Designing Green Roofs for Stormwater Management › wp-content › uploads › 2018 › 11 › Earth... · 2018-11-16 · Designing Green Roofs for Stormwater Management Dr. Karen

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


infiltration

Urban Hydrology


35% evapotranspiration 30% evapotranspiration

30% runoff

55% runoff


infiltration


infiltration


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*


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*


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


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


Install Water Retention Layers

Root Barrier

Retention Fleece


Install Water Retention Layers

Pre‐Vegetated Mats


Vegetation Free Zone

Stone Ballast


Covered Walkway Greening Completed


Upper Roof Section


Upper Roof Section


Upper Roof & Covered Walkway


Lower Roof Section


Lower Roof Section (3 hours later)


Coquitlam Water Treatment Facilities -2 Years Later








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

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Designing Green Roofs for Stormwater Management › wp-content › uploads › 2018 › 11 › Earth... · 2018-11-16 · Designing Green Roofs for Stormwater Management Dr. Karen