SW3231104_Response To Comments_20240318

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ENGINEERS Pi SURVEYORS
March 15, 2024
Ms. Brianna Holland
DEMLR Post-Construction Stormwater Program
512 N. Salisbury Street, Office 625Y
Raleigh, NC 27604
RE: Stormwater Permit No. SW3231104
1st Submittal Response Letter
Please find responses to February 18, 2024 Request for Additional Information letter:
1. Please revise the following items on the Application:
a. Section I, 2: Please revise to 537 N NC 16 Business Hwy Denver, NC 28037.
Response:Address has been updated
b. Section I,4: Revise the coordinates, as they do not correspond to this project.
Response: coordinates have been modified
c. Section IV, 8:The percent impervious area for the project area does not appear to be
calculated correctly.This value can be found by dividing the "Total (sf)" found in the
Impervious Surface Area table by the Total Project area. Project density calculation
information can be found in 15A NCAC 02H .1003(1).
Response: Impervious calculations have been checked and corrected on the plans, calc's and
application.
d. Section IV, 10:The narrative states 0.75 acres of impervious area existed in the area
draining to the SCM, but the impervious surface area table lists 18,605 SF (—0.427 acres.)
Please revise the package for consistency.
Response: The narrative has been revised to reflect 0.75 acres existed prior to 7/07 and
0.427 acres will remain.
e. Section IX is only to be filled out if the applicant is not the property owner, as the applicant is
the property owner, please revise to leave this section blank and complete Section X.
Response: New application has been submitted
2. It appears that a copy of the corrected O&M Agreement was provided, an original,signed copy
will need to be provided (Section VI, 3).
Response: Original provided with current submittal

3. Please show the drainage areas for the proposed SCM to be permitted through SW3231104 on
the main set of plans (Section VI, 8o). Note that the drainage area is the area draining to the
SCM and is dictated by the grading rather than the property line/approximate watershed
boundary shown on plans.
Response: Sheet C5.0 modified to show proposed drainage area to sand filter
4. Please correct the following issues with the Supplement-EZ Form:
a. Cover Page—Line 7-8: While there is no surface water, and therefore no vegetated
setbacks associated with this project, please fill in. Per 15A NCAC 02H .1017 (10), the
vegetated setback shall be at least 30 feet in width.The vegetated setback must remain
vegetated.
Response: Section filled in as requested
b. Drainage Areas Page—"Click to Load Form" was not selected, so the drainage area page
information was not generated, and so the information on this page cannot be reviewed at
this time. Please generate the page and submit it for review.
Response: Drainage Areas Page provided
c. Sand Filter Page—Line 27:The physical depth of the sediment chamber is the depth from
the lowest bypass invert(Line 35)to the excavated bottom of the sediment chamber(Line
25).
Response: Noted
d. Sand Filter Page—Line 28:The ponding depth of the sediment chamber is the depth from
the lowest bypass invert to the top of the permanent pool in the sediment chamber(or to
the excavated bottom of the sediment chamber if there is no permanent pool in the
sediment chamber). For sand filters without a permanentpool, this value is the same as Line
27.
Response: Noted
e. Sand Filter Page—Line 29: Please specify these elevations on the plans (This item is required
per 15A NCAC 02H .1042(2)(h)(i)).
Response: Note added
f. Sand Filter Page—Line 33:The physical depth of the sand chamber is the depth from the
lowest bypass invert(Line 35)to the lowest excavated point of the sand chamber(below the
underdrains).
Response: Noted
g. Sand Filter Page—Line 34:The ponding depth of the sand chamber is the depth from the
lowest bypass invert(Line 35)to the top of the sand layer(Line 31).
Response: Noted
h. Sand Filter Page—Line 37:The SCM appears to have a weir as the mechanism for
distributing flow over the sand surface.
Response: Weir selected
i. Sand Filter Page—Line 39:This value refers to the depth of the sand layer being
provided above the underdrain pipe.
Response: Noted

5. The Supplement-EZ,Sand Filter Page, Line 9 lists a drawdown orifice as the dewatering method,
but a drawdown orifice could not be located on the details.
Response: Pump noted
Per 15A NCAC 02H .1042 (2) (h),the details shall be at a scale of one inch equal to 30 feet or larger
and include:
(i) dimensions, side slopes, and elevations with a benchmark for clean-out if appropriate. Show
the plan view of the sand filter detail to scale and include relevant elevations and dimensions on the
plan and profile view. Please include the SHWT elevation so that required separation can be
confirmed.
Response: Information added to sheets C5.1 and C5.4
6. Per General MDC 4:The inlets 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 immediately downslope of the discharge point during the peak flow from the 10-year
storm event as shown by engineering calculations. Provide sizing of inlet/outlet protection of the
SCMs on the main set of plans that correspond to engineering calculations.
Response: RipRap calculation provided for flow into pond of 10 year storm, along with riprap
sizing noted on the plans. RipRap calculations for flow out of pond over noted for 25 year storm,
in order to provide higher level of protection than required.
7. Clearly show the recorded easement on the main set of plans (Section VI, 8n). Per 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.
Response:The easement is shown on sheet C3.0
8. The provided seasonal high water table determination did not list the elevation of the ground at
the location the boring was taken, only the depth from the ground to the estimated SHWT.
Please indicate the location of the boring on a figure or one of the plan sheets so that the SHWT
elevation can be confirmed (Sand Filter MDC 1).
Response: Noted on sheet C5.1
9. Per Sand Filter MDC 2: Please ensure that the volume stored in each chamber of the sand filter is
equivalent. Please provide a separate stage-storage table for the sediment chamber portion and
a sand chamber portion so that the volumes can be compared.
NOTE:The sand filter can be oversized if required for peak flow mitigation, in these situations,
you need to demonstrate that at least half of the minimum required treatment volume is being
stored in each chamber. For example, if the minimum required treatment volume fora sand
filter is equal to 1,000 cf(this value has already been reduced by 0.75 per Sand Filter MDC 3) an
acceptable sand filter could be as follows:volume stored in the sediment chamber=874 cf,
volume stored in sand chamber= 517 cf(since at least half of the minimum required treatment
volume, 500 cf, is stored in each chamber).
Currently, the sand chamber is much larger than the sediment chamber, which is not
acceptable.

Response: Stage storage table provided, noting over 50%of the required storage is
provided in the sediment chambers and the remaining in the sand filter
10. Per Sand Filter MDC 7:The minimum depth of sand above the underdrain pipe shall be 12
inches. Revise the Surface Sand Filter Section detail accordingly.
Response: 18" of sand is provided
11. Per Sand Filter MDC 8: Provide calculations demonstrating that the sand filter draws down
at a rate of at least 2 in/hr at the sand surface.
Response:ASTM C33 sand has an infiltration rate of 8"/hour+/-. Calculation on stage store
calculation sheet notes the underdrain system needs 14.5—%" holes to meet the 2"/hour
drawdown.The perforated pipe notes 2 rows of%" holes 5" on center.
12. Per 15A NCAC 02H .1003 (5), stormwater outlets (of the site) 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:The ponds have been designed for a 25-year flow control, with the riprap sized
accordingly.
Respectfully,
Dc vidhS. 1_Ut'
David S. Lutz, P.E.
Partner, Principal Civil Engineer