SW1190901_Combined_20190910

There are no Deed Restrictions and Protective Covenants

Processing fee $505 will be sent in via check

Section 1 PROJECT OVERVIEW
1.1 Narrative
The Asheville Regional Airport - Parking Lots project includes the construction of two new parking
lots, Lot A and Lot B, and associated grading, drainage, stormwater, landscaping and erosion control.
This project also includes construction of two dry stormwater management ponds.
This report includes the stormwater design calculations and facilities that will be installed to meet
Buncombe County requirements. The report will also review the stormwater impacts on the
property and downstream to ensure no adverse impacts will result from the construction.
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' 10.38" N, 82° 32' 8.61"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 Lots project site is located
on an existing undeveloped area in the eastern
portion of the airport, located along Airport Road.
9eo Asheville
¢,
a ,... Regional
0
Airport
For stormwater management planning purposes,
the site is comprised of Hydrologic Soil Group (HSG)
A and B soils. Appendix A contains the NRCS Soil
Survey Maps for each parking lot. The existing land
use of the project is grass, gravel, forest, and
impervious. The proposed land use will impervious (parking lots) with a small amount of grass for
landscaping.
As shown in the FEMA FIRM 3700964200K (January 6, 2010) and 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, stormwater management ponds are proposed to reduce post -
construction flow rates to pre -construction levels. The construction of the permanent stormwater
management measures will occur during the grading phase of the project and will be used as
temporary sediment basins to protect downstream receiving stormwater systems from construction
activity. After the site is stabilized, the measures will be cleaned out and re -graded to final design
grades. The cost of pond Al and B1 are 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 3.3 discusses the operation and maintenance of the ponds.
3
ASHEVILLE REGIONAL AIRPORT- PARKING LOTS
INITIAL SUBMITTAL: 8/7/19
STORMWATER MANAGEMENT REPORT REVISION #1: 8/20/19

Section 2
HYDROLOGY
2.1 Project Methodology
The NRCS Curve Number Method (also known as the SCS Method) was used to determine Pre -
Construction and Post -Construction peak flow rates. The NRCS Method applicable to this area is based
on a 24-hour storm event that has a Type II time distribution with a 484 shape factor. The software
program, Hydraflow Hydrographs, was used to route the system through a series of hydrographs and
determine the effects of the proposed improvements.
The following Precipitation Table for Fletcher, NC from NOAA's National Weather Service
Hydrometeorological Design Studies Center Precipitation Frequency Data Server was used for the
hydrologic analysis. See full NOAA Atlas 14 Precipitation Table in Appendix A. The Buncombe County
1-yr storm post -construction requirement was followed for this report but the 25- and 100-yr storms
were checked for capacity and safe conveyance.
24-hr Precipitation Amount (in)
1-yr
25-yr
100-yr
2.91
5.82
7.28
The following curve numbers were used for hydrologic analysis of Lot A:
CN= 61 (Open Space, good cond., HSG B) CN= 98 (Impervious area)
CN= 85 (Gravel, HSG B)
The following curve numbers were used for hydrologic analysis of Lot B:
CN= 39 (Open Space, good cond., HSG A) CN= 98 (Impervious area)
CN= 76 (Gravel, HSG A)
4
ASHEVILLE REGIONAL AIRPORT- PARKING LOTS
INITIAL SUBMITTAL: 8/7/19
STORMWATER MANAGEMENT REPORT REVISION #1: 8/20/19

The Rational Method was used for the parking lots' pipe design on the development. The Intensity
Duration Frequency (IDF) curve for Fletcher, NC below was used for the calculations:
1A
21j00 11 I YF
9-00 to
s-0o a V�
&W
0.na
10 115 20 25 30 35 40 16 60 Ri W
T� (mill)
The following runoff coefficients were also used for hydraulic (storm inlet and pipe) analysis of Lot A:
c = 0.30 (Grass, HSG B)
c = 0.50 (Gravel, HSG B)
c = 0.95 (Impervious area)
The following runoff coefficients were also used for hydraulic (storm inlet and pipe) analysis of Lot B:
c = 0.20 (Grass, HSG A)
c = 0.45 (Gravel, HSG A)
c = 0.95 (Impervious area)
9
ASHEVILLE REGIONAL AIRPORT - PARKING LOTS
INITIAL SUBMITTAL:8/7/19; REVISION 1: 8/20/19
STORMWATER MANAGEMENT REPORT

2.2 Pre -Construction Hydrology
Parking Lot A
The existing site consists of a grassed area east of the airport that contains a cell tower and a road to
service the cell tower. The offsite areas to the north and east that flow through the subject site consist
of a parking lot, a hotel, a gas station, and a car dealership. Overall Basin Al is divided into four (4) sub
basins that all converge at an existing 36" pipe that flows under Highway 280. Basin A1.1 contains the
parking, gas station, hotel, and car dealership. The runoff generated by this basin sheet flows to the
existing drainage network where it is routed to the existing detention pond. Basin A1.2 is the northeast
portion of the grassed area which flows to an existing headwall where it is then routed to the existing
detention pond. Basin A1.3 is the southeast portion of the grassed area; its runoff sheet flows to an
existing headwall where it is then routed to the existing detention pond. Basin A1.4 occupies the
northwest portion of the grassed area and it also flows directly to the existing 36" pipe outlet.
Basin A2 is a separate sub watershed that does not exit the property at the 36" pipe but rather flows
south along Highway 280. Al and A2 converge downstream of airport property and eventually discharge
into the French Broad River but for purposes of this report they are separated outfall points.
Parking Lot B
The existing site consists of a wooded area, a gravel parking lot, and a portion of Wright Brothers Way.
The site is divided into two drainage basins: Basin B1 and Basin B2. Basin B1 is the western portion of the
basin and it is occupied by a portion of the wooded area, the entire gravel parking lot, and a portion of
Wright Brothers Way. The runoff generated by Basin B1 sheet flows to the west and onto Wright Brothers
Way where it is collected by grate inlets along the curb line. The runoff then flows to the south where it
eventually leaves the site in a pipe that runs underneath Wright Brothers Way.
Basin B2 is a separate basin that lies to the southeast of Basin B1. Basin B2 is occupied solely by a wooded
area. The runoff generated by Basin B2 sheet flows to the east where is collected by a ditch along Highway
280. The ditch flows to the south where it exits the site. B1 and B2 flow to the same spot down Wright
Brothers Way and also merge with Al and A2 at the French Broad River.
Appendix B shows the overall Pre -Construction map. The table below illustrates the drainage areas,
weighted curve number, and Times of Concentration.
PRE -CONSTRUCTION
Basin
Total
Area
AC
I mperv.
Grassed
Grassed
Gravel
Gravel
C
CN
Tc
min
HSG A,
(HSGB,
(HSGB.
HSG A.
0.95
98
Lot B)
0.20
39
Lot A)
0.30
61
Lot A)
0.50
1 85
Lot B)
0.50
76
Pre Al
13.2
6.1
6.6
0.5
0.61
79
7
Pre A2
0.4
0.4
0.30
61
5
Pre 131
1.7
0.4
0.9
0.4
0.46
63
5
Pre 132
0.4
0.4
0.20
39
5
TOTAL
15.78
6.53
1.27
7.01
0.42
6
ASHEVILLE REGIONAL AIRPORT - PARKING LOTS
INITIAL SUBMITTAL:8/7/19; REVISION 1: 8/20/19
STORMWATER MANAGEMENT REPORT

2.3 Post -Construction Hydrology
Parking Lot A
The proposed project will replace most of the grassed area with paved parking lots. Post construction
there will be two drainage basins, Basin Al and Basin A2. Basin Al occupies a similar area to pre -
construction basin Al and is divided into three sub basins, Al and A2 and A3. Al is the same runoff
from the north that flows into the existing 30" CMP system. In post -construction the new Parking Lot A
North connects to this system and into the existing pond. The existing pond will be regraded to expand
its capacity and a new Outlet Control Structure (OCS) will be constructed to ensure the post -
development flow is less than the pre -construction flow for the 1-year storm. Basin A2 collects runoff
from the east hotel and undisturbed grass area from the east through new pipes and into the
stormwater pond. A3 will be collected in the new Parking Lot A South drainage system and then routed
to the stormwater pond. The runoff will exit the site out of an existing 36" CMP pipe that flows under
Highway 280.
Basin A2 occupies a similar area to pre -construction Basin A2, but the area is reduced. Because of the
reduction of area, no stormwater management is needed for the region. The runoff from Basin A2 flows
to the south along Highway 280. Al and A2 converge downstream of airport property and eventually
discharge into the French Broad River but for purposes of this report they are separate outfall points.
Parking Lot B
The proposed project will replace the existing gravel parking lot with an expanded paved lot. Post -
construction there will be two drainage basins, B1 and B2. Basin B1 will occupy a similar area to pre -
construction Basin B1 is entirely the proposed Parking Lot B. The runoff generated by Basin B1 will be
collected by a proposed drainage system and routed to a new proposed detention pond. The proposed
pond will contain an Outlet Control Structure (OCS) to ensure the post -construction flow is less than the
pre -construction flow for the 1-year storm. The pond will contain a 15" concrete outlet pipe and will tie
into the existing drainage system and exit the site through a pipe under Wright Brothers Way at the
same location as pre -construction.
Basin B2 will occupy a similar area to pre -construction Basin B2. Basin B2 is comprised of a grassed area
and a small portion of the proposed parking lot. Runoff generated by Basin B2 will sheet flow to the east
where it will be collected by a ditch running alongside Highway 280. The ditch flows to the south where
it exits the site. B1 and B2 flow to the same spot down Wright Brothers Way and merge with Al and A2
at the French Broad River.
7
ASHEVILLE REGIONALAIRPORT-PARKING LOTS
INITIAL SUBMITTAL:8/7/19; REVISION 1: 8/20/19
STORMWATER MANAGEMENT REPORT

Appendix B shows the overall Post -Construction map and Appendix C contains Grading and Drainage
Plans, Stormwater Details, and Pipe Profiles. The table below illustrates the Post -Construction drainage
areas, weighted curve number, and Times of Concentration.
POST -CONSTRUCTION
Basin
Total
Area
AC
Imperv.
Grassed
Grassed
Gravel
Gravel
C
CN
Tc
min
HSG A,
(HSGB,
(HSGB.
HSG A.
0.95
98
Lot B)
0.20
39
Lot A)
0.30
61
Lot A)
0.50
85
Lot B)
0.45
76
Post Al
13.5
8.5
5.0
0.5
0.73
88
7
Post A2
0.2
0.2
0.30
61
5
Post B1
1.7
1.0
0.7
0.64
73
5
Post B2
0.4
0.1
0.3
0.39
54
5
TOTAL
15.78
9.60
1.02
5.16
0.00
ASHEVILLE REGIONALAIRPORT-PARKING LOTS
INITIAL SUBMITTAL:8/7/19; REVISION 1: 8/20/19
STORMWATER MANAGEMENT REPORT

Section 3
STORMWATER CALCULATIONS
3.1 Comparison of Pre -Construction vs. Post -Construction Discharges
Due to the increase in impervious area in Basins Al and B1 the runoff generated in these areas will
increase post -construction. In order to mitigate the effects of this increase, detention ponds were
designed to detain the runoff and discharge it at a rate that is less than the pre -construction rate for the
1-year storm. Due to a reduction in area, Basin A2 will generate less runoff post -construction. While the
impervious area increased in Basin B2, B2 experienced negligible increases in runoff in the 1-year storm
(< 0.1 cfs). Therefore, stormwater management facilities were not proposed for Basin B2. The peak flow
rates for the 1-year storm pre -construction and post- construction can be seen below. The details for
the stormwater management facilities can be found in Appendix C.
Drainaee
Basin
1-Year Pre-
Construction Flow
1-Year Undetained
Post -Development
Detention
Needed?
Post -Construction
Flow
cfs
Increase M
Al
26.3
34.1
29.6%
Yes
A2
0.2
0.1
-61.8%
No
B1
0.9
2.3
146.6%
Yes
B2
0.0
0.0
0.0%
No
As mentioned previously, dry detention ponds were used in Basins Al and B1 to reduce the peak flow.
The new post -construction detained flows are listed below showing the reduced flow rates.
Drainaee
Basin
1-Year Pre-
Construction Flow
1-Year Detained Post-
Post -Development
Detention
Construction Flow
cfs
Increase (%)
Achieved
cfs
Al
26.3
24.1
-8.3%
yes
B1
0.9
0.7
-21.2%
yes
ASHEVILLE REGIONALAIRPORT-PARKING LOTS
INITIAL SUBMITTAL:8/7/19; REVISION 1: 8/20/19
STORMWATER MANAGEMENT REPORT

The stage storage for the ponds are as shown in the tables below and can be found in the Hydraflow
Hydrographs report in Appendix F.
Pond Al (Existing)
Stage
ft
Elev
ft
Proposed
Storage
(cf)
0.0
2129.5
0
1.5
2131.0
74
2.5
2132.0
278
3.5
2133.0
777
4.5
2134.0
1,728
5.5
2135.0
3,339
6.5
2136.0
5,890
7.5
2137.0
9,701
8.5
2138.0
14,952
9.5
2139.0
22,973
Pond Al (Proposed)
Stage
ft
Elev
ft
Proposed
Storage
(cf)
0.0
2129.5
0
0.5
2130.0
487
1.5
2131.0
2,816
2.5
2132.0
5,800
3.5
2133.0
9,365
4.5
2134.0
13,729
5.5
2135.0
19,196
6.5
2136.0
25,905
7.5
2137.0
33,898
8.5
2138.0
43,292
9.5
2139.0
54,299
Pond B1
Stage
ft
Elev
ft
Proposed
Storage
(cf)
0.0
2162.0
0
1.0
2163.0
410
2.0
2164.0
1,017
3.0
2165.0
1,846
4.0
2166.0
2,923
As observed in the two Pond Al tables above, approximately 31,000 cf of extra storage was
generated by excavating the existing pond Al. This extra storage, along with the OCS addition, is
what reduces the peak flow rates out of the pond.
10
ASHEVILLE REGIONALAIRPORT-PARKING LOTS
INITIAL SUBMITTAL:8/7/19; REVISION 1: 8/20/19
STORMWATER MANAGEMENT REPORT

Adequate freeboard is a concern to ensure the 100-year storm does not overtop the ponds and spill
onto the roadways. The tables below illustrate the 1-year, 25-year and 100-year water surface
elevations and freeboard in relation to the top of dams.
Pond Al
Peak
Storm
Freeboard
Elevation
Frequency
(ft)
(ft)
1
2133.1
5.9
25
2136.6
2.4
100
2137.9
1.1
Pond B1
Peak
Freeboard
Storm
Elevation
Frequency
(ft)
(ft)
1
2164.1
1.9
25
2164.7
1.3
100
2165.1
1 0.9
As shown in the tables above, the on -site stormwater management results in a decrease in the flow
for the 1-year storm and keeps the 100-year storm contained in the ponds with adequate freeboard.
Therefore, the construction of the Asheville Regional Airport Parking Lots Project is not anticipated
to have adverse impacts.
11
ASHEVILLE REGIONALAIRPORT-PARKING LOTS
INITIAL SUBMITTAL:8/7/19; REVISION 1: 8/20/19
STORMWATER MANAGEMENT REPORT

3.2 Drainage & Erosion Control Design
Per Buncombe County requirements, the inlets and pipes were designed for the 25-yr storm. Pipe
Charts can be found in Appendix E showing pipe capacity and gutter spread. Energy Dissipators at
the proposed pipe outfalls can be found in Appendix C. Erosion Control design can be found in
Appendix C.
3.3 Operation and Maintenance Manual
The two dry detention ponds require maintenance in order to work properly. An O&M plan has been
developed and can be found in Appendix D.
3.4 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
• Buncombe County Stormwater Management Plan Review Checklist
• County of Buncombe Checklist and Guidelines for the Preparation of the Erosion and
Sedimentation Control Plans
12
ASHEVILLE REGIONALAIRPORT-PARKING LOTS
INITIAL SUBMITTAL:8/7/19; REVISION 1: 8/20/19
STORMWATER MANAGEMENT REPORT

Calculations can be found in the separate Stormwater Management Report, last Revised August 20,
2019.

Plans can be found in the separate 8.5"x14" package included.