SW5231001_Stormwater Report_20231008

RSsH
March 23, 2023 (Revised June 2023)
RDU Stormwater Control Measure (SCM)
Implementation Strategy

RS&H
RDU PE3 Stormwater Control
Measure (SCM) Strategy
March 2023 (Revised June 2023)
Raleigh, NC
RS&H No.: 10510068.002
Prepared by RS&H, Inc. at the
direction of the Raleigh Durham Airport
Authority.

Table of Contents
1 Introduction 1
1.1 Background on SGWs 1
1.2 Benefits of SGWs for the PE3 Project 3
1.2.1 Airport Limitations 4
1.2.2 Geologic Site Constraints 4
1.2.3 Alignment with the RDUAA Sustainability Management Plan 5
1.3 Stormwater Management Regulatory Standards 5
2 PE3 Project Proposed SGW Implementation 6
2.1 Preliminary Design Information 6
2.2 Post Construction Activities 8
2.2.1 Landscaping 8
2.2.2 Maintenance 9
2.2.3 Research and Community Outreach 9
3 SGW Case Studies 10
3.1 Examples from North Carolina 10
3.1.1 City of Greensboro 10
3.1.2 City of Raleigh 12
3.1.3 NCDOT at Falls Lake 12
3.2 SGWs in the Study Region 12
3.2.1 Maryland 12
3.2.2 Delaware 14
3.2.3 Georgia 14
3.3 SGWs Outside the Study Region 14
4 Conclusion 15
5 References 17
Appendix A: SGW Design Guidance Summary 21
March 2023 (revised June 2023) iii

List of Tables
Table 1: Comparison of Treatment Values between the Greensboro SGW and NCDEQ Standards
for a Bioretention Cell with IWS 11
Table 2: States outside of the Study Region with Regulatory Guidance on SGWs 14
Table 3: Design Guidance from State Regulatory Agencies, Permitted Projects, and University
Research. . 22
List of Figures
Figure 1: Rendering of a SGW from the Walnut Creek Wetland Center. Image from City of
Raleigh (2022) 3
Figure 2: Drainage Areas for the RDU PE3 Project and Potential SGW Locations 7
List of Abbreviations and Acronyms
Built Upon Area BUA
Department of Energy, Mineral, and Land Resources DEMLR
Department of Natural Resources and Environmental Control DNREC
Department of the Environment DOE
Department of Transportation DOT
Department of Water Resources DWR
Environmental Site Design ESD
Event Mean Concentration EMC
Federal Aviation Administration FAA
Internal Water Storage IWS
North Carolina Department of Environmental Quality NCDEQ
North Carolina Department of Transportation NCDOT
North Carolina State University NCSU
Parking Economy 3 PE3
Raleigh-Durham Airport Authority RDUAA
Submerged Gravel Wetland SGW
Total Nitrogen TN
Total Phosphorus TP
Total Suspended Solids TSS
United States Environmental Protection Agency USEPA
University of New Hampshire UNH
Water Quality Volume WQV
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RDU SCM Implementation Strategy
1 Introduction
To accommodate existing and future demands for an economy parking product, the Raleigh-
Durham Airport Authority (RDUAA) needs to expand the Parking Economy 3 facility (PE3). To
mitigate the potential for adverse effects of increased stormwater runoff expected as part of the
expansion, and to adhere to the requirements of the RDU Sustainability Management Plan,
RDUAA is developing a project-specific stormwater management plan that includes a more
advanced stormwater control measure (SCM). This SCM, which relies on Submerged Gravel
Wetland (SGW) technology and is rather new to North Carolina, has been previously
demonstrated in many states along the eastern seaboard and is in the process of being
approved for use on projects by the North Carolina Department of Transportation (NCDOT). The
SGW is expected to meet or exceed existing North Carolina SCM requirements as well as help
address airport operations and site location challenges.
The ensuing RDU SCM Implementation Strategy was originally submitted to the North Carolina
Department of Environmental Quality(NCDEQ) for review under the title "RDU Submerged Gravel
Wetland Implementation Strategy"on April 7, 2023. The conceptual lot layout depicted in - Figure
2 (Drainage Areas for the RDU PE3 Project and Potential SGW Locations) that follows has since
been refined through the design process to the form that was evaluated in the ECR.
The following pages introduce the SGW technology, the biogeochemical processes that lead to
stormwater pollutant removal, and a present conceptual design. The next section summarizes
the applicability of the technology to the PE3 project, and the third section outlines how Federal
Aviation Administration (FAA) and North Carolina Department of Environmental Quality
(NCDEQ) stormwater regulatory requirements will be achieved with this SGW stormwater control
measure.
1.1 Background on SGWs
SGWs have traditionally been used for wastewater treatment due to their ability to capture and
retain high levels of pollutants. Until recently, the use of SGWs in stormwater management and
regulatory guidance on the technology were limited. However, research in the early 2000s by the
University of New Hampshire (UNH) Stormwater Center began to demonstrate the application
of the control measure for stormwater management. Through UNH studies, a Stormwater
Center Subsurface Gravel Wetlands Design Specification document was developed in 2009 (UNH
Stormwater Center, 2009). Since that time, many states in the eastern U.S. have developed
regulatory guidance and established design criteria with benchmarks for SGW applications. As
knowledge of the SCM expands, permitted SGW projects are becoming more commonplace.
Although found nationwide, some regionally applicable examples of permitted and publicized
SGW projects are located in North Carolina, Maryland, Delaware, and Massachusetts (Dragonfly
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RDU SCM Implementation Strategy
Pond Works, 2019) (EA Engineering, Science, and Technology, n.d.) (Delaware Business Times,
2021) (USEPA, 2016).
Further, the United States Environmental Protection Agency (USEPA) has promulgated a
Stormwater Best Management Practice Stormwater Wetland fact sheet (USEPA, 2021) that
describes subsurface flow wetlands, of which SGWs are a subset. This endorsement by USEPA
also demonstrates this stormwater treatment technology's viability and efficacy.
The SGW system consists of one or more treatment cells or ponds, which are filled with a gravel
substrate material, topped with wetland soil, and seeded with wetland plants. Through a series
of perforated underdrain pipes and outfalls, stormwater runoff replicates a high groundwater
table within the gravel layer. The outfall for each cell is constructed at a depth that will maintain
the constant water level to create the characteristic saturated soil conditions of a wetland.
Wetland vegetation is rooted in the top layers of soil and choker stone, which is artificially
saturated, allowing for enhanced water quality treatment through biogeochemical processing
and bioremediation of pollutants. Algae, bacteria, and other microbes also establish within the
gravel substrate and provide biological nutrient sequestration. The anaerobic conditions at the
bottom of the SGW can also contribute to denitrification and other treatment processes (Atlanta
Regional Commission, 2016). The SGW design and inflow capacity is planned such that soil
saturation conditions should not result in standing or ponded water above the soil surface
except for brief periods during and immediately after rain events. To further prevent prolonged
surface water inundation after a storm event, perforated riser pipes are used to drain water from
the SGW surface to subsurface gravel and aggregate stone layers.
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RDU SCM Implementation Strategy
Between Storms During a Storm
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Figure 1: Rendering of a SGW from the Walnut Creek Wetland Center. Image from City of Raleigh (2022).
Figure 1 shows the design features of a typical SGW, which includes a perforated riser, a gravel
substrate with differing sizes of stone, an optional impervious layer (a clay liner in this scenario),
and an outflow pipe serving as the SGW's outfall to adjacent receiving waters.
Results from SGW performance research conducted for this study, in conjunction with
conversations between the design team and researchers at North Carolina State University
(NCSU), support the assertion that SGW technology is suitable for site conditions and space
requirements. The following sections highlight the viability of this option for the RDU PE3
project, combining optimal qualities of dry and wet facilities (in alignment with FAA standards)
while meeting and potentially exceeding NCDEQ's performance guidance.
1.2 Benefits of SGWs for the PE3 Project
SGWs are commonly used in retrofit scenarios when current stormwater controls are not
performing adequately, water quality is not improving to the level necessary, or there are issues
with stormwater quantity and flooding. They are also preferred over other SCMs due to their
relatively low land consumption requirements (Atlanta Regional Commission, 2016) (Knox
County Tennessee, 2008). Most often, SGWs are used for treating runoff from mostly contiguous
impervious areas such as parking lots, roadways, and industrial areas, which may have large
pollutant loads.
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RDU SCM Implementation Strategy
1.2.1 Airport Limitations
Rapid surface water drawdown times make SGWs an applicable and preferential method of
airport stormwater control in alignment with the FAA standards (FAA, 2013) (FAA, 2020). Dry
ponds also minimize periods of standing water immediately following storm events and
primarily remain dry; however dry pond facilities are much less effective than other SCMs at
sequestering and removing pollutants of concern, and as such are not typically allowed as
primary SCMs under North Carolina regulations. However, facilities at or near airports may use
dry pond facilities provided the stormwater management minimum criteria are met, but dry
ponds do not achieve the same level of water quality improvement that other SCMs attain; and
therefore, are not preferred for this type of project.
Wet facilities, such as wet ponds and constructed stormwater or scrubbing wetlands, typically
provide greater water quality improvements compared to dry facilities and are also primary
SCMs allowable by NCDEQ. However, wet ponds cannot be used at airports, or for the PE3
project in particular since their designs involve permanent or semipermanent standing water.
These conditions attract birds and other species that create wildlife hazards to airport operations
and present a safety concern for the flying public. The SGW, therefore, is an acceptable SCM
alternative since it functions like a wetland with typical wetland plants, soils, and pollutant
removal processes without the necessity for prolonged surface inundation or standing water
conditions.
1.2.2 Geologic Site Constraints
The PE3 project is located in an area identified during preliminary geologic studies as having
Hydrologic Soil Group C soils (Web Soil Survey, 2023). Hydrologic Soil Group C soils typically
have a higher proportion of clay than A or B soils, which primarily consist of sand and loam.
Because there is more clay, soils are less permeable and water transmission through the soil is
restricted (USDA, 2007). SGWs are recommended for projects where there are poorly drained
soils such as C or D hydrologic soils because they do not rely on infiltration (Maryland
Department of the Environment, 2009). In addition, the PE3 project sits atop a layer of generally
impervious partially weathered rock and bedrock. This natural liner eliminates the need for an
artificial geotextile fabric or clay liner. Installed liners could fail over longer time scales due to
contraction and expansion, or they could be compromised during the installation of the SGWs.
By using a natural liner, which is inherently more stable, the SGWs and the overall project is
more resilient.
Bioretention cells were considered as a project alternative since they also provide similarly
effective pollutant treatment without prolonged periods of surficial inundation or standing
water. Bioretention cells are constructed shallow stormwater basins, which remove pollutants
and reduce runoff quantities through soil and substrate infiltration, filtration, and microbial
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RDU SCM Implementation Strategy
bioremediation. Since these SCMs rely on rapid stormwater infiltration into underlying soil layers
and substrate, slowly permeable soils and impermeable layers such as bedrock are not ideal for
applications of the bioretention cell without underdrainage (Jarrett, 2022). In addition, unlike
SGWs, soils and substrata in bioretention cells without internal water storage (IWS) are not
inherently designed to remain saturated and therefore do not have the same pollutant removal
efficacy as a constructed stormwater wetland system. Therefore, the geologic conditions at the
PE3 project site support the use of SGWs.
1.2.3 Alignment with the RDUAA Sustainability Management Plan
The RDUAA released their Sustainability Management Plan in February 2023, which includes two
focus areas relating to this project: Land Use and Natural Resources, and Water and Stormwater.
The Airport is committed to environmental stewardship and conservation of natural resources,
which include wildlife, surface water, and wetlands. The Airport has also committed to
implementing best practices in stormwater management to minimize flooding and erosion while
protecting water quality in the Crabtree Creek Watershed of the Neuse River Basin (Raleigh-
Durham Airport Authority, 2023). By using SGWs for the management of stormwater runoff from
the parking lot project, the RDUAA is demonstrating its commitment to the Sustainability
Management Plan by reducing pollutant loads in stormwater runoff, implementing wetland
systems with multiple ecosystem functions, and minimizing the potential for flooding and
erosion of the surrounding lands. In general, the use of SGWs exceeds regulatory standards and
requirements for an airport project and advances the sustainability and resiliency of the Airport
as a whole.
1.3 Stormwater Management Regulatory Standards
Stormwater management regulations and standards for this project are defined by the FAA and
the NCDEQ. This project follows the FAA guidance regarding stormwater drainage design and
reducing wildlife hazards (FAA, 2013) (FAA, 2020). To prevent wildlife attraction to stormwater
wetland systems, the SGWs will not have ponded water beyond 48 hours following a rainfall
events.
While SGWs are not yet an approved SCM by the NCDEQ, under 15A North Carolina
Administrative Code 02H.1003(6), the permitting authority, in this case the Department of
Energy, Mineral, and Land Resources (DEMLR), can approve projects that do not comply with all
the provisions of the [stormwater rules] on a case by case basis if the applicant can demonstrate
that the project provides equal or better stormwater control and equal or better protection of
waters of the State than the requirements of this Section (NCDEQ, 2017). Based on case studies
and regulatory guidance from other states, it is clear that SGWs provide equal and in most
cases, better stormwater control and treatment levels than other already approved SCMs.
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RDU SCM Implementation Strategy
Preliminary assessments of the pollutant reduction capacity of SGWs for the RDU PE3 project are
discussed in the following section.
The proposed SGW design at RDU will fulfill the NCDEQ's requirement that SCMs be capable of
containing and treating the first one (1) inch of rainfall within the impervious area over a 48- to
72-hour period. In addition, the project team will communicate with the NCDEQ Department of
Water Resources (DWR), which oversees nonpoint source planning, during the design and
development of the SCM to demonstrate probable treatment capacity.
2 PE3 Project Proposed SGW Implementation
The intent of this section is to demonstrate the SGWs implementation for the PE3 project with
preliminary design information and anticipated post construction activities. The first section
provides an overview of the 60% design of the SGWs including the drainage basin areas, the
number of proposed SGWs, and the SGW strata depths. Proposed treatment values based on
case studies and regulatory guidance are included. The second section describes proposed post
construction activities including the development of a landscaping plan and maintenance and
inspection forms. There will also be a discussion on potential research opportunities and
community outreach related to the PE3 project.
2.1 Preliminary Design Information
The RDU PE3 project involves the collection and treatment of 70 to 80 acres of expanded
parking area stormwater runoff which includes the capture and treatment of some existing
parking areas based on the geometric design. The runoff from this new built upon area (BUA)
must be contained and treated by stormwater controls, which will be the SGWs. At the time of
completion of the Stormwater Implementation Strategy, the 60% design included seven
drainage basins with outfalls to seven SGWs (Figure 2).
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RDU SCM Implementation Strategy
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Figure 2: Drainage Areas for the RDU PE3 Project and Potential SGW Locations.
The drainage basins have the following approximate sizes:
• Basin 1 = 8.5 acres
• Basin 2 = 7.5 acres
• Basin 3 = 6.0 acres
• Basin 4 = 24.0 acres
• Basin 5 = 11.5 acres
• Basin 6 = 6.0 acres
• Basin 7 = 16.5 acres
The associated SGWs (numbered) are located adjacent to the corresponding drainage basin, but
final sizing has not been determined.
Preliminary cross sections for the SGW follow the design guidance from the NCSU team who
constructed the state's first SGW in Greensboro, the design specifications from the UNH
Stormwater Center, and nearby state stormwater regulations including guidance published and
referenced by the Maryland Department of the Environment, the Delaware Department of
Natural Resources and Environmental Control, and the Georgia Environmental Protection
Division (Appendix A). Broadly, the SGWs for the RDU PE3 project will have a two-foot ponding
depth and three main strata:
• 6 in wetland soil
• 6 in of choker stone, and
• 36 in of course gravel media (57 stone).
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RDU SCM Implementation Strategy
Each SGW will also have a sediment forebay, which will contain a portion of the runoff volume
and remove much of the total suspended solids (TSS). The project team expects that treatment
values for the SGW, based on the presumed design, should meet or exceed the effluent event
mean concentration (EMCEffIuent) or median percent removal for the SGW constructed at
Greensboro (Appendix A). Baseline treatment values that would be expected are 96% TSS
removal, 54% total nitrogen (TN) removal, and 58% total phosphorus (TP) removal. In
comparison, the NCDEQ standard for a bioretention cell is 35-40%TN removal and 45% TP
removal (NCDEQ, 2018). The proposed treatment values are based on the findings from fully
sized and studied SGWs at the UNH and informed by permitted projects, regulatory guidance
from other states, and assumed design (Appendix A) (Houle & Ballestero, 2020).
2.2 Post Construction Activities
The project team is developing plans for landscaping and inspection and maintenance of the
SGW to ensure proper function and success of the SCM. While those resources are not
completed at the time of this report, the following sections outline the guidance from other
states' regulatory agencies and departments of transportation (DOTs) that will guide the
development of post construction activity planning documents. Additionally, post construction
activities may include education and research campaigns related to this project in collaboration
with local universities and partner organizations.
2.2.1 Landscaping
Landscaping is critical for the success of the SGWs due to plants' ability to remove pollutants
and aid in the treatment of stormwater as it filters through the SCM. However, improper
landscaping due to use of the wrong plants, improper timing of seeding, or inability for plants to
establish, can impede the function of the SGW and lead to long term maintenance
requirements. Notably, maintenance may be required if plant survivorship is particularly low
after 1-2 years of the SGW construction. Replanting or adjustments to the planting plan (e.g.,
changing species mix) may be required to maintain the proper functioning of the SGW.
Most state guidance for SGWs recommends using native wetland plants and including a
diversity of species (typically at least 3) (Atlanta Regional Commission, 2016) (MDOT State
Highway Administration, 2022) (Maryland Department of the Environment, 2009). Specifically
Maine recommended Juncus effusus, Typha latifolia, and Scirpus species which are all non-
invasive wetland species. Additionally, the City of Raleigh provided insights into their
landscaping strategy and found that proper spacing, sequencing of plant installation, and
avoiding last minute plant substitutions are critical for establishment of wetland plants (Dutra,
2023). This project will utilize landscaping and planting guidance from the NCDEQ Stormwater
Design Manual for C-4 Stormwater Wetlands (NCDEQ, 2020). A North Carolina liscenced
landscaping professional will be consulted to develop a landscaping plan.
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RDU SCM Implementation Strategy
2.2.2 Maintenance
While SGWs are less maintenance intensive over long time scales than bioretention cells and
other SCMs, maintenance will be required in the first couple of years after construction to
ensure proper drainage and function of the SGWs. After a year or two of being constructed,
once wetland vegetation has established, the SGW should support natural hydrologic and
ecologic processes. In comparison, bioretention cells are not as self-sustaining and require
mulching activities 1-2 times a year and vegetation replacement periodically (Jarrett, 2022).
SGWs are advantageous as they have less long-term maintenance requirements, and they allow
the site to function more similarly to a natural wetland which provides other ecological benefits
to the community and aligns with the sustainability goals of the RDUAA.
Most state guidance recommends conducting inspections after every major storm event
(typically greater than 0.5 in) in the first year post construction (Maryland Department of the
Environment, 2009) (Maine Department of Environmental Protection, 2016). During these
inspections, recommended activities include:
• Verifying the system drains within 24-48 hours.
• Evaluating the condition of planted vegetation and conducting replanting if necessary.
• Identifying areas of erosion and making repairs.
• Checking inlets, outlets, and subdrains for proper functioning and cleaning out if necessary.
After the first year post construction, most state guidance recommends conducting inspections
biannually. The following list of items may be indicators that additional controls or maintenance
is needed:
• Evidence of standing water, discolored water, or accumulated sediments.
• Trash or debris
• Structural damage or blocked pipes
• Decaying vegetation
• Invasive species
• Trees and animal activity
Some states have provided Inspection, Operation, and Maintenance Guidance documents
(Atlanta Regional Commission, 2016) (NHDOT, 2015). These documents will inform the
maintenance plan developed by the project team.
2.2.3 Research and Community Outreach
The SGW project presents many opportunities for research and community education on the
ecosystem services provided by wetlands. By implementing the project, researchers at NCSU and
the NCDEQ may have opportunities to sample EMC influent values from the parking lot and
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RDU SCM Implementation Strategy
effluent values from the SGW outfalls. Once performance data has been reviewed, the NCDEQ
may be able to proceed with establishment of the SGW as a primary SCM, which will promote its
use for future projects in the state.
The project also lends itself well to community outreach and education opportunities.
Educational signage on the construction and design of the SGW, its purpose for treating
stormwater, and the general importance of wetlands for improving water quality may be
incorporated into the project site. These opportunities to educate the general public and create
community partnerships is another goal of the RDUAA in its Sustainability Management Plan
and helps promote a culture which values resilience and sustainability (Raleigh-Durham Airport
Authority, 2023). It should be noted that the proposed SGWs for this project will provide large
scale treatment for an area larger than any other previous SGW within the state of North
Carolina.
3 SGW Case Studies
There have been numerous examples of designed and permitted SGWs in North Carolina and
neighboring states in the Southeast and the Chesapeake Bay Watershed. Further, states such as
Maryland, Delaware, and Georgia have developed regulatory guidance for SGWs, which
promotes their implementation as a SCM in those states. The first section discusses the status of
SGWs in North Carolina including case studies from the City of Greensboro, the City of Raleigh,
and the North Carolina Department of Transportation (NCDOT) project. The second section
describes permitted projects and state or local regulatory guidance for SGWs from Maryland,
Delaware, and Georgia. The final section briefly discusses the regulatory guidance for SGWs
beyond the study region in other eastern states. A summary of the design guidance from
permitted projects and state regulatory guidance is provided in Appendix A.
3.1 Examples from North Carolina
Even though NCDEQ and other state agencies are still developing regulatory guidance for
SGWs, three examples of SGWs have been designed and built in the state.
3.1.1 City of Greensboro
The predominant example of the SGW in North Carolina is located at 401 Patton Avenue and
provides stormwater management for the City of Greensboro. The 14,365 square foot basin was
sized to detain the 0.45 in storm from a 14-acre drainage area. Therefore, the SGW surface area
was about 2.4% of the overall drainage area. Design included a wet forebay and one treatment
cell with a bowl storage volume of 11,000 cubic feet where horizontal and vertical flow was used
to filter the stormwater from the 89% impervious watershed. The SGW was designed with three
layers: a 6 inch layer of wetland soil (100% sand), a 3.5 inch layer of choker stone (#78 Stone),
and a 12 inch layer of gravel (#57 Stone). The SGW had a water quality ponding depth above the
March 2023 (revised June 2023) 10

RDU SCM Implementation Strategy
wetland surface of 1 foot and a maximum of 2.5 feet before the emergency spillway was
activated. (Mitchell, Hunt, &Waickowski, 2021).
In the first year after construction, monitoring and inspection found that a layer of
schmutzdecke had formed on the SGW surface. Raking was required to maintain proper
hydrology and was completed twice during the first year after construction. After two years of
operation, the hydraulic conductivity was reduced to 1.59 in/hour which is within the guidance
for North Carolina of 1-2 inches/hour (Mitchell, Hunt, &Waickowski, 2021). If construction
activities are timed with proper seeding and landscaping, there may be less risk of
schmutzdecke accumulation.
Monitoring data was collected in the years after the SGW was constructed. Results are
summarized in the table below.
Table 1:Comparison of Treatment Values between the Greensboro SGW and NCDEQ Standards for a Bioretention Cell
with IWS (Mitchell, Hunt, &Waickowski, 2021) (NCDEQ, 2018)
Metric Greensboro SGW NCDEQ Bioretention Cell
with IWS Standard
TN Median EMCeffluent 0.44 mg/L 0.58 mg/L
TP Median EMCeffluent 0.07 mg/L 0.12 mg/L
TN Percent Reduction 43% 35-40%
TP Percent Reduction 67% 45%
Table 1 shows that the Greensboro SGW median EMCeffluent valuesfor TN and TP were lower than
the NCDEQ standard for a bioretention cell with IWS. Further, the Greensboro SGW median EMC
percent reductions exceeded the NCDEQ standards for bioretention cells with IWS. These data
show that the SGW performs equal to, and more often better than the bioretention cell with
IWS.
Additionally, this SGW was undersized by a factor of 3 as it was only designed to contain a 0.45
in storm, as opposed to the 1 in storm regulatory requirement by NCDEQ (NCDEQ, 2018).
Researchers at the UNH Stormwater Center have found that reductions in water quality volume
(WQV) sizing impacts water quality performance and leads to lower rates of TSS, TN, and TP
removal. Specifically, they found that a SGW designed to treat the 0.1 in storm (>90% reduction
in WQV) had a TN percent removal that was 57% lower than would be expected for a
conventional SGW. The difference in TP percent removal was less substantial, only 8% lower than
would be expected (Houle & Ballestero, 2020). Therefore, it could be expected that with a fully
sized unit, the TN and TP percent removals would have been higher in Greensboro if it had been
fully sized.
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RDU SCM Implementation Strategy
3.1.2 City of Raleigh
In 2021, the City of Raleigh completed a $350,000 SGW green stormwater infrastructure project
at the Walnut Creek Wetland Center, located in south Raleigh. The project was supported by
NCDEQ who helped fund some of the construction costs and the educational components,
which are still in development. The 5300 square foot wetland was designed to capture
stormwater runoff from 1 acre of impervious surface, which included Peterson Street and other
nearby streets, adjacent sidewalks, and buildings. Therefore, the SGW surface area was roughly
12.2% of the contributing drainage area (City of Raleigh, 2022)
The wetland is expected to substantially improve runoff water quality before it drains into
Walnut Creek. The city published that the wetland could remove around 8 pounds of nitrogen
and 135 pounds of suspended solids per year (City of Raleigh, 2022). Due to unforeseen issues
with some of the drainage structures and the pipes, maintenance activities to ensure proper
functioning of the SGW are ongoing, and the actual performance capabilities of the SGW are
unknown at this time (Dutra, 2023).
3.1.3 NCDOT at Falls Lake
In North Carolina, NCDOT designed and constructed a SGW as a part of their stormwater BMP
retrofit program. The SGW is located at the intersection of 1-85 and Gate One Road near Falls
Lake. The SGW was chosen for this project for several reasons. Since SGWs are known to be
highly effective in treating stormwater runoff, this BMP maintains compliance with the Falls Lake
Nutrient Strategy Rules. The high-water table at the site also favored the use of the SGW. Finally,
the SGW has lower maintenance requirements and is a highly resilient BMP. This project was
completed within the past year, but there is no monitoring data yet from the site. However, the
site is highly accessible, and recent inspections have not found any signs of the schmutzdecke
(Mullins, 2023) (McDaniel, 2023).
3.2 SGWs in the Study Region
Multiple neighboring states, which are biogeochemically and ecologically similar to North
Carolina, in the southeastern United States and the Chesapeake Bay Watershed, have permitted
SGW projects and published regulatory guidance on SGWs. These projects serve as additional
proof of concept of the SGW and its utility as a stormwater control measure. This section
describes SGW projects and adopted regulations in Maryland, Delaware, and Georgia.
3.2.1 Maryland
Maryland (MD) provides the best case study due to their extensive regulatory guidance and
permitted SGW projects. The Maryland Department of the Environment (DOE) has recognized
the utility of SGWs for stormwater management since at least 2009. In the 2009 revision of the
Stormwater Design Manual, they defined a SGW as a nonstructural and micro-scale practice
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RDU SCM Implementation Strategy
used to capture and treat stormwater runoff at the source from impervious areas less than one
acre (Maryland Department of the Environment, 2009). However, they recognized that since the
SGW requires the gravel substrate to remain saturated to support the wetland vegetation, the
drainage area may be greater than one acre.
In 2012, the Maryland DOE published Stormwater Design Guidance specific to SGWs (Maryland
Department of the Environment, 2012). The document describes that stormwater management
regulations require environmental site design (ESD), which is composed of two main criteria.
First, micro-scale practices such as SGWs provide pollutant treatment close to the source of
runoff. Second, ESD practices must mimic natural hydrologic conditions. There is no drainage
area requirement, but runoff controls should be distributed uniformly across the site and mimic
local hydrology. In summary, ESD can treat equal or larger volumes of runoff compared to
traditional stormwater BMPs, indicating that Maryland promotes utilizing practices such as
SGWs in series to effectively manage stormwater. In 2018, the Maryland DOE released a fact
sheet about SGWs as a Stormwater BMP with pollutant removal efficiencies (Maryland
Department of the Environment, 2018).
In addition to guidance released from the Maryland DOE, the Maryland DOT and the City of
Annapolis have published their own guidance for SGWs. The Maryland DOT State Highway
Administration published design guidance drawings in 2022 for the SGW (MDOT State Highway
Administration, 2022). These design guides include plan view, profile view, and detail drawings
for the observation well and the stone or gabion basket weir. In addition, the City of Annapolis
developed a Stormwater Maintenance Fact Sheet for SGWs that describes maintenance and
monitoring activities for SGWs (City of Anapolis, n.d.).
Not only was Maryland a frontrunner for recognizing and using this control measure for
managing stormwater, but they now have many examples of designed, permitted, and
constructed SGWs throughout the state. One of the more publicized examples of a SGW in
Maryland was the Graham Avenue Submerged Gravel Wetland project in Berlin, MD designed by
EA Engineering (EA Engineering, Science, and Technology, n.d.). The purpose of the project was
to alleviate flooding concerns and improve stormwater quality runoff. Construction began in
2019 and when completed, the basin provided 15,000 cubic feet of stormwater storage from a
drainage area of 155,000 square feet or 3.55 acres of impervious area. Runoff drained primarily
from heavily trafficked roadways and small commercial businesses and was filtered through the
SGW before draining into the Hudson Branch Waterway (Town of Berlin, 2019). The design team
published that nutrient load reductions included 165 pounds of nitrogen, 16 pounds of
phosphorus, and 8400 pounds of TSS annually.
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RDU SCM Implementation Strategy
3.2.2 Delaware
Like Maryland, Delaware also has many examples of SGWs designed, permitted, and constructed
in the state. In 2021, Delaware saw its first approved and permitted SGW in partnership with the
Delaware Natural Resources and Environmental Control (DNREC) and the Kent Conservation
District. Designed by Becker Morgan Group, the Kenton Dollar General Project was completed in
2021 and chose to use a SGW due to the site's high groundwater table and environmentally
sensitive surrounding landscape (Delaware Business Times, 2021). The project received state
acclaim and was recognized by the Governor's office with the Agricultural and Urban
Conservation Award.
The Delaware DNREC considers SGWs a design variant of a constructed wetland (12-D)
according to their BMP Standards and Specifications (DNREC, 2019). The agency has also
released revised Post Construction Stormwater BMP Construction Review Checklists, which are
currently out for public review. This new guidance includes user guides and checklists for
submerged gravel wetlands (DNREC, 2023).
3.2.3 Georgia
The Atlanta Regional Commission, alongside the state Environmental Protection Division, the
state Environmental Finance Authority, the Technical Advisory Group, and engineering
consulting firms, developed the Georgia Stormwater Management Manual (Atlanta Regional
Commission, 2016). Volume II of this document contains a Table of Key Considerations,
Pollutant Removal Capabilities, Application and Site Feasibility Criteria, Planning and Design
Criteria, Design Procedures, and Inspection and Maintenance Requirements for SGWs. The
manual also contains an Operations and Guidance Document for SGWs, which describes routine
maintenance activities and suggested schedules and includes an inspection form. The Georgia
Environmental Protection Division refers to this manual under its sections on Coastal
Stormwater, Municipal Stormwater, and the Georgia Water Planning Council. The Atlanta
Regional Council also developed a SGW Feasibility Checklist, which includes stormwater
management credits, site feasibility, and site applicability (Atlanta Regional Commission, n.d.).
3.3 SGWs Outside the Study Region
SGWs have been used numerous times outside of the study region in other east coast states. In
addition, many states have developed regulatory guidance as it pertains to SGWs. A table
describing SGW regulatory guidance from Tennessee, New Jersey, West Virginia, and Maine is
below. These examples provide further support of this SCM's capacity to adequately manage
and treat stormwater.
Table 2: States outside of the Study Region with Regulatory Guidance on SGWs
March 2023 (revised June 2023) 14

RDU SCM Implementation Strategy
State Regulatory Guidance Approach to SGWs
The Tennessee Department of Environment and SGW is a variation of a constructed
Conservation, in collaboration with the University wetland for stormwater treatment
of Tennessee, developed the Tennessee Permanent (Tennessee Stormwater Management,
Stormwater and Design Guidance Manual. 2015).
Tennessee
Knox County developed a Stormwater Section 4.4.3 of the Knox County
Management Manual. Manual describes submerged gravel
wetlands as a limited application
stormwater BMP (Knox County
Tennessee, 2008).
Stormwater BMP Manual: Subsurface Gravel Requires a waiver or variance to address
New
Wetlands (NJ Department of Environmental stormwater quality.
Jersey
Protection, 2021)
West Virginia Stormwater Management and Design Identifies three basic design variations
Guidance Manual (West Virginia Department of of stormwater wetlands:Wetland basin
West Environmental Protection, 2012) (Level 1), Multi-cell wetland or pond/
Virginia wetland combination (Level 2), and
Subsurface gravel wetland (Modified
Level 2).
Stormwater BMPs (Maine Department of Chapter 7.4—Gravel Wetlands in the
Environmental Protection, 2016) Stormwater BMP Manual.The City of
Maine
City of South Portland Stormwater Manual (City of Portland manual includes Gravel
South Portland, n.d.) Wetlands.
4 Conclusion
By utilizing SGWs, RDU is taking an innovative approach to stormwater management that is
beyond the regulatory baseline and provides community benefits such as watershed
conservation, visual enhancement, research opportunities, and education. To summarize, SGWs
offer many benefits to airports since they drain quickly and should not have permanent standing
water. Further, SGWs support the PE3 project at RDU since they function well in poorly draining
soils and in areas where there is an impermeable layer. Since this SCM does not require
infiltration, it is more feasible for a project of this scale. Additionally, the use of SGWs aligns with
the RDUAA Sustainability Management Plan, which has created initiatives and targets for
responsible stormwater and natural resources management and community outreach and
collaboration. Through this project, the RDUAA demonstrates their commitment to sustainability
and resiliency in the Neuse River Basin and to engagement with local communities and partners
in the surrounding cities.
The SGW design will align with guidance from NCSU, UNH, and regulatory documents from
surrounding states. Based on a presumptive approach and data from other projects, SGWs
March 2023 (revised June 2023) 15

RDU SCM Implementation Strategy
should treat the stormwater runoff from the PE3 lot equal to or potentially better than other
approved SCMs in the state. Post construction activities will be necessary for maintaining proper
function and treatment capacity of the SCM and will include landscaping and maintenance. The
RDUAA may also collaborate with nearby research institutions and local partners to conduct
research on the SGWs and develop signage and education opportunities for airport patrons and
community members.
SGWs, while fairly new to North Carolina, are not a new concept and have been used extensively
in other east coast states to improve water quality. In addition, other states such as Georgia,
Maryland, Delaware, and others have developed regulatory guidance for the design and
treatment capacity of SGWs. These projects and state stormwater guides prove that SGWs are
an effective stormwater control. The success of this project may further demonstrate the
effectiveness of this tool for future uses in North Carolina, and increased adoption of innovative
SCMs such as SGWs will lead to better stormwater management throughout the state.
March 2023 (revised June 2023) 16

RDU SCM Implementation Strategy
5 References
Atlanta Regional Commission. (2016). Georgia Storm water Management Manual. Retrieved
February 12, 2023, from https://cdn.atlantaregional.org/wp-content/uploads/gsmm-
2016-final.pdf
Atlanta Regional Commission. (n.d.). Submerged Gravel Wetlands Feasibility Checklist. Retrieved
February 12, 2023, from https://cdn.atlantaregional.org/wp-
content/uploads/19submerged-gravel-wetland-checklist.pdf
City of Anapolis. (n.d.). Storm water Maintenance Fact Sheet:Submerged Gravel Wetland.
Retrieved February 12, 2023, from
https://www.annapolis.gov/DocumentCenter/View/9598/Submerged-Gravel-Wetlands-
Fact-Sheet-PDF?bidld=
City of Raleigh. (2022). Walnut Creek Gravel Wetland. Retrieved February 12, 2023, from Projects:
https://raleighnc.gov/projects/walnut-creek-gravel-wetland
City of South Portland. (n.d.). City of South Portland Storm water Manual Design Specifications:
Gravel Wetland. Retrieved February 12, 2023, from
https://www.southportland.org/files/3716/5641/6931/Gravel_Wetland.pdf
Delaware Business Times. (2021, September 27). Becker Morgan Group honored for a Delaware
first. Retrieved February 12, 2023, from News Briefs:
https://delawarebusinesstimes.com/news-briefs/becker-morgan-sept-2021/
DNREC. (2019, February). Delaware Post Construction Storm water BMP Standards and
Specifications. Retrieved February 16, 2023, from
https://documents.dnrec.delaware.gov/Watershed/Sediment-Stormwater/Regulatory-
Guidance/BMP%20Stds%20and%20Specs%20-%20EFF%2OFEB%202019.pdf
DNREC. (2023, March). Submerged Gravel Wetland Construction Checklist. Retrieved February 16,
2023, from https://documents.dnrec.delaware.gov/Watershed/Sediment-
Stormwater/Construction-Review/Proposed-BM P-Construction-Checklist.pdf
Dragonfly Pond Works. (2019, November 19). Subsurface Gravel Wetland:Stormwater
Management Innovation. Retrieved February 12, 2023, from Education:
https://www.dragonflypondworks.com/blog/subsurface-gravel-
wetland#:—:text=In%20July%202019%2C%20Dragonfly%20Pond%20Works%2C%201ake
%20and,and%2ONC%20CIean%20Water%20Management%20Trust%20Fund%20sponsor
ship.
Dutra, H. (2023, 02 27). Email Correspondance about Walnut Creek Gravel Wetland. (C. Gantt,
Interviewer)
EA Engineering, Science, and Technology. (n.d.). Graham Avenue Submerged Gravel Wetland.
Retrieved February 12, 2023, from Projects: https://eaest.com/projects/graham-avenue-
submerged-gravel-wetland/
FAA. (2013, 8 15). Airport Drainage Design.AC150/5320-5D. Retrieved March 10, 2023, from
https://www.faa.gov/documentLibrary/media/Advisory_Circular/150_5320_5d.pdf
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RDU SCM Implementation Strategy
FAA. (2020, February 21). Hazardous Wildlife Attractants on or Near Airports.AC 150/5200-33C.
Retrieved March 12, 2023, from
https://www.faa.gov/documentLibrary/media/Advisory_Circular/150-5200-33C.pdf
Houle, J., & Ballestero, T. (2020). Some Performance Characteristics of Subsurface Gravel
Wetlands for Stormwater Management. World Environmental and Water Resource
Congreess 2020.
Jarrett, A. (2022, August 24). Rain Gardens (BioRetention Cells) - a Storm water BMP. Retrieved
March 10, 2022, from PennState Extension: https://extension.psu.edu/rain-gardens-
bioretention-cel is-a-stormwater-bmp
Knox County Tennessee. (2008, January).4.4.3 Submerged Gravel Wetland. Retrieved February
12, 2023, from
https://www.knoxcounty.org/stormwater/manual/Volume%202/Chap4/Section_4_4/4_4_
3_submerged_gravel_wetland.pdf
Maine Department of Environmental Protection. (2016, February). Chapter 7.4 - Gravel Wetlands.
Retrieved Februar 12, 2023, from
https://www.maine.gov/dep/land/stormwater/stormwaterbmps/vo13/chapter7_4.pdf
Maryland Department of the Environment. (2009, May). Maryland Storm water Design Manual.
Retrieved February 16, 2023, from
https://mde.maryland.gov/programs/water/StormwaterManagementProgram/Document
s/www.mde.state.md.us/assets/document/sedimentstormwater/M D%20SWM%20Volum
e%201.pdf
Maryland Department of the Environment. (2012, June). Storm water Design Guidance -
Submerged Gravel Wetlands. Retrieved February 12, 2023, from
https://mde.maryland.gov/programs/Water/StormwaterManagementProgram/Documen
is/ESDM EP%20Design%20Guidance%20SGW%20(2).pdf
Maryland Department of the Environment. (2018, May). Facts About:Submerged Gravel
Wetlands. Retrieved February 12, 2023, from
https://mde.maryland.gov/programs/Water/StormwaterManagementProgram/Documen
is/Fact%20Sheets/M DE_Fact_Sheet_ESD_SGW.pdf
McDaniel, A. (2023, 02 21). Conversation about the NCDOT SGW at Falls Lake. (C. Gantt,
Interviewer)
MDOT State Highway Administration. (2022). Storm water Management Design Resources.
Retrieved February 12, 2023, from Stormwater and Erosion Control Resources:
https://www.roads.maryland.gov/mdotsha/pages/Index.aspx?Pageld=432
Mitchell, C., Hunt, W., &Waickowski, S. (2021, December 6). Quantifying the Water Quality and
Hydrologic Performance of a Submerged Gravel Wetland Treating Industrial Runoff in
Greensboro, North Carolina. Retrieved February 12, 2023
Mullins, R. (2023, 02 28). Conversation about the NCDOT SGW near Falls Lake. (C. Gantt,
Interviewer)
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RDU SCM Implementation Strategy
NCDEQ. (2017, April 18). NCDEQ Stormwater Design Manual. Part F: NEST Program. Retrieved
March 10, 2023
NCDEQ. (2018, 11 7). North Carolina Stormwater Control Measure Credit Document. Retrieved
03 02, 2023, from https://deq.nc.gov/media/11819/download
NCDEQ. (2020, 11 20). C-4. Stormwater Wetland. NCDEQ Stormwater Design Manual. Retrieved
March 14, 2023, from https://deq.nc.gov/media/17538/download
NHDOT. (2015, February 4). Design and Maintenance of Subsurface Gravel Wetlands. Retrieved
March 7, 2023, from
https://www.nh.gov/dot/org/projectdevelopment/materials/research/projects/document
s/15680W-FI NALREPORT.pdf
NJ Department of Environmental Protection. (2021, March). 11.5 Subsurface Gravel Wetlands.
Retrieved February 14, 2023, from
https://www.nj.gov/dep/stormwater/bmp_manual/NJ_SWBM P_11.5-su bsurface-gravel-
wetlands.pdf
Raleigh-Durham Airport Authority. (2023). Sustainability Management Plan. Retrieved 03 10,
2023, from http://www.rdu.com/wp-content/uploads/2023/02/RDU-SMP-Draft-
02.02.23.pdf
Tennessee Stormwater Management. (2015, January 9). 5.4.11 Stormwater Treatment Wetlands.
Retrieved February 12, 2023, from
https://tnpermanentstormwater.org/manual/20%20Chapter%205.4.11%20Stormwater%2
OTreatment%20Wetlands.pdf
Town of Berlin. (2019, October 8). Berlin Graham Avenue Submerged Gravel Wetland Project to
Begin. Retrieved 02 21, 2023, from https://berlinmd.gov/2019/10/08/berlin-graham-
avenue-project/
UNH Department of Civil Engineering. (2015, February 4). Design and Maintenance of Subsurface
Gravel Wetlands. Retrieved February 14, 2023, from
https://www.nh.gov/dot/org/projectdevelopment/materials/research/projects/document
s/15680W-FI NALREPORT.pdf
UNH Stormwater Center. (2009, June). Subsurface Gravel Wetland Design Specifications.
Retrieved February 12, 2023, from
https://www.unh.edu/unhsc/sites/unh.edu.unhsc/files/pubs_specs_info/unhsc_gravel_wet
land_specs_6_09.pdf
UNH Stormwater Center. (2016, June). Subsurface Gravel Wetland Design Specifications.
University of New Hampshire. Retrieved from
https://scholars.unh.edu/cgi/viewcontent.cgi?article=1013&context=stormwater
UNH Stormwater Center. (2022, January). UNHSC Subsurface Gravel Wetland Design
Specifications. Retrieved March 10, 2023, from https://scholars.unh.edu/stormwater/74/
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RDU SCM Implementation Strategy
USDA. (2007, May). Chapter 7 Hydrologic Soil Groups. Part 630 Hydrology:National Engineering
Handbook. Retrieved March 10, 2023, from
https://d i rectives.sc.egov.usda.gov/Open NonWebContent.aspx?content=17757.wba
USEPA. (2016, February). Operation and Maintenance Plan for Subsurface Gravel Wetland BMP
Retrofit for Control of Nitrogen in Storm water:An EPA Green Infrastructure Demonstration
and Outreach Project for Oyster Pond and the Town of Chatham, MA. Retrieved February
12, 2023, from https://19ja n ua ry2021snapshot.epa.gov/sites/static/files/2016-
03/documents/chatham-om-plan.pdf
USEPA. (2021, December). Storm water Best Management Practice:Stormwater Wetland. Retrieved
February 12, 2023, from https://www.epa.gov/system/files/documents/2021-11/bmp-
stormwater-wetland.pdf
West Virginia Department of Environmental Protection. (2012, November). West Virginia
Storm water Management and Design Guidance Manual. Retrieved February 12, 2023,
from
https://dep.wv.gov/WWE/Prog rams/stormwater/MS4/Documents/West_Virgi nia_Stormw
ater_Management_and_Design_Guidance_Manual_FULL_November_2012-v2.pdf
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RDU SCM Implementation Strategy
Appendix A: SGW Design Guidance Summary
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RDU SCM Implementation Strategy
Table 3: Design Guidance from State Regulatory Agencies, Permitted Projects, and University Research. Sources: (Mitchell, Hunt, &Waickowski, 2021) (Atlanta
Regional Commission, 2016) (Maryland Department of the Environment, 2009) (Maryland Department of the Environment, 2018) (DNREC, 2019) (Tennessee
Stormwater Management, 2015) (Knox County Tennessee, 2008) (West Virginia Department of Environmental Protection, 2012) (NJ Department of Environmental
Protection, 2021) (Maine Department of Environmental Protection,2016) (UNH Stormwater Center, 2022) (Houle&Ballestero, 2020).
Number of Choker Minimum Maximum Flood
Typical Drainage TSS TN
Wetland Gravel Total Depth Total Depth Elevation Retention TP Percent
State Layer Soil(in) Stone (in) of SGW of SGW Ponding POnding Area to Time(hr) Percent Percent Removal
Strata (in) Layers(in) Layers(in) Depth(in) Depth ft SGW Ratio Removal Removal
Y Y p ( )
North 3 6 3.5 12 21.5 36 5 2.4%
Carolina' z 92% 43% 67%
North
Carolina3
Georgia 2 12 24-48 36 60 2-3% 80% 20% 50%
Maryland 1 24-48 24 48 80% 56%4 66%
Delaware 3 8 4 24-48 36 24 80% 30% 40%
Tennessee 1 24 24 80% 20% 60%
West Virginia 1 24 24 95% 95%5 55%
New Jersey 3 8 3 24 35 24 90% 90%
Maine 3 8 6 24 38 18 3 5% 24-48
New 3 8 3 24 35 24-30 96% 54% 58%
Hampshire'
s Permitted project for the City of Greensboro.
2Calculated based on the project information and is not included in recommended guidance from NCSU.
3 NCDOT is planning to publish guidance on the SGW in their BMP toolbox this year.
4As part of a larger system or treatment train.
5On1y includes dissolved inorganic N.
6 Design information is from the UNH Stormwater Center and is not approved guidance from the state.
March 2023 (revised June 2023) 22