SWA000227_Response To Comments_20231221

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610 East Morehead Street P 704.602.8600
T I M M O N S GROUP Suite 250 F 704.376.1076
YOUR VISION ACHIEVED THROUGH OURS. Charlotte, NC 28202 www.timmons.com
December 20, 2023
NCDEQ Stormwater Comments
Subject: Response to Comments—Waxhaw Volunteer Fire Department
NCDEQ Project# SWA000227
Timmons Group received comments from NCDEQ Stormwater on November 27th, 2023
regarding the Waxhaw Volunteer Fire Department project. Timmons has modified the
construction documents to address the below comments and offers the following responses.
The initial comments are black italic while Timmons responses are shown in bold type.
Stormwater - Jim Farkas
[e-mail]
Review Comments
1. "This project is located within the Waxhaw Creek Watershed and is subject to the Goose Creek
Rules. The Goose Creek Rules require the control and treatment of the difference in the pre-&post-
development 1-year,24-hour storm (instead of the 1.0"storm as in other parts of the State)and
requires infiltrating SCMs(such as infiltration trenches, bioretention cells(in A or B soils),
infiltrating permeable pavement, and Silva Cell unless it is demonstrated that these SCMs are not
practicable for use on-site. Please ensure that your design will meet the Goose Creek Rules upon
completion."Simply stating that there are C and D soils on-site is not sufficient justification for not
meeting 15A NCAC 02B.062(a), especially since the provided soil boring information indicates
that SHWT1 boring is representative of the Cecil soil series(which is HSG B). Demonstrating that
the on-site soil hydraulic conductivity is very low(typically less than 0.25 in/hr) and that it is not
practicable to locate the SCM in a portion of the site that has better infiltration potential could be
used to show that it is not practical to use infiltration methods for this project. NOTE:If it is not
practicable to use infiltration methods for this project,you may want to select an SCM that has a
higher percent annual runoff treatment by ET&1(please refer to the SCM Credit Document
https://www.deq.nc.gov/energy-mineral-and-land-resources/stormwater/bmp-manual/2023-nc-
stormwater-control-measure-credit-document/download?attachment)as it will provide some
groundwater recharge.
Response: Infiltration testing has been scheduled to take place in the next few weeks.We
will coordinate and address this comment after receiving the results.
Design
1. Sand Filter MDC3-Please note that the design volume of the SCM must be able to be stored
between the bottom of the SCM(top of the sand layer&bottom of the sediment chamber)and
below the invert of the lowest bypass device(2.5"orifice with invert at 578.21').Storage provided
within the sand layer, in a permanent pool in the sediment chamber, or above the invert of the
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bypass device does not count towards the design volume of the SCM.As designed, this SCM does not
appear to have a sufficient design volume.
Response: The SCM design has been revised.Storage within the sand filter has been taken
out of the design volume,and the lowest bypass device has been raised accordingly.
End of Comments&Responses
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