SW1230601_Stormwater Narrative_20231030

Section 1 PROJECT OVERVIEW
1.1 Narrative
The Asheville Regional South Parking Lot project includes the construction of a new parking lot,
driveway, sidewalk, and associated grading, drainage, landscaping and erosion control.
This report includes the computations that show compliance with the NCDEQ High Density Project.A
Secondary SCM (Dry Pond) 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
New Airport Road (35° 25' 42.94" N, 82° 32'
6.81"W).
The area that includes airport related operations
encompasses approximately 900 acres. For a larger
vicinity map, see Appendix A. The Asheville South
Parking Lot project site is located is a wooded area Asheville
in the southwest region of the airport,located along Regional
New Airport Road. Airport
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 will impervious(parking lot, driveway,and sidewalk) and grass for landscaping and
bioretention.
As shown in the FEMA FIRMs 3700964300K and 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 dry detention pond is proposed as a secondary stormwater
conveyance measure(SCM).The rough outline of the permanent dry detention pond 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 dry detention pond design
grades. No mechanical stabilization will occur.The cost of the dry detention pond 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 pond.
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Section 2 HYDROLOGY
2.1 Project Methodology
In order to qualify for a secondary Stormwater Conveyance Measure (SCM) for a Dry Pond, several
minimum design criteria (MDC) must be met. The recommendations will also be met to maximum
extent practical.
NCDEQ Stormwater Design Manual Appendix E-4 Airports, Table 1 "SCMs that do not Pond Water
with Suggestions for Airport Application". Below is a list justifying why the suggest SCMs were
implausible for this site:
Bioretention Cell requires 2ft of separation between its bottom and the SHWT along with a
minimum of 2.5ft of soil underneath the surface layer. According to the soil report, SHWT was
found 1-5ft below the surface.Therefore, with the required depth of soil and bedrock so shallow
beneath the surface, a bioretention cell is not plausible.
Infiltration System requires 2ft of separation between its bottom and the SHWT.According to the
soil report, SHWT was found 1-5ft below the surface. Therefore, a layer of stone would not be
plausible which is insufficient for an adequate infiltration system.
Sand Filter requires 1-2ft of separation between its bottom and the SHWT along with a minimum
of 1.5ft of filter bed + 1ft of sand above the underdrain pipe. According to the soil report, SHWT
was found 1-5ft below the surface. Therefore, with the required amount of filter bed + sand, a
sand filter is not plausible.
Rainwater Harvesting is infeasible because the proposed project is only a parking lot that does
not include a building, so roof drainage will not occur on the site.
Permeable Pavement is not feasible with the amount of snow at this location. Airport
maintenance requires salts and deicers for safety precautions. The use of these additives is
prohibited for permeable pavement and therefore makes this SCM infeasible.
Treatment Swale (Secondary) the minimum inlet to outlet flow length of 100ft could not be
achieved due to the limited green space that this project provides.
Neither a Wet Pond nor a Stormwater Wetland are recommended at airports. These SMCs create
standing water that attracts wildlife that pose safety risks as they may damage aircrafts. See Figure 3
in Appendix E-4 of the NCDEQ Stormwater Design Manual for data relating to this potential hazard.
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Below are the MDC for the proposed Dry Pond SCM, and how they have either been met or could
not be met:
MDC 1:Separation from the SHWT
The existing ground surface(EGS)elevation is 2081, and the estimated SHWT depth from the EGS
is 20 inches, meaning the estimated depth is at 2079.3. The dry pond bottom elevation is 2080.5,
therefore,the lowest point of the dry pond will be at least 6 inches above the SHWT.
MDC 2:Temporary Pool Depth
There is no temporary pool in this dry detention pond due to its location at an airport (See
Appendix E-4 of the NCDEQ Stormwater Design Manual, therefore the maximum depth of 10ft is
not a concern.
MDC 3: Uniform Grading and Positive Drainage
The bottom of the dry pond will be graded uniformly to flow toward the outlet control structure
without low or high spots, per the grading pond and detail.
MDC 4: Location of Inlet(s) and Outlet
The inlets and outlet of the pond are in a manner that will avoid short circuiting.The outlet control
structure is the furthest point of the pond thus providing the longest flow path of water until the
outlet location. See appendix C for outlet protection calculations performed by AVCON.
MDC 5: Pretreatment
A forebay is provided as pretreatment to settle sediment and prevent erosion before the runoff
enters the dry detention pond.
MDC 6: Drawdown Time
The pond outlet control structure has an 2.5" orifice to provide drawdown of 40 hours. The
calculations show an orifice size of 2.3", but a 2.5" will be used for constructability. See
calculations below:
Drawn Down Calcs
Draw Qavg Start
Vol Vol Vol g Draw Down
Down Qavg Ext. Qcp. Havg A Detention
Pond ID Provided Bottom Top C (ft/ Orifce and
Time (cfs) Det. (cfs) (ft) (ft^2) Orif/Weir
(cf) Elev. Elev. s^2) Elevation
(hr) (cfs) @ Elev.
Dry Pond 27,195 2080.5 2084.5 40 0.19 0.0 0.19 2.0 0.6 32.2 0.028 2.3 @ 2080.5 2084.5
MDC 7: Protection of the Receiving Stream
The addition of the parking lot, sidewalk, and roads results in an increase of 5.49acres of
impervious in the onsite drainage area. 0.16 acres of this proposed impervious will bypass the
pond. The pond detention resulted in a decrease of the post project flow rate in the 1 year, 24-
hour storm. See delineations and flow rates in the following tables:
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PRE-CONSTRUCTION
Grassed Forest
Total I mperv.
Drainage Area Area (HSG B) (HSG B) C CN Tc
(AC) 0.95 0.30 0.15 (min)
98 69 55
Pre 1 6.80 0.10 0.92 5.88 0.18 58 10
Pre 2 326.46 166.49 152.14 7.82 0.63 83 60
POST-CONSTRUCTION
Grassed Forest
Total I m pe rv.
Drainage Area Area (HSG B) (HSG B) C CN Tc
(AC) 0.95 0.30 0.15 (min)
-
98 69 55
Post 1(to Dry Pond) 6.80 5.43 1.37 0.00 0.82 92 5
Post 2 (Pond Bypass) 326.46 166.65 151.98 7.82 0.62 83 60
TOTAL 333.26 172.08 153.36 7.82
Drainage area 2 includes the addition of 0.16ac of impervious surface.This 0.16ac will bypass the
drainage area 1 dry pond SCM. Drainage area 2 is shown in these tables and on sheet SW-06 to
show that this 0.16ac of proposed impervious will not change the existing curve number in this
basin and therefore result in no adverse impacts at the study point or downstream. The 0.16ac
increase is within a 326.5ac watershed, so the increase is negligible. This drainage area 2 is not
included in the Supplemental EZ form or the Stormwater Management Permit Application form
as it is not a drainage area to a proposed SCM.
PEAK FLOW RATES
Storm Return Pre-Construction Post-Construction Post Project
Study Point Frequency Flow Total Flow Total Decrease
(yr) (cfs) (cfs) (%)
1 1 1.0 0.6 46.3
MDC 8: Outlet
There is no permanent pool in this dry detention pond due to its location at an airport. A trash
rack is provided to prevent trash and debris from entering the outlet system.
MDC 9:Vegetation
A note on detail sheet SW-03 with the MDC 9 dry pond vegetation requirements as show in the
manual. Fescue is proposed in the parking lot landscape areas and the dam.
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AVL SOUTH PARKING LOT INITIAL SUBMITTAL:APRIL 2023,REVISION 1:OCTOBER 2023
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Recommendation 1: Freeboard
The pond will provide more than one foot of freeboard above the design pool elevation. The
elevation of the design pool volume is 2083.2 and the pond top of dam is 2086.0.
The dry pond was designed to treat all of the proposed parking lot.See below for volume calculations,the
proposed dry pond will provide adequate volume per the table below:
Design Volume - Provided
Design Volume Design Volume
Drainage Area Required Provided
(cf) (cf)
Dry Pond 18,644 27,195
Dry Pond
Proposed
Stage Elev
Storage
ft ft (cf)
0.0 2080.5 0
0.5 2081.0 207
1.5 _ 2082.0 3,215
2.5 _ 2083.0 10,369
3.5 2084.0 21,096
4.0 _ 2084.5 27,195
4.5 2085.0 33,294
5.0 _ 2085.5 40,160
5.5 2086.0 47,025
The Rational Method was used for the parking lot design on the development. Onsite drainage
calculations were performed by AVCON and not Aulick Engineering.The Intensity Duration Frequency
(IDF) curve for Fletcher, NC below was used for the calculations.:
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AVL SOUTH PARKING LOT INITIAL SUBMITTAL:APRIL 2023,REVISION 1:OCTOBER 2023
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I tc•Mw
2L00 2100 1e.n
1iD0 10_00 ;,
15.50 _ _ 15A0 2S Y,
1200 - 1200 18-Yr
S AD 9.00 S n
Om 5A0 2 Y.
3.00 3.0o 115
0m 1 0,00
0 5 10 15 20 25 30 75 40 45 50 55 W
Tine(mil)
The following runoff coefficients were also used for hydraulic (storm inlet, pipe) analysis of this
project:
c=0.15 (Forest, HSG B)
c=0.30 (Grass, HSG B)
c=0.95 (Impervious area)
2.2 Drainage& Erosion Control Design
Drainage designed was performed by AVCON but a brief description is provided here. 24" RCP and
36" RCP is proposed along the west edge of the parking lot.Stormwater runoff from the entire parking
lot will sheet flow into curb inlets along the west edge. Water will then be conveyed through the 24"
RCP and eventually 36" RCP into a dry pond forebay, then into the dry pond itself. Stormwater will
outfall from the dry pond into the existing creek through an OCS and 18" RCP. These pipes have been
designed to safely convey the 25-year storm. AVCON pipe capacity computations can be found in
Appendix C3.
2.3 Operation and Maintenance Manual
The dry pond 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 2021
• AutoCAD Civil 3D 2021
• Microsoft Office 2021
The following design manuals and literature were used in analysis and development of the stormwater
documents:
• NCDEQ Storm water Design Manual—2017 Edition
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AVL SOUTH PARKING LOT INITIAL SUBMITTAL:APRIL 2023,REVISION 1:OCTOBER 2023
STORM WATER MANAGEMENT REPORT