SW3240205_Response To Comments_20240816

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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.timmonscom
August 16,2024
NCDEQ Stormwater Permit No.SW3240205
Subject: Response to Comments-Courtyards On The Greenway
Timmons Group received comments from NCDEQ Stormwater on July 31,2024,regarding the Courtyards
On The Greenway submittal.Timmons has modified the Courtyards On The Greenway Plans to address
the below comments and offers the following responses.The initial comments are black italic while
Timmons responses are shown in bold type.
Application Review (Reviewed by Jacob Beeker) [e-mail]
1. General
a. 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&I
(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
and will help meet the intent of 15A NCAC 02B.0602(a).
Response:Acknowlegded.
b. The design volume of a wet pond is the amount of storage that is provided between
the elevations of the permanent pool surface and the invert of the lowest bypass. The
invert of the lowest bypass can be moved to adjust the design volume to more closely
resemble the minimum treatment volume, but the design volume of the SCM will
always be defined the same. The design volume is indicated correctly on Line 35 of
the Wet Pond page of the Supplement-EZ form but is not used in the provided
drawdown calculations. Please revise the package as necessary.
Response: For Wetpond#1,a 4"orifice was added at invert 625.25'.This puts the provided
water quality elevation at 625.25',which corresponds to a provided water quality volume of
146,038 cu-ft.For Wetpond#2,the provided water quality elevation is 628.50'.This reflects
a provided water quality volume of 64,318 cu-ft.
2. Supplement-EZ
a. Lines 36-39- These lines refer to the main device through which the design volume
is drawn down.According to the plans and calculations, the outlet device seems to be
an orifice. Please revise the Supplement-EZ to indicate this, as well as indicating the
desired diameter of said orifice. Lines 38 and 39 can also be marked as N/A.
b. Line 40- The calculations and plans indicate orifice diameters will be used that do
not correspond to the diameters that were back-calculated from the required flow rate
to meet the drawdown times listed here. For instance,for Wet Pond 1, a required
orifice diameter of 3.09 inches was calculated. This value was then rounded to 3
inches.A 3-inch diameter orifice would have a drawdown time greater than 5 days,
which would not meet the MDC.Please ensure that the drawdown times listed here
correspond to the final diameter that will be used for the drawdown orifice.
CIVIL ENGINEERING I ENVIRONMENTAL I SURVEYING I GIS I LANDSCAPE ARCHITECTURE I CONSTRUCTION SERVICES

Response: See response above.The drawdown times have been updated in the calculations
and supplement-EZ form.The supplement-EZ form has been updated with the correct orifice
information for Wetpond#1.For Wetpond#2,the top of weir is the lowest bypass. Since the
weir is open-top,the height was left as"n/a"and the width is the available weir length on all
four sides.
3. Calculations
a. Please ensure that the calculations reflect the actual drawdown time of the orifice
diameters selected. If the calculated diameters are not the final diameters that will be
used,please show that the final diameters will draw down the design volume within
two to five days to show compliance with Wet Pond MDC 7. The given drawdown
orifice does not draw down the design volume of the wet pond between two and five
days, as required by Wet Pond MDC 7. In addition,please ensure that Line 40 of the
Supplement-EZ form is updated accordingly. (See 2b.)
NOTE: While not required, it is recommended to use a value of H0/3 when
calculating drawdown through an orifice.
Response: For calculating drawdown,the calculations have been updated to utilize Ho/3.The
drawdown orifice has been revised for Wetpond#1.Both wetponds drawdown within 2-5
days.
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