svEPA
United States
Environmental Protection
Agency
2012 GREEN INFRASTRUCTURE TECHNICAL ASSISTANCE PROGRAM
City of Sanford
Sanford, Maine
Conceptual Green Infrastructure Design for
Washington Street, City of Sanford
April 2016
EPA 832-R-15-007
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About Green Infrastructure Technical Assistance Program
Stormwater runoff is a major cause of water pollution in urban areas. When rain falls in undeveloped
areas, soil and plants absorb and filter the water. When rain falls on our roofs, streets, and parking lots,
however, the water cannot soak into the ground. In most urban areas, stormwater is drained through
engineered collection systems and discharged into nearby water bodies. The stormwater carries trash,
bacteria, heavy metals, and other pollutants from the urban landscape, polluting the receiving waters.
Higher flows also can cause erosion and flooding in urban streams, damaging habitat, property, and
infrastructure.
Green infrastructure uses vegetation, soils, and natural processes to manage water and create healthier
urban environments. At the scale of a city or county, green infrastructure refers to the patchwork of
natural areas that provides habitat, flood protection, cleaner air, and cleaner water. At the scale of a
neighborhood or site, green infrastructure refers to stormwater management systems that mimic nature
by soaking up and storing water. These neighborhood or site-scale green infrastructure approaches are
often referred to as low impact development.
The U.S. Environmental Protection Agency (EPA) encourages using green infrastructure to help manage
stormwater runoff. In April 2011 EPA renewed its commitment to green infrastructure with the release of
the Strategic Agenda to Protect Waters and Build More Livable Communities through Green Infrastructure.
The agenda identifies technical assistance as a key activity that EPA will pursue to accelerate the
implementation of green infrastructure. In October 2013 EPA released a new Strategic Agenda renewing
the Agency's support for green infrastructure and outlining the actions the Agency intends to take to
promote its effective implementation. The agenda is the product of a cross-EPA effort and builds upon
both the 2011 Strategic Agenda and the 2008 Action Strategy.
In February 2012, EPA announced the availability of $950,000 in technical assistance to communities
working to overcome common barriers to green infrastructure. EPA received letters of interest from over
150 communities across the country, and selected 17 of these communities to receive technical
assistance. Selected communities received assistance with a range of projects aimed at addressing
common barriers to green infrastructure, including code review, green infrastructure design, and cost-
benefit assessments. The City of Sanford was selected to receive assistance identifying green
infrastructure opportunities and a conceptual design for Washington Street's storm drain system between
Main Street and Pioneer/Riverside Avenue.
For more information, visit http://water.epa.gov/infrastructure/greeninfrastructure/gi support.cfm.
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Acknowledgements
Principal USEPA Staff
Tamara Mittman, USEPA
Christopher Kloss, USEPA
Key Sanford Stakeholders
Jim Gulnak, City of Sanford
Michael Casserly, City of Sanford
Charles Andreson, City of Sanford
Lee Burnett, City of Sanford
Consultant Team
Jonathan Smith, Tetra Tech
Bobby Tucker, Tetra Tech
Garrett Budd, Tetra Tech
This report was developed under EPA Contract No. EP-C-11-009 as part of the 2012 EPA Green
Infrastructure Technical Assistance Program.
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Contents
Introduction 1
Report Purpose 3
Benefits of Green Infrastructure 3
Washington Street Site 5
Existing Site Conditions 5
Gateway Park Site 7
Existing Site Conditions 7
Storm Drainage System 8
Green Infrastructure Conceptual Design 10
Design Goals 10
Stormwater Management Toolbox 10
Bioretention Facilities 10
Permeable Pavement 13
Conceptual Layout 14
Green Infrastructure Sizing 18
Stormwater Control Measure Technical Specifications 21
Common Elements 21
Operations and Maintenance 26
Bioretention 26
Permeable Pavement 27
Stormwater Control Measure Cost Estimates 28
Conclusions 30
References 31
Appendix A. SWMM Analysis 33
Hydrology and Hydraulics 33
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Tables
Table 1. Studies Estimating Percent Increase in Property Value from Green Infrastructure 4
Table 2. SCM Sizing for Bioretention Cells 18
Table 3. Conceptual Design Hydrology/Hydraulics at Truck-Line Junctions 19
Table 4. Percent Change in Hydrology/Hydraulics from Green Infrastructure 19
Table 5. Bioretention Performance Summary: 2-yr, 24-hr Storm 20
Table 6. Bioretention Performance Summary: 10-yr, 24-hr Storm 20
Table 7. Traditional Bioretention Specifications 23
Table 8. Permeable Pavement Specifications 24
Table 9. Bioretention Operations and Maintenance Considerations 26
Table 10. Permeable Pavement Operations and Maintenance Considerations 27
Table 11. Cost Estimate for Washington Street Green Infrastructure SCMs 28
Table 12. Cost Estimate for Gateway Park Green Infrastructure SCMs 29
Table A-l. SWMM LID Input Values for Bioretention 34
Table A-2. Existing Drainage Area Runoff Rates 35
Table A-3. Existing Condition Hydrology/Hydraulics at Trunk-Line Junctions 35
Table A-4. Main Street Drainage Area 36
Table A-5. Proposed Condition Hydrology/Hydraulics at Trunk-Line Junctions 37
Table A-6. Percent Increase in Hydrology/Hydraulics from Main Street Drainage Area 37
Figures
Figure 1. Location Map of Washington St. Project 2
Figure 2. Site Conditions for Washington Street 6
Figure 3. Site Conditions for Future Gateway Park Site 7
Figure 4. Washington St., facing north 8
Figure 5. Future park site, facing south 8
Figure 6. Hydraulic Profile of Main Trunk-Line 9
Figure 7. Bioretention Incorporated into a Right-of-Way 11
Figure 8. Bioretention Incorporated into Traditional Parking Lot Design 11
Figure 9. Planter Box within Street Right-of-Way 12
Figure 10. Flow-through Planter Box Attached to Building 12
Figure 11. Tree Box at Roberts St. Sanford, Maine 12
Figure 12. Pervious Concrete Parking Stalls 14
Figure 13. Permeable Paver Installation in Sanford 14
Figure 14 Conceptual Layout for Washington Street Green Infrastructure SCMs 16
Figure 15. Conceptual Layout for Gateway Park Green Infrastructure SCMs 17
Figure 16. Permeable interlocking concrete 25
Figure 17. Pervious concrete 25
IV
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Introduction
The Mousam River corridor has historically served as the economic and social heart of the City of Sanford,
Maine. The central feature of the waterfront is the Sanford Mill Yard, an early 20th century textile mill
complex located along the river. The Mill Yard was long a primary employment provider for the
City. Beginning in the mid-20th century, with the closure of the mill the waterfront area has had
reduced importance to the City. As an aging brownfield, the Mill Yard contains areas of potential and
identified contamination from previous industrial uses. The Mousam River is listed by Maine's
Department of Environmental Protection's (DEP) 2012 303(d) list as impaired due to nutrients, metals,
BOD and E. Coli (MEDEP, 2012). Maine DEP had identified the Mousam as one of eight coastal rivers that
are a priority for cleanup from non-point source pollution impairment.
In recent years the City of Sanford worked to restore the importance and function of the Mousam
waterfront to serve economic, social and recreational needs of the community. The City secured a number
of grants to clean up contaminated industrial sites, rebuild housing, and rebuild infrastructure. Other
activities within the waterfront area include ongoing historic rehabilitation of two mill buildings,
waterfront infrastructure improvements, and partnerships with non-profit organizations to improve
water quality.
In 2010 the Sanford Mill Yard complex initiated an extensive community planning process as part of an
EPA Brownfields Area-Wide Planning Pilot Project (EPA, 2014). This process provided the City with a
strategy to attract sustained private investment in the Mill Yard through the integration of green
infrastructure and healthy living opportunities. The City now seeks to implement this strategy in part
through the development of a suite of green infrastructure practices for the 32-acre Mill Yard Site.
Through the EPA Green Infrastructure Technical Assistance Program, EPA worked with the City of Sanford
to identify opportunities for Green Infrastructure implementation at the Mill Yard site, quantify green
infrastructure benefits, and develop a conceptual design for two areas along Washington Street. Due to
their proximity to the visually striking Mousam River spillway, the project sites selected for conceptual
design development have been identified by the City of Sanford as a focal point for the Sanford Mill Yard
redevelopment efforts. The following report reflects the results of this technical assistance effort.
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,0-i Sanford^
North Berwick
Legend
Street Centerline
Mousam River Corridor
Town of Sanford
Washington Street
Green Infrastructure Design
Figure 1. Location Map of Washington St. Project
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Report Purpose
The purpose of this report is to identify opportunities for green infrastructure implementation in the City
of Sanford waterfront district. The report outlines ways to utilize green infrastructure to support the wider
urban renewal effort occurring along the Mousam River waterfront, and more particularly the brownfield
redevelopment project at the Sanford Mill Yard complex. Washington Street Corridor and Gateway Park
were both identified during preliminary site visits as potential green infrastructure project areas that could
enhance existing brownfield restoration by improving aesthetics, drainage and road infrastructure, as well
as improvements to water quality. These sites were chosen by the project team due to their potential to
integrate green infrastructure into planned redevelopment or potential drainage improvement projects
which can result in more cost effective green infrastructure applications. These sites were also selected
so that they could demonstrate to local residents and other stakeholders how green infrastructure could
be adopted into the waterfront setting and foster additional green infrastructure implementation
elsewhere in the Mill Yard.
After discussing general community benefits associated with green infrastructure practices, this report
explores site-specific opportunities to implement green infrastructure practices in the Washington Street
corridor and Gateway Park area, respectively. A conceptual stormwater management design and cost
estimate for each of the two proposed project areas is included in this report. These designs include
specifications for green infrastructure practices, as much as practicable, to meet state and local
stormwater design criteria.
Note: Final stormwater management designs should be completed by a stormwater management
professional in conjunction with final design of the street and park redevelopment. Stormwater
management professionals charged with design for the site should use the proposed selection, layout,
and sizing of stormwater control measures (SCMs) presented in this report as an initial conceptual design.
Final design will need to take into account actual site/building layout, soil infiltration rates, and detailed
survey information, which will dictate the final layout, sizing, and outlet control of green and gray
infrastructure practices.
Benefits of Green Infrastructure
The Sanford Mill Yard Complex presents an opportunity to include green infrastructure practices in a land
redevelopment initiative with relative ease while providing multiple benefits to the surrounding
community. The environmental, social, and economic benefits that green infrastructure can provide
include, but are not limited to:
Increased enjoyment of surroundings, walkability: Residents living in apartment buildings surrounded by
vegetated areas or features reported significantly more use of the area just outside their building than did
residents living in buildings with less vegetation (Hastie 2003; Kuo 2003). Research has found that people
in greener neighborhoods judge distances to be shorter and make more walking trips (Wolf 2008).
Implementing green infrastructure practices that enhance vegetation within the neighborhood will help
to create a more pedestrian-friendly environment that encourages walking and physical activity.
Increased pedestrian safety, traffic calming and reduced crime: Researchers examined the relationship
between vegetation and crime for 98 apartment buildings in an inner city neighborhood. The study found
the greener a building's surroundings are, the fewer total crimes (including violent crimes and property
crimes), and that levels of nearby vegetation explained 7 to 8 percent of the variance in crimes reported
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by building (Kuo 2001a). Studies also show that the traffic-calming effects of trees are also likely to reduce
road rage and improve the attention of drivers. Green streets can also increase safety. Generally, if
properly designed, narrower green streets decrease vehicle speeds and make neighborhoods safer for
pedestrians (Wolf 1998; Kuo 2001a).
Increased property values: Many aspects of green infrastructure can potentially increase property values
by improving aesthetics, drainage, and recreation opportunities. These in turn can help restore, revitalize,
and encourage growth in the economically distressed areas around Pittsburgh. Table 1 summarizes the
recent studies that have estimated the effect that green infrastructure or related practices have on
property values. The majority of these studies addressed urban areas, although some suburban studies
are also included. The studies used statistical methods for estimating property value trends from observed
data.
Table 1. Studies Estimating Percent Increase in Property Value from Green Infrastructure
Source
Ward et al. (2008)
Shultz and Schmitz (2008)
Wachter and Wong (2006)
Anderson and Cordell
(1988)
Voicu and Been (2008)
Espey and Owasu-Edusei
(2001)
Pincetl et al. (2003)
Hobden, Laughton and
Morgan (2004)
New Yorkers for Parks and
Ernst & Young (2003)
Percent
increase in
Property Value
3.5 to 5%
0.7 to 2.7%
2%
3.5 to 4.5%
9.4%
11%
1.5%
6.9%
8 to 30%
Notes
Estimated effect of green infrastructure on adjacent
properties relative to those farther away in King County
(Seattle), WA.
Referred to effect of clustered open spaces, greenways and
similar practices in Omaha, NE.
Estimated the effect of tree plantings on property values for
select neighborhoods in Philadelphia.
Estimated value of trees on residential property (differences
between houses with five or more front yard trees and those
that have fewer), Athens-Clarke County (GA).
Refers to property within 1,000 feet of a park or garden and
within 5 years of park opening; effect increases over time
Refers to small, attractive parks with playgrounds within 600
feet of houses
Refers to the effect of an 11% increase in the amount of
greenery (equivalent to a one-third acre garden or park)
within a radius of 200 to 500 feet from the house
Refers to greenway adjacent to property
Refers to homes within a general proximity to parks
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Washington Street Site
Existing Site Conditions
The Washington Street drainage area consists of Approximately 7.6 acres of roadway and adjacent
commercial properties which are served by a storm drain system between Main Street and
Pioneer/Riverside Avenue. Of this drainage area, approximately 6.6 acres are considered impervious
(88%) with an average slope of 3.1%. The land use is predominantly roadway and mixed commercial,
although several medium-high density residential lots are located in the eastern side of the drainage area.
Figure 2 shows a watershed map including the sub-drainage area delineations and the existing storm
drainage system. Note that the study area for the green infrastructure design is the drainage system
between the upper catch basin (CB-1) on Washington Street and the blind junction (J-4) located near the
intersection with Riverside/Pioneer Ave. Figure 2 also shows a stormwater drainage area on Main Street
(MAIN-1) that currently contributes to a combined sewer system in this area. This is one of the few
remaining areas within the city served by a combined sewer and the city is currently considering
undertaking a sewer separation project. Sewer separation would require diverting the stormwater runoff
from this area into an existing drainage network with sufficient capacity or, if sufficient capacity is not
available replacing or upgrading an existing culvert system.
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Legend
Project Catch Basins
Project Manholes
Project Storm Sewers
1-m Contours
Project Drainage Areas
Washington St. Green Infrastructure Project
Downtown Sanford, Maine
Figure 2. Site Conditions for Washington Street
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NRCS soil data classifies the majority of the watershed as 'Urban/ although a portion of the residential
drainage area (MIX-1 and MIX-2) are considered 'Adams-urban land complex' with a Hydrologic Soil Group
(HSG) classification of'A.'
Gateway Park Site
Existing Site Conditions
The Gateway Park site sits at the intersection of Washington St. and Riverside Avenue directly bordering
the south bank of the Mousam River (Figure 3). The site consists of two commercial properties currently
occupied by an unutilized fuel station and restaurant facility. The site has been identified by the City of
Sanford for a future municipal park although a park plan or specific park amenities have not been
identified. The drainage area for the site includes half of the adjacent roadways (Riverside Ave. and
Washington St.), and all of the existing project area. The site is intersected by the downstream section of
the Washington St. drainage network that discharges to the river to the north. One catch basin (CB-510)
connected to the main drainage network is located within the park boundary. Based on site observations,
it is suspected that overflow from the Washington St. drainage network drains via surface flow to the site
before eventually discharging to the river.
Project Storm Sewers
Parcels
Mousam River
Project Drainage Areas
Gateway Park Green Infrastructure Project
Downtown Sanford, Maine
Figure 3. Site Conditions for Future Gateway Park Site
NRCS soil data classifies the entire park drainage area as 'Urban'. The existing imperviousness of the site
is almost 100%, although the future park renovation will likely convert most of the impervious area in the
parcel boundary to grass or landscaped areas. Photographs of the project area are included in Figure 4
and Figure 5.
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Figure 4. Washington St., facing north
Figure 5. Future park site, facing soi
Storm Drainage System
An important tool for evaluating and optimizing infrastructure solutions for storm flooding and water
quality challenges involves continuous hydro-simulation models. The project team selected the
Stormwater Management Model (SWMM; Rossman 2010) for the purposes of assessing both flooding and
water quality impacts, and evaluating stormwater infrastructure solutions that help achieve the project
goals. SWMM is a dynamic precipitation-runoff simulation model designed for discrete event or
continuous representation of hydraulics, hydrology, and water quality. It is optimized and designed for
storm event flow management in urban area drainage systems. Precipitation and other meteorological
input data are used to drive the hydrologic response in the simulation. SWMM 5 represents land areas as
a series of subcatchments, with properties that define retention and runoff of precipitation, infiltration,
and (optionally) percolation to a shallow aquifer. Subcatchments are connected to the drainage network,
which may include natural watercourses, open channels, culverts and storm drainage pipes, storage and
treatment units, outlets, diversions, and many other elements of an urban drainage system. Nodes and
links are used in SWMM 5 to define the connectivity and control within the drainage network. Additional
information on the SWMM analysis for the Washington Street storm drainage system is provided in
Appendix A.
The Washington Street storm drainage system consists primarily of a reinforced concrete pipe (RCP) trunk-
line that starts at the intersection of Washington and Main. There are multiple culverts that intersect the
trunk-line along the Washington Street study area, consisting of PVC, RCP, or vitrified clay (VC) pipes. Most
of the junctions were visible manholes (labeled with an "MH" prefix), although several were blind
junctions (labeled with a "J" prefix). Inverts for the blind junctions were interpolated based on the two
adjacent manholes with surveyed inverts.
An estimated profile of the main trunk-line system is shown below in Figure 6. The node labels represent
manholes, catch basins, or blind junctions along the trunk-line that were included in the SWMM model.
There is approximately 240 feet of 12" RCP between the upstream catch basin (CB-1) and MH-2. Between
MH-2 and MH-4, the trunk-line changes to a 15" RCP that is approximately 312 feet in total length. At the
junction labeled as MH-4, the trunk-line becomes a relic 17" RCP sanitary sewer. Connectivity beyond MH-
4 remains uncertain. Field investigations by City of Sanford staff revealed deviation between on-site
drainage conditions and GIS data. For the modeling purposes of this project, uncertainty in downstream
connectivity in the drainage system does not affect the hydrologic and hydraulic results and the
conceptual design configuration.
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Drainage Profile: Node CB-1 - OUT
400 450 EDO
Detance (ft)
BOO S50
Figure 6. Hydraulic Profile of Main Trunk-Line
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Green Infrastructure Conceptual Design
Design Goals
Specific design goals for this project include quantifying the hydrologic benefits of the proposed green
infrastructure practices (linear bioretention and permeable pavement) in reducing peak flow rates within
the Washington Street drainage network. The project further assesses the impact of these peak
reductions on the feasibility of routing stormwater contributions from the Main Street combined sewer
system into the Washington Street drainage system. For the Washington Street drainage area, proposed
linear bioretention cells were designed to maximize water quality volume treatment and hydraulic
function. At Gateway Park, the design objective was to use the green infrastructure practices to treat the
entire water quality volume from the park's drainage area (which includes offsite roadway runoff), and
utilize a detention feature to mitigate the 2-year, 24-hour flood volume from the Washington Street
drainage area. To the extent practicable all of the green infrastructure practices were designed according
to specifications in the Maine Stormwater Manual (MEDEP, 2014).
Stormwater Management Toolbox
The green infrastructure practices identified as appropriate for the Washington Street and Gateway Park
project areas as well as throughout the mill yard complex include bioretention facilities, planter boxes,
and permeable pavement. To assist planners and designers in going forward with these conceptual
designs, the following discussion addresses constraints and opportunities associated with each applicable
green infrastructure practice.
Bioretention Facilities
Bioretention facilities are shallow, depressed areas with a fill soil and vegetation that infiltrates runoff and
removes pollutants through a variety of physical, biological, and chemical treatment processes. The
depressed area is planted with small to medium sized vegetation including trees, shrubs, grasses, and
perennials, and may incorporate a vegetated groundcover or mulch that can withstand urban
environments and tolerate periodic inundation and dry periods. Bioretention may be configured
differently depending on site context and design goals. This section summarizes general design
considerations for bioretention facilities, and then describes two configurations designed for dense urban
areas: planter boxes and tree boxes. Note that use of these practices within the public right-of-way will
need prior approval from the City.
Bioretention is well-suited for removing stormwater pollutants from runoff, particularly for smaller (water
quality) storm events, and can be used to partially or completely meet stormwater management
requirements on smaller sites. Bioretention areas can be incorporated into a development site to capture
roof runoff and parking lot runoff and within rights-of-way to capture sidewalk and street runoff (Figure
7 and Figure 8).
For unlined systems, maintain a minimum of 5 feet between the facility and a building and at least
10 feet with a basement.
A surface dewatering time of no greater than 12 hours, either through infiltration with soils of
sufficient percolation capacity or with an underdrain system and outlet to a drainage system. Use
of an underdrain system is very effective in areas with low infiltration capacity soils.
Planted with native and noninvasive plant species that have tolerance for urban environments,
frequent inundation, road salt application, and Maine's cold winter climate.
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Inclusion of an overflow structure with a non-erosive overflow channel to safely pass flows that
exceed the capacity of the facility or design the facility as an off-line system.
Inclusion of a pretreatment mechanism such as a grass filter strip, sediment forebay, or grass
swale upstream of the practice to enhance the treatment capacity of the unit.
Figure 7. Bioretention Incorporated
into a Right-of-Way.
Figures. Bioretention Incorporated into
Traditional Parking Lot Design.
Planter Box: Planter boxes are bioretention facilities contained within a concrete box, allowing them to
be incorporated into tighter areas with limited open space. Runoff from a street or parking lot typically
enters a planter box through a curb cut, while runoff from a roof drain typically enters through a
downspout. Planter boxes are often categorized either as flow-through planter boxes or infiltrating
planter boxes. Infiltrating planter boxes have an open bottom to allow infiltration into the underlying soils.
In brownfield settings such as the Mousam Mill Yard area, an evaluation of subsurface conditions should
be conducted to determine the potential for existing contamination of subsoil or groundwater. Flow-
through planter boxes are completely lined and have an underdrain system to convey flow that is not
taken up by plants to areas that are appropriate for drainage away from building foundations. Planter
boxes are well-suited to narrow areas adjacent to streets and buildings (Figure 9 and Figure 10).
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Figure 9. Planter Box within Street
Right-of-Way.
Figure 10. Flow-through Planter Box
Attached to Building.
Tree Box: Tree boxes are bioretention facilities configured for dense urban areas that use the water-
uptake benefits of trees. They are generally installed along street corridors with curb inlets (Figure 11).
Tree boxes can be incorporated immediately adjacent to streets and sidewalks with the use of a structural
soil, modular suspended pavement, or underground retaining wall to keep uncompacted soil in its place.
Tree boxes typically contain a highly engineered soil media to enhance pollutant removal while retaining
high infiltration rates. The uncompacted media allows urban trees to thrive, providing shade and an
extensive root system for water uptake. For low to moderate flows, stormwater enters through the tree
box inlet and filters through the soil. For high flows, stormwater will bypass the tree box if it is full and
flow directly to the downstream curb inlet.
Figure 11. Tree Box at Roberts St. Sanford, Maine.
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Permeable Pavement
Conventional pavement results in increased surface runoff rates and volumes. Permeable pavements, in
contrast, allow streets, parking lots, sidewalks, and other impervious surfaces to retain the underlying
soil's natural infiltration capacity while maintaining the structural and functional features of the materials
they replace. Permeable pavements contain small voids that allow water to drain through the pavement
to an aggregate reservoir and then infiltrate into the soil. If the native soils below the permeable
pavements do not have enough percolation capacity, underdrains can be included to direct the
stormwater to other downstream stormwater control systems. Permeable pavement can be developed
using modular paving systems (e.g., concrete pavers, grass-pave, or gravel-pave) or poured-in-place
solutions (e.g., pervious concrete or permeable asphalt).
Permeable pavement reduces the volume of stormwater runoff by converting an impervious area to a
treatment unit. The aggregate sub-base can provide water quality improvements through filtering, and
enhance additional chemical and biological processes. The volume reduction and water treatment
capabilities of permeable pavements are effective at reducing stormwater pollutant loads (Collins et. al.,
2010).
Permeable pavement can be used to replace traditional impervious pavement for most pedestrian and
vehicular applications. Composite designs that use conventional asphalt or concrete in high-traffic areas
adjacent to permeable pavements along shoulders or in parking areas can be implemented to meet both
transportation and stormwater management needs. Permeable pavements are most often used in
constructing pedestrian walkways, sidewalks, driveways, low-volume roadways, and parking areas of
office buildings, recreational facilities, and shopping centers (Figure 12 and Figure 13).
General guidelines for applying permeable pavement are as follows:
Permeable pavements can be substituted for conventional pavements in parking areas, low-
volume/low-speed roadways, pedestrian areas, and driveways if the grades, native soils, drainage
characteristics, and groundwater conditions of the paved areas are suitable.
Permeable pavement is not appropriate for stormwater hotspots where hazardous materials are
loaded, unloaded, or stored unless the sub-base layers are completely enclosed by an
impermeable liner.
The granular capping and sub-base layers should provide adequate construction platform and
base for the overlying pavement layers.
If permeable pavement is installed over low-permeability soils or temporary surface flooding is a
concern, an underdrain should be installed to ensure water removal from the sub-base reservoir
and pavement.
The infiltration rate of the soils or an installed underdrain should drain the sub-base in 24 to 48
hours.
An impermeable liner can be installed between the sub-base and the native soil to prevent water
infiltration when clay soils have a high shrink-swell potential or if a high water table or bedrock
layer exists.
Measures should be taken to protect permeable pavements from high sediment loads,
particularly fine sediment, to reduce maintenance. Typical maintenance includes removing
sediment with a vacuum truck.
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Figure 12. Pervious Concrete Parking Stalls.
Figure 13. Permeable Paver Installation in Sanford.
Conceptual Layout
As with most green infrastructure retrofit projects, available space for SCMs is often limited. To address
this constraint, many green infrastructure retrofits are designed to optimize small footprints and narrow
right-of-ways through the use of vertical retaining walls (rather than gradual side slopes), such as along
sidewalks and traffic/parking lanes. Another strategy is to modify road patterns, travel lane
configurations, and street widths to provide space for green street SCMs and reduce impervious surface
area. Furthermore, coordinating green infrastructure implementation with larger redevelopment or
roadway improvement projects can significantly reduce implementation costs relative to projects where
green infrastructure is the only element.
The following sections summarize the factors influencing the conceptual designs for the Mill Yard project
sites and describe in detail the conceptual design configurations. The proposed layouts are based on
overall project goals and site specific priorities that were conveyed by City of Sanford staff as discussed
earlier. Detailed design information is also included below to assist with final design of the SCMs.
Washington Street
The proposed green infrastructure approach for Washington Street involves reducing the existing three-
lane road configuration to two-lanes. The western lane will convert to a combination of bioretention cells
and new parking stalls, while most of the existing parking stalls along the eastern side of the street will be
converted to bioretention cells. Both the installation of the vegetated bioretention system and removal
of the middle turning lane would most likely slow traffic through the study area, which would increase
walkability and improve pedestrian/traffic safety. This impact will need to be further evaluated and
discussed among stakeholders prior to final design.
The proposed conceptual plan was designed to maximize water quality volume treatment (per the Maine
DEP sizing requirements) and hydraulic function along the Washington Street drainage area. However,
during the final design process stakeholder input may justify design modifications that focus on other
aspects of green infrastructure implementation, including aesthetics, public parking, safety concerns,
constructability, construction cost, etc. For example, if additional hydrologic and water quality impacts
are desired, additional media depth can be incorporated into the bioretention cells with the overflow
discharging to an underground detention/retention facility beneath the BMP system. Engineered planter
boxes and tree boxes can be installed in small footprints (e.g., curbsides and store-fronts) to provide
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further treatment and improved aesthetics along Washington Street. If additional on-street parking is
desired, installation of permeable pavement can be incorporated in place of one or more bioretention
areas.
Gateway Park
Since the City of Sanford plans to redevelop this site into a "focal point" for the Sanford Mill Yard
brownfield project, as well as to potentially serve as a public congregating/recreation area, the
stormwater improvements at the site were designed for aesthetics, multifunction, and adaptability. The
City has yet to develop a design for Gateway Park, so the stormwater plan leaves most of the internal park
area open for subsequent park planning efforts.
Linear bioretention is proposed in the park along both Riverside Ave. and Washington St. to treat the
water quality volume from the adjacent roadway and park area. The bioretention cells were designed with
6'-wide media beds and an underdrain layer that connects to the existing storm drainage system. The cells
were also segregated to provide multiple pedestrian access points to the park. Planting plans for the
bioretention areas can be later adopted to meet the specific needs of the park (e.g., visibility goals,
maintenance needs, site aesthetics, etc.).
Permeable walkways are also proposed for the entire perimeter of the park site to provide pedestrian
access along the waterfront and roadway, and to reduce the required area for the bioretention cells. The
permeable walkways can be designed with porous asphalt, porous concrete, or a paver system. Currently,
the permeable walkways are design without an underdrain to encourage infiltration (although this feature
can be incorporated later if required by soil and site conditions).
The park's stormwater plan also includes two grassed detention areas within the park. Although both
basins include visible outlet structures, the grassed basins will be unobtrusive in the landscape while
simultaneously providing a grassed surface for public use and recreation during dry weather. The shallow
side slopes were designed for mower access and easy maintenance. Figure 14 and Figure 15 shows the
proposed placement and relative sizing of the green infrastructure SCMs along Washington Street and
within Gateway Park, respectively.
15
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Catch Basins
1-m Contours
Sanitary Sewer
> Storm Sewers
- - New Travel Lanes
Washington Street
Green Infrastructure Design
Figure 14 Conceptual Layout for Washington Street Green Infrastructure SCMs
16
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1-m Contours
Sanitary Sewer
Storm Sewers
Proposed Culvert
Possible Tree Locations
Box Riser Structure
Permeable Walkway
Landscaped Area
Grassed Storage Area
Bioretention
Gateway Park
Green Infrastructure Design
Figure 15. Conceptual Layout for Gateway Park Green Infrastructure SCMs
17
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Green Infrastructure Sizing
The primary green infrastructure practice proposed as part of the conceptual designs is bioretention filter
cells. According to Chapter 7.2.3 of the Maine BMP Technical Design Manual, bioretention cells must be
sized to capture a treatment volume that is equal to 1.0 inch times the impervious area within the
catchment, plus 0.4 inches times the catchment's landscaped area. In addition, the surface area of the
bioretention filter must be no less than the sum of 7% of the impervious area and 3% of the landscaped
area within the catchment. After using this method to size each bioretention cell according to its
respective catchment area, SWMM was used to simulate hydrologic performance for the 2- and 10-year,
24-hour storm events for the overall drainage network along Washington St. and Gateway Park.
Washington Street
Based on the locations of existing catch basins and road entrances, six separate linear bioretention cells
are proposed for the Washington Street study area. Each cell was sited just up-gradient of an existing
catch basin to easily connect underdrains and bypass overflows into the existing storm sewer network.
Table 2 shows the surface area dimensions of each SCM, designed according to the Design Manual sizing
criteria. The cells have either a 10-foot or 8-foot width and range in length between 80 feet and 130 feet.
Each cell will contain 18 inches of bioretention media and a 14" gravel layer with 4" slotted PVC underdrain
that connects to the existing manholes and catch basins along Washington Street. Due to site constraints,
bioretention cell 04 is undersized according to the design guidance and is only 64% of the recommended
surface area.
Table 2. SCM Sizing for Bioretention Cells
SCM ID
01
02
03
04
05
06
Drainage
Area (Ac)1
0.14
0.11
0.12
0.23
0.09
0.12
%
Impervious
100
100
100
100
100
100
width
(Ft)
10
10
10
8
8
8
Length
(Ft)
102
80
90
130
85
100
Surface Area
(Sqft)
1016
800
900
1040
680
800
Ponded Storage
Volume (Cu ft)
508
400
450
520
340
400
1 Includes BMP footprint
Table 3 shows the hydrologic and hydraulic impacts at each of the trunk-line nodes as a result of adding
the proposed bioretention cells along Washington Street. To better illustrate the impacts, Table 4 shows
the percent change due to the proposed infrastructure.
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Table 3. Conceptual Design Hydrology/Hydraulics at Truck-Line Junctions
Junction
Node
CB-l
MH-1
MH-2
J-l
MH-3
J-2
J-3
MH-5
MH-4
J-4
2-year, 24-hour
Q(cfs)
0.13
0.21
4.16
8.15
8.16
8.7
11.35
13.65
9.71
9.88
Hours
Surcharged
0
0
0.17
0.36
0.16
0.37
0.56
0.4
0.7
0.57
Hours
Flooded
0
0
0
0
0
0
0
0.29
0.01
0
10-year, 24-hour
Q(cfs)
1.4
1.3
6.84
12.32
12.32
9.06
12.82
16.39
9.71
10.12
Hours
Surcharged
0.22
0.23
0.37
0.67
0.37
0.65
0.81
0.7
0.94
0.82
Hours
Flooded
0.01
0
0.14
0
0.26
0
0
0.55
0.17
0
Table 4. Percent Change in Hydrology/Hydraulics from Green Infrastructure
Junction
Node
CB-l
MH-1
MH-2
J-l
MH-3
J-2
J-3
MH-5
MH-4
J-4
2-year, 24-hour
Q(cfs)
0%
-34%
0%
0%
-2%
-4%
-2%
-2%
0%
-1%
Hours
Surcharged
N/A1
N/A1
-11%
0%
-20%
-10%
-7%
-9%
-5%
-7%
Hours
Flooded
N/A1
N/A1
N/A1
N/A1
N/A1
N/A1
N/A1
-6%
-50%
N/A1
10-year, 24-hour
Q(cfs)
10%
7%
8%
0%
0%
0%
0%
0%
0%
0%
Hours
Surcharged
5%
5%
0%
2%
-3%
-4%
-5%
-1%
-3%
-5%
Hours
Flooded
N/A1
N/A1
40%
N/A1
0%
N/A1
N/A1
-4%
-11%
N/A1
1 No flooding/surcharge calculated for both existing and proposed conditions
Table 5 and Table 6 show the performance summary for the six bioretention areas as estimated by
SWMM. Surface overflow represents the volume of runoff that bypasses the bioretention area and is not
treated. Drain outflow represents the volume of runoff that infiltrates through the bioretention media
and discharges through the underdrain, while the final storage is the runoff volume remaining in the
bioretention area at the end of the simulation period, which will eventually be treated.
19
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Table 5. Bioretention Performance Summary: 2-yr, 24-hr Storm
Bioretention
Cell
01
02
03
04
05
06
Total Inflow
(in)
17.8
17.3
17.5
27.9
17.4
19.9
Surface
Overflow (in)
1.9
1.6
1.7
8.2
1.7
3.1
Drain Outflow
(in)
12.5
12.3
12.4
15.8
12.3
13.3
Final Storage
(in)
3.5
3.4
3.4
4.0
3.4
3.6
Percent
Treated
89.5%
90.7%
90.2%
70.7%
90.4%
84.5%
Table 6. Bioretention Performance Summary: 10-yr, 24-hr Storm
Bioretention
Cell
01
02
03
04
05
06
Total Inflow
(in)
27.4
26.7
27.0
43.0
26.8
30.7
Surface
Overflow (in)
7.7
7.2
7.5
19.2
7.4
10.0
Drain Outflow
(in)
15.8
15.6
15.7
19.2
15.6
16.7
Final Storage
(in)
3.9
3.9
3.9
4.6
3.9
4.0
Percent
Treated
71.8%
72.9%
72.4%
55.3%
72.6%
67.4%
As expected, the total percentage of runoff volume treated by the bioretention cells is significant. For the
2- and 10-yr storm events, the treatment percentages range from 71 to 91%, and 55 to 73%, respectively.
However, the impacts from green infrastructure on hydraulic performance and peak flow reduction within
the Washington St. drainage network were less evident. Although SWMM results predict a 34% peak flow
reduction at one of the upstream manholes (MH-1) during the 2-year event, peak reductions at the
remaining storm structure locations only ranged between 0 and 4%. In addition, the 10-year storm
simulation predicts a noticeable increase in peak flow events at the three upstream storm structures (CB-
1, MH-1, MH-2) of up to 10%. One possible cause for this observation relates to the "area-conversion"
method that SWMM uses to model LID practices like bioretention, which may limit the accuracy for
hydraulic routing and peak flow estimation at an event scale (versus annual hydrology). Also, the
implementation of bioretention cells along Washington Street might influence the curb/bypass flooding
that occurs during existing conditions, and allow for additional ponding and driving head at some of the
higher elevation nodes that would increase simulated flow rates through the structure. In other words,
the locations of the existing flood occurrences was re-distributed (not reduced) throughout the
Washington St. drainage network as a result of the bioretention cells.
Gateway Park
Since hydraulic capacity analyses were not required for the Gateway Park green infrastructure, SWMM
was not used to design the proposed BMP system. Chapter 7.2 in Volume III of Maine's BMPs Technical
Design Manual was used to size the bioretention cells in Gateway Park.
20
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Approximately 0.5 acres of catchment area directly drain to the proposed bioretention cells sited along
the park's roadside perimeter. The majority of the catchment area, which is approximately 70%
impervious, includes off-site roadway from Riverside Ave. and Washington Street. Most of the pervious
area in the catchment includes the proposed permeable pavement within the park and adjacent
landscaped areas. For the purposes of required treatment volume calculations, the BMP surface area was
treated as pervious landscaped area for the Gateway Park design. Based on the ME DEP sizing guidelines,
the required treatment volume for the bioretention cells is 1,480 ft3. With a 6" ponding depth, the
recommended bioretention area is approximately 3,000 ft2.
The grassed storage areas within the park's open space area were sized to retain the 2-year, 24-hour
flooding volumes from Washington Street. Based on SWMM output, the internal outflow from
Washington Street with the proposed green infrastructure improvements (without Main St. drainage) is
0.097 ac-ft, or 4,225 ft3. Assuming a 1-ft maximum ponding depth in the storage areas, the required
footprint area is 4,225 ft2. As currently proposed, the storage area footprint is 4,840 ft2 to account for
volume reductions associated with the shallow side slopes.
Stormwater Control Measure Technical Specifications
The purpose of this section is to provide guidance for designing the SCMs during final design. Design
criteria for the bioretention cells are based on Chapter 7.2 of Maine Stormwater Manual. Additional
design guidance for bioretention and permeable pavements is presented in Table 7 and Table 8 at the
end of this section along with accompanying figures showing cross-sections of typical roadside
bioretention and permeable pavement details.
Common Elements
Soil Media
Soil media is typically specified to meet the growth requirements of the selected vegetation while still
meeting the hydraulic requirements of the system. The system must be designed to drain the surface
storage volume in no less than 24 hours and no more than 48 hours. The expected infiltration rate should
range from 2.4 to 4 in/hr after compaction to 90-92% standard proctor (ASTM D6998).
The engineered soil mixture shall be a blend of a silty-sand soil or soil mixture that is 20-25 percent (by
volume) moderately fine aged bark fines or wood fiber mulch. Organic matter is considered an additive
to help vegetation establish and contributes to sorption of pollutants. Organic material should not consist
of manure or animal compost. Newspaper mulch has been shown to be an acceptable additive.
Gradation analyses of the blended material, including hydrometer testing for clay content and
permeability testing of the soil filter material, should be performed by a qualified soil testing laboratory
and submitted to the project engineer for review. Particle gradation tests should conform to ASTM
C117/C136 (AASHTO T11/T27) and the blended material should have no less than 8% passing the 200
sieve and shall have a clay content of less than 2%. Other soil media design criteria include:
pH should be between 6-8, cation exchange capacity (CEC) should be greater than 5
milliequivalent (meq)/100 g soil.
High levels of phosphorus in the media have been identified as the main cause of bioretention
areas exporting nutrients. All bioretention media should be analyzed for background levels of
nutrients. Total phosphorus should not exceed 15 ppm.
21
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Geotextile fabric of Mirafi 170n or equivalent may be placed between the sides of the filter layer
and adjacent soil to prevent surrounding soil from migrating into the filter and clogging the outlet.
Overlap seams must be a minimum of 12 inches.
Underdrain
An underdrain is required in areas where existing soils have an infiltration rate less than 0.5 in/hr and
should meet the following criteria:
The underdrain piping should be 4" of 6" slotted, rigid Schedule 40 PVC or SDR35. The total
opening area exceeds the expected flow capacity of the underdrain and does not limit infiltration
through the soil media. The perforations can be placed closest to the invert of the pipe to achieve
maximum potential for draining the facility. Structure joints shall be sealed so they are watertight.
At least one line of underdrain should be placed for every 8 feet of filter area's width (i.e., placed
no further than 8 feet apart)
Underdrain pipes must be bedded in 12 to 14 inches of clean, well-graded coarse gravel that
meets the MEDOT specification 703.22 Underdrain Type B for Underdrain Backfill.
A choking layer composed of 2" of washed sand and 2" of #8 stone should be placed above the
gravel layer to prevent the underdrain from clogging from migrating media particles.
The underdrain must drain freely and discharge to the existing stormwater infrastructure.
Plant Selection
For the SCM to function properly as stormwater treatment and blend into the landscape, vegetation
selection is crucial. Appropriate vegetation will have the following characteristics:
1. Plant materials must be tolerant of drought, ponding fluctuations, and saturated soil conditions
for 10 to 48 hours.
2. It is recommended that a minimum of three tree, three shrubs, and/or three herbaceous
groundcover species be incorporated to protect against facility failure from disease and insect
infestations of a single species.
3. Native plant species or hardy cultivars that are not invasive and do not require chemical inputs
are recommended to be used to the maximum extent practicable.
4. Refer to Appendix B, Volume 1 of the Maine Stormwater Manual for a list of appropriate
bioretention plant species for the City of Sanford.
5. After planting, the filter area should be mulched with 2-3 inches of triple-shredded hardwood
mulch. Do not fertilize after planting.
Geotechnical Investigation
A full geotechnical investigation is necessary to characterize the soils prior to final design. Pertinent
information includes permeability at each bioretention site, hydrologic soil group type, depth to water
table, and the presence of expansive soils. If expansive soils are present, bioretention design should
include an impermeable barrier since the proposed bioretention cell locations are adjacent to
infrastructure such as roads and buildings.
As a result of the historic industrial use of the gateway park parcel and areas adjacent to Washington
Street an investigation of potential soil and groundwater contaminants should be conducted in these
areas before the implementation of any infiltration based treatment practices. In the event that
subsurface investigations reveal the presence of contaminants, infiltration limiting elements such as
impermeable synthetic or clay liners may be incorporated into any green infrastructure practices. While
22
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the use of liners will certainly alter the runoff volume reducing properties of these practices, pollutant
and flow rate reduction benefits will be partly retained.
Maximizing Infiltration
SCMs implemented over soils with low permeability can be hydrologically connected to SCMs
implemented over high permeability soils through the underdrain systems. Hydrologically connecting the
SCMs where infiltration will be limited to locations where infiltration will be higher will maximize the
treatment capacity of the site providing a greater overall infiltration capacity. Note that when infiltration
is concentrated via a subsurface fluid distribution system, it may be considered a Class V well and will
need a permit.
Table 7. Traditional Bioretention Specifications
1. Siting Setbacks
Pavement
No requirement
Building
No requirement with lined bottom; otherwise,
Basement: > 10 feet
No Basement: > 5 feet
Property
lines/ROW
> 2 feet / > 0 feet
2. Volume
Bottom slope
Flat
Side slopes
Bioretention: 2H:1V or flatter
Planter Box: Vertical retaining wall
Freeboard
6 to 12 inches
3. Vertical Component
Surface Storage
6 to 12 inches
Growing Layer
> 12 inches soil media;
3 inches of mulch, max
2 to 4 inches of clean medium sand (ASTM c-33) over 2 to 3 inches of #8 or #78 washed
stone when drainage layer is used
Filter Layer
Drainage Layer
Recommended 12 to 30 in. of clean coarse aggregate AASHTO #4, #5, or equivalent
Native Material
Test infiltration; > 1/2 in/hr if designing with infiltration
4. Drainage
Inlet
Curb inlet; sheet flow through grass filter strip;
downspout w/ energy dissipation
4-inch perforated PVC placed to meet dewatering requirement if needed; cleanout at
terminal ends and every 250-300 feet
Underdrain
Outlet
Required to meet release rates
_ ,., Downstream inlet or catch basin set 6 to 12 inches above soil surface and connected to
Overflow . .
storm drainage network
23
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4. Drainage (cont.)
Infiltration Meet water quality volume requirement
Surface: < 24 hours
Dewatermg
Sub-surface: < 72 hours
5. Composition
Surface Treatment Vegetation and mulch
Soil Media With or without an underdrain, meets dewatering requirement; supports plant growth
Side Slopes Grass or mulch
Mulch Triple-shredded hardwood
6. Pollutant
Pretreatment Required. May include grass filter strip, stone trench, forebay, sump inlets
7. Maintenance
Access Able to be accessed by a vehicle
Requirements Designed and maintained to improve water quality; Maintenance plan should be in place
Table 8. Permeable Pavement Specifications
1. Siting Setbacks
Pavement No requirement
No requirement with lined bottom; otherwise,
Building Basement: > 10 feet
No Basement: > 5 feet
Property lines/ROW > 2 feet / > 0 feet
2. Volume
slope Less than 0.5 percent
Side slopes Not applicable
Freeboard Not applicable
3. Vertical Component
Surface Layer Interlocking Concrete Pavers; Concrete Grid Pavers; Plastic Grid Pavers; Concrete; Asphalt
Growing Layer Not applicable
1) Perm. Interlocking Cone. Pavers: 1.5 to 3 inches of #8 or #78 washed stone
Bedding 2) Concrete and Plastic Grid Pavers: 1 to 1.5 inches of bedding sand
3) Permeable Concrete and Asphalt: None
12 to 30 in. of clean aggregate AASHTO #56 or equivalent; thickness depends on
strength/storage needed; install 30 mil geotextile liner where aggregate meets soil
Native Material Compacted as sub-base
24
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4. Drainage
Inlet
Pavement surface
Outlet
Required to meet release rates
Overflow
Downstream inlet
Infiltration
Meet water quality volume requirement
Dewatering
< 72 hours
5. Composition
For interlocking or grid-type pavers use fine aggregate, coarse sand, or top soil & grass in
Surface Treatment
openings
6. Pollutant
Pretreatment
Divert runoff from sediment sources away from pavement
7. Installation and Maintenance
Installation
Per manufacturer's recommendation
Load Bearing
Designed for projected traffic loads using AASHTO methods
Requirements
Designed and maintained to improve water quality; Maintenance plan should be in place
Notes: A reinforced concrete transition width (12-18 inches) is required where permeable pavement meets adjacent non-
concrete pavement or soil.
structural. Layer
(washed IA.O systole)
Pen/lows
qeotextlle
(As Ktc\u-irtd)
-dtsturbec) Base soil
Figure 16. Permeable interlocking concrete
ivMl, washed
structural. Layer
Figure 17. Pervious concrete
25
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Operations and Maintenance
This section provides recommendations for the maintenance of green infrastructure practices applicable
to the conceptual design at the Washington Street and Gateway Park sites. Maintenance tasks and the
associated frequency of the tasks are included for the practices incorporated into the conceptual designs.
Bioretention
Maintenance activities for bioretention are generally similar to maintenance activities for any public
garden or landscaped area. The focus is to remove trash and monitor the health of the plants, replacing
or thinning plants as needed. Over time, a natural soil horizon should develop which will assist in plant
and root growth. An established plant and soil system will help in improving water quality and keeping
the practice drained. Irrigation for the landscaped practices may be needed, especially during plant
establishment periods or in periods of extended drought. Irrigation frequency will depend on the season
and type of vegetation. Native plants often require less irrigation than non-native plants.
In winter climates experiencing heavy snowfall, such as southern Maine, the plowing of snow on to the
bioretention area should be avoided if possible. Over time the snow and ice can compact the bioretention
media reducing infiltrative capacity and overall function. In addition sand which is spread on roadways to
provide vehiculartraction can accumulate on the top of the media bed near inlets and should be removed
after each snow season using hand tools.
Table 9. Bioretention Operations and Maintenance Considerations.
Task
Monitor infiltration and
drainage
Pruning
Mowing
Mulching
Mulch removal
Watering
Fertilization
Remove and replace dead
plants
Inlet inspection
Frequency
1 time/year
1-2 times/year
2-12 times/year
1-2 times/ year
1 time/2-3 years
1 time/2-3 days for first 1-
2 months; sporadically
after establishment
1 time initially
1 time/year
Once after snow season,
then monthly during the
remainder of the year.
Maintenance notes
Visually inspect drainage time (12 hours). Might
have to determine infiltration rate (every 2-3 years).
Turning over or replacing the media (top 2-3 inches)
might be necessary to improve infiltration (at least
0.5 in/hr).
Nutrients in runoff often cause bioretention
vegetation to flourish.
Frequency depends on the location, plant selection
and desired aesthetic appeal.
Recommend maintaining l"-3" uniform mulch layer.
Mulch accumulation reduces available water storage
volume. Removal of mulch also increases surface
infiltration rate of fill soil.
If drought conditions exist, watering after the initial
year might be required.
One-time spot fertilization for first year vegetation.
Within the first year, 10% of plants can die. Survival
rates increase with time.
Check for sediment accumulation to ensure that
flow into the bioretention area is as designed.
Remove any accumulated sediment.
26
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Task
Outlet inspection
Underdrain inspection
Miscellaneous upkeep
Frequency
Once after the snow
season then monthly
during the remainder of
the year
Once per year
12 times/year
Maintenance notes
Check for erosion at the outlet and remove any
accumulated mulch or sediment.
Check for accumulated mulch or sediment. Flush if
water is ponded in the bioretention area for more
than 12 hours.
Tasks include trash collection, plant health, spot
weeding, and removing mulch from the overflow
device.
Permeable Pavement
The primary maintenance requirement for permeable pavement consists of regular inspection for clogging
and sweeping with a vacuum-powered street sweeper. If interlocking concrete permeable pavers are
installed, the small aggregate used to fill the void between pavers must be replaced following vacuum
sweeping. However, if use of the proposed permeable walkways in Gateway Park is limited to foot traffic
only, actual maintenance requirements will be much less than are typically recommended in Table 10.
Table 10. Permeable Pavement Operations and Maintenance Considerations
Task
Impervious to Pervious
Interface
Vacuum street sweeper
Replace fill materials
(applies to pervious
pavers only)
Miscellaneous upkeep
Monitor infiltration and
drainage
Frequency
Quarterly
Twice per year as needed
1-2 times per year (and
after any vacuum truck
sweeping)
4 times per year or as
needed for aesthetics
After each rainfall event
Maintenance notes
Check for sediment accumulation to ensure that
flow onto the permeable pavement is not restricted.
Remove any accumulated sediment. Stabilize any
exposed soil.
Portions of pavement should be swept with a
vacuum street sweeper at least twice per year or as
needed to maintain infiltration rates.
Fill materials will need to be replaced after each
sweeping and as needed to keep voids with the
paver surface.
Tasks include trash collection, sweeping, and spot
weeding.
Visually inspect pavement surface for signs of
surface ponding.
27
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Stormwater Control Measure Cost Estimates
The construction cost estimates for implementing the Green Infrastructure SCMs along Washington Street
are found in Table 11. Costs for the project are estimated based on the existing site conditions and account
for the potential necessity of underdrains and providing detention for 1" of runoff from impervious
surfaces. The construction cost estimate for implementing the Gateway Park Green Infrastructure SCMS
is included separately in Table 12. These costs do not include the cost associated with removal of the
existing buildings and pavement surfaces. The costs for both project areas include both construction of
the SCMs as well as site preparation, mobilization, etc., but do not account for utility removal/rerouting
that may be required upon site survey and final design. It is also assumed that all construction is retrofit.
Table 11. Cost Estimate for Washington Street Green Infrastructure SCMs
Item No
1
2
3
4
5
6
7
8
9
Description
Preparation
Traffic Control
Site Preparation
Curb and Gutter Removal
Excavation and Removal
Traditional Bioretention
Fine Grading
Soil Media
Filter Layer (sand and No. 8 stone)
Vegetation
Mulch
Curb and Gutter
Quantity
15
591
680
5238
291
65
5238
32
1290
Unit
day
LF
CY
SF
CY
CY
SF
CY
LF
Unit Cost
$1,000.00
$3.30
$45.00
$0.72
$40.00
$45.00
$4.00
$55.00
$22.00
Construction Subtotal
10
11
12
Planning (20% of subtotal)
Mobilization (10% of subtotal)
Construction contingency (20% of subtotal)
Construction Total
13
Design (30% of Construction Total)
Total Cost
Total
$15,000
$1,951
$30,582
$3,771
$11,639
$2,910
$20,950
$1,778
$28,389
$116,970
$23,394
$11,697
$23,394
$175,454
$52,636
$228,091
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Table 12. Cost Estimate for Gateway Park Green Infrastructure SCMs
Item No
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Description
Preparation
Traffic Control
Site Preparation
Excavation and Removal
Traditional Bioretention
Fine Grading
Soil Media
Filter Layer (sand and No. 8 stone)
Vegetation
Mulch
Curb and Gutter
Permeable Pavement
Permeable Concrete
Structural Layer (washed no 57 or no 2 stone)
Grassed Detention
Fine Grading
Concrete Box Riser Structures
Precast Concrete Junction Box
18" CMP
Quantity
15
574
5969
169
38
3040
19
478
4420
82
4840
2
1
105
Unit
day
CY
SF
CY
CY
SF
CY
LF
SF
CY
SF
EA
EA
LF
Unit Cost
$1,000.00
$45.00
$0.72
$40.00
$45.00
$4.00
$55.00
$22.00
$12.00
$50.00
$0.72
$3,500.00
$1,200.00
$31.26
Construction Subtotal
15
16
17
Planning (20% of subtotal)
Mobilization (10% of subtotal)
Bond (5% of subtotal)
Construction contingency (10% of subtotal)
Construction Total
18
Design (40% of Construction Total)
Total Cost
Total
$15,000
$25,817
$4,298
$6,756
$1,689
$12,160
$1,032
$10,516
$53,040
$4,093
$3,485
$7,000
$1,200
$3,282
$77,267
$15,453
$7,727
$773
$15,453
$116,673
$46,669
$163,343
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Conclusions
Evaluation of the Sanford Mill Yard complex revealed numerous opportunities to integrate green
infrastructure primarily within the existing transportation right of ways and on publically owned spaces.
Permeable pavement, bioretention, and tree box filters were identified as the most applicable practices
within the project area; additionally, grassed detention is suitable for the Gateway Park site. Through
hydrologic modeling, the incorporation of these practices was shown to provide significant treatment of
the total runoff volume for the 2- and 10-year storm events (between 55% and 91% total volume). In
addition, a small amount of peak flow reduction is also provided for the 2-year storm event, particularly
at the higher elevation storm structures. Although a majority of design storm flows were treated and
discharged through the drainage network, implementation of the green infrastructure retrofits alone
did not mitigate the increase in peak flow along Washington St.; with or without the addition of the
Main Street drainage diversion.
However, the Washington Street project site is ideal for demonstrating a variety of green infrastructure
practices incorporated into a redeveloping brownfield to address water quality concerns. Because of its
location along the Mousam Mill Yard and adjacent to the visually stimulating dam for Millpond Number 1
there is potential for much interest from local officials, developers, and residents. Implementation of
green infrastructure in the project area will enhance ongoing redevelopment activities in the Mill Yard
while reducing the impact of stormwater runoff to the Mousam River. In addition, the green infrastructure
practices prescribed for the Mill Yard Complex and the integration of these practices in the project site
may serve as examples to other similar communities in the northeastern region or elsewhere in the U.S.
where aging brownfields are being converted into residential and commercial uses.
As the City of Sanford moves forward with development of the Gateway Park and potential modifications
to Washington Street and its associated drainage system, the concept plans contained in this report can
serve as a basis for development of final design documents. In order to ensure project success the
community should conduct detailed subsurface investigations to verify conditions are suitable for the
green infrastructure practices proposed. Furthermore as the city begins planning for these project areas
the conceptual design plans should be provided to any stakeholder groups or planning consultants so that
they can be incorporated into the planning process.
30
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Center for Neighborhood Technology and American Rivers. 2010. The Value of Green Infrastructure: A
Guide to Recognizing Its Economic, Environmental and Social Benefits.
Collins, K.A., W.F. Hunt, J.M. Hathaway. 2010. Side-by-Side Comparision of Nitrogen Species Removal for
Four Types of Permeable Pavement and Standard Asphalt in Eastern North Carolina. Journal of
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Correll, M.R., J.H. Lillydahl, and L.D. Singell. 1978. The Effects of Greenbelts on Residential Property
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http://www.naturewithin.info/UF/TreeBenefitsUK.pdf. Accessed September 2010.
Hobden, D., G. Laughton. and K. Morgan. 2004. Green Space Bordersa Tangible Benefit? Evidence
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Manual, Accessed January
30, 2015.
Maine Department of Environmental Protection. 2012. Integrated Water Quality Monitoring and
Assessment Report. Available at
www.state.me.us/dep/water/monitoring/305b/2012/appendices-final.pdf
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Appendix A. SWMM Analysis
Hydrology and Hydraulics
SWMM Background
The EPA's Stormwater Management Model (EPA SWMM v5.0) was utilized to assess the existing
conditions of the Washington Street stormwater infrastructure and evaluate green infrastructure
opportunities. SWMM is a dynamic precipitation-runoff simulation model designed for discrete event
or continuous representation of hydraulics, hydrology, and water quality. It is optimized and
designed for storm event flow management in urban area drainage systems. First developed in
1971, SWMM has undergone numerous updates and enhancements; the current public version of
SWMM as of this writing is 5.0.022 (released April 2011).
SWMM represents land areas as a series of subcatchments, with properties that define retention and
runoff of precipitation, infiltration, percolation to a shallow aquifer, and discharge from the aquifer.
Subcatchments are connected to the drainage network, which may include natural watercourses, open
channels, culverts and storm drainage pipes, storage and treatment units, outlets, diversions, and many
other elements of an urban drainage system. Nodes and links are used in SWMM to define the connectivity
and control within the drainage network. Precipitation and other meteorological input time series are
used to drive the hydrologic and water quality response in the simulation. Subcatchment runoff is directed
to junction nodes, which are connected to the channel network. The channel network is represented with
a series of condu/te (a type of link) and junction nodes, allowing for the specification of variation in channel
properties.
Green infrastructure improvements were represented using the LID component recently introduced in
SWMM 5. LID is modeled in SWMM using a layered configuration, and it allows a great deal of flexibility
in representing various types of practices, including bioretention, swales, infiltration devices, permeable
pavement, rain barrels, cisterns, etc. Green-Ampt infiltration parameters can be defined for filtering
media, and the model tracks evaporation and soil moisture allowing infiltration rates during runoff events
to be dynamic. Table A-l provides the assumptions used in the configuration for bioretention utilized in
the LID control feature of SWMM. The primary design parameters for modeling hydraulic performance of
bioretention in SWMM include the surface storage depth, planting soil depth and void space ratio,
aggregate reservoir depth and void space ratio, and native soil infiltration rate.
33
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Table A-l. SWMM LID Input Values for Bioretention
Property
Surface ponding (in)
Media thickness (in)
Porosity (fraction)
Field Capacity (fraction)
Wilting Point (fraction)
Conductivity (in/hr)
Conductivity slope
Suction head (in)
Storage layer (in)
Storage void ratio
Bottom infiltration rate (in/hr)
Underdrain?
Underdrain offset
Underdrain coefficient *
Underdrain exponent *
Bioretention
6
36
0.437
0.105
0.047
1.18
7
2.4
14
0.54
0
Yes
0
5
1
* Values used reflect no limitation on underdrain outflow (i.e., outflow rate is limited by media filtration rate)
Existing Hydrology and Hydraulics
To assess the existing capacity of the Washington Street drainage system and to evaluate the feasibility
of using green infrastructure SCMs to mitigate the increase in proposed drainage area, multiple hydrologic
calculations were required. Rainfall/runoff simulations were calculated for the 2-year, 24-hour and 10-
year, 24-hour storm events which were identified by the project team as appropriate design storms to
evaluate the performance of piped storm system conveyance. According to Chapter 2, Volume III of
Maine's BMPs Technical Design Manual, rainfall depth estimates for Sanford, Maine are 3 inches and 4.6
inches, respectively, for the two design storms. Rainfall distributions for the Type III, 24 hour storm were
applied to the rainfall depths to develop an event-based time series for the model with 6 minute time
intervals.
SWMM automatically calculates the runoff coefficient (C) based on a subcatchment's percent impervious
area. Runoff coefficients are used to estimate initial abstraction from the drainage areas and calculate the
fraction of rainfall that becomes runoff. Table A-2 shows the calculated runoff coefficients and the
estimated peak runoff rates for the existing drainage network in the study area.
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Table A-2. Existing Drainage Area Runoff Rates
Subcatchment
Com-l
Com-2
RD-1
RD-2
RD-3
Mix-1
Mix-2
Com-3
Res-1
Rd-4
c
0.978
0.992
0.92
0.992
0.914
0.89
0.872
0.657
0.873
0.992
Peak Runoff
2-year, 24-hour
(cfs)
3.96
2.3
0.13
0.63
0.37
4.22
2.65
0.82
2.39
0.34
Peak Runoff
10-year, 24-hour
(cfs)
6.11
3.57
0.21
0.97
0.58
6.71
4.26
1.42
3.86
0.52
Results from the hydraulic routing through the existing drainage system are displayed in Table A-3 for
both design storms. Reported parameters include the peak flow through the junction, the amount of time
that the node is surcharged (depth of water is above the highest conduit), and the amount of time the
node is flooded (depth of water is above rim elevation of the structure). These parameters were most
important in mitigating the impact from the additional drainage area using the proposed green
infrastructure practices.
Table A-3. Existing Condition Hydrology/Hydraulics at Trunk-Line Junctions
Junction
Node
CB-l
MH-1
MH-2
J-l
MH-3
J-2
J-3
MH-5
MH-4
J-4
2-year, 24-hour
Q(cfs)
0.13
0.32
4.14
8.13
8.34
9.05
11.64
13.94
9.68
9.98
Hours
Surcharged
0
0
0.19
0.36
0.2
0.41
0.6
0.44
0.74
0.61
Hours
Flooded
0
0
0
0
0.01
0
0
0.31
0.02
0
10-year, 24-hour
Q(cfs)
1.27
1.21
6.32
12.32
12.32
9.07
12.84
16.41
9.68
10.11
Hours
Surcharged
0.21
0.22
0.37
0.66
0.38
0.68
0.85
0.71
0.97
0.86
Hours
Flooded
0
0
0.1
0
0.26
0
0
0.57
0.19
0
35
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Total runoff from all of the drainage areas was estimated as 1.66 ac-ft and 2.86 ac-ft, respectively, for the
2- and 10-year storm events. However, not all of this runoff is conveyed through the drainage system and
discharged through the outlet. As shown above, some of the junctions experience periodic flooding during
the 2-year storm event and greater. Based on SWMM results, approximately 0.3 ac-ft and 0.7 ac-ft of
runoff is considered internal outflow for the 2- and 10-year events. Internal outflow is the total volume of
runoff that exceeds the capacity of the underground conveyance system and leaves the system via non-
outfall nodes (e.g., surface flow). Based on the steep gradient and curb-inlet configurations within the
Washington Street study area, the model was configured to assume no ponding and temporary storage
at the storm sewer junctions.
Proposed Hydrology and Hydraulics
SWMM was also utilized to evaluate the hydraulic impacts to the drainage system by adding runoff from
Main Street as a part of a subsequent combined-sewer separation project. The proposed drainage area
on Main Street consists of approximately 1 acre of impervious roadway between the Washington Street
and Berwick Road intersections. The drainage area, as detailed in Table A-4, is modeled in SWMM to
connect to the Washington Street drainage system at CB-1. This catch basin is approximately 6 feet deep,
which is more than sufficient depth to receive drainage from a storm sewer installed down the proposed
415-foot long Main Street drainage area.
Table A-4. Main Street Drainage Area
Property
Area (ac)
Flow path (ft)
Slope (%)
Imperv. (%)
Outlet node
Main-1
0.99
415
0.1%
100
CB-1
Results from the SWMM model (Table A-5) show the impacts to the existing drainage system junctions on
Washington Street by adding the Main Street drainage area at CB-1. As shown in Table A-6, Significant
increases in peak flow and time of surcharge/flooding are predicted at the upper junctions (e.g., CB-1,
MH-1, MH-2) where flooding was not previously occurring for the existing conditions simulation.
However, less impact to the drainage system is incurred at the lower junctions since these junctions
already experience flooding. Since the model assumes no ponding at these junctions, extra runoff to these
locations is added to the internal outflow in the model.
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Table A-5. Proposed Condition Hydrology/Hydraulics at Trunk-Line Junctions
Junction
Node
CB-l
MH-1
MH-2
J-l
MH-3
J-2
J-3
MH-5
MH-4
J-4
2-year, 24-hour
Q(cfs)
2.15
2.16
6.1
10.32
10.33
9.14
11.64
13.94
9.67
9.98
Hours
Surcharged
0.17
0.17
0.31
0.54
0.29
0.54
0.71
0.56
0.83
0.7
Hours
Flooded
0
0
0
0
0.15
0
0
0.4
0.01
0
10-year, 24-hour
Q(cfs)
3.5
2.83
8.25
12.32
12.32
9.18
12.84
16.41
9.67
10.11
Hours
Surcharged
0.4
0.39
0.59
0.8
0.53
0.8
0.96
0.81
1.11
0.94
Hours
Flooded
0.23
0
0.19
0
0.34
0
0
0.67
0.18
0
Table A-6. Percent Increase in Hydrology/Hydraulics from Main Street Drainage Area
Junction
Node
CB-l
MH-1
MH-2
J-l
MH-3
J-2
J-3
MH-5
MH-4
J-4
2-year, 24-hour
Q(cfs)
1554%
575%
47%
27%
24%
1%
0%
0%
0%
0%
Hours
Surcharged
N/A1
N/A1
63%
50%
45%
32%
18%
27%
12%
15%
Hours
Flooded
0%
0%
0%
0%
1400%
N/A1
N/A1
29%
-50%
N/A1
10-year, 24-hour
Q(cfs)
176%
134%
31%
0%
0%
1%
0%
0%
0%
0%
Hours
Surcharged
90%
77%
59%
21%
39%
18%
13%
14%
14%
9%
Hours
Flooded
N/A1
0%
90%
0%
31%
0%
0%
18%
-5%
0%
No flooding calculated for existing conditions; divisible-by-zero error
37
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