Tisbury MA Impervious Cover Disconnection (ICD) Project an Integrated Stormwater Management Approach for Promoting Urban Community Sustainability and Resilience a Technical Direct Assistance Project Funded by the U.S. EPA Southern New England Program (SNEP) Task 4c. Opti-tool Application for Two Pilot Drainage Areas (Outfall #2 and #7) to Evaluate Source Area Contributions and GI SCM Reduction Benefits


Tisbury MA Impervious Cover Disconnection (ICD) Project: An Integrated Stormwater
Management Approach for Promoting Urban Community Sustainability and Resilience

A TECHNICAL DIRECT ASSISTANCE PROJECT FUNDED BY THE U.S. EPA SOUTHERN NEW ENGLAND

Program (SNEP)

Task 4C. Opti-Tool Application for Two Pilot Drainage Areas (Outfall #2 and #7) to
Evaluate Source Area Contributions and GISCM Reduction Benefits

U. S. EPA Region 1 Tisbury MA

Prepared for:

Martha's Vineyard Commission

MassDOT



MARTHA'S VINEYARD

COMMISSION

mass DOT

Massachusetts Department of Transportation

Paradigm Environmental

Prepared by:
UNH Stormwater Center

Great Lakes Environmental Center

PARADIGM

ENVIRONMENTAL

sc

STORMWATER CENTER

GleC

Blanket Purchase Agreement: BPA-68HE0118A0001-0003
Requisition Number: PR-R1-18-00375
Order: 68HE0118F0011

1

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CC:

Date:

Re:

To:
From:

Ray Cody, Mark Voorhees (US EPA Region 1)

Khalid Alvi, David Rosa, Ryan Murphy (Paradigm Environmental)

Project Technical Team

2/19/2020

Opti-Tool Application for Two Pilot Drainage Areas (Outfall #2 and #7) to Evaluate
Source Area Contributions and Green Infrastructure (GI) and Stormwater Control
Measure (SCM) Benefits (Task 4C)

1 EXECUTIVE SUMMARY

This memorandum presents the technical approach for the application of Opti-Tool (U.S. EPA, 2016) to the
evaluation of the stormwater quantity and quality at two outfalls in Tisbury, MA under existing conditions
and the expected benefits of implementing Green Infrastructure (GI) and Stormwater Control Measures
(SCM) in the outfalls' drainage areas. The approach is supported by a rainfall analysis that assessed the
number of discharge-producing days that could be eliminated by capturing and infiltrating surface runoff
through implementing GI SCM opportunities for a range of storm sizes. The study demonstrates that
distributed GI SCM practices can provide cost-effective solutions that achieve volume and load reduction
targets while also effectively integrating within urbanized landscapes. An analytical framework and
summary metrics are provided which can be readily customized and applied in other settings to inform
stormwater management planning efforts. A comparison of flow volumes, flow duration curves, and total
nitrogen (TN) loads delivered at the two selected outfall locations before and after the implementation of GI
SCM opportunities is presented. Cost-effectiveness curves are provided to visualize the level of investment
needed to obtain a range of flow volume and TN load reductions. Summary tables present the optimal level
of SCM implementation for various land uses.

Summarized study results are presented in Table 1. The results suggest that GI SCM practices can infiltrate
approximately 50.7 million gallons of stormwater volume within the combined catchments of outfall #2 and
#7 (129 acres) if sizing those infiltration practices to capture 0.35 inches of runoff from the impervious cover.
This equates to an 80% reduction in annual stormwater volume compared to existing, baseline conditions.
The total estimated cost to achieve this overall reduction in both outfalls was approximately $1,160,000.
This cost represents an optimization goal of reducing stormwater volume. The solution would achieve a co-
benefit of approximately 90% reduction in TN. Additionally, assuming a moderate infiltration rate of 1.02
in/hr (U.S. EPA, 2019a), the optimized solution would also result in a 62%-75% reduction in average annual
bacteria loading. The estimated cost for flow volume reduction was $0.02 per gallon for both outfalls. The
implementation of GI SCM practices was also optimized for TN reduction, a target solution that achieved
a 91% reduction in loading would also have the co-benefit of achieving an 80% reduction in annual
stormwater volume. The estimated cost to achieve this overall reduction in both outfalls was approximately
$1,174,000. Cost estimates assume no cost-sharing or use of town labor and equipment, which could help
lower costs. The costs for TN load reduction varied by outlet, the cost for removing a pound of TN was
between $1,700 and $2,000. While actual costs may vary depending on local conditions, the cost estimates
provide a useful comparison of relative differences in optimization scenarios. Overall, it appears that an
optimized solution that focuses on either stormwater volume or TN load reduction can achieve similar
reductions for both benefits for approximately the same costs.

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Table 1 Summary of Analyses Results for Tisbury, MA Outlets #2 and #7,



Outfall #2

Outfall #7

Baseline Average Flow Volume (gallons/yr)

23,193,061

40,174,307

Baseline Average TN Load (Ibs/yr)

261.87

420.63

Flow Volume Removed (gallons/yr)

18,551,813

32,192,534

TN Load Removed (Ibs/yr)

233.27

386.14

Cost per Gallon Flow Removed ($)

$0.02

$0.02

Cost per Pound TN Removed ($)

$1,727

$1,996

Total Cost

$406,122

$753,076

Strategically optimizing the selection and placement of distributed SCMs within highly urbanized settings
through continuous simulation can help to develop management strategies that are more cost-effective than
the traditional approach of sizing SCMs at fixed locations to treat a design storm. The flood mitigation
benefits of GI SCM are especially valuable in urbanized areas with poor stormwater transmission where
even relatively small storms can result in flooding. The relatively small size of distributed GI facilities
substantially increases the feasibility of treating runoff from impervious surfaces in constrained developed
spaces and achieving meaningful water quantity and quality benefits. This application of Opti-Tool
demonstrates that relatively small GI facilities and SCMs can provide a cost-effective stormwater
management approach in an opportunity-limited, urban setting like Tisbury, MA. Additionally, this study
highlights the value of conducting strategic planning to address stormwater impacts for achieving multiple
water resource goals. The results of this study are based on an assessment of a twenty-one-year time series
of simulated overland flow. The modeling focused on watershed-scale hydrologic processes including the
conversion of rainfall to runoff and the capture and infiltration of that runoff. The modeling did not include
an explicit representation of Tisbury's stormwater conveyance network, therefore hydraulic processes such
as transportation losses and pipe surcharge are not simulated. Despite these limitations, the modeling
provides valuable insight into the existing conditions in Tisbury and the potential benefit of GI SCM
opportunities.

2 RAINFALL ANALYSIS FOR TISBURY GAUGE

Green infrastructure and SCM opportunities can be built to capture a range of storm sizes. Prior to running
an Opti-Tool-based optimization, a simplified, spreadsheet-based analysis was conducted to assess the
potential benefits of implementing GI SCM opportunities over a range of sizes designed to capture runoff
depths ranging from 0.1 to 2.0 inches.

A twenty-one year (Jan 1998 - Dec 2018) hourly precipitation timeseries was analyzed to determine the
average annual number of daily precipitation events and their respective depths in order to assess the benefits
of implementing GI SCM opportunities of various sizes. A dry year (year-2001), wet year (year-2018), and
an average year (year-2012) were also estimated based on the total precipitation and the number of rain days
(Table 2). Based on the analysis, an event exceeding 1.5 inches in 24 hours is very likely to occur in Tisbury,
MA in any given year (Figure 1). A less frequent event, one which exceeds 4.2 inches, has an approximately
10% chance of occurring in any given year. While these numbers represent probabilities for annual
maximum 24-hr rainfall, surprisingly, over 50% of total annual precipitation events (24-hr rainfall) in Tisbury
are 0.1 inches or less in-depth (T able 3). F igure 2 shows the number of precipitation days that can be captured
by implementing infiltration GI SCM opportunities over a range of sizes. Since over 50% of annual events
are 0.1 inches or less in-depth, sizing infiltration GI SCM opportunities throughout the community to
capture 0.1 inches of runoff can be expected to reduce the number of discharge days by the same amount.

The rainfall analysis provides important results at the conceptual level that highlight the benefit of
implementing small, distributed GI SCM. The analysis is especially applicable in communities where
occurrences of flooding, algal blooms, and bacteria-related beach closings may occur multiple times a year.
For Tisbury, implementing relatively small infiltration systems designed to capture 0.2 inches is estimated

3

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Table 2. Number of rain days, maximum daily, and annual rainfall depth for 21 years (1998-2018) in Tisbury, MA.

Year

Total Rainfall
(in./year)

Maximum Rainfall
(in./day)

No. of Rain Days

1998

44.5

2.54

145

1999

36.3

2.57

133

2000

39.9

2.81

153

2001

27.5

2.65

147

2002

40.1

1.51

153

2003

41.5

3.40

135

2004

37.3

3.84

129

2005

42.6

3.03

132

2006

43.4

4.16

141

2007

33.7

2.12

135

2008

39.2

2.43

138

2009

42.8

3.70

143

2010

46.3

4.18

119

2011

43.8

4.26

133

2012

40.5

2.33

137

2013

40.4

1.92

147

2014

40.3

2.27

122

2015

37.5

2.71

115

2016

30.8

2.02

100

2017

46.5

3.05

133

2018

51.8

3.13

137

Long-Term Average:

40.3

2.88

134

1 I Dry Year 1 1 Average Year 1 1 Wet Year



4.50



4.00



3.50



3.00

c





2.50







2.00

'ra



01



1.50



1.00



0.50



0.00



























• •• a







































0%	20%	40%	60%	80%	100%

Exceedance probability {%)

Figure 1. Exceedance probability for maximum daily rainfall depths for 21 years (1998-2018) in Tisbury, MA.

4

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Table 3. The number of storms captured/retained and percent of discharge days eliminated with infiltration SCMs of
various sizes.

Infiltration SCM Size to Capture
Runoff Depth from Impervious
Surfaces (in.)

Captured Number of 24-hour
Storms (per year)

% Number of Discharge Days
Eliminated (per year)

0.1

73

54%

0.2

88

66%

0.3

98

73%

0.4

105

78%

0.5

110

82%

0.6

114

85%

0.7

118

88%

0.8

120

90%

0.9

123

92%

1.0

125

93%

1.1

126

94%

1.2

128

96%

1.3

129

96%

1.4

129

96%

1.5

130

97%

1.6

131

98%

1.7

131

98%

1.8

132

99%

1.9

132

99%

2.0

132

99%

5

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100%
90%
g 80%

c
o

70%
* 60%

aj

0 50%

O)

aj

S 40%
b

2 30%

aj
.Q

E

^ 20%
10%
0%

0



























































































































0 0.5 1.0 1.5 2.0 2.5
BMP Size as Runoff Depth (in.) from Impervious Cover

Figure 2. Comparison of SCM size to percent number of discharge days reduction.

6

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to eliminate 66% of the days that would have otherwise resulted in stormwater discharge. The results also
provide a strong foundation on which additional analyses using Opti-Tool optimization and continuous
simulation can provide further insights into the benefits of GI SCM implementation.

3 OUTFALLS (#2 AND #7) CATCHMENTS CHARACTERISTICS	

The study areas were adjacent catchments draining to two stormwater outfalls (Figure 3) in the town of
Tisbury MA. The sub-catchments to each catch basin within the study area were auto-delineated using 1-
meter high-resolution elevation data in ArcGIS software (Figure 4). The outfalls are located off the shore of
the municipality. The catchments varied in size and land cover. The area distribution for Hydrologic
Response Units (HRUs), unique land segments with an attribute of land use, land cover, soil, and slope
combinations, in these two catchments is shown in Table 4. The catchment draining to outfall #2 was
approximately 66% impervious surfaces, while the larger catchment draining to outfall #7 was
approximately 40% impervious surfaces (Table 5). The previously completed Task 4B memo (U.S. EPA,
2019b) provides a detailed discussion on the development of Hydrologic Response Units (HRUs) for the
area, including summary figures of hydrologic soil groups, land use, land cover, and slope in the area.

Table 4. HRU area distribution in drainage catchments to selected two outfall locations.





Catchment Area (acres)

HRU

Land Use

Catchment
#2

Catchment
#7

Total

1001

Forest

0.060

1.611

1.670

2001

Agriculture

-

-

-

3001

Commercial

17.973

11.378

29.352

4001

Industrial

-

-

-

5001

Low Density Residential

-

-

-

6001

Medium Density Residential

2.010

21.760

23.770

7001

High Density Residential

0.918

0.900

1.818

8001

Transportation

0.473

1.937

2.410

9001

Open Land

0.002

0.449

0.451

11110

Developed Pervious-A-Low

1.118

18.037

19.156

11120

Developed Pervious-A-Med

2.336

22.628

24.963

11130

Developed Pervious-A-High

0.935

6.880

7.814

11210

Developed Pervious-B-Low

-

-

-

11220

Developed Pervious-B-Med

-

-

-

11230

Developed Pervious-B-High

-

-

-

11310

Developed Pervious-C-Low

4.390

0.476

4.866

11320

Developed Pervious-C-Med

1.634

0.490

2.123

11330

Developed Pervious-C-High

0.429

0.115

0.545

11410

Developed Pervious-D-Low

0.000

0.006

0.006

11420

Developed Pervious-D-Med

-

0.033

0.033

11430

Developed Pervious-D-High

-

0.010

0.010

12110

Forest Pervious-A-Low

0.069

3.402

3.470

12120

Forest Pervious-A-Med

0.116

4.480

4.596

12130

Forest Pervious-A-High

0.079

1.745

1.824

12210

Forest Pervious-B-Low

0.020

-

0.020

12220

Forest Pervious-B-Med

0.012

-

0.012

7

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Catchment Area (acres)

HRU

Land Use

Catchment
#2

Catchment
#7

Total

12230

Forest Pervious-B-High

-

-

-

13110

Agriculture Pervious-A-Low

-

-

-

13120

Agriculture Pervious-A-Med

-

-

-

13130

Agriculture Pervious-A-High

-

-

-

13210

Agriculture Pervious-B-Low

-

-

-

13220

Agriculture Pervious-B-Med

-

-

-

13230

Agriculture Pervious-B-High

-

-

-

Total Area

32.573

96.336

128.908

Table 5. Pervious and impervious areas for catchments draining to outfalls #2 and #7



Total Area
(acres)

Impervious Area (acres)

Pervious Area
(acres)



Roofs

Other
Impervious

Total
Impervious

Outfall #2

32.6

6.2



15.2

21.4

11.1

Catchment

(19.0%)



(46.8%)

(65.8%)

(34.2%)

Outfall #7

96.3

12.4



25.6

38.0

58.3

Catchment

(12.9%)



(26.6%)

(39.5%)

(60.5%)

8

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^Outfall

^Outfall #2
5JP

Outlet #2 catchment area
Outlet #7 catchment area

Legend

* Storm drain outfall

#7

—	Outlet #2 storm drains

—	Outlet #7 storm drains

Figure 3.Storm drains, outfalls, and catchment areas

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Legend

Storm drain outfall

— Outlet #2 storm drains
Outlet #7 storm drains
I I Catchment boundaries

I I Subcatchment boundaries

Land Use

Residential
Industrial
Commercial
Agriculture

Developed Pervious
Forest

Transportation
Open Land
Water

Figure 4. Sub-catchment delineation and major land uses in the drainage areas to outfall #2 and #7.

10

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4 TECHNICAL APPROACH

The Opti-Tool provides the ability to evaluate options for determining the best mix of GISCM opportunities
to achieve water quantity and quality goals. The tool incorporates long-term runoff responses in the form of
HRU timeseries for regional climate conditions that are calibrated to regionally representative stormwater
data and annual average pollutant load export rates from nine major land uses. The tool uses regionally
representative SCM cost functions and regionally calibrated SCM performance parameters for various
pollutants, including total nitrogen (TN), to calculate long-term cumulative load reductions for a variety of
structural controls. Green infrastructure and SCMs simulated by the tool include infiltration systems, bio-
filtration, and gravel wetlands.

The technical approach for applying the Opti-Tool is organized into three general steps:

1.	Develop stormwater management categories for SCMs known to be highly effective at reducing
storm flows and removing nitrogen (e.g., shallow filtration, infiltration, biofiltration) based on the
site suitability analysis of GIS layers;

2.	Estimate the available opportunity by SCM type (i.e., physical footprint area) within each
management category and summarize the upstream impervious drainage area that can be managed
for each management category, and

3.	Set up and run the Opti-Tool application to identify the most cost-effective combination of SCM
options that achieve the desired management objectives.

4.1 Stormwater Management Categories

Spatial data analyses were previously conducted (U.S. EPA, 2018) to characterize watershed features and
identify the corresponding stormwater management categories that were suitable for application with the
Opti-Tool for the two outfall catchments. The GIS data used for the evaluation of stormwater management
categories for the Tisbury catchments included: land use coverage, impervious cover, Hydrologic Soil Group
(HSG), and LiDAR-derived Digital Elevation Model (DEM) for ground slopes. All data are from
Massachusetts GIS (MassGIS) data layers.

Table 6, previously presented in U.S. EPA (2018), presents the siting criteria used for all potential GI SCM
opportunities in Tisbury, which were derived from GIS analysis. Based on the dominant HSG of'A' within
the two catchments, the assessed GI SCM opportunities all fell under the "infiltration" management
category (Figure 5). For this pilot study, it was assumed that rooftops could be disconnected by redirecting
their runoff to infiltrations trenches, while all other types of impervious areas, such as roads and driveways,
could be disconnected by diverting their runoff to infiltration basins. Both public and private property were
assumed to be available for GIS SCM implementation.

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Table 6. Potential stormwater management categories and SCM types in the Opti-Tool

Land
Use

Landscape
Slope (%)

Within
100 feet of
Coastline?

Within
25 feet of
Structure?

S
Gr



Management
Category

SCM Type(s) in
Opti-Tool

Pervious
Area

<= 15

Yes

Yes

All

Less likely for
onsite SCM

-

No

No

A/B/C

Infiltration

Surface
Infiltration Basin
(e.g., Rain
Garden)

D

Biofiltration

Biofiltration (e.g.,

Enhanced
Bioretention with
ISR and
underdrain
option)

> 15

-

-

-

Less likely for
onsite SCM

-

Impervious
Area

<= 5

Yes

Yes

All

Less likely for
onsite SCM

-

No

No

A/B/C

Infiltration

Infiltration Trench

D

Shallow filtration

Porous Pavement

>5

-

-

-

Less likely for
onsite SCM

-

12

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Legend

0 Storm drain outfall 	Outlet #2 storm drains

[ I Infiltration opportunities Outlet #7 storm drains

¦i Outlet #2 catchment area
Outlet #7 catchment area

Outfall #7

—.Outfall #2

Figure 5. Infiltration-based Gl SCM opportunities in the two outfall catchments.

13

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4.2 Estimating SCM Footprints and Drainage Treatment Areas

The distribution of the SCM opportunity areas (i.e., SCM footprints) was estimated by land use category
group. This distribution represents the maximum available SCM footprint in the pilot watersheds, based on
GIS spatial data analysis, and does not necessarily represent the feasibility of such opportunity areas. The
treated impervious areas by land use group were split into two categories; roofs and others (Table 7). The
total drainage treatment area was 59 Acres of impervious surface, this represents all impervious surfaces in
the study catchment (Table 5). While all impervious surfaces were routed to an SCM, treatment was
contingent on the SCM size. For this case study, the maximum SCM footprints that could be considered
during optimization were limited the capture up to 2 inches of runoff from the impervious drainage areas by
land use group (Table 8).

The GI SCM types are derived from five land uses having the possibility of either an infiltration trench or
an infiltration basin placed on it, due to most land uses having both roofs and other types of impervious
areas. However, the transportation land use only included impervious road surfaces associated with it,
therefore the land use category contained no roofs and no opportunities for infiltration trenches.

Table 7. SCM-treated impervious area (drainage treatment area)

Land cover/Land use

Impervious Type

Drainage Treatment Area (acres)

Catchment 2

Catchment 7

Total

Forest

Roofs

0.048

0.237

0.285

Other

0.011

1.373

1.384

Commercial

Roofs

4.893

3.678

8.571

Other

13.080

7.700

20.78

Medium Density

Roofs

0.915

7.953

8.868

Residential

Other

1.095

13.807

14.902

High Density Residential

Roofs

0.283

0.358

0.641

Other

0.635

0.543

1.178

Transportation

Other

0.473

1.937

2.41

Open Land

Roofs

-

0.026

0.026

Other

-

0.423

0.423

Total

Roofs

6.491

12.437

18.928

Other

14.942

25.598

40.54

Table 8 Potential SCM opportunity areas (maximum footprints) in the two outfall catchments

Land cover/Land

Impervious

SCM

Maximum Footprint (acres)

use

Type

Type

Catchment 2

Catchment 7

Total

Forest

Roofs

Infiltration trench - A

0.003

0.014

0.017

Other

Infiltration basin - A

0.001

0.114

0.115

Commercial

Roofs

Infiltration trench - A

0.220

0.211

0.431

Other

Infiltration basin - A

0.257

0.642

0.899

Medium Density

Roofs

Infiltration trench - A

0.039

0.457

0.496

Residential

Other

Infiltration basin - A

0.091

1.150

1.241

High Density

Roofs

Infiltration trench - A

0.001

0.021

0.022

Residential

Other

Infiltration basin - A

0.027

0.045

0.072

Transportation

Other

Infiltration basin - B

0.039

0.161

0.2

Open Land

Roofs

Infiltration trench - A

-

0.001

0.001

Other

Infiltration basin - A

-

0.035

0.035

Total

Roofs

Infiltration trench

0.289

0.728

1.017

Other

Infiltration basin

0.389

2.123

2.512

14

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4.3 Opti-Tool Setup

The following steps were performed to set up the Opti-Tool for the pilot sub-watershed.

1. Establish baseline condition: Unit-area HRU timeseries for the period of interest (Jan 1998 - Dec
2018) were used as the boundary condition to the SCM simulation model. The Opti-Tool provides
a utility tool that runs the SWMM models, calibrated to Region 1 specific land use average annual
loading export rates, and generates the HRU hourly time series in the format needed for the Opti-
Tool. The HRU hourly timeseries were developed using the hourly rainfall and temperature data
from a local rain gage located at the Martha Vineyard's airport.

2. Set Management objective: The management objective was to identify the most cost-effective
stormwater controls (types and sizes) for achieving a wide range of TN loading, stormwater volume,
and storm flow rate reductions at the two outfall locations.

3. Set Optimization target: Cost effectiveness-curves for average annual TN load and average annual
stormwater volume reduction were developed.

4. Incorporate Land use information: The area distribution for the major land use groups within the
pilot watershed was estimated. Each land use group in the model was assigned the corresponding
unit-area HRU timeseries.

5. Incorporate SCM information: Two SCM types, infiltration trench and infiltration basin, were
selected for six major land use categories based on the Management Category analysis. SCM
specifications were set using the default parameters and SCM cost function available in the Opti-
Tool (Table 9). Impervious drainage areas were assigned to be treated by each SCM type in the
model.

6. Rim optimization scenario: The simulation period (Jan 1998 - Dec 2018), the stormwater metrics
of concern (flow volume and TN loading), the objective function (minimize cost) were defined and
input files were created for the optimization runs. The optimization was performed using the
continuous simulation SCM model to reflect actual long-term precipitation conditions that included
a wide range of actual storm sizes to find the optimal SCM storage capacities that provided the most
cost-effective solution at the watershed scale. Each optimization run generated a CE-Curve showing
the optimal solutions frontier for a wide range of stormwater volume and TN load reduction targets.

Table 9. SCM design specifications

General

SCM Parameters

Infiltration

Information

Trench - A

Basin - A

Basin - B

SCM Dimensions

Surface Area (ac)

Table 8

Orifice Height (ft)

Orifice Diameter (in.)

Surface Storage

Rectangular or Triangular
Weir

Rectangular

Configuration

Weir Height (ft)/Ponding

0.5

Depth (ft)

£-

Crest Width (ft)

Depth of Soil (ft)

Soil Properties

Soil Porosity (0-1)

0.4

Vegetative Parameter A

0.9

-------
General
Information

SCM Parameters

Infiltration
Trench - A

Infiltration
Basin - A

Infiltration
Basin - B



Soil Infiltration (in/hr)

8.27

8.27

2.41

Underdrain
Properties

Consider Underdrain
Structure?

No

No

No

Storage Depth (ft)

0

0

0

Media Void Fraction (0-1)

0

0

0

Background Infiltration
(in/hr)

8.27

8.27

2.41

Cost Parameters

Storage Volume Cost
($/ft3)

$12.49

$6.24

$6.24

Cost Function
Adjustment

SCM Development Type

New SCM in
Developed
Area

New SCM in
Developed
Area

New SCM in
Developed
Area

Cost Adjustment Factor

2

2

2

Decay Rates

TN (1/hr)

0.13

0.27

0.27

Underdrain
Removal Rates

TN (%, 0-1)

0

0

0

16

-------
5 RESULTS

5.1 Outfall #7

5.1.1 Stormwater Volume

The optimal mix of GI SCM types and sizes was assessed for the management objective of flood mitigation
through a reduction in stormwater volume. Figure 6 presents the cost-effectiveness curve (CE-Curve) for the
stormwater volume reduction objective for outlet #7. The blue diamonds form the most cost-effective
combination of GI SCM configurations for reducing flow volume. The grey dots on the curve are inferior
solutions; compared to these solutions, cheaper alternatives exist that would achieve the same flow volume
reduction. The red triangle presents a theoretical target solution. The target solution generally represents
some environmentally beneficial, socially acceptable, and economically feasible goals. The cost estimates
are based on regional unit cost information for the control types, a 35% add-on for engineering and
contingencies and a site factor multiplier to account for anticipated difficulties associated with installations.
For this analysis, a multiplier of 2X was assumed for all controls.

The target solution presented in Figure 6 shows that it would cost $750,000 to achieve an 80% reduction in
annual average flow volume. All costs presented in Opti-Tool derived CE-Curves are intended for planning
level purposes and meant to highlight relative cost differences between various solutions. The CE curve
presented in Figure 6 demonstrates how relative cost differences are relatively lower for reductions of 0% to
approximately 80%, but the rate at which solutions become more expensive quickly increases for reductions
higher than 80%.

Table 10 presents the optimized mix of GI SCM opportunity implementation which achieved an 80%
reduction in annual flow volume. Design depths ranged from 0.10 to 1.47 inches. Overall, the solution was
equivalent to a total design storage volume of 0.35 inches. Based on the rainfall analysis presented in section
2, the target solution would result in an 76% reduction in the annual number of runoff discharge days from
the impervious surfaces being treated. While it is important to note that a reduction in annual discharge days
is not directly comparable to a reduction in annual flow volume, both metrics provide valuable quantification
of the potential benefits of GI SCM implementation.

The reduction in peak flows resulting from achieving the target solution, which focused on flow volume, can
be seen in Figure 7. Peak flows across the driest, wettest and average years were all reduced compared to the
baseline simulation reflecting existing conditions. Figure 8 highlights the impact of the target solution to
storm hydrographs over selected periods of rainfall and runoff. A storm occurring on 5/17/2012 had the
peak flow reduced from approximately 17 cubic feet per second (cfs) in the baseline condition to
approximately 4.5 cfs in the optimized solution, a reduction of close to 74%. Other storms, occurring in
March 2012, had their respective runoff contribution from treated impervious surfaces eliminated due to the
optimized GI SCM implementation.

The impact of the target solution on the entire range of flow rates was also assessed. Figure 9 presents flow
duration curves for both the baseline and optimized solutions. The curves characterize the storm flows of
various magnitudes discharging from the outlet. The analysis assumes that the outlet is in good condition
and not clogged or otherwise obstructed. The graph only includes data from days in which rainfall and
discharge occurred. The graph demonstrates that for the same exceedance probability, the optimized
scenario had lower flows for all but the largest and most infrequent storms. For storms that occur only 5%
of the time (infrequent larger storm events that cause runoff), the optimized solution reduced the total flow
at Outfall #7 from about 9 to 2 cfs, a reduction of about 78%. For more frequently occurring storms, whose
flows exceeded more than 20% in baseline conditions, the total flow at Outfall #7 was reduced from 3 to
0.06 cfs, a reduction of about 98%. From the curve, the larger reductions occur for the more frequent
comparatively smaller storm events, meaning that overall, more precipitation is being infiltrated and
recharging the aquifer.

-------
AllSolutions ~Best Solutions A Target Solution

3.5
3
2.5
2
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1
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10%

20%

30%

40%

50%

60%

70%

80%

90% 100%

% Reduction
Average Annual Flow Volume

Figure 6. Opti-Tool Outfall cost effectiveness curve for annual average flow volume for outfall #7

Table 10. Optimized Gl SCM opportunities for achieving an 80% reduction in annual average storm volume at outfall
#7

SCMID

SCM Type

Land Use

Treated
Impervious
Area
(acres)

Runoff
Depth
(in.)

SCM
Storage
Capacity
(gallon)

SCM Cost
($)

SCM1

Infiltration Basin - A

Forest

1.37

0.30

11,186

$18,662

SCM 2

Infiltration Trench - A

Forest

0.24

0.89

5,803

$19,380

SCM3

Infiltration Basin - A

Commercial

7.7

0.40

83,636

$139,532

SCM4

Infiltration Trench - A

Commercial

3.68

0.40

39,951

$133,410

SCM5

Infiltration Basin - A

Medium Density Residential

13.81

0.40

149,968

$250,196

SCM6

Infiltration Trench - A

Medium Density Residential

7.95

0.20

43,191

$144,228

SCM7

Infiltration Basin - A

High Density Residential

0.54

0.10

1,474

$2,460

SCM8

Infiltration Trench - A

High Density Residential

0.36

0.40

3,884

$12,970

SCM9

Infiltration Basin - B

Transportation

1.94

0.30

15,778

$26,322

SCM 10

Infiltration Basin - A

Open Land

0.42

0.10

1,149

$1,916

SCM11

Infiltration Trench - A

Open Land

0.03

1.47

1,198

$4,000

Total

38.04

0.35

357,217

$753,076

18

-------
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Figure 7. Rainfall and runoff for the driest (top), wettest (middle), and average years (bottom) for outfall #7. Grey

area highlights the wettest week for the time period shown.

19

-------
¦ Rainfall (in./hr)

Wettest Week

Wettest Week Baseline —Optimized Solution

Wettest Week Baseline —-Optimized Solution

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-------
AllSolutions ~ Best Solutions A Target Solution

% Reduction
Average Annual TN Load

Figure 10. Opti-Tool cost effectiveness curve for TN annual average load reduction for outfall #7

Table 11. Optimized Gl SCM opportunities for achieving a 92% reduction in annual TN loading at outfall #7

SCMID

SCM Type

Land Use

Treated
Impervious
Area
(acres)

Runoff
Depth
(in.)

SCM
Storage
Capacity
(gallon)

SCM Cost
($)

SCM1

Infiltration Basin - A

Forest

1.37

0.30

11,186

$18,662

SCM 2

Infiltration Trench - A

Forest

0.24

0.49

3,224

$10,766

SCM 3

Infiltration Basin - A

Commercial

7.7

0.20

41,818

$69,766

SCM4

Infiltration Trench - A

Commercial

3.68

0.40

39,951

$133,410

SCM5

Infiltration Basin - A

Medium Density Residential

13.81

0.30

112,476

$187,648

SCM6

Infiltration Trench - A

Medium Density Residential

7.95

0.40

86,381

$288,458

SCM7

Infiltration Basin - A

High Density Residential

0.54

0.30

4,423

$7,378

SCM8

Infiltration Trench - A

High Density Residential

0.36

0.89

8,738

$29,180

SCM 9

Infiltration Basin - B

Transportation

1.94

0.20

10,519

$17,548

SCM 10

Infiltration Basin - A

Open Land

0.42

0.20

2,297

$3,832

SCM11

Infiltration Trench - A

Open Land

0.03

1.47

1,198

$4,000

Total

38.04

0.31

322,211

$770,650

-------
5.2 Outfall #2

5.2.1 Stormwater Volume

Outfall # 2 was also assessed for the optimal mix of GISCM types to achieve the management objective of
flood mitigation through a reduction in stormwater volume. Figure 11 presents the CE-curve for the
stormwater volume reduction objective for outlet #2. The target solution presented in Figure 11 shows that
it would cost $410,000 to achieve an 80% reduction in annual average flow volume. The same percent
reduction was estimated to cost approximately $750,000 for outlet #7. The estimated costs are useful for
planning purposes because they suggest that it would cost twice as much to obtain an 80% reduction in storm
volume for outlet #7 as it would for outlet #2 because of almost double impervious footprints in the
contributing drainage area to outlet #7. It is important to note that outlet #2 has a smaller contributing
drainage area.

Table 12 presents the optimized mix of GI SCM opportunity implementation which achieved an 80%
reduction in annual flow volume. Design depths ranged from 0.15 to 1.75 inches. Like outfall #7, the
solution for outfall #2 was equivalent to a total design storage volume of 0.35 inches (weighted average of
design depths based on impervious area treated).

The impact of achieving the target solution, which focused on flow volume, on peak flows, can be seen in
Figure 12. Peak flows across the driest, wettest and average years were all reduced compared to the baseline
simulation reflecting existing conditions. Figure 13 highlights the impact of the target solution to storm
hydrographs over selected periods of rainfall and runoff. The same storm assessed for outlet #7, which
occurring on 5/17/2012 had the peak flow reduced from approximately 11 cfs in the baseline condition to
approximately 3 cfs in the optimized solution, a reduction of about 73%. Other storms had their respective
discharge eliminated due to the optimized implementation.

The impact of the target solution on the entire range of flow rates was also assessed. Figure 15 presents flow
duration curves for both the baseline and optimized solutions. The curves characterize the storm flows of
various magnitudes discharging from the outlet. The graph only includes data from days in which rainfall
and discharge occurred. The graph demonstrates that for the same exceedance probability, the optimized
scenario had lower flows for all but the largest and most infrequent storms. For storms that occur only 5%
of the time (infrequent larger storm events that cause runoff), the optimized solution reduced the total flow
at Outfall #2 from about 3 to 1 cfs, a reduction of about 67%. For more frequently occurring storms, whose
flows exceeded more than 20% in baseline conditions, the total flow at Outfall #2 was reduced from 1.75 to
0.01 cfs, a reduction of about 99%. From the curve, the larger reductions occur for the more frequent
comparatively smaller storm events, meaning that overall, more precipitation is being infiltrated and
recharging the aquifer. This not only reduces flooding in the Commercial district but helps to restores the
hydrologic and hydrogeologic imbalance caused by the relatively high percentage (66%) of impervious cover
that characterizes the catchment draining to Outfall #2

During all days which had rainfall and discharge, the baseline conditions show that 7 3% of flows were equal
to or greater than 0.1 cfs. The optimized solution reduced the frequency of 0.1 cfs or greater flows to
approximately 15%. A 0.1 cfs flow was as frequent in the optimized scenario as a 2 cfs flow was in the
baseline scenario. The flow rate over all wet days was reduced by an average of 57% due to GI SCM
implementation.

-------
AllSolutions ~ Best Solutions A Target Solution

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0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

% Reduction
Average Annual Flow Volume

Figure 11. Opti-Tool Outfall cost effectiveness curve for annual average flow volume for outfall # 2

Table 12. Optimized Gl SCM opportunities for achieving an 80% reduction in annual average storm volume at outfall
#2

SCMID

SCM Type

Land Use

Treated
Impervious
Area
(acres)

Runoff
Depth
(in.)

SCM
Storage
Capacity
(gallon)

SCM Cost
($)

SCM1

Infiltration Basin - A

Forest

0.01

1.32

359

$600

SCM 2

Infiltration Trench - A

Forest

0.05

1.75

2,382

$7,954

SCM3

Infiltration Basin - A

Commercial

13.08

0.38

133,886

$223,366

SCM4

Infiltration Trench - A

Commercial

4.89

0.24

31,206

$104,208

SCM5

Infiltration Basin - A

Medium Density Residential

1.1

0.50

14,867

$24,802

SCM6

Infiltration Trench - A

Medium Density Residential

0.92

0.30

7,428

$24,804

SCM7

Infiltration Basin - A

High Density Residential

0.63

0.21

3,573

$5,962

SCM8

Infiltration Trench - A

High Density Residential

0.28

0.15

1,113

$3,716

SCM9

Infiltration Basin - B

Transportation

0.47

0.50

6,420

$10,710

Total

21.43

0.35

201,234

$406,122

-------
s Rainfall (in./hr)

Wettest Week

2.0
1.5

100
90
80
70
60
50
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90
80
70
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50
40
30
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-------
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Wettest Week

lllll

Wettest Week Baseline —Optimized Solution

20 i—

Wettest Week ^—Baseline Optimized Solution

20 i—

Figure 13. Selected periods of rainfall and runoff for outfall #2 during 2012, a year representing an average amount

of precipitation for Tisbury, MA.

-------
•Baseline

Optimized Solution

1,000

100

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-------
AllSolutions ~ Best Solutions A Target Solution

1.2
1

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o
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0.4
0.2
0

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

% Reduction
Average Annual TN Load

Figure 15. Opti-Tool Outfall cost effectiveness curve for TN annual average load reduction for outfall #2

Table 13. Optimized Gl SCM opportunities for achieving an 89% reduction in annual average total nitrogen loading at
outfall #2

SCMID

SCM Type

Land Use

Treated
Imperviou
s Area
(acres)

Runoff
Depth
(in.)

SCM
Storage
Capacity
(gallon)

SCM Cost
($)

SCM1

Infiltration Basin - A

Forest

0.01

0.44

120

$200

SCM 2

Infiltration Trench - A

Forest

0.05

0.78

1,059

$3,536

SCM3

Infiltration Basin - A

Commercial

13.08

0.26

92,047

$153,564

SCM4

Infiltration Trench - A

Commercial

4.89

0.39

52,010

$173,680

SCM5

Infiltration Basin - A

Medium Density Residential

1.1

0.50

14,867

$24,802

SCM6

Infiltration Trench - A

Medium Density Residential

0.92

0.22

5,571

$18,604

SCM7

Infiltration Basin - A

High Density Residential

0.63

0.37

6,253

$10,432

SCM8

Infiltration Trench - A

High Density Residential

0.28

0.12

879

$2,934

SCM9

Infiltration Basin - B

Transportation

0.47

0.70

8,987

$14,994

Total

21.43

0.31

181,792

$402,746

-------
5.3 Outfall Summary

Table 14 presents a summary of the optimized solutions for reducing storm flow and TN at outlets #7 and
#2. The cost of removing one gallon of stormwater volume from either of the outfalls was $0.02 while the
cost for removing a pound of TN was $1,727 for outfall #2 and $1,996 for outfall #7. Outfall #7 had higher
runoff and TN loading in the baseline conditions. This can be mainly attributed to the larger catchment area
(almost double impervious footprints) contributing the Outfall #7. The percent reduction in TN load was
similar for both outlets, GISCM implementation reduced TN loading 89% for outfall #2 and 92% for outfall
#7.

Table 14. Cost and effectiveness of LID SCM implementation within the catchments of two stormwater outlets in
Tisbury, MA

Outfall #2

Outfall #7

Baseline Average Flow Volume (gallons/yr)

23,193,061

40,174,307

Baseline Average TN Load (Ibs/yr)

261.87

420.63

Flow Volume Removed (gallons/yr)

18,551,813

32,192,534

TN Load Removed (Ibs/yr)

233.27

386.14

Cost per Gallon Flow Removed ($)

$0.02

Cost per Pound TN Removed ($)

$1,727

$1,996

6 SUMMARY

The results of this pilot study provide quantitative results to support watershed-based GI management
planning. Opti-Tool analyses helped to identify optimal stormwater controls, including GI SCM types and
sizes, that could guide retrofitting strategies in the developed catchments of two stormwater outfalls in
Tisbury, MA. This study highlights the computational power of optimization algorithms in Opti-Tool for
evaluating thousands of possible GI SCM combinations to identify the most cost-effective solutions over a
range of target reductions.

Eleven GI SCM opportunity types were considered which treated stormwater runoff from impervious
surfaces associated with a variety of land uses. For both catchments, GI SCM implementation resulted in
reduced flow volume, peak flows, and TN loading. Comparison of baseline and optimized flow duration
curves demonstrate reduced flow magnitudes across nearly the entire range of flow storm flows, with only
the largest, most infrequent storms generating approximately the same amount of runoff despite GI SCM
implementation. A visual assessment of hydrographs for dry, wet, and average precipitation years
demonstrated a reduction in peak flows. The impact on peak flows ranged from relatively small reductions
for some large storms, to eliminating runoff and therefore peak flows for several smaller storms. Since the
area underneath a hydrograph represents flow volume, the shape of the baseline optimized solution
hydrographs also demonstrated reduced stormwater volume. The cost of removing a gallon of water from
storm flows was estimated to be $0.02 for both outfalls. The average cost to remove a pound of TN was
between $1,700 and $2,000. Whether optimizing for a management objective of reduced stormwater volume
or reduce TN loading, the resulting cost-benefit analyses suggests that an approximately 80% reduction in
volume and a 90% reduction in TN loading can be achieved at a total cost around $1,160,000 - $1,173,000
for implementing distributed infiltration practices sized to capture 0.35 inches (weighted average) of runoff
from the impervious cover.

-------
7 RECOMMENDATIONS

This study relied on surface runoff modeling to make conclusions about the benefits of GI SCM
implementation for achieving goals of storm volume and TN load reduction. A more robust assessment
would take into account the Tisbury stormwater routing network, including lengths and sizes of storm drains,
as well as any reduced capacity in the system, such as clogged catch basins. Additionally, flow-related
monitoring data can help inform design options and provide valuable data for future modeling efforts. For
example, in-system flow depth, flow rate, and rainfall monitoring to document the frequency and duration
of flooding events would be a practical and inexpensive approach to more fully inform Tisbury of the
potential benefits of the GI SCM approach described herein. However, the data presented in this report
provides strong support for the town of Tisbury to begin pursuing the implementation of GI SCM
opportunities on both public and private lands.

Specific recommendations for goals are presented below.

Near-term goals (1 to 6 months)

• Review candidate locations for a pilot GI SCM opportunity installation. Consider design options,
including rain gardens, infiltration trenches, rain barrels for rooftop disconnection that can be readily
implemented in these drainage areas and throughout Tisbury.

• Consider development and adoption of implementation strategies to opportunistically incorporate
GI SCMs into all feasible infrastructure projects on municipal lands and rights of ways and through
typical redevelopment and urban renewal projects. This may involve an evaluation of local
bylaws/ordinances relating to stormwater management.

• Begin recording flood events, including smaller-scale nuisance flooding. Information to record
includes date and location of flooding, total rainfall depth, duration, pictures of the affected area.
This information can help better characterize flooding in town with valuable qualitative and
quantitative information.

• Clean out catch-basins and other components of the stormwater conveyance system.

• Consider a more frequent and consistent catch basin cleaning schedule.

Intermediate goals (6 months to 1.5 years)

• Adopt generic GI SCMs design templates suitable for Tisbury and gain experience through
installation of pilot stormwater GI SCMs using town labor and equipment or local contractors
Further, investigate optimal site design and supply chain opportunities.

• Adopt long-term GI SCM strategies for opportunistically implementing controls as part of municipal
infrastructure related work and private redevelopment projects.

• Continue community engagement and outreach, use pilot SCM(s) to facilitate community adoption.
Enlist community members (e.g., students) for planting rain gardens.

• Update stormwater infrastructure datasets to facilitate future hydraulic modeling of the system.
Municipal GIS stormwater infrastructure datasets lack some data and appear to show some
discrepancies with on-the-ground observations. Additional information that would facilitate
hydraulic modeling include dimensions, such as depth, width, and invert elevations of catch basins,
conveyance pipes, and outlets. Update the attribute table describing catch basins that are 'good',
'need cleaning' and 'need repair'.

• To the extent possible, incorporate the results of this project into Federal Emergency Management
Agency (FEMA) Hazard Mitigation Plans (HMP). Although not all green infrastructure projects
meet FEMA funding criteria, small-scale GI is potentially eligible for FEMA funding if the project
meets certain requirements. Implementation of small-scale GI can demonstrate a tangible effect on
flooding, particularly when implemented town-wide or areawide. In addition, DPW personnel can
implement small-scale GI flexibly and cost-effectively. As stormwater-related flooding is highly
correlated with impervious cover, projects or other efforts (e.g., ordinance/bylaws) aimed at

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reducing impervious cover, particularly in combination with green infrastructure, can be effective
strategies to include in Hazard Mitigation Plans.

Long-term goals (1.5-5 years)

• Use lessons learned from the pilot GI SCM implementation site to facilitate additional
implementations on both private and public land.

• Continue to implement long-term strategies for installing GI SCMs throughout Tisbury as
opportunities arise (e.g., municipal infrastructure work and redevelopment projects)

• Ensure that GI SCM opportunities receive adequate maintenance.

• Conduct a detailed hydrologic and hydraulic study that incorporates the rainfall-runoff analysis,
simulation of the GI SCM being installed on the ground, and flow routing through the storm drain
system accounting the backwater effects due to tidal influence at the outfall locations.

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REFERENCES

U.S. EPA. 2019a. Develop Planning Level GISCM Performance Curves for Estimating Cumulative Reduictions in
SW-Related Indicator Bacteria (Task 4d). Prepared for: U.S. EPA Region 1, Boston, MA. Prepared by:
Paradigm Environmental, Fairfax, VA.

U.S. EPA. 2019b. Opti-Tool Analyses for Quantifying Stonnwater Runoff Volume and Pollutant Loadings from
Watershed Source Areas (Task 4b). Prepared for: U.S. EPA Region 1, Boston, MA. Prepared by:
Paradigm Environmental, Fairfax, VA.

U.S. EPA. 2018. Watershed Characterization & Spatial Data Analysis (Task 4a). Prepared for: U.S. EPA Region
1, Boston, MA. Prepared by: Paradigm Environmental, Fairfax, VA.

U.S. EPA. 2016. Opti-Tool-Opti-Tool for Stonnwater and Nutrient Management User's Guide. Prepared for: U.S.
EPA Region 1, Boston, MA. Prepared by: Tetra Tech, Inc. Fairfax, VA.

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