1

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

3

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

5

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

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)

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)

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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

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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

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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

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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