SW3210801_Response To Comments_20211124

H THOMAS & HUT TO N
1020 EOCLIDAVENUE
CHARLOTTE, NC 29203 1 980.201.5505
WWW.THOMA5AN DH UTTON.COM
November 17, 2021
Attn: Jim Farkas
NCDEQ - Stormwater
512 N. Salisbury Street, Office 640K
Raleigh, NC 27604
Re: 1-85 Commerce Center
NCDEQ - Stormwater Review
Salisbury, NC
J-29101.0000
To whom it may concern:
This letter serves as acknowledgement that we have revised the previously submitted package to
address all comments below as follows:
Original Comment 4 — "The percent impervious cover of the project area appears to
be incorrectly calculated (Section IV, 8). The percent impervious cover is calculated
as follows: [% Impervious Cover] [Total Proposed B UA] / [Project Area] " Per the
information in the submission, the total amount of proposed BUA within the project
area is 29.43 ac (20.16 ac is shown in the calculation of this item) and Section IV, 7
shows the project area as 41.87 ac (40.35 ac is used in the calculation of this item).
Also, performing the calculation as shown for this item [(20.16 / 40.35) * 100%]
results in
a %BUA of 50%, not the 69.8% as shown. Please revise as needed for correctness and
consistency.
The total drainage area to the proposed pond is 39.2 acres. The project area (property
area) is 41.87. The Impervious area in the drainage area is 29.8 acres. The percent
impervious using the areas listed is 71.2% = (29.8/41.87) x 100
2. Original Comment 12 — "The design of the wet pond could not be fully reviewed for
Lul«pi lance with the A�DCs since stage -storage tables for the wet pond were not
provided... The stage -storage tables are required to ensure compliance with Wet Pond
MDCs 1, 2, 3, 5, 6, & 7. " The following calculation errors were noted from reviewing
the provided stage -storage tables:
The stage storage tables are added to Appendix F (page 98 in pdf) of the stormwater
report.

a. 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 sediment storage zone elevations have been lowered to provide adequate
storage volume. See stage storage volumes for the main pool in the
Stormwater Report Appendix F (page 98 in pdf).
b. The calculated forebay volumes appear to be incorrect and may be too small.
The forebay volume is the volume of water that can be stored in the forebay
between the top of the sediment storage zone and the permanent pool. NOTE: I
did not notice a detail for the berm separating the forebay from the main pool in
the plans. Without this detail, it is assumed that the berm is made up of pervious
stone which would allow water stored in the forebay to seep into the main pool
and equalize at the permanent pool elevation. If the berm is made up of
something impervious that would prevent this from happening, then you would
be able to count the forebay volume up to the point at which it spills over into
the main pool as "forebay volume".
A pervious stone forebay detail has been added to the construction set. See
sheet C3.9 and C3.12. The forebays have been adjusted so the entire volume
between the top of the sediment storage and the permanent pool is used as
forebay in pond 1 and 4. Ponds 1 and 4 will serve as forebays for ponds 2 and
3. The combined forebay volume is 38,022 c.f. and the combined main pool
volume is 207,368 c.f. See Appendix F (page 102 in pdf) in the stormwater
report for a breakdown of volumes at different storage elevations.
c. The cross -sectional area of section 4 wet pond 4 at elevation 770 appears to be
incorrect. The cross -sectional area of this portion of the wet pond (21,185.3 sf)
is smaller than the cross -sectional area of this portion of the wet pond at
elevation 769 (26,033.2 sf). Cross -sectional areas of a wet pond should
increase as the elevation increases.
The cross -sectional area at elevation 770 has been corrected to 28,851. See
Appendix F in the stormwater report (page 101 in pdf) .

d. The temporary pool surface elevation (765.26) is located above the invert of
the bypass weir located in the outlet structure (at elevation 765.2). Strictly
speaking, this is allowable and not against the MDCs for a wet pond, however
it is a bad design practice since the design volume of the wet pond should be
stored below any bypass devices and allowed to be drawn down via the
drawdown orifice. If you will be leaving the bypass weir below the temporary
pool surface elevation, please make sure to account for its impact on the
drawdown time for the design volume.
The bypass wier had been raised to elevation 765.6' to prevent the design
volume to bypass the draw down orifice. See sheet C3.10 in the construction
set for the updated control structure.
3. Original Comment 20 — "Please correct the following issues with the O&M
ant Form... An updated version of the O&M Agreement Form was not
provided in this resubmission. Please provide this item with the corrections requested
from the prior letter.
The O&M form has been sent with the package.
4. Original Comment 21.b.iv.2 — "Please correct the following issues with the
Supplement-EZ Form... Drainage Area Page... Lines 5-7 The `drainage area' of the
entire site is the entire project area and should correspond to the value shown in
Section IV, 7... " The "drainage area" information entered into the entire site column
does not correspond to the total project area information from Section IV, 10 of the
Application. The entire site column is not merely a sum of the information in the
drainage area columns, but rather an accounting of the entire project area.
The areas in Section IV, 10 match the areas entered in the EZ form.
5. Original Comment 22 — "Provide pdfs of all revisions..." Please upload the
electronic files from the 10/7/2021 submission to the link provided below.
Electronic files are required per 15A NCAC 02H .1042(2).
Electronic files have been submitted to NCDEQ
6. There appears to be 17,921 sf of "Other" BUA listed in the Application that does
not appear in the Supplement-EZ Form. Please revise as needed for correctness
and consistency.
The 17,921 sf of imperious area (curb and gutter) has been accounted for in the
EZ form.

7. The amount of BUA within the drainage area to the proposed wet pond is shown as
28.16 ac in the calculations and as 29.43 ac in the Application and Supplement-EZ
Form (This may impact the design volume of the SCM). Please revise as needed for
correctness and consistency.
The BUA area within drainage area has been updated to 29.8 acres. This matches the
area in the EZ from. This assumes all the offsite drainage area will be impervious
(15,465 sf).
8. Any off -site areas that drain to the proposed wet pond must be accounted for at their
full build -out potential (per 15A NCAC 02h .1003(3)(b)). It is recommended to
bypass these off -site areas around the proposed SCM since the Applicant has no
control over future development of these off -site areas and can become non -complaint
with this permit at no fault of their own. If it is not possible to divert this off -site
drainage area around the proposed SCM, it is recommended to have the off -site
property owner and the Applicant enter into a legal agreement limiting the amount of
BUA and drainage area that the off- site property owner can direct to the proposed
SCM. If this option is not feasible, the off- site area can be accounted for at the full
build -out potential of 100% BUA. The off -site area being developed to the point
where it becomes 100% BUA may not seem like a likely outcome, but it is possible
and therefore the full build -out potential of this area.
The drainage area proposed to the pond will have 15,465 sf of offsite drainage. We
have assumed that all the offsite drainage will be 100% BUA.
9. Please correct the following issues with the Wet Pond Page of the Supplement-EZ Form:
a. Line 26 — Please only include the surface area of the main pool (not the
forebay) at the permanent pool elevation for this item. Please revise.
The forebay area has been excluded from the surface area of the permanent
pool
b. Line 27 — Similar to the above comment, the volume of the main pool should
only include the main pool and not the forebay. Please revise.
The forebay volume has been excluded from the volume of the permanent pool
c. Line 28 — Excluding the sediment storage zone from the main pool volume
calculations will result in a smaller average depth of the main pool than what
is currently shown. Please revise.
The sediment storage zone has been excluded from the main pool volume.
The average depth calculation has been revised accordingly.

d. Line 32, Column 3 — The plans do not indicate that section 3 of the wet pond
has a forebay. Please revise/clarify.
This has been corrected. Only section 1 and 4 have forebays.
e. Line 35 — The sum of the provided design volumes only equals 92,563 cf
whereas the minimum required treatment volume is shown as 98,919 cf. Please
revise.
The sum of the updated design volumes is 108,999 cf, The minimum required
is 104,471 cf.
f. Line 40 —The calculations indicate that the drawdown time for this wet pond is
2.2 days (52.81 hours) whereas the drawdown time is shown as 4.6 days.
Please revise. NOTE: Nothing was provided in Appendix G of the
calculation booklet.
The drawdown time has been updated to be 2.1 days. Appendix G (page 104
in pdf) in the stormwater report has the drawdown graph.
10. While not required, it is strongly recommended to more proportionately distribute the
forebays between the two main points of inflow into the proposed wet pond. This is
not required per the MDCs, but an "undersized" forebay will need more frequent
maintenance than a more appropriately sized forebay (and section 4 of the wet pond
has the room for an enlarged forebay).
The forebay in pond 4 has been increased per your recommendations and to meet water
quality requirements.

11. 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. Pdfs must be uploaded using the form at:
httl2s:,[/edocs.deq..nc.gov/Forms/SW-Sulplemental-Up1oad
b. Hard copies must be mailed or delivered to the following address:
i. For FedEx/UPS:
Suzanne McCoy
512 N. Salisbury Street, Office
640K Raleigh, NC 27604
ii. For LISPS:
Suzanne McCoy
1612 Mail Service
Center Raleigh, NC
27699-1612
While it is not required, it is recommended that the off -site drainage from upstream
properties be diverted around the SCM since the Applicant cannot maintain control over the
development of property that they do not own. If the Applicant elects to account for off -site
drainage, there must be a recorded legal agreement in place between the Applicant and the
Owner of the upstream property in which the Owner of the upstream property agrees to record
a deed restriction limiting the minimum/maximum amount of BUA on the upstream property
that will be treated in the Applicant's SCM, the locations and sizes of access and drainage
easements, how construction and/or ongoing maintenance costs will be handled, maintenance
responsibility, a list of legal recourses available to each party should one party fail to hold up
their end of the agreement, and any other related legal issues.
We look forward to receiving the approved certification for the referenced project to
allow us to proceed. If you have any questions, comments, or desire additional information, please
contact our office at [phone].
Sincerely,
THOMAS & HUTTON
William Bradley Smith, PE