SW3220306_Response To Comments_20230216 (2)

LandDes*n
CREATING PLACES
THAT MATTER.
February 15, 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 1 - LID PN 1020233
Stormwater Permit No. SW3220306
Dear Mr. Jim Farkas,
1. Original Comment 1.a.ii — Per 15A NCAC 02H .1003(3)(b), any off -site areas not bypassed around the
SCM must be accounted for at their full build -out potential which can either be determined by entering into a
legal agreement between the property owners or by assuming that the off -site area is 100% BUA (see original
6/2/2022 Request for Additional Information Letter). It is noted that, in this latest submission, the off -site BUA
values are determined based on allowable zoning densities, however, this is not an acceptable method since the
total amount of off -site BUA directed to the SCM may be in excess of the provided value. For example, if the off -
site property that partially drains to an SCM for this project is limited to a total of 60% BUA per zoning, that off -
site property owner could put more BUA on the portion of their property that drains to this project's SCM than
was accounted for by just assuming that 60% of the off -site drainage area could be BUA (so long as the off -site
property owner's overall density remains below 60%). Another situation is that the off -site property owner could
re -grade their property to increase the amount of drainage directed from their property to the SCM for this
project. Either way, we require the off -site area to be considered as 100% BUA if no legal agreement is in place.
The SCMs appear to be adequately sized to take this into account so this is more of an accounting issue than a
design issue. Please revise.
Response: Pond calculations were revised to reflect the offsite drainage area as 100% BUA. Sheets
were revised accordingly.
Original Comment 9.a —The following electronic files do not appear to have been uploaded:
o Calculation booklet (10/22 submission)
Response: Revised calculation book will be uploaded, update to reflect latest comments.
o Response to comments (6/22 submission)
Response: 6/22 comments response will be uploaded as well as comment response to latest
comments.
o Deed Restriction Document (6/22 submission)
Response: 6/22 Deed Restriction will be uploaded as well as latest Deed Restriction.
3. Application:
Section IV, 10 — Please revise the Off -site Drainage Area and Off -site BUA values per earlier
comment. Please also provide a hard copy of the page with Drainage Areas 5 & 6 on it as this
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was not provided in the latest submission (The electronic file had this extra page so it was
reviewed).
Response: Pond calculations were revised to reflect the offsite drainage area as 100% BUA.
Sheets were revised accordingly. Revised calculation book and sheets will be uploaded and
hard copies will be provided.
Section IV, 11—See earlier comment, revise as needed.
Response:Acknowledged
4. Supplement-EZ Form:
o Drainage Areas Page:
■ Entire Site Column —There appears to be an issue with the numbers in this column.
Line 8 should be the sum of Lines 9 & 10, but it is not (1,741,052 sf ;t 1,319,061 sf+
427,975 sf). Please revise any submittal items as needed to address this issue.
Response: Acknowledged, EZ form was revised.
■ Drainage Area Columns 2-6, Lines 7, 11, & 18—See earlier comment, revise as
needed.
Response: Acknowledged, EZ form was revised.
■ Drainage Area Columns 5 & 6, Line 20 —The provided values do not seem correct.
Please revise as needed.
Response: Acknowledged, EZ form was revised.
• For Wet Pond 5, the provided value is 24,917 cf. The lowest bypass
elevation of the outlet structure (temporary ponding surface) is at
elevation 605.5' and the provided stage -storage table indicates that the
provided storage between the permanent pool (604.0') and the temporary
ponding surface (605.5') is approximately 30,000 cf.
Response: Acknowledged, EZ form was revised.
• For Wet Pond 6, the provided value is 21,073 cf. The lowest bypass
elevation of the outlet structure (temporary ponding surface) is at
elevation 613.75' and the provided stage -storage table indicates that the
provided storage between the permanent pool (612.0') and the temporary
ponding surface (613.75') is approximately 25,000 cf.
Response: Acknowledged, EZ form was revised.
■ Line 22—See earlier comment, revise as needed.
Response: Acknowledged, EZ form was revised.
Wet Pond Page:
■ Line 2—See earlier comment, revise as needed.
Response: Acknowledged, EZ form was revised.
■ Line 28—See earlier comment, revise as needed.
Response: Acknowledged, EZ form was revised.
■ Line 35, Columns 5 & 6 — See earlier comment, revise as needed.
Response: Acknowledged, EZ form was revised.
■ Line 40—See earlier comment, revise as needed.
Response: Acknowledged, EZ form was revised.
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5. Calculations:
Drawdown —The drawdown calculations should be based on the design volume (Line 35) and
the temporary ponding elevation (Line 25). The provided design configurations appear to draw
down within the required window when the design volume is used so this is more of an
accounting issue than a design issue. Please revise.
Response: Acknowledged, EZ form was revised. See calculation book.
Average Depth & Minimum Main Pool Surface Area —The minimum surface area calculations
do not appear to be correct. For example, using Equation 2 to calculate the average depth for
Wet Pond 2 (Main Pool surface area = 6,266 sf and volume = 20,418 cf) results in an average
depth of 3.26 ft (20,418 cf / 6,266 sf) while the calculations show a value of 3.18 ft. Since the
minimum required surface area calculations are based on the average depth, these
calculations are also off. The average depths and main pools appear to be sufficiently sized
when calculated correctly so this is more of an accounting issue than a design issue. Please
revise.
Response: Acknowledged, EZ form was revised.
Sincerely,
Frank McMahan, PE for LandDesign, Inc.
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