SW3220702_Response To Comments_20231103

LandDesign.
CREATING PLACES
THAT MATTER.
Nvember 3, 2023
Jim Farkas
Environmental Engineer
North Carolina Department of Environmental Quality
Division of Energy, Mineral, & Land Resources—Stormwater Program
512 N. Salisbury Street
1612 Mail Service Center
Raleigh, NC 27699-1612
RE: Preserve at Forest Creek Phase 2
Stormwater Permit No. SW3220702
Dear Mr. Jim Farkas,
1. Original Comment 8, Prior Comment 3— "There appears to be issues with the BUA
accounting...""...This issue was not resolved, the Deed Restriction document still appears
to show that the amount of BUA allocated to the individual lots is greater than... Please revise
as needed."Adding up the per lot BUA allocations as shown in the deed restriction document
results in a total of 1,412,906 sf whereas the total amount of BUA allocated to the individual
lots shown on the Supplement-EZ Form is 1,412,904 sf(this appears to be a rounding error,
however these values must match). Please revise as needed. If you are updating the deed
restriction document, please email me a spreadsheet of the per lot BUA allocations so that
the values may be easily added and verified. NOTE: While not required, it was noted that lots
411, 413, 425, 429, 436, 444, &453 are allocated more BUA than the size of the lot.
LDI Response: Deed Restriction sheet has been updated.
2. Oriainal Comment 9.b.iii.. Prior Comment 4— "Lines 25 & 35...""The design volume of a
wet pond is the volume that can be stored between the permanent pool surface elevation and
the invert of the lowest bypass device (typically a weir in the outlet structure)...This is the
volume that should be reported on Line 35 and used to calculate the drawdown time of the
pond. The invert of the lowest bypass device should be used to set the temporary pool
surface elevation (Line 25). Please revise as needed."The design volume for Wet Ponds 9,
10, & 11 is less than the minimum required treatment volume, these wet ponds are
undersized for their drainage areas. This is required per General MDC 1. For example, for
Wet Pond 9, the minimum required treatment volume is 42,986 cf, but the provided design
volume is only 40,957 cf. Please revise as needed. Please also refer to the later comment
with regard to the drawdown calculations as the temporary pool (determined by the invert of
the bypass device) needs to be used for these calculations.
LDI Response: Wet Ponds 9, 10 and 11 were updated. See Supplement EZ form.
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3. Prior Comment 7— "Off-site components were added to the drainage areas for SCMs 9, 10,
11... Any off-site areas that are not bypassed around the SCM must be accounted for at their
full built-out potential per 15A NCAC 02H.1003(3)(b). The full build-out potential of an off-site
area can either be limited by having the property owners enter into a legal agreement limiting
the amount of drainage area and BUA draining from the off-site area to the SCM or by
assuming that the off-site area is 100% BUA (its full build-out potential(more information can
be found following these comments). Please revise as needed."Please correct the following:
a. SCM Design: Similar to the above comment, SCMs 9, 10, & 11 do not appear to account
for the entire drainage area of the SCM. SCMs 9, 10, & 11 are undersized for their
drainage area (the design volume is less than the minimum required treatment volume).
The surface area of the main pool of SCM 9 appears to be undersized based on the
SA/DA calculations. Per the calculations, the minimum required main pool surface area
is 18,212 sf, whereas the provided stage-storage table only indicates a main pool
surface area of 15,571 sf. Please revise as needed.
LDI Response: Drainage areas for these ponds are accounted for per the Pre/Post development
sheets. Existing contours added to Post Development sheet for better visual of drainage pattern.
Ponds have been increased in size to account for the minimum SA/DA calculations.
b. BUA Accounting: Please ensure that the percent BUA values shown in the Application
and Supplement-EZ Form take into consideration the entire drainage area to the SCM.
For example, the drainage area to SCM 10 is shown as 20% BUA. The total amount of
BUA shown draining to the SCM is 363,468 sf and the total drainage area to the SCM is
521,855 sf(percent BUA would be closer to 70%). NOTE: These values are correctly
shown in the calculations.
LDI Response: BUA on application and Supplement EZ form updated.
4. Prior Comment 8.a,— `Please correct the following issues with the Wet Pond Page of the
Supplement-EZ Form:Line 40—Please recalculate these values in accordance with the
earlier comment."There still appears to be issues with the drawdown calculations. For
example, for the drawdown calculations for Wet Pond 10, the cross-sectional area at the
temporary pool surface elevation (shown as 656.49 ft) is shown in the calculations as 24,894
cf, however, the provided stage-storage table shows a cross-sectional area of 17,201 sf @
656.0 ft and 19,173 sf @ 657.0 ft. Another example would be the cross-sectional areas used
for the drawdown calculation for Wet Pond 9. The cross-sectional area at the permanent pool
is shown in the calculations as 18,211 sf whereas the stage-storage table shows 18,676 sf.
The cross-sectional area at the temporary pool is shown in the calculations as 20,628 sf
whereas an interpolated value from the stage-storage table would be closer to 21,700 sf. In
both examples, the temporary pool surface elevation should be determined by the invert of
the lowest bypass elevation (For SCM 9, the calculations use an elevation of 655.84 ft
whereas the invert is shown at 656.0 ft and for SCM 10, the calculations use an elevation of
656.49 ft whereas the invert is shown at 656.35 ft. Cross-sectional areas and elevations are
used to determine the volume and head used in the drawdown calculations so issues with the
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cross-sectional areas and elevations will result in errors in the calculations. Please re-check
your drawdown calculations and ensure that they are correctly done and representative of the
proposed design. Please revise as needed.
LDI Response: This has been corrected on all SCM design sheets.
5. Prior Comment 8.b,— "Please correct the following issues with the Wet Pond Page of the
Supplement-EZ Form:...Line...35...— See earlier comments"These values will need to be
revised, see earlier comment. Please revise as needed.
LDI Response: Noted, revised per previous comment.
6. 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.
LDI Response: Noted. Revised plans will be uploaded as well as hard copies sent.
a. PDFs must be uploaded using the form at:
https://edocs.deq.nc.gov/Forms/SW-Supplemental-Upload
b. Hard copies must be mailed or delivered to the following address:
i. For FedEx/UPS:
Jim Farkas
512 N. Salisbury Street, Office 640M
Raleigh, NC 27604
ii. For USPS:
Jim Farkas
1612 Mail Service Center
Raleigh, NC 27699-1612
iii. Hand Delivery:
Please reach out to me prior to hand delivering a submission to make sure that I (or someone
else in my group)will be able to receive the submission.po not leave the packaae in the
foyer with the security guard.
NOTE: Hard copies should ma be sent to a Regional Office. Doing so will delay the review
process and the submission package may be lost while being sent from the Regional Office to
me in the Central Office
Sincerely,
Frank McMahan, PE for LandDesign, Inc.
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