&EPA
United States
Environmental Protection
Agency
2014 GREEN INFRASTRUCTURE TECHNICAL ASSISTANCE PROGRAM
City of Norfolk
Norfolk, VA
Restoring Knitting Mill Creek through Green
Infrastructure
A plan for adapting green infrastructure to a shoreline community subject to
sea level rise
December 2015
EPA832-R-15-012
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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, v\s\t http://www.epa.Qov/Qreeninfrastructure
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Acknowledgements
U^lifA, "]"'-• am
Jamie Piziali, USEPA
Christopher Kloss, USEPA
Eva Birk, ORISE, USEPA
Ken Hendrickson, USEPA Region 3
John Stewart, Lafayette Wetlands Partnership
Lisa Renee Jennings, Friends of Norfolk Environment
Justin Shafer, City of Norfolk
Jonathan Smith, Tetra Tech
Martina Frey, Tetra Tech
Christy Williams, Tetra Tech
Kelly Meadows, Tetra Tech
Bobby Tucker, 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.
Cover illustration by Tetra Tech.
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Contents
1 Executive Summary 1
2 Introduction 2
2.1 Water Quality Issues 3
2.2 Project Overview 6
2.3 Project Goals and Benefits 6
2.4 Local Challenges 6
3 Green Infrastructure Opportunity Analysis 7
3.1 General Observations 9
3.2 Colley Avenue Commercial Corridor 11
3.3 Mayflower Road and Shoreline 12
3.4 Residential Neighborhoods 13
3.5 Soil Conditions 14
4 Design Approach 15
4.1 Stormwater Toolbox 15
4.1.1 Bioretention 15
4.1.2 Permeable Pavement 17
4.2 Stormwater Design and Performance Standards 18
4.3 Community Involvement in Green Infrastructure 20
4.3.1 Community Workshop 20
4.4 Project Selection 20
5 Conceptual Design 23
5.1 35th Street and Colley Avenue Green Street 23
5.2 Mayflower Road and Shoreline 29
5.3 Cost Estimate 32
6 Conclusion 33
7 References 34
Appendix A: Site Investigations Summary A-l
Appendix B: 35th Street and Colley Avenue Green Street Conceptual Design B-l
Appendix C: Mayflower Road Shoreline Conceptual Design C-l
Appendix D. Green Infrastructure Conceptual Design Detailed Cost Estimates D-l
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Figures
Figure 2-1. Knitting Mill Creek watershed and the surrounding waters 3
Figure 2-2. Knitting Mill Creek watershed displaying the stormwater drainage system 5
Figure 3-1. Potential green infrastructure retrofit locations identified by desktop screening 8
Figure 3-2. The majority of stormwater enters the southern end of Knitting Mill Creek near the
intersection of Mayflower Road and 42nd Street 9
Figure 3-3. The rain garden at 47th Street enhances the community aesthetic 10
Figure 3-4. Colley Avenue exhibits extensive paving and limited vegetation 11
Figure 3-5. Knitting Mill Creek's shoreline is slated to be armored with a living shoreline; the
concrete bulkhead is visible in the background 12
Figure 3-6. Directly connected residential roof drains appear to have been disconnected or are
otherwise clogged 13
Figure 3-7. Residential streets in Colonial Place are often characterized by a wide grassed verge between
the road curb and sidewalk 14
Figure 4-1. Bioretention located at the Knitting Mill Creek Community Garden 15
Figure 4-2. Example of bioretention integrated within a right of way vegetated fringe in Toledo, OH 16
Figure 4-3. Example permeable paver side-street parking in Garden City, ID 17
Figure 4-4. Watershed area treated by the selected green infrastructure retrofit projects 21
Figure 5-1. Before and after images of Colley Avenue green street design 24
Figure 5-2. Drainage areas for proposed Colley Avenue/35th Street retrofits 26
Figure 5-3. Before and after representations of Mayflower Road green infrastructure design 30
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Tables
Table 4-1. HRPDC design modifications for coastal plain bioretention practices 19
Table 4-2. HRPDC design modifications for coastal plain permeable pavement 19
Table 5-1. Drainage area and runoff volumes for Colley Avenue/35th Street green street retrofits 27
Table 5-2. Design parameters for 35th Street bioretention curb extensions 27
Table 5-3. Design assumptions for permeable concrete paver reservoir depth calculation 28
Table 5-4. Design parameters for permeable concrete parking lanes 28
Table 5-5. Drainage area and design parameters for Mayflower Road bioswales 31
Table 5-6 Suggested Bioswale plant species 31
Table 5-7. Summary of planning-level implementation costs 32
Table D-l. Cost estimate for 35th Street bioretention D-l
Table D-2. Cost estimate for Colley Avenue green street D-2
Table D-3. Mayflower Road shoreline project D-3
¥1
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I Executive Summary
Green infrastructure design is an adaptable and multi-functional approach to stormwater management
that includes an evolving list of practices which can be integrated into any community across the United
States. Norfolk, Virginia, - a highly-developed coastal Chesapeake Bay community with limited
opportunity to treat stormwater runoff - faces challenges to both mitigating stormwater pollution from
existing urban areas, as well as long-term geologic subsidence and future sea level rise due to its
geographic location. In an effort to address these challenges, a green infrastructure plan was developed
for Norfolk's Knitting Mill Creek watershed. The plan incorporates green infrastructure practices into
two locations: (1) a green street retrofit of three blocks at the intersection of a residential and
commercial corridor and (2) a planned shoreline stabilization project with considerations for sea-level
rise. Specific practices included roadside bioretention cells, permeable parking stalls/walkways, and an
extensive bioswale system along the Knitting Mill Creek shoreline. Although the concept designs include
practices that are typically adopted throughout the Bay region, this project highlights the adaptability of
green infrastructure to a community's changing environmental conditions and provides a template for
how these practices can be implemented throughout urban areas of the Chesapeake Bay shoreline.
The green street design is proposed for a single long residential block along 35th Street and two
commercial blocks along Colley Avenue. Runoff from these areas currently collects in roadside gutters
and is routed to a set of curb inlets at the western extent of 35th Street which are connected to the
subsurface drainage network. As part of the green street retrofits, seven roadside bioretention planter
boxes are proposed for the 35th Street block. Planter box areas for 35th Street will treat between 25%
and 30% of a 1" rainfall event from their respective drainage areas. The Colley Avenue green street
design includes five roadside planter boxes and four permeable concrete paver parking areas. Planter
areas for Colley Avenue will treat between 69% and 85% of the 1" rainfall event. The concrete paver
parking spaces will treat 100% of the 1" rainfall event from their respective drainage areas. The
proposed green street retrofit for these two street areas will be hydraulically connected via underdrains
and surface overflows, providing a "treatment-train" system to any area where centralized stormwater
management is unfeasible due to space limitations.
The shoreline stabilization project is located along Mayflower Road and borders Knitting Mill Creek. The
concept design presents an approach for improving the long-term function of stormwater infrastructure
within shoreline environments that are subject to tidal influences and a rising water table. In this case, a
planned succession of an infiltration-based practice (e.g., bioswale) into a best management practice
(BMP) type that requires a shallow water table (e.g., wetlands and wet ponds) was proposed. This
transition can simply occur through manual vegetation replacement, or via a natural succession of plant
species overtime as subsurface conditions change.
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2 Introduction
The City of Norfolk is located in the heart of the Hampton Roads metropolitan area, at the mouth of the
Chesapeake Bay in southeast Virginia. Norfolk is home to the world's largest naval base, Naval Station
Norfolk, along with the North American Headquarters for the North Atlantic Treaty Organization
(NATO). The City is bordered on three sides by water: the Bay to the north and the Elizabeth River to the
west and south. In addition, the Lafayette River flows through the City and separates the industrial and
downtown southern portions of Norfolk from the Naval Station to the north. Overall, Norfolk contains
144 miles of freshwater and marine shoreline, including seven miles along the Bay. Much of this
shoreline is located in residential neighborhoods.
Clearly, water is an important feature of Norfolk and to its estimated 246,139 residents (U.S. Census
Bureau 2013) on a number of levels; protecting water resources is a critical task. To that end, the City
adopted a general plan to guide decision making regarding physical development and public
infrastructure (plaNorfolk2030) on March 26, 2014. One section of the plan focuses solely on promoting
environmental sustainability and encourages "a sustainable environment that is not simply protected,
but enhanced." The first key issue in this section is to ensure high quality natural resources that will
enhance water quality in the City's waterways and reservoirs, including the Chesapeake Bay and its
tributaries.
One such tributary, and the focus of this report, is Knitting Mill Creek, a small tidal creek within the City
and a tributary to the Lafayette River. (See Figure 2-1.) Land use within the Knitting Mill Creek
watershed is dominated by residential neighborhoods, with a commercial corridor along Colley Avenue
that bisects the watershed along its north/south axis. Knitting Mill Creek is typical of the many creeks
throughout Norfolk in that it is surrounded by a highly developed watershed of mostly residential land
uses and it has a history of endemic water quality issues.
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Legend
] Knitting Mill Creek Watershed
j Norfolk City Limits
Knitting Mill Creek
TETRA TECH
Source: Tetra Tech, Inc.
Figure 2-1. Knitting Mill Creek watershed and the surrounding waters
2.1 Water Quality Issues
Like many urban communities located along the Chesapeake Bay shoreline, Norfolk is impacted by water
quality contaminants endemic to the Bay watershed. The City also has numerous water quality issues in
its many creeks and rivers which originate within or flow through its own borders. For example, due to
high levels of fecal coliform, shellfish areas within the entire Lafayette River and its tributaries have been
condemned since the 1930's by the Virginia Department of Health (VDH), prohibiting the harvest of
shellfish from the area for any purpose (VDH 2014). The Virginia Department of Environmental Quality
(VA DEQ) has also listed certain segments of the river as impaired or not meeting the current surface
water quality standards of the Commonwealth (VA DEQ 2014). VA DEQ's Final 2012 305(b)/303(d)
Integrated Report included the following impairments in the Lafayette River:
• Aquatic life and open water aquatic life uses due to dissolved oxygen;
• Primary contact recreational use due to enterococcus;
• Shellfish condemnation due to fecal coliform bacteria; and
• Fish consumption due to PCBs in fish tissue.
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Existing efforts toward improved water quality, however, are making a difference. A recent draft report
from the DEQ recommends a partial delisting from the 305(b)/303(d) list, but VDH is awaiting PCB
analysis before making a determination and possibly modifying the oyster moratorium.
The mixture of land uses within the Knitting Mill Creek Watershed creates a patchwork of impervious
surfaces that limits natural infiltration of stormwater. (See Figure 2-2.) A large portion of stormwater
runoff from the watershed is collected in a storm drainage system and discharged into the creek without
treatment. Due to the creek's bathymetry and configuration relative to the river, contaminants are often
retained in the creek. As a result, it is a source of algal blooms that eventually spread downstream into
the Lafayette River. High levels of the bacteria Escherichia coli (E. coli) have also been observed
throughout the creek, particularly at its head waters.
In addition, like most creeks within Norfolk, the Knitting Mill shoreline has been structurally modified
throughout the City's history and does not represent original or natural conditions. Much of the
shoreline has been armored with bulkheads and other hard engineering, a significant portion of which is
deteriorating.
Finally, the City faces a relatively unique challenge in addressing water quality issues along shoreline
neighborhoods such as Knitting Mill Creek; Norfolk is subject to both rising sea level and land
subsidence. Scientists from the Virginia Institute of Marine Science (VIMS) warn that relative sea levels
(the combination of rising water and sinking land) could rise by 1.5 feet in the next 20 to 50 years (VIMS
2013). Flooding has become an increasingly common occurrence for residents, businesses, and the naval
base. According to FEMA, the City has a total of 12,360 flood insurance policies (FEMA 2012), the second
highest in the state of Virginia.
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Legend
Stormwater Pipes
Knitting Mill Creek Watershed
Knitting Mill Creek
NAD_1983_StalePlane_Virginia_South_RPS_4502_F
Map Produced 05-07-2015 -A Porteous
Source: Tetra Tech, Inc.
Figure 2-2. Knitting Mill Creek watershed displaying the stormwater drainage system
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The City of Norfolk has identified green infrastructure approaches to stormwater management as a way
to address water quality in highly impervious areas such as the Knitting Mill Creek watershed. To
prevent costly damage and a relocation of its residents, the City simultaneously seeks to become more
resilient to the impacts of sea level rise and support wise choices in redevelopment efforts. As a result,
any green infrastructure retrofits must consider both the short-term conditions (i.e., possible flooding, a
high water table) and potential long-term changes in the hydrology (i.e., changes in sea level). To
explore the potential for green infrastructure to support city resiliency goals and serve as a template for
other waterfront neighborhoods, a green infrastructure plan for Knitting Mill Creek was developed.
This green infrastructure plan identifies, evaluates, and describes conceptual designs for green
infrastructure retrofits that will protect and improve the water quality of Knitting Mill Creek. EPA and
the community team selected green infrastructure techniques based on pollutant removal effectiveness,
as well as the practices' ability to function effectively over time in an area with a high water table that is
subject to impacts from sea level rise. Conceptual designs for selected retrofit opportunities have been
developed to support the City's efforts to pursue further funding and approval for project
implementation. EPA and the community team also educated residents of the Knitting Mill Creek
watershed about how these green infrastructure practices can help mitigate water quality issues
endemic to the project area and may be designed so that they are resilient to projected sea level rise.
This plan establishes the foundation for a number of green infrastructure retrofit projects that can be
further developed by the City and its partners to improve water quality in Knitting Mill Creek and to
achieve water quality and community goals. This project also supports Principle Two of the City's Central
Hampton Boulevard Plan (2010): "Create safe, walkable and distinctive public realm." The projects
described in this green infrastructure plan are also expected to attract business and residential
development within the Knitting Mill Creek watershed through benefits which are ancillary to water
quality improvements such as improved walkability, improved aesthetics, and pedestrian safety. In other
words, restoring the Knitting Mill Creek shoreline and creating a healthy waterway will also improve
neighborhood livability. The practices described in this plan are also intended to help make the City
more resilient to the long-term challenges it faces with sea level rise and land subsidence.
The biggest challenge is uncertainty regarding the extent to which sea level rise and land subsidence will
impact the City in coming years. Norfolk has one of the highest frequencies of nuisance flooding in the
country (NOAA 2014), which has forced the City to establish a vast array of both simple and complex
solutions. The City has also undergone over a century of redevelopment, filling, and potential
contamination of soils, which reduces the effectiveness of some practices within these disturbed areas
by increasing subsurface compaction and creating unpredictable heterogeneity of soil profiles. Lastly,
Knitting Mill Creek is a federally protected navigation channel, which limits the use of living shoreline
applications as an alternative to the hardened shoreline that currently exists.
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3 Green Infrastructure Opportunity Analysis
In order to begin the green infrastructure project selection and siting process, a desktop screening
evaluation of the watershed using GIS data resources was conducted to identify potential green
infrastructure retrofit opportunities. The evaluation identified potential retrofit locations (see Figure
3-1) with a specific focus on sites where the absence of buildings, utilities and other infrastructure,
coupled with the presence of adjacent runoff-generating impervious surfaces, offered opportunities to
capture stormwater using green infrastructure practices. The screening identified 27 locations for
potential retrofits, including open space/parkland, residential streets, commercial streets, and storm
sewer outfall locations.
On June 26, 2014, the project team conducted an extensive site evaluation of the watershed
accompanied by several local stakeholders. The purpose of the visit was to gain insight on the unique
features of the watershed, identify green infrastructure practice types suitable for consideration, and
visually assess the feasibility of the 27 previously identified green infrastructure retrofit sites. Three
additional sites not previously identified during the desktop screening were identified in the field and
also assessed. A summary table of site observations and recommendations for these 30 sites is provided
in Appendix A.
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Legend
• Potential Retrofit Sites
Stormwater Pipes
Knitting Mill Creek Watershed
Potential Retrofit Sites
NAD_19a3_StatePlane_Virgima_Soulh_FIPS_4502_Fee1
Map Produced 05-0^-2015-A Porteous
Source: Tetra Tech, Inc.
Figure 3-1. Potential green infrastructure retrofit locations identified by desktop screening
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3.1 General Observations
The Knitting Mill Creek Watershed encompasses 340 acres in south-central Norfolk. The watershed is
generally bounded by Old Dominion University to the west, 27th Street and the Norfolk Southern rail line
to the south, Newport Avenue to the east, and the Lafayette River to the north. The dominant features
of the watershed are the Knitting Mill Creek branch of the Lafayette River and the adjacent Colley
Avenue commercial corridor (see Figure 2-2).
Given its location in the Atlantic Coastal Plain, the topography across the project area is quite flat with
surface elevations ranging from 13 feet along the watershed headwaters to approximately 1 foot along
the shoreline (mean sea level within the watershed is 1.3 ft.). Drainage of stormwater runoff from the
watershed is primarily provided via network of storm sewers discharging directly into Knitting Mill
Creek. One main system serves the majority of the watershed and discharges into the southern end of
Knitting Mill Creek (Figure 3-2), while multiple smaller systems serve the areas adjacent to the creek
primarily along the eastern shoreline. Along the western shoreline, where there are few drainage
systems, runoff flows directly into the creek via surface flow.
Photo credit: Tetra Tech, Inc.
Figure 3-2. The majority of stormwater enters the southern end of Knitting
intersection of Mayflower Road and 42nd Street
ill Creek near the
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Historically, no green infrastructure practices (to reduce flooding or improve water quality) were
incorporated into the Knitting Mill Creek stormwater drainage system. In recent years, however, the City
and local citizen groups have implemented a number of pilot stormwater retrofit projects within the
watershed. These have included a shoreline rain garden at 47th Street (see Figure 3-3), an urban
stormwater wetland at 46th Street, and a stormwater rain garden at the Knitting Mill Creek Community
Garden at Georgia Avenue and Mayflower Road. These pilot projects, although small in scale, have
raised awareness with local residents that green infrastructure solutions can address water quality and
quantity concerns and be incorporated into urban settings in a way that enhances community
aesthetics.
Photo credit: Tetra Tech, Inc.
Figure 3-3. The rain garden at 47th Street enhances the community aesthetic
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3.2 Colley Avenue Commercial Corridor
Colley Avenue runs parallel to Hampton Road and helps form the major north/south vehicular corridor
that connects the Norfolk Naval Station with the southwest portions of the City. It is flanked along both
sides by primarily light commercial uses and residential areas. The commercial businesses and
restaurants are frequent destinations for the area residents throughout the watershed, as well as Old
Dominion University students, faculty, and staff who walk or drive from the nearby campus. The Colley
Avenue right-of-way (ROW) is approximately 60 feet wide and includes two travel lanes (one in each
direction), double lanes or turn lanes at some intersections, side-street parking, and paved sidewalks.
Vegetation along Colley Avenue is highly variable with sections exhibiting mature trees located in well-
managed grass verges and other areas devoid of vegetation and a fully paved ROW (see Figure 3-4 for an
example of the latter). This scenario, when coupled with the over-wide roadway cross section, provides
an ideal configuration for incorporation of green street elements.
Photo credit: Tetra Tech, Inc.
Figure 3-4. Colley Avenue exhibits extensive paving and limited vegetation
A unique feature of Colley Avenue, relative to other streets within the watershed, is the relative lack of
subsurface storm drainage infrastructure to collect and convey excess runoff. Based on observed site
conditions, excessive runoff from the roadway and adjacent parcels sheet flows into the roadside gutter
system and then is conveyed along surface grade into one of the east/west oriented cross streets before
surface discharging to Knitting Mill Creek.
II
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3.3 Mayflower Road and Shoreline
Mayflower Road is a two-lane residential road that parallels the entire eastern shoreline of Knitting Mill
Creek. The eastern shoreline runs immediately adjacent to Mayflower Road, in contrast to the western
shoreline that adjoins many private residences and businesses and is separated from public
infrastructure. Along the northern section of Mayflower Road, the shoreline is well protected by a solid
concrete bulkhead of recent construction. Along the southern section, the shoreline exhibits ad-hoc
armoring consisting of concrete debris, an aging/failing brick wall, and what appears to be irregularly
placed concrete washouts. The southern section of the shoreline is slated to be armored in the coming
year with a federally funded shoreline armoring/living shoreline project. The living shoreline component
will consist of a shallow offshore stone breakwater, creating a near-shore shallow area for establishment
of native shoreline vegetation (see Figure 3-5).
Photo credit: Tetra Tech, Inc.
Figure 3-5. Knitting Mill Creek's shoreline is slated to be armored with a living shoreline; the
concrete bulkhead is visible in the background
Vegetation along the shoreline is variable. Several large live oaks are located near the southern terminus
of the shoreline, along with intermittent shrubbery. Throughout the length of Mayflower Road, the open
area between the roadway and the shoreline is covered in poorly established grass. There are several
areas devoid of ground cover, likely due to either foot traffic or soil characteristics insufficient for plant
growth. One specific characteristic of this area (noted by local stakeholders) is the regular encroachment
by the creek's surface waters during "Nor'easter" storm events. In these storms, wind-driven tides push
water along the shore due to its north/south orientation and long fetch north into the Lafayette River.
These conditions, though infrequent, create unique challenges for managing stormwater in this area,
and will influence the selection of green infrastructure techniques and plant species.
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3.4 Residential Neighborhoods
Houses within the residential area date to the early 20th century and consist of single family homes on %-
to ]4-acre lots. Typical of this time period and configuration of construction, there are widespread
indicators that residential rooftops historically were directly connected to the street drainage network
(see Figure 3-6). However, field observations revealed that most if not all of these rooftop connections
had either been disconnected or have clogged with soil and debris, rendering them non-functional.
Disconnection of rooftops and other isolated impervious areas is often considered a low-cost, high-
return green infrastructure retrofit, as long as the roof runoff can be directed across a well-vegetated
area and not endanger structural foundations. Although there were no observed candidates for rooftop
disconnection or other widespread green infrastructure practices appropriate for individual lots, local
residents should be encouraged to participate in the River Star Homes program, a partnership between
the Elizabeth River Project and City of Norfolk to encourage residential practices which improve and
protect water quality.
Photo credit: Tetra Tech, Inc.
Figure 3-6. Directly connected residential roof drains appear to have been disconnected or are
otherwise clogged
The street network throughout the watershed (including residential areas) is laid out in a grid pattern.
Within the street rights of way, residential streets are characterized by two-way vehicular travel, side-
street parking, a vegetated fringe, and sidewalks on both sides of the road. (See Figure 3-7.) While
mature trees or ornamental shrubbery are located within the fringe in most areas, portions of many
streets are devoid of any vegetation within the fringe areas except grass or other low-growing
ornamentals. These characteristics make the residential streets throughout the Knitting Mill Creek
watershed candidates for incorporating green street concepts such as permeable pavement or side
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street bioretention elements, which are both described in the Stormwater Toolbox (Section 4 below).
Additional opportunities for green infrastructure implementation occur at the small outfalls and areas
where stormwater sheet flows directly into Knitting Mill Creek.
Photo credit: Tetra Tech, Inc.
Figure 3-7. Residential streets in Colonial Place are often characterized by a wide grassed verge between
the road curb and sidewalk
3.5 Soil Conditions
Soil classifications within the project area vary between Tomotley-Urban land complex (0 to 2% slopes)
and Urban Land based on the USDA Soil Survey (SSURGO). These classifications are considered poorly
drained with Hydrologic Soil Group (HSG) values of "D.'^Tomotley soils have a dark gray, fine sandy
loam surface layer. Subsurface layers include a gray fine sandy loam (4-15 inches), and a gray sandy clay
loam (15-58 inches), as reported in the most recent soil survey (NRCS 2009). Local stakeholders
indicated that observed surface permeability was somewhat variable, possibly due to extensive dredging
and fill operations during development of the waterfront. It is believed that much of the shoreline along
Knitting Mill Creek resulted from filling of the historic shallow shoreline with material left over from
development activities on upland areas.
1 For guidance on designing Gl practices within heavy clay soils, refer to the EPA document "Soil Constraints and Low Impact
Development - Careful Planning Helps LID work in Clay Soils" {httc>://water.e[>a.aov/[>olwaste/areen/u[>load/bbfs8clav.[>df)
14
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4 Design Approach
4.1 Stormwater Toolbox
Green infrastructure practices and approaches were pioneered in the mid-Atlantic region beginning in
the 1990s as a strategy for restoring water quality within the Chesapeake Bay's highly developed
watershed. As result, the City of Norfolk has a well-developed toolbox of stormwater best management
practices that have been implemented within similar climate, soil, and site conditions, either as retrofit
situations or during new construction. Since green street retrofits were identified as part of the Knitting
Mill Creek project, this section focuses on urban bioretention practices and permeable pavement.
4.1.1 Bioretention
Bioretention is a practice that employs a depressed vegetated area underlain by a shallow layer of soil
media suitable for plant growth and 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. Bioretention is
already in use in the Knitting Mill Creek watershed; see Figure 4-1.
Photo credit: Tetra Tech, Inc.
Figure 4-1. Bioretention located at the Knitting
Creek Community Garden
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While bioretention has seen widespread application in parking medians and larger public open spaces,
the practice is also being increasingly adapted to urban rights-of-way as a component of green street
design (Figure 4-2). Regardless of where in the urban environment bioretention is implemented, the
selection of vegetation type and density must consider site-specific conditions. For example, in areas of
heavy pedestrian use, the potential for occasional foot traffic may warrant selection of vegetation that
serves as a barrier or is hardy to compaction. Likewise, areas near brackish waters may require plants
suitable for such conditions.
Photo credit: Tetra Tech, Inc.
Figure 4-2. Example of bioretention integrated within a right of way vegetated fringe in Toledo, OH
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4.1.2 Permeable Pavement
Permeable pavement refers to pavement systems that incorporate a permeable paving surface (typically
paver block, pervious asphalt, pervious concrete) that allows rainfall to infiltrate into the pavement
system, then into an open graded storage/structural layer beneath the pavement, and ultimately into
the subgrade. Because of the structural requirements (i.e., being subject to vehicular loading),
permeable pavement must be designed to meet both hydrologic and structural criteria. In general,
permeable pavement systems are best located on low-traffic surfaces such as sidewalks, parking stalls,
and lightly used driveways. See Figure 4-3 for an example of permeable pavement. Selecting the most
appropriate permeable pavement surface type depends on aesthetics, structural loading and
implementation costs. While each surface type has similar hydrologic characteristics, the performance
of each type can vary depending on the storage/structural layer and underlying soils that support it.
Photo credit: Tetra Tech, Inc.
Figure 4-3. Example permeable paver side-street parking in Garden City, ID
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Design guidance for the proposed green infrastructure retrofits in the Knitting Mill Creek watershed was
primarily based on guidance from the VA DEQ Stormwater Design Specifications retrieved from the
Virginia Stormwater BMP Clearinghouse. For stormwater curb extensions, Appendix 9-A (Urban
Bioretention) of the VA DEQ Design Specification No. 9 was used for conceptual design and sizing. VA
DEQ Design Specification No. 7 was referenced for permeable pavement sizing. Note that the
conceptual-level permeable pavement sizing was only performed relative to hydraulic requirements. A
structural analysis for anticipated traffic loads will eventually need to be conducted as part of a more
detailed design.
There are a number of Norfolk-specific guidelines and design standards that may be applicable
elsewhere. Chapter 6 of the City's Stormwater Design and Construction Manual (City of Norfolk 2014)
addresses projects implemented to "improve the quality of runoff leaving an existing developed site or
from upstream developed areas and not designed to treat stormwater runoff from new land
development or redevelopment." The manual encourages designers to use the design standards of the
Virginia Stormwater BMP Clearinghouse web site (Virginia OCR 2009) to the extent practicable.
The Norfolk Stormwater Design and Construction Manual specifies several modifications to the BMPs
described in the Virginia BMP Clearinghouse Manual. For bioretention (including urban bioretention2),
these modifications include the following:
a. Table 9.3
i. Subsoil Testing - Level 2 Design Criteria shall be utilized
ii. Underdrain - Level 2 Design shall incorporate an underdrain
b. Section 6.2 (pg. 18 of 54) - Notwithstanding the following sentence: "Soil testing is not needed
for Level 1 bioretention areas where an underdrain is used." All infiltration and bioretention
practices in the City of Norfolk must include site specific infiltration testing at the proposed
location of the practice.
c. Section 6.7 (pg. 25 of 54)- Notwithstanding the following sentence: "Some Level 2 designs will
not use an underdrain (where soil infiltration rates meet minimum standards; see Section 6.2
and Section 6.3 design tables)." all bioretention and infiltration practices proposed in the City of
Norfolk must include an appropriately sized underdrain.
Construction of water quality retrofits is also subject to other standards regarding land-disturbing
activities in the City of Norfolk such as those in the City of Norfolk Design and Specification Manual (City
of Norfolk 2014) and Section 1 of the Hampton Roads Planning District Commission (HRPDC) report Land
and Water Quality in Hampton Roads, Phase II (HRPDC 2013). The HRPDC report provides design
modifications for, and exemptions from, the VA DEQ design specifications that apply to the City of
Norfolk for stormwater management practices implemented within Virginia's Coastal Plain. Table 4-1
and Table 4-2 show the Coastal Plain design modifications that apply to bioretention and permeable
pavement, respectively.
- The City of Norfolk does not specify design modifications for permeable pavement.
-------
Table 4-1. HRPDC design modifications for coastal plain bioretention practices
Design Limitation
Coastal Plain Modifications
Maintain a minimum underdrain slope of .5% and
tie into a ditch or conveyance system.
Utilize linear approach of multiple storage cells to
conserve hydraulic head.
Minimum depth of filter bed is 18 inches for Level 1
and 24 inches for Level 2.
Underdrains should be connected to the
stormwater drainage system.
Obtain media from an approved vendor to ensure
nutrient content of the soil and compost is within
acceptable limits.
Depth to groundwater can be reduced to 1 foot if a
large diameter (6 inches) underdrain is utilized.
Avoid using on-site soils in the coastal plain, unless
soil tests show low nutrient concentrations.
Limit surface ponding to 6 to 9 inches.
Select plant species that reflect coastal plain plant
communities and are wet-footed and salt-tolerant.
Designers can utilize a turf cover rather than mulch
for shallower facilities, but they should follow the
design specifications and pollutant removal values
for dry swales.
Table 4-2. HRPDC design modifications for coastal plain permeable pavement
Design Limitation
Coastal Plain Modifications
Vertical separation from bottom of system to water Avoid using permeable pavement if the site is near
table = 2 feet. sandy soils to minimize clogging.
Maintain a minimum slope of 0.5% for underdrains
to ensure proper drainage.
The design and sizing objective for all green infrastructure practices was to capture and treat the
calculated water quality volume for a 1" rainfall event using the Runoff Reduction Method which is
applicable statewide. The central component of the Runoff Reduction Method is treatment volume (Tv),
which is calculated by multiplying Virginia's "water quality" rainfall depth (P = 1") by the three site cover
runoff coefficients (forest, disturbed soils, and impervious cover) present within each site's drainage
area.
Note that with most urban green infrastructure retrofit projects, existing site constraints yield BMPs that
often are undersized based on local water quality treatment volume targets. However, even undersized
linear treatment systems can provide cost-effective solutions for mitigating runoff volumes and
pollutant loads.
19
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The restoration of waterways and protection of water quality frequently rely heavily on cooperation
among various stakeholders in a community, and these partnerships are strong in the Knitting Mill
watershed. For example, in Norfolk, more than 33 wetland restoration projects throughout the City have
been completed with assistance from various community partners such as the Lafayette Wetlands
Partnership and the Elizabeth River Project. In 2011, the Elizabeth River Project partnered with the
Chesapeake Bay Foundation and created The Plan for Restoring the Lafayette River: Strategies for
Community Wide Action. This plan sets out three goals: reduce harmful bacteria to levels that are safe
for swimming; achieve healthy, plentiful wildlife in the river; and encourage river stewardship in the
community through public awareness and action.
In previous projects throughout the Knitting Mill Creek watershed, public participation has likewise been
extensive. Citizens in the Colonial Place and Highland Park neighborhoods have participated in water
quality monitoring programs, conducted regular shoreline and inland clean-ups, volunteered to do
"Adopt-a-Spot" monitoring of selected shoreline locations, supported City-operated street sweeping
programs with education and encouragement for residents, and participated in urban wetland
restoration projects along the creek. This project aims to include appropriate approaches suggested by
the watershed's residents and seek the support of the public concerning the final design of these
practices.
On June 26, 2014, the project team took advantage of stakeholder resources in the watershed and
conducted a green infrastructure workshop and public meeting at the Ernie Morgan Center in Norfolk.
The purpose of the workshop and public meeting was to inform stakeholders within the Knitting Mill
Creek watershed about the benefits of green infrastructure stormwater practices and obtain valuable
initial feedback on preferences for retrofit opportunities and configurations within the community.
Attendees included City staff, representatives from various non-profit organizations with an interest in
protecting Knitting Mill Creek, and local neighborhood representatives of the Colonial Place, Highland
Park, Riverview, and Park Place neighborhoods. The project team presented various benefits that green
infrastructure could provide to the community with regard to water quality protection, resilience to sea
level rise, and quality of life. The project team presented potential green infrastructure retrofit locations
within the Knitting Mill Creek watershed and implications of sea level rise on green infrastructure
function and resiliency. Attendees provided valuable feedback, which was utilized during the selection
of projects and the development of conceptual designs.
Following the field investigations and public meeting, the project team discussed opportunities for green
infrastructure retrofits for the purpose of development of conceptual designs. Two locations within the
Knitting Mill Creek Watershed were selected for green infrastructure retrofits (Figure 4-4).
-------
Legend
Mayflower St. Project Drainage Area
| | 35th St. Project Drainage Area
Knitting Mill Creek Watershed
BMP Project Areas
NAD_1983_SlalePlane_Virgima.South_FIPS_4502_Feel
Map Produced 05-07-2015 -A. Porteous
Source: Tetra Tech, Inc.
Figure 4-4. Watershed area treated by the selected green infrastructure retrofit projects
21
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The project team proposes a green street concept design for the blocks north, south, and west of the
35th Street/Colley Avenue intersection. The BMP retrofits include permeable pavement within the
parking lanes on Colley Avenue and bioretention curb extensions sited throughout the project area
where allowed by roadway access and utility conflicts within the right of way. The selection of green
street practices was informed primarily by the project goals for the site, which include hydrologic and
water quality enhancement, improved aesthetics, and improved access for pedestrian and bicycle
transportation.
A second project area consists of the Mayflower Road shoreline from the southern extent of Knitting
Mill Creek north to the mouth of Knitting Mill Creek near the intersection of Mayflower Road and New
Hampshire Avenue. The southern portion of this concept is proposed to be implemented in conjunction
with the proposed shoreline restoration. This green infrastructure concept includes a stormwater
treatment system that can be installed between the existing edge of curb and the proposed shoreline
armoring/living shoreline. Due to the shallow water table, soil conditions, and projected sea level rise, a
modified wet swale/bioretention practice was configured to allow for functions and geometries that can
be adapted as sea levels rise over the next few decades. (Note that adapting stormwater management
practices and design standards to account for impacts from climate change over the next decades is a
new but growing practice, especially in coastal communities.) Furthermore, a permeable pavement
sidewalk is proposed to parallel the shoreline to alleviate pedestrian compaction, which was observed as
one cause of poor vegetative cover. The northern portion of the concept design will extend the
permeable sidewalk and associated shoreline buffer north of the shoreline armoring/living shoreline
project to the mouth of Knitting Mill Creek. In addition to improved water quality, the selection of the
Mayflower Road project also strives to improve an important and highly visible public amenity and serve
as a demonstration for how green infrastructure can be designed to be resilient to sea level rise.
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5 Conceptual Design
Conceptual designs were developed for the two selected green infrastructure retrofit locations using
appropriate local or regional design methodologies adapted to local site conditions.
Existing ROW widths and roadway configurations strongly influence the specific design geometries for
green street retrofit practices. The ROW width for the 35th Street block is approximately 64-65' with a
curb-to-curb width of 50'. Existing sidewalks are approximately 4-5' wide with a 2' landscape strip
adjacent to the curb. Currently, both sides of the road are used for parallel parking, leaving two
excessively wide travel lanes.
A 7-foot wide bioretention cell is proposed for each curb extension on the 35th Street block. The curb
extensions, which are designed to help calm traffic and ease transitions into the parking stalls and driveway
accesses, also include a 6"-wide roadside curb, a 6" concrete edge along the sidewalk, a 40 sq. ft. cobble
energy dissipation pad on the up-gradient end, a raised landscape strip on the down-gradient end, and two
curb cuts (one inlet and one overflow outlet). The bioretention component contains a 3" top layer of
triple-shredded hardwood mulch, 2 feet of soil media, a 2" choker layer of ASTM C33 washed sand, and
a 12" drainage layer of washed #57 stone surrounding a perforated PVC underdrain. The curb extensions
vary in length from 26' to 46' (measured from the end of each curb transition location).
Since part of the bioretention cell will be constructed within the existing landscape strip to minimize
impacts to parking and travel lanes, the green street retrofit for 35th Street would yield a minimum curb-
to-curb width of 40'. As a result, future roadway modifications could accommodate dual bicycle lanes
(each 5'-wide) and either (2) 10'-wide travel lanes with a center turn lane, or two 15'+ travel lanes. Of
the estimated 32 available parking spaces currently located along 35th Street block, implementation of
the proposed curb extensions would reduce the existing roadside parking by approximately 14 spaces.
The existing ROW width along the Colley Avenue corridor varies, but is typically 58' wide between 34th and
36th Streets. For the block north of 35th Street, curb-to-curb width is typically 36' while the block south of
the 35th Street is 40' wide. Other variations along the corridor include the extent and width of landscape
strips, building frontage setbacks, and sidewalk widths. Based on the Commercial Street template from the
Downtown Norfolk Street Plan (City of Norfolk 2009), lO'-wide travel lanes and 8'-wide parking stalls are
appropriate dimensions for both sides of Colley Avenue. These general dimensions are reasonable for the
north Colley Avenue block and allow for two 12'-wide travel lanes along the south block.
The proposed Colley Avenue green street design includes four separate permeable interlocking concrete
paver parking lanes installed along the north side of each block wherever there are no access driveways.
Interlocking concrete pavers were selected as the preferred pavement surface during field
investigations. Each 8'- wide concrete paver section will be approximately 100' long and contain both a
washed stone bedding and a reservoir layer below the concrete paver course. The concrete paver
system will also contain an underdrain within the reservoir layer and a raised concrete curb edge on the
sidewalk to prevent non-roadway runoff from draining onto the parking lane. Several bioretention curb
extensions were also sited around the roadway entrances along the southern half of each Colley Avenue
block. The curb extensions are similar in geometry and configuration to the ones proposed on 35th
Street, except the bioretention cell widths are increased up to 9.5' to maximize footprint area due to the
wider sidewalk and landscape strip dimension on Colley Avenue. Before and after representations of
Colley Avenue are shown in Figure 5-1 and a detailed concept design is provided in Appendix B.
23
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Source: Tetra Tech, Inc.
Figure 5-1. Before and after images of Colley Avenue green street design
24
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Figure 5-2 shows the drainage areas for the proposed green retrofits in the 35th Street/Colley Avenue
project area. For the 35th Street block, drainage areas were delineated for the entire block using the
roadway centerline for subdivision. The bioretention cells, which were maximized based on available
area, will treat runoff from each drainage area by allowing untreated overflow bypass from the highest
curb extension to drain to the next down-gradient cell, and so on. An underdrain will connect all the
curb extensions on each side of the road before connecting to the existing catch basins at Morton
Avenue. Drainage areas for the Colley Avenue corridor were delineated for each separate BMP practice.
Raised curb edges along the permeable concrete paver parking lanes will limit the drainage area ratios
to 2:1 or below, per VA BMP specifications.
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Stormwater Structures
Stormwater Pipes
"Wffify Bioretention Areas
[/^//] Permeable Pavement
| | Project Site Drainage Area
Green Street Project
NAD_1983_StatePlane_Virgima_Soulh_FIPS_4502_Feet
Map Produced 12-12-2014 -A Porteous
NO 40 80
160
•
0 12.5 25
50
• Meters
TETRATECH
Source: Tetra Tech, Inc.
Figure 5-2. Drainage areas for proposed Colley Avenue/35th Street retrofits
26
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Table 5-1 provides the drainage area properties and the water quality treatment volume (Tv) calculated
for each catchment.
Table 5-1. Drainage area and runoff volumes for Colley Avenue/35th Street green street retrofits
Attribute
DA (ac)
Imperv. (%)
Tv(cf)
BR #1-4 BR# 5-7 BR# 8-9
1.25
85
3843
0.99
80
2900
0.19
100
650
BR#10
0.15
100
509
BR#11
0.16
100
557
BR#12
0.14
98
468
PP#1
0.07
100
231
PP#2
0.05
100
176
PP#3
0.05
100
156
PP#4
0.05
100
179
a. Estimate from aerial photography
b. Treatment volume calculated using Runoff Reduction Method
Table 5-2 displays the design parameters for the proposed bioretention curb extensions on 35th Street.
The total storage depth includes both the 6" surface ponding and the instantaneous void storage within
the soil media underdrain layer. BMP footprint areas represent the cumulative surface areas of all the
curb extensions within each drainage area. As shown, the undersized retrofits provide a range of
treatable volumes between 25% and 85% of the calculated 1" water quality volume from their
respective drainage areas.
Table 5-2. Design parameters for 35th Street bioretention curb extensions
Parameter
Total storage depth1 (ft)
Required footprint (s.f.)
Design footprint (s.f.)
Percent of Tv treated (%)
BR#l-4 BR#5-7 BR#8-9 BR# 10 BR #11 BR #12
1.4
2745
700
25
1.4
2071
630
30
1.4
464
320
69
1.4
363
284
78
1.4
557
284
71
1.4
466
284
85
1 Includes surface ponding and storage in media voids; Vr-soil = 0.25; Vrgravel = 0.4
Given the adherence to the permeable pavement's drainage area/surface area maximum design ratio,
the permeable concrete paver parking lanes were adequately sized to capture their targeted treatment
volume. As previously mentioned, the concrete paver system was only designed based on hydraulic
loading. Although advanced design will need to account for traffic loading per the guidance provided in
VA DEQ Design Specification No. 7', the gravel reservoir depths proposed for the Colley Avenue design
will certainly help provide additional structural integrity along this commercial corridor.
Hydraulic design is performed to ensure adequate storage of the water quality treatment volume within
the reservoir layer. The depth of the reservoir layer (dp) is calculated using the following equation, which
assumes outflow through the underdrain (and not through the underlying soil):
{(dc x fi) + p - (5- x tr) - (qu x tr)}
vr
The assumed input parameters for the permeable concrete paver design are shown in Table 5-3.
27
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Table 5-3. Design assumptions for permeable concrete paver reservoir depth calculation
Parameter Design Value Unit
Depth of runoff (dc)
Infiltration rate (i)
Void ratio (Vr)
Fill time (Tf)
Drain time (Td)
Reservoir hydr. conductivity (k)
Underdrain slope (m)
Underdrain flow (qu)
1.0
1
0.4
0.083
1.5
100
0.005
0.5
inch
ft/day
n/a
day
day
ft/day
ft/ft
ft/day
The maximum allowable depth of the reservoir layer (dp-max) is constrained by the maximum allowable
Drain time, and is calculated by the following:
dp-max
x
x
The final design parameters for the Colley Avenue permeable parking lanes are shown in Table 5-4.
Table 5-4. Design parameters for permeable concrete parking lanes
Parameter
Direct drainage area (DA) (sf)
Concrete paver footprint area
(PA) (sf)
DA/PA Ratio
Reservoir depth, dp (ft)
Max depth, dp-max(ft)
PP #1 PP #2
2127
790
1.7
2.6
3.8
PP#3
1458
761
0.9
2.5
3.8
PP#4
1218
750
0.6
2.4
3.8
1438
827
1.7
2.6
3.8
Through the incorporation of streetside bioretention and permeable pavement parking lanes within the
Colley Avenue and 35th Street ROW corridors, the City of Norfolk can reduce the volume of stormwater
discharged to Knitting Mill Creek from the project area by encouraging infiltration into subsurface soils.
Additional pollutant removal will be accomplished for that portion of runoff which exceeds the capacity
of underlying soils and is discharged through underdrains connected to the existing sewer system. Other
benefits of the green street conceptual design include a reduction of impervious surface, modest
reduction of peak flow rates, improved street aesthetics, and a more inviting pedestrian connection
between residential neighborhoods and commercial areas. The green street concept design detailed
above can also serve as a template for the integration of roadway green infrastructure practices both
within the Colley Avenue commercial corridor and elsewhere in the Knitting Mill Creek watershed.
28
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5.2
The Mayflower Road project area highlights today's challenges with implementing green infrastructure
within coastal plain and shoreline environments. The area comprising the 4200-4600 blocks of
Mayflower Road was designated for retrofit with a series of bioswales installed between the roadway
and the planned bulkhead/shoreline protection revetment on Knitting Mill Creek. In addition, a riparian
buffer and pervious walkway will be installed along the shoreline throughout the entire 2400 ft. eastern
shore of Knitting Mill Creek, including areas north of the section proposed for bioswale retrofit, which
are not suitable for incorporation of bioswales due to the presence of a recently constructed concrete
bulkhead and less suitable topographic conditions. Although the section of Mayflower Road suitable for
bioswale implementation extends for 1,250 feet, only 500 linear feet of bioswale is required to treat the
contributing roadway in accordance with the selected design criteria. As a result, final configuration of
the bioswales and the alignment of the pervious walkway can be adjusted to accommodate existing and
future landscape and infrastructure elements such as the live oak trees at the southern end of the
project. Curb cuts installed directly upslope of the existing catch basins will allow runoff from the
western half of Mayflower Road to enter the bioswale system, which will treat an 80% impervious
drainage area that is approximately 28,000 sq. ft. A small embankment with buffer planting will be
constructed between the bioswale and the shoreline to ensure that peak flood volumes overflow onto
Mayflower Road and discharge through the existing catch basins. Pedestrian access paths, sited at all
the existing culvert crossings, will hydraulically separate each bioswale and provide convenient locations
for the underdrains to connect into the drainage network.
Hydraulic restrictions due to shallow gradients between the roadway and the mean high water elevation
limit the depth of the bioswales and their media layer. As specified in the design guidance provided by
the HRPDC for bioretention systems installed in the Coastal Plain, the depth to ground water can be
reduced to 1 foot if a 6" diameter underdrain is utilized. Therefore, the minimum elevation of the
bioswale underdrain is approximately 2.3' Mean Sea Level (MSL) along the project reach. Existing catch
basin rim elevations along the west side of Mayflower Road vary between 4.4' and 5.9' MSL. Assuming a
6" ponding depth and an 8" gravel underdrain layer, the bioretention media depths will range from 1.0'
to 2.4', depending on adjacent curb elevation. Due to the shallow media depths, a sodded turf grass is
proposed for the internal side slopes to limit rooting depth and potential underdrain clogging. Several
varieties of turf grass perform well in bioretention cells (provided they adequately drain between storm
events), including bermuda varieties. Before and after representations of the Mayflower Road project
area are shown in Figure 5-3 and a detailed concept design is provided in Appendix C.
29
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Source: Tetra Tech, Inc.
Figure 5-3. Before and after representations of Mayflower Road green infrastructure design
Table 5-5 shows the conceptual design parameters for the bioswale system. The footprint area required
to treat the water quality runoff volume is based on a minimum media depth of 1.0', although actual
footprint areas could be decreased to account for additional storage volume provided in the deeper
bioswale sections. Note that the required bioswale area also accounts for ponding volume within the 3:1
side slopes.
30
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Table 5-5. Drainage area and design parameters for Mayflower Road bioswales
Attribute
Value
Drainage area (ac)
0.65
Imperv. (%)
80
Tv(cf)
1898
Req'd bioswale footprint area (s.f.)1 1502
Design width (ft)
3.0
Minimum design length (ft)
500
1 Includes 6" surface ponding and void storage in 1' of media (Vr = 0.25) and 8" gravel (Vr = 0.4)
Extra consideration was given to account for forecasted sea level rise. Under this future conditions
scenario, the bioswale system will eventually function as a seasonal, subsurface wetland since drainage
through the underdrain will be hindered during parts of the year. Although these anaerobic conditions
may provide additional nitrogen removal through de-nitrification processes, the raised water table may
continually decrease infiltration rates, limiting the initial plant selection to those unaffected by shallow
water tables (including shallow-rooted turf grass). Regardless, observation and feedback in plant
response will be necessary to manage the continuum in bioswale function and supported plant species
as conditions evolve (e.g., raised water table, higher salinity levels, more frequent surface ponding,
lower infiltration rates, etc.). In the case that deleterious conditions hinder the survival of turf grass, the
internal bioswale zones can be replanted (or even over-seeded) with an advanced succession of native
herbaceous species that are better adapted to the altered in-situ conditions.
Table 5-6 provides a brief list of suggested plant species for the bioswale that are native to eastern
Virginia. These species were selected for their medium (M) or high (H) tolerance to salinity, and their
adaptability to a wide range of moisture requirements (JDry, Moist, or Wet). The plant selections were
limited to grasses, lower-growing perennials, and shrubs due to the limited bioswale dimensions and
potential water table rise. In addition to changing environmental factors, the flexibility in plant species
and ground cover type within the bioswale provide options depending on the community's desired
aesthetics and maintenance needs.
Table 5-6 Suggested Bioswale plant
Species
Red fescue (Festuca rubra)
Big Bluestem (Andropogon girardii)
Switch grass (Panicum virgatum)
Indian grass (Sorghastrum nutans)
Talus slope penstemon
(Penstemon digitalis)
Inkberry holly (///ex glabra)
High-tide bark (A/a frutescens)
Highbush blueberry
(Vaccinium corymbosum)
species
Salinity
M
M
M
M
M
M
H
H
Moisture
D/M/W
D/M/W
D/M/W
D/M
D/M
M
M/D
D/M/W
Comments
Cool-season turf grass
Warm-season bunchgrass; H 36"-
Warm-season clump grass; H 36"-
Tall clump grass; H 30"- 72"
72"
60"
Clumping, drought-tolerant; H 24"- 48"
Tolerates occasional flooding; H 6'- 8'
Grows in brackish/salt marshes; H
Edible, shallow-rooted; H 6'- 12'
4'- 10'
31
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The Mayflower Road green infrastructure concept design incorporates an adapted bioswale in a setting
that falls outside of the standard design criteria (due to a shallow water table). By modifying the
bioswale geometries to both fit the existing condition and retain functionality in projected future
conditions, the concept design serves to ensure that this infrastructure improvement is resilient to
potential sea level rise. The use of a permeable pavement walkway3 serves to protect vegetative fringe
around the bioswale and along the shoreline, as well as provide an inviting recreational trail for
residents. The integration of both of these practices with a planned living shoreline project provides an
example of how green infrastructure practices can be implemented in communities subject to sea level
rise in a manner that is both resilient and adaptive.
5.3 Cost Estimate
Planning-level cost estimates were prepared for each the proposed Knitting Mill Creek green
infrastructure retrofits. Unit costs were developed using RSMeans Construction Cost Data (2014) specific
to Norfolk, supplemented with engineer's estimates (based on comparable bid summaries) where
available. A summary of implementation costs for the three green infrastructure projects is provided in
Table 5-7 and detailed planning-level costs for each scenario are provided in Appendix D.
Table 5-7. Summary of planning-level implementation costs
Scenario Implementation Costs
35th Street $92,215
Colley Avenue $182,255
Mayflower Road Shoreline $90,674
3 Note that the use of permeable pavement for the walkway surface is optional, and may be replaced with bituminous asphalt
or conventional concrete as a cost saving measure with limited impact on stormwater management function.
32
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6 Conclusion
Norfolk, like many communities along the Chesapeake Bay shoreline, faces a significant challenge in
addressing stormwater pollutant contributions to the Bay from existing urban areas. In the extensive
residential neighborhoods that border the City's shores, stormwater runoff often discharges directly into
the Bay system without treatment. The extensive development along the shoreline limits opportunities
to implement stormwater treatment systems at the outfall. Given its coastal setting and location in an
area of long-term geologic subsidence, the City is further challenged with high rates of future sea level
rise which could endanger existing and new infrastructure.
In recent years, green infrastructure practices have been identified as part of a suite of solutions that
can address long-term pollutant issues within the Chesapeake Bay. These practices, including those
proposed in the conceptual designs for Knitting Mill Creek, have been successfully implemented
throughout the Bay watershed. Design criteria for these practices, however, typically recommend deep
groundwater levels that may not exist in certain coastal settings. Furthermore, in areas where significant
sea level rise is projected, stormwater infrastructure along the shoreline may experience a shorter
functional lifespan or provide reduced level of service due to encroaching waters. One possible solution
to this challenge, as presented in the Mayflower Road concept design, is to account for the planned
succession of infiltration-based practices into BMP types that require shallow water tables (i.e., wetlands
and wet ponds). In most cases, this transition simply occurs through manual vegetation replacement or
via a natural succession of plant species over time as subsurface conditions change.
The green infrastructure concept designs developed for the Knitting Mill Creek watershed are typical of
practices being adopted throughout the Bay. The Colley Avenue/35th Street green street concept design
provides a template for implementing green infrastructure in the rights of way throughout Knitting Mill
Creek, as well as in other streets elsewhere in Norfolk and in other shoreline communities in the region.
The Mayflower Road concept design serves as an example of how standard green infrastructure practice
criteria, in this case for bioswales and permeable pavement, can be adapted to the shoreline
environment in a way that is resilient to future sea level rise.
33
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City of Norfolk Department of Public Works. 2014. Norfolk City Design Standards. NCDS2014.06
Federal Emergency Management Agency (FEMA). 2012. Flood Insurance Policies and Community Rating
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Hampton Roads Planning District Commission (HRPDC). 2013. Land and Water Quality in Hampton
Roads, Phase II. November 2013.
National Oceanic and Atmospheric Administration (NOAA). 2014. Sea Level Rise and Nuisance Flood
Frequency Changes around the United States. NOAA Technical Report NOS CO-OPS 073. June 2014.
http://tidesandcurrents.noaa.Qov/publications/NOAA Technical Report A/OS COOPS 073.pdf.
Natural Resources Conservation Service (NRCS). 2009. Soil Survey of Tidewater Cities Area, Virginia. USDA.
RSMeans. 2014. Version 5.1.1. Accessed December 23, 2014. http://rsmeansonline.com/.
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January 16, 2015. http://quickfacts.census.gov/qfd/states/51/51710.html.
United States Department of Agriculture (USDA). 2014. Web Soil Survey. Accessed December 18, 2014.
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Virginia Department of Conservation and Recreation (OCR). 2009. Various stormwater management
BMP specifications. Virginia Stormwater BMP Clearinghouse web site:
http://www.vwrrc.vt.edu/swc/. Richmond, VA.
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9.-Bioretention. Version 1.9. March 1, 2011
Virginia Department of Environmental Quality. 2011. Virginia DEQ Stormwater Design Specification No.
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Virginia Department of Environmental Quality (VA DEQ). 2014. Final 2012 305(b)/303(d) Water Quality
Assessment Integrated Report. January 2014.
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sessments/2012305(b)303(d)lntearatedReport.aspx.
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Number 056-007, Hampton Roads. January 2014.
http://www.vdh.virainia.aov/EnvironmentalHealth/shellfish/closure/cond056-007.pdf.
Virginia Institute of Marine Science (VIMS). 2013. Recurrent Flooding Study for Tidewater Virginia.
Report submitted to the Virginia General Assembly. January 2013.
http://ccrm.vims.edu/recurrent flooding/Recurrent Flooding Study web.pdf.
34
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Appendix A: Site Investigations Summary
Site
number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
Location
Carolina Circle at
intersection of Carolina
and Newport Avenues
Delaware Circle at
intersection of Delaware
and Gosnold Avenues
Rhode Island Circle at
intersection of Rhode
Island and Newport
Avenues
Munson Park at W 27th
and Munson Place
Intersection of New York
and Newport Avenues
Intersection of W 28th St.
and Gosnold Ave.
East of Post Office
Parking Lot
W 46th Street west of
Col ley Ave
Eastern extent of W 46th
Street at Knitting Mill
Creek
Outfall at Mayflower
Road and Delaware
Avenue
Upstream
(southernmost) extent of
Knitting Mill Creek
Western extent of W 43rd
Street, at Knitting Mill
Creek
Western extent of W 50th
Street at Knitting Mill
Creek
Intersection of
Mayflower Road and W
41s Street
Open area along
Mayflower Road north of
Carolina Avenue
Western shoreline of
Knitting Mill Creek south
of W 45th Street
Shoreline at Tidewater
Boat Club
Commercial at Colley
Avenue and W 45th
Street
Industrial building at
Newport Avenue and W
23rd Street
Observation
Circular park within intersection, slightly elevated relative to
adjacent roadways surface drainage away from sites.
Circular park within intersection, slightly elevated relative to
adjacent roadways surface drainage away from sites.
Circular park within intersection, slightly elevated relative to
adjacent roadways surface drainage away from sites.
Large block scale open park, slightly elevated relative to
adjacent streets surface drainage away from site.
Relatively wide streets (particularly Newport Avenue), curb
inlets at all corners, narrow verge with some utility conflicts
and mature vegetation.
Moderate street width, curb inlets at every corner and some
utility and mature trees in ROW.
Very small landscaped area.
Narrow street with curb inlets on both sides. Multiple
driveway entrances.
Existing wetland restoration project.
Site of proposed shoreline armoring incorporating a section
of living shoreline. Outfall serves adjacent Delaware
Avenue and some portion of Mayflower Road.
Large RCP outfall with adjacent (failing) retaining wall
surrounded by Mayflower Road, a small grassed area, and
an adjacent parking lot.
Gravel parking lot serving marina with corrugated metal
pipe outfall into creek.
Surface flows from street discharge across narrow gently
sloped shoreline with native shrub vegetation.
15-20 ft wide grassed area with moderate slope situated
between Mayflower Road and Parking area behind Pancho
& Luigi's.
Gently sloping grassed area, potions of adjacent parking
drain to site.
Narrow grassed area between gravel parking lot and
shoreline. Runoff sheet flows across grassed area.
Impervious road/lot along shoreline.
One story building with impervious road street frontage
surrounded by very small amount of pervious area.
Industrial building currently in use as self-storage facility.
Surrounded by narrow grassed building setback. Two roads
without curb and gutter.
Recommendation
None
None
None
None
Curb bump out planter
boxes.
Curb bump out planter
boxes.
None
None
None
Implement shoreline
bioswale, using curb cuts
to divert western half of
Mayflower Road drainage.
Implement bioretention
area in grassed space
serving adjacent parking
lot.
Convert parking area to
permeable pavement or
install bioretention.
None
Small bioswale to treat
parking area.
Consider bioswale or
roadside bioretention area,
Possible daylighting of
creek in this area.
Convert parking area to
permeable pavement or
install bioretention.
None, insufficient space
for retrofitting.
Consider incorporating
planter box or other
landscape elements into
street frontage.
Investigate green roof for
building and consider
bioswale along Newport
Avenue and W 23rd Street.
A-1
-------
Site
number
Location
Observation
Recommendation
Industrial building at
20 Newport Avenue and W
23rd Street
Building has been demolished.
Explore Gl elements as
part of redevelopment.
21
Gosnold Avenue
between W. 25th Street
and W 24th Street
Relatively wide street with inset parallel parking and wide
grassed verge.
Convert northernmost
parking spaces to
permeable pavement
(adjacent catch basins
allow use of underdrains).
22
W 41st Street Between
Colley and Killam
Avenues
Wide street with very wide vegetated fringe. No evidence of
subsurface drains.
Reduce street width
and/or consider roadside
bioswale (dependent on
infiltration or connection to
drainage network).
23
W 42s1 Street Between
Colley and Killam
Avenues
Wide street with very wide vegetated fringe. No evidence of
subsurface drains.
Reduce street width
and/or consider roadside
bioswale (dependent on
infiltration or connection to
drainage network).
24
38th Street playground
Park area between 37th and 38th Streets set in area of low
elevation. Main storm drain runs underneath. Park appears
to be heavily used.
Implement bioretention
areas along 37th and 38th
streets to treat street
runoff before it enters
drainage network.
Alley between W 37th
25 Street and W 36th Street
west of Colley Avenue
Narrow alley between streets lined with narrow grass verge
without curb
Implement roadside
swales to encourage
infiltration.
26
27
Undeveloped lots at
1 020 W 36th Street.
Large gravel lot at 831
W 39th Street
Three undeveloped lots with managed grass vegetation
roadway drainage surface flows along curb to the east.
Large gravel lot which appears to serve adjacent ASCO
facility
None
None
Mayflower Road at New
28 Jersey Avenue
Small outfall along shoreline serving New Jersey Avenue
and small portion of Mayflower Road.
Consider roadside
bioswale serving western
half of Mayflower Road.
29
Knitting Mill Creek
Community Garden
Majority of adjacent restaurant building rooftop discharges
nearest the garden.
Install rainwater cistern to
capture rooftop runoff and
provide irrigation water for
garden.
Western end of W 48th
30 Street at Knitting Mill
Creek
Small wooded area in shoreline buffer where street runoff
discharges into creek.
None
A-2
-------
Appendix B: 35th Street and Colley Avenue Green Street Conceptual Design
B-l
-------
Site Location
Drainage Area Characteristics Proposed Characteristics
Date of Field Visit
6/26/2014
Field Visit Personnel J. Smith
Latitude
Longitude
Major Watershed
Street Address
Lafayette River Landowner
Blocks north, south, and west of
35th St./Colley Ave. intersection
36° 52' 40" N
76° 17' 41" W
City of Norfolk
Drainage Area, acres 3.1
Hydrologic Soil Group D, Urban
Total Impervious, % 86
Design Storm Event, in 1.0
Proposed BMPs BR, PP
Total Detention Vol., ft3 3,500
Bioretention Area, ft2 2,500
Perm. Pavement Area, ft2 3,130
Existing Site Description: The proposed project site includes the blocks immediately
north, south, and west of the Colley Avenue/35th Street intersection. Colley Avenue
is a commercial corridor through Norfolk that is primarily a 2-lane roadway with
parking lanes and sidewalk on both sides of the street. 35th Street has a similar
configuration, but is a residential corridor with significantly wider travel lanes.
Sanitary sewer and water supply lines are located throughout the project area,
although no stormwater drains currently exist directly within the three blocks.
Proposed Green Infrastructure Description: Proposed BMPs within the right-of-
way (ROW) include bioretention curb extensions on 35th St., and both porous
asphalt parking lanes and bioretention curb extensions along Colley 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.
Stonmwatei Pipes
;/J Bioretention Areas
Permeable Pavement
Feet i I Project Site Drainage Area
Norfolk City Limits
Legend
| Kmtiing Mil! Watershed Boundary
Legend
O Stormwater Structures
Stormwater Pipes
Underdrains
CO CD 7s
o z: ^
O —
> co
CD
O
m
^
c
m
>>
a m
om
m
~0
Section A-A'
b
— >
g
1
5.0'
SIDEWALK
' 7' BIORETENTION CELL
___,
•
0.5'
0.5'
~^
'
h
DRAIN
5.0'
BIKE
xl
10.0'
TRAVEL
I/
0.5'
7' BIORETENTION CELL
-DRAIN
5.01
SIDEWALK
Section B-B'
i^Stf"
5.0'
SIDEWALK
8.0' PERMEABLE PAVEMENT AREA
/
DRAIN
4.01
8.0'
•o
0.5'
8.0'
PARKING
^_
j
V
10.0'
TRAVEL
LANE
\
_^
f l
10.0'
TRAVEL
\
,. '
J
a*
^.
8.0'
PARKING
LANE
36.0'
DRAIN
0.5'
5.0'
SIDEWALK
-------
Appendix C: Mayflower Road Shoreline Conceptual Design
c-i
-------
Site Location
Drainage Area Characteristics Proposed Characteristics
Date of Field Visit
6/26/2014
Field Visit Personnel J. Smith
Latitude
Longitude
Major Watershed
Street Address
Lafayette River Landowner
4200-5000 blocks of Mayflower Rd
36° 53' 7" N
76° 17' 36" W
City of Norfolk
Drainage Area, acres 0.65
Hydrologic Soil Group n/a (fill)
Total Impervious, % 80
Design Storm Event, in 1.0
Proposed BMPs BR, PP
Total Detention Vol., ft3 1,900
Bioretention Area, ft2 1,500
Perm. Pavement Area, ft2 7,200
Existing Site Conditions: Mayflower Road borders the eastern shore of Knitting Mill
Creek. The northern section of shore consists of a new concrete bulkhead and the
southern section of shore consists of a badly eroding shoreline which is planned to
be protected by a living shoreline/shoreline armoring project in the coming years.
Throughout this entire extent there is a 10-15 ft wide grass verge between the road
edge and the shoreline exhibiting signs of heavy foot traffic and poor plant health.
Stormwater runoff from Mayflower Road directly discharges into Knitting Mill Creek
via curb and gutter directed to subsurface drainage system.
Proposed Green Infrastructure Description: Approximately 500 feet of bioswale
will be installed directly adjacent to the existing curb to capture , via curb cuts, and
treat runoff along the southern section of roadway. A pervious walkway is
proposed between the bioswale and shoreline to provide pedestrian access and
necessary freeboard and will also extend along the northern section of roadway.
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
• Stormwater Structures
Stcrmwater Pipes
^J Project Site Drainage Area
Norfolk City Limits
Legend
I I Knit!
Legend
O Stormwater Structures
Stormwater Pipes
Bioswale Strip (2-5 ft from curb)
Pervious Walkway (8-11 ft from curb)
Turfgrass/Native Vegetation
CO
2 > r=
^ CO "~
m —i O
g o m
o ^ z
co m O
5 o 5!
^ o TS
^ m -
^ ~o <
Existing conditions along Mayflower Rd.,
view looking north
Artist's rendering of proposed future conditions
Cross-section of
bioswale/permeable pavement
along Mayflower Rd.
PROPOSED SHORELINE
PROTECTION-
MEAN HIGH
WATER TABLE
PROPOSED EMBANKMENT-
MIN. CURB EL.4.4'
-------
"1
Legend
Stormwater Structures
Stormwater Pipes
Bioswale Strip (2-5 ft from curb)
Pervious Walkway (8-11 ft from curb)
Turfgrass/Native Vegetation
Legend
• Stormwater Structures
Stormwater Pipes
Pervious Walkway (8-11 ft from curb)
| Turfgrass/Native Vegetation
C/
g > r=
^ C/} "~
m —i O
XI XI X>
XI C m
o m
m
o
'= m
-------
Appendix D. Green Infrastructure Conceptual Design Detailed Cost Estimates
Table D-l.
Item No.
Cost estimate for 35th Street bioretention
Description
Reference
Quantity
Unit
Unit Cost
Total
Preparation
1
Traffic Control
21
day
$1 ,000.00
$21 ,000
Site Preparation/Earthwork
2
3
4
5
6
7
Saw-cut asphalt
Asphalt Removal, BMP
Excavation, BMP
Excavation, underdrain trench
Curb removal
Haul and disposal
RSMeans
RSMeans
RSMeans
RSMeans
RSMeans
1407
239
199
540
302
199
LF
SY
CY
LF
LF
CY
$2.31
$4.18
$16.24
$1.13
$4.30
$8.55
$3,250
$999
$3,233
$610
$1 ,299
$1 ,702
Bioretention Curb Extension
8
9
10
11
12
13
14
15
16
17
Bioretention Media - 2' Depth
Filter Layer (washed concrete sand)
Drainage stone (washed #57 stone)
Grouted River Rock
Curb Cuts
Hardwood mulch (triple shredded)
Concrete Curb (6" vertical, straight)
Concrete Curb (6" vertical, radius)
4" SCH 40 perforated PVC cleanout
Vegetation
RSMeans
RSMeans
RSMeans
Engineer's estimate
Engineer's estimate
Engineer's estimate
RSMeans
RSMeans
Engineer's estimate
Engineer's estimate
99
8
49
6
14
12
262
65
7
1330
CY
CY
CY
CY
EA
CY
LF
LF
EA
SF
$31 .31
$60.82
$47.91
$150.00
$125.00
$55.00
$11.73
$19.03
$100.00
$1.00
$3,085
$499
$2,360
$933
$1 ,750
$674
$3,069
$1 ,245
$700
$1 ,330
Underdrain
18
19
Construction
20
21
22
23
Construction
24
Total Cost
6" PVC Underdrain, perforated
6" PVC Underdrain, solid
Subtotal
Planning (20% of subtotal)
Mobilization (10% of subtotal)
Bond (5% of subtotal)
Construction contingency (10% of subtotal)
Total
Design (20% of Construction Total)
RSMeans
RSMeans
190
540
LF
LF
$10.80
$5.94
$2,052
$3,208
$52,997
$10,599
$5,300
$2,650
$5,300
$76,846
$15,369
$92,215
D-l
-------
Table D-2. Cost estimate for Colley Avenue green street
Item No. Description
Reference
Quantity
Unit
Unit Cost
Total
Preparation
1 Traffic Control
21
day
$1 ,000.00
$21,000
Site Preparation/Earthwork
2 Saw-cut asphalt
3 Asphalt Removal, BMP
4 Excavation, BMP
5 Excavation, underdrain trench
6 Curb removal
7 Haul and disposal
RSMeans
RSMeans
RSMeans
RSMeans
RSMeans
1779
564
585
580
591
585
LF
SY
CY
LF
LF
CY
$2.31
$4.18
$16.24
$1.13
$4.30
$8.55
$4,109
$2,359
$9,496
$655
$2,541
$5,000
Bioretention Curb Extension
8 Bioretention Media - 2' Depth
9 Filter Layer (washed concrete sand)
10 Drainage stone (washed #57 stone)
1 1 Grouted River Rock
12 Curb Cuts
13 Hardwood mulch (triple-shredded)
14 Concrete Curb (6" vertical, straight)
15 Concrete Curb (6" vertical, radius)
16 4" SCH 40 perforated PVC cleanout
17 Vegetation
RSMeans
RSMeans
RSMeans
Engineer's
estimate
Engineer's
estimate
Engineer's
estimate
RSMeans
RSMeans
Engineer's
estimate
Engineer's
estimate
87
7
43
4
10
11
182
46
5
1172
CY
CY
CY
CY
EA
CY
LF
LF
EA
SF
$31.31
$60.82
$47.91
$150.00
$125.00
$55.00
$11.73
$19.03
$100.00
$1.00
$2,718
$440
$2,080
$667
$1,250
$595
$2,140
$868
$500
$1,172
PICP Pavers
18 Porous concrete
19 Bedding layer (No. 8 stone)
20 Reservoir layer (No. 57 stone)
Engineer's
estimate
RSMeans
RSMeans
3128
58
437
SF
CY
TN
$8.00
$40.00
$24.57
$25,024
$2,317
$10,745
Underdrain
21 6" PVC Underdrain, perforated
22 6" PVC Underdrain, solid
Construction Subtotal
23 Planning (20% of subtotal)
24 Mobilization (10% of subtotal)
25 Bond (5% of subtotal)
26 Construction contingency (1 0% of subtotal)
Construction Total
27 Design (20% of Construction Total)
Total Cost
RSMeans
RSMeans
521
580
LF
LF
$10.80
5.94
$5,623
$3,445
$107,744
$20,949
$10,474
$5,237
$10,474
$151,879
$30,376
$182,255
D-2
-------
Table D-3. Mayflower Road shoreline project
Item No. Description Reference
Quantity
Unit
Unit Cost
Total
Preparation
1 Traffic Control
14
day
$1 ,000.00
$14,000
Site Preparation/Earthwork
2 Excavation, BMP
3 Curb removal
4 Haul and disposal
5 Finish Grade
341
14
341
2083
CY
LF
CY
SY
$16.24
$4.30
$8.55
$1.10
$5,539
$60
$2,916
$2,292
Bioswale
6 Bioretention Media - 2' Depth
7 Filter Layer (washed concrete sand)
8 Drainage stone (washed #57 stone)
9 Grouted River Rock
10 Curb Cuts
1 1 4" SCH 40 perforated PVC cleanout
12 Turf sod
13 Native planting for riparian buffer
94
9
37
3
7
7
4
12
CY
CY
CY
CY
EA
EA
MSF
MSF
$31.31
$60.82
$47.91
$150.00
$125.00
$100.00
$640.00
$1,000.00
$2,957
$563
$1 ,783
$467
$875
$700
$2,560
$12,000
Underdrain
14 6" PVC Underdrain, perforated
500
LF
$10.80
$5,400
Porous Walkway
1 5 Porous concrete
16 Reservoir layer (No. 57 stone)
Construction Subtotal
17 Planning (20% of subtotal)
18 Mobilization (10% of subtotal)
19 Bond (5% of subtotal)
20 Construction contingency (1 0% of subtotal)
Construction Total
21 Design (20% of Construction Total)
Total Cost
7200
132
SF
TN
$4.00
$24.57
$28,800
$3,243
$52,112
$10,422
$5,211
$2,606
$5,211
$75,562
$15,112
$90,674
D-3
------- |