Holistic Watershed Management for Existing and Future Land
Use Development Activities: Opportunities for Action for Local
Decision Makers: Phase 2 - Flow Duration Curve Application

Modeling (FDC 2A Project)

SUPPORT FOR SOUTHEAST NEW ENGLAND PROGRAM (SNEP)

communications strategy and technical assistance

Task 5: Opti-Tool Enhancements: Green Roofs and
Temporary Run-off Storage with IC Disconnection

March 17,2021

Prepared for:
U.S. EPA Region 1

Prepared by:

Paradigm Environmental

PARADIGM

ENVIRONMENTAL

Great Lakes Environmental Center

GleC

Blanket Purchase Agreement: BPA-68HE0118A0001-0003
Requisition Number: PR-R1-20-00322
Order: 68HE0121F0052


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Table of Contents

1.	Introduction	2

2.	Green Roof.	2

3.	IC Disconnection with storage	6

4.	IC Disconnection without Storage	11

5.	Summary	13

6.	References	14

Figures

Figure 2-1. Conceptual schematic of a typical green roof system as represented in the Opti-Tool	3

Figure 2-2. Green roof BMP setup windows within the Opti-Tool	5

Figure 3-1. Typical residential rain barrel application highlighting key geometry parameters	6

Figure 3-2. Typical cistern release curve used in Opti-Tool	7

Figure 3-3. Rain barrel setup window within the Opti-Tool	9

Figure 3-4. Cistern setup window within the Opti-Tool	10

Figure 3-5. Cistern release curve input window within the Opti-Tool	10

Figure 4-1. Impervious disconnection schematic	11

Figure 4-2. IC Disconnection BMP setup window within the Opti-Tool	12

Tables

Table 2-1. Suggested parameter values for representation of green roofs in Opti-Tool	3

Table 3-1. Suggested parameter values for representation of IC disconnect with rain barrel storage in Opti-
Tool	7

Table 3-2. Suggested parameter values for representation of IC disconnect with cistern storage in Opti-Tool

	8

Table 4-1. Suggested parameter values for representation of IC disconnect without storage in Opti-Tool. 12


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1. INTRODUCTION

This memorandum outlines four new green infrastructure stormwater control measures (GI SCM),
incorporated into the Opti-Tool to support management alternative analyses involving disconnection of
impervious cover (IC). The Opti-Tool was configured to simulate (1) green roof technologies (2) IC
disconnection with temporary runoff storage (i.e., rain barrels or cisterns), and (3) IC disconnection without
temporary storage. Schematics of these GI SCMs are presented to illustrate the key simulation processes.
This memorandum is organized into the following sections:

•	Green Roofs (Section 2)

•	IC Disconnection with Temporary Storage (Section 3)

•	IC Disconnection without Temporary Storage (Section 4)

2. GREEN ROOF

A green roof GI SCM was added as an option for modeling the conversion of conventional roofs to green
roofs as a method for impervious disconnection. Green roofs provide benefits including runoff peak and
volume reduction as well as water quality improvement under well-designed systems. They also produce
several other benefits beyond stormwater including improvement of building energy efficiency, creation of
quality habitat, and reduction in heat fluxes that mitigate the effects of urban heat islands. The Opti-Tool
green roof CSM focuses solely on modeling the stormwater processes and does not include explicit
representations of these secondary benefits. However, outputs from the Opti-Tool may be used to support
calculations outside of the model to estimate these secondary benefits.

Figure 2-1 presents a conceptual diagram of the key processes occurring with the green roof system. The
processes are grouped into four layers (far left) to better organize their relationship to the overall water budget
and make links to key parameters in the Opti-Tool. External water balance inputs and outputs occur on the
top vegetation layer. The growth layer represents the soil media in which the plants grow. This layer includes
the plant roots and has available pore storage for the retention of stormwater. The drainage layer represents
an underdrain system that ensures the roof can properly drain when the growth layer becomes saturated.
Impermeable roof decking sits below the drainage layer and acts as a barrier between the green roof and the
building below it. The roof deck is represented by a 0.0 inch/hour background infiltration rate which ensures
that all water exiting the drainage layer continues downstream as additional runoff.

Based on the schematic in Figure 2-1, Table 2-1 presents a summary of the default parameter values for
representing the key processes of green roof GI SCMs within Opti-Tool. The summary presents parameters
related to each of the key processes grouped by layer. The green roof setup windows within Opti-Tool are
shown in Figure 2-2. The Green Roof footprint can be optimized using the LENGTH as a decision variable
in Opti-Tool. The Opti-Tool does not simulate the rainfall but instead uses the pre-simulated runoff time
series as boundary conditions. To simulate the direct rainfall on the BMP footprint, the user has to add the
BMP surface area as an impervious HRU area to the BMP drainage area. For example, if 50% of the rooftop
area is converted to the Green Roof and the remaining 50% of the rooftop area is draining to the Green Roof
then the drainage area for the Green RoofwW be 100% of the rooftop area.

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2 Growth Media

3 Drainage Layer

4 Roof Deck

*Energy is
not explicitly
modeled in SUSTAIN

1 Vegetation

Water Balance

Figure 2-1. Conceptual schematic of a typical greeri roof system as represented in the Opti-Tool.
Table 2-1. Suggested parameter values for representation of green roofs in Opti-Tool

Layer Parameter

Description

Value(s)

Units



FOOTPRINT

Roof area available

Varies

ft2

1.

Vegetation

AVEG

Holtan Vegetative Parameter

1

-

Gl

Holtan Growth Index

Seasonal
(0.1-1.0)

--



ETMULT

ET Multiplier1

1.0

-



WEIRH

Weir Height for Overflow

0.1

ft

2.

Growth
Media

SDEPTH

Soil Media Depth 2

1

ft

POROSITY

Media Porosity 3

78

%

FINFILT

Media Infiltration Rate 3

51.8

in/hr

3.

UDDEPTH

Drainage Depth 4

0.1

ft

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Layer

Parameter

Description

Value(s)

Units

Drainage
Layer

VOID

Void Space 5

50

%

QUALPCTREM

Treatment Efficiency (TSS)6

88.7

%

Treatment Efficiency (TN)6

55

%

Treatment Efficiency (TP)6

-248

%

Treatment Efficiency (TZn)6

70

%

Treatment Efficiency (£. coli)6

10.8

%

4.
Roof
Deck

INFILT

Native soil infiltration

0.0

In/hr

-

COST

Unit-volume cost of Green Roof7

70

$/ft3

1.	Recommended Penman Monteith-based ASCE standard reference ET equation (Marasco et al., 2014).

2.	Soil depths can vary between intensive and extensive applications (Razzaghmanesh & Beecham, 2014).

3.	Media porosity and infiltration rate (Omni Ecosystems Green Roof).

4.	Drainage layer depth for extensive green roof application

5.	Typical void space in the drainage media (Baryta et al., 2018).

6.	Treatment efficiency is reported from a monitoring study on two surfaces of a roof in Toronto. The conventional surface is a
131 m2 shingled, modified bitumen roof. The 241 m2 green roof is vegetated with wildflowers. The 14cm growth medium is
composed of crushed volcanic rock, compost, blonde peat, cooked clay, and washed sand (Seters et al., 2009).

7.	Average cost based on Omni Ecosystems Green Roof pricing matrix, 2018.

Infiltration through the growth media is governed by the Holtan infiltration method as described in the
following equation (USEPA 2009, USEPA 2012):

F = GI * AVEG * (Soil Media Storage)A1.4 + FINFILT

Where:

F is the final infiltration rate in inches per hour
GI is the growth index that varies monthly
A VEG is the Vegetative A parameter
FINFILT is the saturated infiltration rate in inches
Soil Media Storage is the available storage in inches

per hour, and

computed at each model time step

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BMP Dimensions j Substrate Properties | Water Quality Parameters and Cost Function |
[— General Information -
BMP Name

BMP1

BMP Type
Aquifer ID

0

GREENROOF

U

"31

(— Sub watershed Information
BMP Location	| Junctionl

Downstream Junction or
BMP

3

| Junction 1 ~^]

Specify BMP Drainage Area

Basin Dimensions

| BMP Length (ft.)

1 500

Decision Variable J

BMP Width (ft.)

1 10



- Exit Type (Orifice Discharge Coefficient) -

&









r i.o

C 0.61

r 0.5

— Surface Storage Configuration -

Orifice Height (ft.)

Orifice Diameter (m.)

- Weir Configuration -
(* Rectangular Weir

Weir Height (ft.)

Crest Width (ft.)

C Triangular Weir

\Ti

Best Management Practices

BMP Dimensions j' Subsfrate PropertieF]! Water Quality Parameters and Cost Function |

vey. parameter







(A) 	^ jrnL











Ds

soil porosity

^ "'T . •• *•,













soil f „
c

J /

1 ' : ' :

4 , X



Du



y y H ^1 -













( /





* , h^rktjrmirid f



| 0.71

Soil Properties —
DS - Depth of Soil (ft)

Soil Porosity (0-1)
Vegetative Parameter A
FC - Soil Infiltration (in/hr)

If underdrain is off, then the FC - Soil Infiltration (in/hr)
is the FC - Background Infiltration On/hr).

- Underdrain Properties —

W Condiser Underdrain Structure? Update Eff. Depth
DU - Storage Depth (ft)

Media Void Fraction (0-1)





FC - Background Infiltration
(in/hr)

Figure 2-2. Green roof BMP setup windows within the Opti-Tool.

Default Parameters

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3. IC DISCONNECTION WITH STORAGE

To model IC disconnection where temporary storage is available, rain barrel and cistern BMPs were added
to the Opti-Tool. These BMPs include a basic tank volume for the storage of stormwater, IC runoff from
roofs, or other impervious surfaces, when possible, are routed to the tank. During runoff events, available
storage within the tank will be filled; if the tank storage is exceeded, the additional volume is routed
downstream as untreated runoff. The difference between rain barrels and cisterns within Opti-Tool is based
on how the stored water is consumed. For rain barrels, the stored water is drained to represent non-portable
water use after a preset user-defined duration of dry days. Stored water within cisterns is drained based on a
user-defined per-capita consumption curve and the number of people. These uses can be routed out of the
system to represent a consumptive use and do not contribute to the untreated runoff volume.

IC disconnection simulates the storage and processes associated with a rain barrel and cistern features which
typically receive runoff from residential roofs. Key process parameters include the volume of the available
storage, the height of the tank, the size of the contributing area that the storage is designed to receive, and
parameters related to the draw-down and usage of captured water including the hose orifice height and
diameter. Draw-down of captured water is necessary to ensure that storage is again made available to capture
subsequent storms. These BMPs are assumed to be fully enclosed and, therefore, do not simulate
evapotranspiration (ET). A typical residential rain barrel application highlighting some key geometric
parameters is presented in Figure 3-1. A typical cistern release curve is shown in Figure 3-2.

Table 3-1 and Table 3-2 present fee proposed default values for IC disconnection using rain barrels and
cisterns, respectively, as temporary storage, including the geometry parameters highlighted in Figure 3-1.
Typically, costs associated with implementing rain barrels include the cost of the actual rain barrel and any
costs needed for proper installation. Since residential rain barrels are often installed by the homeowner or
through community-group or local government outreach programs, including installation costs is likely not
necessary. The rain barrel and cistern setup windows within the Opti-Tool are shown in Figure 3-3 and
Figure 3-4. The cistern's default release curve values can be updated through the user interface shown in
Figure 3-5.

:igure 3-1. Typical residential rain barrel application highlighting key geometry parameters.

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0.3 |
0.25

Q.

03

0.2

-C

m""

~ 0.15

o
+-»

Q_

E 0.1

d

to

c

8 0.05

0 1



































































































































































































































































































L 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Hour

Figure 3-2. Typical cistern release curve used in Opti-Tool.

Table 3-1. Suggested parameter values for representation of IC disconnect with rain barrel storage in Opti-Tool

Parameter

Description

Default
Value

Units

VOLUME

Storage volume of rain barrel

50

gallons

WEIRH

Total height of rain barrel

2.5

feet

DIAM

Diameter of the release orifice

1

inches

OHEIGHT

Height of the release orifice

0.5

feet

DDAREA

Maximum designed contributing area

Varies based on the
rain barrel capacity

acres

DDAYS

Number of dry days before release begins

3

days

ETMULT

ET Multiplier

0.0

-

DECAY

Pollutant Decay Rate (TSS)

0.0

1/day

Pollutant Decay Rate (TN)

0.0

1/day

Pollutant Decay Rate (TP)

0.0

1/day

Pollutant Decay Rate (TZn)

0.0

1/day

Pollutant Decay Rate (£. coli)

0.0

1/day

NUMUNIT1

Number of Rain Barrels

Varies

-

COST2

Cost of Rain Barrel + Disconnection

216

$/unit (50 gallons)

1.	The number of rain barrels (NUMUNIT) is a decision variable.

2.	Unit Cost ($/50 gallon) (Ref: https://www.lid-stormwater.net/raincist_cost.htm).

Rain barrels normally are not designed to meet the water quality objectives, so the pollutant decay rates
default to zero as a placeholder in the Opti-Tool and those can be easily updated if the observed data is
available.

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Table 3-2. Suggested parameter values for representation of IC disconnect with cistern storage in Opti-Tool

Parameter

Description

Default
Value

Units

VOLUME

Storage volume of the cistern

200

gallons

WEIRH

Total height of the cistern

4

feet

DIAM

Diameter of the release orifice

1

inches

OHEIGHT

Height of the release orifice

0.5

feet

DDAREA

Maximum designed contributing area

Varies based on the
cistern capacity

acres

PEOPLE

Number of people served by a cistern

3

-

ETMULT

ET Multiplier

0.0

-

DECAY

Pollutant Decay Rate (TSS)

0.0

1/day

Pollutant Decay Rate (TN)

0.0

1/day

Pollutant Decay Rate (TP)

0.0

1/day

Pollutant Decay Rate (TZn)

0.0

1/day

Pollutant Decay Rate (£. coli)

0.0

1/day

NUMUNIT1

Number of cisterns

Varies

-

COST2

Cost of cistern + Disconnection

225

$/unit (200 gallons)

1.	The number of cisterns (NUMUNIT) is a decision variable.

2.	Unit Cost ($/200 gallon) (Ref: https://www.lid-stormwater.net/raincist_cost.htm).

Cisterns normally are not designed to meet the water quality objectives, so the pollutant decay rates default
to zero as a placeholder in the Opti-Tool and those can be easily updated if the observed data is available.

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BMP Dimensions j Substrate Properties | Water Quality Parameters and Cost Function |

f— General Information	

BMP Name

BMP Type

BMP1

RAINBARREL

Aquifer ID

31

"31

r~ Basin Dimensions -
Diameter (ft.)



Decision Variable

— Surface Storage Configuration -

Orifice Height (ft.)
| 0.5

Orifice Release

Orifice Diameter (m.)

- Sub watershed Information
BMP Location	| Junctionl

Downstream Junction or
BMP



Junction 1

Specify BMP Drainage Area

- Exit Type (Orifice Discharge Coefficient)

- Weir Configuration -

(* Rectangular Weir	C Triangular Weir

Weir Height (ft.)
Crest Width (ft.)



Release Options -

Number of Dry Days

Release Curve

Figure 3-3. Rain barrel setup window within the Opti-Tool.

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BMP Dimensions j Substrate Properties | Water Quality Parameters and Cost Function |

f— General Information	

BMP Name

BMP Type

BMP1

CISTERN

Aquifer ID

3

"31

- Surface Storage Configuration -

Orifice Release

Orifice Height (ft.)
| 0.5

Orifice Diameter (m.)

- Sub watershed Information
BMP Location	| Junctionl

Downstream Junction or
BMP



| Junction 1

| Specify BMP Drainage Area

Basin Dimensions





Diameter (ft.)

1 3



1 Number of Units 1

h

Decision Variable







- Exit Type (Orifice Discharge Coefficient)

- Weir Configuration -

(* Rectangular Weir	C Triangular Weir

Weir Height (ft.)
Crest Width (ft.)

Release Options



Number of People

I 3



| Release Curve



Figure 3-4. Cistern setup window within the Opti-Tool

Cistern Release

Hour 1
Hour 2
Hour 3
Hour 4
Hour 5
Hour 6
Hour 7
Hour 8

X

Hourly water release per capita from the Cistern Control (ft^3/hr/capita)
Hour 9	Hour 17

Hour 9
Hour 10
Hour 11
Hour 12
Hour 13
Hour 14
Hour 15
Hour 16

Hour 18
Hour 19
Hour 20
Hour 21
Hour 22
Hour 23
Hour 24

Figure 3-5. Cistern release curve input window within the Opti-Tool.

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4. IC DISCONNECTION WITHOUT STORAGE	

To model IC disconnection where temporary storage is not used, routing over a pervious land segment was
added as a BMP to the Opti-Tool. A nonlinear reservoir routing algorithm is applied to route the surface
runoff from the impervious cover to the pervious area where depression storage and infiltration reduce the
volume of runoff contributed by the impervious cover.

This approach approximates reality where the runoff from the impervious area is routed to and simulated
on the pervious area. The runoff from the disconnected impervious area is captured by the modeled pervious
land through infiltration and surface storage. The pervious land does not simulate additional ET since
surface runoff already accounts for it in the HRU time series as boundary conditions. Surface runoff occurs
only when the surface water depth exceeds the maximum surface storage depth, where surface runoff is
calculated using Manning's equation and infiltration rate is calculated using the Horton infiltration method.

Figure 4-1 presents a plan view schematic of impervious area routing to an adjacent pervious area where key
overland flow parameters are used to simulate storage and infiltration processes. Table 4-1 presents the
recommended default values for representing the pervious land segment used for disconnecting IC.
Typically, the cost associated with implementing IC disconnection to pervious land is limited only to the
physical disconnection process from the MS4. Since homeowners can typically handle this process with
limited technical guidance, these capital costs may be negligible. However, the cost associated with
conveying the runoff from the impervious cover as a sheet flow or a level spreader could be expressed as a
per linear foot (LENGTH) of pervious area perpendicular to the flow path as shown in Figure 4-1. Figure
4-2 shows the IC Disconnection BMP setup window within the Opti-Tool.



Drainage
Impervious Cover
(IC)

Pervious Area
Receives Runoff
from IC

WIDTH

Figure 4-1. Impervious disconnection schematic

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Table 4-1. Suggested parameter values for representation of IC disconnect without storage in Opti-Tool

Parameter

Description

Default
Value

Units

LENGTH 1

Length of the pervious area over which flow
occurs (perpendicular to the flow path)

Varies

feet

WIDTH

Width of the pervious area over which flow
occurs (flow length)

Varies

feet

DSTORAGE

Surface depression storage of the pervious
area

0.15

inches

SLOPE

Overland slope of the pervious area

0.01

feet/feet

MANNING_N

Overland Manning's roughness coefficient

0.13

-

SATJNFILT

Saturated infiltration rate

Varies based on the
hydrologic soil group

inch/hour

DECAY

Pollutant Decay Rate (TSS)

0.0

1/day

Pollutant Decay Rate (TN)

0.0

1/day

Pollutant Decay Rate (TP)

0.0

1/day

Pollutant Decay Rate (TZn)

0.0

1/day

Pollutant Decay Rate (£. coli)

0.0

1/day

COST

Cost per foot of pervious length

?

$/ft

1. The LENGTH is the decision variable.

Best Management Practices

BMP Dimensions I Substrate Properties | Water Quality Parameters and Cost Function |
|— General Information -
BMP Name

BMP1

BMP Type
Aquifer ID

0

ICDISCONNECTION

D

T3

Surface Storage Configuration -

Drainage
Impervious Cover
(IC)

Pervious Area
Receives Runoff
from IC

Manning's N
| 0.13

Slope

|— Sub watershed Information	

BMP Location	| junction 1 ^

Downstream Junction or |	,

BMP	| Junction 1 H

Specify BMP Drainage Area

Basin Dimensions

| BMP Length (ft.)

I 100

Decision Variable J

BMP Width (ft.)

I 30



• Weir Configuration —

(* Rectangular Weir	C Triangular Weir

Depression Storage On.)	| o7l5

Figure 4-2. IC Disconnection BMP setup window within the Opti-Tool.

Default Parameters

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5. SUMMARY

Adding the four GI SMCs described in the previous sections provides a powerful new suite of modeling
options within the Opti-Tool framework. The IC disconnection with and without storage is particularly
attractive from a cost and programmatic standpoint as these types of initiatives require only basic technical
knowledge and funding, meaning that homeowners are often able to implement them with little or no expert
guidance. The parameters presented in the tables represent basic, midrange default values consistent with
the Region 1 climate and landscape. Parameters that are highly site-specific, such as slope and infiltrate rate,
can and should be adjusted as necessary for each modeling application. The Opti-Tool does not simulate the
rainfall but instead uses the pre-simulated runoff time series as boundary conditions. To simulate the direct
rainfall on the BMP footprint, the user has to add the BMP surface area as an impervious HRU area to the
BMP drainage area. For example, if 50% of the rooftop area is converted to the Green Roofmd the remaining
50% of the rooftop area is draining to the Green Roof then the drainage area for the Green Roof will be 100%
of the rooftop area. Similarly, IC disconnection to the pervious area has a significant footprint, so to account
for the direct precipitation on the pervious area, an equal amount of impervious HRU area should be added
to the drainage area. For example, if 0.5 acres of IC is disconnected and routed to 1.0 acres of pervious area
then 1.5 acres of IC should be used as drainage area to the pervious area.

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6. REFERENCES

Baryta, A., Karczmarczyk, A., Brandyk, A., & Bus, A. (2018). The influence of a green roof drainage layer
on retention capacity and leakage quality. Water Science and Technology, 77(12), 2886-2895.
https://doi.org/10.2166/wst.2018.283

Marasco, D. E., Hunter, B. N., Culligan, P. J., Gaffin, S. R., & McGillis, W. R. (2014). Quantifying

evapotranspiration from urban green roofs: A comparison of chamber measurements with commonly
used predictive methods. Environmental Science and Technology, 48(17), 10273-10281.
https://doi.org/10.1021/es501699h

Razzaghmanesh, M., & Beecham, S. (2014). The hydrological behaviour of extensive and intensive green
roofs in a dry climate. Science of the Total Environment, 499(1), 284-296.
https://doi.Org/10.1016/j.scitotenv.2014.08.046

Seters, T. van, Rocha, L., Smith, D., & Macmillan, G. (2009). Evaluation of Green Roofs for Runoff Retention,
Runoff Quality, and Leachability.

USEPA (United States Environmental Protection Agency). 2012. Report on Enhanced Framework

(SUSTAIN) and Field Applications to Placement ofBMPs in Urban Watersheds. U.S. Environmental
Protection Agency, Washington, DC, EPA/600/R-11/144, 2012.

USEPA (United States Environmental Protection Agency). 2009. SUSTAIN - A Framework for Placement of
Best Management Practices in Urban Watersheds to Protect Water Quality. U.S. Environmental Protection
Agency, Washington, DC, EPA/600/R-09/095, 2009.

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