SW3181002_Response To Comments_20240112

V.3-
January 11, 2024 SOUTHEAST
Mr.James Farkas
State Stormwater Engineer
NCDEQ
512 N. Salisbury St. Office 640M
Raleigh, NC 27604
RE: Cottages West Single Family I Request for Additional Information
Dear Mr. Farkas,
We are in receipt of your review letter, dated December 12, 2023 regarding the above subject project. In
response to your comments, we offer the following:
Comment 1: Prior Comments 1, 4, 6.b.i.1., & 6.b.ii.1. -There still appear to be
accounting/consistency issues throughout the submission. Please see Attachment A for
more information on this. Please revise the submittal items as needed so that the drainage
area information and BUA accounting is consistent throughout the submission. Please
note that the SCM design cannot be fully reviewed for compliance with the
rules/requirements until these inconsistencies are resolved.
Response:Attachment A has been reviewed and calculations have been checked and edited as needed,
see updated submission.
Comment 2: Prior Comment 2 &6.b.iii.1-"Please ensure that the off-site portions of the drainage
areas to the SCMs are correctly accounted for at their full build-out potential..."The
revised Supplement-EZ Form indicates that there is a total of 80,598 sf of off-site area
draining to SCM 2 and that there is a total of 81,022 sf of off-site BUA draining to the
SCM (which is 424 sf larger than the off-site drainage area). Please revise as needed.
Response:Calculations have been revised, see updated submission.
Comment 3: Prior Comment 3.a.-"As designed,the proposed sand filters do not appear to meet the
following MDCs... Sand Filter MDC 1-Please provide the existing ground surface
elevations for the provided soil borings. It is noted that the depth from the existing
ground surface to the SHWT is indicated in the report, but without the elevation of the
existing ground surface,the elevation of the SHWT cannot be determined. NOTE: A
perched water table is considered to be a SHWT unless it is demonstrated that the
confining layer will be removed/modified to no longer produce the perched condition." It
is unclear where the provided values (<668' for SCM 1 &<650'for SCM 2) came from.
Per the provided soil borings, boring B-9 (SCM 1) has an existing ground surface
elevation of 673' and is 15' deep (bottom of boring at elevation 658') and boring B-8
3700 South Boulevard, Suite 200, Charlotte, NC 28209 1704.940.2883
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Page 2 of 4
Mr.James Farkas
January 11, 2024
(SCM 2) has an existing ground surface elevation of 665' and is 15' deep (bottom of
boring at elevation 650'). Per the findings, a perched SHWT was observed to a depth of
180 inches (@ elevation 658')for B-9 (SCM 1) and a perched SHWT was observed to a
depth of 90 inches (@ elevation 657.5')for B-8 (SCM 2).The lowest excavated elevation
of SCM 1 is shown as 665.5' and for SCM 2 as 653.5'.A minimum of 2' of vertical
separation is required per the MDC(this separation can be reduced to no less than 1'for
open bottom designs provided that a hydrogeologic evaluation is provided per the MDC).
Please revise as needed.
Response: Water elevations have been updated within submission.An EPDM liner has been added to the
SCM 2 detail sheet to prevent groundwater seepage into the basin.
Comment 4: Prior Comment 3.b.—"...Sand Filter MDC 2—Please ensure that the volume provided
in the sediment chamber is equivalent to the volume provided in the sand chamber..." Per
the information provided in the Supplement-EZ Form, the minimum required treatment
volume for SCM 1 is 11,975 cf(half of which is 5,987.5 cf)The sediment chamber of
this SCM is shown to only hold 5,733 cf. Please revise as needed to ensure that at least
half of the minimum required treatment volume is being stored in each chamber. In
addition, the sum of the volumes provided in each chamber for SCM 2 (12,586 cf+9,545
cf= 22,131 cf) is larger than the available volume shown in the stage-storage table
between elevations 657.0' & 659.5' (approximately 21,750 cf). Please revise as needed.
Response: The rip-rap forebay has been shifted within SCM 1 to split the sediment chamber and the sand
chamber.
Comment 5: Prior Comment 5—"The water quality volume of the SCMs do not appear to be
calculated correctly..." Revised calculations for the water quality volume were not provided.
Response: Water quality calculations were provided via online portal, being that there have been some
changes, they will be resent with the updated submission.
Comment 6: Prior Comment 6.c.i—"Please correct the following issues with the Supplement-EZ
Form... Sand Filter Page... Line 2—See earlier comment." Revised calculations for the
water quality volume were not provided so this item could not be verified. Please revise
as needed.
Response:See above comment#6.
Comment 7: Prior Comment 6.c.ii—"...Line 18—See earlier comment. If, for example,the boring
terminates at elevation 100' and the SHWT is not encountered in the boring, this item
would be expressed as "<100"." See earlier comment, the provided values of"<668" &
"<650" do not appear to correspond to the soil boring information in the report. Please
revise as needed.
3700 South Boulevard, Suite 200, Charlotte, NC 28209 1704.940.2883
V3 I Visio, Vertere, Virtute ... The Vision to Transform with Excellence

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Mr.James Farkas
January 11, 2024
Response:See above comment#3.
Comment 8: Prior Comment 6.c.iii.—"...Line 23—This item refers to the volume that can be stored
between the lowest excavated elevation (bottom) of the sediment chamber and the invert
of the bypass weir/orifice. For example, per the plans for SCM #1,the bottom of the
sediment chamber appears to be at elevation 668.0' (although it is not specifically called
out on the plans) and the invert of the bypass weir is at elevation 671.0'. NOTE:The
provided stage-storage table for the sediment chamber starts at elevation 669.0'. Please
revise as needed." See earlier comment with regard to more available storage being
shown in the Supplement-EZ Form for SCM 2 (sum of Lines 23 &30)than is shown in
the stage-storage table for this SCM. Please revise as needed.
Response:Calculations have been updated, see updated submission.
Comment 9: Prior Comment 6.c.v.—"Line 30—Similar to the sediment chamber,this item refers to
the volume that can be stored between the top of the sand layer(bottom) of the sand
chamber and the invert of the bypass weir/orifice. For example, per the plans for SCM
#1,the bottom of the sand chamber is shown at elevation 668.0' and the invert of the
bypass weir is at elevation 671.0'. NOTE: Since there is a stone layer provided up to
elevation 688.25', please ensure that it is accounted for in the volume. NOTE:The sum
of Lines 23 & 30 result in the design volume of the sand filter(Line 20 on the Drainage
Area Page). determination." See earlier comment with regard to the volume provided
within the SCM. Please also ensure that the porosity of the stone layer is taken into
account if the stone layer is being incorporated in the sand chamber storage zone.
Response: The porosity of the stone layer within the sand chamber storage zone has been accounted for,
see updated submission.
Comment 10: Prior Comment 6.c.vi.—"Line 31—As mentioned earlier, the bottom of the sand
chamber is the top of the sand layer." Per the plans,the bottom of the sand chamber for
SCM 1 is shown at elevation 668.0' (667.0' is shown). Please revise as needed.
Response:Plans/calcs has been updated, see updated submission.
Comment 11: Prior Comment 6.c.vii.—"Line 32—The area of the sand chamber is the cross-sectional
area of the sand chamber at the bottom elevation, not necessarily the cross-sectional
area of the sand layer(we do recommend that the entire surface area of the sand
chamber contain sand)." For SCM 1, the combined surface area of the sediment chamber
(1,911 sf) and the sand chamber(2,777 sf) is greater than the cross-sectional area of the
bottom of the sand filter as shown in the stage-storage table, 3,720 sf(1,911 sf+2,777 sf
=4,688 sf= 3,720 sf).This is also true for SCM 2 (5,034 sf+3,278 sf=8,312 sf= 7,643
sf). Please revise as needed.
3700 South Boulevard, Suite 200, Charlotte, NC 28209 1704.940.2883
V3 I Visio, Vertere, Virtute ... The Vision to Transform with Excellence

Page 4 of 4
Mr.James Farkas
January 11, 2024
Response: Values have been updated, see updated submission.
Comment 12: Prior Comment 6.c.viii.—"Lines 33 &34—The ponding depth in the sand chamber is
the distance from the bottom of the sand chamber to the invert of the bypass weir/orifice.
The physical depth of the sediment chamber is the distance from the excavated bottom of
the sand chamber(i.e., under the underdrain)to the invert of the bypass weir/orifice."
SCM 1, Line 34—The provided value (25") does not correspond to the sand filter
geometry. Per the plans, the bottom of the sand chamber is at elevation 668.0' and the
bypass weir is at elevation 671.0' (difference of 3' or 36"). Please revise as needed or
clarify.
Response: Values have been updated, see updated submission.
Comment 13: Prior Comment 8—"Please upload the electronic files from the original hard copy
submission at the following address: https://edocs.deq.nc.gov/Forms/SW-Supplemental-Upload.
Electronic files are required per 15A NCAC 02H .1042(2).
Response:Electronic files were originally submitted to the portal, see attached confirmation email.
Updated calculations have been re-uploaded to the portal.
Comment 14: Please note that, as of 12/6/23, no response was provided to the comments issued on
8/22/23 for the transfer of this project (please refer to 8/22/23 email for list of comments). Please note
that we will be unable to issue the approval for this major modification until
the transfer submitted jointly with this modification has been approved.
Response:Please see this updated response.
Comment 15: 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.
Response:See updated submission
Sincerely,
V3 Southeast
(\411,2_, Up&
Nicholas Vesely, PE
Senior Project Engineer, North Carolina
3700 South Boulevard, Suite 200, Charlotte, NC 28209 1704.940.2883
V3 I Visio, Vertere, Virtute ... The Vision to Transform with Excellence