11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
A green roof experimental site in the Mediterranean climate
A. Palla*, L.G. Lanza and P. La Barbera
Department of Civil, Environmental and Architectural Engineering,
University of Genoa, Via Montallegro 1, 16145 Genoa, Italy
*Corresponding author, e-mail [email protected]
ABSTRACT
In January 2007 the Department of Civil, Environmental and Architectural Engineering
(DICAT - University of Genoa), in collaboration with the Italian Green Roofs Association
(AIVEP) and the Municipality of Genova, transformed the existing green roof of its hydraulic
laboratory in an experimental site to investigate the green roof control potential on storm
water management. The objectives of this study are to provide detailed information about
green roof performances in the Mediterranean climate and to identify synthetic parameters for
describing the associated hydraulic behaviour (retained volume, peak flow reduction, runoff
delay).
Two different approaches were implemented to simulate the hydraulic response of the
experimental green roof: the Hydrus – 1D model, that solves the Richards' equation for onedimensional saturated-unsaturated water flow, and a simplified linear reservoir model. In the
latter model, three reservoirs are used to simulate percolation in the growing medium, and
drainage from the saturated and unsaturated zones.
Data colleted during the initial monitoring campaign (from May to November 2007) are
presented in the paper, together with preliminary results obtained in simulating the storm
water source control potential at the experimental site.
KEYWORDS
Green roofs; runoff source control; infiltration models.
INTRODUCTION
Up to the present day the practice of green roofs has been prominently developed in regions
where the climatic conditions, and especially the rainfall regime along the course of the year,
are favourable to the grow up of vegetation. International research efforts were consequently
addressed with reference to experimental installations in continental or sub-artic climates
(Bengtsson et al., 2005; Köheler et al., 2005; Carter and Rasmussen, 2006; Villareal and
Bengtsson, 2004) while as far as the Mediterranean countries are concerned, and especially in
Italy, scientific studies are scarce and so are the experimental projects aimed to evaluate the
effectiveness of green roofs as a tool for sustainable urban drainage. Aiming at the assessment
of the influence of green roofs on the management of storm water runoff in the urban
environment of the Mediterranean regions, the Department of Civil, Environmental and
Architectural Engineering (DICAT) of the University of Genova exploited the existing green
roof of its hydraulic laboratory as an experimental site for the quantitative monitoring of the
hydrologic behaviour of a vegetated roof.
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11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
METHODS
Experimental Site
Site Description and Data collected. The Hydraulic Laboratory rooftop is essentially a flat
roof on three independent levels, with a surface area of approximately 1000 m2. The existing
green roof system (constructed in 1969) was made of a drainage layer in bricks for a depth of
8 cm and a growing medium of clayey ground for a depth of 35 cm.
In May 2007 DICAT realized on the central part of this green roof a new substrate system
with an extension of about 350 m2. The new solution consists in a protection layer (300 gr/m2
non woven), a drainage layer (realized by lapillus of granulometry 3/16 mm for a depth of 15
cm), a filter layer (100 gr/m2 non woven fabric) and a growing medium with mixed soil
(lapillus, pumice, zeolite and 200 l/m3 of peat) for a depth of 20 cm. The central plot was
divided in two equal halves for research purposes; in one half the growing medium is 70%
lapillus, 30% pumice and peat and in the other half is 70% lapillus, 20% pumice, 10% zeolite
and peat.
The central plot is fully monitored: the site is equipped with a meteorological station for rain
data (at one minute time resolution), air temperature and humidity, solar radiation and air
pressure, and with a suitable hydraulic device for continuous flow monitoring. The
monitoring campaign started in May 2007 and is still in progress.
Models and Governing Equations
Numerical Model for the Impermeable Control Roof. The impervious system was modelled
by employing the EPA Storm Water Management Model (SWMM). The domain is simplified
in six sub-catchments, four junctions and five conduits. The flow routing method is based on
the kinematic wave model and the infiltration model is the Soil Conservation Service Curve
Number method (SCS, 1972).
Conceptual Hydrological Model for the green roof. The green roof system is simulated by
means of three linear reservoirs representing infiltration in the growing medium and drainage
from the saturated and unsaturated zones respectively (Palla and Lanza, 2008). The behaviour
of the first reservoir is regulated by the pair of equations (1) and (2) where apexes indicate the
instant in time; subscripts indicate the number of the reservoir; K is the reservoir constant [T1
]; s is the threshold; q is the specific effluent per unit surface area [L/T]; h is the water depth
[L] and P is precipitation [L/T].
qIi = 0
hIi < sI
qIi = K I ⋅ (hI − sI ) hIi ≥ sI
hIi = P i + hIi−1 − qIi−1
(1)
(2)
The behaviour of the second reservoir is regulated by the pair of equations (3) and (4), while
analogue equations describe the behaviour of the third reservoir.
qIIi = 0
hIIi < sII
(3)
qIIi = K II ⋅ (hII − s ) hIIi ≥ sII
hIIi = qIi + hIIi−1 − qIi−1
2
(4)
A green roof experimental site in the Mediterranean climate
11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
The total specific flow discharged by the green roof, qGreenModel [L/T], is calculated as a linear
combination (5) of the effluents from the second and third reservoirs.
i
qGreenModel
= β ⋅ qIi + (1 − β ) ⋅ qIIi
(5)
Hydrus 1-D Model for the green roof. The Hydrus-1D code (Simunek et al., 1998) was
employed to simulate the infiltration process and water content profile in 1D variably
saturated media. The governing flow equation is the one-dimensional form of the Richards’
equation:
∂θ (ψ ) ∂
∂ψ
=
K (ψ ) ⋅
+1
∂t
∂z
∂z
(6)
where θ is the volumetric water content [L3L-3]; ψ is suction head [L]; K is the unsaturated
hydraulic conductivity [LT-1], and the assumptions are made that the air phase plays a
negligible role in the liquid flow movement and that water flow due to thermal gradients can
be neglected. To obtain the hydraulic conductivity function in terms of soil water retention
parameters, the Van Genuchten (1980) soil-hydraulic functions with the statistical pore
distribution model of Mualem (1976) are implemented. The Van Genuchten relationships are:
θ (ψ ) =
θr +
θs
K (ψ ) = K S S e
θ s −θ r
ψ <0
[1 + αψ ]
n m
1/ 2
(
1 − 1 − Se
(7)
ψ ≥0
1
m
)
m
2
(8)
where θr and θs are saturated and residual water content [L3L-3]; S e =
θ −θ r
is the effective
θ s −θ r
saturation; α is an empirical constant; n and m are the dimensionless parameters with
m = 1− 1 ; and Ks is the saturated hydraulic conductivity [L/T].
n
The soil vertical profile (a drainage layer of 20 cm in lapillus and a growing layer of 20 cm in
Vulcaflor), is discretized employing 100 elements (each one of 4 mm depth) connected by
101 nodes.
The boundary conditions at the soil-atmosphere interface (9) may change from a prescribed
flux (unsaturated soil conditions) to a prescribed head type condition (for saturated soil) and
are given by:
∂ψ
− K (ψ ) = P
∂z
=0
− K (ψ )
ψ z =0
ψ z =0 < 0
(9)
While at the soil-concrete foundation interface, the boundary conditions may change from a
zero flux condition (for unsaturated soil conditions) to a zero head condition (for saturated
soil conditions) and are given by:
∂ψ
− K (ψ ) = 0
∂z
=0
− K (ψ )
ψ Z =− L
Palla et al.
ψ z =− L < 0
(10)
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11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
As shown in (7) and (8) the Hydrus-1D code requires the estimation of five parameters (θr, θs,
n, α, Ks) whose values mainly depend on the soil type.
The drainage process in the saturated media and the convolution process in the drainage pipe
to the gauge station are simulated with two linear reservoirs respectively.
RESULTS AND DISCUSSION
Monitoring campaign
The monitoring campaign can be dived in two phases, the first one with collecting data from
the impervious roof and the second one from the new green roof system.
The first phase of the monitoring campaign was carried out in May 2007 during the
retrofitting works for the new green roof system, when the rooftop was only covered with the
impermeable layer. Only three events were collected and two of them produced some
significant runoff (see Table 1).
Table 1. First phase of the Monitoring Campaign for the impervious system.
Event
(yyyy/mm/dd)
2007/05/02
2007/05/03
2007/05/04
Rain Depth
(mm)
27.2
1.6
43.2
Flow Peak
(l/s)
5.7
0.13
3.9
Calibration and Verification of the Numerical Model for the Impermeable Control Roof. Out
of the three events collected in the first phase of the monitoring campaign, the 2 May event
was the most intense, resulting in a well-defined hydrograph as illustrated in Figure 1. This
event was selected as the calibration event to determine hydraulic resistance and depressions
depth for the system. The obtained values for such parameters are summarized in Table 2.
Table 2. Calibration parameters for the impermeable roof components.
Sub-catchments
Conduits
n Manning
(-)
0.012
0.01
Depression Depth
(mm)
0.2
-
The simulation overestimates the total effluent volume and the flow peak by respectively 15%
and 2.6% as shown in Figure 1(a). The calibration strategy involved comparing the predicted
and measured subsurface hydrographs using two indices: outflow hydrograph volume and
peak outflow rate. The ImpModel calibration was validated on the 4 May event.
The simulation results, as illustrated in Figure 1(b), underestimate the total effluent volume
and the peak flow by respectively 14.4 % and 17%.
The second phase of the monitoring campaign started on 22 May 2007, just the day after
completion of the new green roof, and is still in progress. Four out of the eleven events
monitored produced no subsurface runoff, 5 produced runoff with a peak lower than 0.01 l/s
and only two events produced a significant subsurface runoff with peaks greater than 1 l/s.
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A green roof experimental site in the Mediterranean climate
11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
The rainfall volume for each event was completely infiltrated (no surface runoff occurred) and
only partially exfiltrated. The synthetic parameters (retained volume and peak flow reduction)
used for describing the green roof hydraulic performances are listed in Table 3.
evol = -15%
epeak= -2.6%
8
Q [l/s]
04 May
0
evol = 14.4%
epeak= 17 %
25
50
6
75
4
i [mm/h]
02 May
10
100
2
125
0
0
4.0
0
6.0
0
8.0
Time [h]
.00
10
Rain
.00 .00
12 13
Q-Measured
.00
15
.00
.00
17
19
Time [h]
.00
21
150
Q-ImpModel
Figure 1. Comparison of measured and simulated hydrographs from the impermeable model
and errors on the total volume and the peak flow for: (a) the 2 May 2007 event; (b) the 4 May
2007 event.
Table 3. Events observed during the monitoring campaign events and percentage of retained
volume, peak flow reduction and run-off delay.
Event
(yyyy/mm/dd)
2007/05/26
2007/05/28
2007/06/01
2007/06/05
2007/08/08
2007/08/09-10
2007/08/20
2007/08/21
2007/09/27
2007/11/21
2007/11/22-23
Rain Depth
(mm)
9
12.4
42.4
41.2
13.2
14
15.2
32.6
28.6
8
138.2
Flow Peak
(l/s)
No outflow
No outflow
0.02
1.31
No outflow
< 0.01
< 0.01
0.04
0.02
No outflow
1.27
Retained Vol.
(%)
100
100
99
41
100
95
95
96
99
100
9.5
Peak Reduction
(%)
100
100
99
87
100
98.7
99.9
99
99.6
100
79
The peak flow reduction calculated as the percentage difference between impermeable peak
flow and measured peak flow ranges between 80% and 100%, with an average value of 97%.
The retained volumes calculated as the percentage difference between the volume of rain and
discharged volume ranges between 10% and 100%, with an average value of 85%.
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11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
The runoff delay calculated as the difference between the hyetograph and hydrograph
centroids for the events with outflow peaks greater than 1 l/s, are 50 min for the 05 June event
and 148 min for the 22-23 November event.
These delay values are relevant in view of the usual concentration times of urban catchments.
From these data it clearly emerges that a green roof system is able to significantly reduce
storm water runoff generation in Mediterranean regions in terms of runoff volume reduction,
peak attenuation and increase of concentration time. If these results are transferred to the
spatial scale of the urban watershed, green roof installations can become helpful tool to
prevent flooding phenomena in the urban areas and to limit the impact of storm water on
waste water treatment plants (Carter and Rasmussen, 2006; Palla et al., 2008).
Calibration and Verification of Models for the green roof. Both the conceptual hydrological
model (GreenModel), and the Hydrus-1D model (Hydrus+conv) were calibrated using the 5
May 2007 event. Out of the eleven collected events, the 5 June event was the most intense
with a maximum intensity on 5 minutes of 110 mm/h and with a duration of 157 min. This
event was selected to determine the GreenModel parameters (the reservoirs constants, Ki; the
reservoir thresholds, si and the linear combination coefficients, β) as summarized in Table 4.
Table 4. Parameters of the green roof Linear Reservoir model
1/K
(min)
32
322.6
33.3
Reservoir I
Reservoir II
Reservoir III
β
s
(mm)
16
2
2
(%)
24
76
The hydraulic parameters (θr, θs, α, n, Ks) required by Hydrus-1D for each green roof
components are listed in Table 5: the α and n values are literature data from Carsel and
Parrish (1988), while θr , θs and Ks parameters were calibrated for the 5 June 2007 event.
The conceptual parameters Kd (reservoir constant for the drainage process) and Kc (reservoir
constant for the convolution) were calibrated: the inverse of Kd value (1/Kd) is 50 min and the
inverse of Kc value (1/Kc ) is 10 min.
On May 26th 2007, the starting day for the simulation, the green roof system was just
installed and therefore initial dry soil conditions were assumed for the water content, as
illustrated in Figure 3.
The calibration strategy involved comparing the predicted and measured subsurface out flow
hydrographs using two indices: discharge volume and peak outflow rate.
Table 5. Hydraulic parameters of the green roofs Substrates.
1. Lapillus
2. Vulcaflor
Depth
(cm)
-40 ÷ -20
-20 ÷ 0
θr
(-)
0.04
0.05
θs
(-)
0.47
0.5
α
(1/cm)
0.145
0.075
n
(-)
2.68
1.89
Ks
(cm/s)
1
0.1
The GreenModel simulation, as shown in Figure 2, underestimates the total effluent volume
and the flow peak by respectively 7% and 15%; while the outflow hydrograph predicted by
the Hydrus+conv simulation, also plotted in Figure 2, compares favourably with the measured
effluent.
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A green roof experimental site in the Mediterranean climate
11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
05 June
20
0
100
10
150
5
I [mm/h]
50
15
Q [l/s]
Rain
Q-ImpModel
Q-Measured
Q-GreenModel
Q-Hydrus+conv
GreenModel evol = 7%
GreenModel epeak= 15%
Hydrus+conv evol = 0.8%
Hydrus+conv epeak= 0%
200
0
250
12.00
14.00
Time [h]
16.00
Figure 2. Comparison of green roof measured and simulated hydrographs and the
impermeable roof simulated hydrograph with errors on the volume and peak flow operated by
GreenModel and by Hydrus+conv model for the 5 June 2007 event.
The evolution in time of the water content distribution inside the green roof under transient
hydraulic forcing was modelled with the Hydrus-1D for the 5 June event. The variation of
water content with time and depth illustrates the variably saturated flow conditions inside the
system. As shown in Figure 3, the water could not saturate the pores of the Vulcaflor layer
even under the maximum intensity rainfall because of the high drainage capacity of the
system.
0
Initial Condition
26 May 00.00
5 Jun.11.00
5 Jun. 12.30
5 Jun. 13.30
5 Jun. 20.00
Vulcaflor θs = 0.5
Depth [cm]
-10
Lapillus θs = 0.47
-20
-30
-40
0.0 0.2 0.4 0.6 0.0 0.2 0.4 0.6 0.0 0.2 0.4 0.6 0.0 0.2 0.4 0.6 0.0 0.2 0.4 0.6
θ
θ
θ
θ
θ
Figure 3. Water content profiles at different time steps for the 5 June 2007 event.
Both the GreenModel and the Hydrus+conv model were validated on the 22-23 November
event. The 22-23 Nov. event is a long lasting event (2870 min) and the hyetograph shape is
very complex, so it represents a good test for the calibrated parameters.
As shown in Figure 4, the water content was assigned at a residual level for the initial time 09
Nov. 00.00, and it was assumed that a warming-up period of 13 days is a sufficient duration to
loose memory of the initial conditions.
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11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
The GreenModel simulation, as shown in Figure 3, overestimates the total outflow volume by
4% and underestimates the flow peak by 19%; while the Hydrus+conv simulation, also
plotted in Figure 3, correctly predicts the total effluent flow and underestimates the flow peak
by 9.2 %.
22-23 November
0
8
40
6
80
4
120
2
160
13.00
21.00
Time [h]
Rain
Q-ImpModel
Q-Measured
Q-GreenModel
Q-Hydrus+conv
GreenModel evol = -4%
GreenModel epeak= 19%
Hydrus+conv evol = 0.7%
Hydrus+conv epeak= 9.2%
200
13.00
0
5.00
I [mm/h]
Q [l/s]
10
5.00
Figure 4. Comparison of the green roof measured and simulated hydrographs and the
impermeable roof simulated hydrograph with errors on volume and peak flow operated by
GreenModel and by Hydrus+conv model for the 22-23 November 2007 event.
The evolution in time of the water content distribution inside the green roof under transient
hydraulic forcing was modelled using the Hydrus-1D model for the 22-23 November event.
The displacement of the wetting zone was similar to the 05 June event, except obviously for
timing. As illustrated in Figure 5 the wetting zone moves rapidly downward, the water content
was not evenly distributed with saturation reached at the bottom interface while the upper part
of the system never reached saturation.
0
Initial Condition
9 Nov. 00.00
22 Nov. 01.00
22 Nov. 07.00
22 Nov. 23.30
23 Nov. 16.00
Vulcaflor θs = 0.5
Depth [cm]
-10
Lapillus θs = 0.47
-20
-30
-40
0.0 0.2 0.4 0.6 0.0 0.2 0.4 0.6 0.0 0.2 0.4 0.6 0.0 0.2 0.4 0.6 0.0 0.2 0.4 0.6
θ
θ
θ
θ
θ
Figure 5. Water content profiles at different time steps for the 22-23 November 2007 event.
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A green roof experimental site in the Mediterranean climate
11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
CONCLUSIONS
The hydrologic behaviour of an experimental green roof in the Mediterranean town of Genoa
was examined on an event basis over a six month period. In this study, the 1D numerical
unsaturated flow model Hydrus-1D and a conceptual hydrological model were used to
examine the green roof hydraulic performances. The discharge hydrograph profile, volume
and timing predicted by numerical and conceptual model matched experimental
measurements, however further events are necessary to better validate both models. Further
additional field measurements (water content measures) are needed to complete the study:
water content measures could be used to determine the initial conditions and to verify the
water content profile predicted by the numerical model.
Using the complete numerical unsaturated model it was possible to identify the most
important factors where experimental or field work is needed. The following are the three
most important variables governing the flow regime through the green roof: rainfall forcing
(rate, duration); substrate media (type, hydraulic conductivity and degree of saturation) and
drying process (evapo-transpiration). The role of evapo-transpiration during the interevent
duration must be quantified for each historical event under the measured meteorological
conditions and compared with the null evaporation conditions, to better estimate the initial
water content. However, for the 5 June event and for the 22-23 November event, the green
roof was not yet vegetated so the key drying process is just evaporation.
Although the numerical model predictions reported here are more accurate than conceptual
predictions, the latter appear to be reasonable and easily implemented, so the goal is to
identify suitable procedures for parameters estimation – limited in number with respect to the
rigorous problem formulation – starting from the design details of the green roof system
(substrates depth, soil texture, vegetation type and rooftop slope).
The performances of the green roof as a device for storm water control appear excellent, with
an average percent retained volume of 85% and a percent peak reduction of 95%. However, in
the framework of the assessment of the environmental benefits (prevention of flooding
phenomena and reduction of the impact on waste water treatment plants), it is necessary to
extend the investigation horizon from the spatial scale of the single rooftop to the one of the
entire watershed.
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11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
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A green roof experimental site in the Mediterranean climate