SW3231001_Response To Comments_20240209

RESPONSE TO COMMENT LETTER
Subject: Stormwater Permit No. SW3231001
McBride Subdivision,High Density Wet Pond/Sand Filter
Union County
Date: 02/09/2024
1. General Comments:
a. Supplement-EZ, Drainage Areas
i. Lines 21-22: Per the page 58 of the provided calculations, the"Simple Method"
was used to determine the volume of runoff from the 1" storm. Please revise Line
21 to "Simple" and revise Line 22 to provide clarification on the "Future" and
"Other"BUA. It appears the text provided in Line 22 is truncated,please ensure
any additional information can be read. If there is not enough room for the note in
the cell, the area below can be utilized.
Response: Line 21 has been revised to Simple and information in line 22 has been
removed as it was redundant.
b. Calculations
i. Clarify the source of the flow values used in the riprap sizing calculations to
demonstrate that the riprap at the inlets and outlets have been adequately sized to
prevent erosion during the 10-year storm per 15A NCAC 02H .1050(4).
Response: Storm pipes have been designed for the 25-yr storm and therefore the rip rap
aprons have also been size for the 25-yr storm. We have added a narrative on the FES
& Rip Rap Calculation page describing where the flows used in the apron calcs
originate.
ii. Clarify if the riprap sizing calculation listed as "Idlewild" is intended to be the
apron associated with"FES RD1" on Sheet C4.1.
Response: The calculation has been revised to change Idlewild to FES RD1.

2. Wet Pond Comments:
a. Calculations
i. The volume of the main pools and the forebays should not include the sediment
storage zone since this portion of the pond is intended to be filled up with sediment
and can therefore not count towards provided storage volume. (The cumulative
volume for elevation 631 should be"0" instead of 13,328 CF at elevation 631 in
the stage storage calculations. Please revise.)
Response: The calculation has been revised to provide a 0 at elevation 631.
ii. Please provide drawdown calculations for the 4" orifice so that Wet Pond MDC 7
can be confirmed. "The design volume shall draw down to the permanent pool
level between two and five days."Please revise the stage storage calculations to
show the surface area and volume at the temporary pool elevation so that
drawdown calculations can be verified.
Response: The calculation has been revised to provide the drawdown
calculation.
b. Plans
i. The plan details shall include side slopes per 15A NCAC 02H .1042 (2) (h) (i),
please include on the cross section or plan view of the wet pond detail.
Response: we have added notes in plan view and in the cross section to label
the slopes as 3:1.
ii. Please indicate on the plans that the entrance of the forebay is at a lower elevation
than the exit of the forebay(Wet Pond MDC 5b) for each forebay. Only a single
elevation for the bottom of the forebay is provided and the forebay bottom appears
to be flat. Please revise as needed.
Response: We have shown both in plan view and in the cross section that the
elevation at the forebay entrance is lower than the elevation at the outlet.
iii. The entirety of the SCM shall be shown and labeled within the permanent recorded
easement per 15A NCAC 02H .1050 (9). The SCM system includes the dam
embankment which is not clearly shown within the easement in lots 49-54.
Response: The easement runs along the rear property lines of lots 49-54. While
there is a slope into lots 49-52, this slope is a cut and is above the top of dam
elevation and therefore is not part of the SCM to be maintained. We had rather
keep the lots free of easement. To make this clearer, we have shown the easement
line along and over the rear property lines.
iv. The Supplement-EZ indicates that fescue will be utilized on the dam and
embankment, please specify on plans as required per MDC 11(a) and 15A NCAC
02H .1042 (2)(h)(iii).
Response: On sheet C4.9, we have specified that all zones above water that are
not hatched for platting shall be fescue.
v. Provide a planting plan on the plans as required per 15A NCAC 02H .1042 (2)(i)
that meets the requirements of Wet Pond MDC 11.
Response: Planting Plan has been provided on sheet C4.9.
c. Supplement-EZ, Wet Pond Page
i. Lines 27, 28, & 32—These items will need to be recalculated(to exclude the
sediment storage zone). Note on Line 27, this value is the volume between the top
of sediment storage and the main pool elevation as shown in the stage storage
calculations, and not the volume calculated by the Simple Method.
Response: Lines 27, 28 and 32 have been updated to reflect the removal of the
sediment storage zone.
ii. Line 30—This value is the perimeter of the main pool, but it appears that the
surface area of the main pool was provided. Please revise as this affects the

calculation for the average depth of the main pool.
Response: Line 30 has been corrected to reflect the area of the main pool.
iii. Line 33 —Per Wet Pond MDC 5 (a), "Forebay volume shall be 15 to 20 percent of
the volume in the main pool."Please demonstrate this in the calculations and
ensure that both forebays are accounted for.
Response: Line 33 has bee corrected to yes. The volume is 17.65% and this value
is listed on the wetpond calculation
r^
Pxm Weer%..r4O •05 10n= 636.5
Ci4d/lu Vy4t Fpeby
Main Pod50104* Vdvne Fog Percentage
61rveeon Area 6 ng Cumwlr w peYn 6dbp 9or4ce 0... Vo4mF)15Q05.
Fde6e,S(sl.) Vdv17Icn 8I Foretop Is 1.) Vdvre(c4 0e44.e0)
(w9) R7
630 31370 0 2494 0
631 34512 0 3618 0
632 37723 36.116 4689 4.254
633 41002 75.460 6312 9.854
634 44350 118.156 7882 16 951
635 47768 164214 9602 25.693
636 51250 213.722 11472 36.230
836.5 54473 240.153 13125 42.379 17.65
838.5 86803 243,735 0 0
iv. Line 34—The cleanout depth of the forebay is the distance from the permanent
pool surface elevation to the top of the sediment storage zone. Please revise.
Response: Line 34 has been corrected to 636.50-631.00= 5.5'66 inches.
v. Line 35 -For a wet pond, the design volume is the amount of storage that is
provided in the wet pond above the permanent pool surface and below the
elevation of the lowest bypass weir/orifice in the outlet structure (The bypass
orifice is not the same as the drawdown orifice, the invert of the lowest bypass
orifice appears to be at elevation 637.37') and must be at least as large as the
minimum required treatment volume. Please revise accordingly.
Response: Line 35 has been revised to 63,396 c.f. This is an interpolated
value of volume at elevation 637.37 b/t elevations 637 and 638.
vi. Lines 42 & 43 —The forebay entrance shall be deeper than the forebay exit per
Wet Pond MDC 5. Please revise. Note: These depths provided do not appear to
correlate with what is shown on the plans.
Response: Lines 42 and 43 have been revised to show the required varying
forebay depths.
d. O&M Agreement, Wet Detention Pond Design Summary:
i. Forebay- The invert of the bypass device sets the temporary pool surface elevation
and appears to be 637.37 per the outlet structure details. Please indicate the bypass
device at 637.27 on the plans or revise the package accordingly.
Response: The Temp. pool elevation has been revised to 637.37.
ii. Main Pond- The temporary pool elevation of the main pond is listed as 627.7,
which is below the permanent pool elevation, and not consistent with the
remainder of the package.
Response: The Temp. pool elevation has been revised to 637.37.

3. Sand Filter Comments:
a. The provided soils report indicates that the SHWT was not encountered at depths of
9.7 and 14.6 ft below the existing ground surface for Borings SW-1 and SW-2. Please
indicate this elevation as the SHWT elevation on the cross-sectional detail and revise
the Supplement-EZ, Sand Filter Page, Line 18 with this value. Separation from the
SHWT is required per Sand Filter MDC 1.
Response: The existing ground elevation of SW-2 is 638.65. We have shown on the
cross section a SHWT of 638.65-14.6 = 624.05, and have likewise updated line 18 of
the Supplement-EZ form.
b. Per Sand Filter MDC 2, storage volume in the sediment chamber and sand chamber
shall be equivalent. The sand filter must be designed so that 50% of the treatment
volume can be stored in the sand chamber below the first bypass device. If the sand
filter is designed to attenuate peak flows, additional surface area may be added in the
sediment chamber only.
Response: The basin has been revised to relocate the forbebay berm to get the
forebay volume and sand chamber volume roughly the same volumes. At elevation
638.50, the forebay volume is 2,755 cf and the sand chamber volume is 2,244 cf
c. Sand Filter MDC 9, the ends of each underdrain pipe should be capped to prevent
clogging of the underdrain system, the BMP (Sand Filter) Forebay Berm Detail seems
to show a capped cleanout, but please specifically call out the capped pipe to ensure
compliance with this MDC requirement.
Response: We have added the following specification to the cleanout in cross
section. "Vertical cleanout pipes to be solid PVC. Terminate with threaded female
adapter and 6" screw cap with protruding square nut."
d. Supplement-EZ, Sand Filter Page
i. Line 9—Please clarify"Other" in Line 43.
Response: This has been changed to Pump. This pond will be dry in between rain
events as long as it is working properly. Should it become clogged and need
maintenance, the undrained volume will be pumped out.
ii. Line 13 —Please revise to "N/A" as the SCM does not appear to be located on a
single-family lot.
Response: The field has been changed to N/A.
iii. Lines 23 & 30—For the sediment chamber, this value corresponds to the volume
that can be stored above the lowest excavated point of the sediment chamber and
below the invert of the lowest bypass weir/orifice. For the sand chamber, this value
corresponds to the volume that can be stored above the top of the sand layer and
below the invert of the lowest bypass weir/orifice. Each of these volumes should
be roughly equivalent(Sand Filter MDC 2) and the sum of which is equal to the
design volume (which must be at least as large as the minimum required treatment
volume).
Response: Line 23 remains unchanged at 3,003 cf. Line 30 has been changed to
3,879 cf.
iv. Line 26—This value refers to the cross-sectional area of the sediment chamber at
the bottom elevation of the sediment chamber(Line 25).
Response: Since the sediment chamber slopes to the forebay, this number has
been changed to 1.0 s.f.
v. Line 27—The physical depth of the sediment chamber is the depth from the lowest
bypass invert to the excavated bottom of the sediment chamber.

Response: Line 27 has been changed to 637.50-636.37 = 1.13 ft= 14 inches.
vi. Line 33 —The physical depth of the sand chamber is the depth from the lowest
bypass invert(Line 35)to the lowest excavated point of the sand chamber(below
the underdrains).
Response: Line 33 has been changed to 637.50-633.11 =4.39 ft= 53 inches
vii. Line 34—The ponding depth of the sand chamber is the depth from the lowest
bypass invert(Line 35)to the top of the sand layer(Line 31).
Response: Line 34 has been changed to 637.50-636.0= 1.50 ft= 18 inches
viii. Line 35, 36—This value (640') does not correspond to the invert of the lowest
bypass device as shown in the plans (12"weir opening @ 637.50').
Response: Lines 35 has been changed to 637.50. Line 36 has been changed
to list 12"weir.
4. Provide PDFs of all revisions, 2 hardcopies of revised plan sheets, 1
hardcopy of other documents, and a response to comments letter briefly
describing how the comments have been addressed.
a. Please ensure the electronic repository is complete, the following items
were noted as missing: the documentation showing the LLC is an active
corporation in good standing with the NC Secretary of State and official
documentation showing the titles and positions held by the person who
signed the application.
Response: Hard copies of the LLC sheets requested have been included
in the resubmitted documents and have also been loaded to the
electronic repository.