SW3180905_Response To Comments_20210712

LandDesign.
CREATING PLACES
THAT MATTER.
July 12, 2021
NC Department of Environmental Quality
Division of Energy, Mineral and Land Resources
512 North Salisbury Street
Raleigh, NC 27699-1612
RE: Request for Additional Information
Stormwater Permit No. SW3180905
Twin Lakes
Union County
Dear Mr. Anen,
Thank you very much for recent coordination and correspondence on the Twin Lakes Project. After
several meetings together, we have revised a complete Stormwater Resubmittal for your review
and approval. This package includes:
• Revised Low Density Permits and O&M EZ forms (signed and notarized)
• Revised Sheet C205: Project Area and Impervious Area Summary (dated 7/2/2021)
• Revised Chapter 4: Storm Drainage and BMP Plans + Details (dated 7/2/2021)
• Stormwater Swale Drainage Area Exhibit
• Revised Strom and Erosion Control Calc Book (dated 7/2/2021)
• Copies of Deeds
Please find the below responses to the feedback below from NCDEQ, dated May 14, 2021, for the
above -mentioned project. These responses reflect the proposed changes to the originally approved
low -density plan as discussed at our virtual meetings. Please do not hesitate to reach out with any
questions or concerns.
1. Please address the following comments with regard to the low -density area:
a. Why was the low -density area broken up into two areas on the design
plans? This should be handled as one area as it was on the application
form.
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LD Response: The drainage boundary is very similar to the previously
approved delineation, breaking the drainage areas at the existing berm
between the two lakes. Low Density Area #1 routes into the Upper Lake
(which acts as a detention pond in PondPack routing), while Low Density
Area 42 bypasses it and discharge to the downstream POI. The drainage
areas remain separated for the detention analysis but have been
combined for the BUA and density analysis for clarity.
b. The low -density area is designed with 27% BUA according to the
application form (26% for LD 1 and 28% for LD2 according to Supplement
EZ). This amount of built -upon area exceeds the 24 % threshold for low
density. The built -upon area in the low -density area must be reduced or
the area will be required to be designed as high density (15A NCAC 02H
.1003(2)(a) and. 1017(4)].
LD Response: The design has been revised to reflect the entire site as
low density. Calculations have been updated to reflect that the entire
project area meets 24% maximum impervious cover for law density
development.
c. The streets in the low -density area use curb and gutter and pipe runoff
away from the site. This is not allowed under the low -density regulations,
which specify that "Stormwater that cannot be released as dispersed flow
shall be transported by vegetated conveyances. A minimal amount of
non -vegetated conveyances for erosion protection or piping for driveways
or culverts under a road shall be allowed by the permitting authority when
it cannot be avoided." The design does not meet the low -density
standards and shall be revised to remove the piping and propose
vegetated conveyances or the area will be required to be designed as
high density £15A NCAC 02H .1003(2)(c)].
LD Response: After receiving feedback that the originally approved
design does not meet the low density criteria, we have revised the
original design as much as feasible based on the site grading and
roadway design to better fit DEQ low density standards. We
accomplished this is two ways:
First we identified all locations where the proposed pipes storm drainage
outlets could be switched to curb outlet systems. We identified 4 locations
where we are now proposing the curb outlets that directly discharge to
vegetative swales, as preferred by DEQ.
Where we could not revise the system to utilize a curb outlet, we made
every effort to reduce the amount of pipe proposed throughout the site.
We reviewed every proposed outfall and were able to reduce the
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proposed pipe length total by a reduction of approximately 1,533 LF of
pipe from the previous design.
All vegetated swales meet the minimum required 100 LF from the outfall
towards the existing vegetated low areas onsite. The vegetative swales
meet all criteria for length, slope and outfall velocity outlined in the NC
State Low Density requirements throughout the low -density area.
We understand DEQ's position that "a minimal amount of piping does not
indicate the minimum amount of piping required to make this site plan
work." We have made great efforts to improve upon the previously
approved design to better align with the preferences and feedback that
were shared with us. Due to this project being a revision to a previously
approved plan, we are constrained by the limit of available areas for
revisions but strongly believe we have improved the system and better
meet the intention of low -density development.
LandDesign will use this feedback in future projects and applications to
DEQ where we have the ability to revise the overall site design, and
sincerely appreciates any flexibility in this application, where we are
revising a previously approved low -density design that is already partially
under construction.
d. A portion of the low -density area at the southeastern part of the
site falls in an area labeled "Future Senior Living Area". Note that
the application does not specify any future built -upon area in the
low -density area. Any future development in this area will require a
major modification to the permit. This area cannot be used in the
density calculations for any future development £15A NCAC 02H
.1002(38) j.
LD Response: No impervious area for this future development is
being accounted for in this submittal. Despite having the same
ownership, this project boundary does not include this Future Senior
Living Development. We understand the future development in this
area will need to treat their impervious area in a separate system.
That application will be submitted and permitted with NCDEQ
separately.
Please see sheet C205 for the Project Area Boundary limits.
e. Part of the development on the western side of the project was not
included in the low -density area (parts of lots 45-48). This area should be
included regardless of where the runoff drains because it is an area
regulated under this permit.
LD Response: The project area boundary has been updated to include
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the entirety of these lots (see Sheet C205). The drainage area sheet
remains the same, reflecting the existing drainage patterns of the site.
2. Each wet pond detail (Sheets C416, C417, and C418) have a note on the wet
pond in a text box with design information for the wet pond. The label for "TOP
OF BERM" on each of these details in incorrect.
LID Response: We have reviewed and confirmed the Top of Berm elevations are
shown correctly on the plan and section views on the referenced sheet. Contour
Labels will be added to plan views to better clarify. Please note that (2) BMPs have
been removed from this revised submission as we are not long proposing a mixed
density application.
3. The drain times for the wet ponds should be calculated using the orifice
equation and methodology in Part B: Stormwater Calculations of the
Stormwater Design Manual. When calculating the drawdown time using this
standard calculation both Wet Pond 2 and Wet Pond 3 drain in less than two
days. Wet ponds are required to drain the runoff from the design storm in 2-5
days f 15A NCAC 02H .1053(7)J.
4.
LID Response: We calculated drawdown time based on the Bernoulli Equation as
shown in Step 3 of the Wet Pond Design Sheets in the provided Calc Book, showing
detention of the WQ Storm event in 2.90 days. Land Design has confirmed that the
10-yr and 25-yr storm also discharge in the 2-5 day timeframe via the orifice equation
in Part B of the Stormwater Design Manual. This has been provided on a
supplemental sheet of calculations in the Calc Book on Page 144,
5. Please show an easement and maintenance access for each SCM that
complies with General MDC 8 and 9 [15A NCAC 02H .1050(8) & (9)J,
LID Response: Easements and Maintenance Access are proposed surrounding each
BMP and all proposed storm drainage pipes. Additional labels will be added to BMP
Sheets for clarity easement limits.
6. Please use the most up-to-date version of O&M EZ, which includes the option to
specify areas of low density.
LID Response: Land Design has provided the most up-to-date version of the O&M EZ,
dated 12/15/2020.
7. Please upload an electronic copy of the property deed. This is required on all
submittals and is currently missing from the permit file.
LID Response: We believe this was provided but will confirm it is provided in
resubmittal upload to link below.
8. Provide pdfs of all revisions, 2 hardeopies of revised plan sheets, and 1 hardcopy
of other documents. Pdfs must be uploaded using the form at:
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https://edocs.deq.nc.gov/Forms/SW Project Submittal
LD Response: LandDesign will provide all documents and uploads are requested for
resubmittal.
We very much appreciate your review of this summary response to the preliminary feedback. Any
clarifications or direction you can provide to the boldlhighiighted requests above is welcomed as
we revised the plans for resubmission.
Please do not hesitate to reach out to me directly at any point if there is anything that I can clarify.
I can be reached at [e-mail] or [phone] x1011 anytime.
Thank you,
�1��
Mega chultz, PE
Senior Civil Designer
LANDDESIGN.COM