SW1240603_Stormwater Narrative_20240830

Section 1 PROJECT OVERVIEW
1.1 Narrative
The Greater Asheville Regional Airport Authority ATCT and TRACON project includes the construction
of a perimeter road, a new parking lot, a TRACON building, and associated grading, drainage,
landscaping, and erosion control.
This report includes the computations that show compliance with the NCDEQ High-Density Project.A
Primary SCM (Bioretention cell) will be used to treat the stormwater runoff from the proposed site.
1.2 Introduction and Site Description
The site is located at Greater Asheville Regional Airport in Fletcher, NC, south of Asheville, North
Carolina near Airport Road(35°26' 17.43" N,82°32'
13.40"W).
The area that includes airport-related operations
encompasses approximately 580 acres. For a larger
vicinity map,see Appendix A. The Greater Asheville
Regional Airport Authority ATCT and TRACON Asheville
project site is located on an undeveloped area in Regional
the western portion of the airport, located at the Airport
end of Westfeldt Drive. f
For hydrologic planning purposes, the site is
comprised of Hydrologic Soil Group (HSG) B soils.
Appendix A contains the NRCS Soil Survey Maps for
the parking lot.The existing land use of the project
is grass,gravel,forest,and impervious.The proposed land use is impervious(perimeter road,TRACON
building, and parking lot) with some grassed area for landscaping and bioretention.
As shown in FEMA FIRM 3700964300K(January 6, 2010) in Appendix A,the disturbance is not located
within 200ft of a FEMA floodplain.
1.3 Construction Sequence and Cost
As explained later in the report, a bioretention cell is proposed as a primary stormwater conveyance
measure (SCM). The rough outline of the permanent bioretention area will occur during the grading
phase of the project and a portion of it will be used as a temporary sediment trap to protect
downstream receiving stormwater systems from construction activity.After the site is stabilized, the
sediment trap will be cleaned out and re-graded to final bioretention design grades. No mechanical
stabilization will occur. The cost of the bioretention cell is unknown at this time and will be finalized
during bidding. The Asheville Regional Airport will be the end-user and responsible for annual
inspection, maintenance, and repairs of the system. Section 2.3 discusses the operation and
maintenance of the cell.
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ASHEVILLE REGIONAL AIRPORT-ATCT AND TRACON PROJECT INITIAL SUBMITTAL:MAY 2024
STORMWATER MANAGEMENT REPORT

Section 2 HYDROLOGY
2.1 Project Methodology
In order to qualify for a primary Stormwater Conveyance Measure (SCM) for Bioretention, several
minimum design criteria (MDC) must be met.The recommendations will also be met to the maximum
extent practical.
MDC 1 Separation from the SHWT
Boring logs for the project area observed SHWT 7ft to 10ft below the existing grade surface. The
lowest point of the bioretention cell will be a minimum of two feet above the SHWT.
MDC 2 Maximum Ponding Depth for Design Volume
The maximum ponding depth for the bioretention cell is 12 inches.
MDC 3 Peak Attenuation Volume
The maximum depth of the entire cell is 18 inches for peak attenuation. See plans with outlet
control structure weirs at 12-inch and 18-inch depths.
MDC 4 Underdrain
An underdrain with internal water storage is designed. See the plan view on SW-01 and details
on SW-02 and SW-03 showing the underdrain location and details.
MDC 5 Media Depth
The media depth is 36 inches, see detail on SW-02 showing media depth.
MDC 6 Media Mix
The media shall be a homogeneous soil mix engineered media blend with approximate volumes
of:
(a) 75 to 85 percent medium to coarse washed sand (ASTM C33,AASHTO M 6/M 80,
ASTM C330,AASHTO M195, or the equivalent)
(b) 8 to 15 percent fines (silt and clay); and
(c) 5 to 10 percent organic matter (such as pine bark fines).
See detail on SW-02 showing media mix.
MDC 7 Media P-Index
The phosphorus index(P-index) for the media shall not exceed 30 in NSW waters as defined in
15A NCAC 02B .0202 and shall not exceed 50 elsewhere.
MDC 8 No Mechanical Compaction
The media shall not be mechanically compacted. It is recommended to either water it or walk on
it as it is placed.This note is on the detail, sheet SW-02.
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ASHEVILLE REGIONAL AIRPORT-ATCT AND TRACON PROJECT INITIAL SUBMITTAL:MAY 2024
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MDC 9 Maintenance of Media
The bioretention cell shall be maintained in a manner that results in a drawdown of at least one
inch per hour at the planting surface. This is accomplished by not installing until the end of the
project phase, installing the correct soil type as outlined above,and not mechanically compacting.
MDC 10 Planting Plan
A planting plan can be found in Appendix C2 by Landart.
MDC 11 Mulch
Water will enter the bioretention at less than 3 fps (see Recommendation 1 narrative below).
Therefore, sod is proposed in the bioretention area.
MDC 12 Clean-Out Pipes
A minimum of one clean-out pipe shall be provided on each underdrain line.Clean-out pipes shall
be capped. As seen on the plan and details, sheets SW-01 through SW-03, a cleanout is provided
in the bioretention cell.
Recommendation 1 Dispersed Flow or Energy Dissipation
The parking lot, building, and associated impervious runoff will enter the bioretention via pipe
flow into two proposed forebays.The forebays are flat settling areas with spillways to spread the
flow evenly into the bioretention portion, at an outlet velocity of less than 3 fps.
Recommendation 2 Pretreatment
The parking lot, building, and associated impervious runoff will enter the bioretention via pipe
flow into a proposed forebay. The forebay will be 18 inches deep.
The Rational Method was used for the parking lot design on the development.The Intensity Duration
Frequency(IDF) curve for Fletcher, NC below was used for the calculations:
mac•
21_00 21.00 1st Yr
1s-00
15_00 \ - - _ 15.00 25 Yt
1200 12 00 1 Lri
9.00 9.00 yri
6m 6_00 n
3m 3.00
OAO I I I 10 00
0 5 10 15 20 25 30 35 40 45 50 55 60
Tie(moo)
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ASHEVILLE REGIONAL AIRPORT-ATCT AND TRACON PROJECT INITIAL SUBMITTAL:MAY 2024
STORMWATER MANAGEMENT REPORT

The following runoff coefficients were also used for hydraulic (storm inlet, pipe) analysis of this
project:
c=0.30 (Grass, HSG B)
c=0.95 (Impervious area)
The bioretention was designed to treat a total of 6.13 acres. See below for volume calculations, the
proposed bioretention cell will provide adequate volume per the tables below:
Design Volume-Required
Drainage Impervious Rv RD(Design Design Volume
Area Area IA(Impervious Fraction) (0.05*0.9 Storm Depth) (3630*RD*Rv*A)
(ac) (ac) *IA) (in) (cf)
Bioretention 1 6.13 1.12 0.18 0.21 1.0 4,772
Design Volume-Provided
Design Volume Surface Area Surface Area Design Volume
Depth Provided
Required (ft) Required Provided Provided
(cf) (sf) (sf) (cf)
Bioretention 1 4,772 1.0 4,772 5,899.0 7,251
2.2 Drainage & Erosion Control Design
On the northeastern side of the project area, 54" RCP is proposed in the ditch on the southwest side
of the existing gravel road. The 54" RCP line continues south along the ditch and eventually turns
south under Perimeter Road after the proposed Westfeldt round-a-bout. 15" and 18" RCP systems
are proposed along the eastern side of Perimeter Road and west of the proposed building and parking
area. Each of the lines collects water via curb inlets and travels south. At the southeastern corner of
the parking lot, the 18" eastern line turns west and outfalls into the bioretention area. The 15" RCP
from the western side of the project site travels south and outfalls into the northwestern side of the
bioretention area.Additionally, a 36" RCP is proposed from the bioretention outlet control structure.
This 36" RCP flows southwest and ties into the 54" RCP line, which continues along the proposed
Westfeldt Drive. These pipes have been designed to safely convey the 25-year storm. Pipe profiles
and Pipe capacity computations can be found on sheet SW-01 of the stormwater plans(Appendix C1).
2.3 Operation and Maintenance Manual
The bioretention area requires maintenance to work properly. An O&M plan has been developed and
can be found in Appendix D.
2.4 Design Software and Manuals
The following design software programs were used in the analysis and development of the
stormwater and documents:
• Hydraflow Hydrographs for AutoCAD Civil 3D 2021
• AutoCAD Civil 3D 2023
• Microsoft Office 2021
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ASHEVILLE REGIONAL AIRPORT-ATCT AND TRACON PROJECT INITIAL SUBMITTAL:MAY 2024
STORMWATER MANAGEMENT REPORT

The following design manuals and literature were used in the analysis and development of the
stormwater documents:
• NCDEQ Storm water Design Manual—2017 Edition
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ASHEVILLE REGIONAL AIRPORT-ATCT AND TRACON PROJECT INITIAL SUBMITTAL:MAY 2024
STORMWATER MANAGEMENT REPORT