SW3220901_Stormwater Narrative_20220914

AMERICAN
Engineering
Blythe Mill Townhomes Subdivision
Stormwater Calculations
Site Narrative
American Engineering Associates -Southeast, PA.
8008 Corporate Center Drive, Suite 110
Charlotte, NC 28226
OFFICE: 704.376.2438
www.American-EA.com
The proposed project, Blythe Mill Townhomes Subdivision, is a 118-lot townhome development located in Union
County, North Carolina, in the Town of Waxhaw. The proposed project is a 17-acre development, with
approximately 18 acres of denuded area. The proposed development is located along Blythe Mill Road, east of the
intersection of Blythe Mill Road and NC HWY 16. The eastern boundary of the site is low density residential
homes. The western boundary of the site is undeveloped woods and open field. The northern boundary of the site
is an undeveloped parcel and Waxhaw Parkway. The southern boundary of the site is Blythe Mill Road.
This construction site includes the clearing and grubbing of existing wooded land, grading of the existing site, and
the construction of 118 townhome units, roads, sidewalks, and associated improvements.
Site development and grading will occur in three phases. All sediment basins have been designed for both stages
of construction. Erosion and sediment control measures were sized using North Carolina Department of
Environmental Quality (NCDEQ) requirements.
Pre -Development Conditions
The project site is located within an area having a zone designation X by the Federal Emergency Management
Agency (FEMA) on Flood Insurance Rate Map (FIRM) Number 3710447300J, with a date of identification of
October 16, 2008. The existing property consists of woods and open space with several structures.
Existing stormwater drains off the site to the north, east and south as shown on the pre development drainage
map located in the appendix. The area to the north and east drains off the property and ultimately towards Blythe
Creek. The area to the south drains to Blythe Mill Road.
The soil types identified within the project area are BdC2 (Badin channery silty clay loam), TbB2 (Tarrus gravelly
silty clay loam) and TuB (Tarrus-Urban land complex). The soil types identified were used in determining Curve
Numbers (CNs) for the onsite and offsite runoff.
Post -Development Conditions
The proposed site will consist of paved streets with curb & gutter and concrete sidewalks. The 118 lots
will consist of townhomes with concrete driveways. Common open areas are located throughout the
proposed development. The site will be mass graded and the denuded area is approximately 18 acres.

AMERICAN
Engineering
Blythe Mill Townhomes Subdivision
Stormwater Calculations
American Engineering Associates -Southeast, PA.
8008 Corporate Center Drive, Suite 110
Charlotte, NC 28226
OFFICE: 704.376.2438
www.American-EA.com
The hydrologic analysis conducted for the proposed project included the study of both onsite and offsite drainage
areas for the wet pond. The analysis was conducted using the Soil Conservation Service (SCS) Hydrograph Method,
as outlined in SCS Technical Release 55 (TR-55). Hydrographs calculated using Hydraflow software were used to
compute peak rates of runoff for the pre -development and post -development conditions. The wet pond has been
designed to meet pre -construction conditions for the 2-year and 10-year storm events and minimum 80%
trapping efficiency as required by the NCDEQ and the Town of Waxhaw. The wet pond was sized to meet the pre
development conditions for drainage flowing off the site to the east as the pond outfalls to the east of the site.
This was to ensure that the pond would not add additional water for the 2-year and 10-year events in this
location.
Post -development drainage conditions can be viewed on the drainage area map located in the appendix.
Development of the site will include a wet pond to account for water quality storage volumes. The wet pond will
maintain a permanent pool of water, with storage volumes provided above this elevation. The wet pond will
promote sedimentation and pollutant treatment of stormwater runoff. Treated flow will be discharged back to
the eastern boundary of the site. The total tributary area draining to the proposed wet pond is 14.51 acres. The
tributary areas for the pond is summarized in the table below. The undetained areas are also included in the table
below. The undetained areas are areas that drain to the points of analysis that are not routed through the ponds.
These areas are modeled and added to the areas routed through the ponds to get a total post development
runoff.
POND NO. 1 TRIBUTARY AREAS
Impervious
Open
Woods
TOTALTO
POND
Undetained*
Drainage Area (ac)
8.04
6.47
0.00
14.51
2.85
The contents of this Stormwater Pollution Prevention Plan (SWPPP) were prepared for MTS CLT, LLC as the
primary permittee seeking coverage under North Carolina's National Pollutant Discharge Elimination System
(NPDES) General Permit for Stormwater Discharges from Construction (CGP). Unless required by the owner, this
plan may also be used by secondary permittees to obtain coverage under the CGP.
Summary
The stormwater control measures have been designed to meet pre- and post -development requirements for the
2-year and 10-year storm events. Furthermore, the stormwater control measures have been designed to achieve

AMERICAN
Engineering
Blythe Mill Townhomes Subdivision
Stormwater Calculations
American Engineering Associates -Southeast, PA.
8008 Corporate Center Drive, Suite 110
Charlotte, NC 28226
OFFICE: 704.376.2438
www.American-EA.com
a minimum 80% trapping efficiency as required by the NCDEQ and meet the latest NPDES General Permit for
Stormwater Discharge requirements.
Upon removal of the sediment basins, level spreader devices shall be installed at the proposed storm drainage
outfalls. Upon exiting the level spreader devices, stormwater from the various systems will discharge into off the
site at the eastern boundary.
In conclusion, due to the existing site conditions and proposed runoff control measures, the proposed project is
not anticipated to have any negative impacts on any downstream or adjacent properties.
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