Storm water retention and actual evapotranspiration performances of experimental green roofs in French oceanic climate

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Storm water retention and actual
evapotranspiration performances of
experimental green roofs in French
oceanic climate
D. Yilmaz
ac
, M. Sabre
a
, L. Lassabatère
d
, M. Dal
c
& F. Rodriguez
b
a
CAPE, Centre Scientifique et Technique du Bâtiment, Nantes,
France
b
LUNAM Université, IFSTTAR, GERS, EE, IRSTV, Bouguenais, France
c
Engineering Faculty, Civil Engineering Department, University of
Tunceli, Tunceli, Turkey
d
UMR5023 Laboratoire d’Ecologie des Hydrosystèmes Naturels
et Anthropisés, Université Lyon 1, ENTPE, CNRS, Vaulx-en-Velin,
France
Published online: 06 May 2015.
To cite this article: D. Yilmaz, M. Sabre, L. Lassabatère, M. Dal & F. Rodriguez (2015): Storm
water retention and actual evapotranspiration performances of experimental green roofs
in 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 performances
of experimental green roofs in French oceanic climate
D. Yilmaza,c
*, M. Sabrea
, L. Lassabatèred
, M. Dalc
and F. Rodriguezb
a
CAPE, Centre Scientifique et Technique du Bâtiment, Nantes, France; b
LUNAM Université,
IFSTTAR, GERS, EE, IRSTV, Bouguenais, France; c
Engineering Faculty, Civil Engineering
Department, University of Tunceli, Tunceli, Turkey; d
UMR5023 Laboratoire d’Ecologie des
Hydrosystèmes 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 both
rainwater management and microclimatology. They are considered as a storm water
mitigation technique and may also favour evapotranspiration fluxes, which can be
beneficial for urban comfort during summer periods. In France, however, water reten-
tion performance of green roofs remains unknown, and published values are often
unsuitable. Six experimental roofs, including two thicknesses of growing media,
three types of vegetation cover and bare surfaces, were monitored for two years in
Nantes and compared to an experimental gravel flat roof. The thickest media
combined with the most densely vegetated cover yields the best results in terms of
storm 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 design
tools 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 area
consumed and population density. This urban evolution has caused water management
problems, namely the sealed surface of urban areas has led to storm runoff increases
capable of generating considerable property damage and environmental pollution. Since
modern urban infrastructure was designed several decades ago, storm water management
practices must adapt to these urban expansion trends. For new buildings today, French
regional institutions now prescribe limited runoff outflows. In some locations, connect-
ing new buildings to the storm water network may even be forbidden. Green roofs
represent an emerging strategy for mitigating storm water runoff (Moran, 2004;
Monterusso, 2004; VanWoert et al., 2005). Retrofitting older structures with a green roof
could 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 during
summer periods increases air temperature on urban areas and generates additional
*Corresponding author. Email: dyilmaz@tunceli.edu.tr
© 2015 Taylor & Francis
European Journal of Environmental and Civil Engineering, 2015
http://dx.doi.org/10.1080/19648189.2015.1036128

energy consumption due to air conditioning, thus contributing to global warming. Green
roofs 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 the
ambient air (Bass, Stull, Krayenjoff, & Martilli, 2002; Dimoudi & Nikolopoulou, 2003;
Mentens, Raes, & Hermy, 2006; Rosenfeld, Akbari, Romm, & Pomerantz, 1998; Von
Stülpnagel, 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 retaining
rain water in the roof media) due to geographical location. For a one-year monitoring
period on similar experimental roofs, Berthier, De Gouvello, Archambault, and Gallis
(2010) and Palla, Gnecco, and Lanza (2010) found retention capacities of respectively
65% 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). Values
from 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 the
German studies and found that the annual retention capacity of extensive green roofs
varied from 27 to 81%. Scholz-Barth (2001) stated that the mean storm water retention
for the United States was approx 65% for green roofs; this estimation was based on case
studies from various cities, e.g. Chicago, Philadelphia and Portland.
It is also a very difficult exercise to compare retention capacities from one study to
the next since green roof components differ and affect water retention capabilities differ-
ently. For example, Stovin, Vesuviano, and Kasmin (2012) studied a single-layer green
roof, while Getter, Rowe, and Andresen (2007) focused on a green roof containing a
retention fabric used as a water reservoir.
It is also known that during summer periods, evapotranspiration and green roof
water retention capacity both increase (Mentens et al., 2006; Villarreal & Bengtsson,
2005). Stovin et al. (2012) found lower retention capacity values during the spring than
in the summer. Thus, seasonal variation is a parameter that affects the green roof
performances.
Many authors have pointed out that vegetation contributes to a reduction in outflow
volumes at the annual scale due to evapotranspiration (Bengtsson, 2005; Gregoire &
Clausen, 2011; Köehler, 2005; Palla et al., 2010; Stovin, 2010). Only a few studies have
quantified the actual evapotranspiration (AET) by means of experimental measurement
(Gregoire & Clausen, 2011; MacIvor & Lundholm, 2011). Other several studies have
calculated AET from numerical modelling (Hilten, Lawrence, & Tollner, 2008;
Metselaar, 2012). Temperature reduction at the green roof surface, in comparison with
conventional 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 of
water removed through evapotranspiration by green roofs and the reduction in surface
temperature 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 decreasing
peak flow, while on the other, green roofs may induce greater evapotranspiration than
more common impermeable roofs, thus helping refresh the urban environment in sum-
mer. The main obstacles to retrofitting older traditional construction with green roofs are
cost-related. Only a few French regions have voted to provide financial assistance for
green 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
2 D. Yilmaz et al.

be compiled for potential subsequent research to test green roof retrofitting scenarios in
France. Green roof impacts on both storm water and UHI mitigation will be studied in
greater depth through the use of numerical modelling tools like TEB for climate and
URBS 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 several
experimental green roof configurations in a French oceanic climate by estimating their
respective 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 these
experimental green roofs will then be discussed in comparison with conventional flat
gravel roofs and across the various experimental configurations. Emphasis will also be
placed 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 at
this 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 with
overall dimensions of 1500 mm × 1500 mm were constructed. All platforms were built on
wooden supports with null slope, and a protective coat of paint was applied. This
construction process respected relevant French building codes. The wood support was
covered with a commercial-grade bituminous vapour barrier (5 mm thick). The insulation
layer was installed using 60-mm high polystyrene blocks. 5-mm high PVC membranes
were introduced for waterproofing. The discharge outlet was designed with a 30-mm
diameter PVC tube placed at one corner of the roof platform (120 mm from the border) to
respect 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. Their
metabolism provides the benefits of greater resistance during dry periods. These plants
offer the most appropriate species for green roofs (Emilsson, 2008). CRITT Horticole, a
French association contributing to the research and development on green roofs in
France, has tested several plant species in a roof environment. Upon the advice of this
association, three species have been selected herein. The first two types are Sedum
album and Festuca glauca, both of which are commonly used in green roof design. The
third species is Dianthus deltoides, which has yet to be used on a green roof; this plant
is a perennial and aesthetically appealing with a long flowering period extending from
June to September.
2.1.3. Growing media properties
Extensive commercial-grade green roof media were used (Star, Forges, France). The
growing media was composed of 70% mineral materials and 30% organic matter.
European Journal of Environmental and Civil Engineering 3

The mineral parts were a mix of 3/20-mm pumice stones with 3/7 and 7/15-mm
Pozzolana. The organic matter was produced with composted bark from a maritime
pine, 3/20-mm black peat and 3/20-mm blond peat. The particle size distribution of
these 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 pycnometer
method 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 and
specific ρs densities via the following equation:
n ¼ 1 À
qb
qs
(1)
The saturated volumetric water content θs is assumed to equal the porosity n of the
media: 68.1%. The field capacity is the maximum water amount that can retain a porous
media. 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 are
composed of growing media with no vegetation cover. Specimen Nos. 3 and 4 are
green roofs planted with S. album. Lastly, roofs 5 and 6 have been greened with
F. glauca and D. deltoides, respectively. There is no irrigation system in the
experimental roofs.
Gravel was directly laid on the PVC impermeable layer on roof no. 0. For all other
specimens, 40-mm thick extruded polystyrene (Siplast, France) was used as a drainage
system on the impermeable layer. A 200 g/m² geotextile was introduced in order to
avoid plant roots and small particles of growing media from penetrating into the
underlying layers. According to the configurations listed in Table 2, the growing media
was installed over the textile. Experimental roofs 3 through 6 initially remained
10 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
4 D. Yilmaz et al.

The experimental roofs were placed on a small road sloped at 1.5° (see Figure 1). A
meteorological (MET) station records the rainfall, air temperature, relative humidity of
air, atmospheric pressure, global radiation and wind speed, plus the direction at a 1-min
frequency. 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 outlet
through the drainage layer. The water outflow on each roof is discharged into a tipping
bucket mechanism (switch activated for every 20 ml of water). In this study, we will
refer to runoff as discharge. Two temperature probes (thermocouple type T) were placed
in 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 placed
300 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 by
Rüdiger et al. (2010). For each roof studied, the average volumetric water content was
estimated from the average value of four probes. Note that the volumetric water content
was 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 No
1 Yes Growing media 80 No
2 Yes Growing media 120 No
3 Yes Growing media 80 Sedum album
4 Yes Growing media 120 Sedum album
5 Yes Growing media 120 Festuca glauca
6 Yes Growing media 120 Dianthus deltoides
Figure 1. Location of Experimental roofs at CSTB Nantes site.

2.3. Hydrological performance evaluation
Experimental green roof performance in terms of water retention was evaluated using
two distinct methods. The first was based on calculating an average mean retention
value for each roof tested over a monitoring period: summer, winter or overall. The
output values were then compared to one another to further investigate roof configura-
tion behaviour. The second method consisted of calculating the mean retention value for
each experimental roof and for each rain event, subsequent to which an Anova statistical
analysis 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 to
the 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 the
mean retention per monitoring period, defined as the percentage of cumulative storm
water retained in the experimental roof over the monitoring period.
Mean retention ð%Þ ¼ 1 À
Cumulated runoff volume
Cumulated rain volume

Â 100 (2)
where Cumulative rain volume is the total volume of rainfall recorded during the
monitoring period, and Cumulative runoff volume is the total volume of water flowing
through the experimental roof outlet during the monitoring period.
Figure 2. CS616 water content probes location.
6 D. Yilmaz et al.

2.3.2. Method no. 2: performance evaluation using the Anova test
The hydrological performance of experimental roofs is analysed using the Anova
statistical 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 cumulative
rain depth is above a threshold value of .5 mm. The end of a Rain event is
considered to occur when no rain has fallen for a full hour.
• Runoff events are considered to take place whenever runoff ﬂow is occurring. The
end of a runoff event occurs once the cumulative runoff depth drops below a
threshold value of .5 mm over two consecutive hours, except for experimental
gravel 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 linked
Runoff event. If there is no runoff during two hours after the Rain event, the end
of episode is the end of the Rain event.
• The mean retention per episode is calculated for each episode by the following
equation:
Mean retention per episode ð%Þ ¼ 1 À
Runoff event volume
Linked rain volume

Â 100 (3)
Figure 3. Monthly mean rain depth (mm): in situ measurements (blue histogram), and
1981–2010 historical records for Nantes (red).

All steps for detecting Rain event, Runoff event, episode, and calculation for mean
retention 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 roof
behaviour (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 study
by VanWoert et al. (2005). The growing media thickness on the roof was broken down
into 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 into
two categories as winter and summer. The ANOVA statistical test implemented in the
R©
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 are
considered 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 volumetric
water content in gravel media was not monitored) on urban microclimate can be dis-
cussed by calculating the ratio of cumulative Daily Actual Evapotranspiration (Daily
AET) for the summer periods (2011 and 2012) to the cumulative rain depths for these
same periods.
Daily AET (in mm) is calculated from daily water balance values during the summer
period according to the following equation:
AET ¼ P À R À n Dh Á L (4)
where P is the daily rain depth (mm), R the daily runoff depth (mm), Δθ the daily
variation in volumetric water content (−), n the porosity of the growing media, and L
the 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 of
three 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 ASD
fieldspec®
III Hi-Res portable spectroradiometer. The reflectance probe was located
1.2 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 from
probes placed at the middle of the media for each experimental roof. The net radiation
and atmospheric air temperature were measured in situ by the weather station. Nighttime
period was considered to occur at zero net radiation, while a daytime period was
assumed for any positive net radiation value. Mean daytime and night-time temperatures
for summer period 2011 could then be calculated. The temperature difference is
considered as the difference between roof media temperature and air temperature.
8 D. Yilmaz et al.

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 France
weather agency) averaged 820 mm. The 2011 and 2012 cumulative annual rain depths
measured in situ were, respectively, 727 and 888 mm. Figure 3 shows the monthly rain
depth for each study period. The rain depth for summer 2011 equals 238 mm, while the
mean 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 a
cumulative 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 average
observations, are respectively, dryer and equivalent.
Figure 4 presents the monthly mean temperatures measured in situ along with the
monthly mean temperature recorded for Nantes from 1981 to 2010. The temperature
recordings are in good overall agreement with typical observations.
Due to technical difﬁculties, the monitoring of roofs 0, 1 and 2 ceased at the end of
March 2012. The mean retention results per monitoring period are summarised in
Table 3.
3.1. Comparison with the reference gravel roof
The mean retention of the experimental gravel roof (no. 0) equals 28.4% for the total
study period. In comparison with other studies, both VanWoert et al. (2005) and
Mentens et al. (2006) found mean retention values of, respectively, 27 and 30% for
similar ﬂat gravel roofs. The scatter plot of runoff vs. rainfall per episode shows that
such a gravel roof exhibits rather linear hydrological behaviour (Figures 5 and 6),
except during lighter events characterised by low runoff. This result may be correlated
with the hydrological behaviour of common urban surface coverings. All other roofs
display a different behaviour with less runoff production than the gravel roof, and even
no 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 for
Nantes from 1981 to 2010 (red line).

The mean retention rate of all experimental roofs with growing media is much
higher than the value of experimental roof no. 0. Experimental roofs 1 and 3 yield
retention 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 1
through 6, in comparison with the reference gravel roof: roof retention with growing
media is greater in summer than in winter, especially for vegetated roofs, since roof 0
displays an opposite trend. This result proves the higher retention capacity of roofs with
a growing media during the summer period, which is of much greater value with the
presence of a vegetation cover.
3.2. Impact of growing media thickness
For this discussion, roofs 1 and 3 will be compared, respectively, to experimental roofs
2 and 4. The mean retention rates during the summer 2011 and winter 2012 periods for
experimental 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 for
S. 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 93.7 238.4
Winter 2012 33.6 66.1 71.1 66.4 71.5 71.0 71.1 270.2
Summer 2012 – – – 88.2 95.7 82.3 89.3 207.0
Winter 2013 – – – 62.7 70.1 65.5 56.1 340.0
Global 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.
10 D. Yilmaz et al.

contribution from increasing the growing media is similar for bare and sedum-covered
surfaces, as evidenced by a similar water retention benefit.
Observations of the runoff vs. rainfall scatter plot (Figures 5 and 6) indicate that
hydrological behaviour is roughly the same for the 80 and 120 mm thicknesses during
light and medium rain events, with many events generating zero or little runoff. The
sensitivity 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 generate
less runoff on the 120-mm thick roof than on the 80-mm roof, regardless of the
vegetation status.
Similar results were found in the literature; for example, Palla et al. (2010) states
that increasing the growing media thickness yields higher water retention rates, though
this gain was relatively minor. Scholz-Barth (2001) observed that increasing growing
media thickness from 20 to 150 mm produced a 24% gain in mean retention.
The influence of growing media thickness was detectable on vegetation development
during the first year. From an aesthetic standpoint, the flowering of S. album was more
pronounced on experimental roof 4 than on roof 3 (Figure 7). Thicker growing media
promotes 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) for
experimental roofs 5 and 6 were 78.9 and 81.7%, respectively. For an 80-mm growing
thickness (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 rate
increases are 7.5, 5.4 and 9.1%, respectively. These results confirm that greening the
growing media increases the retention capacity of a flat roof, yet this increase always
remains less than 10%.
Figure 6. Runoff (mm) vs. rainfall (mm) for roofs 3 through 6 for the full data set (overall
period).

The above results are in agreement with the study by VanWoert et al. (2005), whose
observations focused on an experimental roof featuring a 25-mm media thickness
combined with a 25-mm fabric retention system extending over a 14-month monitoring
period. These authors recorded mean retention rates of 50 and 61%, respectively, for a
non-vegetated growing media and sedum species cover. An 18% increase in total
retention 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 present
similar 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 maintenance
and offering less resistance.
3.5. Impact of seasonal variations
This part is devoted to investigating the impact of seasonal variations on the retention
performance of experimental roofs. We noticed that the summer precipitation distribu-
tion is clearly different from summer 2011 to summer 2012 (Figure 3). In particular, at
August 2011, a big storm occurred. For roofs 3 and 4, the storm water retention
performance increased from summer 2011 to summer 2012, while the performance of
roofs 5 and 6 are slightly decreasing over the same period. The fact that sedum roof
performance increases is probably due to two main reasons: The ﬁrst reason is due to
the lower precipitation and intensity of the rain in summer 2012 in comparison with
summer 2011 and the second reason is due to the fact that the sedum species are more
invasive and require more time for their roots to become established in the porous
media. During the second year and thereafter, sedum species were still developing and
hence improving the level of roof performance.
Figure 7. Sedum development with 80 mm (left) and 120 mm (right) of growing media
thickness at the end of summer 2011.
12 D. Yilmaz et al.

From summer 2011 to winter 2012, the mean retention rate decreased for all
experimental roofs (except gravel roof no. 0). The observed values demonstrate that the
winter 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 for
the 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 dropped
drastically, whereas the mean retention value of roof 4 held relatively steady from the
previous 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 better
water retention performance during winter for sedum roofs may be explained by the fact
that 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 of
various rain events. The distribution of events is shown in Figure 9, which is typical of
the most common meteorological conditions in the study area, i.e. an oceanic climate
characterised by a majority of smaller events (with a depth less of than 4 mm) and few
heavy showers. For these rain events, the mean retention distribution reveals that a basic
impermeable 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 a
wide 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 for
medium-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, although
rain 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 analysed
according 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.

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 significant
effects 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 to
be lower (from the p-value). As for growing media thickness, the Anova test did not detect
any 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% rise
in water retention rate due to increased thickness might be too small for detection by
Anova. Similarly, Voyde, Fassman, and Simcock (2010) did not identify any statistically
significant 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 No
Surface cover .008013 **
Rain type (depth) 2.2e-16 ***
Season 2.2e-16 ***
Note: *** and ** make reference to significant effects.
14 D. Yilmaz et al.

3.8. AET performance and temperatures variations
The ratios of cumulative AET to cumulative rain for the summer period and for each
experimental roof are presented in Table 5. For summer 2011 and the bare roof, let us
observe that roof no. 2 evaporates slightly more than roof 1, which is correlated with
the 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 of
vegetation, given its better development on roof 4 and greater storm water retention in
the growing media, thus proving that vegetation activates the evapotranspiration func-
tion of the roof since the growing media thickness does not exert any influence. The
performance observed naturally differs considerably when drawing comparisons between
the bare roof and the 120-mm vegetated roof (no. 4). No difference is noticeable in the
evapotranspiration 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 consistent
with the highest retention performance of this roof.
In summer 2012, we were unable to compare evapotranspiration values between the
bare surface and vegetated surface due to technical problems that occurred on roofs 1
and 2. For the vegetated roofs, the observed behaviour is very similar to that identified
in summer 2011, with a distinct impact of growing media thickness on evapotranspira-
tion performance. The evapotranspiration fluxes on roof 4 (sedum cover), however, have
become 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 sedum
roof increases over time, which again corresponds to the fact that this roof was not fully
developed 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 in
depth. However, an analysis of the media temperature measured on experimental roofs
is still instructive: during the day, the media temperature of roof no. 0 tends to be higher
than the other experimental roofs (Table 6), which indicates that during the day green
roofs 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) is
higher during the day, compared to roofs with vegetation covering (3 through 6), though
at night no difference is found. We can conclude that greening a roof allows reducing
the 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 gravel
roof has been estimated at .40, while the bare surface albedo of growing media is .22
and S. album surface albedo is .14. The surface albedo for F. glauca and D. deltoides
are, 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 September
2011. 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 .90
Summer 2012 – – .75 .91 .79 .87

of their colour have changed. A green roof is clearly less sensitive to temperature
variations than a basic gravel roof, and this statement confirms the positive role of green
roofs 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 and
winter 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 test
roofs have been estimated and analysed with regard to various variables, including rain
type, growing media thickness, vegetation species and season. We found that a thicker
growing media slightly improves storm water mitigation and offers a more attractive
appearance to the vegetation as well. The vegetation layer may increase this retention
impact, albeit to a rather limited extent. The sensitivity of both the retention and
cumulative AET of green roofs to the type of vegetation species has also been
identified. During winter periods, the performance of all roofs was decreasing. Despite
performing extremely well during summer, the D. deltoides species revealed a dramati-
cally worse performance in winter. This study has shown that sedum species are best
adapted 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 highly
detailed answers regarding UHI mitigation. This micro-scale study has, however,
produced some pertinent results on the role of green roofs both in decreasing surface
temperature during the day and in increasing evapotranspiration. The entire energy
budget has not monitored herein, and a more extensive study could be implemented
either 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 of
green roofs on urban climatology will require larger scale modelling approaches.
Moreover, this study has exposed the role of flat roof greening with respect to roof
hydrological and energy mitigation performance. Building green roofs with a thicker
growing media, is however, debatable since the mitigation performance was slightly bet-
ter in our study. Another point worth mentioning concerns the maintenance of green
roof systems, given that well-maintained vegetation seems to be necessary to ensure
steady storm water mitigation and high evapotranspiration rates. Green roof maintenance
may 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 3.2
Roof 1 3.4 2.3
Roof 2 4.8 1.4
Roof 3 4.9 .6
Roof 4 3.8 .5
Roof 5 4.0 .6
Roof 6 5.3 .7
16 D. Yilmaz et al.

environmental benefit in the case of artificial watering or vegetation pruning. The
vegetation species tested here (D. deltoides and F. glauca) might be a suitable
alternative to the more commonly used sedum plants; however, they are probably more
vulnerable, especially during winter, and their implementation could require more
intensive 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 urban
practitioners and planners in evaluating their project to green the urban environment.
Acknowledgement
The 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 also
extended to the CRITT Horticole and ONERA organisations for their collaboration, in addition to
the entire technical staff at CSTB.
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
This study was sponsored by France’s National Research Foundation (ANR) under contract
ANR-09-VILL-0007 (VegDUD).
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