SW6240102_Response To Comments_20240507

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May 6, 2024 civil engineering I land surveying
NCDEQ Division of Water Quality f
Attn: Brianna Holland
512 N. Salisbury Street
Raleigh, NC 27604
RE: Storm water resubmittal, Raeford Vass Road Dollar General (SW6240103), Raeford,
North Carolina
Ms. Holland;
The review comments for the above project dated April 17, 2024 have been addressed
as noted below.
1. An outlet structure is shown on Sheet C-3.1 — Drainage Area Map, but it is not shown
on the other plan sheets. Is there an outlet structure for the SCM? If so, show on the
SCM detail sheets per 15A NCAC 02H .1042 (2) (h) and provide calculations showing
the outlet is non-erosive with the maximum velocities allowed. If not, provide the
maximum allowable velocity for the material in the provided 10-year storm spillway
outlet calculations per 15A NCAC 02H .1050 (4) and show the elevation of the bypass
on the detail sheet (15A NCAC 02H .1042 (2) (h)). The outlet structure you are
referring to on sheet C-3.1 is the skimmer for the basin while it is functioning as an
erosion control measure. There is no outlet structure other than the concrete
spillway as noted on detail #9 sheet C-6.2. The notch elevation is shown in the
spillway. The overflow from the spillway during a 10 yr storm is 6.40 cfs. The
10'spillway will flow 4.2" deep for a velocity of 1.83 ft/s which is less than the
maximum of 5 ft/s. The skimmer has been removed from sheet C-3.1.
2. Per Infiltration MDC 2: "The lowest point of the infiltration system shall be a
minimum of two feet above the SHWT. However, the separation may be reduced to
no less than one foot if the applicant provides a hydrogeologic evaluation that
demonstrates that the water table will subside to its pre-storm elevation within five
days or less." There are two primary reasons for the two-foot separation
requirement; the first is the inherent uncertainty in pinpointing the exact location of
the SHWT in the field, and the second is the potential for mounding of the
groundwater table in the vicinity of the infiltration system. If the mounding becomes
severe enough, the infiltration will not function as designed. See Part A-2 for
guidance on what should be provided in a hydrogeologic evaluation. The lowest
point in the forebay does not meet this requirement. Including the 4" of clean sand
shown (bottom of which is 231.77), it does not appear the infiltration system meets
this requirement outside of the forebay as well. The sand has been removed from
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the bottom of the basin. I did not see a requirement for sand in the bottom of the
basin. Therefore, the invert of the infiltration basin at 232.10' is more than 2' above
the SHWT of 229.83'. Per the comment below, the forebay is provided in addition to
and is not part of the infiltration system. As such, the forebay is not subject to the
separation requirement of the infiltration system.The forebay is designed to collect
sediment and will therefore not infiltrate like the main body of the basin. There was
no change to the basin except for removing the 4" of sand in the bottom. All MDC
requirements have been met.
3. Please ensure that the forebay area & volume are not included in the Infiltration
System surface area &volume (A forebay is provided in addition to, not as part of,
an Infiltration System), Infiltration MDC 4. The forebay was not included with the
infiltration system sizing or routing calculations. 2,600 sf is the bottom of the basin
minus the forebay. The linework on the basin has been modified to eliminate
confusion about the location of the bottom of the basin.
4. 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). A response to each of the comments has
been provided.
5. Please note that electronic copies were not received and are required, per 15A
NCAC 02H .1042 (2). A full electronic copy was uploaded as part of the resubmittal.
6. 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. Done
I look forward to working with you on this project. If you have any questions or
concerns, please contact me at your earliest convenience.
Sincerely,
4D Site Solutions, Inc
Scott Brown, PE
[e-mail]
Enclosure
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