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Storm water retention and actualevapotranspiration performances ofexperimental green roofs in Frenchoceanic climateD. Yilmazac, M. Sabrea, L. Lassabatèred, M. Dalc & F. Rodriguezb
a CAPE, Centre Scientifique et Technique du Bâtiment, Nantes,Franceb LUNAM Université, IFSTTAR, GERS, EE, IRSTV, Bouguenais, Francec Engineering Faculty, Civil Engineering Department, University ofTunceli, Tunceli, Turkeyd UMR5023 Laboratoire d’Ecologie des Hydrosystèmes Naturelset Anthropisés, Université Lyon 1, ENTPE, CNRS, Vaulx-en-Velin,FrancePublished online: 06 May 2015.
To cite this article: D. Yilmaz, M. Sabre, L. Lassabatère, M. Dal & F. Rodriguez (2015): Stormwater retention and actual evapotranspiration performances of experimental green roofsin French oceanic climate, European Journal of Environmental and Civil Engineering, DOI:10.1080/19648189.2015.1036128
To link to this article: http://dx.doi.org/10.1080/19648189.2015.1036128
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Storm water retention and actual evapotranspiration performancesof experimental green roofs in French oceanic climate
D. Yilmaza,c*, M. Sabrea, L. Lassabatèred, M. Dalc and F. Rodriguezb
aCAPE, Centre Scientifique et Technique du Bâtiment, Nantes, France; bLUNAM Université,IFSTTAR, GERS, EE, IRSTV, Bouguenais, France; cEngineering Faculty, Civil EngineeringDepartment, University of Tunceli, Tunceli, Turkey; dUMR5023 Laboratoire d’Ecologie desHydrosystèmes Naturels et Anthropisés, Université Lyon 1, ENTPE, CNRS, Vaulx-en-Velin, France
(Received 14 November 2014; accepted 26 March 2015)
Green roofs are promising urban management tools from the standpoint of bothrainwater management and microclimatology. They are considered as a storm watermitigation technique and may also favour evapotranspiration fluxes, which can bebeneficial for urban comfort during summer periods. In France, however, water reten-tion performance of green roofs remains unknown, and published values are oftenunsuitable. Six experimental roofs, including two thicknesses of growing media,three types of vegetation cover and bare surfaces, were monitored for two years inNantes and compared to an experimental gravel flat roof. The thickest mediacombined with the most densely vegetated cover yields the best results in terms ofstorm water mitigation and actual evapotranspiration. In winter, the rainwater reten-tion performance is clearly dependent on the type of experimental roof vegetation.This kind of experimental set-up is well suited to assisting urban planners designtools for storm water mitigation in buildings.
Keywords: green roofs; storm water retention; runoff; evapotranspiration
1. Introduction
Cities are continuously expanding in many parts of the world, in both their land areaconsumed and population density. This urban evolution has caused water managementproblems, namely the sealed surface of urban areas has led to storm runoff increasescapable of generating considerable property damage and environmental pollution. Sincemodern urban infrastructure was designed several decades ago, storm water managementpractices must adapt to these urban expansion trends. For new buildings today, Frenchregional institutions now prescribe limited runoff outflows. In some locations, connect-ing new buildings to the storm water network may even be forbidden. Green roofsrepresent an emerging strategy for mitigating storm water runoff (Moran, 2004;Monterusso, 2004; VanWoert et al., 2005). Retrofitting older structures with a green roofcould offer an opportunity to mitigate storm water effects (Castleton, Stovin, Beck, &Davison, 2010) and allow institutions to take less drastic measures.
The concentration and expansion of urbanised areas have given rise to another prob-lem with the urban heat island (UHI) phenomenon, which is occurring mainly duringsummer periods increases air temperature on urban areas and generates additional
*Corresponding author. Email: [email protected]
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energy consumption due to air conditioning, thus contributing to global warming. Greenroofs in urban areas might also provide an opportunity to reduce UHI effects by increas-ing the evapotranspiration of water, process that consumes energy and thus cool theambient air (Bass, Stull, Krayenjoff, & Martilli, 2002; Dimoudi & Nikolopoulou, 2003;Mentens, Raes, & Hermy, 2006; Rosenfeld, Akbari, Romm, & Pomerantz, 1998; VonStülpnagel, Horbert, & Sukopp, 1990; Wong, Tay, Wong, Ong, & Sia, 2003).
The hydrological and energy behaviour of green roofs depends on many parameters,including climate, vegetation species and structural components (Czemiel Berndtsson,2010). Studies have pointed to differences in retention capacity (capacity of retainingrain water in the roof media) due to geographical location. For a one-year monitoringperiod on similar experimental roofs, Berthier, De Gouvello, Archambault, and Gallis(2010) and Palla, Gnecco, and Lanza (2010) found retention capacities of respectively65% for Paris (France) and 51% for Genoa (Italy). Despite a thicker growing media,Palla et al. (2010) found a lower retention capacity than Berthier et al. (2010). Valuesfrom the literature tend to show very different retention values depending on the loca-tion, roof components and season when studied. Mentens et al. (2006) summarised theGerman studies and found that the annual retention capacity of extensive green roofsvaried from 27 to 81%. Scholz-Barth (2001) stated that the mean storm water retentionfor the United States was approx 65% for green roofs; this estimation was based on casestudies from various cities, e.g. Chicago, Philadelphia and Portland.
It is also a very difficult exercise to compare retention capacities from one study tothe next since green roof components differ and affect water retention capabilities differ-ently. For example, Stovin, Vesuviano, and Kasmin (2012) studied a single-layer greenroof, while Getter, Rowe, and Andresen (2007) focused on a green roof containing aretention fabric used as a water reservoir.
It is also known that during summer periods, evapotranspiration and green roofwater retention capacity both increase (Mentens et al., 2006; Villarreal & Bengtsson,2005). Stovin et al. (2012) found lower retention capacity values during the spring thanin the summer. Thus, seasonal variation is a parameter that affects the green roofperformances.
Many authors have pointed out that vegetation contributes to a reduction in outflowvolumes at the annual scale due to evapotranspiration (Bengtsson, 2005; Gregoire &Clausen, 2011; Köehler, 2005; Palla et al., 2010; Stovin, 2010). Only a few studies havequantified the actual evapotranspiration (AET) by means of experimental measurement(Gregoire & Clausen, 2011; MacIvor & Lundholm, 2011). Other several studies havecalculated AET from numerical modelling (Hilten, Lawrence, & Tollner, 2008;Metselaar, 2012). Temperature reduction at the green roof surface, in comparison withconventional roofs, has been demonstrated in many works (DeNardo, Jarrett, Manbeck,Beattie, & Berghage, 2005; Jaffal, Ouldboukhitine, & Belarbi, 2012; Susca, Gaffin, &Dell’Osso, 2011; Teemusk & Mander, 2009; Wong et al., 2003). The percentage ofwater removed through evapotranspiration by green roofs and the reduction in surfacetemperature are two indicators of the potential for green roofs to reduce UHI effects.
On one hand, green roofs mitigate storm water effects by retaining and decreasingpeak flow, while on the other, green roofs may induce greater evapotranspiration thanmore common impermeable roofs, thus helping refresh the urban environment in sum-mer. The main obstacles to retrofitting older traditional construction with green roofs arecost-related. Only a few French regions have voted to provide financial assistance forgreen roof retrofitting. To help institutions decide in favour of green roof development,a number of guidelines are needed. A French database on green roof performance must
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be compiled for potential subsequent research to test green roof retrofitting scenarios inFrance. Green roof impacts on both storm water and UHI mitigation will be studied ingreater depth through the use of numerical modelling tools like TEB for climate andURBS for hydrology (Benzerzour, Masson, Groleau, & Lemonsu, 2011; Lemonsu,Pigeon, Masson, & Moppert, 2006; Rodriguez, Andrieu, & Morena, 2008).
In this context, the aim of this study is to characterise the performance of severalexperimental green roof configurations in a French oceanic climate by estimating theirrespective retention capacity and cumulative AET. Two growing media thicknesses,three vegetation species and the contribution of roof vegetation will all be studied.Storm water mitigation and the potential reduction of UHI effects thanks to theseexperimental green roofs will then be discussed in comparison with conventional flatgravel roofs and across the various experimental configurations. Emphasis will also beplaced on analysing the influence of vegetation species.
2. Material and methods
2.1. Site description and green roof design
2.1.1. Roof design
This study was conducted on the CSTB (Centre Technique et Scientific du Batiment –Scientific and Technical Center for Building) Nantes site in western France. The climate atthis site is oceanic; rainfall is frequent but not very intense, with an annual average precip-itation of 820 mm, average potential evapotranspiration of 867 mm and average daily tem-perature of 12.5 °C for the last 30 years. A total of seven experimental roof platforms withoverall dimensions of 1500 mm × 1500 mm were constructed. All platforms were built onwooden supports with null slope, and a protective coat of paint was applied. Thisconstruction process respected relevant French building codes. The wood support wascovered with a commercial-grade bituminous vapour barrier (5 mm thick). The insulationlayer was installed using 60-mm high polystyrene blocks. 5-mm high PVC membraneswere introduced for waterproofing. The discharge outlet was designed with a 30-mmdiameter PVC tube placed at one corner of the roof platform (120 mm from the border) torespect the French rules of roof construction (French code DTU 43.1).
2.1.2. Choice of plant species
The plants commonly used to compose green roofs are varieties of succulents. Theirmetabolism provides the benefits of greater resistance during dry periods. These plantsoffer the most appropriate species for green roofs (Emilsson, 2008). CRITT Horticole, aFrench association contributing to the research and development on green roofs inFrance, has tested several plant species in a roof environment. Upon the advice of thisassociation, three species have been selected herein. The first two types are Sedumalbum and Festuca glauca, both of which are commonly used in green roof design. Thethird species is Dianthus deltoides, which has yet to be used on a green roof; this plantis a perennial and aesthetically appealing with a long flowering period extending fromJune to September.
2.1.3. Growing media properties
Extensive commercial-grade green roof media were used (Star, Forges, France). Thegrowing media was composed of 70% mineral materials and 30% organic matter.
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The mineral parts were a mix of 3/20-mm pumice stones with 3/7 and 7/15-mmPozzolana. The organic matter was produced with composted bark from a maritimepine, 3/20-mm black peat and 3/20-mm blond peat. The particle size distribution ofthese media is shown in Table 1. The growing media contain 3 kg/m3 of long-term min-eral fertilizers composed of 5% nitrogen, 3.5% anhydrous phosphoric acid and 8%potassium oxide. The fertilizers were contained in small balls set-up for a slow release;they provided necessary nutriments to the plants for nearly a 6-month period.
The specific density ρs of the growing media was measured using the pycnometermethod at equal to 1.88 ± .04 g cm−3. The dry bulk density ρb was measured at .60± .06 g cm−3. The porosity n could then be estimated from both the dry bulk ρb andspecific ρs densities via the following equation:
n ¼ 1� qbqs
(1)
The saturated volumetric water content θs is assumed to equal the porosity n of themedia: 68.1%. The field capacity is the maximum water amount that can retain a porousmedia. It is evaluated in the laboratory at 49.4%, according to FLL guidelines(For- schungsgesellschaft Landschaftsentwicklung Land-schaftsbau e.V, 2008).
2.2. Experimental configuration and data acquisition
The set of seven experimental green roof types were all built in April 2011 (Table 2).Experimental roof no. 0 is a conventional flat white gravel roof. Numbers 1 and 2 arecomposed of growing media with no vegetation cover. Specimen Nos. 3 and 4 aregreen roofs planted with S. album. Lastly, roofs 5 and 6 have been greened withF. glauca and D. deltoides, respectively. There is no irrigation system in theexperimental roofs.
Gravel was directly laid on the PVC impermeable layer on roof no. 0. For all otherspecimens, 40-mm thick extruded polystyrene (Siplast, France) was used as a drainagesystem on the impermeable layer. A 200 g/m² geotextile was introduced in order toavoid plant roots and small particles of growing media from penetrating into theunderlying layers. According to the configurations listed in Table 2, the growing mediawas installed over the textile. Experimental roofs 3 through 6 initially remained10 weeks in a greenhouse to both plant the vegetation and accelerate its sprouting.
Table 1. Growing media particle size distribution.
Diameter (mm) % of particles
>31.5 0>16 4.1>8 50.9>4 32.3>2 5.3>1 3>.2 .5>.063 .6>.02 1.7>.002 .9<.002 .7
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The experimental roofs were placed on a small road sloped at 1.5° (see Figure 1). Ameteorological (MET) station records the rainfall, air temperature, relative humidity ofair, atmospheric pressure, global radiation and wind speed, plus the direction at a 1-minfrequency. All meteorological data are stored on a CR10X Campbell Scientific Datalog-ger every 10 min. Rainfall infiltrates into each roof and then drains into the outletthrough the drainage layer. The water outflow on each roof is discharged into a tippingbucket mechanism (switch activated for every 20 ml of water). In this study, we willrefer to runoff as discharge. Two temperature probes (thermocouple type T) were placedin the middle and on the surface of the growing media. Four water content probes(CS616, Campbell Scientific), based on a domain frequency response, were used to esti-mate volumetric water content inside the growing media. The probes were placed300 mm from each corner of the experimental roof and 50 mm from the border(Figure 2). All green roof data were recorded every 10 min on a data logger (CR3000,Campbell Scientific).
The volumetric water probes were calibrated according to the method proposed byRüdiger et al. (2010). For each roof studied, the average volumetric water content wasestimated from the average value of four probes. Note that the volumetric water contentwas not monitored for gravel roof no. 0.
Table 2. Experimental roof configuration.
Roof number Drainage Media type Thickness (mm) Vegetation
0 No Gravel 5/15 mm 80 No1 Yes Growing media 80 No2 Yes Growing media 120 No3 Yes Growing media 80 Sedum album4 Yes Growing media 120 Sedum album5 Yes Growing media 120 Festuca glauca6 Yes Growing media 120 Dianthus deltoides
Figure 1. Location of Experimental roofs at CSTB Nantes site.
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2.3. Hydrological performance evaluation
Experimental green roof performance in terms of water retention was evaluated usingtwo distinct methods. The first was based on calculating an average mean retentionvalue for each roof tested over a monitoring period: summer, winter or overall. Theoutput values were then compared to one another to further investigate roof configura-tion behaviour. The second method consisted of calculating the mean retention value foreach experimental roof and for each rain event, subsequent to which an Anova statisticalanalysis was conducted to investigate the most influential parameters on roof behaviour.
2.3.1. Method no. 1: seasonal and overall performance measures
The hydrological performance of experimental roofs will first be discussed relative tothe given monitoring period (Figure 3):
• Summer 2011• Winter 2012• Summer 2012• Winter 2013• Overall (full data set).
For this purpose, the water retention performance of each roof is calculated using themean retention per monitoring period, defined as the percentage of cumulative stormwater retained in the experimental roof over the monitoring period.
Mean retention ð%Þ ¼ 1� Cumulated runoff volume
Cumulated rain volume
� �� 100 (2)
where Cumulative rain volume is the total volume of rainfall recorded during themonitoring period, and Cumulative runoff volume is the total volume of water flowingthrough the experimental roof outlet during the monitoring period.
Figure 2. CS616 water content probes location.
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2.3.2. Method no. 2: performance evaluation using the Anova test
The hydrological performance of experimental roofs is analysed using the Anovastatistical test with respect to the set of variables a priori affecting green roof behaviour,i.e. media type, growing media thickness, type of cover, category of rainfall and season.
The mean retention per episode is calculated according to the following procedure:
• Rain events are selected if the rain intensity exceeds .2 mm/h and the cumulativerain depth is above a threshold value of .5 mm. The end of a Rain event isconsidered to occur when no rain has fallen for a full hour.
• Runoff events are considered to take place whenever runoff flow is occurring. Theend of a runoff event occurs once the cumulative runoff depth drops below athreshold value of .5 mm over two consecutive hours, except for experimentalgravel roof no. 0, for which this duration lasts one hour.
• For experimental roofs on which runoff has occurred, a runoff event is tied to cor-responding rain events. Several rain events may be tied to a single runoff event.
• An episode is considered as the beginning of a Rain event and the end of linkedRunoff event. If there is no runoff during two hours after the Rain event, the endof episode is the end of the Rain event.
• The mean retention per episode is calculated for each episode by the followingequation:
Mean retention per episode ð%Þ ¼ 1� Runoff event volume
Linked rain volume
� �� 100 (3)
Figure 3. Monthly mean rain depth (mm): in situ measurements (blue histogram), and1981–2010 historical records for Nantes (red).
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All steps for detecting Rain event, Runoff event, episode, and calculation for meanretention per episode were done through a script using Scilab software (Campbell,Chancelier, & Nikoukhah, 2006).
To analyse the respective influence of the different variables affecting green roofbehaviour (i.e. growing media type, growing media thickness, rainfall type, roof cover,season), the rain events were divided according to amount of rainfall into three cate-gories: light (<2 mm), medium (2–6 mm) and heavy (>6 mm), as suggested in the studyby VanWoert et al. (2005). The growing media thickness on the roof was broken downinto two categories: 80 and 120 mm. Four categories were used to describe the experi-mental roof cover: bare, sedum, festuca and dianthus. The season variable was split intotwo categories as winter and summer. The ANOVA statistical test implemented in theR© software (Team, 2010) was run in order to analyse the effects of these variables.The statistical test threshold was set to a typical value of .05 (Getter et al., 2007;Nagase & Dunnett, 2011; Stovin et al., 2012). Test results below this threshold areconsidered to exert a significant impact on the mean retention of the experimental roof.
2.4. AET performance evaluation
The impact of the experimental roof (except for roof no. 0, given that the volumetricwater content in gravel media was not monitored) on urban microclimate can be dis-cussed by calculating the ratio of cumulative Daily Actual Evapotranspiration (DailyAET) for the summer periods (2011 and 2012) to the cumulative rain depths for thesesame periods.
Daily AET (in mm) is calculated from daily water balance values during the summerperiod according to the following equation:
AET ¼ P � R� n Dh � L (4)
where P is the daily rain depth (mm), R the daily runoff depth (mm), Δθ the dailyvariation in volumetric water content (−), n the porosity of the growing media, and Lthe growing media thickness.
This ratio is calculated as follows:
RatioAET ¼ Cumulated Daily AET of experimental roof for summer period
Cumulated Rain for monitored summer period(5)
Surface albedo measurements were performed during October 2011 once per replicate ofthree measurements for each experimental roof surface type. The measurements were car-ried out in situ by the French Aerospace Lab (ONERA, Toulouse, France) using an ASDfieldspec® III Hi-Res portable spectroradiometer. The reflectance probe was located1.2 m below the roof surface, and the covered surface was a 100-mm-diameter disk.
2.5. Growing media temperature evaluation
The growing media temperatures considered in this evaluation were recorded fromprobes placed at the middle of the media for each experimental roof. The net radiationand atmospheric air temperature were measured in situ by the weather station. Nighttimeperiod was considered to occur at zero net radiation, while a daytime period wasassumed for any positive net radiation value. Mean daytime and night-time temperaturesfor summer period 2011 could then be calculated. The temperature difference isconsidered as the difference between roof media temperature and air temperature.
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3. Results and discussion
The results presented herewith correspond to the four study periods: summer 2011,winter 2011–2012, summer 2012 and winter 2012–2013.
The annual amount of rainfall recorded in Nantes from 1981 to 2010 (Meteo Franceweather agency) averaged 820 mm. The 2011 and 2012 cumulative annual rain depthsmeasured in situ were, respectively, 727 and 888 mm. Figure 3 shows the monthly raindepth for each study period. The rain depth for summer 2011 equals 238 mm, while themean rain depth in Nantes from 1981 to 2010 for this same period was only 153 mm.Summer 2011, therefore, was wetter than usual, whereas summer 2012 displayed acumulative rain depth similar to typical observations. For winter 2012 and winter 2013,the rain depth measured in situ was 270 and 340 mm, which compared to averageobservations, are respectively, dryer and equivalent.
Figure 4 presents the monthly mean temperatures measured in situ along with themonthly mean temperature recorded for Nantes from 1981 to 2010. The temperaturerecordings are in good overall agreement with typical observations.
Due to technical difficulties, the monitoring of roofs 0, 1 and 2 ceased at the end ofMarch 2012. The mean retention results per monitoring period are summarised inTable 3.
3.1. Comparison with the reference gravel roof
The mean retention of the experimental gravel roof (no. 0) equals 28.4% for the totalstudy period. In comparison with other studies, both VanWoert et al. (2005) andMentens et al. (2006) found mean retention values of, respectively, 27 and 30% forsimilar flat gravel roofs. The scatter plot of runoff vs. rainfall per episode shows thatsuch a gravel roof exhibits rather linear hydrological behaviour (Figures 5 and 6),except during lighter events characterised by low runoff. This result may be correlatedwith the hydrological behaviour of common urban surface coverings. All other roofsdisplay a different behaviour with less runoff production than the gravel roof, and evenno runoff for a large proportion of rain events.
0,00
5,00
10,00
15,00
20,00
25,00
Jun-11
Aug-11
Sep-11
Nov-11
Jan-12
Feb-12
Apr-12
May-12
Jul-12
Sep-12
Oct-12
Dec-12
Feb-13
Figure 4. Monthly mean temperature (°C) in situ (blue line), and monthly mean temperature forNantes from 1981 to 2010 (red line).
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The mean retention rate of all experimental roofs with growing media is muchhigher than the value of experimental roof no. 0. Experimental roofs 1 and 3 yieldretention values of 67.9 and 72.8%, while those with 120-mm thick growing media(i.e. Nos. 2, 4, 5 and 6) have higher values, varying from 74.9 to 80.2%.
Moreover, seasonal hydrological behaviour differs quite substantially for roofs 1through 6, in comparison with the reference gravel roof: roof retention with growingmedia is greater in summer than in winter, especially for vegetated roofs, since roof 0displays an opposite trend. This result proves the higher retention capacity of roofs witha growing media during the summer period, which is of much greater value with thepresence of a vegetation cover.
3.2. Impact of growing media thickness
For this discussion, roofs 1 and 3 will be compared, respectively, to experimental roofs2 and 4. The mean retention rates during the summer 2011 and winter 2012 periods forexperimental roofs 3 and 4 are 73.2 and 80.5%, respectively.
Let us observe that increasing the growing media thickness from 80 to 120 mm forS. album species offers a 10.3% mean retention gain. For bare surfaces (roofs 1 and 2),the mean retention increase amounts to approx 10.1%. This result shows that the
Table 3. Mean retention per monitoring period (%): method no. 1.
Experimental roof 0 1 2 3 4 5 6 Rain (mm)
Summer 2011 22.6 77.0 79.1 80.9 90.8 87.9 93.7 238.4Winter 2012 33.6 66.1 71.1 66.4 71.5 71.0 71.1 270.2Summer 2012 – – – 88.2 95.7 82.3 89.3 207.0Winter 2013 – – – 62.7 70.1 65.5 56.1 340.0Global 28.4 67.9 74.9 72.8 80.2 75.3 74.9 1055.6
Figure 5. Runoff (mm) vs. rainfall (mm) for roofs 0, 1 and 2 for summer 2011 and winter 2012.
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contribution from increasing the growing media is similar for bare and sedum-coveredsurfaces, as evidenced by a similar water retention benefit.
Observations of the runoff vs. rainfall scatter plot (Figures 5 and 6) indicate thathydrological behaviour is roughly the same for the 80 and 120 mm thicknesses duringlight and medium rain events, with many events generating zero or little runoff. Thesensitivity of runoff response to growing media thickness may still be observed: how-ever, for the heavy rain event data set, in which the highest-volume event may generateless runoff on the 120-mm thick roof than on the 80-mm roof, regardless of thevegetation status.
Similar results were found in the literature; for example, Palla et al. (2010) statesthat increasing the growing media thickness yields higher water retention rates, thoughthis gain was relatively minor. Scholz-Barth (2001) observed that increasing growingmedia thickness from 20 to 150 mm produced a 24% gain in mean retention.
The influence of growing media thickness was detectable on vegetation developmentduring the first year. From an aesthetic standpoint, the flowering of S. album was morepronounced on experimental roof 4 than on roof 3 (Figure 7). Thicker growing mediapromotes healthier plants with greater biomass (Rowe, Getter, & Durhman, 2012).
3.3. Impact of vegetation as opposed to a bare surface (July 2011–March 2012)
The overall mean retention rates over this period (summer 2011 and winter 2012) forexperimental roofs 5 and 6 were 78.9 and 81.7%, respectively. For an 80-mm growingthickness (roof no. 3), the addition of vegetation raises the mean retention rate by 7.8%.For a 120-mm growing thickness and for roofs 4, 5 and 6, the mean retention rateincreases are 7.5, 5.4 and 9.1%, respectively. These results confirm that greening thegrowing media increases the retention capacity of a flat roof, yet this increase alwaysremains less than 10%.
Figure 6. Runoff (mm) vs. rainfall (mm) for roofs 3 through 6 for the full data set (overallperiod).
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The above results are in agreement with the study by VanWoert et al. (2005), whoseobservations focused on an experimental roof featuring a 25-mm media thicknesscombined with a 25-mm fabric retention system extending over a 14-month monitoringperiod. These authors recorded mean retention rates of 50 and 61%, respectively, for anon-vegetated growing media and sedum species cover. An 18% increase in totalretention capacity can be attributed to this vegetation cover.
3.4. Impact of vegetation type
In this part, experimental roofs 4, 5 and 6 are compared to one another. The mean reten-tion of experimental roof no. 4 is higher than roofs 5 and 6. We observed that for sum-mer periods 2011 and 2012, roofs with D. deltoides and F. glauca species presentsimilar mean retention values, while the roof with S. album species has higher values.In the two winters 2011–12 and 2012–13, sedum tends to show a higher mean capacity.
Let us also note that sedum species are more resistant and require less maintenance,whereas F. glauca and D. deltoides are just the opposite: in need of more maintenanceand offering less resistance.
3.5. Impact of seasonal variations
This part is devoted to investigating the impact of seasonal variations on the retentionperformance of experimental roofs. We noticed that the summer precipitation distribu-tion is clearly different from summer 2011 to summer 2012 (Figure 3). In particular, atAugust 2011, a big storm occurred. For roofs 3 and 4, the storm water retentionperformance increased from summer 2011 to summer 2012, while the performance ofroofs 5 and 6 are slightly decreasing over the same period. The fact that sedum roofperformance increases is probably due to two main reasons: The first reason is due tothe lower precipitation and intensity of the rain in summer 2012 in comparison withsummer 2011 and the second reason is due to the fact that the sedum species are moreinvasive and require more time for their roots to become established in the porousmedia. During the second year and thereafter, sedum species were still developing andhence improving the level of roof performance.
Figure 7. Sedum development with 80 mm (left) and 120 mm (right) of growing mediathickness at the end of summer 2011.
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From summer 2011 to winter 2012, the mean retention rate decreased for allexperimental roofs (except gravel roof no. 0). The observed values demonstrate that thewinter season exerts an impact by reducing the retention capacity of experimental roofs.A comparison of experimental roofs with the same growing media thickness(i.e. comparing 1 with 3, and 2 with 4, 5 and 6) proves that mean retention values forthe winter period are similar. Moreover, the water retention of vegetated roofs is com-parable to bare surface retention during winter.
From summer 2012 to winter 2013, the water retention value of roof 6 droppeddrastically, whereas the mean retention value of roof 4 held relatively steady from theprevious winter. This same trend is observed for the mean retention of roof no. 5,though the decrease is less pronounced when compared to roof 6. For the second year,results show that the winter season reduces green roof retention capacity. The betterwater retention performance during winter for sedum roofs may be explained by the factthat S. album species were less wilted than D. deltoides and F. glauca in winter(Figure 8).
3.6. Impact of rain distribution
Retention performance may be summarised by analysing the mean retention rates ofvarious rain events. The distribution of events is shown in Figure 9, which is typical ofthe most common meteorological conditions in the study area, i.e. an oceanic climatecharacterised by a majority of smaller events (with a depth less of than 4 mm) and fewheavy showers. For these rain events, the mean retention distribution reveals that a basicimpermeable flat roof exhibits poor retention performance once the event reaches 2 mm.Placing growing media on a flat roof may significantly increase this performance for awide range of rain events, excluding the very small events (See roof 2 in Figure 9).Vegetating a flat roof may increase retention to an even greater extent, especially formedium-volume events, yet the difference between vegetation species is not substantial.Let us also note that retention tends to be greater during high-volume events, althoughrain samples may be too limited to analyse this result in any further detail.
3.7. ANOVA test results (method no. 2)
The mean retention rates per episode for experimental roofs 1 through 6 were analysedaccording to the ANOVA test (Table 4).
Figure 8. Photos of experimental roof species: S. album (left), D. deltoides (centre) and F.glauca (right) in winter 2012.
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The ANOVA statistical test shows that media type (i.e. gravel vs. growing media),amount of rain depth (rain type) and season (summer vs. winter) all exert significanteffects on experimental roof retention values. The surface covering of these roofs (bare,sedum, festuca and deltoides) also plays an important role, but this effect is clearly seen tobe lower (from the p-value). As for growing media thickness, the Anova test did not detectany effect specific to the thickness. This sample of just two thicknesses (80 and 120-mm)was perhaps not large enough for such a statistical test. As discussed above, the 10% risein water retention rate due to increased thickness might be too small for detection byAnova. Similarly, Voyde, Fassman, and Simcock (2010) did not identify any statisticallysignificant differences in retention relative to the growing media thickness.
Figure 9. Rain event distribution and mean retention performance vs. rain event depth (mm) –the mean retention is estimated for each rain event depth subsample.
Table 4. Anova test relative to retention value.
p-value Retention effect
Media type <2.2e-16 ***Growing media thickness .204211 NoSurface cover .008013 **Rain type (depth) <2.2e-16 ***Season <2.2e-16 ***
Note: *** and ** make reference to significant effects.
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3.8. AET performance and temperatures variations
The ratios of cumulative AET to cumulative rain for the summer period and for eachexperimental roof are presented in Table 5. For summer 2011 and the bare roof, let usobserve that roof no. 2 evaporates slightly more than roof 1, which is correlated withthe thicker growing media. This difference is more pronounced between roofs 3 and 4,which have been vegetated with S. album. This finding is mainly due to the type ofvegetation, given its better development on roof 4 and greater storm water retention inthe growing media, thus proving that vegetation activates the evapotranspiration func-tion of the roof since the growing media thickness does not exert any influence. Theperformance observed naturally differs considerably when drawing comparisons betweenthe bare roof and the 120-mm vegetated roof (no. 4). No difference is noticeable in theevapotranspiration performance of the various species (nos. 4, 5 and 6) during this sum-mer period, but the D. deltoides roof (no. 6) had the highest ratio, a finding consistentwith the highest retention performance of this roof.
In summer 2012, we were unable to compare evapotranspiration values between thebare surface and vegetated surface due to technical problems that occurred on roofs 1and 2. For the vegetated roofs, the observed behaviour is very similar to that identifiedin summer 2011, with a distinct impact of growing media thickness on evapotranspira-tion performance. The evapotranspiration fluxes on roof 4 (sedum cover), however, havebecome greater than those on roof 6, as opposed to what occurred in summer 2011.This finding underscores the fact that the evapotranspiration performance of a sedumroof increases over time, which again corresponds to the fact that this roof was not fullydeveloped as of the first year.
Since all energy fluxes have not been estimated as part of this experimental proce-dure and since the cumulative AET has not been calculated for experimental roof no. 0,the potential influence of a green roof on the energy budget cannot be analysed indepth. However, an analysis of the media temperature measured on experimental roofsis still instructive: during the day, the media temperature of roof no. 0 tends to be higherthan the other experimental roofs (Table 6), which indicates that during the day greenroofs contribute more than the conventional gravel roof to reducing roof surface tem-perature. At night, the temperature in gravel roof no. 0 is less than the vegetated roofs.The temperature inside the growing media of roofs with bare surfaces (1 and 2) ishigher during the day, compared to roofs with vegetation covering (3 through 6), thoughat night no difference is found. We can conclude that greening a roof allows reducingthe surface temperature range during the day, relative to the conventional gravel roof.
This analysis may be strengthened with a surface albedo examination (Akbari &Konopacki, 2005; Susca et al., 2011). The measured albedo surface of a white gravelroof has been estimated at .40, while the bare surface albedo of growing media is .22and S. album surface albedo is .14. The surface albedo for F. glauca and D. deltoidesare, respectively, .19 and .21. These values are quite low especially for the S. album:This could be due to the fact that measurements were done at the end of September2011. The S. album species are at the maturation stage, and probably the characteristic
Table 5. Ratio of cumulative AET to cumulative rain for the two summer periods.
Experimental roof 1 2 3 4 5 6
Summer 2011 .65 .67 .75 .85 .84 .90Summer 2012 – – .75 .91 .79 .87
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of their colour have changed. A green roof is clearly less sensitive to temperaturevariations than a basic gravel roof, and this statement confirms the positive role of greenroofs in the UHI mitigation previously discussed in other studies (Zinzi & Agnoli,2011).
4. Conclusion
Data on rainfall, runoff and water content have been monitored during the summer andwinter seasons for two consecutive years on various green roof configurations in Nantes,France. This study was based on micro-scale green roofs specially built for research pur-poses. The mean retention and cumulative AET (for summer periods only) of these testroofs have been estimated and analysed with regard to various variables, including raintype, growing media thickness, vegetation species and season. We found that a thickergrowing media slightly improves storm water mitigation and offers a more attractiveappearance to the vegetation as well. The vegetation layer may increase this retentionimpact, albeit to a rather limited extent. The sensitivity of both the retention andcumulative AET of green roofs to the type of vegetation species has also beenidentified. During winter periods, the performance of all roofs was decreasing. Despiteperforming extremely well during summer, the D. deltoides species revealed a dramati-cally worse performance in winter. This study has shown that sedum species are bestadapted to both mitigating storm water flows and increasing the daily latent heat flux.The monitoring of experimental roofs is still ongoing and will be introduced in subse-quent research to confirm the observations derived from this study.
From an energy budget point of view, this study is not intended to provide highlydetailed answers regarding UHI mitigation. This micro-scale study has, however,produced some pertinent results on the role of green roofs both in decreasing surfacetemperature during the day and in increasing evapotranspiration. The entire energybudget has not monitored herein, and a more extensive study could be implementedeither by using more experimental devices on a micro-scale, like in Jim and He (2010),or by applying modelling approaches on a larger scale. Estimating the real influence ofgreen roofs on urban climatology will require larger scale modelling approaches.
Moreover, this study has exposed the role of flat roof greening with respect to roofhydrological and energy mitigation performance. Building green roofs with a thickergrowing media, is however, debatable since the mitigation performance was slightly bet-ter in our study. Another point worth mentioning concerns the maintenance of greenroof systems, given that well-maintained vegetation seems to be necessary to ensuresteady storm water mitigation and high evapotranspiration rates. Green roof maintenancemay indeed entail watering or irrigation, which could subsequently decrease the overall
Table 6. Temperature difference (in °C) between roof media temperature and air temperature.
Summer 2011 Night Day
Roof 0 2.2 3.2Roof 1 3.4 2.3Roof 2 4.8 1.4Roof 3 4.9 .6Roof 4 3.8 .5Roof 5 4.0 .6Roof 6 5.3 .7
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environmental benefit in the case of artificial watering or vegetation pruning. Thevegetation species tested here (D. deltoides and F. glauca) might be a suitablealternative to the more commonly used sedum plants; however, they are probably morevulnerable, especially during winter, and their implementation could require moreintensive surveillance and maintenance. The results of this study contribute to the cre-ation of a (French) database of green roof performance and moreover may assist urbanpractitioners and planners in evaluating their project to green the urban environment.
AcknowledgementThe authors would like to express their thanks to France’s National Research Foundation (ANR)for sponsoring this study under contract ANR-09-VILL-0007 (VegDUD). Our gratitude is alsoextended to the CRITT Horticole and ONERA organisations for their collaboration, in addition tothe entire technical staff at CSTB.
Disclosure statementNo potential conflict of interest was reported by the authors.
FundingThis study was sponsored by France’s National Research Foundation (ANR) under contractANR-09-VILL-0007 (VegDUD).
ReferencesAkbari, H., & Konopacki, S. (2005). Calculating energy-saving potentials of heat-island reduction
strategies. Energy Policy, 33, 721–756.Bass, B., Stull, R., Krayenjoff, S., & Martilli, A. (2002). Modelling the impact of green roof
infrastructure on the urban heat island in Toronto. The Green Roof Infrastructure Monitor, 4,2–3.
Bengtsson, L. (2005). Peak flows from thin sedum-moss roof. Lyngby: Nordic Association forHydrology.
Benzerzour, M., Masson, V., Groleau, D., & Lemonsu, A. (2011). Simulation of the urban climatevariations in connection with the transformations of the city of Nantes since the 17th century.Building and Environment, 46, 1545–1557.
Czemiel Berndtsson, J. (2010). Green roof performance towards management of runoff waterquantity and quality: A review. Ecological Engineering, 36, 351–360.
Berthier, E., De Gouvello, B., Archambault, F., & Gallis, D. (2010). Bilan hydrique des toituresvégétalisées: vers de meilleures compréhension et modélisation [Water balance of green roofs:Contributions to better understanding and simulation]. Techniques sciences méthodes, 6, 39–47.
Campbell, S., Chancelier, J. P., & Nikoukhah, R. (2006). Modeling and simulation in scilab/sci-cos. New York, NY: Springer.
Castleton, H. F., Stovin, V., Beck, S. B. M., & Davison, J. B. (2010). Green roofs; buildingenergy savings and the potential for retrofit. Energy and Buildings, 42, 1582–1591.
DeNardo, J. C., Jarrett, A. R., Manbeck, H. B., Beattie, D. J., & Berghage, R. D. (2005).Stormwater mitigation and surface temperature reduction by green roofs. Transactions of theASAE, 48, 1491–1496.
Dimoudi, A., & Nikolopoulou, M. (2003). Vegetation in the urban environment: Microclimaticanalysis and benefits. Energy and Buildings, 35, 69–76.
Emilsson, T. (2008). Vegetation development on extensive vegetated green roofs: Influence ofsubstrate composition, establishment method and species mix. Ecological Engineering, 33,265–277.
For- schungsgesellschaft Landschaftsentwicklung Land-schaftsbau e.V. (2008). Guideline forplanning, construction and maintenance of green roofing. Bonn: Author.
European Journal of Environmental and Civil Engineering 17
Dow
nloa
ded
by [
Tun
celi
Uni
vers
itesi
] at
04:
29 0
8 M
ay 2
015
Getter, K. L., Rowe, D. B., & Andresen, J. A. (2007). Quantifying the effect of slope on extensivegreen roof stormwater retention. Ecological Engineering, 31, 225–231.
Gregoire, B. G., & Clausen, J. C. (2011). Effect of a modular extensive green roof on stormwaterrunoff and water quality. Ecological Engineering, 37, 963–969.
Hilten, R. N., Lawrence, T. M., & Tollner, E. W. (2008). Modeling stormwater runoff from greenroofs with HYDRUS-1D. Journal of Hydrology, 358, 288–293.
Jaffal, I., Ouldboukhitine, S.-E., & Belarbi, R. (2012). A comprehensive study of the impact ofgreen roofs on building energy performance. Renewable Energy, 43, 157–164.
Jim, C. Y., & He, H. (2010). Coupling heat flux dynamics with meteorological conditions in thegreen roof ecosystem. Ecological Engineering, 36, 1052–1063.
Köehler, M. (2005). Urban storm water management by extensive green roofs. Basel: WorldGreen Roof Congress.
Lemonsu, A., Pigeon, G., Masson, V., & Moppert, C. (2006). Sea-town interactions overMarseille: 3D urban boundary layer and thermodynamic fields near the surface. Theoreticaland Applied Climatology, 84, 171–178.
MacIvor, J. S., & Lundholm, J. (2011). Performance evaluation of native plants suited to extensivegreen roof conditions in a maritime climate. Ecological Engineering, 37, 407–417.
Mentens, J., Raes, D., & Hermy, M. (2006). Green roofs as a tool for solving the rainwater runoffproblem in the urbanized 21st century? Landscape and Urban Planning, 77, 217–226.
Metselaar, K. (2012). Water retention and evapotranspiration of green roofs and possible naturalvegetation types. Resources, conservation and recycling, 64, 49–55.
Monterusso, M. A. (2004). Runoff water quantity and quality from green roof systems. ActaHorticultirae, 639, 369–376.
Moran, A. C. (2004). A North Carolina field study to evaluate greenroof runoff quantity, runoffquality, and plant growth. (Thesis). North California State University, Raleigh.
Nagase, A., & Dunnett, N. (2011). The relationship between percentage of organic matter insubstrate and plant growth in extensive green roofs. Landscape and Urban Planning, 103,230–236.
Palla, A., Gnecco, I., & Lanza, L. (2010). Hydrologic restoration in the urban environment usinggreen roofs. Water, 2, 140–154.
Rodriguez, F., Andrieu, H., & Morena, F. (2008). A distributed hydrological model for urbanizedareas – Model development and application to case studies. Journal of Hydrology, 351,268–287.
Rosenfeld, A. H., Akbari, H., Romm, J. J., & Pomerantz, M. (1998). Cool communities:Strategies for heat island mitigation and smog reduction. Energy and Buildings, 28, 51–62.
Rowe, D. B., Getter, K. L., & Durhman, A. K. (2012). Effect of green roof media depth onCrassulacean plant succession over seven years. Landscape and Urban Planning, 104,310–319.
Rüdiger, C., Western, A. W., Walker, J. P., Smith, A. B., Kalma, J. D., & Willgoose, G. R. (2010).Towards a general equation for frequency domain reflectometers. Journal of Hydrology, 383,319–329.
Scholz-Barth, K. (2001). Green roofs: Stormwater management from the top down. EnvironmentalDesign & Construction, 4, 63–69.
Stovin, V. (2010). The potential of green roofs to manage Urban Stormwater. Water andEnvironment Journal, 24, 192–199.
Stovin, V., Vesuviano, G., & Kasmin, H. (2012). The hydrological performance of a green rooftest bed under UK climatic conditions. Journal of Hydrology, 414–415, 148–161.
Susca, T., Gaffin, S. R., & Dell’Osso, G. R. (2011). Positive effects of vegetation: Urban heatisland and green roofs. Environmental Pollution, 159, 2119–2126.
Teemusk, A., & Mander, Ü. (2009). Greenroof potential to reduce temperature fluctuations of aroof membrane: A case study from Estonia. Building and Environment, 44, 643–650.
VanWoert, N. D., Rowe, D. B., Andresen, J. A., Rugh, C. L., Fernandez, R. T., & Xiao, L.(2005). Green roof stormwater retention. Journal of Environment Quality, 34, 1036–1044.
Villarreal, E. L., & Bengtsson, L. (2005). Response of a Sedum green-roof to individual rainevents. Ecological Engineering, 25, 1–7.
Von Stülpnagel, A., Horbert, M., & Sukopp, H. (1990). The importance of vegetation for theurban climate. In Urban ecology plants and plant communities in urban environments(pp. 175–193). The Hague: SPB Academic Publication.
18 D. Yilmaz et al.
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vers
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] at
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Voyde, E., Fassman, E., & Simcock, R. (2010). Hydrology of an extensive living roof undersub-tropical climate conditions in Auckland, New Zealand. Journal of Hydrology, 394,384–395.
Wong, N. H., Tay, S. F., Wong, R., Ong, C. L., & Sia, A. (2003). Life cycle cost analysis ofrooftop gardens in Singapore. Building and Environment, 38, 499–509.
Zinzi, M., & Agnoli, S. (2011). Cool and green roofs. An energy and comfort comparisonbetween passive cooling and mitigation urban heat island techniques for residential buildingsin the Mediterranean region. Energy and Buildings, 55, 66–76.
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