SW5231001_Stormwater Narrative_20240119

STORMWATER MANAGEMENT PLAN
FOR
RALEIGH-DURHAM AIRPORT AUTHORITY PE3 EXPANSION
PROJECT DESCRIPTION, PURPOSE, AND SUMMARY
This Project, located at the Raleigh Durham International Airport (RDU) in Wake County, North
Carolina, will consist of a proposed expansion of an existing surface parking lot containing 3,827
spaces to approximately 10,800 spaces. The additional parking lot will consist of approximately
7,000 new parking spaces, drive aisles, associated storm drainage and storm water
management facilities. The proposed expansion will have a total land disturbance of
approximately 120 AC. The proposed development primarily drains to Haley's Branch to the
East. Haley's Branch is part of the Crabtree Creek basin which is part of the Neuse River Basin.
The Project will be constructed in two main phases. The first phase will consist of the clearing
and grubbing of the area for the project. The second phase will consist of the remaining portion
of the expansion project which will encompass the final grading, curb & gutter, paving, lighting,
storm drainage, and SCM installation for the proposed lot expansion areas.
RDU falls under the jurisdiction of the State of North Carolina for permitting activities other than
building permits, which is through Wake County, NC. RDU is required to adhere the NCDEMLR
Post Construction Stormwater minimum requirements which consist of capturing and treating the
first one (1) inch of rainfall over the impervious area of the project and releasing the runoff at a
rate no greater than the pre-developed 1 YR-24HR storm event.
This project is on airport property, where SCMs that require standing water are not allowed per
FAA guidance due to the increase in potential for wildlife attractants, primarily birds. Additionally,
this project was intentionally designed to avoid impacts to existing stream and wetlands, creating
a "unique" parking lot shape. Other considered SCMs that meet the wildlife attractant criteria
either do not provide adequate treatment for the project's goals or are not feasible due to site
conditions.
The Raleigh-Durham Airport Authority(RDUAA) has also pursued a lengthy environmental review
policy and proposes to significantly exceed the minimum requirements set forth by the NCDEMLR
BMP manual for post construction stormwater requirements. RDUAA has not only set forth a plan
to treat the first one (1) inch of rainfall across the impervious area but to treat the 1-yr, 24-hour
storm event (2.83 inches). The SCMs shall also capture and release the following post
construction storm flows at or below the respective pre-developed design storm flow rates: the 1
YR, 2 YR, 5 YR, 10 YR, 25 YR, 50 YR and 100 YR storm events. As a part of this Project, RDUAA
is pursuing Envision certification through the Institute of Sustainable Infrastructure (ISI) to further
their commitment to sustainable development and low impact design (LID).
RDUAA will be using an innovative approach to water treatment and storage goals for the different
storm events. The proposed primary treatment measure will be a Submerged Gravel Wetland
(SGW). While this measure is currently adopted in various forms throughout several other states,
it is currently not a standalone SCM in North Carolina. However, NCDOT is adopting the practice
currently and several pilot programs have occurred throughout the state. These stormwater
controls have been used in surrounding states for storm water treatment and nutrient removal
with great success, achieving treatment results at or above current North Carolina standards in
most scenarios. The proposed measures will not only treat the runoff through the use of the
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Authority PE3 Expansion

forebay, the wetland plants, and the microorganisms in the gravel substrate, but will also provide
the storage within the SGW to contain and release stormwater at or below the pre-developed
storm events.
A Submerged Gravel Wetland Implementation Strategy (Strategy) was developed and shared
with NCDEQ earlier this year after a promising discussion with the NCDWR non-point source
group, who expressed great interest in this technology's application for this project. The Strategy
discusses the proposed approach for this project as well as synthesizes data on SGWs from North
Carolina and other states. It is attached to this application.
RDUAA worked with Dr. William Hunt at North Carolina State University regarding the use of the
SGW for this project. Dr. Hunt and his graduate students have studied this technology for several
years and have confirmed its effectiveness. Additionally, RDUAA will be engaging a local
university or consultant to monitor the installation and observe the efficiency of the measures for
research use by NCDEMLR.
RDUAA is seeking approval for this stormwater management approach as a one-off application
for a project that can provide not only improved water quantity and quality management, but also
valuable data for NCDEQ to use to hopefully adopt the practice for statewide use. RDUAA is
looking forward to working with NCDEQ on this matter. NCDEQ's existing primary design criteria
for bioretention cells and constructed wetlands have both been modified for application in this
project.
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EXISTING CONDITIONS
This project lies within the Neuse River Basin. The existing terrain of the project location
consists of a variety of differing slopes (2-30% slopes) and is currently woodland.
Runoff from this area collects in a series of drainage areas that drain to Haleys
Branch to the east and an unnamed tributary to the west. Haleys Branch is part of the
Crabtree Creek Basin which is part of the Neuse River Basin.
The soils within the proposed development area are classified by soils surveys as the
following hydrologic soils group: CID soils. For this analysis, Group C soil type was used
to be more conservative. A detailed Soils Survey Map for the Project is attached within
this document.
A Geotechnical Report for the proposed development has been performed by the
RDUAA. The Seasonal High-Water Table (SHWT) Geotechnical Report is included in this
document under Season High-Water Table at the locations for the SGWs. No
groundwater was encountered in the bore holes.
PROPOSED DEVELOPMENT
This development program will consist of the installation of the proposed parking lot,
lighting, storm drainage and submerged gravel wetlands. The total drainage area to
the proposed seven (7) SGWs will be approximately 78 AC. The total site is
approximately 145 AC. The BUA area for this development will equal
approximately 70% upon completion.
EXISTING REGULATORY REQUIREMENTS
This Stormwater Management Plan does fall in a NPDES Phase II Area. The RDUAA is
a quasi-municipal government and is required to adopt the minimum stormwater
requirements for the NPDES Phase II regulations as part of the Airport Authorities
Stormwater Management Program. The requirements for the Stormwater Management
Plan for this development require that the first 1" of rainfall over the site must be captured
and released over a 2 — 5-day period.
The Stormwater Management Plan quantity controls have been designed in accordance
with the NCDEMLR BMP manual, current edition. The Project is not subject to the Post
Construction ordinance water quantity provisions of the surrounding counties or
municipalities.
STORMWATER QUANTITY AND QUALITY ANALYSIS
In addition to the minimum requirement for the treatment of the first one (1) inch of rainfall,
RDUAA is proposing to treat the 1 YR-24 HR Storm. RDUAA would like to include the
provision of capturing the post developed peak runoff rate for the 1-YR, 2-YR. 5-YR,
10-YR, 25-YR, 50-YR and 100-YR 24-hour storm events back to, or below the pre-
developed rates for the same storm events. This design has also made provision
for the post
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developed 100 YR storm event volume to be attenuated in the management system and
released over a 2 — 5-day period.
The SGW design is based on the requirements for the bioretention cell with elements of
constructed wetlands. For the 1-YR storm event, water will be contained above the
surface of the plantings and wetland soil media and be drawn down within 48 hours
through the media into an underdrain system and discharged through an upturned elbow
within the outlet structure. Subsequent storm events will be controlled through a
rectangular weir placed above the elevation of the 1 YR storm stage. All the remaining
storm events use the rectangular weir and the media and have discharges that are well
below the pre-developed flow rates for the differing storm events. The SCMs will each
have an impervious liner beneath the gravel media layer and under the forebay areas.
The different outlet structures consist of concrete riser structures with a primary 6" or 4"
diameter drawdown orifice and a rectangular weir above the treatment volume for the 1-
YR storm. Varying class III reinforced concrete pipe sizes have been designed for the
outlet pipes. The outlet will drain onto a riprap outlet protection and overland flow to
existing drainage features that drain to Haleys Branch or the unnamed tributary. The
outlet pipes from the SGWs are sized to attenuate the 100 YR Storm Flow. The riprap
aprons for the outlet pipes from the SGWs are sized for the 100-YR storm event. The
SGWs will not have an earthen emergency spillway; the emergency spillway will be the
top of the concrete riser structure. The SGWs have been designed as a water quality and
quantity measure to reduce the overall post developed flow for the 1-YR, 2-YR. 5-YR, 10-
YR, 25-YR, 50-YR and 100-YR 24-hour storms discharging from the site. The 100 YR
storm frequency has a 1% chance of occurring in any given year. The SGW will pass the
1 YR storm event through the media without the use of the weir. The SGW will pass the
2-YR storm event with minimal use of the weir, and the majority of the storm event will
pass through the media.
A Geotechnical Report of the Seasonal High-Water Table (SHWT) for the Proposed
Development has been performed and no ground water was encountered. The seasonal
high-water table information is in the section called Seasonal High-Water Table. The soils
on the project site typically have a SHWT greater than 6 ft. and no groundwater or wet
soil was encountered during any of the borings for the site.
The storm water management basins were analyzed using Hydraflow software. The
drainage basins were modeled using the NRCS method to calculate runoff. The Pre-
Developed NRCS curve numbers were determined based upon the existing conditions.
The Post Developed NRCS number was determined based upon the proposed
impervious areas and open spaces. TR-55 methodology was used to best model the Time
of Concentration (Tc) for the pre-Developed drainage areas. The Post Developed
drainage areas were determined using the Kirpich Method for developed areas with a
minimum time of concentration of 5 minutes. The SGWs have been designed to capture
and release the post developed 1-YR, 2-YR. 5-YR, 10-YR, 25-YR, 50-YR and 100-YR
24-hour storm events at an equal to or less flow rate than the pre-developed storms. The
SGWs for the post developed drainage areas are designed to have a release rate at or
below the pre-developed runoff rates.
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CONCLUSION
The Project drains by concrete curb & gutter to a storm drainage system and onto the
SGWs through a forebay area. The stormwater is collected in an underdrain system in
the SGWs, exits through the outlet structure, and released as dispersed flow at the edge
of the existing drainage features up stream of Haleys Branch and the unnamed tributary
to Haley's Branch near the property lines.
The following table contains the Pre-Developed Drainage Area Flow, Post-Developed
Flow, Submerged Gravel Wetland Routed Flow combined with Post Developed Drainage
along with the Water Surface Elevation for each of the required storm events.
Summary of 1" Treatment Volume for
Submerged Gravel Wetland Ponds
Submerged 1" Treatment Water Surface
Gravel Volume Elevation
Wetland (CF)
POND 1 42,575.00 324.07
POND 2 47,738.00 321.07
POND 3 23,069.00 323.75
POND 4 46,830.00 340.05
POND 5 45,595.00 351.61
POND 6 24,241.00 337.46
POND 7 58,393.00 341.42
Summary of Pre/Post Peak Discharges
Submerged Gravel Wetland Pond 1
Storm Event Pre-Developed Post Developed Routed Post Water Surface
Flow Flow Developed Flow Elevation
1-YR 7.44 CFS 45.17 CFS 0.000 CFS 324.45
2-YR 11.03 CFS 58.21 CFS 0.070 CFS 324.92
5-YR 16.62 CFS 74.47 CFS 2.747 CFS 325.24
10-YR 21.25 CFS 87.20 CFS 6.577 CFS 325.62
25-YR 27.67 CFS 104.21 CFS 11.51 CFS 326.23
50-YR 32.83 CFS 117.57 CFS 19.22 CFS 326.67
100-YR 38.12 CFS 131.08 CFS 28.33 CFS 327.08
Summary of Pre/Post Peak Discharges
Submerged Gravel Wetland Pond 2
Storm Event Pre-Developed Post Developed Routed Post Water Surface
Flow Flow Developed Flow Elevation
1-YR 21.86 CFS 53.29 CFS 0.000 CFS 321.70
2-YR 31.89 CFS 65.78 CFS 0.844 CFS 321.94
5-YR 47.37 CFS 83.64 CFS 3.077 CFS 322.20
10-YR 60.10 CFS 97.63 CFS 5.587 CFS 322.60
25-YR 78.00 CFS 116.34 CFS 8.298 CFS 323.32
50-YR 92.38 CFS 131.04 CFS 12.27 CFS 323.83
100-YR 107.10 CFS 145.92 CFS 19.70 CFS 324.26
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Summary of Pre/Post Peak Discharges
Submerged Gravel Wetland Pond 3
Storm Event Pre-Developed Post Developed Routed Post Water Surface
Flow Flow Developed Flow Elevation
1-YR 10.73 CFS 25.75 CFS 0.000 CFS 324.40
2-YR 15.76 CFS 31.79 CFS 0.445 CFS 324.62
5-YR 23.54 CFS 40.42 CFS 1.893 CFS 324.82
10-YR 29.97 CFS 47.18 CFS 4.554 CFS 325.15
25-YR 38.84 CFS 56.22 CFS 7.311 CFS 325.78
50-YR 45.95 CFS 63.32 CFS 10.73 CFS 326.27
100-YR 53.24 CFS 70.51 CFS 17.42 CFS 326.68
Summary of Pre/Post Peak Discharges
Submerged Gravel Wetland Pond 4
Storm Event Pre-Developed Post Developed Routed Post Water Surface
Flow Flow Developed Flow Elevation
1-YR 9.617 CFS 55.13 CFS 0.000 CFS 340.57
2-YR 14.21 CFS 68.52 CFS 0.859 CFS 340.76
5-YR 21.34 CFS 87.66 CFS 3.072 CFS 340.97
10-YR 27.24 CFS 102.65 CFS 6.542 CFS 341.22
25-YR 35.40 CFS 122.67 CFS 14.26 CFS 341.65
50-YR 41.95 CFS 138.40 CFS 22.66 CFS 342.02
100-YR 48.67 CFS 154.30 CFS 31.79 CFS 342.39
Summary of Pre/Post Peak Discharges
Submerged Gravel Wetland Pond 5
Storm Event Pre-Developed Post Developed Routed Post Water Surface
Flow Flow Developed Flow Elevation
1-YR 25.73 CFS 50.90 CFS 0.000 CFS 352.20
2-YR 34.89 CFS 62.83 CFS 0.843 CFS 352.45
5-YR 48.83 CFS 79.89 CFS 3.300 CFS 352.69
10-YR 60.21 CFS 93.25 CFS 6.982 CFS 353.07
25-YR 75.84 CFS 111.11 CFS 12.06 CFS 353.70
50-YR 88.34 CFS 125.16 CFS 22.43 CFS 354.14
100-YR 101.11 CFS 139.36 CFS 35.09 CFS 354.55
Summary of Pre/Post Peak Discharges
Submerged Gravel Wetland Pond 6
Storm Event Pre-Developed Post Developed Routed Post Water Surface
Flow Flow Developed Flow Elevation
1-YR 9.557 CFS 28.54 CFS 0.000 CFS 338.30
2-YR 14.15 CFS 35.47 CFS 0.559 CFS 338.52
5-YR 21.29 CFS 45.38 CFS 2.602 CFS 338.73
10-YR 27.21 CFS 53.13 CFS 6.279 CFS 339.08
25-YR 35.47 CFS 63.50 CFS 17.62 CFS 339.74
50-YR 42.12 CFS 71.64 CFS 28.07 CFS 340.14
100-YR 48.95 CFS 79.87 CFS 38.84 CFS 340.49
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Summary of Pre/Post Peak Discharges
Submerged Gravel Wetland Pond 7
Storm Event Pre-Developed Post Developed Routed Post Water Surface
Flow Flow Developed Flow Elevation
1-YR 8.198 CFS 65.19 CFS 0.000 CFS 342.00
2-YR 12.14 CFS 80.47 CFS 1.051 CFS 342.18
5-YR 18.27 CFS 102.31 CFS 3.854 CFS 342.43
10-YR 23.35 CFS 119.42 CFS 7.135 CFS 342.80
25-YR 30.44 CFS 142.30 CFS 10.51 CFS 343.44
50-YR 36.15 CFS 160.29 CFS 13.80 CFS 343.95
100-YR 42.01 CFS 178.48 CFS 20.55 CFS 344.37
The post-developed flows for all post-developed drainage areas discharge at a rate less
than the pre-developed storm flows. The outflow from the SGWs will discharge at a non-
erosive velocity and will not have a negative impact to Haleys Branch or the unnamed
tributary to Lake Crabtree. The post developed 100-YR 24 HR Storm is attenuated within
all the proposed SGWs. See attached calculations.
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