SW5240104_Response To Comments_20240522

FLM ENGINEERING
May 22, 2024
Brianna Holland
Environmental Engineer
Post-Construction Stormwater Permitting Group
North Carolina Department of Environmental Quality
512 N. Salisbury Street, Office 640E
Raleigh, NC 27604
Reference: Green Hill Subdivision
Stormwater Permit No. SW5240104
Request for Additional Information Resubmittal
Dear Ms. Holland:
In response to your letter dated April 23, 2024, we have addressed your comments as noted below.
1. NOTE: It is noted that the project name on the Application and Supplement-EZ is "Green
Hill Subdivision" and "Green Hill Subdivision Remaining Phases" on the plans. Using
"Green Hill Subdivision" as the project name in the permit can lead to future confusion
on what areas the permit covers, as "Green Hill Subdivision Phase 1" is an existing
permit. We are going to rename this project in our system to "Green Hill Subdivision
Phases 2-4" to reflect this. You are not required to update your submittal items (as this
letter serves as a record of this change/decision), but you can if you choose to do so.
Green Hill Subdivision Phase 1 is not an existing permit. Phase 1 was submitted for permitting
in March of 2021, but we were advised by Jim Farkas that it would be better if we included all
phases in one permit, which is what this submittal does. The permitting for Phase 1 was never
completed, and a permit was never issued.
2. Please ensure that this project is meeting the definition of "Runoff Treatment" per
15ANCAC 02H .1002(43). In order for a project to meet Runoff Treatment, the net
increase in BUA must be captured and treated in one or more primary SCMs. If it is not
practicable to capture the net increase in BUA, the portions of the site that do not drain
to the SCM can be permitted as a low-density area (in accordance with 15A NCAC 02H
.1003(1)(d)) provided that the area meets the low-density requirements outlined in 15A
NCAC 02H .1003(2) (NOTE: If pursuing this option, you will need to revise the
Supplement-EZ Form, Application, O&M Agreement, etc... to reflect the low-density
area). If it is not practicable to capture the net increase in BUA and the uncaptured areas
do not meet the low-density requirements, we will need to discuss a variance for the
project. Since this project drains to the Tar River (a Nutrient Sensitive Watershed),
please ensure that the project is also compliant with the Tar-Pamlico Nutrient Strategy
Rules 15A NCAC 02B .0731.
The net increase in BUA is captured by the primary SCMs. The existing impervious surface
area of the site is 123,447 SF, and the proposed impervious surface area of the site is
1,547,410 SF. The net increase is 1,423,963 SF. The impervious surface area captures by
the primary SCMs is 1,506,439 SF, which is greater than the net increase.
3. This project is located within 5 miles of Triangle North Executive Airport. While not
required, it is recommended to use SCMs that do not promote standing water in
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PO Box 91727, Raleigh, NC 27675 p 1919.610.1051

accordance with G.S. 143-214.7(c3) (https://www.ncleq.qovi EnactedLegislation/
Statutes/HTML/BySection/Chapter_143/GS_143-214.7.html). This is to prevent
accidents involving birds and airplanes. Alternative SCMs that can be used within the
5 mile radius of an airport are outlined in Part E-4 of the Manual
(https://deq.nc.gov/media/9977/download).
The proximity to the airport is addressed on sheets C-18 through C-23 by the following note
that is in the plan view of each SCM:
DUE TO THE PROJECT BEING WITHIN A 5 MILE RADIUS OF THE TRIANGLE NORTH
EXECUTIVE AIRPORT, BIRD REPELLANTS SHALL BE INSTALLED AROUND THE
WETLAND.
The wetlands were selected as primary SCMs because they provide they provide the best
combination of water quality treatment and rate of runoff control with a minimal permanent
pool of water and are able to handle the large drainage areas associated with this project.
4. Clearly identify and delineate the top of banks of the streams/rivers in the main set of
plans and the normal pool elevation of the ponds (Section VI, 8h). If it is not possible to
delineate the banks of the surface water (as would be the case for a narrow stream),
please show the centerline of the surface water and provide a callout listing the surface
area of that surface water that is within the project boundary. Note that the surface area
of the surface water is the area between the top of the banks of the streams and normal
pool of impounded structures. Ensure the vegetated setback is measured from the
normal pool elevation of impounded structures, and top of bank of each side of streams
or rivers, perpendicular to the shoreline for all perennial and intermittent surface waters
within and adjacent to the site. Ensure the vegetated setbacks are clearly shown on the
main set of plans for all surface water, including surface water adjacent to the site. 15A
NCAC 02H.1003(4)(a), 15A NCAC 02H.1042(2)(g)(iii).
The tops of banks of surface waters and associated vegetated setbacks have been clearly
shown and labeled as requested. Please refer to sheets C-6 and C-7.
5. Please clarify the project area for the project currently being permitted (Section IV, 4-8).
a. Change the cover sheet to refer to the current phases being permitted (Section VI,
8a).
As noted in response#1 above, while this set of construction plans includes the infrastructure
for the remaining phases of Green Hill Subdivision, the stormwater permitting is for all SCMs
associated with the project. Because the stormwater permit is not the only permit associated
with this project, the cover sheet cannot be changed because it would not accurately reflect
the infrastructure being permitted with these plans.
b. Provide bearings and distances for the extent of the Phase 2-4 areas (Section VI, 8i).
c. The "Entire Site" column of the Drainage Area page of the Supplement-EZ Form
should only show the BUA and area ("drainage area" on the form) for the Phase 2-4
area, as Phase 1 is permitted by SW5210302.
Bearings and distances are provided around the entire project boundary,which is what is being
permitted with this submittal, contrary to the Phase 1 permit that was never issued.
d. Any drainage area that contains BUA that is being directed to Phase 1 will be
considered as being treated by an off-site SCM.
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Phase 1 was never permitted, so this submittal includes all SCMs for Green Hill Subdivision.
e. Any Phase 1 BUA that is being directed to an SCM on this permit, will be considered
off-site BUA.
Phase 1 was never permitted, so this submittal includes all SCMs for Green Hill Subdivision.
6. Delineate the drainage areas (on-site and off-site) of each SCM on the main set of plans
per the Application Section VI, 80. It is recommended to call out the square footage of
each drainage area so that it is clear which drainage area on the plans corresponds to
the drainage areas provided in the Application and Supplement-EZ.
The post-development drainage area map has been added to the plan set as sheet C-69.
7. Please dimension all existing and proposed BUA (Section VI, 8j). The information
shown in the Application and Supplement-EZ Forms should be able to be verified on
the plans in the field.
Existing impervious surface has been dimensioned on sheets C-6 and C-7. The site plan
sheets contain dimensions for the proposed infrastructure with a total linear footage of roadway
listed in the summary information table on sheet C-8. Additional dimensions for the roadway
section are included on sheet C-64.
8. Please revise the following with the Application:
a. Section IV, 10
i. Drainage Area 2: 209,750 SF +122,353 SF +41,361 SF = 373,464 SF < 393,464
SF. Please revise package for consistency.
ii. Drainage Area 3: 98,256 SF +48,889 SF +16,361 SF +20,000 SF = 183,506 SF>
163,506 SF. Please revise package for consistency.
The application was correct as submitted. The 20,000 SF for the "other" BUA was incorrectly
applied to Drainage Area 3 instead of Drainage Area 2 in the supplemental form, which has
been corrected within the supplemental form with this submittal.
9. Please revise the following item with the O&M Agreement:
a. The provided O&M Form is outdated and therefore incomplete. Please use the most
up to date form, which can be found on our website at the following address:
https://www.deq.nc.gov/about/divisions/energy-mineral-and-landresources/
stormwater/stormwater-program/stormwater-design-manual (O&M EZ Form). NOTE:
It is recommended to get a second copy of this form signed & notarized for the
Applicant's records (it will be helpful when they go to record it with the Register of
Deeds).
b. Please describe the location of the SCMs instead of "See Plans."
The most up to date O&M form has been included with this resubmittal. The form does not
allow for enough space to describe the location of six SCMs, so the location note has been
changed to "refer to recorded plat."
10. Please revise the following with the plans:
a. Recorded Maintenance Access and Easement:
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General MDC 8 and 9: All SCMs and associated maintenance accesses shall be located
in permanent recorded easements. The SCM shall be shown and labeled within the
easement. Maintenance accesses shall:
(a) have a minimum width of ten feet;
(b) not include lateral or incline slopes that exceed 3:1 (horizontal to vertical); and
(c) extend to the nearest public right-of-way.
It is noted that easements are shown on the plans, but it is not clear that they
encompass the entirety of the SCMs.
Easements meeting the requirements noted above are provided for all SCMs. Additional
labeling has been provided on the grading plans, sheets C-15 through C-17, and on the SCM
detail sheets, C-18 through C-23, for clarity.
b. Make the following revisions to the detail sheets per 15A NCAC 02H .1042 (2) (h):
i. Label the side slopes of the SCM embankments and vegetated shelf on the
detail sheets.
The SCM embankment side slopes have been labeled on both the plan views and
cross-section views.
ii. Label the following elevations on the cross section/plan view:
peak attenuation above temporary pool (if applicable), temporary pool (top of
the temporary inundation zone), permanent pool (top of the shallow water zone),
bottom of shallow water zone, bottom of forebay deep pool at deepest point (at
forebay entrance), bottom of forebay deep pool at shallowest point (at forebay
exit), bottom of non-forebay deep pool at deepest point, and sediment cleanout
elevation.
The elevations noted have been added to all cross-sections as requested.
iii. All energy dissipaters (rip rap aprons) must be shown and sizing must
correspond to the calculations.
All dissipators have been shown and labeled, and the associated calculations are
included in Appendix M of the stormwater report.
11. Please do not upload entire plan sets as a combined file to the electronic repository,
but rather upload the relevant individual sheets under the appropriate file type.
Selecting the correct file type ensures that the files are properly named in the electronic
repository and it helps expedite the review process. NOTE: We do not require and we
discourage the upload/submission of extraneous plan sheets (such as lighting plans,
utility plans, striping plans, sight distance plans, etc...) as these files are not needed
for review.
This comment is noted.
12. Please revise the following in the calculations:
a. Please provide calculations on the project percentage of impervious area and ensure
the value corresponds to the Application Section IV, 8. 15A NCAC 02H .1042 (2) (f).
The requested calculations have been provided in Appendix Q of stormwater report.
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b. The NRCS Curve Number Method used to determine the design volume is calculated
incorrectly (Calculation guidance is provided in Part B of the Stormwater Design
Manual). The Discrete Curve Number Method requires the runoff volume from the
impervious areas and the pervious areas to be calculated independently and then
added together (instead of using the weighted average shown in the calculations).
Based on the information provided in the submission, the runoff volume for Wetland 1
(for example) would be closer to approximately 23,000 cubic feet (The
houses/sidewalks/etc... are not considered disconnected impervious surfaces unless
they are designed and permitted as such). The design of the stormwater wetlands was
not reviewed for correctness since sizing was based on incorrect design volumes.
The NRCS curve number calculations provided are correct and per calculations previously
approved by Jim Farkas for other stormwater permits. The impervious and pervious areas
were run independently, and no impervious surface was considered to be disconnected.
Appendix J of the stormwater report has been revised to include individual print outs for each
subcatchment for clarity (instead of one print out with all subcatchments, which may have
been what caused the confusion).
General comments about the stormwater wetland that would apply to a stormwater
wetland of any size that came up during the review are shown below:
i. Wetland 1: The deep pool area adjacent to the outlet should have a ring of
"Shallow Water Area" around it similar to the other deep pools.
The ring of shallow water area has been shown as requested.
ii. All Wetlands: Add a note stating that the deep pool plantings should not be
planted in the deep pool adjacent to the outlet structure (to prevent clogging).
The requested note has been added as note #4 in all cross-sections.
iii. All Wetlands: Please provide a detail for the outlet structure showing the
trash rack (Stormwater Wetland MDC 17).
Outlet structure and trash rack details have been provided for all wetlands adjacent to
the cross-section.
iv. All Wetlands: The inlets of SCMs shall be designed to protect the SCM from
erosion resulting from stormwater discharges. The outlets of SCMs shall be
designed so that they do not cause erosion downslope of the discharge point
during the peak flow from the 10- year storm event as shown by engineering
calculations. 15A NCAC 02H .1050 (4). One inlet calculation was provided per
SCM. Provide calculations demonstrating all inlets and outlets to the SCMs are
protected from erosion.
Calculations have been provided for all inlets and outlets in Appendix M of the
stormwater report.
c. Please verify the drainage area for Subcatchment 8S.
The drainage area for 8S is 116.34 acres as noted in Section 4.3 of the stormwater report,
shown on the post-development watershed map in Appendix H of the stormwater report, and
per the hydrologic model in Appendix I of the stormwater report.
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d. Demonstrate that stormwater outlets (of the site) do not cause erosion downstream
of the discharge point during the peak flow from the 10-year storm event through
engineering calculations, 15A NCAC 02H .1003 (5).
Calculations have been provided for all inlets and outlets in Appendix M of the stormwater
report.
e. Per Wetland MDC 10, The design volume shall draw down to the permanent pool
level between two and five days.The design volume used in the drawdown calculations
did not correspond to the design volume stated in the Supplement-EZ.
The design volume was incorrectly reported in the drawdown calculations for Wetland #1 and
has been corrected to match the Supplement-EZ. Refer to Appendix L of the stormwater
report.
f. Please provide a grading table or a "stage-storage" table for the portion of the
stormwater wetland that is located below the permanent pool so the portion of the
stormwater wetland that is located within the different zones can be verified and
Wetland MDCs 6-8 can be confirmed.
Grading tables with the requested information for MDCs 6 — 8 have been added to each
wetland detail sheet.
13. Please revise the following items as needed for consistency:
a.Wetland 3 Drainage Area: Per the Supplement-EZ and Application, this drainage area
is 388,408 SF or 7.77 acres. Per the provided calculations, the drainage area to this
SCM is 8.91 acres.
338,408 SF is equivalent to 8.91 acres, so the Supplement-EX, Application, and provided
calculations are correct.
14. Please correct the following with the Supplement-EZ
a. Cover Page, Line 2: The project area listed here should correspond with the
Application Section IV, 7 and omit the surface water area. 15A NCAC
02H.1003(4)(a).
The project area has been revised as requested.
b. Cover Page, Line 7: This refers to the width of the vegetated setback from any
surface waters that BUA is not allowed to be placed in (Per 15A NCAC 02H .1017(1)
& .1003(4). Per 15A NCAC 02H .1017(10) the minimum required vegetated setback
width is 30 ft, please revise to ensure this requirement is met.
This line has been revised as requested.
c. Assume the following items apply to all Stormwater Wetlands:
d. Line 20: Berms and baffles to improve the flow path were not provided on the plans.
Please revise this item or the plans.
This line has been changed to "no" for all wetlands.
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e. Lines 24-30: Please utilize elevations that correspond to the plans.
Per comment 10.b.ii, the elevations in lines 24-30 have been noted on the plans.
15. Please provide a Response to Comments letter indicating, in a point-by-point manner,
how each of the above comments has been addressed. This item is required per 15A
NCAC 02H .1042(3)(b).
This letter serves as the comment response letter.
Please let me know if you have any questions or need any additional information.
Sincerely,
Jon D. Frazier, PE, LEED AP
Principal
919.610.1051
[e-mail]
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