Sustainable Benefits of Green Roofs
William Retzlaff, Ph.D.
SIUE Department of Biological Sciences
Susan Morgan, Ph.D., P.E.
SIUE Department of Civil Engineering
Serdar Celik, Ph.D.
SIUE Department of Mechanical Engineering
ISTC 2009 – 2010 Seminar Series on Sustainability
“The Path to Curbing Global Warming: Becoming Carbon Neutral”
March 18, 2010
Outline
 Introduction
 Results
 Runoff quantity
 Runoff quality
 Thermal
 Summary
Introduction: What are they?
Why consider them?
Results: Runoff Quantity
60
50
A
A
A
Retention (%)
A
40
30
20
10
B
0
5cmC
5cmP
10cmP
15cmP
20cmP
Treatment
Mean storm water retention (%) for study period 09/05-04/07 for green roof
systems with 5, 10, 15, and 20 cm planted medium depths. (C=Control,
P=Planted. Bars with same letter not significantly different at the p<0.05 level.
Error bars + 1se) (Forrester 2008).
100
A
Retention (%)
80
A
A
15cmP
20cmP
B
60
40
20
C
0
5cmC
5cmP
10cmP
Treatment
Mean storm water retention (%) for study period 04/07-11/07 for green
roof systems with 5, 10, 15, and 20 cm planted medium depths.
(C=Control, P=Planted. Bars with same letter not significantly different
at the p<0.05 level. Error bars + 1se) (Woods 2009).
Results: Runoff Quality – Metals
[Zn] (mg kg-1)
0.17
A
0.16
0.15
B
0.02
B
B
B
B
B
0.00
l
e
t ro
lyt
a
n
k
o
C
Ar
ass
l
G
it
yd
a
H
e
va
La
e
ce
flit
mi
o
u
P
Ro
Planted Media Treatments and Control
April 11, 2008 collection event. Effect of media treatments on zinc concentration (bars
denote mean [zinc] and standard error, n=3-4) measured from leachate collected from
empty built-in-place control roofs and from each planted media placed in 10 cm Green
Roof Blocks; bars with same letter not significantly different at the p <0.05 level.
F = 74.48; p < 0.0001.
Results: Runoff Quality – Solids
Growing Media
Media
Description
Arkalyte
Clay heated to 1000 0C
Bottom ash
Ash from coal power plant
Haydite
Shale heated to 1000 0C
Lava
Volcanic rock
Vegetation
Vegetated Media TSS
600
A
Mean TSS , mg/L
500
Lava-B
Hay-B
Ark-B
Bot Ash-B
400
A
300
AB
A
200
B
100
B
A
BA
B
0
0
2
B
C
C
B
C
C
A
A
A
BA
B
B
BA
B
BA
B
B
B
4
6
Trial Number
A
A
A
B
B
8
120 mg/L
A
A
B
B
20 mg/L
10
Vegetated Media Turbidity
350
A
300
280 NTU
Mean Turbidity , NTU
A
250
Lava-B
Hay-B
Ark-B
Bot Ash-B
A
200
B
150
B
100
B
C
50
B
B
C
C
C
0
0
2
A
A
B
B
A
A
B
B
A
A
B
B
4
6
Trial Number
A
A
B
B
A
A
B
C
8
A
A
B
B
10
Non-Vegetated Media Turbidity
600
A
Mean Turbidity, NTU
500
Lava-B
Hay-B
Ark-B
Bot Ash-B
400
B
300
B
280 NTU
200
A
A
C
100
B
B
0
0
1
2
A
A
A
A
B
B
A
B
B
A
B
B
3
4
Trial Number
5
6
Effect of Vegetation – Haydite
600
A
Veg.
Mean Turbidity, NTU
500
Non-v
400
300
280 NTU
200
A
B
100
A
A
A
A
A
A
A
0
0
1
2
3
Trial Number
4
5
6
Effect of Vegetation – Bottom Ash
300
Mean TSS, mg/L
250
A
Veg.
Non-v
200
A*
150
B
100
A*
50
A
A
120 mg/L
A
A
A
A
20 mg/L
0
0
1
2
3
Trial Number
4
5
6
Results: Thermal
Coupled green roof model
Theory
qradiation  A(GND  Gd  GR )  A(Ts4  T4 )
Reflected irradiation
Diffuse irradiation
qconvection  hA(Ts  T )
A(Ts  Tr )
qconduction 
Rtotal
Normal direct irradiation
Unit thermal resistance
through all layers
Rtotal
L1 L2 L3
 
  ...
k1 k 2 k3
Theory
qconduction  qradiation  qconvection
• Adiabatic side walls
• 1-D heat transfer
 (Ts  Tb ) 
q 

 R p g  ij
"
ij
Heat flux through a
selected combination
Vegetation
Growing Media
Media
Description
Arkalyte
Clay heated to 1000 0C
Lava
Volcanic rock
Pumice
Volcanic Rock
August 2007 Data for All Combinations
Peak Hours on August 12, 2007
Heat Flux on August 12, 2007
Energy Cost of a 50,000 m2 Roof
during Peak Hours
Summary: Runoff Quantity
 Green roof systems reduce storm water runoff in the
Midwest.
 When saturated, stormwater retention of green roof systems
is approximately 40 - 50%.
 At 10-14 day intervals between rainfall events, green roofs
can retain as much as 0.75 - 1.0 inch of stormwater (no
through-flow).
 During a “normal” St. Louis growing season, stormwater
retention of green roof systems can reach 80%.
Summary: Runoff Quality
 There has been no heavy metal contamination in runoff.
 TSS and turbidity of the growing media and the vegetated
media decrease over time.
 TSS and turbidity vary between media.
 Vegetation provides a reduction in TSS and turbidity only
during the first flush.
Summary: Thermal
 The color of the growth media and the vegetation affects the
thermal radiation to/from the green roof blocks significantly.
 The more condensed the leaves are, the better shading effect
occurs, resulting in better insulation in air-conditioning
seasons.
 Fully covered blocks act as better insulators compared to
partially covered ones, due to having a higher reflectivity and
a better shading effect.
Questions?
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