SW3190703_Stormwater Permit Response 2019-12-19_12/19/2019

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December 19, 2019
Division of Energy, Minerals, and Land Resources
North Carolina Department of Environmental Quality
512 North Salisbury Street
Raleigh, NC 27699-1612
Re: Hemby Place Single -Family Subdivision
Union County, NC
Permit No: SW3190703
LID PN: 1018416
Attn: Corey Anen
Stormwater Infiltration Response
Site/BMP Overview
The subject property consists of approximately 43.2 acres of land located on the southeast corner of Providence
Road and Hemby Road in Weddington, North Carolina. The existing three parcel numbers are 6147009,
6147010, and 6147011. The existing site consists of a woods and grass mix, with field grass, isolated trees and
woodlands. The terrain consists of typical piedmont rolling hills split into two areas. The western portion of the
site consists of CeB2 soils with gentle slopes, the majority ranging from 2 to 8 percent. The eastern portion of
the site consists of CeC2 soils with hilly slopes, ranging from 8 to 15 percent. There is an existing stream that
flows on the eastern portion of the site that flows into a tributary of Sixmile Creek.
The site will be mass graded with a total disturbed area of 25.2 acres. The project will consist of approximately
35 single-family homes, as well as common open space and on -site roadway. There will be a main entrance off
Hemby Road and a secondary exit off Providence Road. Storm drainage will be treated in one wet pond east of
the development, where the site natural drains to before entering the stream. The wet pond will be designed to
detain to the pre -developed conditions for the 1, 2, 10, 25, 50, and 100-year storm events. It will remove an 85%
average annual amount of Total Suspended Solids, draw down treatment volume between two and five days,
and will discharge the storm volume at a rate less than pre -developed conditions for the one-year, 24-hour storm.
The engineering calculations have been updated to show this, and the values are shown on Sheet C-6.05-C6.06
of the storm drainage plans.
Feasibility of Infiltration System
Summit Engineering, Laboratory and Testing, P.C. conducted a Geotechnical Report for the Hemby Place
Subdivision (formerly Patterson Property) site. Feasibility for infiltration is based off these findings. While the
site did consist of CeB2 and CeC2 soils according to the Web Soils Surveys, Summit found in their boring report
that the soils at the BMP location were not suitable for infiltration. Very stiff lean clay (CL) was found at the
boring location, as well as elastic silt. These soils make infiltration virtually impossible and at best, extremely
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difficult. Typical hydraulic conductivities of these soils range from 1 x 10-6 to 1 x 10-8 cm/sec, much lower than
the minimum required for infiltration. These soils are conducive to a wet pond, as it holds water very well and is
often used for landfill liners, earth dam cores, pond liners, etc. The low -plasticity, stiff clay soils combined with
the hilly slopes found around the BMP render infiltration extremely difficult.
We have enclosed a full set of plans and engineering calculations for your review, as well as a second copy of the
updated Stormwater Plans. Summit's Geotechnical Report and emails referenced above are also attached. Should
you have any questions or need any additional information, please contact me at [phone]. Thank you.
Sincerely,
Dale Stewart
223 North Graham Street
Charlotte, NC 28202
V: 704.333.0325
F . 704.375A 138
www.LandDesign.com
Washington DC
Charlotte INC
Dallas TX
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