SW1210502_C-RPT-1901780-Project Narrative-2021-05-14_20210519

EALand
'Architecture
Engineering
.�
Environmental
Surveying
Companies
An Employee -Owned Company
Project Narrative
For the proposed development located at:
394 Fanning Fields Road
Mills River, North Carolina
Prepared for Submission to:
Town of Mills River, North Carolina
May 14, 2021
Prepared for:
Samet Corporation
P.O. Box 8050
Greensboro, NC 27419
Prepared by:
.�' Architecture
Engineering
Environmental
.� Land Surveying
Companies
BL Companies
3420 Toringdon Way, Suite 210
Charlotte, NC 28277
[phone]
BL Project Number: 1901780

. ' Architecture
Engineering An Employee -Owned Company
Environmental Stormwater Management Report
D
� Land Surveying
Companies
This narrative has been prepared in support of a Final Fast Track ATC Stormwater Permit Application
submission to the North Carolina Department of Environmental Quality (NCDEQ) by Samet Corporation
for the proposed development at 394 Fanning Fields Road. The subject property is approximately 34.23
acres in size and comprised of tax parcel 49975227 and the land to be acquired as part of the land exchange
agreement with tax parcels 4800374 and 49968359. The parcels that were previously undeveloped
woodland, are now developed with a ±110,866 s.f. Amazon warehouse and distribution facility, associated
parking, loading areas, utilities, stormwater systems, and landscaping to accommodate the tenant's
operations. Several pockets of wetlands and streams, including a USGS "blue -line" stream, reside on the
eastern and western portions of the property, and have been left undisturbed by the development. The USGS
"blue -line" stream, also known as "Stream I", requires a 30-foot buffer. However, no development impacts
any of the streams, associated buffers, or wetlands located on -site. All streams and wetlands are tributaries
that discharge to McDowell Creek and ultimately the French Broad River basin.
The proposed stormwater management system is designed to be in compliance with the 2013 NC Erosion
and Sediment Control Planning and Design Manual and the NCDEQ Stormwater Design Manual for a high -
density project.
A HydroCAD model, using TR-55 methodology, was developed to evaluate the existing and proposed
drainage conditions of the property. The results of the analysis demonstrate that there will not be an increase
in peak stormwater runoff rates for the 2-, 10- and 25-year storm events.
The stormwater runoff from the proposed development for all intensities and durations of rainfall up to the
25-year storm event shall not exceed that of the existing conditions. Infiltration has conservatively been
considered in the drainage calculations using exfiltration rates recommended by NRCS Web Soil Survey
data. The exfiltration rate utilized is 0.2 in/hr.
The NCDEQ stormwater regulations require high density projects to treat the design storm depth in a
stormwater control measure. The stormwater control measures incorporated into this project include eight
(8) underground stormwater infiltration/detention systems with isolator rows and seven (7) BayFilter
systems to remove sediment and nutrients. SWMB-2, 3, 4, 5, 6, 7 are ADS MC-3500 detention chambers
and SWMB-1 is comprised of ADS SC-740 detention chambers. The proposed Stormwater Management
Basin 45 (SWMB-5) system handles all of its water quality through the isolator row, and therefore does not
require a BayFilter to meet NCDEQ TSS removal requirements. For all stormwater management systems,
the impervious area stormwater runoff will be captured by catch basins with deep sumps for pretreatment.
Stormwater will then be conveyed through the proposed storm sewer system and discharge into several
subsurface detention/infiltration basin facilities. The detention/infiltration systems have been designed to
dewater the 1" design volume within 72 hours or less. The clean stormwater will then outlet through the
combined BayFilter and outlet control structures and discharge upgradient of the onsite wetlands and
streams or connect to the drainage facilities along Boylston Highway. All other landscaped areas will
continue to flow as they currently do today either directly into the streams or directly into the Boylston
Highway storm sewer system.
6135 Park South Drive • Charlotte, NC 28210 • [phone] • www.blcompanies.com

. ' Architecture
Engineering An Employee -Owned Company
Environmental Stormwater Management Report
. � Land Surveying
Companies
The drainage study analyzed fourteen (14) proposed drainage areas that discharge to six (6) points of interest
to which the peak flow rates were maintained from existing, undeveloped conditions. A comparison of
existing versus developed peak flow rates can be found in Table 1 below.
Peak Flow (cfs)
Design Storms
Analysis Point
2-YR
10-YR
25-YR
100-YR
POI-1
Existing
1.88
3.29
4.15
5.52
Proposed
0.98
1.75
2.22
2.98
Percent Change
-47.87%
-46.81%
-46.51%
-46.01%
POI-2
Existing
9.81
19.00
24.93
34.96
Proposed
7.73
16.64
22.18
36.10
Percent Change
-21.20%
-12.42%
-11.03%
3.26%
POI-3
Existing
5.53
9.94
12.68
17.13
Proposed
5.38
9.51
12.47
17.92
Percent Change
-2.71%
-4.33%
-1.66%
4.61%
POI-4
Existing
1.37
2.66
3.49
4.90
Proposed
1.23
2.60
3.37
5.73
Percent Change
10.22%
-2.26%
-3.44%
16.94%
POI-5
Existing
35.55
70.26
92.59
129.95
Proposed
31.32
66.92
91.24
136.41
Percent Change
11.90%
-4.75%
-1.46%
4.97%
POI-6
Existing
2.22
4.88
6.66
9.75
Proposed
2.22
4.88
6.66
9.75
Percent Change
0.00%
0.00%
0.00%
0.00%
Overall Total
Existing
56.36
110.03
144.50
202.21
Proposed
48.86
102.30
138.14
208.89
Percent Change
-13.31%
-7.03%
-4.40%
3.30%
6135 Park South Drive • Charlotte, NC 28210 • [phone] • www.blcompanies.com