The rapid urbanisation and industrialisation involve an unsustainable usage of natural systems and creates assorted jobs in metropoliss. The urban hydrological system has to get by with a extremely fluctuating sum of surface overflow H2O which may go highly high during periods of heavy rainfall. It has been suggested that green roofs significantly extenuate storm H2O overflow coevals even in tropical clime. Green roofs have become popular due to its proved benefits by extenuating urban heat island effects and protecting biodiversity. The one-year rainfall and overflow relationship for green roofs is determined by the deepness of the substrate. Dry or wet substrates affect the H2O keeping capacity of the green roof. For dry status, by and large 6 to 12 millimeter of rain is required to originate overflow while for wet conditions the response is about consecutive. Besides, there are some other factors impacting overflow kineticss such as type of green roof and its incline ; age of green roof ; type of flora ; dirt wet features and conditions. This reappraisal indicates that there is a demand for more research into a green roofs public presentation over storm H2O overflow direction. This paper reviews and addresses the function of green roofs in urban drainage H2O direction.
Nowadays the planetary population is concentrating more in urban countries. Urban countries are invariably spread outing in footings of infinite and denseness. Grassland and woods are displaced by the imperviable surfaces of streets, private roads and edifices due to continual demand for urbanisation. The degree of urbanisation is lifting and expected to make 83 % by 2030 in developed states [ 1, 2 ] . Urbanization is greatly intensifying storm H2O overflow, decreasing groundwater recharge and heightening watercourse channel and river eroding. This involves an unsustainable usage of natural systems and creates legion jobs both within and outside metropoliss. The urban hydrological system faces extremely fluctuating sum of surface overflow H2O all through twelvemonth ; which creates a negative impact for some metropolis substructure and environing environment. In most metropoliss, conventional storm H2O substructure failed to convey storm H2O rapidly from the metropolis to receivers.
Since the 1990s, urban storm H2O substructure has been exposed to major rethinking when the thought of sustainable development gained more and more land. A metropolis can be benefited by proper use of storm H2O. Storage reservoirs, pools every bit good as green countries are effectual tools for cut downing high overflow during rainfall. A alteration of bing hydrological system is required to get by the state of affairs so that it can play a more active and positive function. Most of the large metropoliss have the high sum of imperviable surfaces [ 3, 4 ] and the high land monetary values make the creative activity of green countries in urban parts really expensive and sometimes impossible. Solving the high storm H2O overflow, a dearly-won and riotous manner is to enlargement or enlargement of storm H2O substructure. Several metropoliss, are commissioning advisers to analyze environmentally friendly, cost-efficient options to work out that jobs. Low impact development ( LID ) and Water sensitive urban design ( WSUD ) attacks are used in USA and Australia severally for minimising imperviable screen and maximising infiltration of rainfall. Although in most locations those attacks are non widely implemented and the huge bulk of new storm H2O direction in US and Australia remains in the signifier of conventional storm cloacas with limited intervention by keeping basins [ 5 ] .
In older urban countries, there is a deficiency of suited land for making sustainable storm H2O substructure, green roofs and perpendicular green walls can be a variable option at that place. Green roof or vegetated roof can be implemented in the immense sum of fresh roof country ( about 40-50 % of the impermeable surfaces in urban countries [ 6 ] . Vegetated roofs can play an of import function in modern urban drainage because of their ability to decelerate down and cut down overflow response. Green roof provides legion ecological and economic benefits, including energy preservation, ramp H2O direction, extenuation of noise and air pollution, extenuation of the urban heat island consequence and increased length of service of roofing membranes every bit good as improved biodiversity [ 7-13 ] . Out of all proved benefits, the decrease of storm H2O overflow is the greatest environmental service that green roofs provide.
In a green roof system, much of the precipitation is captured in the media or flora and finally evaporates from the dirt surface or is released back into the ambiance by transpiration. This is a uninterrupted procedure. Figure 1 shows the simple mass balance used to cipher roof overflow.
Fig. 1. Simple mass balance used to cipher roof overflow.
Depending on the type of green roof system ( design, substrate deepness and works species ) , research has shown decreases of 60-100 % in overflow [ 10, 13, 14 ] . Bengtsson [ 15 ] has reported that high evapotranspiration from a vegetated roof can cut down the one-year overflow to less than the half precipitation. Local urban implosion therapy and combined cloacas floods ( CSOs ) can be lessened as H2O is stored ab initio in the dirt and flora which reduces extremum flow and extends the clip of concentration. Urban runoff pollution can be reduced by green roof as it absorbs the pollutants of moisture and dry atmospheric deposition. This pollution overflow is dependent on some factors like the type of environing country ( industrial, residential or commercial ) and local pollution beginnings ( strength of traffic, type of heating system ) . When it rains, H2O comes out of conventional roofs and paved countries mixes with deposited pollutants and carries to rivers and local H2O beginnings. Contaminants in storm H2O overflow can include fertilisers, weedkillers and insect powders ; oil and lubricating oil from roads and energy production installations. The ensuing taint non merely harms aquatic life but besides make H2O insecure for human. It besides reduces the diverseness of insect and fish populations. Sometimes local H2O intervention systems can be overloaded due to ramp H2O overflow and in many instances it treats healthful sewerage from showers and lavatories and storm H2O in the same installations. During the heavy rainstorm, H2O may transcend the system ‘s capacity and discharges assorted sewerage and storm H2O straight into local lakes and rivers. Green roofs non merely minimise the storm H2O overflow but besides act as pollution adsorbents and filters. This survey aims to look into the influence of a green or vegetated roof on Storm H2O overflow direction. Green roof engineering, factors impacting storm H2O keeping and relationship between green roof and overflow are besides discussed.
Green Roof Technology
Green roof is a superimposed system comprising of a sealing membrane, turning medium and the flora bed itself. Green roofs typically have four beds of building. A waterproofing membrane sits instantly on top of the structural roof deck to forestall wet from come ining the edifice. Typically, above this membrane is a root barrier bed that is designed to forestall roots from perforating the waterproofing membrane and the structural roof. A drainage bed is following. The drainage bed ( realized with either some engineered coarse grained porous media or plastic profiled elements ) is typically designed to transport extra overflow to roof drains, and to hive away H2O for the workss in dry periods. Next, a filter cloth is installed to forestall dirt from rinsing off and compromising the drainage bed as H2O drains from the roof. Finally, the turning workss and associated substrate or turning medium ( a blend of mineral stuff enriched with organic stuff ) complete the green roof. The substrate is frequently a lightweight man-made dirt that is porous and inherently inert, with foods added for works growing. Figure 2 shows a typical green roof system.
Fig. 2. Typical green roof system.
Based on the deepness of the planting medium and care two chief types of green roof are normally distinguished ; they are intensive and extended green roofs.
– Intensifier green roofs are established with deep dirt beds. They can back up workss and shrubs and typically require care in the signifier of weeding, fertilising and irrigating. By and large intensive green roofs can back up complex flora like ground covers, little trees and bush which has deeper rooting. Intensive roofs involve a greater burden of more than 150 kg/m2 and have more than 200 millimeter of substrate with higher sum of organic stuff than extended systems [ 16 ] . Intensive green roofs are typically installed on roofs with a incline of less than 10- .
– Extensive green roofs have a thin substrate bed with low degree planting, typically sedum or lawn. They are planted with smaller workss which in the concluding phase are expected to supply full coverage of the vegetated roof. Extensive roofs are intended to be self sustaining and necessitate minimum care. It can be distinguished by being low cost, lightweight ( 50-150 kg/m2 ) and with thin stuff substrate of up to 150 millimeter. This type may besides be installed on aslant surfaces. The incline angle can be every bit high as 45- .
Factors impacting storm H2O keeping
Green roof features
Green roof features such as figure of beds and type of stuffs, dirt thickness, dirt type, flora screen, type of flora, roof geometry: incline, length of incline, roof place and roof age affects the public presentation of storm H2O keeping. Table 1 presented that the one-year overflow is chiefly determined by the roof type and may be every bit high as 91 % for a traditional non-greened roof and every bit low as 15 % for an intensive green roof ; and 19 % for an extended green roof. The different surveies on incline influence on green roofs overflow keeping capacity bring different consequences. While some surveies find no correlativity between roof incline and overflow [ 13, 15 ] , the others observe that overflow keeping may depend on inclines. Mentens [ 17 ] stated that the one-year precipitation, roof type, deepness of substrate beds is significantly correlated with the annual overflow while they did non happen any important relation with the age of the green roof, incline angle and length.
Table 1. Substrate bed deepness ( millimeter ) and overflow ( % of entire one-year precipitation ) features on an one-year degree [ 17 ] .
Substrate Layer ( millimeter )
Runoff ( % )
Intensive Green Roof
Extensive Green Roof
Gravel covered Roof
Getter [ 18 ] has reported that keeping values decreased as incline increased and was important for inclines between 2 % and 15 % every bit good as between 2 % and 25 % . They found that organic affair content and pore infinite of dirt doubled in 5 old ages clip ( from 2 % to 4 % and from 41 % to 82 % , severally ) .
Weather conditions such as length of continuing dry period, season/climate ( air temperature, air current conditions, humidness ) , features of rain event ( strength and continuance ) besides affect the storm H2O keeping. Kaufmann [ 19 ] compared the overflow per centum during winter and summer for both 5 centimeter crushed rock roof and 1cm extended green roof and found the overflow was significantly higher during winter clip, 86 % for crushed rock roof and 80 % for green roof ; where as overflow 70 % in the summer for crushed rock roof and 52 % in the summer for extended green roof. Carter and Rasmussen [ 20 ] found that peak discharge for little storm was much lower from the vegetated roof than a conventional roof but this consequence was much reduced for larger storms ; 57 % of extremums on a vegetated roof were delayed up to 10 min as compared with extremums from a conventional roof. Green roofs by and large delayed overflow ( peak to top out ) by 10 min [ 8, 10 ] . DeNardo [ 21 ] showed that green roofs reduced the peak strengths from an mean rainfall strength of 4.3 mm/h to an mean green roof overflow rate of 2.4 mm/h. Moran [ 22 ] reported that 90 % of rain events holds of overflow were observed and minimal 30 min hold for 60 % of rain events was observed. Bengtsson [ 15 ] showed that conditions conditions ( dry or moisture ) affected the keeping capacity of studied green roof ; for dry conditions 6-12 millimeter rain were required to originate overflow ; for moisture conditions the response was about consecutive.
Green Roof- Storm H2O overflow relationship
Green roof reduces storm H2O overflow compared with that from a difficult roof through take downing and detaining the extremum overflow. A certain H2O volume is detained in a green roof and its substrate beds. A part of detained H2O will run out and a part matching to field capacity will be retained. The maintained H2O will vaporize or be used by workss and parts of it will transpirate. Through vaporization and transpiration of H2O overflow volume decrease from green roofs occurred.
Since green roofs retain storm H2O, they can extenuate the effects of imperviable surface overflow. Peck [ 23 ] estimated that if 6 % of all edifices in Toronto had green roofs, it would ensue in the same storm H2O keeping impact as constructing a $ 60 million ( CDN ) storage tunnel. Likewise, in Washington, DC, if 20 % of all edifices that could back up a green roof had one, they would add over 71 million liters ( 19 million gallons ) to the metropolis ‘s storm H2O storage capacity and shop about 958 million liters ( 253 million gallons ) of rainwater in an mean twelvemonth [ 24 ] . In the United States, the combined cloaca floods ( CSOs ) discharge about 850 billion gallons ( 3.2 trillion Liters ) of untreated sewerage and storm H2O in 30 two provinces and the District of Columbia every twelvemonth [ 25 ] . New York Harbour entirely receives more than 27 billion gallons ( 1 trillion Liters ) of sewerage and polluted overflow from an norm of 460 CSOs every twelvemonth [ 26 ] . In Washington, Seattle metropolis, 815 million gallons ( 3.1 billion L ) from 80 seven CSOs were discharged into local H2O organic structures from June 2007 to May 2008. Sanitary systems can besides go overladen during heavy storms and discharge sewerage ; the EPA estimates that this happens approximately 40,000 times every twelvemonth.
In some extremely urbanised societies like Japan, Singapore, Germany and Belgium the advantages of green roofs have already resulted in inducements from the authorities to promote or even enforce the usage of green roofs [ 6, 27, 28 ] . Figure 3 illustrates the decrease in extremum overflow from a green roof, as observed in Belgium during a rainstorm.
Fig. 3. Typical cumulative overflow from a non-greened roof ( 20 ISslope ) and an extended green roof ( 20 ISslope ) as observed in Leuven ( Belgium ) during the 24h period of a 14.6 millimeter rain shower ( April 2003, 5 pm-5 autopsy on the following twenty-four hours ) [ 17 ] .
The green roof undertaking at the Fencing Academy of Philadelphia is a 3,000 square-foot vegetated screen installed and monitored by Roofscapes, Inc. , on top of an existing construction. The theoretical account ( Figure 4 ) predicted a 54 % decrease in one-year overflow volume.
Fig.4. Runoff fading efficiency for a 0.4-inch rainfall event with concentrated media.
The theoretical account besides predicted fading of 54 % of the 24 hr, 2 twelvemonth Type II storm event and 38 % of the 24 hr, 10 twelvemonth Type II storm event. Additionally, supervising at a pilot-sized undertaking for existent and man-made storm events was conducted by Roofscapes, Inc. for a period of 9 months at 14 and 28 square-foot trays. The most intense storm monitored was a 0.4 inch, 20 minute electrical storm. The storm event occurred after an drawn-out period of rainfall had to the full saturated the system. Although 44 inches of rainfall was recorded during this period, merely 15.5 inches of overflow was generated from the trays. Runoff was negligible for storm events with less than 0.6 inch of rainfall.
Liu [ 29 ] reported that extended green roof reduced storm H2O overflow by 54 % ( April to September ‘ 2002 ) . Gutteridge [ 30 ] estimated that 3.6 million three-dimensional metres ( 127 million three-dimensional pess ) of storm H2O can be retained if 6 per centum of the entire roof country in Toronto is covered with green roofs ( 6.5 million square metres or 70 million square pess ) . Many European metropoliss, every bit good as several metropoliss in the United States, now charge developers and edifice proprietors fees for hook up to the storm H2O system, based on the sum of discharge produced by the site. In 1996, the State of Illinois passed a jurisprudence that promotes the planting of buffer zones at class, to cut down storm H2O overflow, in return for a decrease in belongings revenue enhancements. There is besides a gradual move in North America toward storm H2O user fees, which are based on the grade of impermeable surfaces on a given site.
The metropolis of Cologne, Germany, receives 27 % more rainfall than environing countries. In metropoliss already plagued by overextended storm H2O systems and combined sewerage floods, the jobs caused by terrible rainfall are likely to decline with planetary clime alteration. If sufficiently implemented in an urban country, green roof systems can assist to better storm H2O direction. German surveies from 1987 to 2003 as summarized by Mentens [ 17 ] reported that intensive green roofs showed one-year overflow decrease equal to 85 % -65 % of one-year precipitation ( 100 % ) and for extended roofs the corresponding values were 81 % -27 % .
Some surveies find no correlativity between roof incline and overflow [ 13, 15, 17 ] whereas the others observe that overflow keeping may depend on inclines [ 9, 10, 18 ] . The consequence of the incline on overflow keeping combines with the consequence of other factors as the physical belongingss of the roof substrates, length and strength of precipitation event studied and flow conditions ( saturated or unsaturated, overland flow or non ) , the design of green roof beds and different type drainage stuffs or systems [ 31 ] .
This reappraisal paper addresses the function of green roofs in extenuating urban storm H2O overflow job. In the literature, possible environmental benefits are common but scientific grounds of some benefits is non sufficient. Some contradictory consequences are presented by different writers in their survey. This is due to differences in the survey conditions, conditions, type of green roofs and workss. On a per roof footing, Fig. 3 shows the storm H2O mass balance theoretical account predicted that an extended green roof can cut down roof overflow volumes by about 65 % , while an intensive roof can cut down overflow by 85 % . Using a combination of 80 % extended and 20 % intensive ratio across all green roof-ready edifices in the District, roof overflow volume would diminish by every bit much as 69 % as compared to conventional rooftops ( Figure 5 ) .
Fig. 5. Comparison of roof overflow for conventional roofs and green roofs [ 32 ] .
From the extended literature reappraisal, it is clear that storm H2O keeping capableness may run from 75 % for intensive green roofs to 54 % for extended green roof ( Table 1 ) . Although intensive green roofs retain highest H2O due to its dirt thickness and physical belongingss, it can non be easy retrofitted into the bing roofs. For that ground it is non a suited option. Intensive green roofs are suited for new construction which can transport comparatively higher burden than extended green roof. For bing edifice extended green roof can be an ideal solution for storm H2O overflow direction.
The reappraisal indicates that there is still necessitate of more research on green roof public presentation in urban environment. It is obvious that green roof entirely will ne’er to the full work out the urban overflow job and it needs to be combined with other overflow decrease steps such as storage reservoirs, updated storm H2O substructure, rainwater cisterns and an addition of green countries. Vertical gardens are a utile tool as it interrupt rainfall and detain overflow to some extent, particularly during heavy rainstorms with strong air currents. Most storm H2O direction represents a cost-based activity and if it widely implemented green roofs and walls will supply new chances to turn to urban storm H2O overflow job.