1. Introduction
In recent years, high-speed urbanization led to a rapid increase in the impervious area of the surface, and the natural hydrologic cycle changed greatly [
1,
2]. Due to these changes, urban flooding and runoff pollution occur frequently, which are also caused by extreme climate events [
3,
4]. China is a society with severe urban flooding problems, and damages caused by flooding are exponentially increasing [
5]. A survey indicated that 62% of Chinese cities suffered from floods, with direct economic losses amounting to
$100 billion from 2011 to 2014 [
6]. However, as developed countries in North America and Europe previously rapidly urbanized, they faced and addressed stormwater problems earlier than China [
7]. Urban drainage systems aim at draining surface runoff out of urban areas. However, traditional urban drainage systems are not able to meet the current requirements for protection from urban flooding [
8]. Because of the frequent occurrence of flood disasters in China and the precedents of other countries, it is of great significance for China to propose new suitable policies to deal with these hazards [
7].
The Chinese government launched sponge city (SPC) construction to address these challenges [
9]. The concept of SPC indicates that a city can function as a sponge, whereby it absorbs, stores, infiltrates, and purifies stormwater, and releases it for reuse when necessary [
9,
10,
11,
12]. Sponge city construction aims to realize effective control and utilization of city rainwater; it is a new concept that controls rainwater based on green infrastructure, such as rain gardens, green roofs, and so on [
13,
14,
15]. The sponge city (pilot) problem was launched under the guidance and support of the Ministry of Housing and Rural–Urban Development (MOHURD), Ministry of Finance (MOF), and Ministry of Water Resources (MWR) at the end of 2014. As of now, there are more than 30 pilot sponge cities set up in China. Although some of them provided benefits to urban environment, a range of challenges are still present, such as technical challenges, physical challenges, and financial challenges [
5,
9]. On the whole, there are three ways for sponge city construction: (1) protection of the original urban ecosystem, (2) ecological remediation and restoration, and (3) low-impact development (LID) [
16]. LID systems are characterized by a series of micro-scale stormwater devices which are near to or located at the source of runoff [
2]. These LID techniques infiltrate, retain, and purify stormwater at the source by reducing imperviousness of urban areas [
17,
18,
19]. They mainly include green roofs, permeable pavements, concave greenbelts, rain gardens, and so on [
20].
There are many other urban water terms around the world which are similar to SPC, such as sustainable drainage systems (SuDs), water-sensitive urban design (WSUD), and green stormwater infrastructure (GSI) [
2,
5,
21]. SuDs were developed in the United Kingdom in late 1980s and consisted of a diverse range of technologies used to drain stormwater in a more sustainable manner than conventional solutions. These technologies aim to replicate as closely as possible the natural flow of water and to offset the excess runoff caused by urbanization at source [
2,
5]. The term WSUD was first used in Australia in the 1990s and underwent several upgrades since then. It mainly aims at the protection and management of the urban water circle [
2,
5]. GSI is a suite of interventions which comprises artificial and natural materials. These materials utilize vegetation to mitigate surface runoff [
21]. GSI projects include natural elements (i.e., wetlands, street trees, and rain gardens) and engineered techniques (i.e., green roofs, bio-retention trenches, and permeable pavements), which mimic the hydrology of the natural landscape [
22]. The term GSI is widely used in the stormwater literature, and is synonymous with LID [
2]. Bai et al. [
23] found that LID facilities based on infiltration performed better in runoff reduction than LID facilities based on storage. It was noted that there are many researches on LID techniques.
Green roofs are building roofs which are covered with vegetation, planted on waterproofing membranes. A green roof has a soil layer on a special drainage mat material that can convey excess rainwater off the building’s roof; it is also an important part of sponge city construction [
4,
21]. Alfredo et al. [
24] reported that green roofs can delay peak time, with the reduction rate of peak flow ranging from 30% to 78% compared to conventional roofs. Dietz’s [
25] experimental results indicated that a green roof can reduce flood volume by 60–70% compared with a traditional roof.
Permeable pavements are filled with gravel, paved with a porous or bituminous concrete, allowing for rainwater to flow through it. In addition to reducing surface runoff, permeable pavements can filter pollutants from rainwater, thereby improving the environment [
26]. Permeable pavements are recommended as an effective way of managing runoff from the surface and controlling pollutants in the soil [
27,
28]. They are usually applied to roads, paths, squares, and parking lots. Qin et al. [
8] reported that the distributions of peak flow and volume were statistically different between asphalt catchment runoff and permeable pavement underdrain discharge; peak flow from permeable pavement underdrain was less flashy and tended to show less variation overall compared to asphalt runoff, which usually mirrored spikes in rainfall intensity. Abbott and Comino-Mateos [
29] found that, on the permeable pavement of a parking lot, an average of 22% of the runoff left the system during the storm.
Concave greenbelts are a type of bio-retention cell containing vegetation such as green grasses. Therefore, they can provide infiltration, storage, and evaporation of rainwater from surrounding areas [
23]. Luan et al. [
30] evaluated the simulated effectiveness of surface runoff reduction with three concave greenbelts scenarios with a concave depth of 10 cm and concave greenbelt ratios of 50%, 70%, and 90%. The runoff reduction ranged from 5.2% to 57.3% in all types of rainfall events.
Rain gardens (stormwater gardens) usually refer to extensive vegetated depressions that collect urban runoff from surrounding impervious areas, thereby promoting evapotranspiration and infiltration [
21,
31]. The plants, such as small trees, wildflowers, and ferns, make excess rainwater feasible to flow into a rain garden. In order to make maintenance requirements desirable, rain gardens are to be located near institutions, such as community centers and schools [
21]. Avellaneda et al. [
32] showed that the flood hygrograph shifted downward after adding green infrastructure facilities such as green gardens.
To quantify LID benefits, researchers should develop some rainfall-runoff models. The US Environmental Protection Agency (EPA) Storm Water Management Model (SWMM) is a dynamic rainfall-runoff simulation model which is used to simulate the formation of urban runoff, and is also widely used for LID simulation [
8,
13,
20,
23]. SWMM was developed in 1971 [
33], and the current edition is Version 5.1. SWMM 5.1 set up eight LID controls in the LID module, namely green roofs, permeable pavements, bio-retention cells, rain gardens, roof closure facilities, infiltration trenches, rainwater tanks, and grassed swales [
34].
This paper analyzed the hydrologic effects of various LID scenarios in Sucheng district, Suqian city, China. A comprehensive evaluation system based on an analytic hierarchy process (AHP) [
13] was proposed to quantify the performance of each LID technique, and we used the SWMM model to obtain simulated results. Three indicators were selected: runoff reduction, peak flow reduction, and economical cost. Finally, we provided an optimal scheme for sponge city construction in the case study. The simulation results can provide useful guidance for the selection of LID techniques, and can also provide technical support for sponge city construction in urban areas. Some suggestions for sponge city construction are given in the conclusions.
4. Conclusions
The utilization of LID measures is increasingly important for sponge city construction. To provide a reasonable layout of LID measures in Sucheng district, a comprehensive evaluation system was proposed to quantify the performance of different LID scenarios with regard to the environment and economy. Based on the results of this project, several conclusions can be made as follows:
(1) For environmental benefits, results show that runoff reduction varies with different LID facilities; the green roof performed best, while the rain garden performed worst. The performance of each LID facility in runoff and peak reduction varied with the LID proportion of maximum construction area. As the LID proportion of maximum construction area became smaller, the capacity for stormwater management of each LID facility was limited. Specifically, with the increase of the LID proportion of maximum construction area, green roofs performed better than permeable pavements in reducing runoff volume and peak flow, and permeable pavements performed better in peak reduction than runoff reduction. Compared with the results of Liao et al. [
7] and Li et al. [
46], the influence of LID proportion of maximum construction area on hydrological performance was evaluated. Overall, LID facilities performed better in the reduction of runoff volume than peak flow.
(2) For economic costs, the unit cost of each LID facility was ranked as follows: rain garden > green roof > permeable pavement > concave greenbelt. The rain garden was the most expensive, while the concave greenbelt was the cheapest.
(3) The comprehensive benefits presented various trends with different proportions of LID facilities. Increased surface area of green roofs, permeable pavements, and concave greenbelts, with fewer rain gardens are suggested for maximum comprehensive benefits; however, combined scenario 2 (A* 40%, B* 40%, C* 40%, D* 40) could also be an alternative. These results can be the reference for the optimization of LID facilities in sponge city construction in Sucheng district. Sponge city construction in the study area can achieve desirable comprehensive performance based on the proposed evaluation system. However, more detailed modeling studies should be taken into account before practical uses of these results, so as to apply LID schemes effectively. In this case, some neglected factors, such as the social factor and water quality factor, might lead to limitations in evaluating the comprehensive benefits of each LID scenario. Despite the limitations, the conclusions are valuable for sponge city construction. For future research, we can carry out a systematic study based on a more integrated methodology, as well as in accordance with specific conditions and actual planning of a sponge city.