water Article Comprehensive Performance Evaluation System Based on Environmental and Economic Benefits for Optimal Allocation of LID Facilities Yiran Bai, Yuhong Li *, Ruoyu Zhang, Na Zhao and Xiaofan Zeng School of Hydropower and Information Engineering, Huazhong University of Science and Technology, Wuhan 430074, China; [email protected] (Y.B.); [email protected] (R.Z.); [email protected] (N.Z.); [email protected] (X.Z.) * Correspondence: [email protected]; Tel.: +86-138-8610-1077 Received: 7 January 2019; Accepted: 13 February 2019; Published: 18 February 2019 Abstract: In recent years, urban flooding occurred frequently because of extreme rainstorms. Sponge city construction can effectively mitigate urban flooding and improve urban rainwater utilization. Low-impact development (LID) is regarded as a sustainable solution for urban stormwater management. In this project, a comprehensive evaluation system was developed based on environmental and economic benefits using the analytical hierarchy process (AHP) and the Storm Water Management Model (SWMM) of the United States (US) Environmental Protection Agency (EPA). The performance of four LID scenarios with the same locations but different sizes of green roof, permeable pavement, concave greenbelt, and rain garden were analyzed in the Sucheng district of Jiangsu province in China. Results illustrate that the green roof performed best in reducing runoff, while the rain garden performed worst. The LID combination scenario (1) that contained more green roof, permeable pavement, and concave greenbelt facilities, but fewer rain gardens had the better comprehensive performance on the basis of environmental and economic benefits. The combined scenario (2) (LID proportion of maximum construction area was 40%) could also be an alternative. This study provides a guide to optimize LID layouts for sponge city construction, which can also provide optimal selection for other sponge city constructions. Keywords: sponge city construction; LID; comprehensive evaluation system; AHP; SWMM 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, Water 2019, 11, 341; doi:10.3390/w11020341 www.mdpi.com/journal/water Water 2019, 11, 341 2 of 14 and purifies stormwater, and releases it for reuse when necessary [9–12]. 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 [13–15]. 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 [17–19]. 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. Water 2019, 11, 341 3 of 14 Water Rain2019, 11 gardens, x FOR PEER (stormwater REVIEW gardens) usually refer to extensive vegetated depressions3 thatof 14 collect urban runoff from surrounding impervious areas, thereby promoting evapotranspiration infiltrationand infiltration [21,31]. [21, 31The]. Theplants, plants, such such as assmall small trees, trees, wildflowers, wildflowers, and and ferns, ferns, make make excess excess rainwater rainwater feasible to flow flow into into a a rain rain garden. garden. In In order order to to make make maintenance maintenance requirements requirements desirable, desirable, rain rain gardens gardens are to be located near institutions, such as communi communityty centers and schools [21]. [21]. Avellaneda Avellaneda et al. [32] [32] showed that the flood flood hygrograph hygrograph shifted downward downward after adding green infrastructure facilities such as green gardens. To quantifyquantify LIDLID benefits,benefits, researchersresearchers shouldshould developdevelop somesome rainfall-runoffrainfall-runoff models.models. The US Environmental Protection Agency (EPA) (EPA) Storm Water Management Model (SWMM) is a dynamic rainfall-runoff simulation model which is used to si simulatemulate the formation of urban runoff, and is