&EPA
United States
Environmental Protection
Agency
2014 GREEN INFRASTRUCTURE TECHNICAL ASSISTANCE PROGRAM
Ada County Highway District
Garden City, ID
Fairview Avenue Green Street Conceptual Design
Evaluating Off-site Mitigation through Green Infrastructure within a Public
Right-of-way
Rendering: Tetra Tech, Inc.
July 2015
EPA832-R-15-011
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About the 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. Green infrastructure can be a cost-effective approach for
improving water quality and helping communities stretch their infrastructure investments further by
providing multiple environmental, economic, and community benefits. This multi-benefit approach
creates sustainable and resilient water infrastructure that supports and revitalizes urban communities.
The U.S. Environmental Protection Agency (EPA) encourages communities to use green infrastructure to
help manage stormwater runoff, reduce sewer overflows, and improve water quality. EPA recognizes
the value of working collaboratively with communities to support broader adoption of green
infrastructure approaches. Technical assistance is a key component to accelerating the implementation
of green infrastructure across the nation and aligns with EPA's commitment to provide community
focused outreach and support in the President's Priority Agenda Enhancing the Climate Resilience of
America's Natural Resources. Creating more resilient systems will become increasingly important in the
face of climate change. As more intense weather events or dwindling water supplies stress the
performance of the nation's water infrastructure, green infrastructure offers an approach to
increase resiliency and adaptability.
For more information, visit http://www.epa.Qov/cireenmfrastructure
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Acknowledgements
Principal USEPA Team
Jamie Piziali, USEPA
Christopher Kloss, USEPA
Matt King, USEPA
Krista Mendelman, USEPA Region 10
Community Team
Joan Meitl, Ada County Highway District
Erica Anderson-Maguire, Ada County Highway District
Justin Bledsoe, Ada County Highway District
Rob Bousfield, City of Boise
Robbin Finch, City of Boise
Jason Korn, Ada County Highway District
Sarah Schafer, City of Boise
Jay Story, West End Project Coordinator
Consultant Team
Jonathan Smith, Tetra Tech
Martina Frey, Tetra Tech
Maureen Harris, Tetra Tech
Kelly Meadows, Tetra Tech
Alex Porteous, Tetra Tech
Adam Orndorff, Tetra Tech
This report was developed under EPA Contract No. EP-C-11-009 as part of the 2014 EPA Green
Infrastructure Technical Assistance Program.
in
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Contents
Executive Summary vi
1 Introduction 1
1.1 Water Quality Issues/Goals 2
1.2 Project Overview and Goals 2
1.3 Project Benefits 3
1.4 Local Challenges 4
2 Site Conditions 5
2.1 Site Selection 7
3 Design Approach 8
3.1 Site-Specific Constraints/Considerations 8
3.2 Stormwater Standards 9
3.3 Stormwater Toolbox 10
3.3.1 Permeable Pavement 11
3.3.2 Bioretention 13
3.3.3 Tree Filter 16
4 Conceptual Design 17
4.1 Concept Plan 17
4.2 Calculations 19
4.3 Costs 20
5 Off-site Mitigation Policy Approach 22
5.1 Off-site Mitigation Models 25
5.1.1 Bilateral Trading 25
5.1.2 Citywide Trading 26
5.1.3 Public Aggregation 27
5.2 Preferred Off-site Mitigation Approach for Boise 29
6 Conclusion 31
7 References 32
Appendix A: Fairview Avenue Green Street Conceptual Design 33
Appendix B: Fairview Avenue Green Street Conceptual Design Calculations 35
Appendix C: Fairview Avenue Green Street Conceptual Cost Estimates 37
IV
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Figures
Figure 1. The Boise Metro Area is located along the scenic Boise River 1
Figure 2. Map of the Main-Fairview sub-district project area 3
Figure 3. Many parcels along Main St. and Fairview Ave are vacant and have converted to informal
parking areas 5
Figure 4. Main Street traffic density is well under design capacity 6
Figure 5. The Fairview Avenue drainage system discharges directly into the Boise River 6
Figure 6. Fairview Avenue looking east from 25th Street 7
Figure 7. Existing Fairview Avenue cross-section 8
Figure 8. Fairview Avenue Preferred Street Section 9
Figure 9. This infiltration basin contrasts with the surrounding residential neighborhood 10
Figure 10. Infiltration basins often become overgrown with weeds and other vegetation 11
Figure 11. Permeable paver side street parking installed on 36th Street, Garden City, ID 12
Figure 12. Private interests in Boise have adapted permeable pavement into parking systems 12
Figure 13. Bioretention located in a parking median at the Ustick Library 13
Figure 14. A bioswale along N 14th Street provides stormwater management for a commercial carwash .. 14
Figure 15. Bioretention implemented into a roadway to treat road runoff 15
Figure 16. Bioretention integrated within a right-of-way vegetated fringe 15
Figure 17. Example of a tree system which utilizes the area under the sidewalk to infiltrate stormwater
while allowing adequate room fortree root growth 16
Figure 18. Proposed green street conceptual design cross-section 17
Figure 19. View of the 2400 Block of Fairview Avenue looking east (top) and an artist's rendering
of the green street design (bottom) 18
Tables
Table 1. Summary of green street design geometry and retention volume 19
Table 2. Summary of green street conceptual design cost estimates 20
Table 3. Summary of conventional infiltration basin cost estimate for case study project 20
Table 4. Unit cost of retention comparison between green street and
conventional infiltration practice estimates 21
Table 5. Impact of property value on conventional stormwater management unit cost 21
Table 6. Advantages and disadvantages of off-site mitigation 24
Table 7. Comparison of off-site mitigation models 29
Table 8. Pertinent green street conceptual design parameters 36
Table 9. Detailed cost estimate for Fairview North 37
Table 10. Detailed cost estimate for Fairview South 38
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Executive Summary
Ada County Highway District and the City of Boise are among six co-permittees subject to a National
Pollutant Discharge Elimination System Municipal Separate Storm Sewer System (MS4) permit covering
Boise and Garden City in southwest Idaho. While stormwater retention standards for new development
activities have been in place in some communities for over two decades, the most recent permit, issued
in 2012, expands this standard to also cover redevelopment activities and establishes the minimum
retention standard for all co-permittees (retention of runoff from 0.6 inches of rainfall in 24 hours).
In areas of new development the retention standard typically is addressed by implementing an
infiltrating stormwater facility in a set-aside area. In redevelopment areas, existing infrastructure and
space limitations make dedicating high-value parcel area for stormwater management less desirable to
developers and can be a disincentive to redevelopment. An example of this scenario occurs just west of
downtown Boise, specifically along Main Street and Fairview Avenue. These streets historically served as
the primary east-west corridor for commercial and commuter traffic between downtown Boise and
points west of the city. In the last 20 years, however, the construction of a nearby interstate bypass has
significantly reduced traffic volume along both streets, resulting in dwindling commercial activity on
adjacent parcels and high vacancy.
As a part of district-wide planning efforts in this area, the City of Boise identified Main Street and
Fairview Avenue as priority locations for redevelopment with a vision to create a pedestrian-friendly
retail/commercial corridor that link the downtown to the nearby Boise Greenbelt. The proposed plan—
which calls for fewer travel lanes and the addition of streetside parking, trees, and other pedestrian-
friendly amenities in the right-of-way—provides an opportunity to incorporate a green street concept
into an existing roadway corridor while also managing stormwater.
The green street conceptual design developed for one block of Fairview Avenue identifies green
infrastructure practices that are applicable for the right-of-way. Practices incorporated into the concept
plan include permeable pavement, curbside bioretention planter boxes, and tree box systems to accept
runoff from adjacent impermeable areas. The concept design is evaluated to determine the extent to which
the green street features can provide off-site credit/mitigation for stormwater retention requirements in
the adjacent property. The case study shows that the green street features can offset a portion of the
stormwater requirements of the adjacent parcels through direct management of runoff within the right-
of-way. The green street offset option is cost competitive when compared to on-site stormwater
management and provides the additional community benefits associated with green infrastructure.
The concept design project serves as a demonstration project that will introduce green infrastructure
practices to developers and the public. It will provide additional evidence of the feasibility and
functionality of green infrastructure practices in a semi-arid climate where multiple jurisdictions share
responsibility for stormwater management.
Options for development of a regional scale off-site mitigation program that can be undertaken by local
agencies to coordinate on-site and off-site stormwater retention projects for improved flexibility and
environmental benefit are also included. This report describes advantages and disadvantages of off-site
mitigation and outlines several program models that could be considered for the region (bilateral
trading, citywide trading, and public aggregation). One option that Boise area stakeholders could
consider is a version of the public aggregation model in which green street projects are undertaken in
upstream parts of the storm sewershed to create capacity in the drainage system and help the agencies
meet their total maximum daily load (TMDL), MS4 permit, and other water quality obligations.
VI
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I Introduction
Boise is the capital and most populous city in Idaho with a population of 205,671 (2010 Census). The City
is located on a broad, flat plain bordered by the Boise Mountains to the north and the Owyhee
Mountains to the south. It is in the southwestern portion of the state and lies within the Intermountain
West geographic region. The Boise River flows through the City and is a popular destination for tubers,
floaters, and fishermen. A 25-mile "greenbelt" runs along the river and provides scenic views, wildlife
habitat, and pedestrian access to many of the city's popular riverside parks. The Greenbelt also serves as
an alternative transportation route for cyclist and pedestrian commuters. The City is also home to Boise
State University.
Photo credit: Tetra Tech, Inc.
Figure I. The Boise Metro Area is located along the scenic Boise River
Ada County Highway District (ACHD) is a Phase I National Pollutant Discharge Elimination System
(NPDES) co-permittee with Boise City, Idaho Transportation Department (ITD), Boise State University
(BSU), Ada County Drainage District 3 (DD3), and Garden City for an area within the corporate boundary
of the City of Boise and Garden City, Idaho. ACHD is also the sole permittee under a Phase II permit
which includes all areas within the Boise Urbanized Area that are not covered by the Phase I permit. The
ACHD is responsible for 2,200 miles of public roads within Ada County. Boise is located within ACHD's
boundaries and adjacent to the Boise River.
Both ACHD and Boise have required on-site retention of stormwater since the early 1980s, with the
exception of those areas with an existing storm drain system. Several neighborhoods in and around
downtown Boise date back to the early 20th century, when stormwater management requirements were
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minimal. Many of these neighborhoods discharge stormwater directly to the Boise River without
treatment. This project aims to explore the potential for green infrastructure placed in the right-of-way
of one such neighborhood to provide off-site mitigation of stormwater to adjacent parcels subject to
potential redevelopment. Through this project, ACHD and the City of Boise also hope to integrate the
concept of green infrastructure into future roadway improvements and educate developers, engineers,
and other stakeholders on the benefits of green infrastructure in the right-of-way.
I. I Water Quality Issues/Goals
The City is located on the water-quality limited Boise River, which has total maximum daily loads
(TMDLs) for sediment and bacteria. Designation of a water segment as water-quality limited indicates
that water quality does not meet and/or is not expected to meet applicable standards. Currently a TMDL
is under development for phosphorus, and a TMDL for temperature is expected to be developed shortly
thereafter. Several local agencies are co-permittees because multiple entities are responsible for
overseeing stormwater management. The primary stormwater drainage system within the Phase I
NPDES Permit area comprises the ACHD-owned and -operated municipal separate storm sewer system
(MS4), privately owned on-site drainage facilities, and systems operated by the other NPDES
co-permittees. While on-site retention and treatment requirements for stormwater have been in place
since the 1980s, redevelopment projects have been allowed to discharge to ACHD's drainage network
with treatment and permission. However, the most recent NDPES Phase I permit issued for the
co-permittees requires redevelopment areas to also meet retention criteria. The Phase I permit also
contains an off-site mitigation requirement as follows: "For projects that cannot meet 100%
infiltration/evapotranspiration/reuse requirements onsite, the Permittees' program may allow off-site
mitigation within the same subwatershed, subject to siting restrictions established by the Permittee."
(USEPA Region 10 2012).
1.2 Project Overview and Goals
This project will help ACHD and the City of Boise evaluate off-site stormwater mitigation techniques and
concepts (Sections 3 and 4), as well as explore several off-site mitigation program approaches
(Section 5) to support green infrastructure in the Main-Fairview sub-district of the 30th Street Urban
Renewal District, shown in Figure 2. It will identify and prioritize approaches that use green
infrastructure practices within the right-of-way that meet public and private parcel stormwater
management requirements. Other goals include:
• Incorporating green infrastructure into redevelopment efforts to meet multiple objectives
• Mitigating stormwater impacts from the adjacent neighborhood
• Establishing a safe pedestrian environment
• Protecting the water quality of the Boise River
• Proposing a set of off-site mitigation options that can be used where retention is technically or
physically unachievable.
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Legend
Water Feature
> Storm Drain
Wfestend Parcels
City Owned Parcel
Area of Interest
ROW
Source: Tetra Tech, Inc.
Figure 2. Map of the Main-Fairview sub-district project area
1.3 Project Benefits
With the expansion of the Interstate 184 Boise Connector in the early 1990s, the project area now
experiences a relatively low volume of traffic and has streets that are overly wide. Installing streetscape,
which can include green infrastructure, is a key step toward transforming the area's character and
creating a setting that will attract mixed use redevelopment. Using green infrastructure for stormwater
management will meet the objectives of stormwater quality improvement, Boise River protection, and
community livability by improving aesthetics, providing green space, and contributing to community
cohesion. Urban greening will encourage use of outdoor spaces for social activity and attract businesses
and the public into this area as it redevelops, demonstrating the multiple benefits of green
infrastructure.
The development of an off-site mitigation program to fund off-setting projects will maximize the use of
future green infrastructure projects in the area. The design project will serve as a demonstration project
that will introduce green infrastructure practices to developers and the public. It will provide evidence of
the feasibility and functionality of green infrastructure practices in a semi-arid climate where multiple
jurisdictions have responsibility of managing stormwater.
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IA Local Challenges
ACHD currently uses green infrastructure practices on a limited basis. This is in part due to the additional
work needed to adapt such practices to the local semi-arid climate, develop design standards, and gain
public acceptance. Additionally, identification of off-site mitigation opportunities and sources of funding
is needed to support green infrastructure implementation in areas where on-site retention options are
limited.
Any approach selected for this project needs to address the multiple jurisdictions and varied legal
authorities that share responsibility in the area. The Main Street storm drainage, which includes parts of
Main Street and Fairview Avenue, has an older storm drain system which discharges directly to the Boise
River with very little existing on-site retention and limited stormwater treatment. Additional retrofits
that disconnect this system from impervious surfaces or treat runoff before it enters the system might
be necessary to significantly improve stormwater runoff discharging to the river.
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2 Site Conditions
The Main-Fairview drainage sub-district project area consists of a 125-acre area bordered on the north
by West Idaho Street, on the south by Interstate 184, on the west by the Boise River, and on the east by
West Grove Street. The area is transected by Main Street and Fairview Avenue, four-lane one-way
avenues which previously served as the primary east-west commuter corridor connecting downtown
Boise with areas to the west. These corridors are bordered by commercial parcels, many of which are
vacant as a result of a gradual decline of the sub-district's economic vitality over the last 15 years since
the construction of the Interstate 184 connector. While many parcels are vacant, they typically retain
impervious cover (Figure 3). Additionally the reduction in vehicular traffic in recent years results in a
roadway that operates well under its intended capacity (Figure 4).
Topography within the sub-district is generally flat, with a gradual slope to the west in the direction of
the Boise River. Stormwater drainage within the sub-district is provided by three storm sewer networks
which discharge directly into the Boise River (Figure 5). Surface soils in the area are underlain by
alluvium of the Boise River consisting primarily of sandy cobble gravel, which grades to sandy pebble
gravel with no clay. Soils investigations of sites within the sub-district revealed groundwater at
approximately 10 feet below the surface or deeper and soil permeability approaching 12 inches per hour
(Brown 2013; Wright 2002). These conditions provide an ideal environment for the use of infiltration for
stormwater management.
Photo credit: Tetra Tech, Inc.
Figure 3. Many parcels along Main St. and Fairview Ave are vacant and have converted to informal
parking areas
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Photo credit: Tetra Tech, Inc.
Figure 4. Main Street traffic density is well under design capacity
Photo credit: Tetra Tech, Inc.
Figure 5. The Fairview Avenue drainage system discharges directly into the Boise River
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2.1 Site Selection
In order to evaluate the potential for a green street retrofit within the project area, the project team
conducted an on-site walking tour in July 2014. As a result of the characteristics described in Section 2,
Main Street and Fairview Avenue were identified as the focus area for selection of a concept design site.
The primary objective of the tour was to observe existing conditions within the right-of-way of both
streets with a particular focus on identifying utility conflicts, selecting green infrastructure elements
suitable for the corridors, and evaluating potential locations to implement the conceptual design.
After subsequent discussion within the project team and additional GIS analysis of site conditions, the
2400 block of Fairview Avenue (Figure 6) was selected as the preferred location for the development of
the green street conceptual design. The primary factor in selecting this area was the presence of two
city-owned parcels on the south side of the block (comprising 2.5 acres) that are currently occupied by
an unutilized parking lot. The public parcels may be considered for sale or redevelopment in the near
future, providing a logical location to evaluate how a green street design can serve to off-set on-site
stormwater requirements. Preliminary information indicates that potential redevelopment of these
parcels would utilize driveway access along 24th and 25th streets rather than along Fairview. Additionally,
the private parcels to the north side of the street are currently being utilized by two used car dealerships
which have expressed plans to retain their existing entrance driveways. This relative stability provides
some assurance that green infrastructure practices, if installed, will not be impacted by construction in
the near future. Minimal infrastructure within the right-of-way also makes the selected block an optimal
location to explore green infrastructure implementation.
Photo credit: Tetra Tech, Inc.
Figure 6. Fairview Avenue looking east from 25th Street
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3 Design Approach
3.1 Site-Specific Constraints/Considerations
The 2400 block of Fairview Avenue selected for development of the green street conceptual design is
typical of conditions along both Main Street and Fairview Avenue throughout the sub-district. The block
consists of four eastbound lanes with a narrow, marked bike lane on the southern side of the block
(Figure 7). Observed vehicular traffic on the block during the site visit was light even during morning
rush hour, while adjacent land uses are vacant or lightly utilized commercial businesses.
•••••••••••^H
L e- L 7' U 1
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, Paved Side ,
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so-
ft L<*> L 7- 1 6-
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Source: Boise City Planning and Development Services and Capital City Development Corporation, 2012.
Figure 7. Existing Fairview Avenue cross-section
The existing storm drainage system for the Fairview Avenue drainage watershed is located under the
center of Fairview Avenue while electric utilities are above ground along the northern side of the block.
The sanitary sewer system in the area runs along north-south streets rather than east-west streets.
Information on other utilities was not available. As noted in Section 2, soils in the area exhibit very high
permeability rates. The relative low potential for utility conflicts and high potential for stormwater
infiltration provide near ideal conditions for the incorporation of green infrastructure infiltration
practices.
As described above, there are two parcels along the southern side of the block that are currently vacant
and publicly owned. The condition of these parcels is primarily impervious with minimal areas of
landscaping or gravel along the outer edges. Stormwater runoff originating in these two parcels moves
as surface flow across the parking surface to the surrounding street where it enters the Fairview Avenue
drainage network at one of several curb edge grate inlets. The parcels along the north edge of the block
are also largely characterized by impervious surfaces. Stormwater runoff from these businesses
currently sheet flows to the right-of-way, then flows along the curb gutter before it enters into the Main
Street drainage system. Fairview Avenue is center-crowned so that surface runoff from the southern
side of the street is routed via curb gutters to the Fairview Avenue drainage system, while runoff
originating from the northern side of the right-of-way is routed to the Main Street drainage network.
As a part of the City of Boise 30th Street Area Master Plan, a preferred street section for Fairview Avenue
(Figure 8) was developed which incorporates modifications of the street to accommodate a reduction of
travel lanes, sidestreet parking, and other modifications to make the corridor more inviting to
pedestrians. This street section provides a template upon which to develop a green street concept
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incorporating the proposed lane widths and other street features and green infrastructure practices
identified as appropriate for this location. It should be noted that while this section represents the
preferred configuration of the vehicular, parking, and pedestrian elements within the right-of-way as
determined through the master planning process, implementation through reduction of travel lanes and
incorporation of side street parking is subject to ACHD Commission approval.
, ! 9 L 6.5'
1
15.5'
. 8.5' L
r
Sidewalk/ ' Parking
3 5. Landscape
t ' y
T
ft
11.5'
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L
T
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ir
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'l ]
^ Jr
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6.5' L 9"
/
15.5'
Parking ^ Sidewalk /
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1
3.5'
y
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Source: Boise City Planning and Development Services and Capital City Development Corporation, 2012.
Figure 8. Fairview Avenue Preferred Street Section
Easement
3.2 Storm water Standards
The City's current stormwater management standards require that new and redevelopment projects
manage the peak flow of runoff to match predevelopment hydrology for a 50 or 100-year storm.
Additionally, projects within the City's regulatory authority (commercial development within Boise city
limits) are required to provide treatment of the 0.34 inch rainfall event (equivalent to the 80th percentile
storm). In those areas where regulatory authority rests with ACHD (areas within the right-of-way and
residential development), stormwater management includes matching predevelopment hydrology for
the 2, 25, and 50-year storms, as well as other water quality and retention standards.
One important caveat of the stormwater management requirements of both ACHD and the City of Boise
is the requirement that developers obtain permission prior to discharging stormwater into existing
drainage networks. In areas where no drainage network exists, this requirement effectively requires
retention of nearly all stormwater. Both agencies have set threshold rainfall depths for purposes of
retention system design of 1 inch and 1.15 inches for City of Boise and ACHD, respectively.
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The most recent NPDES Phase I permit issued in 2013, however, requires co-permittees to adopt a
retention standard of 0.6 inches for both new and redevelopment areas, regardless of options for
downstream discharge. This requirement is in the process of being adopted by both ACHD and the City
of Boise, both of which will likely require a retention standard greater than 0.6 inches for new
development so that volume control will also be addressed. The impact of these regulations on areas of
redevelopment, such as the Main-Fairview sub-district, is that development of these properties will
require the same level of stormwater management as undeveloped parcels on the perimeter of the
municipal area (retention of 0.6 inches of rainfall at a minimum).
3.3 Stormwater Toolbox
Historically, stormwater management throughout the Boise region has been addressed through the
design and implementation of centralized structural infiltration practices such as infiltration basins,
seepage beds and infiltration swales. These practices, while functional relative to the stormwater
management criteria, require some special design considerations. Surface infiltration basins take up
valuable area which could be dedicated to other uses, can be unsightly, and may provide little other
community function other than stormwater management (Figure 9 and Figure 10). Developers and
designers may incorporate green infrastructure approaches into infiltration basin design to address
these shortcomings. These approaches include the distribution of infiltration practices throughout a site
rather than use of a single large facility and incorporation of vegetation within the infiltration practice.
Photo credit: Tetra Tech, Inc.
Figure 9. This infiltration basin contrasts with the surrounding residential neighborhood
10
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Photo credit: Tetra Tech, Inc.
Figure 10. Infiltration basins often become overgrown with weeds and other vegetation
In recent years, both ACHD and the City of Boise have seen an increased interest in the use of green
infrastructure approaches and practices to address stormwater management needs throughout the
community, while also improving community livability by improving aesthetics and providing green
space. Bioswales are a design option in both ACHD and the City of Boise design manuals and have been
utilized in the community for many years. Given the areas relatively high in-situ infiltration capacity and
low annual rainfall, other infiltration practices such as bioretention, tree filters, and permeable
pavement are logical practice types to consider. These practices were included in ACHD's recent Green
Stormwater Infrastructure Stormwater Guidance Manual (ACHD 2014c) and detailed design guidance
will be included for these and other green infrastructure practices in the ACHD Policy Manual revisions
currently under development. These practices were determined by the project team to be suitable for
consideration in the Main-Fairview roadway corridor, both at the conceptual design site and elsewhere
in the project area.
3.3.1 Permeable Pavement
Permeable pavement refers to pavement systems which incorporate a porous paving surface, typically
paver blocks or pervious concrete, to allow rainfall to infiltrate into the pavement system then into an
open graded storage/structural layer beneath the pavement and ultimately into the subgrade. In
general, permeable pavement systems are located in areas of light traffic loading such as sidewalks,
parking stalls, and lightly used driveways (Figure 11 and Figure 12). Because of the structural
requirements of being subject to vehicular loading, permeable pavement must be designed for both
hydrologic and structural criteria.
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Photo credit: Tetra Tech, Inc.
Figure I I. Permeable paver side street parking installed on 36th Street, Garden City, ID
-". -''-'-.'--'•-'"'^•~ '" '•'• •'.'^•T-"'':" ••«-'— ''':"fl''~. ~" f' s- ' ~'/ £-*-?"- ---* A
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Photo credit: Tetra Tech, Inc.
Figure 12. Private interests in Boise have adapted permeable pavement into parking systems
12
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3.3.2 Bioretention
Bioretention consists of a depressed vegetated area underlain by a shallow layer of soil media suitable
for plant growth through which accumulated stormwater can filter. The filtering of the stormwater
results in removal of pollutant constituents. In areas of restrictive underlying soils, bioretention requires
the use of structural underdrains routed to a drainage network to prevent long term saturation of the
bioretention bottom and potential issues with plant growth. Due to the high permeability of the soil in
the Main-Fairview sub-district, however, structural underdrains were not incorporated into the
conceptual design. Examples of bioretention are shown in Figure 13, Figure 14, Figure 15, and Figure 16.
Photo credit: Tetra Tech, Inc.
Figure I 3. Bioretention located in a parking median at the Ustick Library
13
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Photo credit: Tetra Tech, Inc.
Figure 14. A bioswale along N 14th Street provides stormwater management for a commercial carwash
Bioretention application in the Boise region to date primarily consists of open landscaped areas and
parking medians. The practice is being increasingly adapted to ultra-urban right-of-way contexts as a
component of green street design. Such applications often utilize planter boxes or tree filters. The
selection of vegetation type and density for use in bioretention must consider the local environment,
availability of irrigation supply, maintenance, and in the case of bioretention planter boxes, the potential
for occasional pedestrian foot traffic within the vegetation itself.
14
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Surface —.
can vary j\
Profile can be
parabolic or flat
slope can be
optional depending
on stno&t conditions
Source: ACHD2014c.
Figure 15. Bioretention implemented into a roadway to treat road runoff
&
Photo credit: Tetra Tech, Inc.
Figure 16. Bioretention integrated within a right-of-way vegetated fringe
15
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3.3.3 Tree Filter
Tree systems incorporate urban trees growing within a subsurface soil media generally contained
partially or entirely by a structural enclosure. These characteristics allow tree system installation in
compact urban environments. Tree filters collect runoff from the adjacent roadway or pedestrian
sidewalk areas and filter it through the soil media, providing water and nutrients for the tree. Tree filters
are very similar to bioretention but include additional structural components which protect adjacent
utilities and surface infrastructure from excessive moisture and intrusive tree roots. Their increased use
of hardened structural elements results in increased costs relative to bioretention or bioswales. Figure
17 shows an example installation.
Source: ACHD2014c.
Figure 17. Example of a tree system which utilizes the area under the sidewalk to infiltrate stormwater
while allowing adequate room for tree root growth
16
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4 Conceptual Design
4.1 Concept Plan
Based on discussions with the community team and in consideration of site conditions, a green street
conceptual design was developed that builds on the framework provided by the preferred street section
detailed in the 30th Street District Master Plan. Green infrastructure elements determined as potentially
applicable to the project site included permeable pavers within the parking lane and incorporation of
bioretention cells between the sidewalk and proposed curb edge. As noted previously, implementation
of the preferred street section for Fairview Avenue, which was developed through the master planning
process, is subject to ACHD Commission approval. In the event that the ACHD Commission approves a
modified section, the green infrastructure practices can be adapted with modifications to sizing,
performance calculations, and implementation costs.
Incorporation of these two green infrastructure practices on the same side of the street to manage
right-of-way runoff would be duplicative, as each practice is capable of providing retention of the
0.6-inch rainfall event for one-half the width of the right-of-way on which the practice is placed.
Alternatively, if runoff from the adjacent parcels is directed to the edge of right-of-way, through the
pedestrian sidewalk, and into a bioretention area, both practices may be implemented along the street
side to manage both right-of-way runoff and some fraction of private lot runoff. In such a scenario,
sufficient bypass conveyance capacity must be provided to avoid street flooding during extreme events.
In order to provide alternative configurations for stakeholder consideration, the determination of
hydrologic performance, and cost projections, the conceptual design utilizes permeable pavement on
the south side of the street and bioretention on the north side. Each configuration represents a
template that may be applied to suit block specific needs. Similarly the green infrastructure and
streetscape geometries may be adjusted to accommodate site specific constraints or objectives. In the
event that management of additional runoff from adjacent private parcels is desired these
configurations may be combined into a single section incorporating both permeable pavement and
bioretention. A cross-section for the proposed green street is provided in Figure 18. A more complete
exhibit of the green street concept is provided in Appendix A.
Source: Tetra Tech, Inc.
Figure 18. Proposed green street conceptual design cross-section
17
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Figure 19 shows a current view of Fairview Avenue and the projected view after implementation of the
green street design.
Photo credits: Tetra Tech, Inc.
Figure 19. View of the 2400 Block of Fairview Avenue looking east (top) and an artist's rendering
of the green street design (bottom)
18
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4.2 Calculations
Three reference manuals were used to inform the conceptual design process: the ACHD Policy Manual
(Section 8200, revised April 2014), the City of Boise Stormwater Design Manual (Storm Water
Management:A Design Manual, August 2010 version), and the ACHD Green Stormwater Infrastructure
Guidance Manual (June 2014). The City of Boise 30th Street Area Master Plan (September 2012) was also
consulted for the preferred street section for Fairview Avenue, which served as the basis for the
conceptual design.
The appropriate design storm criterion was discussed with ACHD and the City of Boise, and the 0.6 inch
rainfall depth was selected to comply with minimum design storm requirement in the recent NPDES
permit for on-site Stormwater retention. The total contributing area of 0.7 acre for the Fairview Avenue
conceptual design was calculated using the preferred street section per the 30th Street Master Plan to
calculate total right-of-way (ROW) width. Contributing ROW length was calculated as the distance along
Fairview Avenue between the centerlines of 24th and 25th Streets. Fairview Avenue is crowned at the
center of the roadway (travel lanes) such that one half of the contributing area drains to the north side
and one half to the south side. Per City of Boise requirements, the Rational Method is the preferred
method for runoff volume calculation for sites less than 10 acres in size with slopes less than 15 percent.
The conceptual design includes bioretention cells installed on the north side of Fairview Avenue within
the area between the sidewalk and parallel parking lane, and permeable pavers installed on the south
side within the parallel parking lane. To determine the feasible area available for green infrastructure
installation, the total ROW length was adjusted appropriately. For Fairview Avenue north, the width of
two existing driveways was subtracted, as well as additional length to allow for pedestrian access (2 feet
every two parking spaces) and sight lines for vehicles entering and exiting the driveways. The feasible
length for permeable pavement installation on the south side of Fairview Avenue was determined by
subtracting the estimated length of a planned transit stop, as well as areas where parallel parking is not
feasible (e.g., near the transit stop and close to street corners). Feasible widths for permeable pavement
and bioretention were determined based on site constraints, including the need for pedestrian access
and safety, as well as required structural components as shown in the conceptual street cross-section.
Table 1 contains a summary of the design information and further information is provided in
Appendix B.
Table I. Summary of green street design geometry and retention volume
Project
Area
Fairview
Ave. N
Fairview
Ave. S
Green
Infrastructure
BMP Type
Bioretention
Permeable
Pavement
Approx.
Contributing
Area (acres)
0.35
0.35
Target
Retention
Volume* (ft3)
689.2
689.2
Design BMP
Width (ft)
3.5
7.5
Design BMP
Length (ft)
256
235
Storage
Volume
Provided (ft3)
716.8
616.9
*Retention volume for 0.6 inch precipitation event
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Planning-level cost estimates for the green infrastructure elements were developed based on the
structural components and dimensions presented in the conceptual design. Construction cost item
quantities were estimated using the conceptual design and based on bid items reported in the ACHD Bid
Averages Report (May 2014 update). Where available, unit costs were derived directly from that
document. Applicable unit costs were taken as the weighted average of the three lowest bids for each
design component. Unit costs not provided within the Bid Averages Report were derived from the
RSMeans construction cost database using Boise, Idaho as the reference location. Cost estimates for the
green street conceptual design are summarized in Table 2 and provided in more detail in Appendix C.
Table 2. Summary of green street conceptual design cost estimates
Green Street Green Infrastructure Element Implementation Cost
Fairview North (irrigated) Bioretention Planter Boxes $38,373
Fairview South Permeable Paver Parking Lane $56,789
Total $95,162
Fairview North (non-irrigated alternative) Bioretention Planter Boxes $31,425
In order to provide a means to directly compare the unit cost for green infrastructure stormwater
retention to the cost of conventional retention through seepage beds, a cost analysis was completed for
the adjacent 2.5-acre city-owned parcels that will be redeveloped. The analysis assumed that the
2.5-acre parcels would retain their current level of imperviousness and that the selected stormwater
practice would be an underground seepage bed with a storage layer depth of 5 feet. Seepage beds are
the most commonly utilized stormwater management practice within the Main-Fairview sub-district.
The 5-foot-deep storage layer was selected as maximum storage layer depth which would accommodate
the typical groundwater depth in the region and required groundwater separation criteria as indicated
in the City of Boise Stormwater Design Manual.
Unit cost for seepage beds were extracted from the ACHD Bid Averages Report and supplemented with
project-specific information provided upon request to ACHD. Bid summaries were provided for seven
projects in which seepage bed (or an equivalent term) was a bid item. Analysis of these summaries
revealed that unit cost for seepage beds ranged from $4-$15 per cubic foot of storage with the depth of
storage layer having the most significant impact on cost. Three projects included seepage beds with
storage layers approximately 5 feet. The weighted average of unit costs for these three projects was
$8.06 per cubic foot of storage. This value provided a basis to project the unit and total cost of
construction for a conventional on-site stormwater management system (Table 3).
Table 3. Summary of conventional infiltration basin cost estimate for case study project
Required Retention Volume Unit Cost
Implementation Cost
Conventional Practice ft per ft
Seepage Trench 4,860 $8.06 $39,171
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Unit cost in terms of volume retained for the green street elements are computed based on the
estimated construction costs and the volume of retention provided by each design component. A similar
analysis was conducted for the conventional infiltration system assumptions for the adjacent public
parcel. This information is shown in Table 4.
Table 4. Unit cost of retention comparison between green street and
conventional infiltration practice estimates
Stormwater Practice Cost/ ft3 of Stormwater Retained
Fairview North (irrigated) $54
Fairview North (non-irrigated) $44
Fairview South $92
Conventional Infiltration Basin $8
On a direct cost comparison basis, off-site mitigation costs of green infrastructure exceed potential
onsite Stormwater management costs by a factor of more than 5. The unit cost comparison above does
not include the opportunity cost associated with the dedication of space within the development parcel
for the placement of the seepage bed or other similar management system. Seepage beds may not be
placed under structures, although they may be placed under landscaping or parking areas. To estimate
seepage bed facility size the 5-foot storage layer was assumed to be filled with bulk stone exhibiting a
porosity of 0.35, necessitating a seepage bed footprint of 2,777 ft2 to accommodate the required
retention volume. Determining land values associated with Stormwater practice locations for areas
subject to redevelopment can be a challenging task. The two subject properties being considered in this
case study do not have assessed tax values according to the Ada County Tax Assessor's website
(http://www.adacounfyassessor.orq/propsys/). Investigation of assessed values for nearby vacant
parcels revealed a range from $300,000 to $500,000 per acre. When redevelopment efforts in the sub-
district are successful, it is reasonable to assume that property values may increase. To evaluate the
impact of future property value increase on onsite Stormwater management unit costs, an analysis was
performed in which the assumed property value was incrementally increased from $300,000 to
$1,000,000. The results of this analysis are reported in Table 5.
Table 5. Impact of property value on conventional Stormwater management unit cost
Assumed Property Value ($/Ac)
$300,000
$400,000
$500,000
$600,000
$700,000
$800,000
$900,000
$1,000,000
Land Cost/ft3
$3.94
$5.25
$6.56
$7.87
$9.18
$10.49
$11.81
$13.12
Construction Cost/ ft3
$8.06
$8.06
$8.06
$8.06
$8.06
$8.06
$8.06
$8.06
Total Cost/ft3
$12.00
$13.31
$14.62
$15.93
$17.24
$18.55
$19.87
$21.18
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5 Off-site Mitigation Policy Approach
In addition to developing a conceptual design, the project also involved an analysis of the policy
approaches available to ACHD to implement an off-site mitigation program. The permit requirement for
the retention of runoff from 0.6 inches of rainfall in 24 hours requires both new and re-development to
consider urban siting factors such as limited space, site conditions unsuitable for infiltration, high land
value, and, in the case of redevelopment, the expense of retrofitting existing infrastructure (Maupin and
Wagner 2003). In addition, development patterns may not always correspond to high-priority surface
water management zones. Therefore, allowing developers some flexibility to achieve stormwater
management targets through a combination of on-site and off-site mitigation practices can benefit local
water resources and habitat as well as the economy by allowing development to occur in areas that are
best suited to accommodate it.
There are several advantages to the use of off-site mitigation or credit purchases (CH2M Hill 2001). They
allow permittees and regulators to:
• Target stormwater management, habitat enhancement, or other environmentally beneficial
projects to locations where they are most likely to succeed, needed, or valued, within the
framework of land use planning;
• Pool funding for projects in a way that minimizes transaction costs—particularly for mitigation
banking—which allows more money to be allocated to the projects;
• Increase the likelihood of long-term monitoring and achievement of performance standards;
• Combine many small mitigations into larger, more environmentally meaningful and sustainable
mitigation actions; and
• Protect and restore designated uses, habitats, and ecosystems by increasing assimilative
capacity throughout the watershed.
Guidelines for Off-site Mitigation Projects
In general, off-site mitigation projects should be located in the same subwatershed or storm sewershed as the
development project or provide regional water quality benefits.
Depending on site-specific opportunities for on-site retention, developers can implement BMPs to meet some of the
required retention and use off-site mitigation to manage the remainder.
The off-site projects should provide an equivalent or greater stormwater volume reduction or water quality
improvement and maximize environmental benefits.
Payments in lieu of onsite mitigation can be prorated for the amount of unmitigated stormwater.
The local authority can require the developer to justify the need for off-site mitigation with measurements and
calculations to demonstrate infeasibility and document that the off-site project meets the remainder of the retention
requirement.
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Off-site mitigation or credit purchases, whether partially or completely off-site, provide additional
opportunities to select and site mitigation projects according to the value of the environmental benefits
they provide relative to the ecosystem and watershed's priority needs. For example, when developers
pay fees instead of meeting on-site retention requirements (i.e., payment in lieu programs), local
municipalities can use that revenue as a funding stream for regional water management projects that
may have a greater positive effect on water quality and enhanced environmental benefits than
stormwater features located at dispersed development sites. Local and regional water quality managers
can identify and rank opportunities for green infrastructure projects and retrofits on the basis of capital
improvement schedules and community development plans and use developer off-site mitigation
payments to complete those projects.
Despite offering additional flexibility and control over stormwater management decision-making, off-
site mitigation programs can place additional responsibilities on municipalities to set up the program;
mediate decisions about facility siting, design and funding; and ensure long-term maintenance of
facilities. Municipalities need to establish the legal authority and objectives of the program and
potentially undertake pilot projects to establish the feasibility and viability of the chosen mitigation
approach, which require local agency time and resources. Table 6 summarizes some advantages and
disadvantages of off-site mitigation programs from the perspective of a municipality.
Determining Off-site Mitigation Eligibility
In all off-site mitigation models, the local authority should develop criteria for determining the circumstances under
which off-site mitigation will be allowed. The determination should not be based solely on the difficulty or cost of
implementing measures but must include multiple criteria that would rule out an adequate combination of BMPs
(e.g., high groundwater table, existing contamination, soil conditions). The following are possible criteria to determine
off-site mitigation applicability:
• Too small a lot outside of the building footprint to create the necessary infiltrative capacity even with amended
soils
• Soil instability as documented by a thorough geotechnical analysis
• A site use that is inconsistent with capture and reuse of stormwater
• Too much shade or other physical conditions that preclude adequate use of plants
Property owners should be required to provide technical justification as to the infeasibility of on site management to be
evaluated by the local authority.
Local authorities may require that off-site projects mitigate more than the balance of unmitigated on-site stormwater.
For example, the local authority might require that off-site projects manage 1.25 times the amount of stormwater not
managed on site to help mitigate the effects of existing development via new development and redevelopment
opportunities. Enhanced requirements such as these might apply city-wide or to high-priority drainage areas where the
local authority wants to incentivize on-site mitigation. The District of Columbia Department of the Environment uses
enhanced requirements for development sites using off-site credits in the Anacostia Waterfront Development Zone
(CWP 2014).
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Table 6. Advantages and disadvantages of off-site mitigation
Advantages
Disadvantages
Performance Off-site location may allow more space-intensive but
superior performing technologies such as constructed
wetlands or bioretention that can provide greater
habitat, stormwater quality, and flood control
benefits than can be achieved with multiple small-
scale practices on sub-optimal sites.
If soil permits, infiltration technologies can
perform best if decentralized throughout the
basin—performance relies on sound
maintenance practices.
Planning Municipality has an opportunity to strategically locate
investments to address priority water body or known
water quality issues. Off-site green infrastructure can
be used to strategically expand habitat corridors and
work in synergy with other stormwater, flood control,
and environmental enhancement projects.
The municipality must take on the responsibility
of determining where to site a facility based on
priorities, environmental benefits, and
opportunities.
Large regional facilities may be difficult to site in
urban areas.
Funding Partnering may open up additional revenue sources
to fund a more effective regional facility that
addresses known environmental issues and provides
greater benefit than public funding would allow.
Partnering may complicate facility financing and
not fully fund the facility.
Maintenance The municipality allocates staff to maintenance of a
few public facilities, rather than to review, inspection
and enforcement of multiple private facilities.
Increased assurance of maintenance over time will
help to ensure long-term environmental benefits of
stormwater investments.
Maintenance responsibilities are shifted to the
municipality, including disposal of hazardous
waste material.
Liability
N/A
The municipality takes on the responsibility for
managing the risk associated with changing the
location and party responsible for implementing
water quality requirements.
Innovative local regulations or funding may draw
legal challenges or present permit compliance
issues.
Community In facility siting and design, the municipality can assist
in implementing community development plans for
open space, aquatic health, water quality, increased
habitat, and urban centers.
Decisions regarding facility locations and design
can result in community disagreement about use
of public resources and siting. Ensuring equal
environmental benefits might be challenging
when weighing multiple community priorities.
Source: Maupin and Wagner 2003
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5.1 Off-site Mitigation Models
There are three main approaches to allow off-site stormwater mitigation: bilateral trading, citywide
training, and public aggregation. Each model is driven by supply (opportunities for BMP implementation)
and demand (regulatory requirements for new and redevelopment projects). Buying and selling of
stormwater retention credits can occur directly between parties or through a bank of projects and
associated credits. Guidelines for the market can be set by contract, legislation, rulemaking, policy, or
guidance, as dictated by the models chosen and local authority preferences. Off-site mitigation models
may be more or less regulated by the local authority (CH2M Hill 2001).1
The three models are not mutually exclusive and could be designed to work together. For example,
bilateral trades could take place within the framework of a citywide trading program, as could a public
aggregation option. The concept of tradable instruments or stormwater retention credits is relevant for
all models and can facilitate off-site mitigation program operation.
The mitigation model characterizations below are adapted from Creating Clean Water Cash Flows:
Developing Private Markets for Green Stormwater Infrastructure in Philadelphia (NatLab 2013). The
descriptions are tailored for the Boise region with additional information added for applicability to a
broader audience interested in off-site mitigation. At the end of this section, Table 7 presents a side-by-
side comparison of the three models.
5.1.1 Bilateral Trading
Description
Bilateral trading involves an exchange of stormwater retention credit between a property owner with
site constraints (buyer) and another property owner who can supply stormwater BMPs (seller). In this
scenario, the buyer who needs to meet his or her stormwater requirement using off-site mitigation
enters into an agreement with the seller to invest in a BMP on the seller's property. The buyer's
investment yields the associated benefits of meeting part or all of the buyer's regulatory requirements
for on-site retention.
Institutional Aspects
The local authority would need to recognize or certify the off-site benefits of the bilateral transaction for
the buyer to use the credit against his or her stormwater requirements. The local authority could
recognize the BMP and factor the amount of stormwater managed at the seller's site into the review of
the buyer's development plans and calculations, or the local authority could issue a tradable instrument
or off-site credit. If a tradable instrument is used, the seller would register the BMP with the local
authority for approval and certification, after which the local authority would issue a credit to the seller
to be transferred to the buyer to offset his or her stormwater management requirement. An inspection
program would be needed to verify that the BMP remains in place and is maintained over the long term.
1 The term "local authority" is used to represent a municipality or quasi-governmental agency that oversees stormwater
management. In the Boise region, local authority refers to a partnership between the City of Boise, which has land use planning
authority, and ACHD, which operates and maintains the municipal separate storm sewer system.
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Financing Options
Buyers and sellers can work together to fund and implement BMPs off-site, or sellers can install BMPs
for credits independently and sell them to interested buyers. Transactions would be negotiated between
the two parties, and if a sufficient market exists, brokers can be used to make connections and facilitate
transactions. In jurisdictions in which a stormwater fee and a credit system for on-site stormwater
mitigation exist, bilateral transactions can achieve financial benefits associated with a stormwater fee
reduction, which a buyer can use to offset his or her own stormwater fee.
Advantages and Disadvantages
Bilateral trading programs are relatively simple and easy to deploy from the local authority's perspective
because they require minimum intervention and infrastructure investment. Sellers can use bilateral
agreements to fund BMP installation on their own sites. On the other hand, bilateral transactions may
not be fully transparent and do not take full advantage of the market mechanism to maximize efficiency
and achieve least-cost solutions. Unless a mature market with brokers exists, buyers and sellers will
have to seek each other out on a one-on-one basis. If a relationship is not already established between
the two parties, there is little to no infrastructure available to facilitate a new relationship based on
trading. In addition, buyers and sellers will have to invest in advertising their needs.
5.1.2 Cit/wide Trading
Description
Citywide trading expands the bilateral trading model with improved efficiency and broader
participation. As with the bilateral trading model, sellers would build stormwater BMPs that are certified
by the city. The city would issue retention credits that can be split or aggregated depending on the
needs of buyers. The citywide trading model allows both buyers and sellers greater flexibility in how
credits are used; the seller can trade credits earned from a single project to more than one buyer, and
buyers can accumulate credits from multiple sellers to meet their onsite retention requirements.
Unused credits can be resold in an open marketplace.
Institutional Aspects
In a citywide trading program, a developer of an off-site mitigation project would seek certification from
the city, and upon successful certification would be issued a tradable instrument that could be held or
sold. The seller can sell all or part of his or her credits to any other party, either citywide or within a set
geographic boundary (subwatershed or storm sewershed). The buyer could choose to use those credits
against his or her own on-site retention requirements or resell the credits to another buyer. In this
system, the credit is separated from the original seller. It is essential that the local authority has a
program to recognize or certify the credit and to verify that the BMP remains in place and is maintained
over the long term.
Financing Options
Property owners pay for BMP implementation and sell the credits in the citywide marketplace. In
jurisdictions in which a stormwater fee and a credit system for on-site mitigation exist, financial benefits
associated with a stormwater fee reduction can be realized by the credit holder.
Property owners, credit holders, or both parties could be responsible for ensuring long-term
maintenance of the BMP that generated the credit. If the property owner were responsible, the
stormwater credits would need to be priced to account for those long-term costs. If the credit holder,
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who has the long-term financial stake in ensuring that the BMP operates in perpetuity, is responsible for
maintenance, the credit trade would need to include a maintenance agreement and easement to allow
access. To ensure that maintenance agreements are not overlooked when credits are traded, the local
authority can request this documentation when the credits are reviewed and approved.
Advantages and Disadvantages
A citywide trading program uses the market to encourage property owners to invest in BMP installation
at or above the baseline. The citywide approach has more buyers and sellers, increasing liquidity, and
offers more options for property owners with site constraints to connect with suppliers of off-site
mitigation projects. An open market is more transparent in terms of the financial value of credits,
allowing property owners to make informed investment decisions and encouraging cost-efficient BMPs.
A citywide program might require more local authority resources for administration, credit certification,
and oversight than a bilateral program.
5.1.3 Public Aggregation
Description
Under a public aggregation scenario, the local authority or designated entity2 would act as an aggregator
to develop mitigation projects on private and public land that could be offered in the form of
stormwater retention credits to property owners with site constraints. This off-site mitigation model
includes an option for payment in lieu, where constrained property owners can pay a fee commensurate
with the amount of stormwater not mitigated on-site, to be used to fund the development of future
projects (or reimburse the cost of projects already implemented).
Institutional Aspects
A public aggregation scenario places the greatest administrative burden on the local authority compared
to bilateral or citywide trading. The local authority would need to identify mitigation projects on private
and public land, set up a stormwater retention credit market and/or a mechanism to collect fees, enact
policy, and incrementally improve the marketplace (if necessary). A private marketplace could evolve
out of effective policy and the ability to establish a sufficient credit supply, which could lessen the local
authority's regulatory burden overtime.
One important consideration is to avoid competition between the public and private sectors for credits
where the public aggregating mechanism would underbid the private developers. Therefore the local
authority should consider limiting the deployment of public aggregation to constrained property owners
and consider placing a limited term on the aggregation mechanism that would serve to jump-start the
program and offer flexibility to constrained property owners early. If unsuccessful, the program could
phase out while bilateral or citywide trading continued.
2 The local authority might choose to designate an entity to act on its behalf. For example, the local authority could designate
an entity to collect on its behalf and then distribute the money directly into a program that facilitates BMP installations and
retrofits. This program could be implemented by the local authority, by this same entity, or by a private contractor
administered by either of the former options.
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Financing Options
A property owner who needs stormwater retention
credits would be able to purchase credits from the local
authority once they were created (using capital
improvement funds, funds from special tax districts,
grants, private funding, or a combination). Property
owners could also pay up-front into a fund that the
local authority would use to develop stormwater
installation or retrofit projects (payment in lieu). Fees
collected from property owners could be used to offset
the costs of stormwater improvements, possibly
allowing for more projects to be implemented in the
region.
For payment in lieu programs, it is important that the
stormwater projects are implemented in a timely
manner to avoid long periods in which stormwater
from completed new development and redevelopment
projects is unmitigated, increasing the likelihood of
water resource impacts.
Advantages and Disadvantages
Public aggregation would provide significant flexibility
and a single contact for property owners seeking credits
to participate in off-site mitigation. Moreover,
purchasers would have the assurance that credits
generated by the local authority would continue to
function and not become invalidated due to poor
maintenance, thereby reducing risk to the purchaser.
The local authority in turn would be able to leverage
economies of scale by aggregating demand from
several property owners and investing in stormwater
management interventions at a larger scale and on
property that might otherwise not be available to a private developer, such as vacant lands, and will be
able to prioritize BMPs in areas that will produce the greatest water quality benefit.
Case Study: Off-site
Mitigation in the Right-of-
Way in Los Angeles
Off-site mitigation is allowed in the City of Los
Angeles for the amount of runoff that cannot
feasibly be managed on-site. Developers can
implement off-site projects in the public right-
of-way immediately adjacent to the subject
development and at other sites within the
same sub-watershed as the proposed project.
The City's rules stipulate that construction of
off-site mitigation projects "shall achieve at
least the same level of water quality
protection as if all of the runoff were retained
on-site and also be sized to mitigate the
volume from the on-site and the tributary
area from the adjacent street (from the
crown of the street to the curb face for the
entire length of the development site)" (City
of Los Angeles 2011).
Construction work for off-site projects in the
public right-of-way is the responsibility of the
developer and requires a "Revocable Permit"
from the City. The City assists the developer
with permitting and implementation of LID
BMP projects within the public right-of-way.
To ensure long-term maintenance, the
developer is also required to file a covenant
and agreement with the county recorder's
office to assume full responsibility for
perpetual maintenance of the on-site and off-
site BMP(s) executed by a covenant and
agreement (City of Los Angeles 2011).
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Table 7. Comparison of off-site mitigation models
Model
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Description
An exchange of
stormwater retention
credit between a
property owner who
cannot meet his or her
stormwater retention
requirement on-site
and another property
owner who can supply
stormwater BMPs
An exchange of
stormwater retention
credit between two or
more parties within a
city or subwatershed/
storm sewershed
The local authority
aggregates public and
private mitigation
projects, the credits for
which can be
purchased by property
owners who cannot
their stormwater
retention requirement
on-site
Admin.
Needs BMP Financing
Low Property owners
(buyers or
sellers) fund
stormwater
BMPs
Medium Property owners
(buyers or
sellers) fund
stormwater
BMPs
High Stormwater
BMPs funded by
private property
owners, funds
from special tax
districts, grants,
private funding,
or a combination
Payments in lieu
of on-site
retention can
offset the cost of
mitigation
projects
Advantages
• Requires minimum
intervention and
infrastructure investment
by the local authority
• Can serve as a funding
mechanism for BMP
installation
• More buyers and sellers
than bilateral approach
• More options for property
owners seeking credits to
connect with suppliers
• Marketplace transactions
more transparent to inform
investment decisions
• Single contact for property
owners
• Local authority oversight
provides assurance that
BMPs will continue to
function so credits remain
valid
• Economies of scale
achieved by aggregating
demand and investing in
larger scale projects
• Mitigation projects can be
implemented on public or
private lands
• Centralized planning allows
mitigation projects to
contribute to broader
community goals
Disadvantages
• Transactions not fully
transparent
• Transactions do not take
full advantage of the
market to maximize
efficiency and find least-
cost solutions
• Buyers and sellers need to
seek each other out and
advertise their needs
• Requires more local
authority resources for
administration
• Buyers and sellers need to
seek each other out and
advertise their needs
• Requires the most local
authority resources for
administration
• Regional projects may
need to be built with
loans or capital funds
before payments are
received from property
owners seeking
stormwater retention
credit
5.2 Preferred Off-site Mitigation Approach for Boise
Many of the opportunities for green infrastructure implementation in the city center are located in the
public ROW, which is owned and maintained by ACHD. One option local agencies might consider for off-
site mitigation to support redevelopment in the Main-Fairview sub-district and other areas of the city
center is a version of the public aggregation model in which green alley retrofits and other green street
projects are undertaken in upstream parts of the storm sewershed to create capacity in the drainage
system and help the agencies meet their TMDL, MS4 permit, and other water quality obligations. The
ROW improvements can generate a bank of stormwater retention credits that can be used by
developers of individual parcels to help meet the stormwater retention requirement. Local agencies
could implement retrofit projects on streets and alleys proactively on the basis of site suitability and
opportunity, available funding, priority drainage areas (i.e., those with the least capacity or history of
29
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flooding or where the greatest environmental benefit can be realized), and private-sector demand for
credits. The green infrastructure retrofits might be integrated into pedestrian and bicycle safety
improvements for cost-efficiency. ROW improvements could be made and in-lieu-of credits
subsequently purchased by developers.
ACHD, the City of Boise, and other local stakeholders could form a committee to oversee the credit
system, garner stakeholder input on its operation, and ensure proper accounting of new and future
credits and associated costs. The committee would undertake an initial study to evaluate the costs
associated with (1) building green infrastructure in the ROW, (2) long-term maintenance of the
infrastructure, and (3) program administrative costs. This study would help to ensure that the price of
credits reflects the life-cycle costs of the green infrastructure. The study would be updated periodically
to include revised assumptions for life-cycle costs; for example, alleyways converted to green alleys
might be less expensive to maintain over the long term than originally estimated. Updates to the study
would also include a re-evaluation of demand to ensure that the pace of green infrastructure retrofits is
in sync with demand for credits (and that reimbursement is timely).
So as not to discourage the private sector from generating stormwater credits by building more on-site
retention capacity than the local standard requires, ACHD and the City of Boise can work with
developers and property owners to identify such stormwater management opportunities across the
district and incorporate their credits into the overall tracking system. The committee would work
together to determine the priority by which credits would be reimbursed.
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6 Conclusion
Many communities across the country have embraced green infrastructure stormwater practices to help
meet local water quality goals and address water quantity issues. ACHD and the City of Boise have found
that green infrastructure practices can be used in their communities to address these water resource
concerns and provide compliance with pending NPDES Phase I permit post-construction stormwater
requirements. The permit requires runoff reduction of 0.6-inches in 24 hours on both new development
and redevelopment areas. In areas of new development, such requirements can be accomplished by
dedicating appropriate areas of the project site for placement of stormwater infiltration practices. In
areas subject to dense urban redevelopment, such as within the Main-Fairview sub-district, dedication
of already limited site space to stormwater practices may serve as a disincentive to redevelopment
when compared to undeveloped parcels on the urban fringe of Boise. Green infrastructure retrofits
could be undertaken in the right-of-way (green alleys, green streets) to establish stormwater retention
credits that can be purchased to help owners of redevelopment properties meet the 0.6-inch storm
retention standard. The credit system can be administered by a committee of local stakeholders to
ensure fairness and transparency.
A green street conceptual design for a one block section of Fairview Avenue adjacent to vacant lots
under consideration for redevelopment provides a case study for how off-site mitigation of stormwater
management using green infrastructure in a transportation right-of-way can incentivize redevelopment
on adjacent parcels. The analysis revealed that the green street conceptual design could provide
stormwater management mitigation for 0.7 acres of impervious area of the adjacent 2.5 acre private
parcel. On a direct cost comparison basis, off-site mitigation costs of green infrastructure exceed
potential onsite stormwater management costs by a factor of 6.
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7 References
Ada County Highway District (ACHD). 2014a. ACHD Bid Averages.
http://www.achdidaho.orci/Departments/PP/files/Bid Averages Report.pdf
Ada County Highway District (ACHD). 2014b. ACHD Policy Manual. Section 8200 - ACHD Stormwater
Design Tools and Approved BMPs. Accessed November 2014.
http://www.achdidaho.ora/AboutACHD/PolicvExhibits/PDF/Section 8200 Stormwater Design
Manual.pdf
Ada County Highway District (ACHD). 2014c. Green Stormwater Infrastructure Guidance Manual.
Accessed July 2015.
http://www.partnersforcleanwater.ora/media/3050/Boise GC%202014%20-
%20Section%2002%20ACHD App7.pdf.
Boise City Planning and Development Services and Capital City Development Corporation. 2012. 30th
Street Area Master Plan. Accessed June 27, 2014.
http://pds.citvofboise.ora/media/184351/30th master plan fin 07 30 2012.pdf.
CH2M Hill. 2001. Washington Stormwater Management Study: Report and Recommendations from the
Stormwater Policy Advisory Committee. Prepared for the Washington State Department of
Ecology and Washington State Department of Transportation. Accessed June 24, 2014.
http://www.ecy.wa.ciov/procirams/wci/stormwater/commentdocs/finalreport.pdf.
City of Boise, Idaho. 2010. StormWater Management: A Design Manual.
http://www.partnersforcleanwater.orci/media/174240/22196 stormwaterdesicinmanual2010.pdf
City of Los Angeles. 2011. Development Best Management Practices Handbook, Part B Planning Activities
(Low Impact Development Manual). Fourth Edition. Accessed June 24, 2014.
http://www.lastormwater.org/wp-content/files mf/lidhandbookfinal62212.pdf
Center for Watershed Protection (CWP). 2013. District of Columbia Stormwater Management
Guidebook. Accessed June 24, 2014. http://ddoe.dc.aov/node/610622.
Materials, Testing & Inspection. 2002. Geotechnical Engineering Report of Brady Law Chartered, 3101
Main Street, Boise Idaho. August 16, 2002. Prepared by B. Wright.
Materials, Testing & Inspection. 2013. Geotechnical Engineering Report of Medical Office Building, 3003
West Main Street, Boise, Idaho. September 12, 2013. Prepared by E. Brown.
Maupin, M., and T. Wagner. 2003. Regional Facility vs. On-site Development Regulations: Increasing
Flexibility and Effectiveness in Development Regulation Implementation. City of Seattle,
Washington. Accessed June 24, 2014.
http://water.epa.aov/polwaste/nps/stormwater/upload/2003 03 26 NPS natlstormwaterOS 22
Maupin.pdf.
NatLab. 2013. Creating Clean Water Cash Flows: Developing Private Markets for Green Stormwater
Infrastructure in Philadelphia. Accessed June 24, 2014.
http://www.nrdc.ora/water/stormwater/files/areen-infrastructure-pa-report.pdf.
USEPA Region 10. 2012. Permit for Storm Water Discharges from Small Municipal Separate Storm Sewer
Systems. Accessed February 12, 2015.
http://www.epa.gov/reaionlO/pdf/permits/npdes/id/ids027561 ms4 fp 2012.pdf.
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Appendix A: Fairview Avenue Green Street Conceptual Design
33
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Date of Field Visit
Field Visit Personnel
Major Watershed
Street Address
Site Location
7/14/2014 Latitude
J. Smith, M. Frey Longitude
Boise River Landowner
2400 block of
Fairview Avenue
Drainage Area Characteristics Proposed Characteristics
43° 37' 14" N
116° 13' 14" W
City of Boise
Drainage Area, acres 0.7
Hydrologic Soil Group D, Urban
Total Impervious, % 100
Design Storm Event, in 0.6
Proposed BMPs BR, PP
Total Detention Vol., ft3 1,334
Bioretention Area, ft2 896
Perm. Pavement Area, ft2 1,763
Existing Site Description: The proposed project site includes the Fairview Avenue
roadway corridor between 24th Street and 25th Street. Fairview Avenue is a 4-lane
roadway that forms a one-way couplet with Main Street. The existing lanes are 12.5-
ft wide with 5 to 6-ft sidewalks. Fairview Avenue has more capacity than is needed
for current traffic volumes. The project site is served by a separate storm sewer
system managed by Ada County Highway District. Fairview Avenue is currently
slated to undergo improvements within the next few years. Potential improvements
include reducing the number of travel lanes from four to three, adding a bicycle
lane, on-street parallel parking and streetscaping, and widening sidewalks.
Proposed Green Infrastructure Description: Proposed BMPs within the right-of-
way (ROW) include bioretention areas along the north side and permeable pavers
along the south side of Fairview Avenue. These BMPs are designed to capture and
treat runoff from the entire ROW while still allowing pedestrian, vehicle, and
transit access.
BR = Bioretention, PP = Permeable Pavement
*Green Infrastructure characteristics are based on field observations and CIS data resources available at the time of
conceptual design analysis. Note that final design characteristics will be dependent on a detailed site survey and could
vary slightly from conceptual design characteristics.
Project Site Drainage Area
Legend
Storm Drain
~~| Project Drainage Area
Legend
Primary Roads
— Waterways
Legend
Catch Basin
Sand/Grease Trap
Manhole
Storm Drain
FUTURE BUILDING FACADE
permeable pavement
FUTURE BUILDING FACADE
CO CD >
m m >
5E§
m£x
^ co i
> =^ CD
O
O
O
O
CO
>
|—
~D
o
Current Street View (Facing East)
Conceptual Green Street Rendering (Facing East)
Section A-A'
7.5' PERMEABLE PAVEMENT AREA
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Appendix B: Fairview Avenue Green Street Conceptual Design Calculations
The City of Boise Stormwater Design Manual (April 2010) recommends use of the Rational Method for
projects less than 10 acres in size. This method requires determination of contributing drainage area, an
appropriate runoff coefficient (C), and design rainfall intensity for calculation of peak discharge and
volume. Appendix A of the Boise Stormwater Design Manual can be consulted for further guidance,
including a table of recommended values for C. The Rational Method is also the preferred method for
determination of runoff rate and volume per ACHD's Policy Manual (Section 8200).
ACHD's Green Stormwater Infrastructure Guidance Manual (June 2014) also specifies that "all sites
should retain and treat the first 0.6 inch of water quality capture volume from the 24-hour event." This
document should be consulted for further detail on special considerations for Ada County and
appropriate green Stormwater infrastructure practices.
(1) Determine contributing drainage area:
Fairview Avenue North
Approximate contributing ROW length = 355 ft
Approximate contributing ROW width = 43.5 ft
Resulting contributing drainage area = (355 ft x 43.5 ft)/43,560 ft2/acre = 0.35 acre
Fairview Avenue South
Same as Fairview Avenue North.
(2) Calculate required retention volume:
Rational Method Peak Runoff, Q (cfs) = CiA
where C = runoff coefficient (dimensionless)
i = rainfall intensity (in/hr)
A = drainage area (acres)
Fairview Avenue North
Assuming a runoff coefficient of 0.9 for the site
Design rainfall intensity = 0.6 in/hr
Q = (0.9) x (0.6) x (0.35) = 0.189 cfs
Then, required retention volume, V (ft3) = Q (cfs) x 3600 sec/hr
V = (0.189) x (3600) = 689.2 ft3
Fairview Avenue South
Same as Fairview Avenue North.
35
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(3) Calculate storage volume provided by BMP(s):
Fain/lew Avenue North
Bioretention design width = 3.5 ft
Bioretention design length = 256 ft
Bioretention surface area = (3.5 ft) x (256 ft) = 896 ft2
Design soil depth = 2.0 ft
Assumed soil porosity = 0.40
Storage volume provided (ft3) = BMP surface area x soil depth x soil porosity
Storage volume provided by bioretention = 896 ft2 x (2.0 ft x 0.40) = 716.8 ft3
Fain/lew Avenue South
Permeable pavement design width = 7.5 ft
Permeable pavement design length = 235 ft
Permeable pavement surface area = (7.5 ft) x (235 ft) = 1762.5 ft2
Design soil depth = 1.0 ft
Assumed soil porosity = 0.35
Storage volume provided (ft3) = BMP surface area x soil depth x soil porosity
Storage volume provided by permeable pavement = 1762.5 ft2 x (1.0 ft x 0.35) = 616.9 ft3
(4) Summary:
Table 8. Pertinent green street conceptual design parameters
Green
Approx. Retention
Target Design Design
Design
Media
Fairview
Ave. N
Bioretention
Fairview Permeable
Ave. S Pavement
0.35
0.35
0.6
0.6
689.2
689.2
896.0
1762.5
2.0
1.0
0.40
0.35
Storage
Volume
Project , , _ .. . _ . , „ Retention BMP Media
Infrastructure Contributing Rainfall . . ..
Area _..__ , , ,. , Volume Area Depth _ . Provided
BMP Type Area (acre) (in) 3 2 ^ Porosity 3
716.8
616.9
36
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Appendix C: Fairview Avenue Green Street Conceptual Cost Estimates
Table 9. Detailed cost estimate for Fairview North
Item No. Description Reference Quantity Unit
Unit Cost
Total
Preparation
1 Traffic Control ACHD
$2,000
Site Preparation/Earthwork
2 Excavation RSMeans 100 CY
3 Asphalt Removal 0201.4.1. D.I 100 SY
4 Concrete Driveway Approach 0706. 4. I.F.I 35 SY
5 Haul RSMeans 100 CY
$4.37
$3.10
$33.01
$8.55
$435
$309
$1,155
$851
Traditional Bioretention
6 Fine Grading Engineer's Estimate 896 SF
7 Soil Media -2' Depth SSP 25050 66 CY
8 Filter Layer (sand and No. 8 stone) SSP 25049 11 CY
9 Grouted River Rock SP 25002 4 CY
10 Curb Cuts Engineer's Estimate 6 EA
11 Mulch Engineer's Estimate 6 CY
12 Concrete Curb (6" vertical, no gutter) 0706.4.1.A.5 566 LF
13 Vegetation 0307.4.1.A.1 97 SY
14 Sprinkler System Engineer's Estimate 896 SF
Construction Subtotal
15 Estimating Contingency (30% of subtotal)
16 Mobilization (7.5-8% of subtotal, assume 8%)
17 Bond (5% of subtotal)
18 Construction contingency (10% of subtotal)
Construction Total
Design (18-25% of Construction Total,
assume 25%)
Total Cost
$0.72
$26.28
$26.68
$106.27
$125.00
$55.00
$10.80
$34.00
$2.00
$645
$1,744
$295
$378
$750
$303
$6,113
$3,294
$1,792
$20,064
$6,019
$1,605
$1,003
$2,006
$30,698
$7,675
$38,373
NATIVE LANDSCAPING OPTION
Native Landscaping (Optional)
13 Native Desert Landscaping Engineer's Estimate 97 SY
14 Sprinkler System Deduction
15 Vegetation Deduction
Construction Subtotal
16 Estimating Contingency (30% of subtotal)
17 Mobilization (7.5-8% of subtotal, assume 8%)
18 Bond (5% of subtotal)
19 Construction contingency (10% of subtotal)
Construction Total
Design (18-25% of Construction Total,
assume 25%)
Total Cost
$15.00
$1,453
($1,792)
($3,294)
$16,431
$4,929
$1,315
$822
$1,643
$25,140
$6,285
$31,425
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Table 10. Detailed cost estimate for Fairview South
Item No. Description
Reference
Quantity
Unit
Unit Cost
Total
Preparation
1 Traffic Control
ACHD
$2,000
Site Preparation/Earthwork
2 Excavation
3 Asphalt Removal
4 Haul
RSMeans
0201.4.1. D.I
RSMeans
87
196
87
CY
SY
CY
$4.37
$3.10
$8.55
$380
$607
$744
Permeable Pavement
5 Fine Grading
6 Permeable Pavement (Brick Pavers)
7 Structural Layer (washed No. 57)
8 Filter Layer (sand and No. 8 stone)
9 Concrete Curb (6" vertical, no gutter)
Construction Subtotal
10 Estimating Contingency (30% of subtotal)
11 Mobilization (7.5-8% of subtotal, assume 8%)
12 Bond (5% of subtotal)
13 Construction contingency (10% of subtotal)
Construction Total
Design (18-25% of Construction Total,
assume 25%)
Total Cost
Engineer's Estimate
SP 25001
Engineer's Estimate
SSP 25049
0706.4.1.A.5
1763
1763
65
22
235
SF
SF
CY
CY
LF
$0.72
$10.57
$45.00
$26.68
$10.80
$1,269
$18,635
$2,938
$581
$2,538
$29,693
$8,908
$2,375
$1,485
$2,969
$45,431
$11,358
$56,789
38
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