SW3240502_Response To Comments_20240813

L A NGA N Technical Excellence
Practical Experience
Client Responsiveness
9 August 2024
Via FedEx and Email
Brianna Holland
NCDEQ DEMLR Post-Construction Stormwater Management Program
512 North Salisbury Street, Office 625Y
Raleigh, NC 27604
Re: Response to Comments from NCDEQ DEMLR Post-Construction
Stormwater Management Program
Sage at Marvin
Village of Marvin, Union County, North Carolina
Langan Project No.: 281015401
Dear Brianna:
This letter is in response to comments received from North Carolina Department of
Environmental Quality (NCDEQ) Division of Energy, Mineral, and Land Resources (DEMLR) Post-
Construction Stormwater Management Program in a letter dated 30 July 2024. The comments
are identified in italics and Langan's responses are in bold.
1 . SUPPLEMENT-EZ FORM:
1x hard copy of the complete, original Supplement-EZ Form is required per Sections V&
VI, 3 of the Application
a. Cover
i. Lines 2-4, 6-10: Please complete.
Response: Lines 2-4 and 6-10 on the Cover Page portion of the Supplement-
EZ Form have been completed.
b. Drainage Areas
i. Entire Site Column, Off-site Area: Please clarify what the off-site portion of the
entire site is. Typically, there are no off-site portions to the entire site since the
entire site is comprised of the entirety of the site (The "drainage area" of the
entire site column is the project area).
Response: The "Entire Site" column on the Drainage Areas portion of the
Supplement-EZ Form has been updated to only include the on-site
stormwater runoff. This built upon area (BUA), is the BUA for the entire site.
227 W. Trade Street, Suite 320 Charlotte, NC 28202 T: 980.270.3900 F: 980.270.3901 www.langan.com
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Response to Comments from NCDEQ DEMLR Post-Construction 9 August 2024
Storm water Management Program Page 2 of 8
Sage at Marvin
Village of Marvin, Union County, North Carolina
Langan Project No.:281015401
ii. LD 1/LD 2 Columns: In accordance with 15A NCAC 02H . 1003(3)(b), any off-site
areas (areas located outside of the project area) that drain to the SCM must be
accounted for at their full build-out potential. The full build-out potential is
determined by either having the property owners enter into a legal agreement
limiting the amount of 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. Please
see additional information at the end of this letter. Please revise the package as
needed.
Response: The "LD 1" and "LD 2" columns on the Drainage Areas portion of
the Supplement-EZ Form have been updated to only include the stormwater
runoff that is actually going to the stormwater control measures (SCMs).
iii. LD 3 Column, Off-site Area. For Low Density, the Total Drainage Area proposed
is comprised only of an Onsite Drainage Area component. There should not be
"off-site"accounted for in a low-density area.
Response: The Drainage Areas portion of the Supplement-EZ Form has been
updated to only include the on-site stormwater runoff. Therefore, the "LD
3" column has been removed.
iv. Line 12: The size requirements of the SCMs and percent impervious area needs
to be able to be confirmed on the plans. The recorded deed restrictions are the
"full build out potential" of the subdivided lots, and the other items need to
correspond with the BUA shown on plans.
Response: The full buildout potential impervious area has been added as a
line item in the table on the Infiltration Basin Details. See CS507 and CS508.
v. Please revise the Application for consistency when editing the Supplement-EZ.
Response: The Post-Construction Stormwater Management Permit
Application Form (Application) has been updated to be consistent with the
Supplement EZ-Form.
c. Low Density
i. Vallow Column - Typically, we determine the maximum allowable velocity in a
vegetated conveyance based on the type of grass to be used and the type of
underlying soils (A table in Part C-11 of the Stormwater Design Manual
summarizes this information). Please provide some form of technical justification
(i.e., a spec or some other information from the manufacturer) showing the
maximum allowable velocities and/or sheer stress for the permanent conditions
so that we may verify that the conveyances will remain stable for the 10-year
storm event.
LANGAN

Response to Comments from NCDEQ DEMLR Post-Construction 9 August 2024
Storm water Management Program Page 3 of 8
Sage at Marvin
Village of Marvin, Union County, North Carolina
Langan Project No.:281015401
Response: The Vallowable column on the Low Density portion of the
Supplement-EZ Form has been completed based on the manufacturer's
specifications for the North American Green EroNet® P300® Permanent
Erosion Control Blanket that shall line all swales. See CS504.
d. Infiltration System
i. Line 26-27: Please provide.
The drawdown time requirement, in combination with the field-determined soil
infiltration rate, determines the minimum surface area for an infiltration system.
See Equation 1 below.
Equation 2:Minimum Surface Area for an Infiltration System
DV *12
SA = FS * K " T
where: SA = required minimum surface area of infiltration system(ft2)
FS = factor of safety(minimum of 2 is recommended)
DV = design volume(ft3)
K = hydraulic conductivity of soil(in/hr)
T = maximum dewatering time(72 hours)
Response: Lines 26-27 on the Infiltration System portion of the Supplement-
EZ Form have been completed.
2. PLANS:
2x hard copies of revised plan sheets with the following corrections made: (This item is
required per 15A NCAC 02H . 1042(2)(g) and per Section VI, 8 of the Application)
a. Please provide the following items per 15A NCAC 02H . 1042 (2):
(h) signed, sealed, and dated plan details of each SCM in plan view at a scale of one
inch equal to 30 feet or larger and a cross-section view... The plan details shall include:
(i) dimensions, side slopes, and elevations... ;
(ii)all conveyance devices, including inlet device, bypass structure, pretreatment area,
flow distribution device, underdrains, outlet device, energy dissipater, and level
spreader;and
(iii) specification sheets for materials used in the SCM...
Response: An updated Infiltration Basin Detail has been provided to include a
30-scale plan view and a cross-section view of each SCM, along with all
pertinent dimensions, side slopes, elevations and appurtenances. See CS507
and CS508. See CS505 for outlet control structure (OCS) details.
3. CALCULATIONS:
LANGAN

Response to Comments from NCDEQ DEMLR Post-Construction 9 August 2024
Stormwater Management Program Page 4 of 8
Sage at Marvin
Village of Marvin, Union County, North Carolina
Langan Project No.:281015401
1x signed, sealed, and dated hard copy of calculations with the following corrections
made: (This item is required per Section VI, 7 of the Application)
a. Provide drawdown calculations so that the factor of safety, and sizing requirements
can be verified.
i. Note: If the system does not drain within 72 hours throughout its operational life,
then it will be out of compliance with Infiltration MDC 5.
Response: Drawdown calculations have been added to Attachment 1, SCM
Calculations of the Stormwater Management Memo. See page 153 of the
Stormwater Management Memo.
b. Demonstrate compliance with 15A NCAC 02H . 1003 (5) for all outlets of the site,
including those not treated by an SCM. "Stormwater outlets 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. "
Response: Rip rap outlet protection is proposed at all discharge points. See
CG101 and CG102 and page 219 of the Stormwater Management Memo.
c. Demonstrate compliance with General MDC 4. "The inlets SCMs shall be designed
to protect the SCM from erosion resulting from storm water discharges. The outlets
of SCMs shall be designed so that they do not cause erosion immediately downslope
of the discharge point during the peak flow from the 10-year storm event as shown
by engineering calculations. "
Response: Rip rap outlet protection is proposed at all discharge points. See
CG101 and CG102 and page 219 of the Stormwater Management Memo.
d. Please provide a stage storage table so that the volumes can be verified. It appears
that a stage storage table was provided for skimmer basins, but not permanent SCMs.
i. Note: For an SCM, the design volume is the volume that can be stored between
the bottom of the SCM and the invert of the lowest bypass device.
Response: A stage-storage table has been added to the plans and the
Stormwater Management Memo. See CS507 and CS508 and pages 146 and
149 of the Stormwater Management Memo.
4. BUA:
a. The BUA allocated to the individual lots is shown as 13,500 SF/lot for 20 lots per the
deed restrictions, as 0 SF on the Application (Section IV, 10, "On-site Buildings/Lots"
Line of the Application), as 283,532 SF on the Supplement-EZ(Supplement-EZ Form,
Drainage Areas Page, DA 1 Column, Line 9).
LANGAN

Response to Comments from NCDEQ DEMLR Post-Construction 9 August 2024
Storm water Management Program Page 5 of 8
Sage at Marvin
Village of Marvin, Union County, North Carolina
Langan Project No.:281015401
Response: The BUA has been updated to be consistent across the Deed
Restriction, Application, Supplement EZ-Form and plans. There are 20
subdivided lots and three (3) existing lots that will be going to the proposed
SCMs. One (1) subdivided lot will be used for Infiltration Basin 2, and will not
have any built upon area, and one (1) existing lot (Parcel No. 06237003) has an
existing structure to remain and will not have any additional built upon area.
Therefore, 21 lots will have a full buildout potential of 13,500 SF, for a total of
283,500 SF.
b. Provide additional dimensions on the main set of plans so that the BUA information
shown in the Application, Supplement-EZ Form, and calculations can be verified.
Clarify how all impervious areas are being accounted for. This is required per the
Application Section VI, 8i. Note: General MDC 1 requires that the design volume of
SCMs shall take into account the runoff at build out from all surfaces draining to the
system. Drainage from off-site areas may be bypassed. The combined design volume
of all SCMs on the project shall be sufficient to handle the required storm depth.
Response: A Typical Lot Detail has been added to the plans to verify the BUA
information shown in the Application and Supplement EZ-Form. See CS100. The
Proposed CN Calculations also have a separate line item for full buildout
potential BUA. See pages 28 and 30 of the Stormwater Management Memo.
Additionally, as shown on Figure 4, Proposed Watershed Area Map in the
Stormwater Management Memo, Watersheds 1A and 2A account for the full
buildout potential of all proposed lots. All proposed impervious area is going to
a SCM. See page 9 of the Stormwater Management Report.
5. VEGETATED SETBACK (15A NCAC 02H . 1003 (4)):
"Vegetated setback" means an area of natural or established vegetation adjacent to
surface waters, through which storm water runoff flows in a diffuse manner to protect
surface waters from degradation due to development activities. 15A NCAC 02H . 1002
(51).
a. The following buffers are shown on the plans:
STREAM BUFFERS SHOWN: 30-FT STREAMSIDE ZONE PER UNION COUNTY ORDINANCE, 20-FT MANAGED USE ZONE
BUFFER PER UNION COUNTY ORDINANCE, 50-FT UPLAND ZONE PER UNION COUNTY ORDINANCE, AND 200-FT
NCOEQ UNDISTURBED BUFFER.
Please ensure the required vegetated setback/buffer from the surface water is clearly
shown on the main set of plans (per 15A NCAC 02H . 1042(2)(g)(iii)).
Note that the DWR required undisturbed buffer may be different than the vegetated
setback required per 15A NCAC 02H. 1003 (4). The minimum vegetated setback must
be 30 ft; the setback must be measured from the top of bank and not the centerline.
15A NCAC 02H .1017(10), 15A NCAC 02H.1003(4)(a), 15A NCAC 02H.1042(2)(g)(iii).
LANGAN

Response to Comments from NCDEQ DEMLR Post-Construction 9 August 2024
Storm water Management Program Page 6 of 8
Sage at Marvin
Village of Marvin, Union County, North Carolina
Langan Project No.:281015401
Response: The buffers shown on the plan are measured from the top of bank.
The top of bank linework has been modified to be a thick blue line for clarity on
the plans. See CG101 and CG102.
b. The top of bank is clearly shown for Marvin Branch, but please clarify the top of bank
for Six Mile Creek so that the vegetated setback can be confirmed.
Response: The top of bank linework for Marvin Branch and Six Mile Creek has
been modified to be a thick blue line for clarity on the plans. See CG101 and
CG 102.
c. Per 15A NCAC 02H .1003 (4) (b): Vegetated setbacks may be cleared or graded, but
shall be replanted and maintained in grass or other vegetation. Please ensure this
requirement is met, riprap and outlet structures do not meet this requirement,
Response: The outlet pipe, headwall and rip rap for both basins are outside of
the 30-FT vegetative setback from the top of bank. Only a grass-line swale shall
be within the setback. See CG101.
6. INFILTRATION MDC 1: SOIL INVESTIGATION:
a. The plans should show the requirement provided in the soils report (pasted below):
Based on knowledge of the surrounding in-situ soil, it was decided prior to testing to replace the
in-situ soil below the basins with an engineered infiltration media. Therefore, infiltration testing
was not needed nor completed. However,prior to installation,the contractor shall provide testing
from the manufacturer that shows the engineered media has an infiltration rate of 5 inches/hour.
Response: The soil replacement specifications discussed in the Soil Testing
Memo has been added to the Infiltration Basin Details. See CS507 and CS508.
7. INFILTRATION MDC 2: SEPARATION FROM THE SHWT:
a. Please provide the Seasonal High Water Table (SHWT) elevation for the infiltration
basin. The provided soils report indicates the depth from the existing ground surface
elevation to groundwater (which is not the same as the SHWT). Please revise as
needed. Per 15A NCAC 02H. 1002 (45) "Seasonal High Water Table"or"SHWT"means
the highest level of the saturated zone in the soil during a year with normal rainfall.
SHWT may be determined in the field through identification of redoximorphic features
in the soil profile, monitoring of the water table elevation, or modeling of predicted
groundwater elevations. This was not provided in the provided report.
LANGAN

Response to Comments from NCDEQ DEMLR Post-Construction 9 August 2024
Storm water Management Program Page 7 of 8
Sage at Marvin
Village of Marvin, Union County, North Carolina
Langan Project No.:281015401
On 15 April 2024, two(2)test pits were observed by Langan to determine the SHWT(see Figure
1, Test Pit Plan). For Test Pit 1, rock was encountered at approximately 6.5-FT below grade (el.
569.60), and no initial groundwater was observed. For Test Pit 2. no rock was encountered, and
no initial groundwater was observed Both test pits were left open for 24 hours,and groundwater
levels were observed at approximately 6.17-FT below grade (el. 570.00) for Test Pit 1, and at
approximately 16.5-FT below grade(el. 573.00)for Test Pit 2 These final groundwater elevations
were used to design the infiltration basins as outlined below.
Response: There were no indicators of seasonal high water table (SHWT)within
either test pit. Indicators refer to the presence of soil mottling of chroma 2 or
less. Therefore, to be conservative, the observed groundwater seepage was
used as the SHWT for both infiltration basins. Please refer to the attached
updated Soil Testing Memo for additional information.
8. 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).
Response: This comment response letter has been provided to satisfy the
requirements of 15A NCAC 02H .1042(3)(b) and to aid your review.
9. While it is not required, it is strongly recommended to include an emergency spillway for
SCMs. SCMs that overtop without an emergency spillway can fail catastrophically.
Response: An emergency spillway directed to Marvin Branch has been added to
Infiltration Basin 1. Infiltration Basin 2 does not have an emergency spillway
because the basin is in a naturally low lying area with nowhere for the emergency
spillway to be graded or directed to. However, if the basin overtops, the
stormwater will flow into the swale along the road,then to the nearby low point in
the road, and finally make its way to Six Mile Creek. Therefore, the downstream
impacts will likely be minimal. See CS507, CS508 and page 156 in the Stormwater
Management Memo.
10. 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.
a. PDFs must be uploaded using the form at:
https://edocs.deq.nc.gov/Forms/SW-Supplemental-Upload
Response: Acknowledged and will comply.
b. Hard copies must be mailed or delivered to the following address:
LANGAN

Response to Comments from NCDEQ DEMLR Post-Construction 9 August 2024
Storm water Management Program Page 8 of 8
Sage at Marvin
Village of Marvin, Union County, North Carolina
Langan Project No.:281015401
i. For FedEx/UPS:
Brianna Holland
512 N. Salisbury Street, Office 625Y
Raleigh, NC 27604
ii. For USPS:
Brianna Holland
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. Do not
leave the package in the foyer with the security guard.
NOTE: Hard copies should not 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.
Response: Acknowledged and will comply.
Should you have any questions, or should you require additional information, please do not
hesitate to contact me at [phone].
Sincerely,
Langan NC, Inc.
Jonathon Rhine, PE
Senior Project Manager
Enclosure(s): Site Plan Set
Stormwater Management Memo
Soil Testing Memo
Stormwater Management Permit Application Form
Supplement EZ-Form
cc: Brandon Maxwell — Sage Equity
NCDEQ Mooresville Regional Office
North Carolina Business Certificate of Authorization No.2509320
License No.C-5001 (PE/PS;Exp.06/30/20251/C-664(LA;Exp.06/30/2025)
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LANGAN