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Designing Green Roofs for Stormwater Management
Dr. Karen LiuProduct Manager
RCABC 2018‐11‐14
Rural Hydrology
Source: Stream Corridor Restoration: Principles, Processes, and Practices
Natural Ground Cover
40% evapotranspiration
10% runoff
25% shallow infiltration 25% deep
infiltration
10‐20% Impervious Surface
38% evapotranspiration
20% runoff
21% shallow infiltration 21% deep
infiltration
Urban Hydrology
75‐100% Impervious Surface
35% evapotranspiration 30% evapotranspiration
30% runoff
55% runoff
20% shallow infiltration 15% deep
infiltration
10% shallow infiltration 5% deep
infiltration
35‐50% Impervious Surface
Source: Stream Corridor Restoration: Principles, Processes, and Practices
Stormwater Problems in Urban Areas
• Flash flooding causes damage to buildings, infrastructure and erosion to waterways
• Urban runoff carries pollutants to river/ocean
• Combined sewage overflow (CSO) is serious environmental concern
Sponge City Concept –Manage Stormwater by Working with Nature
Source: Drainage Services Department, Hong Kong
Porous Pavement
BioSwale
Rain garden
Stormwater Management –Low Impact Development (LID)
Green Roof
Why Green Roof?
• Reduce stormwater runoff• Lower energy demand• Mitigate urban heat island• Extend roof membrane life• Improve air quality• Enhance biodiversity• Add green amenity space
Project: Coquitlam Water Treatment PlantPartner: Next Level Stormwater Management
Green Roof Policies/Programs in Canada
• Port Coquitlam Green Roof Bylaw– All new commercial & industrial buildings >5,000 m2 must install a green
roof covering at least 75% of roof area
• Richmond Green Roof Bylaw– All new industrial or office buildings outside the city centre >2,000 m2 must
install a green roof covering at least 75% of the roof area
• City of Toronto Green Roof Bylaw & Eco-Roof Incentive Program– All new buildings with gross floor area over 2,000 m2 must install a green
roof covering 20-60% of the roof area depending on GFA
– Industrial Buildings: the lesser of 2,000 m2 or 10% of available roof space
– Penalty of non-compliance: $200/m2
– All buildings not subjected to the Green Roof Bylaw can receive $100/m2
for green roof installed up to $100,000
How does a Green Roof Reduce Runoff?
Vegetation
Growing Medium
Water Retention Layer
Drainage /Filter Layer
Root Barrier
Roof System
– takes up water for photosynthesis & evapotranspiration
– stores rain water,taken up by vegetation or evaporation
– absorbsand stores excess water for later
– stores rainwater and drains off excess
• Depth• Composition• Particle size distribution
Growing Medium
Water Storage Efficiency Comparison
0
5
10
15
20
25
30
Lava / pumice /dolomite mixtures*
Expanded claymixtures*
GM#1** GM#2** GM#3**
Thickn
ess (mm)
Water Storage Efficiency of Typical Growing Media(normalized to 25‐mm thickness)
Dry Componet
Water Storage
* FLL Table 14** Data Sheet
Drainage Layer
Drainage board with reservoirs
Entangled net geocomposite
Dimpled drains
Water Retention Layers
Retention Fleece
Mineral Wool
Partner: Next Level Stormwater Management
Water Retention Comparison
Components Thickness Dry Weight SaturatedWeight
Water Retention
Growing Medium 10 mm 6.2 kg/m2 9.9 kg/m2 3.7 l/m2
Growing Medium 30 mm 18.6 kg/m2 29.7 kg/m2 11.1 l/m2
Retention Fleece 9 mm 1.2 kg/m2 8.7 kg/m2 7.4 l/m2
Mineral Wool 30 mm 2.2 kg/m2 27.5 kg/m2 25.3 l/m2
Water Storage Capacity Comparison
0
5
10
15
20
25
30
35
40
45
Lava / pumice /dolomitemixtures*
Expanded claymixtures*
Retention fleece1200g/m2**
Mineral woolmats**
Shaped hardplastic boards
3D drainagedimpled boards
Growing Media Water Storage Layers Drainage Boards
Weight (kg/m
2 )
Water Storage Capacity (normalized to 25‐mm thickness)
Dry Componet
Water Storage
* FLL Table 14** FLL Table 15 FLL Table 13# Data Sheet
#
2
4
6
8
1
3
5
7
9
0
Weight (lb/sf)
Water Retention to Weight Comparison
0
0.2
0.4
0.6
0.8
1
Lava / pumice /dolomitemixtures*
Expanded claymixtures*
Retention fleece1200g/m2**
Mineral woolmats**
Shaped hardplastic boards
3D drainagedimpled boards
Growing Media Water Storage Layers Drainage Boards
Water‐Reten
tion‐to‐W
eight R
atio
Water‐Retention‐to‐Weight Ratio
* FLL Table 14** FLL Table 15 FLL Table 13# Data Sheets
#
Vegetation – Regular Plants (C3)
• Plants take up water from the roots and release it to the atmosphere from their leaves
• During photosynthesis, plants can lose 97% of the water they uptake to transpiration
• Depletes a green roof’s water storage and creates capacity for the next rainfall
• Plants can become dry and wilted between rainfalls
Plants – Succulents (CAM)
• an adaptation for increased water use efficiency typically found in plants living in arid conditions
• Stomata in the leaves remain shut during the day to reduce evapotranspiration, but open at night to collect CO2
• CAM plants such as sedums are more heat and drought tolerant for rooftop survival
(CAM) Crassulacean Acid Metabolism
British Columbia Institute of Technology
Source: BCIT
GR-1
GR-2
REF
Source: BCIT
Experimental Roof Sections
Rainfall and Runoff in Dry Season(May 20-21, 2005)
0.0
0.5
1.0
1.5
2.0
2.5
1830
2030
2230 30 23
0
430
630
830
1030
1230
1430
1630
1830
2030
Rai
nfal
l / R
unof
f (m
m)
RAINGR-1GR-2
May 20 May 21
Dry Season: Apr – Sep, total rainfall = 231 mm
RAIN = 18 mm in 20 h
GR‐1 = 10 mm (44%)GR‐2 = 3 mm (86%)
Dry Season Event
Delay
Peak
Volume
Source: BCIT
Rainfall and Runoff in Wet Season(Apr 5-6, 2005)
0.0
0.2
0.4
0.6
0.8
1.0
1015
1215
1415
1615
1815
2015
2215 15 21
5
415
615
815
1015
1215
Rai
nfal
l / R
unof
f (m
m)
RAINGR-1GR-2
Apr 5 Apr 6
Wet Season: Oct – Mar, total rainfall = 1277 mm
RAIN = 17 mm in 18 h
GR‐1 = 13 mm (22%)GR‐2 = 14 mm (17%)
Wet Season Event
Source: BCIT
Rainfall and Roof Runoff at GRRF(Jan 1, 2005 - Dec 31, 2005)
0
50
100
150
200
250
300
350
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Rai
n / R
unof
f (m
m)
WS
GR1
GR2
Wet Dry Wet
RAINWet season: reduction=15%Dry season: reduction=90%
Overall Reduction = 28%
=> More growing medium does not necessarily retain more water
Stormwater Runoff Retention
Rainfall and Runoff Statistics
Source: BCIT
Climate Affects Annual Water Retention
• Annual water retention of a green roof with 150 mm(6”) GM – 26% (1117mm or 44”) in Vancouver– 54% (245mm or 9.6”) in Ottawa
0
50
100
150
200
Jan Feb Mar AprMay Jun Jul Aug Sep Oct Nov Dec
Precipita
tion (m
m)
Vancouver (YVR)
0
50
100
150
200
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Precipita
tion (m
m)
Ottawa (YOW)
=> Annual water retention is climate dependent
Case Study –Coquitlam Water Treatment Facilities
Partner: Next Level Stormwater Management
Green Roof System Buildup
Root Barrier
Water Retention
Water Retention
Pre‐vegetated Mat
Properties
Roof Slope 10 ‐ 15°
Thickness 48 mm
Saturated Weight 55 kg/m2
Water Retention 32 l/m2
Materials Arrive on Site
Partner: Next Level Stormwater Management
Install Water Retention Layers
Root Barrier
Retention Fleece
Partner: Next Level Stormwater Management
Install Water Retention Layers
Pre‐Vegetated Mats
Partner: Next Level Stormwater Management
Vegetation Free Zone
Stone Ballast
Partner: Next Level Stormwater Management
Covered Walkway Greening Completed
Partner: Next Level Stormwater Management
Upper Roof Section
Partner: Next Level Stormwater Management
Upper Roof Section
Partner: Next Level Stormwater Management
Upper Roof & Covered Walkway
Partner: Next Level Stormwater Management
Lower Roof Section
Partner: Next Level Stormwater Management
Lower Roof Section (3 hours later)
Partner: Next Level Stormwater Management
Coquitlam Water Treatment Facilities -2 Years Later
Partner: Next Level Stormwater Management
Coquitlam Water Treatment Facilities -2 Years Later
Partner: Next Level Stormwater Management
Coquitlam Water Treatment Facilities -2 Years Later
Partner: Next Level Stormwater Management
Coquitlam Water Treatment Facilities -2 Years Later
Partner: Next Level Stormwater Management
Stormwater Management
• Green roofs delay and reduce peak flow, and reduce the total runoff volume; thus lowering the burden on the sewage system
• Many municipalities have policies or programs to promote LID’s for stormwater management
• There are many options available to increase water storage capacity of green roofs while keeping system weight low
• Use water-retention-to-weight ratio to compare water storage capacity on a per unit weight basis when specifying products
• There is no one-size-fits-all solution; consider local climate when designing green roofs for stormwater management
Thank You!
Karen LiuLow & Bonar / [email protected](B) 778‐737‐4661(C) 604‐561‐5217www.greenroofs.lowandbonar.com/
Project: Seaforth Armory, Vancouver BCPartner: Next Level Stormwater Management