results to obtain the three indicators for evaluating urban stormwater resilience. The quantitative values derived from this method take into account the various alternatives that the scheme may be subjected to in the course of real-life implementation and provide a scientific and reasonable evaluation of the scheme’s capability, as well as a more supportive basis for future retrofitting and planning.
4.3. The Roles of GI in Improving Urban Stormwater Resilience
Spatial differences in the suitability of GI installations lead to differences in the layout of GI installations. As a result, there were some areas that were unsuitable for GI installation even though the surface runoff volume was high. Moreover, in some areas in the eastern area of the study area, insufficient conduit diameter and unsuitability for GI installations resulted in high junction overflow times.
The results emphasized a substantial enhancement in urban stormwater resilience. For this single type of GI installation, rain gardens consistently maintained a high level of regulatory performance, and green roofs were the least effective among the three, which was corroborated by the results of the previous research in the same study area [
70]. In contrast, permeable pavements were able to achieve better resilience enhancement with a smaller area [
48]. In addition, the study found that the GI combination alternatives resulted in a greater increase in resilience enhancement than the alternatives with just a single type of GI, which is also consistent with previous research findings [
86,
87]. Overall, the results suggest that the use of GI is effective in increasing the flood resilience of the resilient system [
51].
4.4. Suggestions for Building Sponge City
Initially, in the evaluation of the GI installation’s suitability, consideration should be given to the constraints posed by engineering conditions. Solely focusing on natural factors in the establishment of GI may lead to the oversight of human activities during urban development and construction. This is particularly pertinent for GI constructed into buildings, such as green roofs, where the impact of engineering technology is substantial. Consequently, it is advisable to incorporate engineering factors in future evaluations of the appropriateness of GI to ensure a comprehensive and accurate evaluation.
Subsequently, when evaluating the role of GI in improving stormwater resilience, the results of various design return periods need to be considered in the evaluation process, combining the performance of multiple alternatives. Resilience also needs to be evaluated by considering the intrinsic features of the resilient system.
Moreover, in the context of selecting GI for sponge city construction, considering the challenges associated with retrofitting GI on private land and the performance of GI implementation, the scenario for installing rain gardens, permeable paving, and green roofs for public buildings proves to be more economically efficient than the scenario for installing all GIs. In situations with limited financial resources, prioritizing the construction of rain gardens can offer relatively better results with constrained funds. It is advisable to avoid exclusively opting for green roofs, as this may represent the least efficient alternative scenario.
Finally, areas unsuitable for GI but experiencing high surface runoff should enhance monitoring, control, and management measures to improve resilience [
40]. Alternatively, augmenting wetlands or open green spaces is advocated to diminish urban impervious areas [
88].
4.5. Limitations and Future Research Directions
This study’s limitations include a focus on the current GI installation layout and resilience evaluation within existing land use without considering potential future changes resulting from ongoing urban development. Consequently, as impervious surfaces increase in the future, it is likely that the current scenario will become less instructive for future scenarios. In addition, the scale of this study is wide, and specific analyses for city streets or lots are missing, especially for the control of runoff performance over time in the draining systems. Moreover, it is important to note that the identified optimal improvement options are tailored specifically to the study area and may only contribute to local stormwater management. But the methodology in this study remains valuable for urban planners, aiding in the identification of optimal GI solutions for enhancing urban resilience. Future research could explore changes in impermeable areas and the impact of climate change on urban stormwater resilience.
Furthermore, this study’s resilience evaluation focuses solely on the effectiveness of resilient system responses concerning stormwater management, neglecting external factors such as government management and public participation in flood response. Incorporating input from various stakeholders in identifying project priorities and garnering public support for improvement plans could enhance collective action during stormwater events [
89]. Additionally, population distribution, particularly among vulnerable groups, is crucial for assessing restorability [
47]. In addition, in the analysis of natural disaster risk evaluation, studies on the vulnerability of the disaster-bearing body have become a hot content [
90]. Future resilience research should encompass external factors like governmental policies, disaster mitigation programs, and public participation while also addressing population distribution and vulnerability to provide a more comprehensive understanding of urban stormwater resilience capacity.
5. Conclusions
This study centered on Fengxi New City, China, and utilized SWMM to evaluate both the suitability of GI and the overall resilience of the resilient system. Our findings offer crucial insights into optimizing the layouts of GI installations and fortifying stormwater resilience.
Firstly, in GI installation suitability analysis and spatial layouts, we integrated engineering and technical factors for a comprehensive evaluation of GI’s suitability. The analysis revealed limited suitability for permeable pavement (2.01% coverage). Suitability areas were concentrated in the western and northern areas of the study areas, which was attributed to the southern and eastern areas being predominantly cultivated and less developed. Secondly, in terms of urban stormwater resilience, we introduced a comprehensive evaluation system comprising absorptivity, robustness, and recoverability. Employing the entropy method, we determined the significance of each rainfall return period and integrated the simulation results under multiple design return periods to achieve the resilience evaluation of each scenario rather than only simulating an extreme design return period or a specific design return period. The results demonstrated a notable improvement in resilience with GI implementations. Lastly, varying performance was observed among GIs, with rain gardens consistently proving effective. The combination of multiple GIs exhibited substantial resilience enhancement, emphasizing the synergistic benefits of diverse GI alternatives. This methodology provides valuable insights for urban planners, offering a robust foundation for deriving optimal alternatives that can maximize the benefits of GI and bolster urban stormwater resilience.