sustainability Article Green Roof for Stormwater Management in a Highly Urbanized Area: The Case of Seoul, Korea Muhammad Shafique 1,2, Reeho Kim 1,2,* and Kwon Kyung-Ho 3 1 Department of Smart City and Construction Engineering, Korea Institute of Civil Engineering and Building Technology, University of Science & Technology (UST), 217, Gajeong-ro, Yuseong-gu, Daejeon 34113, Korea; shafi[email protected] 2 Environmental & Plant Engineering Research Institute, Korea Institute of Civil Engineering and Building Technology, 83, Goyangdae-ro, Ilsanseo-gu, Goyang-si, Gyeonggi-do 10223, Korea 3 Urban Water Cycle Research Center, Korea Institute of Safe Drinking Water Research, Anyang si, Gyeonggi-do 14059, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-31-9100-291 Received: 26 December 2017; Accepted: 21 February 2018; Published: 26 February 2018 Abstract: Urbanization changes natural pervious surfaces to hard, impervious surfaces such as roads, buildings and roofs. These modifications significantly affect the natural hydrologic cycle by increasing stormwater runoff rates and volume. Under these circumstances, green roofs offer multiple benefits including on-site stormwater management that mimics the natural hydrologic conditions in an urban area. It can retain a large amount of rainwater for a longer time and delay the peak discharge. However, there is very limited research that has been carried out on the retrofitted green roof for stormwater management for South Korean conditions. This study has investigated the performance of retrofitted green roofs for stormwater management in a highly urbanized area of Seoul, the capital city of Korea. In this study, various storm events were monitored and the research results were analyzed to check the performance of the green roof with controlling the runoff in urban areas. Results also allowed us to conclude that the retention mainly depends on the intensity and duration of the rain events. From the analysis, average runoff retention on the green roof was 10% to 60% in different rain events. The application of an extensive green roof provides promising results for stormwater management in the highly urbanized area of Seoul. Keywords: green roof; stormwater management; flash flooding; storm events; rainfall runoff 1. Introduction Green Roof as a Mitigation Strategy for Urban Water Related Problems In the natural environment, ground surfaces allow water infiltration [1] which balances groundwater supply and water quality. Surface runoff has decreased to a relatively low level because most precipitation events are not large enough to fully saturate the ground soil. However, urban impervious areas such as roads, buildings and roofs produce significant stormwater runoff under the big storm events that may cause flash flooding and other water-related problems. In the developed urban areas, roof surface areas account for 40–50% of all total impervious surface areas [1,2]. Therefore, it is very important to consider roof areas while designing the drainage system for an urban area. Under these circumstances, the green roof is considered an effective technique to reduce runoff and peak flows because it has the ability to retain stormwater in the soil and the vegetation medium for a long period of time as compared to a control roof approach [3–6]. Green roof may detain the rainwater for the longer period of time, thus can reduce the risks for flash flooding problems in urban areas [6]. A green roof is also generally referred to as vegetated roof/living roof and eco-roof [7–13]. It can retain the runoff for a longer period of time than a common roof [2,6]. It can delay peak discharge Sustainability 2018, 10, 584; doi:10.3390/su10030584 www.mdpi.com/journal/sustainability Sustainability 2018, 10, x FOR PEER REVIEW 2 of 14 roof may detain the rainwater for the longer period of time, thus can reduce the risks for flash flooding problems in urban areas [6]. Sustainability 2018, 10, 584 2 of 14 A green roof is also generally referred to as vegetated roof/living roof and eco-roof [7–13]. It can retain the runoff for a longer period of time than a common roof [2,6]. It can delay peak discharge timetime [[2,6,14]2,6,14] andand cancan alsoalso extenuateextenuate peakpeak dischargedischarge volumevolume [[2,6,14,15].2,6,14,15]. Consequently,Consequently, pastpast researchresearch hashas suggestedsuggested thatthat greengreen roofroof maymay bebe aa usefuluseful approachapproach toto mitigatemitigate thethe combinedcombined sewersewer overflowoverflow (CSO)(CSO) andand flashflash floodingflooding problemsproblems inin urbanurban areasareas [ [6,16,17].6,16,17]. Typically,Typically, a a green green roof roof consists consists of of three three layers: laye vegetation,rs: vegetation, substrate substrate and and drainage. drainage. The greenThe green roof canroof be can classified be classified depending depending on the on depth the depth of the of substrate the substrate layer andlayer can and be can named be named as extensive as extensive roofs androofs intensive and intensive roofs. roofs. Extensive Extensive roofs roofs generally generally have have a substrate a substrate depth depth of less of thanless than 150 mm150 mm and and the intensivethe intensive roofs haveroofs thickerhave substrate.thicker substrate. Green roofs Green provide roofs an provide opportunity an toopportunity retain the stormwater to retain forthe astormwater longer period for ofa timelonger in period urban areasof time (Figure in urban1). At areas first, the(Figure rainwater 1). At isfirst, stored the in rainwater soil media is andstored then in tosoil the media vegetation and then layer to of the the vegetation green roof, layer which of normallythe green helpsroof, towhich reduce normally the peak helps flow to and reduce volume the aspeak compared flow and to volume the conventional as compared roofs to asthe manifested conventional in Figureroofs as1. Greenmanifested roof vegetationin Figure 1. also Green allows roof evapotranspirationvegetation also allows (EV) evapotranspiration [16], which can also (EV) help [16], to reduce which the can runoff also volumes help to asreduce compared the runoff to the conventionalvolumes as compared roofs where to evapotranspiration the conventional (EV)roofs is where almost evapotranspiration zero. Figure1 shows (EV) the runoff is almost peak zero. and volumeFigure 1 delay shows by the using runoff green peak roof and in volume an urban delay area. by As using indicated green below, roof in the an greenurban roofarea. is As an indicated effective approachbelow, the to green decrease roof runoff is an and effective to increase approach the time to ofdecrease concentration runoff as and compared to increase to a conventional the time of roofconcentration in urban areas.as compared In addition, to a itconventional can also help roof to reducein urban the areas. total runoffIn addition, volume it reductioncan also help in the to urbanreduce drainage the total system.runoff volume reduction in the urban drainage system. Figure 1.1. Rainfall runoff responseresponse ofof thethe greengreen roofroof andand conventionalconventional roofroof [[2].2]. OnOn thethe otherother hand,hand, greengreen roofsroofs alsoalso havehave the abilityability to extend the life of building roof [[18],18], improveimprove thethe airair quality,quality, enhanceenhance thethe aestheticaesthetic valuevalue ofof thethe buildingbuilding architecturearchitecture andand improveimprove thethe biodiversitybiodiversity [[19].19]. Green Green roofs ar aree not a new technique (Köhler, 20042004 [[20]),20]), their widespread use, especiallyespecially forfor thethe stormwaterstormwater management provides benefitsbenefits in the urban areas all around the globe. OverOver thethe lastlast 3030 years,years, greengreen roofsroofs havehave becomebecome moremore popular,popular, particularlyparticularly inin mostmost ofof thethe developeddeveloped countriescountries suchsuch as: as: Germany, Germany, Australia, Australia, Switzerland, Switzerland, Austria, Austria, USA, USA, Japan, Japan, Singapore Singapore and South and Korea,South 2 forKorea, example, for example, 14% of the 14% flat of roofs the flat in Germanyroofs in Germany (13.5 km2 (13.5) are supportedkm ) are supported by green roofsby green in 2003 roofs (Herman, in 2003 2003(Herman, [21]). 2003 However, [21]). However, in South in Korea, South theKorea, law the of “lowlaw of carbon “low carbon green green growth growth building building code” code” [22] strongly[22] strongly endorses endorses green green roofs roofs as a Lowas a Low Impact Impa Developmentct Development (LID) (LID) technology technology to achieve to achieve safer safer and moreand more sustainable sustainable cities. cities. Low Low carbon carbon green green growth growth recognized recognized the the guidelines guidelines related related to to the theuse use ofof greengreen roofroof inin thethe buildingbuilding architecturesarchitectures [[22].22]. However,However, thethe useuse ofof greengreen roofroof waswas limitedlimited becausebecause ofof higherhigher constructionconstruction costs, costs, lack lack of properof proper design design and suitableand suitable guidelines guidelines in Korea. in Recently,Korea. Recently, the Korean the governmentKorean government has been has encouraging been encouraging the use of greenthe use roof of bygreen giving roof incentives by giving (by incentives providing (by construction providing costs)construction to the public costs) toto developthe public cities to develop safe,