Green roofs for storm water management:a review

Vol. 11 No. 1Joumal of Ghongqing University (English Edition) [ISSN 1671-8224]March 2012Article ID: 12242020)1000-07Todeos rick ANWAR Mcanmd. RASUL Mohamad G, KHANA MesudM K Creen ros fon stom wale management arvew[] Jogign.ngEdEISSN 1671-8224]1 2012, 1(1 5-11.Green roofs for storm water management: a reviewANWAR Mohammad , RASUL Mohammad G., KHAN Masud M. K.Facully of Sciences, Engineering and Health, cQUniversity, Rockhampton, AustaioAbstract: The rapid urbanization and idnursalization involve an unsustinabla use of natural systems, leading to variousproblems in ctiese. The urban hyrologial system experiences fuctuting amount of srtacc rumoff waler when it rains heavily. Ithas bee sugsed that green rofs sigifcanly mitigte storm water runff generation even in topcal climate. Gree rofs haveTheamnmualrinfallbecome popular due to its proven benefts by mitigating urban heat island efects and protecing biodiversity. The anmual rinfalland rumoff rlationship for green roofs is detemined by the depbh of the substrate. Water retention capacily mosly depends onSubstrate's physcal conditins such as dry or wetness Generally 6 mm to 12 mm ranfll is required for dry subsrate t0 itialerunoff whereas response of wet conditions is mostly straight. Besides, there are some othafecing runoff dynarmics suchas type of a green roof and its slope, age of green roof, type of vegetation, soil moisture carateritics, weather. The reviewindicates that there is not much rescarch in green rofs performace over stom water rnof, hence there is a ned for furtherresearch. This paper reviews and adresses the role of green roofs in urban storm water management.Keywords: storm watr, runof; extensive green roof, urbanCLC number. X922.1Document code: A1Introductionmost cities, conventional storm water infrastructurefailed to convey storm water rapidly from the city toNowadays the global population forces expansion ofrecipients.urban arcas. Due to more opportunities, people startUrban storm water infrastructure has been exposedliving in urban areas. Urban areas constantly lose itsto major rethinking when the idea of sustainablegreenery as demand for building, streets and drivewaysdevelopment gained more ground since 1990s. A cityincreases. Plain land and forests are displaced by thosecan benefit from proper usage of storm water. Storageimpervious surfaces. The level of urbanization is risingreservoirs, ponds as well as green areas are effectiveand expected to reach 83% by 2030 in developedtools for reducing runoff during. a rainfall. Acountries 24. Urbanization not only intensifying stormmodification of existing hydrological system iswater runoff rather it is enhancing stream channel andrequired to cope with this situation. Most big citiesriver erosion, which creates mumerous problems bothhave a large area of impervioussurfacesl3-4J andwithin and outside cities by unsustainable use ofbuilding green areas in urban regions is quite expensivenatural systems. The urban hydrological system facesand sometimes impossible. To solve the proble ofhighly fluctuating amount of surface runoff water alllarge storm water runof, a costly and disruptive way isyear through, which creates a negative impact on someto enlarge or expand storm water infrastructure. Severalxity infrastructure and surrounding environment. Incitiesare employingconsultants to studyenvironmenaly friendly, cost-fective altermatives tosolve these problems. Low impact developmnent (LID)TANWAR Mohammat: m.anwar@cqu.edu au; Tel: +617 4930and water sensitive urban design (WSUD) approaches6371; Fax: +617 4930 9382.are used in USA and Australia respectively for中国煤化工MHCNMHGM. Anwar, et al.Green roofsminimizing impervious cover and maximizinghuman. It also reduces the diversity of insect and fishinfiltration of rainfall, although in most locations thosepopulations. Sometimes local water treatment systemsapproaches are not widely implemented and the vastcan be overloaded due to storm water runoff,majority of new storm water management in USA andparticularly when in many cases sanitary sewage fromAustralia remains in the form of conventional stormshowers and toilets and storm water are treated in thesewers with limited treatment by retention basins5i.same facilities. During a heavy rainstorm, water mayIn older urban areas, there is a lack of suitable landexceed the capacity of a system which then dischargesfor creating sustainable storm water infrastructure; thusmixed sewage and storm water directly into local lakesgreen roofs and vertical green walls can be a variableand rivers. Green roofs not only minimize the stormalternative there. Green oof or vegetated roof can bewater runoff but also act as pollution adsorbents andimplemented in the huge amount of unused roof areafilters. This study aims to investigate the influence of a(about 40% to 50%) of the impermeable surfaces ingreen or vegetated roof on storm water runoffurban areas 0. Vegetated roofs can play an importantmanagement. Green roof technology, factors affectingrole in modem urban drainage because of their abilitystorm water retention and relationships between greento slow down and reduce runoff response. Green roofroof and runoff are also discussed.provides numerous ecological and economic beneftsincluding2 Green roof technologymanagement, mitigation of noise and air pollution,mitigation of the urban heat island effect and increasedGreen roof is a layered system comprising alongevity of roofing membranes as well as improvedwaterproofing membrane, growing medium and thebiodiversityOut of all proven benefits, thevegetation layer itself. Green roofs typically have fourreduction of storm water runoff is the greatestlayers of construction. A waterproofing membrane sitsenvironmental service that green roofs provide. In aimmediately on top of the structural roof deck togreen roof system, much of the precipitation isprevent moisture from entering the building. Typically,captured in the media or vegetation and eventuallyabove this membrane is a root barrier layer that isevaporates from the soil surface or is released back intodesigned to prevent roots from penetrating thethe atmosphere by transpiration. This is a continuouswaterproofing membrane and the structural roof. Aprocess.drainage layer is the next. The drainage layer (realizedDepending on the type of a green roof systemwith either some engineered coarse grained porous(design, substrate depth and plant species), research hasmedia or plastic profiled elements) is typicallyshown reduction of 60% to 100% in runoff 1013141.1designed to carry excess runoff to roof drains, and toBengtsson 05 has reported that high evapotranspirationstore water for plants in dry periods. Next, a filterfrom a vegetated roof can reduce the annual runoff tofabric is installed to prevent soil from being washedless than the half precipitation. Local urban floodingaway and the drainage layer from being compromisedand combined sewers overflows (CSOs) can beas water drains from the roof. Fnally, the growinglessened as water is stored initially in the soil andplants and associated substrate or growing medium (avegetation, which reduces peak flow. Urtban runoffblend of mineral material enriched with organicpollution can be reduced by green roof as it absorbs thematerial) complete the green roof. The substrate ispollutants of wet and dry atmospheric deposition. Thisoften a lightweight synthetic soil that is porous anpollution runoff is dependent on some factors like theinherently inert, with nutrients added for plant growth.type of surrounding area (industrial, residential orFig. 1 shows a typical green roof system.commercial) and local pollution sources (intensity ofBased on the depth of the planting medium andtraffic, type of heating system). When it rains, watermaintenance two types of green roofs are usuallycomes out of conventional roofs and paved areas,distinguished; they are intensive and extensive greenmixes with deposited pollutants and carries them toroofs. Intensive green roofs are established with deeprivers and local water sources. Contarminants in stormsoil layers. They can support plants and bushes andwater runoff can include fertilizers， herbicides,typically require maintenance in forms of fertilizing,insecticides, and oil and grease from roads and energyweeding and watering. Generally intensive green roofsproduction facities. The resulting contamination notcan support complex vegetation like groundcovers,only harms aquatic life but also make water unsafe forsmall trees and shrubs which have deeper rooting.中国煤化工6J. Chongqing Univ. Eng. Ed. [SSN 1671-82.MYHCNMHGM. Anwar, et al.Green roofsIntensive_ roofs involve a greater load of more thanof a green roof. Getter et al. [18] reported that rainwater150 kg/m2 and have more than 200 mm of substrateretention weakened as the slope angle increased and thewith higher amount of organic material than extensiveeffect was for slopes between 2% and 15% as well assystems 16. Intensive green roofs can be installed onbetween 2% and 25%. They found that organic matterroofs of a slope smaller than 10°. Extensive green roofscontent and pore space of soil doubled in 5 years timehave a thin substrate layer with low level planting,(from 2% to 4% and from 41% to 82%, respectively).typically sedum or lawn. An extensive roof is plantedwith relatively smaller plants which in the final stage3.2 Weather conditionsare expected to provide full coverage of the vegetatedroof; it is intended to be self sustaining and requiresWeather conditions such as length of proceeding dryminimal maintenance. It features low cost, small massperiod, season/climate (air temperature, windof (50 to 150) kg/m2 and thin material substrate of upconditions, humidity), characteristics of rain eventto 150 mm. This type can also be installed on sloped(intensity and duration) also affect the storm watersurfaces up to 45°.retention. Kaufmann 9compared the runoffpercentage during winter and summer for both 5 cmgravel roof and 1 cm extensive green roof and found,Vegetationthe runoff was significantly higher during winter time,and water storage86% for gravel roof and 80% for green roof; whereas inthe summer 70% for gravel roof and 52% for extensivegreen roof. Carter and Rasmussen (201 found 57% ofpeaks on a vegetated roof were delayed up to 10 min ascompared with peaks for a conventional roof. Theysummarized peak discharge for a small storm wasInsylationMembrane protectionand root barriermuch lower from the vegetated roof than from aStructural supportconventional roof but this effect was much reduced forFig. 1 Typical green roof system.larger storms. Green roofs generally delay runoff (peakto peak) by 10 min B.10. DeNardo et al. 21 showed thatgreen roofs reduced the peak intensities from an3 Factors affecting storm water retentionaverage rainfall intensity of 4.3 mm/h to an averagegreen roof runoff rate of 2.4 mm/h. Moran et al.3.I Green roof characteristicsreported that runoff delays of 90% rain events wereobserved and minimum 30 min delay for 60% of rainGreen roof characteristics such as number of layersevents was observed. Bengtssonshowed thatand type of materials, soil thickness, soil type,weather conditions (dry or wet) affected the retentionvegetation cover, type of vegetation, and the geometrycapacity of studied green roof; such as for dryincluding slope angle and length, position and age of aconditions 6 mm to 12 mm rain was required to initiateroof affects the performance of storm water retention.runoff and for wet conditions the response was almostTable 1 shows that the annual runoff is mainlystraight.determined by the roof type and may be as high as 91%for a traditional non-greened roof and as low as 15%4 Green-roof and storm water runoff relationshipfor an intensive green roof; and 19% for an extensivereen roof. Different studies related to the slopeA green roof reduces storm water runoff comparedinfluence on green roofs runoff retention capacity bringwith a hard roof by lowering and delaying the runoffdifferent results. While some studies find no correlationpeak. A significant water volume is detained in a greenbetween roof slope and runffl5sis, the others observeroof and its substrate layers. A part of detained waterthat runoff retention may depend on slopes. Mentens etwill drain and a part corresponding to field capacityal.的stated that the annual precipitation, roof type, andwill be retained. The retained water will evaporate ordepth of substrate layers are significantly correlatedbe used by plants and parts of it will transpire. Throughwith the yearly runoff while they did not find anyevaporation and transpiration, water runoff from greensignificant relation with the age, slope angle and lengthroofs reduces.中国煤化工J Chongaing Univ. Eng. Ed. (SN 1671-82JMYHCNMHG7M. Anwar, et al.Green roofsTable 1 Substrate layer depth and rumof caracteristics on an annual level7]Substrate layer/mmRuno17%Type of roofMinimum MaximumAverageMinimum , MaximumIntensive green roof150350250153525Extensive green roof314085197:40Gravel covered roof505688677Non-greened roof6291water, theyobserved was a 0.4 inch from a 20 min thunder-storm.mitigate the effects of impervious surface runoff.The storm event occurred after an extended period ofPeck[23]) etimated that if 6% of all buildings inrainfall had fully saturated the system. Although 44Toronto had green roofs, it would result in the sameinches of rainfall was recorded during this period, onlystorm water retention impact as a 560 million (CDN)15.5 inches of runoff was generated from the trays.storage tunnel. Likewise, in Washington, DC, if 20%Runoff was negligible for storm events with less thanof all buildings that could support a green roof had one,0.6 inch of rainfall.they would add over 7.1x10* m' (19 million gallons) toLiu (29 reported that extensive green roof reducedthe city's storm water storage capacity and storestorm water runoff by 54% (April to September, 2002).approximately 9.58x10 m' (253 million gallons) ofGutteridge 0 estimated that 3.6x10 m' (127 millionrainwater in an average year [241. In the United States,cubic feet) of storm water can be retained if 6% of thethe combined sewer overflows (CSOs) discharge abouttotal roof area in Toronto is covered with green roofs3.2x10' m' (850 billion gallons) of untrcated sewage(6.5x10* m2 or 70 million square feel). Many Europeanand storm water in 32 states and the District ofcities, as well as several cities in the United States, nowColumbia every year 251. New York Harbour alonecharge developers and building owners fees for hookreceives more than 1x109 m' (27 billion gallons) ofup to the storm water system, based on the amount ofsewage and polluted runoff from an average of 460discharge produced by the site. In 1996, the State ofCSOs every year [26]. In Washington, Seattle city,llinois passed a law that promotes planting buffer3.1x10° m3 (815 million gallons) from 87 CSOs waszones at grade, to reduce storm water runoff, in retumdischarged into local water bodies from June 2007 tofor a reduction in property taxes. There is also aMay 2008. Sanitary systems can also becomegradual move in North America toward storm wateroverloaded during heavy! storms and discharge sewage;user fees, which are based on the degree othe EPA (Environmental Protection Agency) estimatesimpermeable surfaces at a given site.that this happens about 40 000 times every year. TheThe city of Cologne, Germany, receives 27% moreproven benefits of green roofs encourage therainfall than surrounding areas. In cities alreadygoveruments of some highly urbanized societies likeplagued by overextended storm water systems andJapan, Singapore, Germany and Belgium to use greencombined sewage overflows, the problems caused byroofs massively (027-28. Fig. 2a shows the reduction insevere rainfall are likely to worsen with global climatepeak runoff from a green roof, as observed in Belgiumchange. If sufficiently implemented in an urban area,during a rainstom.green roof systems can help improve storm waterA 3000 square-foot green roof project at themanagement. German studies from 1987 to 2003 asFencing Academy of Philadelphia predicted a 54 %summarized by Mentens et al. [171 that intensive greenreduction in annual runoff volume (Fig. 2b).roofs showed annual runoff reduction equal to 85% toRoofscapes, Inc. conducted a pilot-sized project fon65% of annual precipitation (100%) and for extensivereal and synthetic storm events for a period of 9 monthsroofs the corresponding values were 81% to 27%。at 14 and 28 square-foot trays. The most severe storm中国煤化工J. Chongqing Univ. Eng. Ed. (ISSN 1671-82.MYHCNMHGM. Anwar, et al.Green roofsab0.20 r15.0■RalnfallRunoff from the traditional roof0.18o Rainfall昌Runoff from the green roof0.16口Runoff; 0.14 t10查0.12 t冒园0.10 t-2.5号0.080.060.040.020.00--m0.17:00 20:00 23:00 02:00 05:00 08:00 11:00 14:00 17:00510152015303540455035606570Time/(h:min)Time/minFig. 2 a) Typical cumulative runoff from a n-greened roof and an extensive green roof at 20° slope 明and b) Runoffattenuation fficiency for a 0.4 inch rainfall event with saturated mediaSome studies find no correlation between roof slopemuch as 69% as compared to conventional rooftops.and runoff [13,15.17] whereas the others observe thatFrom the extensive literature review, it is clear that19-1018)runoff retention may depend on slopesThestorm water retention capability may range from 75%effect of the slope on runoff retention combines withfor intensive green roofs to 54% for extensive greenthe effects of other factors as the physical properties ofroofs (Table 1). Although intensive green roofs retainthe roof substrates, length and intensity of precipitationthe highest amount of water due to its soil thicknessevent studied and flow conditions (saturated orand physical properties, it cannot be easily rtfitedunsaturated, overland flow or not), the design of greeninto existing roofs. For that reason it is not a suitableroof layers and different type drainage materials oroption. Intensive green roofs are suitable for newsystems'31.structures which can carry relatively higher load thanextensive green roof. For an existing building extensive5 Discussiongreen roof can be an ideal solution for storm waterrunoff management.In this review paper the role of green roofs inmitigating urban storm water runoff problem isdiscussed. As seen in the literature, potentialRunof volumeenvironmental benefts are common but scientificevidence of some benefits is not sufficient. Somecontradictory results are presented by different authorsin their studies. This is due to differences in the studyReductionReduetionReautionconditions, weather, type of green roofs and plants. Ona per roof basis, Fig. 3 shows the storm water mass .balance model predicting that an extensive green roofcan reduce roof runoff volumes by approximately 65%,while an intensive roof can reduce runoff by 85%.Coventional roof ExtensiveInensive gren rof 80% Excnsive,Using a combination of 80% extensive and 20%intensive ratio across all green roof-ready buildings inFig.3 Comparison of roof runoff for conventional roofs andthe District, roof runoff volume would decrease by asgreen roofs (32]中国煤化工J Chongqing Univ Eng. Ed. [ISSN 1671-MHCNMHG9M. Anwar, et al.Green rofs6 Concluding remarks[1] Liu K, Minor J. Performance evaluation of anextensive green roof [C]. In: Proceedings of 3rd NorthThe review indicates that there is a need for furtherAmerican Green Roof Conference: Greening Rooftopsresearch on green roof performance in urbanfor Sustainable Communitis, W ashington, DC, May 4environment. It is obvious that the green roof alone6, 2005. Toronto: The Cardinal Group, 2005: 385-398.will never fully solve the urban runoff problem and it[12] Meng Q, Hu W. Roof cooling efct with humid porousneeds to be combined with other runoff reductionmedium [J]. Energy Buildings, 2005, 37: 1-9.measures such as storage reservoirs, updated storm13] Liesecke HJ. Das reientionsverm' ogen von dachbegru"water infrastructure, rainwater cisterns and an increasenungen (water retention capacity of vegetated roofs)of green areas. Vertical gardens are a useful tool as[]. Stadt und Grun, 1998, 47 (1): 46-53.they interupt rainfall and delay runoff to some extent,[14] Moran A, Hunt B, Jennings G. 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