v>EPA
United States
Environmental Protection
Agency
2014 GREEN INFRASTRUCTURE TECHNICAL ASSISTANCE PROGRAM
City of Saint Paul
Saint Paul, Minnesota
West Side Flats Greenway Conceptual Green
Infrastructure Design
Cover Image: West Side Flats, St. Paul, Greenway Concept
EPA832-R-15-004
March 2015
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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.gov/greeninfrastructure.
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Acknowledgements
Principal USEPA Team
Jamie Piziali, USEPA
Christopher Kloss, USEPA
Tamara Mittman, USEPA
EvaBirk-ORISE, USEPA
Bob Newport, USEPA, Region 5
Community Team
Lucy Thompson, City of Saint Paul
Marie Franchett, City of Saint Paul
Wes Saunders-Pearce, City of Saint Paul
Karin Misiewicz, City of Saint Paul
Don Ganje, City of Saint Paul
Anne Weber, City of Saint Paul
Tim Griffin, Saint Paul Riverfront Corporation
Consultant Team
Martina Frey, Tetra Tech
Anne Thomas, Tetra Tech
Jennifer Olson, Tetra Tech
Dan Christian, Tetra Tech
Brad Aldrich, Emmons & Olivier Resources, Inc.
John Kosco, 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.
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Contents
1 Introduction 1
2 Greenway Drainage Area Site Conditions 4
3 Master Plan Framework 7
4 Project Approach 9
4.1 Project Goals 9
4.2 Project Objectives 9
4.3 Key Design Elements 9
5 Stormwater Modeling 11
6 Green Infrastructure Toolbox 15
7 Conceptual Design 18
7.1 Concept 1: Linear and Natural 18
7.2 Concept 2: Central and Urban 23
7.3 Concept 3: Combined 27
8 Conclusion 29
Appendix A: Cost Recovery Options 31
IV
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Figures
Figure 1. Saint Paul Housing and Redevelopment Authority Parcel 4
Figure 2. West Side Flats Apartments 4
Figure 3. Greenway Drainage Area Site Conditions 5
Figure 4. Greenway 6
Figure 5. Saint Paul Housing and Redevelopment Authority Parcel 6
Figure 6. West Side Flats Master Plan Concept 8
Figure 7. Greenway Tributary Drainage Areas 14
Figure 8. Concept 1 Plan View 20
Figure 9. Concept 1 Cross-Section 21
Figure 10. Biofiltration Examples 22
Figure 11. Concept 2 Plan View 24
Figure 12. Concept 2 Cross-Section 25
Figure 13. Urban Pond Feature Examples 26
Figure 14. Concept 3 Plan View 28
Table
Table 1. Retention and Detention Volume Modeling Results 13
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VI
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I Introduction
The West Side Flats Greenway project offers an opportunity for private property owners within the West
Side Flats neighborhood to partner with the City of Saint Paul on a shared stacked-function green
infrastructure concept. In lieu of meeting stormwater requirements on each individual site, this shared
project will have multiple, "stacked" benefits including incentivizing redevelopment, treating
stormwater, providing recreational space and cooling and filtering the air. The greenway will be the first
application of a shared public-private stormwater management facility using green infrastructure in the
city. Additionally, this project will help pilot the shared stacked-function green infrastructure concepts
evaluated as part of the Twin Cities' Light Rail Transit Green Line project [see Strategic Stormwater
Solutions for Transit-Oriented Development (December 2013)]1. The West Side Flats Study Area is
approximately 120 acres and is located directly across the Mississippi River from Downtown Saint Paul.
It is situated between the Mississippi River, Plato Boulevard, Wabasha Street, and Lafayette Road. The
proposed West Side Flats Greenway is located along a working railroad that divides the West Side Flats
Study Area. The parcel, adjacent to the railroad right-of-way, within which the proposed greenway lies is
currently privately-owned and for sale. The City of Saint Paul is exploring options to facilitate creation of
the greenway, including purchasing the greenway parcel independently or in partnership with a private
developer. The City is also evaluating if establishing a regional stormwater management facility
comprised of green infrastructure in the greenway and an expanded tributary storm sewer network can
open new funding options for land acquisition.
Saint Paul is the Capitol and the second-most populous city in
Minnesota with a population of nearly 300,000. It is part of
the Minneapolis-Saint Paul metropolitan area, which includes
a population of about 3.5 million residents. The city
surrounds the confluence of the Mississippi River and the
Minnesota River, a feature integral to the settlement of this
area. Temperatures are typically below freezing during the
winter and 70 to 80 degrees Fahrenheit in the summer.
Annual precipitation is approximately 32 inches.
The history of West Side Flats is marked by a thriving
riverfront neighborhood and market place in the late 1800's
transitioning to an industrial park in the 1960's due primarily
to frequent river flooding. A levee and floodwall were also
built to address flooding during the 1950's. In recent decades
with decreasing river and railway transport, this area has
begun to see new housing and office development, and
industrial uses are changing to meet new market demand.
With its potential to be transformed into an urban riverfront
village, the West Side Flats neighborhood has been targeted
Shared Stacked-Function Green
Infrastructure
Shared stacked-function green
infrastructure means that the project
will not only assist public and private
property owners in meeting local
stormwater management goals, it will
have multiple benefits including:
• incentivizing redevelopment
• treating stormwater
• reestablishing a social
connection to the Mississippi
River
• providing recreational space
• promoting beautification
• cooling and filtering the air
1 Strategic Stormwater Solutions for Transit-Oriented Development (December 2013). Web Address (last accessed: February 3,
2015). http://www.corridorsofoDDortunitv.org/sites/default/files/Strateaic Stormwater Solutions for TOD Final Report.pdf
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for revitalization within various planning documents over the past two decades2'3/ 4. The vision is a
riverfront urban village comprised of a mix of residential, commercial, industrial, office, institutional,
entertainment, and recreational uses. Common goals of the various planning documents are to manage
stormwater naturally and to incorporate a green space network within the urban realm connecting
residents to the Mississippi River. The City of Saint Paul and its collaborators understand the significant
role green infrastructure plays in integrating these two goals.
This project has substantial value in that it will function as a pilot project for implementing shared
stacked-function green infrastructure. It will be a catalytic public investment to stimulate private
investment, and it will help spark development in the area as laid out in the West Side Flats Master Plan
& Development Guidelines, Draft (March 2014). The West Side Flats Master Plan & Development
Guidelines document is expected to be formally adopted by City Council in April 2015. This conceptual
green infrastructure design report has already spurred a purchase offer on a 13.5-acre parcel that
includes the proposed greenway and 780 planned housing units as laid out in the Master Plan.
Although implementation of green infrastructure in this area will likely not address the Mississippi
River's impairments for mercury, PCB, and perfluorooctane sulfonate in fish tissue, it may address the
impairments for fecal coliform and turbidity in addition to reducing nutrient and sediment loads. More
generally, it will demonstrate a shared stormwater management approach for similar areas around the
country targeted for revitalization in the urban environment. To further this project and the City's
overall commitment to green infrastructure, the City of Saint Paul applied for U.S. Environmental
Protection Agency (USEPA) Technical Assistance to explore the technical feasibility of incorporating
green infrastructure within the proposed greenway.
The project focus is to investigate the extent to which the greenway can reasonably host surface water
features and to a lesser extent subsurface storage features to treat stormwater and manage potential
flooding while serving as an amenity to the community. Surface water features refer to practices that
treat and manage stormwater runoff above ground such as bioretention and retention ponds.
Subsurface storage features include below grade storage practices primarily used to manage runoff from
large rain events. Methods to fund the construction, operation, and maintenance of the shared
stormwater management features were also evaluated and are summarized in a memorandum in the
appendix. Prior community engagement efforts revealed that there is an interest in incorporating
surface water features within the West Side Flats study area5. Incorporation of surface water and
subsurface storage features will be defined by community input, topography, West Side Flats master
planning, existing storm sewer attributes, potential soil contamination, groundwater, Saint Paul's
municipal stormwater retention standard, a rate control standard, and other planned park
programming. Providing input on this wide range of subjects, specifically for this USEPA technical
assistance project, was the City's steering committee comprised of staff from a variety of departments
within the City of Saint Paul and the Saint Paul Riverfront Corporation. This report presents options for
2 West Side Flats Master Plan and Development Guidelines (2001). Web Address (last accessed: February 3, 2015).
h ttp://www.stp aul.ao v/in dex. asm ?NID=344 6
3 West Side Flats Master Plan and Development Guidelines, Draft (March 2014). Web Address (last accessed: February 3, 2015).
h ttp://www. stpaul. go v/westsideflats
4 The West Side Community Plan (Addendum to the Saint Paul Comprehensive Plan) (February 2013). Web Address (last
accessed: Februarys, 2015). http://www.st[>aul.aov/index.as[>x?NID=3446
5 May 2013 West Side Flats Design Charrette. Web Address (last accessed: February 3, 2015).
h ttp://www. stpaul. go v/westsideflats
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including green infrastructure concepts within the greenway. Options were informed by both a baseline
stormwater analysis of the drainage area, as well as a series of discussions with the steering committee.
The information contained herein is intended to guide forthcoming phases in the project, which include
the following:
• Securing the 13.5-acre parcel for the greenway and housing development as laid out in the West
Side Flats Master Plan & Development Guidelines
• Procuring a USEPA brownfields area-wide planning grant for the proposed greenway
• Determining the financial mechanisms to establish the greenway and associated tributary storm
sewers.
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2 Greenway Drainage Area Site Conditions
The 120-acre West Side Flats study area is part of the Riverview Subwatershed (3,326 acres). The study
area is further divided amongst the Custer (176 acres) and the Chester (330 acres) subwatersheds,
subsets of the Riverview Subwatershed. To further refine the study area to a "greenway drainage area/'
an analysis of what land area could feasibly drain to the proposed greenway was conducted. The
analysis evaluated approaches to direct stormwater to the greenway including:
• intercepting drainage from existing storm sewers,
• proposing new shallow storm sewers, and
• capturing sheet flow around the perimeter of the greenway.
The resulting greenway drainage area is
approximately 39 acres and is situated entirely
within the Custer Subwatershed (Figure 3). The
drainage area is generally flat with a gradual
slope toward the greenway from northeast to
southwest along Fillmore Avenue and then
toward the levee from Fillmore Avenue. The
existing storm sewers drain toward the greenway
to a 90-inch storm sewer that discharges to the
river just east of Wabasha Street. The Custer lift
station is in service to pump flows from the 90-
inch sewer over the levee when the river level is
higher than the outfall gate. The last time this
situation occurred was in 2011. The drainage
area is roughly between Wabasha Street and
Robert Street and between Plato Boulevard and
the Mississippi River (termed "Riverview West"
by the Saint Paul Port Authority). The drainage
area excludes the West Side Flat Apartments and
the US Bank properties, both recently
redeveloped properties with on-site stormwater
controls. The portion of Fillmore Avenue west of
the railroad and Harriet Island Boulevard adjacent
to the West Side Flats Apartments are publically-
owned rights-of-way. Stormwater runoff from
these roads is treated through the City's tree
trench design. This area was included in the
greenway drainage area as discharge from the
tree trenches can be directed to the greenway.
(looking northeast from Fillmore Avenue)
Figure I. Saint Paul Housing and
Redevelopment Authority Parcel
Figure 2. West Side Flats Apartments
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v\ **ii
\v
\WestSftJe o
,?Flats Apartments*
Figure 3. Greenway Drainage Area Sit
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The proposed greenway delineation is approximately 6 acres with an average width of 175 feet and an
approximate length of 1,450 feet from the levee to Plato Boulevard. There is approximately 600 feet of
existing open space bordering the levee and the existing river walk, which extends from Harriet Island
Regional Park on the west side of Wabasha Street. This land is currently apportioned into three
segments due to road crossings including Fillmore Avenue and the proposed extension of Fairfield
Avenue. The length of the greenway borders a railroad corridor. Except for the City's active lift station in
the northwest portion of the greenway, the land has been vacated and is predominately cleared of
structures. Grasses, brush and a few trees remain. It has unobstructed views of the Saint Paul skyline to
the north as well as the Robert Street bridge and the Wabasha Street bridge.
Properties within the greenway drainage area and the proposed greenway itself are predominately
under private ownership. The City of Saint Paul owns the interior roadways and the Saint Paul Housing
and Redevelopment Authority owns the parcel east of the railroad between Livingston Avenue and
Fillmore Avenue. This parcel is expected to be sold for redevelopment. Robert Street/Hwy 52 is under
Minnesota Department of Transportation jurisdiction, Plato Boulevard is owned by Ramsey County, and
the railroad is owned by Union Pacific Railroad Company.
Limited geotechnical information in this area suggests that the soil is primarily 8 to 10 feet of fill over
sandy deposits with clayey layers. The soil in the greenway and its drainage area are expected to be
contaminated due to former industrial uses in the area. The parcel east of Harriet Island Road and west
of the railroad tracks was recently remediated for lead. The groundwater table is approximately 15 feet
below grade. A recently sealed artesian well (July 2012) is located just outside the southern end of the
greenway6. There are no wellhead protection issues in this area since it does not serve a public water
system. Well testing data is unavailable regarding the possibility of contamination.
(looking southwest from Livingston Ave. and
Fillmore Ave.)
Figure 4. Greenway
(looking northwest from Livingston Ave. and
Fillmore Ave.)
Figure 5. Saint Paul Housing and Redevelopment
Authority Parcel
6 Minnesota Unique Well No. 272117. Web Address (last accessed February 6, 2015).
http://mdh-agua.health.state.mn.us/cwi/cwiViewer.htm
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3 Master Plan Framework
The West Side Flats area has been addressed within several planning documents over the past two
decades. The focus of these efforts has been on revitalizing the area and reconnecting neighborhoods
to the Mississippi River. The vision produced by these various efforts is that of a mixed-use urban village
with both public amenities and an integral connection to the Mississippi River. Relevant planning
documents include the following:
• Saint Paul on the Mississippi Development Framework (1997)
• West Side Flats Master Plan and Development Guidelines (2001)
• The West Side Community Plan (Addendum to the Saint Paul Comprehensive Plan) (February
2013)
• Great River Passage Master Plan (Addendum to the Saint Paul Comprehensive Plan) (April 2013)
• Strategic Stormwater Solutions for Transit-Oriented Development (December 2013)
• West Side Flats Master Plan and Development Guidelines (WSF Master Plan), Draft (March
2014)
• Draft Stormwater Appendix to the West Side Flats Master Plan and Development Guidelines
(April 2014)
With regard to Stormwater management, the various planning documents all emphasize the
incorporation of natural Stormwater management and a network of green spaces. In addition,
community members indicated desire for water quality projects that will reduce pollution to the river.
The most recent documents (Strategic Stormwater Solutions for Transit-Oriented Development and the
WSF Master Plan) focus on the use of green infrastructure as an amenity to meet environmental,
economic, and social goals in shared public-private green infrastructure practices.
The West Side Flats greenway concept is specifically addressed in the WSF Master Plan. In addition, the
draft Stormwater appendix to the WSF Master Plan recommends the use of green infrastructure as
development occurs to reduce the probability of flooding when the river level is above the gravity outfall
and the lift station is not able to keep up with storm flows. The WSF Master Plan outlines a modified
street layout, greenway, and changes to building types/uses within the delineated greenway drainage
area. This initial master plan layout will be carried forward within this report as a basis for presenting
and analyzing alternative green infrastructure concepts for the greenway (Figure 6).
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Figure 6. West Side Flats Master Plan Concept
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4 Project Approach
This section notes the goals, objectives, and key design elements for the greenway project. Using these
goals and objectives as a guide, designers and city staff developed three conceptual design options for
green infrastructure within the greenway. Typical green infrastructure project typologies are also
discussed in this section to help visualize the end product.
With implementation of the West Side Flats Greenway project, the City hopes to achieve the following
goals:
• Attract investors to the West Side Flats area.
• Reduce runoff from the area proposed as greenway, which might otherwise have been largely
impervious.
• Demonstrate the feasibility of creating shared stacked-function green infrastructure practices
for public and private stormwater management within Saint Paul in lieu of managing
stormwater on individual sites. Additionally, this project will help explore and support the
concepts presented in the Strategic Stormwater Solutions for Transit-Oriented Development
(December 2013) and build upon the concepts implemented as part of the Twin Cities' Light Rail
Transit Green Line project.
• Connect the West Side neighborhood to the Mississippi River.
• Provide welcoming green space for residents to enjoy.
• Reduce pollutants to the Mississippi River.
• Help manage local flooding during storm events.
4.2 Project Objectives
The specific objectives of the conceptual green infrastructure design within the proposed greenway are
as follows:
• Provide viable concepts for integrating stacked-function green infrastructure with greenway
programming typical of an active recreational park.
• Determine the feasible public and private tributary drainage areas to the greenway to help
inform recovery of capitol and operation & maintenance costs of the greenway and sizing of the
practices to meet the stormwater design criteria.
• Intercept storm flows from the existing stormwater piping network to the greenway as much as
practicable to avoid the cost of constructing new sewer to convey stormwater to the greenway.
The following represents key design elements that were relevant throughout conceptual design. As
design for this project progresses, some design elements may become more prevalent than others.
• Emphasize surface water features as much as practicable.
• Consider depth of the existing sewer system (varies) and groundwater table (~10 feet below
grade) when determining green infrastructure practice depths and overflow back to the existing
sewer system.
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Consider the WSF Master Plan when determining future land uses.
Allocate approximately 60 percent of the greenway to recreational space. This would not
prohibit subsurface stormwater storage beneath the recreational space.
When analyzing the drainage area, differentiate between private and public areas. Include
areas likely to be developed over the next 10 to 15 years.
Assume soils are contaminated and green infrastructure practices will likely be lined to prevent
infiltration and the migration of pollutants. Further discussion will be needed to determine how
lining a practice due to soil contamination will be reconciled with regard to the existing 1.1-inch
retention standard per the Minnesota Minimal Impact Design Standards (MIDS). 7
Consider the railroad easement (~30 feet) a hydrologic barrier due to the permissions needed
for directing drainage beneath the railroad. Further investigation with the Union Pacific Railroad
Company would be necessary to further pursue this option. If permitted, directing drainage
beneath the railroad could potentially allow the practices along the greenway to be hydraulically
connected with flow toward the river.
Preserve the downtown skyline views.
7 Minnesota Minimal Impact Design Standards (MIDS). Web Address (last accessed February 2, 2015):
http://www.pca.state.mn.us/index.php/water/water-tvpes-and-programs/stormwater/stormwater-minimal-impact-design-
standards-mids.html
10
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5 Stormwater Modeling
An XP-SWMM model of the Riverview Subwatershed, developed in 2010 for a prior project, was
modified and used to calculate the stormwater runoff from the area tributary to the proposed greenway
and to route the flows through a pipe network made of existing and proposed storm sewers. Storm
sewers are proposed as part of this conceptual design to convey flows from the tributary area to the
proposed greenway. The existing pipe network within the West Side Flats neighborhood was reviewed
for locations where shallow pipe could be intercepted and rerouted into the either a surface or
subsurface practice depending on the sewer depth and topography. Figure 7 shows the locations of
proposed storm sewer and existing utilized storm sewer.
The tributary drainage area was divided into three subcatchments, each draining to one of three green
infrastructure practice areas as shown in Figure 7. The existing model used the NRCS curve number
method to calculate runoff. This hydrologic method continued to be used to model the conceptual
design, but the imperviousness and drainage delineations were modified within the 39-acre greenway
tributary area to reflect possible future changes to land use and drainage. Modifications to the original
model also included proposed storm sewers and green infrastructure practices used to retain and detain
stormwater runoff. The proposed storm sewers were sized
for a 5-year design storm assuming no attenuation of
. , The stormwater design
stormwater from the adjacent properties. Because the green standards used for thjs ana|ysis
infrastructure proposed in the greenway is providing a follows-
regional stormwater management system for the tributary
drainage area, the criterion for sizing the proposed storm . Retain u inches of runoff from
sewers differs from the City's standard, which is to design for impervious surfaces (MIDS).
a 5-year design storm assuming a runoff coefficient of 0.4 . |_imit tne discharge rate to 1.64
from the tributary properties. A runoff coefficient of 0.4 cfs/acre per City standard.
represents the required attenuation of stormwater from . Use NOAA Atlas 14 rainfall for
individual properties. Additional modeling will be necessary Saint Paul
to understand how the downstream storm water system (i.e. • Provide analysis for storm events
Custer lift station and deep 90-inch sewer) will function in up to tne | QO-year 24-hour event
response to a regional stormwater management facility
during high river stages.
Three park areas within the greenway, totaling approximately six acres, where the green infrastructure
practices will be located were assumed to have no impervious area, while the remaining catchment
areas contributing to the green infrastructure practices were assumed to have 100 percent impervious
area in accordance with future development plans (Figure 6). Infiltration from the green infrastructure
practices during storm events was assumed to be zero to reflect the likelihood that infiltration will not
be allowed due to soil contamination.
For modeling purposes, each of the proposed green infrastructure practices is sized to retain (not to be
released from the site) the runoff from the first 1.10 inches of rainfall and detain the runoff generated
by the 100-year, 24-hour design storm with a release rate of 1.64 cfs per acre (City of Saint Paul
standard). It is assumed that the green infrastructure practices would be a combination of surface and
subsurface practices to handle the required volume, as surface practices alone would not have capacity.
All rainfalls simulated in the model are based on the rainfall depths in NOAA Atlas 14 Volume 8 Version 2
and use the SCS Type II rainfall distribution. The following design storms were simulated in the model.
II
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• 1.10-inch rainfall (used to quantify the required retention volume)
• 1-year, 24-hour rainfall = 2.45 inches
• 2-year, 24-hour rainfall = 2.80 inches
• 5-year, 24-hour rainfall = 3.49 inches
• 10-year, 24-hour rainfall = 4.18 inches
• 100-year, 24-hour rainfall = 7.40 inches
Water stored in the green infrastructure practices in excess of the retention volume is released back into
the 90-inch diameter storm sewer running parallel to the railroad right-of-way on the east side.
Stormwater within the 90-inch storm sewer discharges into the Mississippi River directly or is pumped
over the levee during high river stages. Table 1 provides the required practice retention and detention
volumes to meet the stormwater design standards for the range of design storms. Volumes are divided
between two categories, "Public" and "Private" to reflect the origination of the runoff.
12
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Table I. Retention and Detention Volume Modeling Results
Description
1.10-inch
1-year, 24-hour
2-year, 24-hour
5-year, 24-hour
10-year, 24-hour
100-year, 24-hour
Practice Area 1
Retention Volume ft3
Private
0
0
0
0
0
0
Public
12,000
12,000
12,000
12,000
12,000
12,000
Detention Volume ft3
Private
0
0
0
0
0
0
Public
0
3,000
5,000
8,000
12,000
32,000
Total Volume ft3
Private
0
0
0
0
0
0
Public
12,000
15,000
17,000
20,000
24,000
44,000
Tributary Drainage Area = 6.6 ac (Private = 0 ac; Public = 3.6 ac; Park=3 ac)
Description
1.10-inch
1-year, 24-hour
2-year, 24-hour
5-year, 24-hour
10-year, 24-hour
100-year, 24-hour
Practice Area 2
Retention Volume ft3
Private
34,000
34,000
34,000
34,000
34,000
34,000
Public
11,000
11,000
11,000
11,000
11,000
11,000
Detention Volume ft3
Private
0
5,000
11,000
19,000
28,000
76,000
Public
0
3,000
4,000
8,000
11,000
27,000
Total Volume ft3
Private
34,000
39,000
45,000
53,000
62,000
110,000
Public
11,000
14,000
15,000
19,000
22,000
38,000
Tributary Drainage Area = 15.1 ac (Private = 10.5 ac; Public = 3.5 ac; Park = 1.1 ac)
Description
1.10-inch
1-year, 24-hour
2-year, 24-hour
5-year, 24-hour
10-year, 24-hour
100-year, 24-hour
Practice Area 3
Retention Volume ft3
Private
31,000
31,000
31,000
31,000
31,000
31,000
Public
18,000
18,000
18,000
18,000
18,000
18,000
Detention Volume ft3
Private
0
4,000
7,000
16,000
25,000
72,000
Public
0
4,000
6,000
10,000
16,000
45,000
Total Volume ft3
Private
31,000
35,000
38,000
47,000
56,000
103,000
Public
18,000
22,000
24,000
28,000
34,000
63,000
Tributary Drainage Area = 17.2 ac (Private = 9.6 ac; Public = 5.7 ac; Park = 1.9 ac)
13
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N
A
Drainage Area =
Water
Flow Direction, Private Property
Flow Direction, Public Property
Drainage Catchment Boundary
Proposed Storm Sewer
— Existing Utilized Storm Sewer
•• Private Impervious
^H Proposed Greenway
Public Impervious
^H Railroad
Figure 7. Greenway Tributary Drainage Areas
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6 Green Infrastructure Toolbox
As the green infrastructure concepts were being developed, precedent projects were used to help
define the function and overall look of potential practices within the greenway. The following projects
are primarily regional park projects that collect stormwater from urban areas adjacent to the park for
treatment and storage within the park. Surface water features are a key element of the designs. The
project team felt that projects with these characteristics most closely represent the vision for the West
Side Flats Greenway, and are presented here as part of a toolbox for shared, stacked-function green
infrastructure. Detailed design information regarding specific practices (e.g. biofiltration, tree boxes,
iron enhanced sand filter, stormwater pond, stormwater wetland) is located in the Minnesota
Stormwater Manual.
Courtesy of Waterfront Toronto
Corktown Common Park - Toronto, Ontario
Set on old riverfront industrial lands, Corktown Common Park is a centerpiece for an emerging urban
neighborhood. Stormwater wetlands are used to treat the adjacent land.
http://www.waterfrontoronto.ca/explore projects2/west don lands/corktown common
15
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Courtesy of Neil Price, Wellington City Council
(Top) Waitangi Park-Wellington, New Zealand
Waitangi Park emphasizes water quality including
daylighting of Waitangi Stream, treatment of
stormwater, and water re-use for irrigation.
http://www. woo/, co. nz/our-proiects/urban/waitan qi-
park/
Courtesy of the City of Vancouver
(Bottom) Hinge Park-Vancouver, British Columbia
The wetlands within the park treat stormwater runoff
and function as an amenity to a vibrant recreational
area.
https://cfapp.Vancouver.ca/parkfinder wa/index.cfm?f
useaction=FAC. ParkDetails&park id=240
16
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Historic Fourth Ward Park-Atlanta, Georgia
A large stormwater pond is featured in this urban park. It provides a venue for concerts and an amenity for the
pedestrian trail.
http://www.h4wpc. com/
17
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7 Conceptual Design
Early discussions with the steering committee revealed the
desire to lay out two distinct concepts for integrating green
infrastructure into the proposed greenway; a linear and
natural waterway concept and a central and urban water
feature concept. Later in the process, a third concept was
developed which combined the linear waterway with the
central water feature. The conceptual green infrastructure
layout and stormwater analysis proceeded with these
concepts to meet the goals, objectives, and key design
elements. A cost analysis was not completed as part of this
conceptual design.
A description, conceptual plan view, cross-section, and
example photo are presented in this section for each
concept. The primary differentiators between the concepts
include the aesthetics of the greenway itself (urban versus
natural) and the resulting available space for other park
programming elements.
7.1 Concept I: Linear and Natural
The linear and natural concept focuses on providing
approximately 1.35 acres of man-made waterway resembling
a natural stream-like water feature with adjacent stormwater
wetlands in Practice Areas 2 and 3 (Figure 8 and Figure 9).
The average storage depth is envisioned to be approximately
2.5 feet providing approximately 146,000 cubic feet of
surface storage. The preferred method for feeding the
stream is restoration of the nearby artesian well. However,
given that the well (i.e. spring) was recently sealed and
restoration of the well is unlikely due to cost, the stream itself
would be completely dependent on stormwater flows. The
waterway would be designed with vegetation adaptive to
variable wet and dry periods. The waterway location is
adjacent to the railroad, which provides a buffer between the
tracks and the proposed active recreational park space and
trail. A bridge is proposed across Fairfield Avenue to
hydraulically connect the southern and central park areas. In
addition, a 0.35-acre shallow biofiltration basin is proposed in
the low area on the northwest end of the greenway, Practice
Area 1.
Linear and Natural Concept
Approximate Surface Practice
Sizes
Surface Practice Area I
Area: 15,000 square feet
Depth: 2.5 feet
Volume: 37,500 cubic feet
Practice Volume/Total Volume: 85%
Surface Practice Area 2
Area: 22,400 square feet
Depth: 2.5 feet
Volume: 56,000 cubic feet
Practice Volume/Total Volume: 38%
Surface Practice Area 3
Area: 36,000 square feet
Depth: 2.5 feet
Volume: 90,000 cubic feet
Practice Volume/Total Volume: 54%
* "Total Volume" is the total runoff for a
100-yr 24-hr storm.
Linear and Natural Concept
Approximate Subsurface
Practice Sizes
Subsurface Practice Area I
Volume: 6,500 cubic feet
Practice Volume/Total Volume: 15%
Subsurface Practice Area 2
Volume: 92,000 cubic feet
Practice Volume/Total Volume: 62%
Subsurface Practice Area 3
Volume: 76,000 cubic feet
Practice Volume/Total Volume: 46%
** Subsurface practice sizes reflect what is
needed for a 100-yr 24-hr storm in addition
to the surface storage.
To keep the proposed practices shallow, stormwater is collected from the adjacent streets through curb
cuts or ribbon curbs and runnels (i.e. a narrow channel). The drainage area beyond the adjacent streets
is served by a proposed stormwater pipe network (and portions of the existing stormwater pipe network
if feasible) discharging to subsurface storage practices within the three practice areas. Within the
collection system, prior to discharging to the practices, pretreatment should be installed to remove
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sediment and trash from the runoff. Pretreatment mechanisms might include a tree trench, swale, or a
hydrodynamic device upstream of each practice. The Linear and Natural Concept provides
approximately 1.7 acres of surface water features equating to approximately 30 percent of the
greenway space. To accommodate a 100-yr 24-hr storm event meeting the design criteria, subsurface
storage practices within the greenway would equal approximately 174,500 cubic feet.
Having a stacked function is a key objective of the green infrastructure practices. In addion to providing
stormwater management, the surface practices are envisioned to be integral to the experience one has
in the greenway with attractive vegetation and access points. Assuming the restoration of the artesian
well is not feasible, these practices would be dry outside of rain events. The subsurface practices,
although not visible, could be used for irrigation and for building functions.
19
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® RESTORED SPRING & STORMWATER WETLANDS
® GREENWAY LAWN
© LOCAL DRAINAGE COLLECTION AND RUNNELS
© PROMENADE EXPANSION & CONNECTION TO GREENWAY
Figure 8. Concept I Plan View
© GREENWAY TRAIL
© BRIDGE STRUCTURE
© BIOFILTRATION FOR STORMWATER
20
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-30' ROW-
EXISTING RAIL
RESTORED SPRING (FEASIBILITY REQUIRED)
WITH STORMWATER WETLAND TREATMENT
GREENWAYTRAILWITH
DISTINCT NATURAL
AREA EDGE
GREENWAY LAWN
(UNDERGROUND STORAGE AREA)
Figure 9. Concept I Cross-Section
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Fairview Park, Lansing Township, Michigan
Towar Gardens, East Lansing, Michigan
Figure 10. Biofiltration Examples
22
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7.2 Concept 2: Central and Urban
In contrast to the linear green infrastructure practices in
Concept 1, the Central and Urban Concept highlights a
central green infrastructure practice in Practice Area 2 that
has a permanent pool and enough storage capacity for the
10-year 24-hour storm event (Figure 11 and Figure 12). The
vision for the 0.6-acre central pond emphasizes an urban
form with a floating island, pond overlook, and tree trench
plaza with spray jet fountains. The feasibility of spray jet
fountains with regard to public health requirements will
need to be further investigated. At flood stage, water in the
pond would come into contact with an iron-sand filter
bench and floodplain forest for further treatment. Six feet
of storage depth provides capacity for about 110,000 cubic
feet of stormwater. Like Concept 1, Concept 2 takes
advantage of the low area in Practice Area 1 by
incorporating 0.35 acre of biofiltration basin. A surface
practice was not included in Practice Area 3 in lieu of leaving
it open for other park programming possibilities. A
drawback in removing the surface practice is that there is
typically less effective water quality treatment with
subsurface practices than with surface practices.
The greater depth of the pond allows for greater tributary
area to the surface feature than the shallow wetland
concept allowed. A shallow conveyance network is
proposed to collect water from the streets to discharge to
the pond. Water in excess of the pond capacity would be
directed to subsurface storage with overflow to the 90-inch
storm sewer.
Concept 2 provides approximately 1 acre of surface water
features equating to approximately 16 percent of the
greenway space. To accommodate a 100-yr 24-hr storm
event meeting the design criteria, subsurface storage
practices within the greenway would equal approximately
210,500 cubic feet.
Similar to Concept 1, the Central and Urban Concept strives
for stacked function green infrastructure. The urban pond
provides considerable flood storage volume and water
quality treatment via settling, the iron-sand filter bench, and
the wetland forest. It is also intended to be a prominent
focal point of the greenway.
Central and Urban Concept
Approximate Surface Practice
Sizes
Surface Practice Area I
Area: 15,000 square feet
Depth: 2.5 feet
Volume: 37,500 cubic feet
Practice Volume/Total Volume: 85%
Surface Practice Area 2
Area: 27,400 square feet
Depth: 6 feet
Volume: I 10,000 cubic feet
Practice Volume/Total Volume: 74%
Surface Practice Area 3
Area: 0 square feet
Depth: 0 feet
Volume: 0 cubic feet
Practice Volume/Total Volume: 0%
* "Total Volume" is the total runoff for a
100-yr 24-hr storm.
Central and Urban Concept
Approximate Subsurface
Practice Sizes
Subsurface Practice Area I
Volume: 6,500 cubic feet
Practice Volume/Total Volume: 15%
Subsurface Practice Area 2
Volume: 38,000 cubic feet
Practice Volume/Total Volume: 26%
Subsurface Practice Area 3
Volume: 166,000 cubic feet
Practice Volume/Total Volume: 100%
** Subsurface practice sizes reflect what is
needed for a 100-yr 24-hr storm in addition
to the surface storage.
23
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® FLOODPLAIN POND - AERATION & FLOATING ISLAND
® GREENWAY LAWN
fo TREE TRENCH PLAZA WITH SPRAY JET FOUNTAINS -
GATHERING STRUCTURES, POND OVERLOOK
© PROMENADE EXPANSION & CONNECTION TO GREENWAY
© GREENWAY TRAIL
fa FLOODPLAIN POND IRON-SAND FILTER
** BENCH-WITH SCULPTURAL METAL WALL
© POND FLOODPLAIN FOREST
® BIOFILTRATION FOR LOCAL STORMWATER
Figure I I. Concept 2 Plan View
24
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tffc,
EXISTING RAIL
FLOOOPLAIN
BOUNCE ZONE
IRON-SAND
FILTER BENCH -
SCULPTURAL STEEL
EDGE
WITH AERATION &
FLOATING ISLANDS
ARCHITECTURAL
RETAINING WALL
AND GREENWAY
TRAIL
TREE TRENCH PLAZA
AND SPRAY JETS
Figure 12. Concept 2 Cross-Section
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Courtesy of Waterfront Toronto
Courtesy of DeepRoot (Flickr: DeepRoot Green
Infrastructure)
(Top) Spray Jet Fountains- Corktown Common Park, (Top) Silva Cell Tree Plaza - University of Calgary EEEL
Toronto, Ontario Building, Calgary, Alberta
(Bottom) Urban Pond - Historic Fourth Ward Park, (Bottom) Iron-Sand Filter - Trout Brook Nature
Atlanta, Georgia Sanctuary, Saint Paul, Minnesota
Figure I 3. Urban Pond Feature Examples
26
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7.3 Concept 3: Combined
Concept 3 is the result of merging the stream-like surface feature in Concept 1 with the central urban
pond feature in Concept 2 (Figure 14). This combination essentially incorporates the highlights from the
first two concepts and provides approximately 1.8 acres of surface water features equating to
approximately 30 percent of the greenway space, similar to Concept 1. To accommodate a 100-yr 24-hr
storm event meeting the design criteria, subsurface storage practices within the greenway would equal
approximately 120,500 cubic feet.
Combined Concept Approximate Surface
Practice Sizes
Surface Practice Area I (from Concept I and 2)
Area: 15,000 square feet
Depth: 2.5 feet
Volume: 37,500 cubic feet
Practice Volume/Total Volume: 85%
Surface Practice Area 2 (from Concept 2)
Area: 27,400 square feet
Depth: 6 feet
Volume: I 10,000 cubic feet
Practice Volume/Total Volume: 74%
Surface Practice Area 3 (from Concept I)
Area: 36,000 square feet
Depth: 2.5 feet
Volume: 90,000 cubic feet
Practice Volume/Total Volume: 54%
Combined Concept Approximate
Subsurface Practice Sizes
Subsurface Practice Area I
Volume: 6,500 cubic feet
Practice Volume/Total Volume: 15%
Subsurface Practice Area 2
Volume: 38,000 cubic feet
Practice Volume/Total Volume: 26%
Subsurface Practice Area 3
Volume: 76,000 cubic feet
Practice Volume/Total Volume: 46%
* "Total Volume" is the total runoff for a 100-yr 24-hr
storm.
** Subsurface practice sizes reflect what is needed for a
100-yr 24-hr storm in addition to the surface storage.
Concept Variations
While developing the three concepts, it was clear that there are design variations that may be
preferred with further discussions and investigation. They include but are not limited to the
following:
• Routing the drainage area west of the railroad tracks between Plato Blvd and Fillmore Avenue
into a subsurface storage within Practice Area I instead of intercepting the existing storm
sewer within Practice Area 3. This may be a more costly option as new storm sewer would
be needed to route flow to the north.
• Assuming the bottom elevation of the pond is fixed due to design constraints, the design
depth of the permanent pool can be adjusted by raising the surface elevation. This would
affect the available capacity above the pool for storm storage.
• With further investigation, it may be feasible to direct stormwater through a culvert beneath
the railroad tracks. This could potentially allow water to flow from the south of Fillmore
Avenue to north of Fillmore Avenue in a continuous stream.
• Further investigation of utilizing the recently capped artesian well along Plato Blvd will help
determine whether the stream-concept is feasible. Either way, the stormwater management
capacity of that surface feature will remain the same.
• Soil investigations within the greenway will be necessary to determine whether the design of
the practices, such as adding an impermeable liner, needs to accommodate contaminated
soils.
27
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® FLOODPLAIN POND-AERATIONS FLOATING ISLAND © GREENWAY TRAIL
® GREENWAY LAWN (f) FLOODPLAIN POND IRON-SAND FILTER
^ BENCH-WITH SCULPTURAL METAL WALL
^ TREE TRENCH PLAZA WITH SPRAY JET FOUNTAINS- _
® GATHERING STRUCTURES, POND OVERLOOK © POND FLOODPLAIN FOREST
© PROMENADE EXPANSIONS CONNECTION TO GREEN WAY ®
Figure 14. Concept 3 Plan View
2i I
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8 Conclusion
The incorporation of green space into the urban fabric amidst pressure to develop and revitalize the
West Side Flats neighborhood is a significant achievement on its own. Enhancing green space to not
only provide water quality treatment and flood management benefits, but also serve as a unique
amenity for residents is an example for cities worldwide.
Key takeaway messages:
• Interdepartmental input is required for successful planning and implementation of green
infrastructure projects within the public realm. The City of Saint Paul created a steering
committee dedicated to the West Side Flats Greenway concept. Steering committee members
included the city's Water Resource Coordinator and staff from the Department of Planning and
Economic Development, Department of Parks and Recreation, and Department of Public Works.
The Saint Paul Riverfront Corporation, an urban design resource for community redevelopment
projects, was also a key member of this steering committee.
• Including green infrastructure as part of a master planning process provides the opportunity to
incorporate the public's vision for green infrastructure and its shared stacked functions. It also
provides the opportunity to understand the important technical aspects of incorporating green
infrastructure, such as defining the tributary drainage area and determining how stormwater
will be conveyed to the green infrastructure practices. As a result of this conceptual design, it
became clear that new shallow storm sewers would be needed to convey stormwater from the
future densely-built area to the proposed greenway. It was also found that some existing
sewers could easily be re-routed to the proposed greenway, a more cost-effective means of
conveying stormwater than installing new sewer.
Additionally, it was recognized that the city's current stormwater design standards should be
reviewed for applicability within the West Side Flats neighborhood. The proposed green
infrastructure within the greenway provides a regional stormwater facility while the existing
design standards were developed for individual on-site stormwater management.
• The technical aspects of the project need to advise the form and vice versa. For example, the
flat topography of the tributary area is not conducive to capturing stormwater runoff from a
large catchment area within a shallow waterway, but the urban pond design allows for deeper
inlets while maintaining a visible amenity for the public. The steering committee recognized this
and indicated preference for the urban pond concept while still trying to incorporate a shallow
waterway from overland flow as much as practicable. This combination of deep and shallow
surface water features is reflected in Concept 3.
To move this project forward, there are several additional items that should be investigated. These
include:
• Completing a detailed XP-SWMM model of the final design concept to determine the impact to
the City's storm sewer infrastructure, including the Custer lift station.
• Developing a fair and equitable funding mechanism that addresses construction, operation and
maintenance, and replacement costs. See Appendix A for a discussion of cost recovery
mechanisms.
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• Completing a project cost estimate for aspects of the greenway that would be funded by private
and public partners. This piece would tie into the funding mechanism for the area.
• Completing a soil investigation throughout the drainage area but particularly within the
greenway where water will be directed. Pertinent information includes the extent and depth of
contamination, the depth to groundwater, soil type, and soil infiltration rate.
• Determining the feasibility of utilizing the recently capped artesian well near the southern end
of the greenway as a constant water source for the proposed stream/wetland.
• Purchase of the greenway parcels by the city or other entity in cooperation with the city.
30
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Appendix A: Cost Recovery Options
Introduction
The City of Saint Paul is evaluating options to install and fund regional shared stormwater facilities in West Side
Flats Greenway. The proposed shared stormwater project will provide for stormwater controls to be used by
contributing properties, potentially in lieu of meeting stormwater requirements on each individual site. The
project will provide flood and rate control and water quality benefits, and will provide recreational, aesthetic, and
other benefits associated with the greenway. The West Side Flats Greenway is assumed to be publically owned in
the future; the Strategic Stormwater Solutions for Transit-Oriented Development report includes a framework for
either public or private ownership of open space for shared, staked green infrastructure.
The drainage area that is tributary to the planned shared stormwater project is comprised of developed parcels in
multi-family, commercial, industrial or vacant use. The parcels are, for all intents, covered fully by impervious
surfaces, and the anticipated redeveloped condition will be similarly dominated by impervious surfaces. Under the
current conceptual plan, contributing and benefiting properties include both potential public and private
ownership (Table A-l and Figure A-l). Properties that are not expected to redevelop in the next 10 to 15 years are
not included, for example the West Side Flats Apartments. The expected total public land area is 18.6 acres
compared with 20.1 acres of privately owned land. An important consideration will be determining which
properties are benefiting from the shared stormwater project. Both private and public properties are required to
implement stormwater management controls for new and re-developed sites through the state general
construction permit; therefore, some level of benefit could be assigned to all contributing parcels. For the
purposes of this analysis, only private contributing and benefiting properties are included.
Another important issue will be timing of construction of the regional stormwater facilities. As new projects are
developed in the contributing area prior to construction of the regional facilities, these projects will be required to
meet the state stormwater construction permit requirements, and will likely no longer be potential benefiting
properties. These properties will likely need to be removed from the contributing and benefiting properties,
potentially increasing the burden on the remaining properties. A possible option could be explored with the state
to determine if stormwater requirements could be deferred in lieu of expected regional facilities.
This memorandum presents an evaluation of the various methods available to the City to fund the construction,
operation and maintenance of a shared regional stormwater facility. The memorandum first describes available
mechanisms for establishing rates and charging properties. This is followed by discussion of the rate structure
options for allocating costs among properties in the contributing area. In addition, examples of existing regional
stormwater programs from both the Twin Cities area and nationally are provided.
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Table A-1. Property ownership of contributing area
Property Ownership
Greenway (park)
Private
Public
Area (acres)
6
20.1
12.8
Water
Plato
125
250
3 Feet
Vegetated Facility
Urban Pond
I Proposed Greenway
I Railroad
Public Property
Private Property
Greenway Contributing
Drainage Area
Saint Paul, Minnesota
Figure A-1. Contributing area and current potential benefiting properties.
Available Implementing Mechanisms
An implementing or funding mechanism should take into account the different funding requirements for a project.
The West Side Flats Greenway stormwater project will require initial capital costs for design and construction
which occur once. Operation and maintenance of the shared stormwater facility are typically recurring annual
costs. Different implementing mechanisms can be better suited towards either one time or recurring costs. A
comparison of the different implementation mechanisms is provided below along with recommendations.
32
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Available implementing mechanisms have been documented in a memo dated May 3, 2012 (LeFevere 2012). Each
relevant funding mechanism is presented below based on the memo. Jurisdiction-wide ad valorem taxes are not
considered as a relevant funding mechanism and are not presented herein.
Stormwater Utility
Cities are authorized to operate storm sewer utilities under MN Statute 444. The City's existing storm sewer utility
has latitude to allocate costs to system users in a just and equitable manner, and charges can be applied to users
based on use of the system, availability of the system to serve a property, and for connection to the system.
Authority to establish utility charges (MN Statute 444.075) encompasses consideration of a parcel's area, land use
classification, and runoff water quality. The City also has authority to modify the charges to reflect adjustments in
runoff, which would be applicable to reflecting various levels of benefit or regional facility use. A reclassification of
properties within the watershed, and an adjustment of rates for parcels within that reclassification reflecting the
benefits received, could be considered to provide greater equity among system users. As noted above, rates can
be charged for system use as well as availability; hence, charges could be applied in advance of a watershed
property undergoing redevelopment. A similar rate surcharge approach has been used for a defined district within
the City of Redmond, Washington to fund planned regional stormwater collection and water quality treatment
improvements to facilitate redevelopment of the downtown area. This surcharge was in addition to the
established City-wide stormwater utility fee.
Special District Ad Valorem Taxes or Charges
Special Assessments: In the state of Minnesota, cities designated as "first class cities" have authority to fund
storm drainage improvements through special assessments against the properties benefiting from the facilities
(MN Statute 435.017-019). The special assessments can be put into effect by council resolution. The city has
latitude to define the types and degrees of benefit to the properties. Benefits in this case would include
stormwater management. There are limitations on assessments against state-owned or city-owned properties.
Special Service Districts: The cost of area-specific stormwater services to watershed parcels may be administered
under provisions of a Special Service District as described in MN Statute 428A. There is flexibility in establishing the
basis for charges as long as they are equitable. Because of petition and veto requirements, this authority requires
substantial support from the businesses and residents affected.
Storm Sewer Improvement District: Cities are authorized to establish storm sewer improvement districts within
the city and levy ad valorem taxes for storm water management projects within the district under MN Statute
Sections 444.16-444.20 (LeFevere 2012). The storm sewer improvement district, locally referred to as an eco-
district, is established by ordinance and requires two-thirds vote and a public hearing process. Because the
affected parcels will likely be impervious following redevelopment, the determinant of a property's use of the
stormwater improvements is its gross area, rather than its assessed valuation. If an assessed valuation is used, the
revenue base (assessed value) will increase over time as the properties redevelop, and the assessment rate would
require periodic readjustment to maintain appropriate revenue levels. This funding mechanism can be applied to
both construction and maintenance of the storm sewer system and related facilities in the district.
Comparative Analysis and Recommendations for Mechanism
Selecting an appropriate funding mechanism should include evaluation of several factors such as complexity,
equity, flexibility in structuring charges, data requirements, and applicability of funding option to capital and
recurring costs for each option (Table A-2). An overall recommendation is provided for each funding option.
Funding options can be combined; for example, a stormwater utility could be used to fund recurring maintenance
costs, while a special assessment could be used to fund construction activities.
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Table A-2. Comparison of funding mechanisms
Fee Structure
Alternative
Complexity
Equity
Flexibility to Structure
Charges
Data Requirements
Applicability to Capital
Costs
Applicability to
Recurring Costs
Overall
Recommendation
Stormwater Utility
Moderate - Existing utility
already in place, would require
coordination amongst several
city departments
Equitable - Depending on rate
structure, can be equitable for
all contributing and benefiting
properties
Moderate - ability to reflect
adjustments in runoff and
water quality
Moderate - Parcel and
assessor's databases, total
project costs, other data needs
dependent on revenue basis
and rate structure
Moderate - can recover costs
over time through rates
High
Recommended
Special District - Special
Assessment
Low
Equitable - Depending on rate
structure, can be equitable
for all contributing and
benefiting properties
High - ability to consider
stormwater- and greenway-
related benefits
Low - Total project costs,
other data needs dependent
on revenue basis and rate
structure
High
Low
Recommended
Special District - Special
Service District
High - requires substantial
support from businesses and
residents
Equitable - Depending on rate
structure, can be equitable
for all contributing and
benefiting properties
High - ability to consider
stormwater- and greenway-
related benefits
Low - Total project costs,
other data needs dependent
on revenue basis and rate
structure
High
Low (property owners
expected to change over
time)
Not Recommended
Special District -Storm Sewer
Improvement District
(Ad Valorem)
Low
Not Equitable - Charges are
poorly correlated to benefits
Low - restricted to assessed
valuation
Low - Total project costs,
assessed valuation
High
High
Not Recommended
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Rate Structure
Depending on the selected funding mechanism, a variety of fee structures can be considered for
allocating costs to the properties within the contributing area.
Flat Rate
The flat rate structure applies a uniform charge to each land parcel, regardless of parcel size, land use,
or improvements. This approach is most appropriately applied to programs largely engaged in planning
efforts across broad areas rather than for construction of local projects.
The primary advantage to the flat rate approach is its ease of application, requiring minimal cost to
implement and administer the charges. The primary shortcoming to the flat rate approach is that it is
not equitable when considered against the relative demands of the parcels, as larger parcels contribute
more runoff to a facility than smaller parcels and consume a greater share of the facilities' capacity.
Ad Valorem
An ad valorem rate structure applies a uniform percentage fee to the assessed value of each parcel. The
assessed value reflects the value of the land itself as well as the value of improvements. Value is partially
related to the size of the property, and hence the amount of runoff contributed to the drainage
facilities; however, it also largely reflects how the property is used, how it is developed, and features of
its location (nearby amenities, infrastructure). As such, this method charges more for developed
properties than similar sized vacant parcels which contribute similar runoff volumes to the drainage
improvements.
Similar to the flat rate structure, the ad valorem rate structure is relatively easy to establish initially, as
the assessed values are readily available from the assessor's office. The ad valorem structure, however,
does not provide an equitable distribution of costs to properties as the assessed value is not closely
related to the relative runoff contributed by respective parcels. Another complication with the use of
the ad valorem method is the evolving rate base: as parcels redevelop, the assessed value increases for
those parcels, which results in a shifting allocation of charges between parcels.
Runoff Contribution
A runoff contribution (sometimes referred to as "graduated") rate structure allocates costs of the
drainage improvements to properties in proportion to the relative proportion of runoff from each parcel
to the facilities. The relative contribution of runoff is typically determined based on the impervious
surface area on each property, as the amount of impervious surface relates directly to the volume and
the rate of runoff discharged from a parcel and to stormwater facilities. This approach is often applied to
the allocation of stormwater utility charges as a strongly equitable means of distributing costs among
stormwater system users and beneficiaries.
A primary advantage of the runoff contribution rate structure is that the resulting charges are related to
the share of facility capacity consumed by runoff from a given parcel. In most instances, the drawback to
the rate structure is the effort and expense required to develop the database of parcel impervious area.
In this instance, however, the parcels are assumed fully impervious, a parcel's impervious area equates
to its gross area, and the charges can be apportioned based on gross area to achieve an equivalent
result. The gross area of each parcel is readily obtainable from assessor data. For the conceptual
modeling, it was suitable and most conservative to assume all contributing parcels are impervious, but
going forward with the rate structure, this assumption may not be applicable.
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Hybrid
To provide a greater level of equity in cost allocation, it may be suitable to employ a hybridized two-
component rate structure. This has been employed in some jurisdictions to allocate the costs from
distinct program elements in different ways to better reflect how parcels benefit from each element. In
Maryland's Prince George's County, the operation and maintenance costs for the watershed retrofit
program are allocated on a flat rate basis, whereas the capital program costs are allocated using the
runoff contribution approach.
Such a hybrid method may provide a greater degree of equity between parcels in the watershed
through:
• Using the Flat Rate method to distribute programmatic, administrative, and maintenance costs
associated with common and shared benefits of improved stormwater management and
greenway-related benefits, and
• Allocating capital costs for facilities construction, the size and capacities of which are driven by
the runoff discharged to them, using the Runoff Contribution approach.
The two rate components could be combined into a single fee charged to the parcel.
Comparative Analysis and Recommendations for Rate Structure
Features of the foregoing alternative fee structures are compared with respect to several criteria,
summarized in Table A-3. This comparison is considered preliminary, and it is recommended the
suitability of the various rate structures be further evaluated using project cost estimates and parcel
data when available.
A hypothetical cost-recovery analysis was completed using a hybrid fee structure (maintenance per
parcel and capital based on impervious area) to determine the potential range of parcel charges
necessary to recover a financial investment (Table A-4). Scenarios are provided for both a 20-year and
30-year bond. The assumptions and inputs to this scenario can be adjusted by the City using the
accompanying spreadsheet.
The following assumptions were made in this analysis:
1. Real estate costs range from $6,000,000 - $8,000,000
2. Construction costs range from $5,000,000 - $10,000,000
3. Annual maintenance is $10,000
4. Design, permitting, legal, administrative, construction management, and contingency are
approximately 25 percent of construction costs
5. 20 benefiting private parcels in the future at 100 percent impervious (total area equals 20.1
acres)
The estimated annual cost per parcel ranges from $31,594 to $67,166 ($0.73 - $1.54 per square foot)
assuming all costs are included, depending on the scenario and life of the bond.
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Table A-3. Comparison of fee structures
Fee Structure Alternative
Description
Flat Rate
Each parcel is charged a
uniform fee, regardless of
gross area, impervious area,
land use, or assessed value
Ad Valorem
Each parcel is charged a
uniform rate based on the
assessed value
Runoff Contribution
Each parcel is charged a
rate based on its
impervious area
Hybrid
Costs for operation and
maintenance, planning,
administration, etc. charged by
the Flat Rate
Costs of capital improvements
charged by the Runoff
Contribution
Implementation
Complexity
Data required to support
Low
Low - Number of parcels
High - Rate base changes
over time as properties are
improved
Moderate- Parcel
database
Moderate
Low - Gross parcel area
(assumes gross parcel area
= impervious area)
Moderate - Segregate costs into
2 components
Low - Number of parcels and
gross parcel area (assumes gross
parcel area = impervious area)
Equity
Between larger and smaller
parcels
Fee related to parcel's
consumption of drainage
improvement capacity
Recommendation
Not Equitable - Does not
distinguish between parcels'
demands for drainage system
capacity
Not Equitable- Poor
correlation between fee
amount and a parcel's runoff
discharge
Not Recommended
Somewhat Equitable - Only
to degree that assessed
value reflects parcel size
and impervious area
Somewhat Equitable -
Assessed value has limited
correlation to parcel's
gross/ impervious area
and, hence, runoff
discharge
Not Recommended
Equitable - Proportional to
parcels' demands for
drainage system capacity/
benefit received
Equitable - Directly related
to parcel's runoff discharge
Recommended
Equitable - Provide high level of
equity reflecting levels of
general benefit and capacity
demand
Equitable - Directly related to
parcel's runoff discharge
Recommended
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Table A-4. Hypothetical rate scenario
1/1
LLJ
cc
D
1-
Q
•z.
LLJ
Q.
X
LLJ
BENEFIT AREA
1/1
LLJ
1-
<
cc
Real Estate Acquisition
Design, permitting, legal/admin, const mgt, contingency
Construction
Total Capital Improvements (1)
Annual Facility Maintenance a (2)
Capital Amount Financed by Bond Sale (3)
Bond Interest Rate (%) b
Bond Term (years)
Annual Debt Service Incurred (4)
Remaining Direct Capital Expenditure (1-3) (5)
Total Annual Expenditures (2+4+5) (6)
Number of Benefitted Parcels (7)
Total Parcel Area Benefitted (acres ) (8)
Total ROW Area Benefitted (acres) (9)
Total Park Area (acres) (10)
Total Tributary Area (acres) (11)
Total Annual Revenue / Parcel (6/7) c (12)
Total Cost over Term of Bond c
Total Annual Revenue / Parcel Acre (6/8) d (13)
Total Cost over Term of Bond d
Total Annual Revenue / Impervious Acre (6/(8+9)) e (14)
Total Cost over Term of Bond e
Hybrid: Maintenance /Parcel; Capital /Acre (16+17) f (15)
Maintenance /Parcel (2/7) f (16)
Capital / Parcel Acre (4+5)/8f (17)
Total Cost over Term of Bond f
ALL CAPITAL COSTS BONDED
LOW-END COST ESTIMATE
20-year Bond
6,000,000
1,250,000
5,000,000
12,250,000
10,000
12,250,000
2.70%
20
800,724
-
810,724
20
20.1
12.8
6.0
38.9
40,536
810,724
40,335
806,691
24,642
492,842
40,337
500
39,837
806,741
30-year Bond
6,000,000
1,250,000
5,000,000
12,250,000
10,000
12,250,000
3.00%
30
624,986
-
634,986
20
20.1
12.8
6.0
38.9
31,749
952,479
31,591
947,740
19,300
579,015
31,594
500
31,094
947,815
HIGH-END COST ESTIMATE
20-year Bond
8,000,000
2,500,000
10,000,000
20,500,000
10,000
20,500,000
2.70%
20
1,339,988
-
1,349,988
20
20.1
12.8
6.0
38.9
67,499
1,349,988
67,164
1,343,271
41,033
820,661
67,166
500
66,666
1,343,321
30-year Bond
8,000,000
2,500,000
10,000,000
20,500,000
10,000
20,500,000
3.00%
30
1,045,895
-
1,055,895
20
20.1
12.8
6.0
38.9
52,795
1,583,842
52,532
1,575,962
32,094
962,822
52,535
500
52,035
1,576,037
DESIGN COSTS EXCLUDED g
LOW-END COST ESTIMATE
20-year Bond
6,000,000
5,000,000
11,000,000
10,000
11,000,000
2.70%
20
719,018
-
729,018
20
20.1
12.8
6.0
38.9
36,451
729,018
36,270
725,391
22,159
443,172
36,272
500
35,772
725,441
30-year Bond
6,000,000
5,000,000
11,000,000
10,000
11,000,000
3.00%
30
561,212
-
571,212
20
20.1
12.8
6.0
38.9
28,561
856,818
28,419
852,555
17,362
520,862
28,421
500
27,921
852,630
HIGH-END COST ESTIMATE
20-year Bond
8,000,000
10,000,000
18,000,000
10,000
18,000,000
2.70%
20
1,176,575
-
1,186,575
20
20.1
12.8
6.0
38.9
59,329
1,186,575
59,034
1,180,671
36,066
721,322
59,036
500
58,536
1,180,721
30-year Bond
8,000,000
10,000,000
18,000,000
10,000
18,000,000
3.00%
30
918,347
-
928,347
20
20.1
12.8
6.0
38.9
46,417
1,392,520
46,186
1,385,592
28,217
846,517
46,189
500
45,689
1,385,667
a. Assumed value
b. Based on National AAA rates, Dec 2014
c. Distributing costs equally between parcels, irrespective of area
d. Distributing costs based on impervious area, excluding ROW
e. Equivalent basis if costs were distributed over both private and public benefitted areas
f. Distributing maintenance cost equally to parcels; distributing capital costs based
on impervious area. Assumes parcel area of 1.0 acres
g. Scenario assumes design, permitting and administrative costs are paid from
outside sources
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Examples
The following examples provide relevant information on cost-recovery approaches for the West Side
Flats Greenway shared stormwater project.
Oakdale, Minnesota
Oakdale maintains a Surface Water Management Fund (Fund) for the purpose of providing for the
acquisition and development of storm water retention areas within the City of Oakdale. The Fund is
funded with a fee on building permits based on the additional runoff generated from a site (commercial,
industrial, or institutional properties only). Additional runoff is based on the difference between the pre-
and post-development condition for the 100-year rainfall event. The City has the option to require
either on-site stormwater facilities, a cash contribution to the Fund, or a combination of both. The
requirements are codified in Section 5-6 of City Code. Operation and maintenance of regional facilities
are funded through the city's stormwater utility.
When the City Council decides that an owner platting property and/or developing commercial, industrial
or institutional property cannot or should not meet all city stormwater requirements on site, that owner
shall pay an amount based on deriving the required storage and the cost associated with providing that
storage elsewhere in the city. The following formulas are used by the City:
The required storage shall be calculated as:
S = A (in acres) x .5 feet x (Q2 - Ql)
(The assumed rainfall of .5 feet or 6.0 inches in 24 hours is comparable to a 100-year frequency,
24-hour duration storm based upon U.S. Weather Bureau statistical data compiled in Technical
Report No. 40, dated May, 1961).
S is the required storage
A means the total area of the development site, measured in acres.
Ql means the composite coefficient of run-off (weighted average) for the entire site or
development area, based upon the predevelopment land use and the coefficient of runoff as
prescribed by the city.
Q2 means the composite coefficient of run-off (weighted average) for the entire site or
development area, based upon the post-development land use and the coefficient of runoff as
prescribed by the city.
Payment = L (acres) x C
Where
L = S/D
L means area of land (in acres) required to provide for the storage of excess surface water run-
off created by the owner's plat or development
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S means the volume of storage required, measured in acre-feet, defined as the increased
surface water run-off and computed as the difference between the calculated surface water
run-off after the development is completed and the calculated surface water run-off from the
site at the time of application.
D means depth of water measured in feet that can be accommodated in a proposed retention
area. When specific information is not available to the Public Works Director/City Engineer for
an accurate determination of such depth, it shall be assumed to be three (3) feet.
C means the cost basis (per acre) for the acquisition and physical development of storm water
retention areas. This amount shall be stated on a per acre basis and shall be determined by and
revised by the City Council from time to time by resolution of the Council.
Redmond, Washington
Becoming an NPDES-regulated community in 2005, the City of Redmond embarked on a two-pronged
approach to complying with development and redevelopment stormwater controls. Most development
and redevelopment projects build stormwater flow control and runoff treatment facilities within their
project site. In some areas, however, regional facilities are being used to meet flow control and runoff
treatment minimum requirements for entire subbasins, effectively treating the entire tributary area as a
"site". In addition to meeting requirements for individual development and redevelopment projects,
these facilities are retrofitting many high pollution-generating land uses (such as roads).
The downtown regional facilities were also viewed as facilitating redevelopment and supporting city
land use policies. Initially the project was supported by a surcharge on the stormwater utility rate for
properties within the tributary subbasin. Later, the costs were translated into a stormwater capital
facilities charge allocated based on the impervious area of a parcel. There are credits available against
this fee for sites that infiltrate stormwater in private on-site systems.
Prince George's County, Maryland
Under Maryland HB 987, the County must establish a watershed protection and restoration program
that includes a Stormwater Remediation Fee ("Fee") and a Local Watershed Protection and Restoration
Fund ("Fund") directed towards restoring water quality in Chesapeake Bay and local receiving waters.
The Fund finances the accelerated rehabilitation of storm water facilities and infrastructure to provide
water quality control of runoff from developed areas currently without, or underserved by, water
quality controls.
Prince George's County elected to implement a hybrid fee structure, allocating operating and
maintenance costs to accounts on a flat fee basis, and distributing the capital program costs based on
impervious area coverage. The hybrid fee structure offered the following features found by the County
to be favorable:
• The hybrid method provided a high level of equity, as all property owners uniformly benefit
from ongoing maintenance and operation, and those properties generating greater volumes of
runoff from larger impervious areas proportionally contribute to the capital solutions for
restoring the watershed.
• The hybrid method is strongly consistent with the proportionality sought in the state legislation.
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• Case studies of the rates resulting from the various fee structures demonstrated that
distributing operation and maintenance costs uniformly, rather than on a graduated basis,
substantially eased the financial burden on larger properties while only slightly increasing costs
for all properties.
• Employing a 3-tiered structure for single family detached residences, based upon zoning,
enhanced equity between the various scales of housing. Offering a lower rate for smaller parcels
was consistent with other County land use policies encouraging smart growth and access to
mass transit.
• A 3-tiered structure for single family detached residences avoided the need to directly measure
impervious area on 88 percent of the accounts, thereby enabling implementation under a very
tight schedule.
Despite the data-intensive effort required to implement an impervious area-based charge across
300,000 accounts, the County implemented the hybrid method due to its high level of equity among
customers.
Philadelphia, Pennsylvania
The Philadelphia Water Department is currently evaluating the use of Stormwater Management
Enhancement Districts (SMED) in the city to support implementation of the department's Long-term
Control Plan for their combined sewer system. SMEDs are areas that could be served by large scale,
centralized green infrastructure. Project work on evaluating SMEDs began in 2012 and is not yet
complete. Part of this work will include evaluation of funding mechanisms.
Cost-Recovery Options Summary
Selection of a cost-recovery option will be dependent on numerous factors including equity, complexity,
flexibility, data requirements, and applicability to capital versus recurring costs. This memorandum
outlines possible options and recommends the following for additional consideration by the city:
• Special District-Special Assessment
• Stormwater Utility
A rate structure based on runoff contribution is recommended, with the possibility of a flat rate per
parcel for annual operation and maintenance costs (hybrid approach). To remain highly equitable, actual
runoff contribution should be determined if parcels are expected to be less than 100 percent
impervious, increasing the complexity and data requirements for this option. A hypothetical cost-
recovery analysis was completed using a hybrid fee structure. Various scenarios are provided to
determine a range of potential costs to benefiting properties. Assumptions should be noted and
adjusted as additional information becomes available.
This analysis did not take into account a developer's fee or fee in lieu such as that being used by
Oakdale. These mechanisms could be further considered if desired by the city.
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References
LeFevere, C, 2012. Central Corridor Stormwater and Green Infrastructure Plan: Governmental Authority
Relating to Stormwater Infrastructure. Memorandum to Central Corridor Stormwater and Green
Infrastructure SAC. May 3, 2012.
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