SW1210901_Stormwater Narrative_20210914

Section 1 PROJECT OVERVIEW
1.1 Narrative
The Asheville Regional Airport - Parking Lot D project includes the construction of a new parking lot,
Lot D, 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 Asheville Regional Airport in Fletcher, NC, south of Asheville, North Carolina near
Airport Road (35° 26' 13.99" N. 82° 32' 8.39"W).
The area that includes airport related operations
encompasses approximately 580 acres. For a larger
vicinity map, see Appendix A. The Asheville
Regional Airport Parking Lot D project site is located
on an existing pad graded area in the eastern
portion of the airport, located along Airport Road.
It is just north of the Lot A project under permitting
in early 2020.
For hydrologic planning purposes, the site is
comprised of Hydrologic Soil Group (HSG) B soils.
Appendix A contains the NRCS Soil Survey Maps for
0
Asheville
Regional
a,....a.m Airport
0
m
the parking lot. The existing land use of the project
is grass, gravel, forest, and impervious. The proposed land use will impervious (parking lot) with a
small amount of grass for landscaping.
As shown in the 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 cell 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 and the cleanouts
and media will be installed. 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 REGIONALAIRPORT—PARKING LOT D
INITIAL SUBMITTAL: JULY 2021
STORMWATER MANAGEMENT REPORT

Section 2
HYDROLOGY
2.1 Project Methodology
In order to qualify for a primary Stormwater Conveyance Measure (SCM) for a bioretention area,
several minimum design criteria (MDC) must be met. The recommendations will also be met to
maximum extent practical.
MDC 1 Separation from the SHWT
Boring logs for the neighboring parking Lot A did not observe water levels at 10-15ft depths so
the season high water table for Lot D is assumed to be similar geology. Therefore, the lowest point
of the bioretention cell shall 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. See plans with spillways
between each cell at the 12 inch depth.
MDC 3 Peak Attenuation Volume
The maximum depth of the entire cell is 24 inches for peak attenuation. See plans with spillways
at the 12 inch depth and the overall dam height of 24 inches.
MDC 4 Underdrain
An underdrain with internal water storage is designed. See plan view on SW-01 and details on
SW-03 showing the underdrain location and details
MDC 5 Media Depth
The media depth is 36 inches, see detail on SW-03 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-03 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-03.
MDC 9 Maintenance of Media
The bioretention cell shall be maintained in a manner that results in a drawdown of at least one
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ASHEVILLE REGIONAL AIRPORT- PARKING LOT D INITIAL SUBMITTAL: JULY 2021
STORMWATER MANAGEMENT REPORT

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 C3 by Landart.
MDC 11 Mulch
Because the bioretention cell is covered in vegetation, triple shredded hardwood mulch shall be
used in the bottom 12 inches. Mulch shall be placed two to four inches deep.
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 plan and details, sheets SW-01 through SW-03, there are 5 cleanouts in the
bioretention cell.
Recommendation 1 Dispersed Flow or Energy Dissipation
The parking lot runoff will enter the bioretention via sheet flow from the parking lot surface.
Recommendation 2 Pretreatment
Runoff from the parking lot flows over 8 inches of gravel followed by 3ft of sod, as seen on Detail
on SW-03.
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:
a& Comm)
21m 21m 111 %
1�00 Is W -1 •;
15_DO 15= 2k-yf
1Z00 WE WyF
sm am S_YF
6_00 G-W 2-VIr
3Lw 3-00 •;I i"
0m - 0.00
0 5 10 1s 20 M 30 35 40 16 W % W
T� (EM)
The following runoff coefficients were also used for hydraulic (storm inlet, pipe) analysis of Lot A:
c = 0.30 (Grass, HSG B)
c = 0.50 (Gravel, HSG B)
c = 0.95 (Impervious area)
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ASHEVILLE REGIONAL AIRPORT- PARKING LOT D INITIAL SUBMITTAL: JULY 2021
STORMWATER MANAGEMENT REPORT

2.2 Drainage & Erosion Control Design
Because the parking lot sheet flows, there is no drainage (pipes, inlets, channels) designed on this
project. Erosion Control design and calcs can be found in Appendix C.
2.3 Operation and Maintenance Manual
The bioretention area requires maintenance in order 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 analysis and development of the stormwater
and documents:
• Hydraflow Hydrographs for AutoCAD Civil 3D 2019
• Hydraflow Storm Sewers for Auto CAD Civil 3D 2017
• AutoCAD Civil 3D 2018
• Microsoft Office 2017
The following design manuals and literature were used in analysis and development of the stormwater
documents:
• NCDEQ Stormwater Design Manual— 2017 Edition
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ASHEVILLE REGIONALAIRPORT-PARKING LOT D
INITIAL SUBMITTAL: JULY 2021
STORMWATER MANAGEMENT REPORT