SW3240503_Response To Comments_20240612

EspMay 10, 2024
North Carolina Department of Environmental Quality
512 N. Salisbury Street, Office 640M
Raleigh,NC 27604
RE: Request for Additional Information Stormwater
Permit No: SW3231101
Project Address: 6308 Weddington Rd, Matthews,NC 28104
Dear Mr. Farkas:,
Please find our responses to the comments dated January 23,2024 below. Should you have
any questions or need any clarification please feel free to reach out to us at 704-990-9429
or by email at [e-mail]
Stormwater Comments:
1. As designed, this project does not meet the requirements to be considered as low-density
per 15A NCAC 02H .1003(2). Please note that a project must meet all of the low-density
requirements specified in 15A NCAC 02H .1003(2), not just the project density
requirement, in order to be considered as low-density. The design of this project
incorporates excessive piping (prohibited via 15A NCAC 02H .1003(2)(c)) and does not
appear to be designed in a manner that maximizes dispersed flow (required per 15A
NCAC 02H .1003(2)(b)). Please revise as needed.
Response: The project was designed as high density.
2. As designed, this project does not appear to meet Runoff Treatment (as defined in 15A
NCAC 02H .1002(43) and as required for high-density projects per 15A NCAC
02H .1003(3)(a)). 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)) and the BUA captured and treated in the SCMs is maximized(allowed with
Division approval per 15A NCAC 02H .1003(1)(d)). NOTE: High-density projects
may alternatively meet Runoff Volume Match(defined in 15A NCAC 02H .1002(44))
however, wet ponds and sand filters are ill-suited to meet this treatment method.
Response: The off-site area draining to SCM#1 is developed (2 lots) and is taken
into account for the design of SCM#1 with remaining off-site area bypassing the SCM
including the runoff from Weddington Road. The off-site area draining to SCM#2 is
minimal and is developed with a portion being bypassed. It is our opinion we meet the
Runoff treatment definition as all BUA is being treated that is draining to the SCM's.
Refer to Drainage Area Map (C4.12) for included off-site areas.
ESP Associates,Inc.
20484 Chartwell Center Drive • Suite D • Cornelius,NC 28031
1.800.960.7317 • 704.990.9428 • fax [phone]
www.espassociates.com

3. As designed, SCM 2 is considered to be just a sand filter and not a dry detention basin&
sand filter(the sand filter SCM includes the sand layer in addition to the storage provided
above the sand layer in the sand chamber and sediment chamber). Please revise the
submittal materials as needed to reflect this.
Response: SCM#2 is designed as a dry detention basin and sand filter. The entire
runoff is being treated within this basin for peak attenuation as well as water quality. The
basin is designed to treat the first flush (1") and the 1, 2, 10 & 25-year storms.
4. Please ensure that off-site portions of the SCM drainage area are accounted for at their
full build-out potential as required per 15A NCAC 02H .1003(3)(b). Full build-out
potential can be determined by having the property owners enter into a legal agreement
limiting the amount of BUA and drainage area draining from the off-site area to the
permitted SCM or by assuming that the off-site area is 100%BUA(its full build-out
potential). It is recommended to bypass off-site areas so that they do not drain to the
SCM as the permittee has no control over property that they do not own. See additional
information following comments for more information.
Response: We have increased the bypass areas for off-site runoff, therefore no BUA
for off-site is required to treat. The off-site area draining to SCM#2 is open space and lots
are built out and draining in opposite direction.
5. There appears to be inconsistencies in the BUA and drainage area accounting for the
project as shown in the submittal items. One such example of this is the on-site/off-site
drainage area values as shown in the Application Form (Section IV, 10) and the
Supplement-EZ Form(Drainage Areas Page). Another such example is that the BUA
amounts and types as shown in the Application do not correspond to the amounts and
types as shown in the Supplement-EZ Form. Please ensure that the information as shown
throughout the submission is consistent and correct. Revise as needed.
Response: The Application and Supplement EZ-Form has been updated.
6. General MDC 4—Please provide calculations showing that the stormwater outlets are
stable in accordance with this MDC. It is noted that the riprap sizing is provided in the
plan set, however, calculations verifying the sizing is adequate is required per 15A
NCAC 02H .1042(2)(f).
Response: Riprap calculations have been included.
7. Please correct the following issues with the design of the wet pond(SCM#1):
a. Wet Pond MDC la—There appears to be an issue with the provided HRT
calculation. When calculating the minimum required main pool volume, the design
volume of the SCM (volume stored between the permanent pool and temporary
pool) needs to be used in the calculation. NOTE: The temporary pool surface
elevation is determined by the invert of the lowest bypass device (per the design,the
weirs at elevation 575.0 ft). This results in a design volume of approximately 41,198
cf(VMPmin=0.87 * 14 days/ 5 days * 41,198 cf= 100,358 cf> 61,032 cf
provided). Please revise as needed.
Response: An orifice has been added at temporary pool surface to meet VMPmin.

b. Wet pond MDC 3 —It is noted that there is a note on the plans indicating to excavate
the pond 0.5 ft below the shown grading provided on the plans to account for the
sediment storage, however the note is not very prominent and could be easily
missed. Please ensure that the plan and cross-sectional view of the SCM shows the
actual proposed grading (including the excavated sediment storage area) and include
the top of the sediment storage area on the cross-sectional view of the SCM. This is
required per 15A NCAC 02H .1042(2)(h).
Response: Added sediment storage contour to plan and profile.
c. Wet Pond MDC 5a—As designed, the forebay volume (13,162 cf) is more than 20%
of the main pool volume (61,032 cf) 13,162 cf/61,032 cf=21.6%>20%.
Response: Adjusted size of forebay to meet threshold.
d. Wet Pond MDC 5b—Please indicate on the plans the elevation of the excavated
forebay bottom near the entrance of the forebay and near the exit of the forebay and
show that the bottom near the entrance is deeper than the bottom near the exit. Per
Wet Pond MDC 3, there should be at least 6 inches of sediment storage provided
above the excavated bottom of the forebay(near the entrance and near the exit).
Response: Revised forebays to indicate entrance lower than exit.
e. Wet Pond MDC 7—Similar to the HRT comment, please ensure that you are using
the design volume (volume stored between the permanent pool and temporary pool)
for this calculation.
Response: Design volume used for draw down time.
f. Wet Pond MDC 1 lb—The provided planting density(4 plants per 100 sf of shelf
area) is less than the minimum required 50 plants per 200 sf of shelf area(4 plants
per 100 sf= 8 plants per 200 sf< 50 plants per 200 sf). Please revise as needed.
Response: Revised plant quantities.
g. While not required, it is strongly recommended to select an SCM that does not pond
water(in accordance with GS 143-214.7(c3)) due to the proximity of this project site
to Charlotte-Monroe Executive Airport.
Response: Because of the large area draining to this section a wet pond was the more
desirable SCM.
8. Please correct the following issues with the design of the sand filter(SCM#2):
a. Sand Filter MDC 1 a—Please provide the SHWT elevation for this SCM. Please note
that the elevation of the SHWT (and not just the depth from the existing ground
surface)needs to be provided (or be able to be calculated from the depth of the
boring and the elevation of the existing ground surface for the boring location).
Please also note that the SHWT is different than encountered groundwater. As
indicated in the provided soils report, groundwater levels fluctuate over time
(whereas the SHWT is more or less constant for a given location). Please revise as

needed.
Response: ESHWT depth shown on profile view. (see report attached)
b. Sand Filter MDC 2—Please ensure that the sand chamber and sediment chamber are
equivalently sized. Please provide a stage-storage table (similar to the one provided
for the wet pond)with the sediment and sand chambers separated so their volumes
can be compared. If the SCM is being designed to attenuate peak flows, the
sediment chamber may be oversized, provided that at least half of the minimum
required design volume is provided in both the sand and sediment chambers. Please
revise as needed. NOTE: The design volume of a sand filter is the volume that can
be stored above the top of the sand layer/bottom of the sediment chamber and below
the invert of the lowest bypass device.
Response: Stage/Storage table added. This pond is being used for attenuation
therefore the storage of sand filter/sediment basin is more than half the required
volume (1/2 design=5,026 cf; sediment storage below orifice=5,265 cf)
c. Sand Filter MDC 3 —NOTE: You are allowed to reduce the minimum required
treatment volume of a sand filter as indicated in the MDC (see MDC for more
information).
Response: Reduced the volume accordingly along with sand filter.
d. While not required, it is strongly recommended to reduce the drainage area to the
sand filter to no more than 5 acres. Sand filters with drainage areas that are larger
than 5.0 acres tend to have more maintenance issues when compared to sand filters
with drainage areas that are less than 5.0 acres. Maintenance issues increase the cost
of ownership of the SCM over the life of the SCM.
Response: We reduced the drainage area as much as possible to this SCM#2 bit it will
remain a sand filter.
e. While not required, it is strongly recommended to reduce the amount of pervious
area draining to the SCM. Pervious areas produce fine particles that tend to clog the
sand filter resulting in more frequent maintenance of the sand filter. More frequent
maintenance increases the cost of ownership of the SCM over the life of the SCM.
Response: We reduced the amount of pervious area to this SCM#2 as much as
possible.
9. Since the Applicant was indicated as a purchaser of the property, please provide a copy of
the pending sales agreement(per Section III, 1.c. of the Application and 15A NCAC 02H
.1042(2)(c)).
Response: Please find the sales agreement attached.
10. Please correct the following issues with the Application Form:
a. Section II, 2—Revise as needed.
b. Section IV, 10—In addition to the earlier comment with regard to consistency,
please ensure that the sum of the impervious surface areas is correct.
c. Section IV, 11 —Please complete this item.

Response: All items revised as noted.
11. Please revise the O&M Agreement Form as needed per the other comments.
Response: Updated O&M Agreement; dry pond will remain as it is our opinion that
this basin was designed as sand filter with dry detention basin.
12. Please correct the following issues with the Supplement-EZ Form(In addition to any
previously mentioned items):
a. Cover Page:
i. Line 4—Please include this item (it should correspond to Section IV, 6 of the
Application).
ii. Line 7—Please refer to 15A NCAC 02H .1017(10) for the minimum
required width of the vegetated setback from surface waters.
iii. Line 22—Please remove this item.
Response: Revised accordingly.
b. Drainage Area Page:
i. The entire site column is an accounting of the entire project area, similar to
how the drainage area columns are an accounting of the areas draining to the
SCMs. Please ensure that all BUA located within the project area, whether or
not it drains to the SCMs, is accounted for in this column. When the entire
site column asks for the "drainage area"please enter the project area
(corresponds to Section IV, 7 of the Application). Typically, there are no off-
site components for the entire site column (the entire site column is not
necessarily a sum of the other columns).
ii. Please ensure that only the BUA allocated to the individual lots is shown on
Line 9. BUA that is allocated to common areas (such as the roadways)
should be shown on Lines 10 & 12 (The sum of the items of Line 12 add up
to Line 10).
iii. Please revise the BUA accounting so it is consistent throughout the
submission.
iv. Line 20—Please revise (see later comments).
Response: Revised accordingly.
c. Wet Pond Page:
i. Line 25 — See earlier comment with regard to the design volume.
ii. Line 34—The clean out depth of the forebay is the distance from the
permanent pool to the top of the sediment storage zone.
iii. Line 35 — See earlier comment with regard to the design volume of the SCM.
iv. Line 40— See earlier comment with regard to the design volume of the SCM
and drawdown time of the SCM.
v. Line 42—There do not appear to be berms or baffles that increase the
flowpath of the SCM.
vi. Lines 43 &44—The depth of the forebay at the entrance is the distance from
the permanent pool to the excavated bottom of the forebay near the entrance
of the forebay. The depth of the forebay at the exit is the distance from the

permanent pool to the excavated bottom of the forebay near the exit of the
forebay.
vii. Line 48—The detail indicates that there is a downturned elbow that draws
down the temporary pool from below the permanent pool.
Response: Revised accordingly.
d. Sand Filter Page:
i. Line 8—There appears to be a bypass mechanism(weir) located in the outlet
structure. This item is required per General MDC 5.
ii. Line 18—Please include this value. If the SHWT elevation was not
encountered in the boring, list the elevation of the lowest point of the boring
and indicate in the additional information section that the SHWT elevation
was not encountered above this elevation.
iii. Lines 23 & 30—The sum of these values equals the design volume (volume
that can be stored above the top of the sand layer/bottom of the sediment
chamber and below the invert of the lowest bypass device).
iv. 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.
v. Line 28—This item is Line 27 minus the depth of any permanent pool
provided in the sediment chamber.
vi. Line 33 —This item refers to the distance from the excavated bottom of the
sand chamber(Line 31) to the invert of the lowest bypass orifice/weir.
vii. Line 34—This item refers to the distance from the top of the sand layer to
the invert of the lowest bypass orifice/weir.
viii. Line 35—Per the design, the elevation of the bypass device is at elevation
576.5 ft(If the cell does not allow for decimal numbers to be entered, enter
the actual value in the additional information section).
ix. Line 39—This item refers to the depth of sand over the underdrain
(minimum of 12 inches per the MDC).
Response: Revised accordingly.
e. Dry Pond Page—Please remove this page.
Response: It is our opinion that the dry detention basin is part of the sand filter
(remained in O&M Agreement).
Please feel free to reach out to me with any questions/clarifications at
[e-mail] or 803-802-2440.
Sincerely,jESP ssociates, In .
Canis E. Simmons, PE
ivil Engineering Director