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Sizing Green Roofs and CisternsFor Stormwater Management
Brian L. Taylor www.mka.comBrian L. Taylor www.mka.com
Overview
! Introduction! Why are civil engineers evaluating green
roofs and cisterns?
! Green Roof Stormwater Potential/Performance
! Green Roof Field Evaluation! Cistern Stormwater Potential
Civil Engineering Crossroads
2
STOP!
Tri-County 4(d) “take” Federal nexus
MRCI rules NMFS ESA USFWS 13limits 24 ESU’s June 2000 Critical habitatSection 7 effect determination
ASCE: “A Better World By Design” An Obligation
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Sustainable Site/Civil Engineering Site Hydrology and Urban Development
Pre-Settlement Conditions
Historical Urban Development
4
Hydrologic Processes Civil Engineering Crossroads
Pre-Settlement Conditions
Historical UrbanDevelopment
Urban Green
Site Hydrology and Urban Development
Pre-Settlement Conditions Historical Urban Development
Urban Green
Urban GreenNarrow SidewalksBetter Tree PitsMore Street TreesBetter LightingNo Tree Pits
SustainableInfrastructureStorm DrainageEco-CorridorsBetter Streets
On SiteEco-RoofCisternBetter
DetentionOpen Space
Wide Sidewalks
Green StreetsEco-Streets
Seattle Green Roof Potential Green Roof Benefits
! Ecology! Social! Noise Reduction! Energy and Heat! Membrane Life! Stormwater
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Agency: “Based on what we know about green roofs, what is their potential to mitigate our stormwater problems?”
Pacific Northwest Data
Photo by: City of Portland
National Climate Data: Design Storms
Commonly Used Synthetic Rainfall Distributions used in Site Hydrologic Analyses
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 240 480 720 960 1200 1440
TIME (min)
Cum
. Por
tion
of 2
4-hr
Sto
rm P
reci
pita
tion
Type I
Type 1A
Seattle Type 1A
Type II (Std.)
Type II-70
Type III
Type 1A Storm - 30% of rainfall is before the peak
New Mexico Storm -Extreme intensity may overwhelm drainage capacity
Type II & III Storm -20% of rainfall is before the peak
Geographical Distribution
Type II-70
Type I Storm -25% of rainfall is before the peak
Rainfall Potential
National Climate Differences: Chicago vs. Seattle
1995 Rainfall: Chicago & Seattle
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
10/1/
1994
11/1/
1994
12/1/
1994
1/1/19
95
2/1/19
95
3/1/19
95
4/1/19
95
5/1/19
95
6/1/19
95
7/1/19
95
8/1/19
95
9/1/19
95
Dai
ly ra
infa
ll (in
)
-30.00
-20.00
-10.00
0.00
10.00
20.00
30.00
40.00
Cum
ulat
ive
rain
fall
(in)
Chicago - daily Seattle - daily Chicago - cumulative Seattle - cumulative
National Climate Data: Evaporation Values
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Green Roof Hydrologic Modeling Green Roof Event HydrologyExample Roof Runoff Hydrograph
0
0.1
0.2
0.3
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0 360 720 1080 1440 1800
Time (min)
Incre. Rainfall (in)
Runoff from equivalentconventional roof (cfs)
Runoff from green roof(cfs)
Green Roof Event Hydrology (cont.)Green Roof Runoff Reduction Potential
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14Media depth (inches)
Equi
vale
nt C
urve
Num
ber
15% 25%
40% NC State
Green Roof Event Hydrology (cont.)Green Roof Runoff Reduction Potential
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14Media depth (inches)
Equi
vale
nt C
urve
Num
ber
15%25%40%Forest (till)Forest (outwash)HardscapeNC StateWA Ecology
Green Roof Event Hydrology (cont.)Green Roof Runoff Reduction Potential (50% AMC)
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14Media depth (inches)
Equi
vale
nt C
urve
Num
ber
15%25%40%Forest (till)Forest (outwash)HardscapeNC StateWA Ecology
Green Roof Event HydrologyExample Roof Runoff Hydrograph
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 360 720 1080 1440 1800
Time (min)
Incre. Rainfall (in)
Runoff from equivalentconventional roof (cfs)
Runoff from green roof(cfs)
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Green Roof Event Hydrology (cont.)
Aggregate drainage layer,
¼:12 slope
Equivalent Time of Concentration
0
60
120
180
240
300
360
420
480
540
600
660
720
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16
Flow (cfs)
(min
)
Green Roof Annual Benefits (Simple Model)
Average Annual Volume Reduction Benefit - Seattle
Average Annual Volume Reduction Benefit - Seattle
Rainfall Retention for Seattle Eco-roofs
0%
5%
10%
15%
20%
25%
30%
35%
40%
45%
50%
0 2 4 6 8 10 12 14Eco-roof thickness, in
1981
rain
fall
rete
ntio
n
Architect: “Brian, you’re the civil engineer…how much of my detention can a green roof mitigate?”
Conventional Urban Stormwater Management
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Green Roof Stormwater Benefits
Volume Reduction:CommunityBenefit
RateReduction:DeveloperBenefit
Flow Control Manual
Flow Control Manual – Green RoofsPredictive Modeling ToolMarch 2003
Green Roof Hydrograph Development Green Roof Design Tool – Validation
Photo by: City of Portland
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Green Roof Design Tool – Validation Typical Project – Seattle Urban Infill
Typical Project – Seattle Urban InfillRoof Plan
Case Study – Results
Case Study – Results (Cont.) Sensitivity Study (40% AMC)Example hydrograph, with drain mat (2-yr MRI, 6-in growing media)
0.0000
0.1000
0.2000
0.3000
0.4000
0.5000
0.6000
0.7000
0 360 720 1080 1440 1800
Time (min)
Flow
(cfs
)
00.010.020.030.040.050.060.070.080.090.1
Rai
n (in
)
Total Eco-roofrelease rate (cfs)
Runoff fromequivalentconventional roof(cfs)
Allowable Release(cfs)
Incre. Rainfall (in)
Example hydrograph, with aggregate drainage (2-yr MRI, 6-in growing media)
0.0000
0.1000
0.2000
0.3000
0.4000
0.5000
0.6000
0.7000
0 360 720 1080 1440 1800
Time (min)
Flow
(cfs
)
00.010.020.030.040.050.060.070.080.090.1
Rai
n (in
)
Total Eco-roofrelease rate (cfs)
Runoff fromequivalentconventional roof(cfs)
Allowable Release(cfs)
Incre. Rainfall (in)
10
Sensitivity Study (80% AMC)Example hydrograph, with aggregate drainage (2-yr MRI, 6-in growing media)
0.0000
0.1000
0.2000
0.3000
0.4000
0.5000
0.6000
0.7000
0 360 720 1080 1440 1800
Time (min)
Flow
(cfs
)
00.010.020.030.040.050.060.070.080.090.1
Rai
n (in
)
Total Eco-roofrelease rate (cfs)
Runoff fromequivalentconventional roof(cfs)
Allowable Release(cfs)
Incre. Rainfall (in)
Example hydrograph, with drainage mat (2-yr MRI, 6-in growing media)
0.0000
0.1000
0.2000
0.3000
0.4000
0.5000
0.6000
0.7000
0 360 720 1080 1440 1800
Time (min)
Flow
(cfs
)
0.000.010.020.030.040.050.060.070.080.090.10
Rai
n (in
)
Total Eco-roofrelease rate (cfs)
Runoff fromequivalentconventional roof(cfs)
Allowable Release(cfs)
Incre. Rainfall (in)
Green Roof Performance: Going beyond the modeling….
Scandinavian Urban Sustainability TourMarch 2004
Scandinavian Green Roof Institute
Scandinavian Green Roof Institute Scandinavian Green Roof Institute
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The Need for a LocalEvaluation Project
Account for Evaporation Differences
Account for Storm Intensity & Magnitude
Commonly Used Synthetic Rainfall Distributions used in Site Hydrologic Analyses
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 240 480 720 960 1200 1440
TIME (min)
Cum
. Por
tion
of 2
4-hr
Sto
rm P
reci
pita
tion
Type I
Type 1A
Seattle Type 1A
Type II (Std.)
Type II-70
Type III
Type 1A Storm - 30% of rainfall is before the peak
New Mexico Storm -Extreme intensity may overwhelm drainage capacity
Type II & III Storm -20% of rainfall is before the peak
Geographical Distribution
Type II-70
Type I Storm -25% of rainfall is before the peak
Account for Inter-storm Duration (Drying Time)
1995 Rainfall: Chicago & Seattle
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
10/1/
1994
11/1/
1994
12/1/
1994
1/1/19
95
2/1/19
95
3/1/19
95
4/1/19
95
5/1/19
95
6/1/19
95
7/1/19
95
8/1/19
95
9/1/19
95
Dai
ly ra
infa
ll (in
)
-30.00
-20.00
-10.00
0.00
10.00
20.00
30.00
40.00
Cum
ulat
ive
rain
fall
(in)
Chicago - daily Seattle - daily Chicago - cumulative Seattle - cumulative
Account for Multitude of Green Roof Systems Seattle Green Roof Evaluation Project –
Sample of Moisture Budget Data
12
Model Verification Timeline
Modeling Tool Development
Begins(March-03)
Scandinavian Sustainability
Tour(March-04)
Evaluation Project
Installation(February-05)
Evaluation Project
Monitoring Complete
(October-06)
InitiateDiscussionwith City
City Declines
Participation
BerlinSustainability
Tour(May-05)
Field EvaluationApril 2004
Plot Analysis and Selection
Seattle Green Roof Evaluation Plot Evaluation Plot in Place
13
Green Roof Evaluation Program Goals
! Provide “hands-on” opportunities! Demonstrate performance and
gather local data! Advance A/E/C/regulatory industry
understanding of green roof implementation
Project Participants and Sites
Diverse Locations in Downtown Seattle
Rainier Tower – Podium at Urban High-Rise Stadium Center – Industrial Warehouse
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Vulcan – Parking Lot in Emerging Area Sellen – Low-Rise Corporate Headquarters
Project Configuration
Test Plot Components
Weather Station Weather Monitoring
Evapotranspiration EstimationWind Speed SensorWind Speed
Evapotranspiration EstimationTemperature/rH SensorTemperature
Evapotranspiration EstimationSilicon PyranometerSolar Radiation
Evapotranspiration EstimationTemperature/rH SensorRelative Humidity
Runoff EstimationTipping Bucket Rain GaugeRainfall
Data Used ForMeasurement DeviceWeather Parameter
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Runoff Monitor Runoff Monitoring
Evapotranspiration ComputationSoil Moisture SensorSoil Moisture
Runoff Temperature Character.Temperature Sensor Runoff Temperature
Runoff ComputationRestrictor Device & Water
Level SensorRunoff Rate
Data Used ForMeasurement DeviceParameter
Flow Measurement Device
Data Transmission
Implementation
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Stadium Center Site Rainier Tower Site
Vulcan Site Sellen Site
Findings to Date
Seattle Green Roof Evaluation Project –Storm Water Runoff Data
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Seattle Green Roof Evaluation Project –Storm Water Runoff Data Sellen Plant Progression
may june
july sept
Seattle Cistern Potential Rainfall Potential
Cistern Event Runoff Reduction PotentialCistern Runoff Reduction Potential
40
50
60
70
80
90
100
0 2 4 6 8 10
Storage (gal/SF roof)
Equi
vale
nt C
urve
Num
ber f
or R
oof
CN
Forest (till)
Forest (outwash)
Hardscape
Annual Benefit - Timing of Rain vs. Demand1995 Seattle Rainfall
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
10/1/
1994
11/1/
1994
12/1/
1994
1/1/19
95
2/1/19
95
3/1/19
95
4/1/19
95
5/1/19
95
6/1/19
95
7/1/19
95
8/1/19
95
9/1/19
95
Dai
ly ra
infa
ll (in
)
-30.00
-20.00
-10.00
0.00
10.00
20.00
30.00
40.00
Cum
ulat
ive
rain
fall
(in)
Seattle - daily Seattle - cumulative
IRRIGATION PERIOD
18
Example: Cistern Runoff Reduction Potential
Supply vs Demand2.5:1 catchment-to-irrigated
11% annual runoff volume retained & used
0
500
1000
1500
2000
2500
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Vol
ume
of W
ater
(cu.
ft.)
Rain Supply
Water Demand
IRRIGATION PERIOD
Example: Cistern Runoff Reduction Potential
Landscape & toilet flushing16.5:1 catchment-to-irrigated
27% annual runoff volume retained & used
-
2,000
4,000
6,000
8,000
10,000
12,000
14,000
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Vol
ume
of W
ater
(cu.
ft.)
Rain Supply
Water Demand
IRRIGATION PERIOD