SW3230302_Response To Comments_20240417

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DICKSONO
community infrastructure consultants
March 28, 2024
Jim Farkas
North Carolina Department of Environmental Quality
RE: Comments on the Submittal Entitled Ivy Ridge Supplemental Storm Drainage
Davis Village Subdivision —Wingate, North Carolina
Project ID: SW3230302
WKD Project No. 2023089500.CL
Dear Jim Farkas:
Enclosed, please find the revised plans for the Davis Village Subdivision project, with comments
from the September 20, 2023, review comments from NCDEQ. Please see below for written
responses to the department's comments.
1. Prior Comment 1 — "As designed, the project does not appear to capture and treat
sufficient BUA in order for the project to meet Runoff Treatment (15A NCAC
02H .1002(43)). Please ensure that the net increase in BUA is being captured and treated
in one or more primary SCMs. If it is not practicable to capture and treat all of the new
Bua, the uncaptured areas of the project can be permitted as a low-density area
(provided that they meet all of the low-density requirements outlined in 15A NCAC
02H .1003(2)). Please also clarify if the provided deed restriction document (3,500 sf per
lot) applies to all of the proposed lots (Lots 1-46 south of Phifer Road and lots 1-[168]
north of Phifer Road). Per the Supplement-EZ Form, the total amount of BUA allocated
to the subdivided lots for the entire site (Entire Site Column, Line 9) is 380,640 sf
whereas the total amount of BUA allocated to the subdivided lots per the deed restriction
document would be [749,000] sf (3,500 sf/lot * [214] lots). Please revise as needed."
The net increase in BUA does not appear to be captured in one or mor primary SCMs. Since
the net increase in BUA is not being captured/treated in one or more primary SCMs, the
areas bypassing the SCMs should be accounted for as a low-density area, but are not. The
BUA per lot issue has not been resolved. It is noted that the drainage area delineations indicate that
the 46 townhomes south of Phifer Road have a BUA limit of 1,720 sf/lot, however, this information
was not revised on the deed restriction documents. If the ownership of these townhomes will be
retained by the permittee (i.e., not subdivided and sold off) then the BUA associated with the
townhomes should not be accounted for as
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subdivided BUA and the BUA accounting should be revised to reflect this. NOTE: The
original comment had a typographical error in it, it listed 186 lots north of Phifer Road
whereas there are only 168 lots proposed north of Phifer Road.
Response: A bypass area has been added to the application as Drainage Area #4, and the
proposed impervious area falls within the low-density threshold. BUA per lot has been
corrected in calculations and deed restriction form.
2. Prior Comment 3.a. — "Please correct the following issues with the Wet Pond Design:
The provided calculations show the drainage area to the wet pond as being 30 acres and
containing 50% BUA whereas the other submittal items (Application & Supplement-EZ
Form) indicate that the drainage area to the SCM as being 25.5 ac and containing 38%
BUA. Please revise as needed."
The Application was not updated to reflect this (the Application still indicates the drainage area to
the SCM as being 25.5 ac and containing 38% BUA). Please revise.
Response: Calculations, Application, and Plans have been updated to match.
3. Prior Comment 3.b. — "...Please provide a cross-sectional view of the wet pond that
clearly shows important slopes and elevations (such as the excavated bottom of the main
pool, the excavated entrance and exit elevations of the forebay, the top of the sediment
storage zone, the cross-slope(s) of the pond, etc...). This item is required per 15A NCAC
02H .1042(2)(h)."
The provided cross-sectional view indicates that the sediment bottom is at elevation 549.0' which is
lower than the excavated bottom of the SCM. The bottom of the sediment storage zone should
correspond to the excavated bottom of the SCM and the top of the sediment storage zone (not
shown on the plans, but required) should be at least 6" higher than the bottom of the sediment
storage zone. For example, if the bottom of the pond/sediment storage zone is at elevation 550.0',
the top of the sediment storage zone could be at elevation 550.5' or higher. Please also provide the
excavated bottom elevations of the forebay near the entrance (appears to be 550.0' per the plans)
and the excavated bottom elevations of the forebay near the exit (appears to be 550.5' per the
plans).
Response: Plans have been revised to show the sediment storage zone, and calculations have
been updated to match what is shown on the plans.
4. Prior Comment 3.c. — "...As designed, the forebay appears to be too large (Wet Pond
MDC 5). The volume of the forebay (volume stored in the forebay above the top of the

sediment storage zone and below the permanent pool surface) must be between [1]5%-20%
of the main pool volume (volume stored in the main pool above the top of the sediment storage
zone and below the permanent pool surface). NOTE: The main pool does not include the forebay.
The forebay is provided in addition to the main pool and, when combined, the forebay and main
pool form the permanent pool. Please revise as needed."
As designed, the forebay is more than 20% of the volume of the main pool. Per the provided
calculations, the forebay is shown to have a volume of 23.5% of the main pool volume. Please note
that the volume of the forebay and main pool used in this calculation should not include the
sediment storage zone. NOTE: The original comment contained a typographical error, the forebay
should be between 15% and 20% of the main pool volume per the MDC, apologies for any
confusion this may have caused.
Response: Forebay has been revised to be between 15%-20% of the main pool volume.
5. Prior Comment 3.e. — "Please use the latest SA/DA tables (provided in Part C-3 of the Manual) in
your calculations."
When using the SA/DA table to determine the minimum required surface area of the main pool of
the wet pond, please ensure that the average depth is used (an average depth of 5.0 ft appears to
be used whereas the average depth of the main pool of the wet pond is closer to 3 ft). if the
average depth of the main pool is not shown in the table, the table value can be interpolated from
the provided values based on the actual average depth, or the average depth can be rounded down
to the nearest average depth value provided in the table). Please also note that the surface area
value used in this determination should correspond to the surface area of the main pool (not the
total permanent pool surface area). Please revise.
Response: The average depth used has been revised to 4', and the calculated average depth is
4.05' using equation #2.
6. Prior Comment 3.f. — "The drawdown orifice (invert at the permanent pool surface elevation) is
shown to have a diameter of 4.0" in the calculations and a diameter of 3.0" in the plans. Please
revise as needed."
The provided calculations appear to still use the cross-sectional area of an orifice with a diameter of
3" as opposed to that of an orifice with a diameter of 4". Please revise.
Response: Plans and Calculations have been revised to show a 4" orifice, which provides a 2.2
day drawdown time.

7. Prior Comment 4.a. — "Please correct the following issues with the Sand Filters: There
appears to be an issue with the drainage area information as shown in the calculations
and as shown in the other submittal items. For example, the calculations show the
drainage area to SCM 2 as 50% BUA whereas the other submittal items show 34% BUA.
Please revise as needed."
The provided calculations show the percent BUA for SCM B as 48%whereas the other submittal
items indicate the percent BUA as 45%. Please also update the Application to reflect the revised
drainage areas.
Response: The previously submitted drainage area and impervious area were incorrect. The
plans, calculations, and applications have been revised accordingly.
8. Prior Comment 4.b. — "Please provide the SHWT elevations and the lowest excavated bottom
elevations for both sand filters so that Sand Filter MDC 1 can be verified."
Please provide either the existing ground surface elevations for the boring locations (so the SHWT
elevation can be determined from the depth to the SHWT from the existing ground surface
elevation provided in the soils report) or the SHWT elevations. The elevations, and not just the
depths from the existing ground surface elevation, are required to ensure sufficient separation
between the bottom of the sand filter and the SHWT.
Response: The existing ground surface has been added to the SHWT Report.
9. Prior Comment 4.c. — "...Please ensure that the storage volume provided in each
chamber of the sand filter is equivalent (Sand Filter MDC 2)."
The provided stage-storage table for SCM B shows the cumulative storage volume provided up to
elevation 578.0'. The invert of the bypass device (weir @ 577.5') sets the upper limit for the
temporary pool/design volume so any storage provided above this elevation doesn't count towards
the design volume. Please revise the stage-storage table to exclude any storage above the invert of
the lowest bypass.
Response: Stage storage has been revised, and Hmax for the 1" storm is calculated to be
1.12'(elevation 576.12), which is below the weir that is at elevation 577.5'
10. Prior Comment 5.b.ii. — "Please correct the following issues with the Supplement-EZ
Form: Drainage Areas Page: Line 20— Please see later comments with regard to the
design volume of the SCMs."

Please list the design volumes instead of the minimum required treatment volumes on this line of
the form.
Response: Updated supplement provided.
11. Prior Comment 5.c.iv. — "... Wet Pond Page: Line 27 — Do not include the forebay volume in this
item."
Please also exclude the sediment storage zone from this item. NOTE: A minimum of 6" of sediment
storage needs to be provided in the main pool.
Response: Sediment storage zone and forebay volumes have been removed from calculation.
12. Prior Comment 5.c.v.— "...Line 28— Please recalculate this value."
The surface area used should only reflect the main pool (not the combined main pool/forebay
surface area) and the volume used should exclude the sediment storage zone of the main pool.
Response: Revised calculation only uses main pool area.
13. Prior Comment 5.c.vi. — "Line 32 — Recalculate as needed."
Recalculate as needed.
Response: Recalculated.
14. Prior Comment 5.d.iv. — "... Sand Filter Page: Line 18 — Please provide the SHWT
elevations from within the footprint of the proposed SCMs."
Please provide these values.
Response: SHWT was not present on BMP#B. BMP#A it was and plans have been revised to
indicate this. SHWT report provided for both locations.
15. Prior Comment 5.d.v.— "...Line 23 —This item refers to the volume that can be stored above the
excavated bottom of the sediment chamber and below the invert of the lowest bypass orifice/weir."

Although this item corresponds to the value shown in the stage-storage table, this item will need to
be revised. Please refer to earlier comment with regard to the design volume relative to the bypass
invert.
Response: Revised per comments.
16. Prior Comment 5.d.vi. — "...Line 26 — ...This item refers to the cross-sectional area of
the sediment chamber at the excavated bottom elevation (Line 25)."
The cross-sectional area of the top of the chamber appears to be provided instead of the bottom
cross-sectional area.
Response: Correct area has been provided.
17. Prior Comment 5.d.vii. — "...Line 27 —This item refers to the distance from the excavated bottom
of the sediment chamber (Line 25) to the invert of the lowest bypass orifice/weir."
SCM B —The provided value, 36" does not correspond to the information shown in the design (Line
25 = 575.0', Invert of bypass = 577.5', difference = 30").
Response: Depth has been revised.
18. Prior Comment 5.d.viii. — "...Line 28 —This item is Line 27 minus the depth of any
permanent pool provided in the sediment chamber."
SCM B — See earlier comment.
Response: Depth has been revised.
19. Prior Comment 5.d.ix. — "...Line 30 —This item refers to the volume that can be stored above the
top of the sand layer in the sand chamber and below the invert of the lowest bypass orifice/weir.
Storage provided within the sand layer is not included."
SCM B —The provided value, 4,934 cf, does not match the plans, 6,311 cf. NOTE: This value and
Line 23 will need to be recalculated, see earlier comment.
Response: Updated per comment
20. Prior Comment 5.d.xv. — "...Line 36—This item refers to the type of bypass device listed on Line
35."

The bypass devices appear to be weirs (although they are labeled as orifices on the plans)
Response: Bypass devices are weirs. Plans updated accordingly.
21. Supplement-EZ Form, Line 40— Please revise the drawdown time (see earlier comment).
Response: Drawdown time updated per design.
22. 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: Completed