water

Review Urban Flooding Mitigation Techniques: A Systematic Review and Future Studies

Yinghong Qin 1,2 1 College of Civil Engineering and Architecture, Guilin University of Technology, Guilin 541004, China; [email protected]; Tel.: +86-0771-323-2464 2 College of Civil Engineering and Architecture, Guangxi University, 100 University Road, Nanning 530004, China



Received: 20 November 2020; Accepted: 14 December 2020; Published: 20 December 2020


Abstract: Urbanization has replaced natural permeable surfaces with roofs, roads, and other sealed surfaces, which convert rainfall into runoff that finally is carried away by the local sewage system. High intensity rainfall can cause flooding when the city sewer system fails to carry the amounts of runoff offsite. Although projects, such as low-impact development and water-sensitive urban design, have been proposed to retain, detain, infiltrate, harvest, evaporate, transpire, or re-use rainwater on-site, urban flooding is still a serious, unresolved problem. This review sequentially discusses runoff reduction facilities installed above the ground, at the ground surface, and underground. Mainstream techniques include green roofs, non-vegetated roofs, permeable pavements, water-retaining pavements, infiltration trenches, trees, rainwater harvest, rain garden, vegetated filter strip, swale, and soakaways. While these techniques function differently, they share a common characteristic; that is, they can effectively reduce runoff for small rainfalls but lead to overflow in the case of heavy rainfalls. In addition, most of these techniques require sizable land areas for construction. The end of this review highlights the necessity of developing novel, discharge-controllable facilities that can attenuate the peak flow of urban runoff by extending the duration of the runoff discharge.

Keywords: urban stormwater management; green roofs; permeable pavements; low-impact development; bio-retention

1. Introduction Most of rainwater reaching the ground surface either infiltrates the soil or returns to the air by evaporation and evapotranspiration. Urbanization has sealed natural soils by pavements, roofs, and other impervious surfaces, constraining natural infiltration and evapotranspiration and converting rainfall into runoff [1,2]. Runoff from open soils in urbanized areas is also increasing because construction activities have compacted soils to behave like impermeable surfaces [3,4]. Traditionally, engineered facilities such as gutters, channels, and pipes are built to convey runoff from sealed surfaces to centralized detention ponds, retention facilities, and nearby streams as quickly as possible. In heavy rainfall, these facilities usually fail to compete with the superimposed peak flows discharging from different sealed surfaces, leading to overflow and flooding [5]. Across the globe, on-site treatment techniques have been recently applied, either separately or in combination, to retain, detain, infiltrate, harvest, evaporate, transpire, or re-use rainwater on the source and thus to reduce both the urban runoff volume and runoff peak discharge. On-site stormwater treatment approaches are termed as Integrated Urban Water Management [6], Water Sensitive Urban Design [7], low impact development [8], Active Beautiful Clean [9], Sponge Cities [10], Sustainable Urban Drainage system [11], and other similar terminologies. While the terms are different, their

Water 2020, 12, 3579; doi:10.3390/w12123579 www.mdpi.com/journal/water Water 2020, 12, 3579 2 of 23 planning and designing principles are similar, with the same goal of restoring pre-development hydrology by treating rainwater at the source [12,13]. While some of these approaches may not be purposed primarily to mitigate urban flooding, they have co-benefits of mitigating urban flooding by reducing the runoff volume and attenuating the peak-flow discharge. Pratt [14] had reviewed rainwater source-control techniques categorizing them into ground-surface and below-ground approaches. Dietz [15] reviewed the practices of low impact development, with a focus on water quality improvement rather than on urban flooding mitigation. Recently, Ahiablame et al. [16] reviewed the benefits related to the practice of low impact development, with a focus on water quality observation and modeling. Which the techniques to mitigate urban flooding are well documented, it is necessary to comprehensively compare the effectiveness of these techniques. This review compares mainstream techniques available to mitigate urban flooding and compiles the wide-range but diffuse literature on the topic of rainwater-runoff reduction and peak-flow attenuation. With this review, engineers, designers, and administrators can overview the mainstream flooding-control techniques. The end of the review highlights the necessity of developing novel, discharge-controllable facilities that can attenuate the peak flow of urban runoff by extending the duration of the runoff discharge.

2. Methodology The review process includes two steps. First, we searched for articles in Google Scholar (https: //scholar.google.com/) using the search strings “urban flooding”, “urban rainwater management”, “runoff reduction”, and “peak flow discharge”. Only journals articles, proceedings, and reports were selected. For those papers published before 2018, only the papers that had been cited more than twice were considered. For those published after 2018, only the papers indexed by Web of Science and/or Engineering Village were candidates. After reading the abstract of each selected paper, the full-text of the paper was downloaded if the abstract is rightly related to the mitigation of urban flooding. The reference section of the downloaded papers was then scanned for any possible related papers on the topic of urban flooding. This process of reading abstract and scanning reference was repeated until no more journals articles, proceedings, and reports related to the topic of urban flooding mitigation were found. Totally, 550 papers were downloaded for deeper reading. Second, each selected paper was read carefully. It is found that most papers reported on techniques to simultaneously reduce the runoff volume and to improve water quality. Only those papers with the core ideas on urban flooding mitigation, runoff volume reduction, and/or peak flow attenuation were selected. In this process, we found that 353 of the downloaded papers fell marginally on the topic of urban flooding mitigation, and that about 14 papers were replicated. Finally, 183 papers were included. Core findings of these papers were then integrated, synthesized, and compiled. We found that mainstream techniques for urban-flooding mitigation included green roofs, non-vegetated roof, trees, permeable pavements, water-retaining pavements, infiltration trenches, rain barrels, rainwater tanks, bioretentions, soakaways, and underground tanks. While different techniques could be grouped differently, the best way to review these techniques was to categorize them into above-ground techniques, ground-level techniques, and underground techniques (Table1). As a review of the mainstream techniques for urban flooding mitigation, we did not intend to compare experimental and modeling data in different articles. Instead, the peak-flow attenuation capacity of each technique was schematically analyzed to illustrate the working mechanism of each technique.

Table 1. Runoff can be controlled above, on and below the ground.

Runoff-Control Sites Facilities Above the ground green roofs, non-vegetated roof trees, pervious pavements, water-retaining pavements, infiltration trenches, On the ground rain barrels, rainwater tanks, bioretentions Below the ground soakaways, and underground cisterns Water 2020, 12, x 3 of 23

Table 1. Runoff can be controlled above, on and below the ground.

Runoff-control sites Facilities Above the ground green roofs, non-vegetated roof trees, pervious pavements, water-retaining pavements, infiltration trenches, On the ground rain barrels, rainwater tanks, bioretentions Below the ground soakaways, and underground cisterns

3. Above-Ground Techniques

3.1. Green Roofs Green roofs, known as vegetated roofs, eco-roofs, and natural roofs, are built by adding a plant layer and a growing medium upon to the roof deck of a traditional roof. A typical green roof has six layersWater including2020, 12, 3579 plants, a growing medium, a filter, a drainage layer, a root barrier, and a waterproof3 of 23 membrane (Figure 1). According to the thickness of the roof layers, a green roof with a substrate depth of 10 cm or less is called as an extensive roof. Intensive green roofs have a thicker substrate and3. Above-Grounda taller plant but Techniques are less commonly used than extensive green roofs. 3.1.Green Green roofs Roofs offer a serial of environmental benefits such as providing thermal insulation for the buildings [17], creating a habitat for wildlife [18], increasing the aesthetic appeal of the landscape [19], mitigatingGreen roofs, urban known heat as islands vegetated [20], roofs, sequestrating eco-roofs, andcarbon natural dioxide roofs, [21], are builtand byothers adding [22]. aplant In addition,layer and a green a growing roof can medium be used upon as a to source-contr the roof deckol offacility a traditional to reduce roof. urban A typical runoff green[23]. Rainwater roof has six thatlayers falls including on a green plants, roof a can growing be detained medium, and a filter, retained a drainage in the layer, layers a rootof green barrier, roofs, and except a waterproof that additionalmembrane rainwater (Figure1 ).enters According downspouts to the thickness and finall ofy thedischarges roof layers, to local a green sewer roof pipes with a[24]. substrate The green depth roofof 10subsequently cm or less is calledevacuates as an the extensive absorbed roof. water Intensive via greensoil evaporation roofs have a thickerand plant substrate transpiration, and a taller resettingplant but for are absorbing less commonly rainwater used during than extensivethe next rainfall green roofs.[25].

roof deck filt er drai nage l ayer downspout root barri er wat erproof me mbrane

FigureFigure 1. 1.RainwaterRainwater falling falling on on a green a green roof roof is isretained retained an andd detained detained by by plants plants and and soils; soils; additional additional rainfallrainfall leads leads to tooverflow. overflow. Green roofs offer a serial of environmental benefits such as providing thermal insulation for the 3.1.1. Extensive Green Roof buildings [17], creating a habitat for wildlife [18], increasing the aesthetic appeal of the landscape [19], mitigatingThe rate urban of rainwater heat islands retained [20], and sequestrating detained by carbon a green dioxide roof is [21 widely], and different, others [22 for]. In instance, addition, 13.8–60.8%a green roof [26], can 12–25% be used [27], as a 35.5–100% source-control [28], facility 32–50.4% to reduce [29], and urban 19–98% runo ff[30].[23 ].Gregoire Rainwater and that Clausen falls on [31]a green summarized roof can the be detainedrainwater and retention retained of in different the layers roofs of green and roofs,found exceptan average that additional retention rainwaterof 56%. Theenters runoff downspouts reduction capacity and finally depends discharges on the to plant local species, sewer pipessubstrate [24]. depth, The greenroof slope, roof subsequentlyantecedent dryevacuates period, therainfall absorbed depth, water as well via as soil the evaporation local climate and [32]. plant Different transpiration, plant species resetting have for different absorbing water-retainingrainwater during capacities the next in rainfall an order [25 ].of grass > sedum > forb [33]. Similar results are reported by [34,35]. Under the plants, a thicker roof substrate retains and detains more water and thus delays the peak3.1.1. flow Extensive further Green[36]. The Roof annual runoff coefficient of a specific roof decreases as the thickness of substrateThe layer rate increases of rainwater [37]. retained The water and retaining detained capacity by a green of a green roof is roof widely is also diff influencederent, for instance,by the roof’s13.8–60.8% configuration [26], 12–25% [38]. [A27 green], 35.5–100% roof with [28 ],a 32–50.4%smaller slope [29], retains and 19–98% more [ 30water]. Gregoire in the substrate and Clausen layer [31 ] andsummarized thus leads the to rainwaterthe less runoff retention volume of diff erent[39]. roofsThe water and found retaining an average capacity retention of the of substrate 56%. The runoalso ff dependsreduction on capacitythe dry period depends before on the the plant rain, species, with a substratelonger antecedent depth, roof dry slope, period antecedent resulting dry in more period, rainwaterrainfall depth,retention as well in the as thesubstrate local climate [40]. The [32]. rati Diffo erentof the plant water species retention have divolumefferent to water-retaining the rainfall volumecapacities decreases in an order as the of rainfall grass > depthsedum increases> forb [33 [4].1]. Similar Similarly, results by aremonito reportedring the by [water-retaining34,35]. Under the capacityplants, aof thicker prototyped roof substrate roofs, Bliss retains et andal. [42] detains found more that water the androofs thus reduced delays the the peakgreatest flow amount further [of36 ]. runoffThe annual in case runo of ffthecoe ffilightcient storm of a specific with a roof short decreases duration. as the As thickness rainfall ofde substratepth and layerduration increases varies [37 ].

The water retaining capacity of a green roof is also influenced by the roof’s configuration [38]. A green roof with a smaller slope retains more water in the substrate layer and thus leads to the less runoff volume [39]. The water retaining capacity of the substrate also depends on the dry period before the rain, with a longer antecedent dry period resulting in more rainwater retention in the substrate [40]. The ratio of the water retention volume to the rainfall volume decreases as the rainfall depth increases [41]. Similarly, by monitoring the water-retaining capacity of prototyped roofs, Bliss et al. [42] found that the roofs reduced the greatest amount of runoff in case of the light storm with a short duration. As rainfall depth and duration varies seasonally, the water-retaining capacity of a green roof also varies seasonally, with a greater value in summer than that in winter [43]. As the grown media retains water in the pores, the peak flow is reduced accordingly. Fassman et al. [44] measured from the water retention of four extensive green roofs and found that the peak flow is 60–90% lower than the control roof. By studying the runoff volume of a green roof and a conventional ballasted roof, Bliss et al. [42] found that the peak flow from the green roof is 5–70% Water 2020, 12, 3579 4 of 23 smaller than that from the ballasted roof. The average runoff coefficient depends highly on the length of the watercourse. As a result, the peak flow can be further reduced if the flow path to the nearby gutter is lengthened and/or if flow-retarding media is used as the substrate [44]. In addition, runoff is delayed because it takes time for the media to be saturated and for rainwater to percolate through the media [45,46]. As the process of a soil being saturated takes hours, a green roof delays the peak flow for hours too [47]. For instance, a delay of half an hour is found in [48,49]; 2 h, in [30,50]; and 2–3 h, in [42]. The true delay of the peak flow depends mainly on the rainfall depth and rainfall intensity [39]. Identification of the peak lag times is difficult and unnecessary because the natural rainfall patterns are irregular and the water storage capacity of a green roof in a specific event varies [51]. A green roof has a high capacity to reduce rainwater especially in case of small rainfall [52]. In case of heavy rainfall, overflow is inevitable after the media are saturated [53]. As urban flooding takes place due to the superposition of the peak flows from different catchments during heavy rainfall, the retention percentage of a green roof at a small rainfall is not important. The importance is the retention depths during a heavy rainfall. As measured by DeNardo et al. [54], the retention and detention depths of a typical green roof are about 1–5 cm, meaning that green roofs inevitably overflow in heavy rainfall. Therefore, green roofs have a marginal ability to mitigate urban flooding in case of frequent, intense, heavy, and extreme rainfall events [55]. There are some contradictory observations related to the water retention of green roofs. Whittinghill et al. [56] observed runoff quantity from traditional sedum- and prairie-covered green roofs over three growing seasons. They found that green roofs covered with prairie reduced a greater amount of runoff than sedum-covered green roofs. Vanuytrecht et al. [33] found that green roofs covered by grass or herb retained rainwater more than green roofs covered by moss sedum did. Similarly, Nagase and Dunnett et al. [35] found that green roofs covered by grass reduce the greatest runoff amount, followed by sedum, and finally forb. It is also found that plants that are taller-stem, larger-canopy, larger-shoots, and deeper-roots retain a greater amount of runoff [35]. Contrary, Buccola and Spolek [36] found that different retention is attributed to the substrate depth rather than plant species, with a thicker roof substrate retaining and detaining a greater amount of rainwater and delaying the peak discharge further. In addition, by quantifying stormwater control of green roofs with different substrate media, Voyde et al. [57] found that the antecedent dry period dictates the water retention of a green roof. This dictation indirectly indicates that the substrate depth dominates the water retention of green roofs because the antecedent dry period determines the moisture content in the substrate soils [58].

3.1.2. Intensive Green Roofs An intensive green roof has a deeper substrate layer for the growth of taller plants. Compared to an extensive green roof, an intensive green roof requires more routine maintenance, is more expensive, and thus is less used [59]. Monitoring runoff from an aged intensive green roof in Manchester, UK during 69 rainfall events, Speak et al. [60] found an average runoff retention of an extensive roof was 67.5%. Kolb [61] monitored the evaporation and runoff coefficients of an intensive roof as well as those of an extensive roof. It is found that compared to an extensive green roof, an intensive green roof evaporates more but discharges a less amount of runoff. Razzaghmanesh and Beecham [62] monitored the hydrologic performance of intensive and extensive green roofs for two years. They found that both roofs reduce the runoff similarly, but that an intensive green roof attenuates the peak flow greater and retard peak flow longer.

3.2. Other Non-Vegetated Roofs Non-vegetated roofs such as ballast roofs or soil roofs reduce runoff as well [63]. Van Woert et al. [39] tested the runoff retention of a vegetated roof, a soil roof, and a gravel roof, of which the substrate depths were the same. The test lasted for 14 months in cases of heavy (>6 mm), medium (2–6 mm), and light (<2 mm) rainfall. While the gravel roof retains the lowest percentage of rainwater, it still Water 2020, 12, 3579 5 of 23

absorbs a sizable amount of rainwater. In addition, both the gravel-covered roof and the soil roof can delay the start of runoff for a quarter to a half of an hour, depending on the rainfall depth and rainfall intensity. A similar finding is reported by Carpenter and Kaluvakolanu [64], who monitored stormwater runoff of stone-ballasted roof over six months. The stone-ballasted captures about 50% of the rainfall volume, attenuates the peak-flow discharge, and delays the peak discharge for 1.2 h. However, during the observations rainfall depths are small. In case of heavy rainfall, non-vegetated roofs also overflow as vegetated roofs do. While non-vegetated roofs are also optional for rainwater management, they are seldom used when compared to vegetated green roofs because vegetated roofs bring more benefits than non-vegetated roofs.

4. Ground Surface Techniques

4.1. Permeable Pavements A permeable pavement system has a permeable surface course laid over a base layer, which is filled with open-graded gravels or stones (Figure2)[ 65]. A filter fabric is usually placed at the bottom of the surface course for preventing water-carrying fine particles from sneaking into the base reservoir [66]. A perforated pipe is optional at the top of the base to bypass the excessive inflow for avoiding waterlogging. Typical permeable pavements include interlocking permeable pavers; Watersoil-filled 2020, 12,grid x pavers; porous asphalt slab; porous concrete slab [67,68]. Design of these permeable6 of 23 pavements can be referred elsewhere, e.g. Mullaney and Lucke [65]. As a stormwater management roof,option, car hoods, permeable or other pavements sources o couldffer multiple saturate benefits the nearby of water permeable purification pavements [69], naturaland lead hydrology to local overflowrestoration [84]. [70 Overflow], and runo fromff reduction a permeable [71], pavement urban heat occurs island when mitigation the subgrade [72], and is tire-pavement of low permeable noise soilsreduction [85]. [73].

1 2 3

4 5 6 1. per meabl e pave ment 2. beddi ng l ayer 3. Filt er l ayer 4. base l ayer 5. geof abri c 6.subgrade soil

Figure 2. Rainwater falling on a permeable pavement is infiltrated to the base, which serves as a Figurereservoir 2. Rainwater to store and falling infiltrate on a rainwater. permeable pavement is infiltrated to the base, which serves as a reservoir to store and infiltrate rainwater. The hydrological performance of a permeable pavement relies on the water storage of the base and on thePermeable saturated concrete hydraulic systems conductivity are prone of the to subgrade be clogged soils. by The fine base particles retains rainwater[86]. Sands, temperately, scraps, organics,which must and drain other timely fine particles for resetting carried the retentionby runoff capacity from other before sources the next sneak heavy into rainfall the cavity [74]. A of thicker, the permeablemore-porous pavement base layer and stores lead ato greater a clogging amount [87]. of Implementation rainwater and thuss of reducespermeable possible pavements surface away runo ff fromto a greatersoil-disturbed extent [75fields]. Where is helpful the soil to infiltrationpreserve the rate permeability is low, the stored of the rainwater pavements spreads [88]. overClogging a large occurredsoil area at to a infiltrate rapid rate through at locations a large areawith [76high]. There coarse are sediments two ways and to circumvent organic sediment this problem. loads One[89]. is Particulatesto increase theclogging thickness in the of the near base surface to restore of morea permeable water during pavement the heavy can rain.be cleared Chai et by al. [vacuum76] found sweeping,that a permeable power washing, shoulder or for a a combination two-lane highway of both needs [90]. aThese 1.5m techniques, aggregate base however, for the are infiltration not useful and torestoration clean the ofclog rainwater. at deep The locations other way [91]. is toIn lineaddition, perforated finer pipes particles in the that base percolate to bypass through the excessive the permeablerainwater surface to the nearby layer waterwayscan finally [settle77,78 ].at Liftingthe aggreg up theate-soil perforated interface, pipe reducing increases the storageinfiltration volume of theand subgrade. the subgrade infiltration, which is helpful to restore local hydrology [79]. The type of permeable pavement significantly influences the hydrological performance [80]. 4.2.Hernandez Water-Retaining et al. [81Pavements] studied four permeable pavement systems with different surface layers and diffAerent water-retaining base materials. pavement Porous concreteis either overlyingan asphalt- a recycledor cement-based aggregate paver base that retains holds a greater water amountat the topof runolayerff preferentiallyand delays the and peak evacuates flow greater the thanwater an via interlock evaporation concrete (Figure pavement 3). Water-retaining over an original pavementsaggregate are base. also Interlock called water-holding, concrete blocks water-rete result inntive, preferential and watered flow through pavements, the gap or other between identical blocks, terminologies [92]. These pavements are fabricated by filling water-retentive media into the pore space of pervious concrete. The fillers can be slag, mortar, moss, hydrophilic tissue, and other water-retentive media [93]. Rainwater falling on this pavement or coming from the nearby sealed surface is held at the surface course, which empties via evaporation. While water-retaining pavements are primarily designed to mitigate urban heat island effect via evaporative cooling [94], they can retain a sizeable amount of water and thus reduce runoff, especially during small rainfall. Depending on the filler materials and the depth of the filler, typically a water-retaining pavement can retain about 15 kg/m2 rainwater [95]. A greater amount can be obtained by optimizing the structure of the water-retaining pavers [96]. This water-retaining capacity means that a dry water-retaining paver can fully absorb a rainfall depth of 1.5 cm without generating runoff [97]. A larger rainfall depth drains to the base or generates runoff when the base and subgrade have saturated. Water-retaining pavements thus also have a limited capacity to reduce the risk of urban flooding.

wat er ret ai ni ng l ayer

base l ayer

subgrade soil

Water 2020, 12, x 6 of 23 roof, car hoods, or other sources could saturate the nearby permeable pavements and lead to local overflow [84]. Overflow from a permeable pavement occurs when the subgrade is of low permeable soils [85].

1 2 3

4 Water 2020, 12, 3579 5 6 of 23 6 1. per meabl e pave ment 2. beddi ng l ayer 3. Filt er l ayer 4. base l ayer reducing the peak-flow lagtime and5. geof a thebri c water retention.6.subg Similarly,rade soil Fontaneda et al. [66] found that although a polymer-modified permeable surface course laid over a crushed-rock base reduces a greater amount of runoff than pervious pavements, the difference is negligible. Except for this small difference, variousFigure permeable 2. Rainwater pavements falling on perform a permeable similarly pavement in term is ofinfiltrated runoff reduction to the base, [79 which,82]. While serves a as permeable a reservoir to store and infiltrate rainwater. pavement delays the infiltration and retains all rainwater in case of small rainfall, it leads to a runoff in case of heavy rain [83]. In addition, rainwater from the roof, car hoods, or other sources could Permeable concrete systems are prone to be clogged by fine particles [86]. Sands, scraps, saturate the nearby permeable pavements and lead to local overflow [84]. Overflow from a permeable organics, and other fine particles carried by runoff from other sources sneak into the cavity of the pavement occurs when the subgrade is of low permeable soils [85]. permeable pavement and lead to a clogging [87]. Implementations of permeable pavements away Permeable concrete systems are prone to be clogged by fine particles [86]. Sands, scraps, from soil-disturbed fields is helpful to preserve the permeability of the pavements [88]. Clogging organics, and other fine particles carried by runoff from other sources sneak into the cavity of the occurred at a rapid rate at locations with high coarse sediments and organic sediment loads [89]. permeable pavement and lead to a clogging [87]. Implementations of permeable pavements away from Particulates clogging in the near surface of a permeable pavement can be cleared by vacuum soil-disturbed fields is helpful to preserve the permeability of the pavements [88]. Clogging occurred sweeping, power washing, or a combination of both [90]. These techniques, however, are not useful at a rapid rate at locations with high coarse sediments and organic sediment loads [89]. Particulates to clean the clog at deep locations [91]. In addition, finer particles that percolate through the clogging in the near surface of a permeable pavement can be cleared by vacuum sweeping, power permeable surface layer can finally settle at the aggregate-soil interface, reducing the infiltration of washing, or a combination of both [90]. These techniques, however, are not useful to clean the clog at the subgrade. deep locations [91]. In addition, finer particles that percolate through the permeable surface layer can 4.2.finally Water-Retaining settle at the Pavements aggregate-soil interface, reducing the infiltration of the subgrade. 4.2.A Water-Retaining water-retaining Pavements pavement is either an asphalt- or cement-based paver that holds water at the top layerA water-retaining preferentially pavementand evacuates is either the anwater asphalt- via orevaporation cement-based (Figure paver 3). that Water-retaining holds water at pavementsthe top layer are also preferentially called water-holding, and evacuates water-rete the waterntive, via and evaporation watered pavements, (Figure3). or Water-retainingother identical terminologiespavements are [92]. also These called pavements water-holding, are fabricated water-retentive, by filling and water-retentive watered pavements, media or into other the identical pore spaceterminologies of pervious [92 ].concrete. These pavements The fillers are can fabricated be slag, by mortar, filling water-retentive moss, hydrophilic media tissue, into the and pore other space water-retentiveof pervious concrete. media The[93]. fillers Rainwater can be falling slag, mortar, on this moss, pavement hydrophilic or coming tissue, from and otherthe nearby water-retentive sealed surfacemedia is [93 held]. Rainwater at the surface falling oncourse, this pavement which empties or coming via from evaporation. the nearby While sealed water-retaining surface is held at pavementsthe surface are course, primarily which designed empties to via mitigate evaporation. urban heat While island water-retaining effect via evaporative pavements cooling are primarily [94], theydesigned can retain to mitigate a sizeable urban amount heat of island water eff andect viathus evaporative reduce runoff, cooling especially [94], they during can retainsmall arainfall. sizeable Dependingamount of on water the filler and thusmaterials reduce and runo theff depth, especially of the during filler, typically small rainfall. a water-retaining Depending pavement on the filler 2 canmaterials retain about and the 15 depth kg/m of rainwater the filler, typically[95]. A greater a water-retaining amount ca pavementn be obtained can retainby optimizing about 15 kgthe/m 2 structurerainwater of [95the]. Awater-retaining greater amount pavers can be obtained[96]. This by water-retaining optimizing the structurecapacity ofmeans the water-retaining that a dry water-retainingpavers [96]. This paver water-retaining can fully absorb capacity a rainfall means de thatpth aof dry 1.5 water-retainingcm without generating paver can runoff fully [97]. absorb A a larger rainfall depth drains to the base or generates runoff when the base and subgrade have rainfall depth of 1.5 cm without generating runoff [97]. A larger rainfall depth drains to the base or saturated. Water-retaining pavements thus also have a limited capacity to reduce the risk of urban generates runoff when the base and subgrade have saturated. Water-retaining pavements thus also flooding. have a limited capacity to reduce the risk of urban flooding.

wat er ret ai ni ng l ayer

base l ayer

subgrade soil

Figure 3. A water-retaining pavement holds water at the surface layer; additional water is drained to the base.

4.3. Trees On urban areas, trees are widely found in parks, parking lots, gardens, streets, nature preserves, greenways, shelter-belts, and many others. Urban trees have multiple benefits such as urban heat island mitigation, air quality amendment, and others [98]. As a co-benefit, urban trees also reduce rainwater runoff by intercepting rainfall in the canopy, evaporating water from the leaves, enhancing the Water 2020, 12, x 7 of 23

Figure 3. A water-retaining pavement holds water at the surface layer; additional water is drained to the base.

4.3. Trees On urban areas, trees are widely found in parks, parking lots, gardens, streets, nature preserves, greenways, shelter-belts, and many others. Urban trees have multiple benefits such as urban heat island mitigation, air quality amendment, and others [98]. As a co-benefit, urban trees also reduce rainwater runoff by intercepting rainfall in the canopy, evaporating water from the leaves, enhancing the infiltration around the trees’ roots, and storing water in trees’ trunks (Figure 4) [99,100]. Runoff at areas with trees is reduced on different timescales. For example, the canopy intercepts rainfall immediately after rainfall starts; throughfall occurs after the canopy saturates; and evapotranspiration lasts for a long time after rainfall stops. During this process, canopy interception and ground infiltration are two main components that reduce rainwater runoff. Evaporation and transpiration of trees return water to the atmosphere, maintain the health of the trees, and restore the interception of the trees before the subsequent rainfall [101]. When rain falls onto a tree, some is intercepted by the canopy, with the remainder reach the ground via throughfall or stemflow. The interception loss is the difference between the throughfall and stemflow from the gross precipitation. The interception loss of trees is found about 10–50% of seasonalWater 2020 or, 12 annual, 3579 total rainfall [102], influenced by tree species [103] and local climate [104].7 of 23 Inkiläinen et al [105] found that the potential reduction of stormwater runoff is about 9.1–21.4%. Xiao [106] found a rainfall interception of 15.3% for a small Jacaranda mimosifolia to 66.5% for a infiltration around the trees’ roots, and storing water in trees’ trunks (Figure4)[ 99,100]. Runoff at areas mature Tristania conferta. Conifers generally intercept more rainwater on the plant canopy than with trees is reduced on different timescales. For example, the canopy intercepts rainfall immediately broadleaf trees do [107], possibly because the former has greater specific areas to intercept rainwater after rainfall starts; throughfall occurs after the canopy saturates; and evapotranspiration lasts for a than the latter. For instance, Xiao et al [104] found that the interception is about 15% of gross long time after rainfall stops. During this process, canopy interception and ground infiltration are two precipitation for a pear tree, but 27% for an oak tree. An interception loss by a tree is also greatly main components that reduce rainwater runoff. Evaporation and transpiration of trees return water to influenced by climate factors such as rainfall intensity, wind speed, rainfall duration, rainfall depth, the atmosphere, maintain the health of the trees, and restore the interception of the trees before the and rainfall frequency [104,108]. As different trees are applicable to different climate, planting the subsequent rainfall [101]. right trees in the right place has to be considered in urban greening [109].

evaporati on

runoff

i nfiltrati on

Figure 4. Rainfall falling on a tree is partitioned to interception, throughfall, infiltration, and soil Figureretention, 4. Rainfall with the falling excessive on a watertree is discharging partitioned as to runo inteffrception,. throughfall, infiltration, and soil retention, with the excessive water discharging as runoff. When rain falls onto a tree, some is intercepted by the canopy, with the remainder reach the groundA tree via further throughfall reduces or stormwater stemflow. Therunoff interception via infiltration loss isand the detention difference when between rainwater the throughfall reaches theand ground stemflow surface. from Armson the gross et al precipitation. [110] studied the The infiltration interception into loss a pit of with trees a issmall found tree about and found 10–50% thatof the seasonal tree reduces or annual about total 60% rainfall of runoff [102 compared], influenced to the by control tree species asphalt [103 lot.] andA tree local also climate enhances [104 ]. theInkiläinen infiltration et of al. the [105 underlying] found that soils the because potential tree reduction roots provide of stormwater conduits runoto conveyff is about rainwater 9.1–21.4%. to deepXiao layers. [106] foundThe soils a rainfall around interception a pit can be ofreplaced 15.3% forby aa smallthick porousJacaranda structural mimosifolia soil,to which 66.5% serves for a mature as a reservoirTristania to conferta retain. Conifersrainwater generally and to interceptsupport morethe growth rainwater of the on thetree. plant By canopyexperimentally than broadleaf studying trees differentdo [107 roots], possibly growth because in structural the former soils hasin the greater lab, Bartens specific et areas al. [111] to intercept found that rainwater the roots than penetrate the latter. For instance, Xiao et al. [104] found that the interception is about 15% of gross precipitation for a pear tree, but 27% for an oak tree. An interception loss by a tree is also greatly influenced by climate factors such as rainfall intensity, wind speed, rainfall duration, rainfall depth, and rainfall frequency [104,108]. As different trees are applicable to different climate, planting the right trees in the right place has to be considered in urban greening [109]. A tree further reduces stormwater runoff via infiltration and detention when rainwater reaches the ground surface. Armson et al. [110] studied the infiltration into a pit with a small tree and found that the tree reduces about 60% of runoff compared to the control asphalt lot. A tree also enhances the infiltration of the underlying soils because tree roots provide conduits to convey rainwater to deep layers. The soils around a pit can be replaced by a thick porous structural soil, which serves as a reservoir to retain rainwater and to support the growth of the tree. By experimentally studying different roots growth in structural soils in the lab, Bartens et al. [111] found that the roots penetrate into compacted soils and that the infiltration of the soils increases about 150% [112]. When designed, a tree pit should be concave to host rainwater from the nearby impervious areas for promoting rainwater natural drawdown. Although urban trees can be designed to reduce the runoff volume, they are effective in intercepting rainwater in case of small rainfall only. In case of heavy rainfall, overflow is inevitable. Urban trees are thus more meaningful to improve urban water quality than to mitigate urban flooding [113]. Water 2020, 12, x 8 of 23 into compacted soils and that the infiltration of the soils increases about 150% [112]. When designed, a tree pit should be concave to host rainwater from the nearby impervious areas for promoting rainwater natural drawdown. Although urban trees can be designed to reduce the runoff volume, they are effective in intercepting rainwater in case of small rainfall only. In case of heavy rainfall, overflowWater 2020 is, 12 inevitable., 3579 Urban trees are thus more meaningful to improve urban water quality than8 ofto 23 mitigate urban flooding [113].

4.4.4.4. Rainwater Rainwater Barrels Barrels A Arain rain barrel barrel is isa asmall small chamber chamber installed installed nearby nearby a aprivate private building building to tocollect collect rainwater rainwater from from a a roofroof downspout downspout for for later later non-potable non-potable uses uses (Figure (Figure 5)5 )[[114].114 ].A Arain rain barrel barrel is isusually usually connected connected to to an an infiltratinginfiltrating lot lot such such as as a arain rain garden garden or or a agravel-filled gravel-filled dry dry well. well. A A case case study study by by Jennings Jennings et et al al. [115] [115 ] 2 2 foundfound that that a 189 a 189 L Lrain rain barrel barrel used used to to store store rainwater rainwater from from a a186 186 m m2 roofroof is issufficient sufficient to to irrigate irrigate 14 14 m m2 gardengarden in inCleveland Cleveland (OH, (OH, USA), USA), reducing reducing 1.4–3.2% 1.4–3.2% annual annual runoff runo fffromfrom the the roof. roof. Similarly, Similarly, a amodel model studiedstudied by by Litofsky Litofsky and and Jennings Jennings [116] [116 found] found that that a 235 a 235 L rain L rain barrel barrel can can reduce reduce 3–44% 3–44% runoff runo fffromfrom a a traditionaltraditional roof, roof, depending depending onon thethe local local weather weather and and the rainfallthe rainfall depth. depth. For small For rainfall,small rainfall, all rainwater all rainwaterwould be would retained be retained in an empty in an barrel. empty But barrel. a rain But barrel, a rain once barrel, filled, once does filled, little does to reducelittle to excessivereduce excessiverunoff and runoff to store and rainwaterto store rainwater in the next in rainfallthe next [ 117rainfall]. In addition,[117]. In additi homeownerson, homeowners are often demandedare often demandedto empty to the empty barrel the routinely barrel routinely to restore to the restore water th retentione water retention of the barrel of the [115 barrel]. The [115]. use of The rain use barrel of rainis thusbarrel challenged is thus challenged not only not by barrelonly by size barrel but alsosize but homeowner also homeowner participations participations [118]. For [118]. instance, For instance,Guo et al.Guo [119 et] foundal [119] that found people that who people are aware who ofare rainwater aware of conservation rainwater areconservation more willing are to more adopt willingrain barrels. to adopt rain barrels.

downspout

barrel overfl ow pi pe

soil

Figure 5. A rain barrel is a small chamber installed nearby a building for collecting rainwater from the Figure 5. A rain barrel is a small chamber installed nearby a building for collecting rainwater from downspout. The overflow can be discharged to the rain garden. the downspout. The overflow can be discharged to the rain garden. 4.5. Infiltration Trenches 4.5. Infiltration Trenches An infiltration trench, also called an infiltration ditch or a percolation trench, is a channel filled withAn stones infiltration or crushed trench, rocks also for called storing an rainwaterinfiltration (Figure ditch 6or)[ a120 percolation]. The cavities trench, of theis a rockschannel and filled stones withdetain stones rainwater or crushed temporarily, rocks for which storing gradually rainwater infiltrates (Figure through 6) [120]. the soil-rockThe cavities interface. of the As rocks the cavities and stonesof the detain rock are rainwater limited, infiltrationtemporarily, trenches which usuallygradually lead infiltrates to overflow through in case the of heavysoil-rock rain. interface. Whether As the thewater cavities in the of trenchthe rock can are be limited, fully evacuated infiltration before trenches thenext usually heavy lead rainfall to overflow depends in case on the of hydrologicalheavy rain. Whetherconductivity the water of the in soils the trench and on can the be depth fully of evacua the groundwaterted before the table. next An heavy infiltration rainfall trench depends is feasible on the to hydrologicalthe areas where conductivity the subsoils of the are highlysoils and permeable on the depth and where of the the groundwater groundwater table. table An and infiltration the bedrock trenchsit some is feasible meters to below the areas the bottom where of the the subso trenchils [are121 ].highly To maintain permeable the permeabilityand where the of groundwater the underlying tablesoils, and rainwater the bedrock runoff shouldsit some be pretreatedmeters below by a swalethe bottom (Figure of6), the by atrench settling [121]. basin beforeTo maintain entering the the permeabilitytrench, or by of an the underdrain underlying pipe soils, carrying rainwater stormwater runoff should into the be trench. pretreated If fine by particles a swale and(Figure settlements 6), by a aresettling not keptbasin o beforeff the trench,entering especially the trench, during or by construction, an underdrain there pipe would carrying be astormwater permanent into clogging. the trench.Warnaars If fine et al.particles [122] monitoredand settlements the runo areff notquantity kept off of infiltrationthe trench, trenchesespecially in during an urbanized construction, area for thereabout would 3 years be and a permanent found that aclogging. slight clog Warnaars has decreased et al. the [122] infiltration monitored of the the trenches. runoff Similarquantity results of infiltrationare reported trenches by Siriwardene in an urbanized et al. [ 123area]. for In addition,about 3 years the use and of found infiltration that a trenchesslight clog is limitedhas decreased by other thetwo infiltration factors. One of the is that trenches. water infiltrationSimilar results may are jeopardize reported the by safety Siriwardene of the nearby et al. buildings.[123]. In addition, The other is that most urban soils are relatively-poor infiltration. Water 2020, 12, x 9 of 23 the use of infiltration trenches is limited by other two factors. One is that water infiltration may jeopardizeWater 2020, 12 the, x safety of the nearby buildings. The other is that most urban soils are relatively-poor9 of 23 infiltration. the use of infiltration trenches is limitedsl o by other two factors. One is that water infiltration may pe i < 5 % jeopardizeWater 2020 ,the12, 3579 safety of the nearby buildings. The other is that most urban soils are relatively-poor9 of 23 infiltration.

sl o pe opein -

open-graded stone

geofabric

geofabric Figure 6. An infiltration trench is a channel filled with stones and gravels to detain runoff for infiltration.

Figure 6. An infiltration trench is a channel filled with stones and gravels to detain runoff for infiltration. 4.6. BioretentionFigure 6. An infiltration trench is a channel filled with stones and gravels to detain runoff for 4.6.infiltration. Bioretention 4.6.1. Rain Gardens 4.6.1. Rain Gardens 4.6. BioretentionRain gardens, or bioretention areas or biofilters, are being adopted in public and private lands in urbanRain areas gardens, to reduce or bioretentionstormwater volume, areas or to biofilters, attenuate are peak being flow, adopted and to in infiltrate public and rainwater private that lands otherwise4.6.1.in urban Rain entersGardens areas local to reduce sewer stormwater pipes without volume, treatments to attenuate [124–126]. peak Typically flow, and a to rain infiltrate garden rainwater is built by that replacingotherwiseRain naturalgardens, enters soils or local bioretention with sewer porous pipes areas media without or biofilteto form treatmentsrs, a are depression being [124– adopted126 shallow]. Typically in publicpit for a rainandvegetated private garden plants lands is built growthin byurban replacing and areas for to natural holding reduce soils stormwaterrunoff with from porous volume, nearby media tosealed toatte formnuate surfaces a depressionpeak (Figureflow, and shallow 7) to[127,128]. infiltrate pit for A vegetatedrainwater rain garden plantsthat shouldotherwisegrowth be andentersdry forrapidly local holding sewerafter runo thepipesff fromrain without for nearby preventi treatments sealedng surfacesthe [124–126]. breeding (Figure Typically of7 )[mosquitoes.127, 128a rain]. A gardenRain rain gardengardens is built should are by oftenreplacingbe drylocated rapidlynatural near afterasoils building, thewith rain porousserving forpreventing media as an endpointto form the breeding afor depression percolating of mosquitoes. shallow the runoff pit Rain forfrom vegetated gardens building are plantsroofs, often patiosgrowthlocated or and lawns. near for a This building,holding process runoff serving reduces from as anlocal nearby endpoint runoff sealed, fordecreases percolating surfaces the (Figure peak-flow the runo 7)ff [127,128].discharge,from building A and rain roofs,recharges garden patios localshouldor lawns.groundwater. be dry This rapidly process Dietz after reduces and the Clausenrain local for runo preventi[129]ff, decreasesbuiltng thea rain thebreeding peak-flowgarden of composingmosquitoes. discharge, of Rain and a 0.6 rechargesgardens m depth are local well-permeableoftengroundwater. located near soil Dietz a withbuilding, and a Clausen perforated serving [129 aspipe] builtan underlain.endpoint a rain garden for They percolating composing found that the of only runoff a 0.6 0.8% m from depth of buildinginflow well-permeable left roofs, the gardenspatiossoil withor as lawns. aoverflow, perforated This processmeaning pipe underlain. reduces that most local They of runoffthe found inflow, decreases that (99.2%) only 0.8%the finally peak-flow of inflow discharges leftdischarge, the as gardens subsurface and recharges as overflow, flow [129].localmeaning Thegroundwater. hydrological that most Dietz of performance the and inflow Clausen (99.2%) of a real[129] finally rain built dischargesgarden a rain also asgarden depends subsurface composing on local flow climate [129 of]. a The [130]0.6 hydrological m and depth the gardenwell-permeableperformance design [131]. ofsoil a realwithWhile rain a perforateda gardenrain garden also pipe dependsretains underlain. a onsizable localThey amount climate found ofthat [130 water ]only and and 0.8% the reduces garden of inflow the design leftrisk the [of131 ]. overflow,gardensWhile aas the rain overflow, reduction garden meaning retains was small a that sizable at most heavy amount of rainfall.the ofinflow water Therefore, (99.2%) and reduces finallya rain the gardendischarges risk ofhas overflow, limitedas subsurface thecapacity reduction flow to reduce[129].was Thesmall urban hydrological at flooding heavy rainfall. [132]. performance Therefore, of a a real rain rain garden garden haslimited also depends capacity on to local reduce climate urban [130] flooding and the [132 ]. garden design [131]. While a rain garden retains a sizable amount of water and reduces the risk of overflow, the reduction was small at heavy rainfall. Therefore, a rain garden has limited capacity to reduce urban flooding [132]. downspout

gr owi ng medi umdownspout

por ous medi a per or at ed pi pe

nat ur al soil gr owi ng medi um sewer

por ous medi a per or at ed pi pe Figure 7. A rain garden is built by replacing natural soils with growth media for vegetated plants Figure 7. A rain garden is built by replacingnat ur a l naturalsoil soils with growth media for vegetated plants growth, which retains and filters rainwater. sewer growth, which retains and filters rainwater. 4.6.2. Vegetated Filter Strips 4.6.2. Vegetated Filter Strips FigureA vegetative 7. A rain filtergarden strip, is built also by called replacing biofiltration natural soils strip, with filter growth strip andmedia bu forffer vegetated strip, is aplants stormwater

controlgrowth, technique which retains that disperses and filters rainwater rainwater. uniformly as an overland sheet flow through a mildly-sloped vegetated area to promote infiltration (Figure8)[ 133]. Typically the vegetated area is a filter strip with 4.6.2. Vegetated Filter Strips dwarf turf grass, grassy meadow or small wood for making the runoff flowing uniformly through the strip surface. Infiltration and retention of a vegetated filter strip are thus influenced by the slope, length, and plant species of the strip [134,135]. However, as a vegetative filter strip is primarily designed to Water 2020, 12, x 10 of 23

A vegetative filter strip, also called biofiltration strip, filter strip and buffer strip, is a stormwater control technique that disperses rainwater uniformly as an overland sheet flow through a mildly-sloped vegetated area to promote infiltration (Figure 8) [133]. Typically the vegetated area is a filter strip with dwarf turf grass, grassy meadow or small wood for making the runoff flowing uniformly through the strip surface. Infiltration and retention of a vegetated filter strip are thus influenced by the slope, length, and plant species of the strip [134,135]. However, as a vegetative filter strip is primarily designed to intercept and slow runoff for improving the quality of the runoff, the ability of a vegetative filter strip to reduce runoff is limited. The vegetated filter strip must be pruned periodically to keep an overland sheet flow on the strip [136]. In addition, a vegetative filter strip requires lands for construction, so the most suitable place for such a strip is the side slope of a roadway embankment [136]. Vegetative filter strips have been refined to level-spreader vegetative filter strips, which introduce a concrete channel with a level spreader that disperses runoff to a vegetated filter strip for further promoting infiltrations [137]. The deployment of a level-spreader vegetative filter strip system can be referred to elsewhere [138]. This system has to be designed upon high permeable soils for infiltrating stormwater effectively [139]. The level spreader slows down rainwater flow rate for promoting infiltration, reducing runoff volume and attenuating peak flow [140]. Hunt et al [141] monitoredWater 2020, the12, 3579 hydrological performance of a level spreader-vegetated filter strip. It is found that10 the of 23 strip can eliminate outflow from 20 of 23 rain events and that the total runoff volume reduction is 85%.intercept Similarly, and Line slow and runo Huntff for [142] improving monitored the quality a level ofspreader-grass the runoff, the filter ability strip of over a vegetative 14 storm filter events strip andto found reduce that runo theff is runoff limited. volume The vegetated is cut by 49% filter and strip that must the be peak pruned flow periodicallyis attenuated to by keep 23%. an In overland some cases,sheet peak flow flows on the were strip [reduced136]. In addition,by an order a vegetative of magnitude filter strip [140]. requires However, lands as for the construction, rainfall depth so the increases,most suitable the peak place flow for cannot such a further strip is thedecrease side slope as the of underlying a roadway soil embankment became saturated. [136].

mil dl y-sl ope subgrade veget ated

outfl ow

Figure 8. A vegetative filter strip is usually built nearby an impervious surface to allow runoff from the Figure 8. A vegetative filter strip is usually built nearby an impervious surface to allow runoff from impervious surface flowing evenly through the strip for intercepting and slowing runoff. the impervious surface flowing evenly through the strip for intercepting and slowing runoff. Vegetative filter strips have been refined to level-spreader vegetative filter strips, which introduce 4.6.3. Swales a concrete channel with a level spreader that disperses runoff to a vegetated filter strip for further promotingSwales, infiltrationsalso called [bioswales,137]. The deployment bio-retention of abasins, level-spreader and ecology vegetative ditch, filter are stripexcavated system lands can be backfilledreferred towith elsewhere a vegetated [138]. surface This system layer and has toa filt beer designed intermediate upon highlayer permeable upon natural soils soils for infiltratingto pond rainwaterstormwater for subsequent effectively [infiltration139]. The (Figure level spreader 9) [143]. slows The filler down in rainwaterthe excavated flow bowl rate is for the promoting storage zoneinfiltration, to retain reducingrainwater runo[144].ff Watervolume passing and attenuating through the peak filler flow infiltrates [140]. to Hunt the subsoils. et al. [141 In] the monitored areas withthe low-permeable hydrological performance soils, underdrained of a level pipes spreader-vegetated are buried in the filter swales strip. to Itdivert is found the excessive that the strip water can toeliminate nearby sewer outflow pipes from [145]. 20 of 23At rainthe eventsareas andwher thate the the groundwater total runoff volume stays shallow, reduction a is groundwater 85%. Similarly, moundLine and usually Hunt forms [142] monitoredunder the swale, a level reducing spreader-grass the infiltration filter strip rate over [146,147]. 14 storm In events this circumstance, and found that perforatedthe runoff pipesvolume are is usually cut by 49%buried and at that the the bottom peak flowof the is attenuatedswale to divert by 23%. excessive In some water cases, to peak nearby flows waterways.were reduced Abida by anand order Sabourin of magnitude [148] found [140]. However,that total as seasonal the rainfall discharge depth increases, from a swale the peak with flow underdraincannot further perforated decrease pipes as the is underlying 2.7–13 times soil becamelower than saturated. the discharge from a swale without underdrain. A swale with a thick filter intermediate layer performs better in peak-flow attenuation and4.6.3. runoff Swales reduction. The retention and infiltration of the filter layer should be optimized to retain water Swales,and to avoid also calledoverflow bioswales, simultaneously bio-retention [149]. basins, While anda swale ecology can ditch,fully intercept are excavated rainwater lands duringbackfilled small with or medium a vegetated rainfall, surface overflow layer andis inev a filteritable intermediate under heavy layer and uponcontinual natural rainfall soils events to pond [71].rainwater The amount for subsequent of overflow infiltration decreases (Figure when 9the)[ 143internal]. The water filler instorage the excavated zone increases bowl is [144]. the storage This zone to retain rainwater [144]. Water passing through the filler infiltrates to the subsoils. In the areas with low-permeable soils, underdrained pipes are buried in the swales to divert the excessive water to nearby sewer pipes [145]. At the areas where the groundwater stays shallow, a groundwater mound usually forms under the swale, reducing the infiltration rate [146,147]. In this circumstance, perforated pipes are usually buried at the bottom of the swale to divert excessive water to nearby waterways. Abida and Sabourin [148] found that total seasonal discharge from a swale with underdrain perforated pipes is 2.7–13 times lower than the discharge from a swale without underdrain. A swale with a thick filter intermediate layer performs better in peak-flow attenuation and runoff reduction. The retention and infiltration of the filter layer should be optimized to retain water and to avoid overflow simultaneously [149]. While a swale can fully intercept rainwater during small or medium rainfall, overflow is inevitable under heavy and continual rainfall events [71]. The amount of overflow decreases when the internal water storage zone increases [144]. This amount also depends on the rainfall duration and the rainfall intensity and on the drainable capacity of the underlying soils [150]. Water 2020, 12, x 11 of 23 amount also depends on the rainfall duration and the rainfall intensity and on the drainable capacity of the underlying soils [150].

The clog of a swale is deleterious to the hydrological performance of the swale. By monitoring the hydraulic conductivity of stormwater biofilters, Le Coustumer et al. [151] found that a biofilter with an initial low conductivity decreases negligibly over time but that with an initial high conductivity drops greatly. The decreases of the conductivity over time are likely caused by sediments depositing at the soil-air interface and by the hydraulic compaction (consolidation) of the growth media. Unlike non-vegetated infiltrated trenches which tend to be clogged permanently due to the settlement of fine particle carried by rainwater [151], bio-retention filter is less likely being fully clogged because vegetated roots provide conduits for water infiltration [152]. Similar findings are reported by Dechesne et al. [153], which found a swale still has good infiltration after 20-year operations.Water 2020, 12The, 3579 risk of clogging can be reduced by installing a check dams on the upstream of11 the of 23 swale to slow the velocity of the runoff and to allow sediments to settle [154].

veget at ed surf ace l ayer

filter intermediate layer natural soil

underdrain

Figure 9. A swale is a depressed vegetated layer over a filter intermediate layer upon natural permeable Figuresoils to9. pondA swale rainwater is a depressed for promoting vegetated infiltration. layer over a filter intermediate layer upon natural permeable soils to pond rainwater for promoting infiltration. The clog of a swale is deleterious to the hydrological performance of the swale. By monitoring the 5. hydraulicUnderground conductivity Techniques of stormwater biofilters, Le Coustumer et al. [151] found that a biofilter with an initial low conductivity decreases negligibly over time but that with an initial high conductivity drops 5.1.greatly. Soakaways The decreases of the conductivity over time are likely caused by sediments depositing at the soil-air interface and by the hydraulic compaction (consolidation) of the growth media. Soakaways are small-scale underground chambers filled with gravel and stones, which detain Unlike non-vegetated infiltrated trenches which tend to be clogged permanently due to the settlement rainwater for subsequent infiltration to the local soils [155]. With respect to water detention and of fine particle carried by rainwater [151], bio-retention filter is less likely being fully clogged infiltration, soakaways are similar to infiltration trenches. However, a soakaway is buried because vegetated roots provide conduits for water infiltration [152]. Similar findings are reported underground, which does not sacrifice lands for construction. Soakaways are built locally in a rain by Dechesne et al. [153], which found a swale still has good infiltration after 20-year operations. The garden or backyard, where water from the downspouts and from other sealed surfaces can enter the risk of clogging can be reduced by installing a check dams on the upstream of the swale to slow the soakaway via gravitational flow (Figure 10) [156]. A soakaway is often combined with a pipe to lead velocity of the runoff and to allow sediments to settle [154]. excessive inflow water to the local sewers for avoiding flooding during extreme rainfall. By slowly releasing5. Underground the detained Techniques rainwater to the local soils, a soakaway reduces local runoff volume and attenuates the peak flow. As soakaways do not sacrifice land uses, they are often recommended to high-density5.1. Soakaways urbanized areas. However,Soakaways there are are small-scale some constraints underground on the chambers use of filleda soakaway. with gravel First, and a soakaway stones, which must detain be constructedrainwater in for a subsequentpermeable soil infiltration for timely to thedraining local soilsthe detained [155]. With rainwater respect before to water the detention next heavy and storminfiltration, events. soakawaysSecond, groundwater are similar tois infiltrationimportant trenches.to consider However, in the implementation a soakaway is buried of soakaways. underground, In thewhich area groundwater does not sacrifice situates lands shallow, for construction. there is a groundwater Soakaways mound are built under locally the insoakaway, a rain garden which or maybackyard, not evacuate where the water detained from the rainwater downspouts before and the from next other storm sealed event surfaces [157,158]. can enterIn this the situation, soakaway pipesvia gravitationalare usually placed flow (Figureat the top 10)[ of156 the]. soakaway A soakaway to carry is often off combined the excessive with water a pipe [159]. to lead Currently, excessive mostinflow of researches water to the are local focusing sewers on for modeling avoiding the flooding hydrological during performance extreme rainfall. of soakaways, By slowly but releasing few reportthe detained the long-term rainwater performance to the local of soils,soakaways. a soakaway The long-term reduces local retention runoff andvolume detention and attenuates of water in the soakawayspeak flow. is Assimulated soakaways by assuming do not sacrifice that the land drainage uses, they of arethe oftensoakaways recommended is changed to high-densityduring its lifetimeWaterurbanized 2020 [159,160]., 12, x areas. However, during the lifetime of a soakaway, fine particles carried by rainwater12 of 23 would settle on the interface between permeable soils and gravel filters, potentially reducing the infiltration capacity of the soakaways. In addition, as most soils in urbanized areas are compacted and low-permeable, there are very few places for construction soakaways for rainwater infiltration.

downspout gutter

overfl ow pi pes per meabl e soil sewer open-graded stone and gravel

i nfiltrati on

Figure 10. Figure 10. AA soakaway soakaway is aa small-scale small-scale underground underground cell cell filled filled with with gravel gravel and stones and stones to store to rainwater store for infiltration. rainwater for infiltration.

5.1. Underground Tanks Different from rain barrels, rainwater tanks are installed to harvest rainwater runoff from roofs and other sealed surfaces for subsequent non-potable water uses [161,162]. Depending on the source of the runoff, the tank can be placed on the ground surface or underground as long as the rainwater can enter the tank via gravitational flow. If the tank is placed on the ground surface, lands must be sacrificed [163] and the overflow from the tank can be connected to a rain garden for infiltration. This further sacrifices the land use so it is preferred to bury the tank underground and to convey the overflow from the tank to the local sewers (Figure 11). Similar to rain barrels, the effectiveness of using a rainwater tank to mitigate urban flooding depends on the tank’s size, the homeowners’ participation, and rain depth [164,165]. For instance, Khastagir and Jayasuriya [166] found that a 100 m2 impervious roof with a 3 kL tank for harvesting water for toilet flushing, garden irrigation, and laundry demand reduces runoff volume about 70%. The runoff reduction volume is also influenced by the use of the harvested rainwater [167]. If the tank is built to harvest rainwater for multi-end water uses, the tank would be more effective than that for a single use [168,169]. In addition, if the use of water during the intermission between two adjacent heavy rainfall cannot empty the tank, the tank would fail to reset for storing rainwater in the next heavy rainfall [170]. While a community accept to install rainwater tanks for rainwater use, it is less interested in maintaining rainwater tanks routinely [171]. At heavy rain, a rainwater tank has limited capacity to reduce the peak flow of the runoff [172]. Therefore, the adoption of rainwater tanks is challenged both by the tank’s size, homeowner participation, and the rain depth.

downspout

settli ng t ank

overfl ow pi pe rai nwater t ank

sewer

Figure 11. An underground tank stores rainwater from the above sealed surfaces for subsequent non-potable uses. Overflow from the tank is led to local sewers.

A rainwater tank is used indispensably with flow regulators, pipes, faucets, a pump, and other devices [173]. Additional energy must be continuously supplied for lifting the rainwater in the tank to the user ends. This lift involves both capital investments and maintenance costs. Gowland and

Water 2020, 12, x 12 of 23

downspout gutter

overfl ow pi pes per meabl e soil sewer open-graded stone and gravel

Water 2020, 12, 3579 i nfiltrati on 12 of 23

Figure 10. A soakaway is a small-scale underground cell filled with gravel and stones to store However, there are some constraints on the use of a soakaway. First, a soakaway must be rainwater for infiltration. constructed in a permeable soil for timely draining the detained rainwater before the next heavy storm events. Second, groundwater is important to consider in the implementation of soakaways. In the 5.1. Underground Tanks area groundwater situates shallow, there is a groundwater mound under the soakaway, which may notDifferent evacuate from the detained rain barrels, rainwater rainwater before tanks the are next in stormstalled event to harvest [157,158 rainwater]. In this runoff situation, from pipes roofs are andusually other placedsealed surfaces at the top for of subsequent the soakaway non-potabl to carrye owaterff the uses excessive [161,162]. water Depending [159]. Currently, on the source most of of researchesthe runoff, arethe focusing tank can onbe modelingplaced on thethe hydrologicalground surface performance or underground of soakaways, as long as but the few rainwater report the canlong-term enter the performance tank via gravitational of soakaways. flow. The If the long-term tank is retentionplaced on and the detention ground surface, of water lands in soakaways must be is sacrificedsimulated [163] by assumingand the overflow that the from drainage the tank of the ca soakawaysn be connected is changed to a rain during garden its lifetimefor infiltration. [159,160 ]. ThisHowever, further sacrifices during the the lifetime land us ofe so a soakaway, it is preferred fine to particles bury the carried tank underground by rainwater and would to convey settle on the the overflowinterface from between the tank permeable to the local soils sewers and gravel (Figure filters, 11). potentially reducing the infiltration capacity of theSimilar soakaways. to rain In barrels, addition, the as effectiveness most soils in urbanizedof using a areasrainwater are compacted tank to mitigate and low-permeable, urban flooding there dependsare very on few the places tank’s for size, construction the homeowners’ soakaways partic foripation, rainwater and infiltration. rain depth [164,165]. For instance, Khastagir and Jayasuriya [166] found that a 100 m2 impervious roof with a 3 kL tank for harvesting water5.2. Undergroundfor toilet flushing, Tanks garden irrigation, and laundry demand reduces runoff volume about 70%. The runoffDifferent reduction from rainvolume barrels, is also rainwater influenced tanks by are the installed use of the to harvestharvested rainwater rainwater runo [167].ff from If the roofs tankand is other built sealedto harvest surfaces rainwater for subsequent for multi-end non-potable water uses, water the uses tank [161 would,162]. Dependingbe more effective on the than source thatof for the a runo singleff, the use tank [168,169]. can be In placed addition, on the if th grounde use of surface water or during underground the intermission as long as between the rainwater two adjacentcan enter heavy the rainfall tank via cannot gravitational empty the flow. tank, If thethe tanktank iswould placed fail on to thereset ground for storing surface, rainwater lands must in thebe next sacrificed heavy [rainfall163] and [170]. the overflowWhile a community from the tank accept can to be install connected rainwater to a rain tanks garden for rainwater for infiltration. use, it Thisis less further interested sacrifices in maintaining the land use rainwater so it is preferred tanks routinely to bury the[171]. tank At undergroundheavy rain, a andrainwater to convey tank the hasoverflow limited fromcapacity the to tank reduce to the the local peak sewers flow (Figureof the runoff 11). [172]. Therefore, the adoption of rainwater tanks is challenged both by the tank’s size, homeowner participation, and the rain depth.

downspout

settli ng t ank

overfl ow pi pe rai nwater t ank

sewer

FigureFigure 11. 11. AnAn underground underground tank tank stores stores rainwater rainwater from from the the above above sealed sealed surfaces surfaces for for subsequent subsequent non-potablenon-potable uses. uses. Overflow Overflow from from the the tank tank is led is led to tolocal local sewers. sewers. Similar to rain barrels, the effectiveness of using a rainwater tank to mitigate urban flooding A rainwater tank is used indispensably with flow regulators, pipes, faucets, a pump, and other depends on the tank’s size, the homeowners’ participation, and rain depth [164,165]. For instance, devices [173]. Additional energy must be continuously supplied for lifting the rainwater in the tank Khastagir and Jayasuriya [166] found that a 100 m2 impervious roof with a 3 kL tank for harvesting to the user ends. This lift involves both capital investments and maintenance costs. Gowland and water for toilet flushing, garden irrigation, and laundry demand reduces runoff volume about 70%. The runoff reduction volume is also influenced by the use of the harvested rainwater [167]. If the tank is built to harvest rainwater for multi-end water uses, the tank would be more effective than that for a single use [168,169]. In addition, if the use of water during the intermission between two adjacent heavy rainfall cannot empty the tank, the tank would fail to reset for storing rainwater in the next heavy rainfall [170]. While a community accept to install rainwater tanks for rainwater use, it is less interested in maintaining rainwater tanks routinely [171]. At heavy rain, a rainwater tank has limited capacity to reduce the peak flow of the runoff [172]. Therefore, the adoption of rainwater tanks is challenged both by the tank’s size, homeowner participation, and the rain depth. A rainwater tank is used indispensably with flow regulators, pipes, faucets, a pump, and other devices [173]. Additional energy must be continuously supplied for lifting the rainwater in the tank Water 2020, 12, 3579 13 of 23 to the user ends. This lift involves both capital investments and maintenance costs. Gowland and Younos [174] studied the feasibility of rainwater-harvesting tanks and found that the pumping cost is the greatest capital investment. They found that the installation of a rainwater tank can be re-paid by harvesting rainwater for non-potable water uses [162]. The payback period is jointly affected by the tank’s size [175], the water usage [176], the local water price [177], the locations [178], and the whole life cost [179].

6. Discussion While some mainstream techniques mentioned above are used in combination for further reducing the runoff volume and attenuating peak flow, a technique that is used in combination with others plays the same role as it works individually [180,181]. Combinations of these techniques are thus not discussed further. While current mainstream stormwater-management techniques can mitigate the urban flooding caused by small rainfalls, in most cases they fail do so for a heavy rainfall. During heavy rainfall, runoff from unsealed and sealed surfaces superimposes, causing the peak flow to overwhelm the carrying capacity of the local sewers. While some of these techniques can be implemented during the retrofit process, the implementation of the rain garden, vegetative filter strip, bioswale would sacrifice lands and tense the land use problems in urbanized areas. Other techniques such as permeable pavements are suitable only to low-volume light-duty paving areas such as parking lots and pedestrian lanes. Therefore, as urban flooding takes place at heavy rainfall events only, current technologies do little to mitigate urban flooding. To find a solution for urban flooding, we must understand that runoffs from sealed and unsealed surfaces merge together and make the superimposed peak flow overwhelm the carrying capacity of local sewers (Figure 12a). For instance, in Figure 12a, the runoff from catchments of B, C, and D merge together, generating a peak flow (hydrograph A) that is substantially greater than the carrying-capacity of the local sewer. To void the peak flow exceeding the carrying capacity of local sewers, the peak flow must be attenuated substantially [182]. Assuming the hydrographs can be simplified to a triangle and the area of the triangle represents the total runoff [5], the peak flow must be attenuated by extending the discharge duration proportionally (Figure 12b). That is, if the peak flow is intended to be attenuated 50%, the discharge duration must be increased twice. While green roofs, permeable pavements, bioswales, and other techniques can extend the discharge duration to some extent, they cannot prolong the discharge duration dramatically, especially during a heavy rainfall. A soakaways can detain a sizeable of water volume and discharge it later to the local soils [157,160], but the detained volume of a soakaway is small because water is mainly stored in the pore of the soakaway’s filler. In addition, the discharge rate from a soakaway is uncontrollable; a fast discharge rate may unlawfully transfer the flooding problems to low-lying locations [183]. Therefore, future research should focus on the development of novel, discharge-controllable facilities to attenuate the peak flow of urban runoff by extending the discharge duration. These facilities must reset their capacity of peak-flow attenuation automatically before the next heavy rainfall. As an example, Qin et al. [182] proposed a leak tank that can temperately detain rainwater and then slowly release it to the nearby sewers. A schematic of the leak tank can be seen Figure 13. Rainwater is routed to the tank though a large pipe that is set above the tank, while the outlet orifice is set at the side of the tank and at a height above the tank’s bottom to avoid being clogged by debris. Rainwater from sealed pavements, rooftops, or other surfaces can be routed to the tank via gravitational flow without directly entering the local sewers. When the tank is empty, rainwater is detained in the tank. When the water in the tank levels above the leak orifice, a portion of inflow remains detained in the tank while the remaining surplus water drains from the leak orifice. After the inflow ceases, the water above the leak orifice continuously releases until the water in the tank levels with the orifice. Finally, the tank is re-set to detain rainwater inflow at the subsequent rain event without manual interventions. Assuming a catchment area of 100 m2, and runoff coefficient of 0.8, a tank base’s area of 2 m, and a tank’s height of 1.5 m and assuming a 10 cm depth of rain whose instantaneous rainfall intensity is Water 2020, 12, 3579 14 of 23 a gammaWater 2020 distribution, 12, x with a duration of 0.25 hour, Qin et al. [182] found that the peak flow14 of 23 can be attenuatedWaterWater 2020 2020 about, 12, 12, x, x 45% and the peak flow is retard about a quarter of hour (Figure 14). As a1414 10 of of cm23 23 rain extremely heavy rain, it is believed that the urban flooding can be greatly mitigated if a serial of depthleak and tanks a 0.25 with hour proper rain sizes duration is designed can to be the deemed right places as an to extremelydetain rainwater heavy on rain, the source. it is believed While that extremelyextremely heavy heavy rain, rain, it it is is believed believed that that the the urba urbann flooding flooding can can be be greatly greatly mitigated mitigated if if a a serial serial of of the urbansuch tanks flooding seem canpromising, be greatly their mitigated long-term if hydrological a serial of leak performances tanks with are proper still awaiting sizes is further designed to leakleak tanks tanks with with proper proper sizes sizes is is designed designed to to the the ri rightght places places to to detain detain rainwater rainwater on on the the source. source. While While the rightstudies. places to detain rainwater on the source. While such tanks seem promising, their long-term suchsuch tanks tanks seem seem promising, promising, their their long-term long-term hydrological hydrological performances performances are are still still awaiting awaiting further further hydrological performances are still awaiting further studies. studies.studies. (a) (b)

(a)(a) (b)(b)

Figure 12. Attenuating the peak flow should be done by extending the discharge time proportionally. (a) The runoff from catchments of B, C, and D merge together, resulting a greater runoff A (b) Peak FigureFigureFigure 12. 12.Attenuating 12. Attenuating Attenuating the the the peak peak peak flowflow flow should should be bebe done done by by by extending extending extending the the thedischarge discharge discharge time time proportionally. time proportionally. proportionally. flow of hydrographs are dwarfed by extending the discharge duration [182]. (a) The(a()a )The runo The runoff ffrunofffrom from from catchments catchments catchments of of B, of B, C, B, C, andC, and and D D mergeD merge merge together, together, together, resulting resulting resulting aa a greatergreater greater runoff runo runoffff AA( A ( b(bb) ))Peak PeakPeak flow of hydrographsflowflow of of hydrographs hydrographs are dwarfed are are dwarfed dwarfed by extending by by extending extending the discharge the the discharge discharge duration duration duration [182 [182]. [182].].

Figure 13. A buffer tank that detains rainwater temperately and releases it slowly at the side of the Figuretank. 13. TheA release buffer orifice tank that is set detains some elevation rainwater above temperately the bottom andof the releases tank to avoid it slowly clog. at the side of the tank.FigureFigure The release13. 13. A A buffer buffer orifice tank tank is setthat that some detains detains elevation rainwater rainwater above temperately temperately the bottom and and releases of releases the tank it it slowly slowly to avoid at at the the clog. side side of of the the tank.tank. The The release release orifice orifice is is set set some some elevation elevation above above the the bottom bottom of of the the tank tank to to avoid avoid clog. clog.

FigureFigure 14. 14.A A leak leak tanktank attenuates attenuates the the peak peak flow flow of runo offf runo andff retardsand retards the peak the flow peak effectively. flow e Theffectively. The simulation assumes assumes a acatchment catchment area area of 100 of 100 m2,m a 2runoff, a runo coefficientff coefficient of 0.8, of a 0.8, tank a base's tank base’s area of area 2 m, of 2 m, FigureFigure 14. 14. A A leak leak tank tank attenuates attenuates the the peak peak flow flow of of runo runoffff and and retards retards the the peak peak flow flow effectively. effectively. The The andsimulation asimulation tank’s height assumes assumes of 1.5a acatchment catchment m and assuming area area of of 100 100 a m 10 m2,2 cm,a arunoff runoff depth coefficient coefficient of rain whose of of 0.8, 0.8, a rainfall atank tank base's base's intensity area area of isof 2 a 2m, gammam,

distribution. Rain duration is 0.25 hour. qi = inflow; qo = outflow; θ = peak flow attenuation rate.

Water 2020, 12, 3579 15 of 23

7. Conclusions This paper briefly reviews mainstream techniques for urban flooding mitigation. Techniques are categorized into aboveground ones, including vegetated and non-vegetated roofs; ground-surface ones, including permeable pavements, water-retaining pavements, trees, bio-retention, infiltration trenches; and underground ones, including soakaways and other underground water storage units. Water barrels and water tanks are used to harvest rainwater for non-potable uses but they must be frequently emptied by homeowners to continually reset them. Vegetated and non-vegetated roofs, permeable pavements, water-retaining pavements, trees, bio-retention, and infiltration trenches have great ability to retain and detain rainwater in case of a small rain. Clogs of permeable pavements, infiltration trenches, and soakaways degrade their hydrological performance, which can be restored by frequent maintenance. Techniques like green roofs, urban trees, swales, and bio-retention can maintain good infiltration over time because roots of vegetated species, urban trees, swale grass, bio-retention plants loosen the growth media for water infiltration. While mainstream techniques effectively retain, store and/or infiltrate rainwater during small rainfalls, they lead to great overflows during heavy rain. As urban flooding takes place when the overflows from different catchments superimpose to a degree that exceeds the carrying capacity of local sewer pipes, current mainstream techniques are insufficient to mitigate urban flooding. To mitigate urban flooding, in case of a heavy rainfall, the peak flow must be attenuated substantially and the discharge duration must be extended proportionally. New studies shall focus on the development of novel, discharge-controllable devices that can attenuate the peak flow in case of heavy rainfalls by extending the duration of the runoff discharge. Leak tanks that can detain rainwater at the source and discharge it slowly for a longer duration than the rain duration are promising options to attenuate the peak flow of urban runoff and to reset tanks automatically. Further studies are necessary to understand the long-term hydrological performance of the leak tanks.

Author Contributions: Y.Q. prepared and wrote all the content of the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: This work is supported by the high-level innovation team and outstanding scholar program in Guangxi colleges (granted to Y. Q.) and by the Science Foundation of Guangxi (Grant no. 2018GXNSFAA294070, no. 2018GXNSFDA138009). Conflicts of Interest: The author declares no conflict of interest.

References

1. Recanatesi, F.; Petroselli, A.; Ripa, M.N.; Leone, A. Assessment of stormwater runoff management practices and BMPs under soil sealing: A study case in a peri-urban watershed of the metropolitan area of Rome (Italy). J. Environ. Manag. 2017, 201, 6–18. [CrossRef][PubMed] 2. Xu, D.; Ouyang, Z.; Wu, T.; Han, B. Dynamic Trends of Urban Flooding Mitigation Services in Shenzhen, China. Sustainability 2020, 12, 4799. [CrossRef] 3. Gregory, J.H.; Dukes, M.D.; Jones, P.H.; Miller, G.L. Effect of urban soil compaction on infiltration rate. J. Soil Water Conserv. 2006, 61, 117–124. 4. Yang, Q.; Zhang, S.; Dai, Q.; Yao, R. Assessment of Community Vulnerability to Different Types of Urban Floods: A Case for Lishui City, China. Sustainability 2020, 12, 7865. [CrossRef] 5. Ferguson, B.K. Storm-Water Infiltration for Peak-Flow Control. J. Irrig. Drain. Eng. 1995, 121, 463–466. [CrossRef] 6. Mitchell, V.G. Applying Integrated Urban Water Management Concepts: A Review of Australian Experience. Environ. Manag. 2006, 37, 589–605. [CrossRef] 7. Wong, T.H.F. Water sensitive urban design—the journey thus far. Australasian. J. Water Resour. 2006, 10, 213–222. 8. Eckart, K.; McPhee, Z.; Bolisetti, T. Performance and implementation of low impact development—A review. Sci. Total Environ. 2017, 607–608, 413–432. [CrossRef] Water 2020, 12, 3579 16 of 23

9. Lim, H.S.; Lu, X.X. Sustainable urban stormwater management in the tropics: An evaluation of Singapore’s ABC Waters Program. J. Hydrol. 2016, 538, 842–862. [CrossRef] 10. Wang, H.; Mei, C.; Liu, J.; Shao, W. A new strategy for integrated urban water management in China: Sponge city. Sci. China Technol. Sci. 2018, 61, 317–329. [CrossRef] 11. Zhou, Q. A Review of Sustainable Urban Drainage Systems Considering the Climate Change and Urbanization Impacts. Water 2014, 6, 976. [CrossRef] 12. Miles, B.; Band, L.E. Green infrastructure stormwater management at the watershed scale: Urban variable source area and watershed capacitance. Hydrol. Process. 2015, 29, 2268–2274. [CrossRef] 13. Qiao, X.-J.; Liao, K.-H.; Randrup, T.B. Sustainable stormwater management: A qualitative case study of the Sponge Cities initiative in China. Sustain. Cities Soc. 2020, 53, 101963. [CrossRef] 14. Pratt, C.J. A Review of Source Control of Urban Stormwater Runoff. Water Environ. J. 1995, 9, 132–139. [CrossRef] 15. Dietz, M.E. Low Impact Development Practices: A Review of Current Research and Recommendations for Future Directions. Water Air Soil Pollut. 2007, 186, 351–363. [CrossRef] 16. Ahiablame, L.M.; Engel, B.A.; Chaubey, I. Effectiveness of Low Impact Development Practices: Literature Review and Suggestions for Future Research. Water Air Soil Pollut. 2012, 223, 4253–4273. [CrossRef] 17. Niachou, A.; Papakonstantinou, K.; Santamouris, M.; Tsangrassoulis, A.; Mihalakakou, G. Analysis of the green roof thermal properties and investigation of its energy performance. Energy Build. 2001, 33, 719–729. [CrossRef] 18. Madre, F.; Vergnes, A.; Machon, N.; Clergeau, P. A comparison of 3 types of green roof as habitats for arthropods. Ecol. Eng. 2013, 57, 109–117. [CrossRef] 19. Jungels, J.; Rakow, D.A.; Allred, S.B.; Skelly, S.M. Attitudes and aesthetic reactions toward green roofs in the Northeastern United States. Landsc. Urban Plan. 2013, 117, 13–21. [CrossRef] 20. Susca, T.; Gaffin, S.R.; Dell’Osso, G.R. Positive effects of vegetation: Urban heat island and green roofs. Environ. Pollut. 2011, 159, 2119–2126. [CrossRef] 21. Getter, K.L.; Rowe, D.B.; Robertson, G.P.; Cregg, B.M.; Andresen, J.A. Carbon Sequestration Potential of Extensive Green Roofs. Environ. Sci. Technol. 2009, 43, 7564–7570. [CrossRef][PubMed] 22. Oberndorfer, E.; Lundholm, J.; Bass, B.; Coffman, R.R.; Doshi, H.; Dunnett, N.; Gaffin, S.; Köhler, M.; Liu, K.K.Y.; Rowe, B. Green Roofs as Urban Ecosystems: Ecological Structures, Functions, and Services. BioScience 2007, 57, 823–833. [CrossRef] 23. Berardi, U.; GhaffarianHoseini, A.; GhaffarianHoseini, A. State-of-the-art analysis of the environmental benefits of green roofs. Appl. Energy 2014, 115, 411–428. [CrossRef] 24. Hutchinson, D.; Abrams, P.; Retzlaff, R.; Liptan, T. Stormwater Monitoring Two Ecoroofs in Portland, Oregon, USA; Int. Conf. on Greening Rooftops for Sustainable Communities: Chicago, IL, USA, 2003; pp. 372–389. 25. Li, S.-x.; Qin, H.-p.; Peng, Y.-n.; Khu, S.T. Modelling the combined effects of runoff reduction and increase in evapotranspiration for green roofs with a storage layer. Ecol. Eng. 2019, 127, 302–311. [CrossRef] 26. Lee, J.Y.; Lee, M.J.; Han, M. A pilot study to evaluate runoff quantity from green roofs. J. Environ. Manag. 2015, 152, 171–176. [CrossRef] 27. Spolek, G. Performance monitoring of three ecoroofs in Portland, Oregon. Urban Ecosyst. 2008, 11, 349–359. [CrossRef] 28. Zhang, Q.; Miao, L.; Wang, X.; Liu, D.; Zhu, L.; Zhou, B.; Sun, J.; Liu, J. The capacity of greening roof to reduce stormwater runoff and pollution. Landsc. Urban Plan. 2015, 144, 142–150. [CrossRef] 29. Palermo, S.A.; Turco, M.; Principato, F.; Piro, P. Hydrological Effectiveness of an Extensive Green Roof in Mediterranean Climate. Water 2019, 11.[CrossRef] 30. DeNardo, J.C.; Jarrett, A.R.; Manbeck, H.B.; Beattie, D.J.; Berghage, R.D. Stormwater mitigation and surface temperature reduction by green roofs. Trans. ASAE 2005, 48, 1491–1496. [CrossRef] 31. Gregoire, B.G.; Clausen, J.C. Effect of a modular extensive green roof on stormwater runoff and water quality. Ecol. Eng. 2011, 37, 963–969. [CrossRef] 32. Talebi, A.; Bagg, S.; Sleep, B.E.; O’Carroll, D.M. Water retention performance of green roof technology: A comparison of canadian climates. Ecol. Eng. 2019, 126, 1–15. [CrossRef] 33. Vanuytrecht, E.; Van Mechelen, C.; Van Meerbeek, K.; Willems, P.; Hermy, M.; Raes, D. Runoff and vegetation stress of green roofs under different climate change scenarios. Landsc. Urban Plan. 2014, 122, 68–77. [CrossRef] Water 2020, 12, 3579 17 of 23

34. Dunnett, N.; Nagase, A.; Booth, R.; Grime, P. Influence of vegetation composition on runoff in two simulated green roof experiments. Urban Ecosyst. 2008, 11, 385–398. [CrossRef] 35. Nagase, A.; Dunnett, N. Amount of water runoff from different vegetation types on extensive green roofs: Effects of plant species, diversity and plant structure. Landsc. Urban Plan. 2012, 104, 356–363. [CrossRef] 36. Buccola, N.; Spolek, G. A Pilot-Scale Evaluation of Greenroof Runoff Retention, Detention, and Quality. Water Air Soil Pollut. 2011, 216, 83–92. [CrossRef] 37. Mentens, J.; Raes, D.; Hermy, M. Green roofs as a tool for solving the rainwater runoff problem in the urbanized 21st century? Landsc. Urban Plan. 2006, 77, 217–226. [CrossRef] 38. Villarreal, E.L.; Bengtsson, L. Response of a Sedum green-roof to individual rain events. Ecol. Eng. 2005, 25, 1–7. [CrossRef] 39. VanWoert, N.D.; Rowe, D.B.; Andresen, J.A.; Rugh, C.L.; Fernandez, R.T.; Xiao, L. Green Roof Stormwater Retention: Effect of Roof Surface, Slope, and Media Depth. J. Environ. Qual. 2005, 34, 1036–1044. [CrossRef] 40. Mangangka, I.R.; Liu, A.; Egodawatta, P.; Goonetilleke, A. Performance characterisation of a stormwater treatment bioretention basin. J. Environ. Manag. 2015, 150, 173–178. [CrossRef] 41. Carson, T.B.; Marasco, D.E.; Culligan, P.J.; McGillis, W.R. Hydrological performance of extensive green roofs in New York City: Observations and multi-year modeling of three full-scale systems. Environ. Res. Lett. 2013, 8, 024036. [CrossRef] 42. Bliss, D.J.; Neufeld, R.D.; Ries, R.J. Storm Water Runoff Mitigation Using a Green Roof. Environ. Eng. Sci. 2009, 26, 407–417. [CrossRef] 43. Schroll, E.; Lambrinos, J.; Righetti, T.; Sandrock, D. The role of vegetation in regulating stormwater runoff from green roofs in a winter rainfall climate. Ecol. Eng. 2011, 37, 595–600. [CrossRef] 44. Fassman-Beck, E.; Voyde, E.; Simcock, R.; Hong, Y.S. 4 Living roofs in 3 locations: Does configuration affect runoff mitigation? J. hydrol. 2013, 490, 11–20. [CrossRef] 45. Czemiel Berndtsson, J. Green roof performance towards management of runoff water quantity and quality: A review. Ecol. Eng. 2010, 36, 351–360. [CrossRef] 46. Liu, X.; Chui, T.F.M. Evaluation of Green Roof Performance in Mitigating the Impact of Extreme Storms. Water 2019, 11, 815. [CrossRef] 47. Getter, K.L.; Rowe, D.B. The Role of Extensive Green Roofs in Sustainable Development. HortScience 2006, 41, 1276–1285. [CrossRef] 48. Carter, T.L.; Rasmussen, T.C. Hydrologic behavior of vegetated roofs. JAWRA J. Am. Water Resour. Assoc. 2006, 42, 1261–1274. [CrossRef] 49. Teemusk, A.; Mander, Ü. Rainwater runoff quantity and quality performance from a greenroof: The effects of short-term events. Ecol. Eng. 2007, 30, 271–277. [CrossRef] 50. Trinh, D.H.; Chui, T.F.M. Assessing the hydrologic restoration of an urbanized area via an integrated distributed hydrological model. Hydrol. Earth Syst. Sci. 2013, 17, 4789–4801. [CrossRef] 51. Stovin, V.; Vesuviano, G.; Kasmin, H. The hydrological performance of a green roof test bed under UK climatic conditions. J. Hydrol. 2012, 414–415, 148–161. [CrossRef] 52. Ercolani, G.; Chiaradia, E.A.; Gandolfi, C.; Castelli, F.; Masseroni, D. Evaluating performances of green roofs for stormwater runoff mitigation in a high flood risk urban catchment. J. Hydrol. 2018, 566, 830–845. [CrossRef] 53. Carter, T.; Jackson, C.R. Vegetated roofs for stormwater management at multiple spatial scales. Landsc. Urban Plan. 2007, 80, 84–94. [CrossRef] 54. DeNardo, J.C.; Jarrett, A.R.; Manbeck, H.B.; Beattie, D.J. Stormwater Detention and Retention Abilities of Green Roofs. In Proceedings of the World Water and Environmental Resources Congress, Philadelphia, PA, USA, 23–26 June 2003; pp. 1639–1645. 55. Lee, J.Y.; Moon, H.J.; Kim, T.I.; Kim, H.W.; Han, M.Y. Quantitative analysis on the urban flood mitigation effect by the extensive green roof system. Environ. Pollut. 2013, 181, 257–261. [CrossRef][PubMed] 56. Whittinghill, L.J.; Rowe, D.B.; Andresen, J.A.; Cregg, B.M. Comparison of stormwater runoff from sedum, native prairie, and vegetable producing green roofs. Urban Ecosyst. 2015, 18, 13–29. [CrossRef] 57. Voyde, E.; Fassman, E.; Simcock, R. Hydrology of an extensive living roof under sub-tropical climate conditions in Auckland, New Zealand. J. Hydrol. 2010, 394, 384–395. [CrossRef] 58. Monterusso, M.A.; Rowe, D.B.; Rugh, C.L.; Russell, D.K. Runoff Water Quantity and Quality from Green Roof Systems; International Society for Horticultural Science (ISHS): Leuven, Belgium, 2004; pp. 369–376. Water 2020, 12, 3579 18 of 23

59. Volder, A.; Dvorak, B. Event size, substrate water content and vegetation affect storm water retention efficiency of an un-irrigated extensive green roof system in Central Texas. Sustain. Cities Soc. 2014, 10, 59–64. [CrossRef] 60. Speak, A.F.; Rothwell, J.J.; Lindley, S.J.; Smith, C.L. Rainwater runoff retention on an aged intensive green roof. Sci. Total Environ. 2013, 461–462, 28–38. [CrossRef] 61. Kolb, W. Good reasons for roof planting-Green roofs and rainwater. In International Conference on Urban Horticulture; Junge-Berberovic, R., Baechtiger, J.-B., Simpson, W.J., Eds.; International Society for Horticultural Science (ISHS): Waedenswil, Switzerland, 2004; pp. 295–300. 62. Razzaghmanesh, M.; Beecham, S. The hydrological behaviour of extensive and intensive green roofs in a dry climate. Sci. Total Environ. 2014, 499, 284–296. [CrossRef] 63. Shafique, M.; Luo, X. Comparison Study of Green Roof, Blue Roof, Green Blue Roof for Storm Water Management: A Review. In Proceedings of the International Conference on Construction and Real Estate Management (ICCREM), Banff, AB, Canada, 21–24 May 2019; pp. 475–482. 64. Carpenter, D.; Kaluvakolanu, P. Effect of Roof Surface Type on Storm-Water Runoff from Full-Scale Roofs in a Temperate Climate. J. Irrig. Drain. Eng. 2011, 137, 161–169. [CrossRef] 65. Mullaney, J.; Lucke, T. Practical Review of Pervious Pavement Designs. CLEAN Soil Air Water 2014, 42, 111–124. [CrossRef] 66. Sañudo-Fontaneda, L.A.; Charlesworth, S.M.; Castro-Fresno, D.; Andres-Valeri, V.C.A.; Rodriguez-Hernandez, J. Water quality and quantity assessment of pervious pavements performance in experimental car park areas. Water Sci. Technol. 2014, 69, 1526–1533. [CrossRef][PubMed] 67. Qin, Y. A review on the development of cool pavements to mitigate urban heat island effect. Renew. Sustain. Energy Rev. 2015, 52, 445–459. [CrossRef] 68. Cheng, Y.-Y.; Lo, S.-L.; Ho, C.-C.; Lin, J.-Y.; Yu, S. Field Testing of Porous Pavement Performance on Runoff and Temperature Control in Taipei City. Water 2019, 11, 2635. [CrossRef] 69. Park, S.B.; Lee, B.J.; Lee, J.; Jang, Y.I. A study on the seawater purification characteristics of water-permeable concrete using recycled aggregate. Resour. Conserv. Recycl. 2010, 54, 658–665. [CrossRef] 70. Andersen, C.T.; Foster, I.D.L.; Pratt, C.J. The role of urban surfaces (permeable pavements) in regulating drainage and evaporation: Development of a laboratory simulation experiment. Hydrol. process. 1999, 13, 597–609. [CrossRef] 71. Gülbaz, S.; Kazezyılmaz-Alhan, C.M. Experimental Investigation on Hydrologic Performance of LID with Rainfall-Watershed-Bioretention System. J. Hydrol. Eng. 2017, 22, D4016003. [CrossRef] 72. Li, H.; Harvey, J.T.; Holland, T.J.; Kayhanian, M. The use of reflective and permeable pavements as a potential practice for heat island mitigation and stormwater management. Environ. Res. Lett. 2013, 8, 015023. [CrossRef] 73. Jiao, L.; Ni, F.; Yang, J. Long-term field performance of porous asphalt pavement in China AU—Yu, Bin. Road Mater. Pavement Des. 2015, 16, 214–226. 74. Imran, H.M.; Akib, S.; Karim, M.R. Permeable pavement and stormwater management systems: A review. Environ. Technol. 2013, 34, 2649–2656. [CrossRef] 75. Park, J.; Park, J.; Cheon, J.; Lee, J.; Shin, H. Analysis of Infiltrating Water Characteristics of Permeable Pavements in a Parking Lot at Full Scale. Water 2020, 12, 2081. [CrossRef] 76. Chai, L.; Kayhanian, M.; Givens, B.; Harvey, J.; Jones, D. Hydraulic Performance of Fully Permeable Highway Shoulder for Storm Water Runoff Management. J. Environ. Eng. 2012, 138, 711–722. [CrossRef] 77. Dreelin, E.A.; Fowler, L.; Ronald Carroll, C. A test of porous pavement effectiveness on clay soils during natural storm events. Water Res. 2006, 40, 799–805. [CrossRef][PubMed] 78. Watanabe, S. Study on storm water control by permeable pavement and infiltration pipes. Water Sci. Technol. 1995, 32, 25–32. [CrossRef] 79. Collins, K.; Hunt, W.; Hathaway, J. Hydrologic Comparison of Four Types of Permeable Pavement and Standard Asphalt in Eastern North Carolina. J. Hydrol. Eng. 2008, 13, 1146–1157. [CrossRef] 80. Alam, T.; Mahmoud, A.; Jones, K.D.; Bezares-Cruz, J.C.; Guerrero, J. A Comparison of Three Types of Permeable Pavements for Urban Runoff Mitigation in the Semi-Arid South Texas, U.S.A. Water 2019, 11, 1992. [CrossRef] Water 2020, 12, 3579 19 of 23

81. Rodriguez-Hernandez, J.; Andrés-Valeri Valerio, C.; Ascorbe-Salcedo, A.; Castro-Fresno, D. Laboratory Study on the Stormwater Retention and Runoff Attenuation Capacity of Four Permeable Pavements. J. Environ. Eng. 2016, 142, 04015068. [CrossRef] 82. Zachary Bean, E.; Frederick Hunt, W.; Alan Bidelspach, D. Evaluation of Four Permeable Pavement Sites in Eastern North Carolina for Runoff Reduction and Water Quality Impacts. J. Irrig. Drain. Eng. 2007, 133, 583–592. [CrossRef] 83. Gomez-Ullate, E.; Castillo-Lopez, E.; Castro-Fresno, D.; Bayon, J.R. Analysis and Contrast of Different Pervious Pavements for Management of Storm-Water in a Parking Area in Northern Spain. Water Resour. Manag. 2011, 25, 1525–1535. [CrossRef] 84. Brattebo, B.O.; Booth, D.B. Long-term stormwater quantity and quality performance of permeable pavement systems. Water Res. 2003, 37, 4369–4376. [CrossRef] 85. Fassman, E.; Blackbourn, S. Urban Runoff Mitigation by a Permeable Pavement System over Impermeable Soils. J. Hydrol. Eng. 2010, 15, 475–485. [CrossRef] 86. Haselbach, L.M.; Valavala, S.; Montes, F. Permeability predictions for sand-clogged Portland cement pervious concrete pavement systems. J. Environ. Manag. 2006, 81, 42–49. [CrossRef][PubMed] 87. Sansalone, J.; Kuang, X.; Ying, G.; Ranieri, V. Filtration and clogging of permeable pavement loaded by urban drainage. Water Res. 2012, 46, 6763–6774. [CrossRef][PubMed] 88. Bean, E.Z.; Hunt, W.F.; Bidelspach, D.A. Field Survey of Permeable Pavement Surface Infiltration Rates. J. Irrig. Drain. Eng. 2007, 133, 249–255. [CrossRef] 89. Pezzaniti, D.; Beecham, S.; Kandasamy, J. Influence of clogging on the effective life of permeable pavements. Proceedings of the Institution of Civil Engineers. Water Manag. 2009, 162, 1–10. [CrossRef] 90. Chopra, M.; Kakuturu, S.; Ballock, C.; Spence, J.; Wanielista, M. Effect of Rejuvenation Methods on the Infiltration Rates of Pervious Concrete Pavements. J. Hydrol. Eng. 2010, 15, 426–433. [CrossRef] 91. Kia, A.; Wong, H.S.; Cheeseman, C.R. Clogging in permeable concrete: A review. J. Environ. Manag. 2017, 193, 221–233. [CrossRef] 92. Nakayama, T.; Fujita, T. Cooling effect of water-holding pavements made of new materials on water and heat budgets in urban areas. Landsc. Urban Plan. 2010, 96, 57–67. [CrossRef] 93. Katsunori, T.; Kazuya, Y. Road Temperature Mitigation Effect of ’Road Cool’, a Water-Retentive Material Using Blast Furnace Slag. JEF GIHO 2008, 19, 28–32. 94. Kinoshita, S.; Yoshida, A.; Okuno, N. Evaporation performance analysis for water-retentive materials, based on outddor heat-bedget and transport properties. J. Heat Isl. Inst. Int. 2012, 7, 222–230. 95. Yamagata, H.; Nasu, M.; Yoshizama, M.; Miyamota, A.; Minamiyama, M. Heat island mitigation using water retentive pavement sprinkled with reclaimed wastewater. Water Sci. Technol. 2008, 57, 763–771. [CrossRef] 96. Qin, Y.; He, Y.; Hiller, J.E.; Mei, G. A new water-retaining paver block for reducing runoff and cooling pavement. J. Clean. Prod. 2018, 199, 948–956. [CrossRef] 97. Bao, T.; Liu, Z.; Zhang, X.; He, Y. A drainable water-retaining paver block for runoff reduction and evaporation cooling. J. Clean. Prod. 2019, 228, 418–424. [CrossRef] 98. Roy, S.; Byrne, J.; Pickering, C. A systematic quantitative review of urban tree benefits, costs, and assessment methods across cities in different climatic zones. Urban For. Urban Green. 2012, 11, 351–363. [CrossRef] 99. Denman, E.C.; May, P.B.; Moore, G.M. The use of trees in urban stormwater management. In The 12th National Street Tree Symposium; Lawry, D., Merrett, B., Eds.; The University of Adelaide: Adelaide, Australia, 2011; pp. 57–66. 100. Song, P.; Kim, G.; Mayer, A.; He, R.; Tian, G. Assessing the Ecosystem Services of Various Types of Urban Green Spaces Based on i-Tree Eco. Sustainability 2020, 12, 1630. [CrossRef] 101. Scharenbroch, B.C.; Morgenroth, J.; Maule, B. Tree Species Suitability to Bioswales and Impact on the Urban Water Budget. J. Environ. Qual. 2016, 45, 199–206. [CrossRef] 102. Roth, B.E.; Slatton, K.C.; Cohen, M.J. On the potential for high-resolution lidar to improve rainfall interception estimates in forest ecosystems. Front. Ecol. Environ. 2007, 5, 421–428. [CrossRef] 103. Van Stan, J.T.; Levia, D.F.; Jenkins, R.B. Forest Canopy Interception Loss Across Temporal Scales: Implications for Urban Greening Initiatives. Prof. Geogr. 2015, 67, 41–51. [CrossRef] 104. Xiao, Q.; McPherson, E.G.; Ustin, S.L.; Grismer, M.E.; Simpson, J.R. Winter rainfall interception by two mature open-grown trees in Davis, California. Hydrol. process. 2000, 14, 763–784. [CrossRef] Water 2020, 12, 3579 20 of 23

105. Inkiläinen, E.N.M.; McHale, M.R.; Blank, G.B.; James, A.L.; Nikinmaa, E. The role of the residential urban forest in regulating throughfall: A case study in Raleigh, North Carolina, USA. Landsc. Urban Plan. 2013, 119, 91–103. [CrossRef] 106. Xiao, Q.; McPherson, E.G. Rainfall interception by Santa Monica’s municipal urban forest. Urban Ecosyst. 2002, 6, 291–302. [CrossRef] 107. Xiao, Q.; McPherson, E.G. Rainfall interception of three trees in Oakland, California. Urban Ecosyst. 2011, 14, 755–769. [CrossRef] 108. Carlyle-Moses, D.E.; Gash, J.H.C. Rainfall Interception Loss by Forest Canopies. In Forest Hydrology and Biogeochemistry: Synthesis of Past Research and Future Directions; Levia, D.F., Carlyle-Moses, D., Tanaka, T., Eds.; Springer: Dordrecht, The Netherlands, 2011; pp. 407–423. 109. Clapp, J.C.; Ryan, H.D.P.; Harper, R.W.; Bloniarz, D.V. Rationale for the increased use of conifers as functional green infrastructure: A literature review and synthesis. Arboric. J. 2014, 36, 161–178. [CrossRef] 110. Armson, D.; Stringer, P.; Ennos, A.R. The effect of street trees and amenity grass on urban surface water runoff in Manchester, UK. Urban For. Urban Green. 2013, 12, 282–286. [CrossRef] 111. Bartens, J.; Day, S.D.; Harris, J.R.; Wynn, T.M.; Dove, J.E. Transpiration and Root Development of Urban Trees in Structural Soil Stormwater Reservoirs. Environ. Manag. 2009, 44, 646–657. [CrossRef][PubMed] 112. Bartens, J.; Day, S.D.; Harris, J.R.; Dove, J.E.; Wynn, T.M. Can Urban Tree Roots Improve Infiltration through Compacted Subsoils for Stormwater Management? J. Environ. Qual. 2008, 37, 2048–2057. [CrossRef][PubMed] 113. Stovin, V.R.; Jorgensen, A.; Clayden, A. Street Trees and Stormwater Management. Arboric. J. 2008, 30, 297–310. [CrossRef] 114. Nachshon, U.; Netzer, L.; Livshitz, Y. Land cover properties and rain water harvesting in urban environments. Sustain. Cities Soc. 2016, 27, 398–406. [CrossRef] 115. Jennings, A.A.; Adeel, A.A.; Hopkins, A.; Litofsky, A.L.; Wellstead, S.W. Rain Barrel-Urban Garden Stormwater Management Performance. J. Environ. Eng. 2013, 139, 757–765. [CrossRef] 116. Litofsky, A.L.E.; Jennings, A.A. Evaluating Rain Barrel Storm Water Management Effectiveness across Climatography Zones of the United States. J. Environ. Eng. 2014, 140, 04014009. [CrossRef] 117. Summerville, N.; Sultana, R. Rainwater harvesting potential in a semiarid Southern California city. AWWA Water Sci. 2019, 1, e1123. [CrossRef] 118. Jarden, K.M.; Jefferson, A.J.; Grieser, J.M. Assessing the effects of catchment-scale urban green infrastructure retrofits on hydrograph characteristics. Hydrol. Process. 2013, 30, 1536–1550. [CrossRef] 119. Gao, Y.; Babin, N.; Turner, A.J.; Hoffa, C.R.; Peel, S.; Prokopy, L.S. Understanding urban-suburban adoption and maintenance of rain barrels. Landsc. Urban Plan. 2016, 153, 99–110. [CrossRef] 120. Chahar, B.R.; Graillot, D.; Gaur, S. Storm-Water Management through Infiltration Trenches. J. Irrig. Drain. Eng. 2012, 138, 274–281. [CrossRef] 121. Locatelli, L.; Mark, O.; Mikkelsen, P.S.; Arnbjerg-Nielsen, K.; Wong, T.; Binning, P.J. Determining the extent of groundwater interference on the performance of infiltration trenches. J. Hydrol. 2015, 529, 1360–1372. [CrossRef] 122. Warnaars, E.; Larsen, A.V.; Jacobsen, P.; Mikkelsen, P.S. Hydrologic behaviour of stormwater infiltration trenches in a central urban area during 23/4 years of operation. Water Sci. Technol. 1999, 39, 217–224. [CrossRef] 123. Siriwardene, N.R.; Deletic, A.; Fletcher, T.D. Clogging of stormwater gravel infiltration systems and filters: Insights from a laboratory study. Water Res. 2007, 41, 1433–1440. [CrossRef][PubMed] 124. Yang, H.; Dick, W.A.; McCoy, E.L.; Phelan, P.L.; Grewal, P.S. Field evaluation of a new biphasic rain garden for stormwater flow management and pollutant removal. Ecol. Eng. 2013, 54, 22–31. [CrossRef] 125. Dunnett, N.; Clayden, A. Rain Gardens: Managing Water Sustainably in the Garden and Designed Landscape; Timber Press, Inc.: Portland, OR, USA, 2007. 126. Amur, A.; Wadzuk, B.; Traver, R. Analyzing the Performance of a Rain Garden over 15 Years: How Predictable Is the Rain Garden’s Response? In Proceedings of the International Low Impact Development Conference, Virtual Online. 20–24 July 2020; pp. 151–162. 127. Guo James, C.Y. Cap-Orifice as a Flow Regulator for Rain Garden Design. J. Irrig. Drain. Eng. 2012, 138, 198–202. 128. Guo, J.C.Y.; Luu, T.M. Operation of Cap Orifice in a Rain Garden. J. Hydrol. Eng. 2015, 20, 06015002. [CrossRef] Water 2020, 12, 3579 21 of 23

129. Dietz, M.E.; Clausen, J.C. A Field Evaluation of Rain Garden Flow and Pollutant Treatment. Water Air Soil Pollut. 2005, 167, 123–138. [CrossRef] 130. Muthanna, T.M.; Viklander, M.; Thorolfsson, S.T. Seasonal climatic effects on the hydrology of a rain garden. Hydrol. process. 2008, 22, 1640–1649. [CrossRef] 131. Yang, H.; Florence, D.C.; McCoy, E.L.; Dick, W.A.; Grewal, P.S. Design and hydraulic characteristics of a field-scale bi-phasic bioretention rain garden system for storm water management. Water Sci. Technol. 2009, 59, 1863–1872. [CrossRef][PubMed] 132. Autixier, L.; Mailhot, A.; Bolduc, S.; Madoux-Humery, A.-S.; Galarneau, M.; Prévost, M.; Dorner, S. Evaluating rain gardens as a method to reduce the impact of sewer overflows in sources of drinking water. Sci. Total Environ. 2014, 499, 238–247. [CrossRef][PubMed] 133. Gavri´c,S.; Leonhardt, G.; Marsalek, J.; Viklander, M. Processes improving urban stormwater quality in grass swales and filter strips: A review of research findings. Sci. Total Environ. 2019, 669, 431–447. [CrossRef][PubMed] 134. Abu-Zreig, M.; Rudra, R.P.; Lalonde, M.N.; Whiteley, H.R.; Kaushik, N.K. Experimental investigation of runoff reduction and sediment removal by vegetated filter strips. Hydrol. process. 2004, 18, 2029–2037. [CrossRef] 135. Otto, S.; Vianello, M.; Infantino, A.; Zanin, G.; Di Guardo, A. Effect of a full-grown vegetative filter strip on herbicide runoff: Maintaining of filter capacity over time. Chemosphere 2008, 71, 74–82. [CrossRef][PubMed] 136. Saleh, I.; Kavian, A.; Habibnezhad Roushan, M.; Jafarian, Z. The efficiency of vegetative buffer strips in runoff quality and quantity control. Int. J. Environ. Sci. Technol. 2018, 15, 811–820. [CrossRef] 137. Brodie, I. Application of level spreader grass filter strips in south east Queensland, Australia for discharge reduction and passive irrigation. In Proceedings of the 12th International Conference on Urban Drainage (ICUD 2011), Porto Alegre, Brazil, 11–16 September 2011; pp. 1–10. 138. Winston, R.J.; Hunt, W.F.; Osmond, D.L.; Lord, W.G.; Woodward, M.D. Field Evaluation of Four Level Spreader-Vegetative Filter Strips to Improve Urban Storm-Water Quality. J. Irrig. Drain. Eng. 2011, 137, 170–182. [CrossRef] 139. Hathaway, J.M.; Hunt, W.F. Field Evaluation of Level Spreaders in the Piedmont of North Carolina. J. Irrig. Drain. Eng. 2008, 134, 538–542. [CrossRef] 140. Winston, R.J.; Hunt, W.F. Field Evaluation of Level Spreader—Vegetative Filter Strips to Improve Hydrology and Water Quality. In Proceedings of the ASABE 2009—Annual International Meeting (ASABE 2009), Reno, NV, USA, 21–24 June 2009; pp. 1–10. 141. Hunt, W.F.; Hathaway, J.M.; Winston, R.J.; Jadlocki, S.J. Runoff Volume Reduction by a Level Spreader—Vegetated Filter Strip System in Suburban Charlotte, NC. J. Hydrol. Eng. 2010, 15, 499–503. [CrossRef] 142. Line, D.E.; Hunt, W.F. Performance of a Bioretention Area and a Level Spreader-Grass Filter Strip at Two Highway Sites in North Carolina. J. Irrig. Drain. Eng. 2009, 135, 217–224. [CrossRef] 143. Khadka, A.; Kokkonen, T.; Niemi, T.J.; Lähde, E.; Sillanpää, N.; Koivusalo, H. Towards natural water cycle in urban areas: Modelling stormwater management designs. Urban Water J. 2020, 17, 587–597. [CrossRef] 144. Winston, R.J.; Dorsey, J.D.; Hunt, W.F. Quantifying volume reduction and peak flow mitigation for three bioretention cells in clay soils in northeast Ohio. Sci. Total Environ. 2016, 553, 83–95. [CrossRef][PubMed] 145. Roy-Poirier, A.; Champagne, P.; Filion, Y. Review of Bioretention System Research and Design: Past, Present, and Future. J. Environ. Eng. 2010, 136, 878–889. [CrossRef] 146. Endreny, T.; Collins, V. Implications of bioretention basin spatial arrangements on stormwater recharge and groundwater mounding. Ecol. Eng. 2009, 35, 670–677. [CrossRef] 147. Bouwer, H. Artificial recharge of groundwater: Hydrogeology and engineering. Hydrogeol. J. 2002, 10, 121–142. [CrossRef] 148. Abida, H.; Sabourin, J.F. Grass Swale-Perforated Pipe Systems for Stormwater Management. J. Irrig. Drain. Eng. 2006, 132, 55–63. [CrossRef] 149. Zhang, L.; Lu, Q.; Ding, Y.; Peng, P.; Yao, Y. Design and Performance Simulation of Road Bioretention Media for Sponge Cities. J. Perform. Constr. Facil. 2018, 32, 04018061. [CrossRef] 150. Barber Michael, E.; King Steven, G.; Yonge David, R.; Hathhorn Wade, E. Ecology Ditch: A Best Management Practice for Storm Water Runoff Mitigation. J. Hydrol. Eng. 2003, 8, 111–122. [CrossRef] 151. Le Coustumer, S.; Fletcher, T.D.; Deletic, A.; Barraud, S.; Lewis, J.F. Hydraulic performance of biofilter systems for stormwater management: Influences of design and operation. J. Hydrol. 2009, 376, 16–23. [CrossRef] Water 2020, 12, 3579 22 of 23

152. Bergman, M.; Hedegaard, M.R.; Petersen, M.F.; Binning, P.; Mark, O.; Mikkelsen, P.S. Evaluation of two stormwater infiltration trenches in central Copenhagen after 15 years of operation. Water Sci. Technol. 2011, 63, 2279–2286. [CrossRef][PubMed] 153. Dechesne, M.; Barraud, S.; Bardin, J.-P. Experimental Assessment of Stormwater Infiltration Basin Evolution. J. environ. eng. 2005, 131, 1090–1098. [CrossRef] 154. Winston, R.J.; Powell, J.T.; Hunt, W.F. Retrofitting a grass swale with rock check dams: Hydrologic impacts. Urban Water J. 2019, 16, 404–411. [CrossRef] 155. Pratt, C.J. Research and Development in Methods of Soakaway Design. Water Environ. J. 1996, 10, 47–51. [CrossRef] 156. Mylevaganam, S.; Chui, T.F.M.; Hu, J. Multiple Regression Model of a Soak-Away Rain Garden in Singapore. Open J. Model. Simul. 2015, 3, 49–62. [CrossRef] 157. Roldin, M.; Locatelli, L.; Mark, O.; Mikkelsen, P.S.; Binning, P.J. A simplified model of soakaway infiltration interaction with a shallow groundwater table. J. Hydrol. 2013, 497, 165–175. [CrossRef] 158. Roldin, M.; Mark, O.; Kuczera, G.; Mikkelsen, P.S.; Binning, P.J. Representing soakaways in a physically distributed urban drainage model—Upscaling individual allotments to an aggregated scale. J. Hydrol. 2012, 414–415, 530–538. [CrossRef] 159. Roldin, M.; Fryd, O.; Jeppesen, J.; Mark, O.; Binning, P.J.; Mikkelsen, P.S.; Jensen, M.B. Modelling the impact of soakaway retrofits on combined sewage overflows in a 3 km2 urban catchment in Copenhagen, Denmark. J. Hydrol. 2012, 452–453, 64–75. [CrossRef] 160. Locatelli, L.; Gabriel, S.; Mark, O.; Mikkelsen, P.S.; Arnbjerg-Nielsen, K.; Taylor, H.; Bockhorn, B.; Larsen, H.; Kjølby, M.J.; Blicher, A.S.; et al. Modelling the impact of retention–detention units on sewer surcharge and peak and annual runoff reduction. Water Sci. Technol. 2015, 71, 898–903. [CrossRef] 161. Petrucci, G.; Deroubaix, J.-F.; de Gouvello, B.; Deutsch, J.-C.; Bompard, P.; Tassin, B. Rainwater harvesting to control stormwater runoff in suburban areas. An experimental case-study. Urban Water J. 2012, 9, 45–55. [CrossRef] 162. Herrmann, T.; Schmida, U. Rainwater utilisation in Germany: Efficiency, dimensioning, hydraulic and environmental aspects. Urban Water 2000, 1, 307–316. [CrossRef] 163. Jones, M.P.; Hunt, W.F. Performance of rainwater harvesting systems in the southeastern United States. Resour. Conserv. Recycl. 2010, 54, 623–629. [CrossRef] 164. Guo, Y.; Baetz B., W. Sizing of Rainwater Storage Units for Green Building Applications. J. Hydrol. Eng. 2007, 12, 197–205. [CrossRef] 165. Vaes, G.; Berlamont, J. The effect of rainwater storage tanks on design storms. Urban Water 2001, 3, 303–307. [CrossRef] 166. Khastagir, A.; Jayasuriya, N.N. Impacts of using rainwater tanks on stormwater harvesting and runoff quality. Water Sci. Technol. 2010, 62, 324–329. [CrossRef][PubMed] 167. Sterren, M.v.d.; Rahman, A.; Dennis, G.R. Implications to stormwater management as a result of lot scale rainwater tank systems: A case study in Western Sydney, Australia. Water Sci. Technol. 2012, 65, 1475–1482. [CrossRef][PubMed] 168. Gowland, D.; Younos, T. Feasibility of Rainwater Harvesting BMP for Stormwater Management; Virginia Tech: Blacksburg, VA, USA, 2008; pp. 1–20. 169. Li, F.; Duan, H.-F.; Yan, H.; Tao, T. Multi-Objective Optimal Design of Detention Tanks in the Urban Stormwater Drainage System: Framework Development and Case Study. Water Resour. Manag. 2015, 29, 2125–2137. [CrossRef] 170. Palla, A.; Gnecco, I.; Lanza, L.G.; La Barbera, P. Performance analysis of domestic rainwater harvesting systems under various European climate zones. Resour. Conserv. Recycl. 2012, 62, 71–80. [CrossRef] 171. Sharma, A.K.; Gardner, T. Comprehensive Assessment Methodology for Urban Residential Rainwater Tank Implementation. Water 2020, 12, 315. [CrossRef] 172. Freni, G.; Liuzzo, L. Effectiveness of Rainwater Harvesting Systems for Flood Reduction in Residential Urban Areas. Water 2019, 11, 1389. [CrossRef] 173. Todeschini, S.; Papiri, S.; Ciaponi, C. Performance of stormwater detention tanks for urban drainage systems in northern Italy. J. Environ. Manag. 2012, 101, 33–45. [CrossRef][PubMed] 174. Rahman, A.; Dbais, J.; Imteaz, M.A. Sustainability of rainwater harvesting systems in multistorey residential buildings. Am. J. Eng. Appl. Sci. 2010, 3, 73–82. [CrossRef] Water 2020, 12, 3579 23 of 23

175. Imteaz, M.A.; Shanableh, A.; Rahman, A.; Ahsan, A. Optimisation of rainwater tank design from large roofs: A case study in Melbourne, Australia. Resour. Conserv. Recycl. 2011, 55, 1022–1029. [CrossRef] 176. Domènech, L.; Saurí, D. A comparative appraisal of the use of rainwater harvesting in single and multi-family buildings of the Metropolitan Area of Barcelona (Spain): Social experience, drinking water savings and economic costs. J. Clean. Prod. 2011, 19, 598–608. [CrossRef] 177. Morales-Pinzón, T.; Rieradevall, J.; Gasol, C.M.; Gabarrell, X. Modelling for economic cost and environmental analysis of rainwater harvesting systems. J. Clean. Prod. 2015, 87, 613–626. [CrossRef] 178. Zhang, Y.; Chen, D.; Chen, L.; Ashbolt, S. Potential for rainwater use in high-rise buildings in Australian cities. J. Environ. Manag. 2009, 91, 222–226. [CrossRef] 179. Roebuck, R.M.; Oltean-Dumbrava, C.; Tait, S. Whole life cost performance of domestic rainwater harvesting systems in the United Kingdom. Water Environ. J. 2011, 25, 355–365. [CrossRef] 180. Schubert, J.E.; Burns, M.J.; Fletcher, T.D.; Sanders, B.F. A framework for the case-specific assessment of Green Infrastructure in mitigating urban flood hazards. Adv. Water Resour. 2017, 108, 55–68. [CrossRef] 181. Huang, C.-L.; Hsu, N.-S.; Liu, H.-J.; Huang, Y.-H. Optimization of low impact development layout designs for megacity flood mitigation. J. Hydrol. 2018, 564, 542–558. [CrossRef] 182. Qin, Y.; Huang, Z.; Yu, Z.; Liu, Z.; Wang, L. A Novel Buffer Tank to Attenuate the Peak Flow of Runoff. Civ. Eng. J. 2019, 5, 2525–2534. [CrossRef] 183. Huang, H.; Chen, X.; Zhu, Z.; Xie, Y.; Liu, L.; Wang, X.; Wang, X.; Liu, K. The changing pattern of urban flooding in Guangzhou, China. Sci. Total Environ. 2018, 622–623, 394–401. [CrossRef][PubMed]

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

© 2020 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).