SW3221008_Response To Comments_20230208

February 8, 2023
Mr. Jim Farkas
State Stormwater Engineer
DEMLR Post- Construction Stormwater Program
Subject: Response for Additional Information
Stormwater Permit No. SW3221008 (Formerly SW3210802)
The Estates of Marvin Branch
Union County
1. SW3210802 Prior Comment 1 — "The SCMmust meet all of the applicable MDCs of an
SCMper I5A NCAC 02B . 0602(b). The provided detention basin is not an SCM, the
information for this "SCM" was not provided in the Supplement-EZ or O&MAgreement
Forms, and sufficient design calculations demonstrating that all of the MDCs were met
were not provided. Please ensure that the provided stormwater treatment device meets all
of the applicable MDCs for an SCM. " As designed, the infiltration SCM does not meet
all of the applicable MDCs:
a. Infiltration MDC 1 — The provided soil report does not indicate the soil infiltration
rate (A value of 0.55 in/hr was provided but it is unclear where this value came from).
Response: The infiltration rate of 0.55 in/hr was an estimated rate provided by the Geotechnical
Engineer for the soils located at the SCM. Infiltration testing was performed and the report has
been provided. The average infiltration rate for the existing soils was found to be 0.31 in/hr. all
documentation has been updated accordingly to reflect 0.31 in/hr.
b. Infiltration MDC 2 — Per the information provided in the soils report, there is
insufficient separation between the bottom of the SCM and the SHWT. The soil report
indicates that the SHWT is at elevation 577.0' while the plans and other submittal
materials indicate that the bottom of the SCM is at elevation 578.0'. Please revise as
needed. NOTE: The provided soils report does not satisfy the "hydrogeologic evaluation
that demonstrates that the water table will subside to its pre -storm elevation within five
days or less" portion of Infiltration MDC 2 since this information was not mentioned in
the report.
Response: The bottom of the proposed Infiltration basin is 581.1, which is not within 2 feet of the
ESWHT (ELEV 577) The callout on the plans was for the amended soils depths to be provided.
We are removing that reference as to not be confusing. We have revised the proposed SCM to
provide enough surface area of existing soils for infiltration at 0.31 in/hr as determined from the
Geotechnical report provided.
Civil & Environmental Consultants, Inc.

c. Infiltration MDC 4 — Pretreatment does not appear to be provided and is required for
this SCM. Please revise as needed.
Response: Pretreatment will be roof gutter screens, gravel/ stone outlet protection, sheet flow
runoff through vegetated lawns on all lots as well as HOA landscaping services that will remove
all potential clogging debris on a biweekly basis.
d. Infiltration MDC 5 — The provided design does not appear to dewater the design
volume within the stipulated timeframe. Per the provided documents, the design
volume of the SCM (DV) is equal to 15,419 cf (NOTE: The design volume of the
SCM must be used for this calculation, not just the minimum required treatment
volume), the provided infiltration surface area (SA) is equal to 5,695 sf, the
infiltration rate of the Soil (K) is equal to 0.55 in/hr (NOTE: It is unclear where this
value came from), and a factor of safety (FS) of 2.0 (This minimum factor of safety
ensures that the SCM will function properly). Rearranging Equation 2 in Part C-1 of
the Manual to solve for time results in a drawdown time of about 118 hours which is
larger than the maximum allowable of 72 hours. Please revise as needed.
Response: Calculations as well as surface areas of the proposed SCM have been revised to 0.31
in/hr in order to meet the requirements for drawdown of provided volume within 72 hours. Based
on solving for Time on Eq-2, the Drawdown time is 71.87 hrs. The volume provided within the
SCM is the volume used for the drawdown calculation.
2. SW3210802 Prior Comment 6 — "The provided detail for the `culvert' swale does not
appear to match the calculations. The calculations indicate that the bottom with of this
swale is 6 ft while the detail shows a bottom with of 2-3 ft." Please clarify the details for
the proposed swales. Plan sheet C-3.0 shows the stormwater swale detail as having a
bottom width of 6 ft and 5:1 side slopes. The culvert swale appears to have side slopes of
3:1 (per the grading plans) as do the curb outlet swales. The curb cuts plan view detail on
this sheet indicates that the bottom width of the curb outlet swales is 1 ft wide whereas a
2 ft bottom width is required per 15A NCAC 02H .1003(2)(d)(iii). It is unclear which
cross-section or methodology is used to determine the 10-year design velocity in the curb
outlet vegetated areas. Please revise as needed. Response: The bypass culvert channel is a 6ft
bottom with 3:1 side slopes, swales 6-10 are 2ft bottom widths and 3:1 side slopes. The vegetated
paths are on natural grade overland at 10:1 side slopes based on natural grade. All details have
been revised to match accordingly.
3. The provided swale calculations appear to be off. For example, the provided calculations
for curb outlet swale 8 (CC#8) shows a drainage area of 0.68 ac (and a design flowrate of

2.59 cfs) however the Supplement-EZ Form shows a drainage area of 0.46 ac (and a
design flowrate of 1.75 cfs). These differences also lead to a discrepancy in design
velocities between the calculations (3.21 fps) and Supplement-EZ Form (2.82 fps). Please
revise as needed for consistency.
Response: The calculations have been revised to depict the correct values for the swales.
4. Please clarify how the actual velocity was determined for the curb outlet vegetated areas
as the calculations do not appear to have been provided.
Response: The velocities were calculated by use of Bentley systems hydraulic software and was
provided in previous calculations. You requested that only information that was revised be
submitted.
5. Please correct the following issues with the Supplement-EZ Form:
a. Drainage Areas Page:
i. Line 2 or 4 — Please include a drainage area for the Infiltration SCM (This form
was not set up with the Goose Creek Rules in mind so feel free to list it as either a
high- or low -density drainage area and explain in the Additional Information area
(Line 22) that the SCM is required to meet the Goose Creek Rules).
Response: Additional information has been added for the SCM drainage area.
ii. New column for the drainage area to the Infiltration SCM — Please fill out the
information in this column.
Response: Additional information has been added for the SCM drainage area.
b. Infiltration Page:
i. Line 2 — Please include the minimum required treatment volume. This is the
difference between the pre- and post -development 1-year 24-hour storm volumes.
Response: Additional information has been added for the minimum required treatment
volume.
ii. Line 8 — This item is required per General MDC 5 and appears to be provided in
the form of the emergency spillway.
Response: Additional information has been added.
iii. Line 13 — Please answer this item as it is required per General MDC 10. If the
SCM is not located on a single-family lot, you can answer "N/a".
Response: N/A provided
iv. Line 20 — Please elaborate on the type of pretreatment provide in the Additional
Information section (Line 46).
Response: Added information to Line 46.

v. Line 25 — This item does not appear to have been provided.
Response: Not provided as we are not within those required parameters of 1-2ft. The
proposed bottom of the basin is at elevation 581.1. ESHWT is at elevation 577.
vi. Line 26 — It is unclear where this value came from.
Response: Proposed K for amended soils.
vii. Lines 32-43 — This SCM configuration appears to be that of an infiltration trench
instead of an infiltration basin (The main difference between the two being that
trenches contain stone or some other material in the storage area while basins do
not). We are proposing a basin with amended soils with no underdrain system. No stone
is proposed. Existing soil will be amended in place to achieve an infiltration rate of lin/hr.
complete Lines 34-43. N/A
viii. Line 45 — Please revise as needed. Revised
c. Low -Density Page:
i. Line 4 — Please answer this item. If multiple side slopes are used, please elaborate
in the Additional Information section (Line 15).
Response: Additional descriptions have been added to line 15 for clarity on the swales
versus the vegetated paths.
ii. Line 7 — Please revise to reflect the maximum slope value from the swale design
table.
Response: Max slope has been revised on line 7, it is 4.11% but cell rounds to 4%.
iii. Swale design table — Please include the design information for "swale" (curb outlet
vegetated area) 5. Response: Information added to table.
Sincerely,
CIVIL & ENVIRONMENTAL CONSULTANTS, INC.
Mason Greeson, RE
Principal