SW6221101_Response To Comments_20230818

July 14, 2023
NCDEQ DEMLR
Post-Construction Stormwater Program
Attn: Jim Farkas
512 N. Salisbury Street, Office 640M
Raleigh,NC 27604
Subject: Response to Comments#2
Stormwater Permit No. SW6221101
Fort Bragg,North Carolina Technical Support Annex, PN79439.2
Cumberland County
Dear Mr. Farkas,
Please consider our response to your comments included in your letter dated June 1, 2023.
Comments have been addressed as follows:
1. Prior Comment 2—"Please QA/QC the submission for consistency..." The following
consistency items were noted during review (there are other issues listed below in
specific comments too):
a. The information in the table in Section IV, 10 of the Application was not updated
in accordance with the Supplement-EZ and other submittal items. Please revise.
RESPONSE: The table in Section IV,10 of the application was updated to match
the Supplement-EZ form.
b. The grading plans were not updated to reflect the revised bottom elevations for
SCMs B3, B4, & B5. The latest submittal items indicate that the bottoms of these
SCMs are located at elevation 267.0' whereas the grading plans indicate that the
bottom elevation of the northernmost SCM is 268.0'. Please revise.
RESPONSE: The bottom elevation of the three ponds B3, B4, &B5 are all 266.
The 266 contour is now shown and labeled in all three ponds. The bottom
elevation is also defined on sheet CG204.
c. The proposed site hydrology plan sheet(CE602)was not revised to reflect the
revised drainage of the site (discharge from SCMs B3, B4, & B5 being directed
around, instead of into, SCM A). Please revise this and any other sheets to show
this new storm drain configuration.NOTE: Please also update the riprap
design/sizing calculations on this plan sheet to reflect the new storm drain
configuration(required per General MDC 4).

RESPONSE:Sheet CE602 was revised to address the last round of comments,
however it appears that the revised sheet was inadvertently not included in the
last submittal. The revised sheet is included in this submittal.
d. The drainage area information for SCMs 4 & 2 on the SWM Pond Drainage Area
Map does not appear to be correct. Per the drainage area map, SCM 4 is not
shown to have an off-site component to its drainage area whereas the drainage
area information in the table indicates that SCM 4 has 5,608 sf of off-site drainage
area. Please revise as needed.
RESPONSE: The titles at the top of the chart were mislabeled. The titles SWM 4
and SWM2 were reversed. This has been corrected. For additional clarity, the
original pond labels were added in parentheses.
2. Prior Comment 3 —The following items were noted with the design of SCM A (SCM A
was changed from a Dry Pond to a Bioretention Cell):
a. Please provide a typical section for SCM A. The typical bioretention cell section
shown on plan sheet CG502 is only specified for SCMs B3, B4, &B5.
RESPONSE:Sheet CG205 has been added to the plan set, which includes a
typical section that is specific to SCM A.
b. There does not appear to be sufficient separation between the SHWT and the
bottom of SCM A per Bioretention MDC 1. Per the boring log information shown
on plan sheets B-100—B-309, there does not appear to be a SHWT boring located
within the footprint of this SCM (The closest soil boring appears to be B-04 and
was excavated to an elevation of 251.0' so it is unclear where this SHWT value
came from). SHWT information should be based on borings conducted within the
footprint of the SCM. Per the information entered into the Supplement-EZ Form,
the SHWT is assumed to be>250' (NOTE: This should be less than"<"250', not
greater than">"250'). Per the outlet structure detail for SCM A provided on plan
sheet CG502,the invert of the underdrain line is at elevation 251.0' (NOTE: The
stone layer surrounding the underdrain would put the bottom of the bioretention
cell lower than this elevation) which is less than 2.0' above the assumed SHWT.
Please revise as needed.
RESPONSE: Two SHWT determinations have been performed within the
footprint of Pond A. The boring logs are attached for reference.
The SHWT at Boring B-1 was determined to be at elevation 236.5.
The SHWT at Boring B-3 was determined to be at elevation 247.0.
The lowest point of the pond is at elevation 250.0, which is three (3)feet above
the highest SHWT value. This pond meets the minimum separation requirement.

c. Please specify the elevation of the IWS for SCM A. The provided outlet structure
detail for this SCM does not specify this information. Per Bioretention MDC 4,
the top of the IWS shall set a minimum of 18 inches below the planting surface.
RESPONSE:A callout was added to the outlet structure detail on sheet CG205
specking the 18 inches separation requirement.
d. Please provide calculations showing the peak attenuation ponding surface
elevation for SCM A. Per Bioretention MDC 3, the peak attenuation ponding
surface elevation cannot be higher than 24" above the planting surface.
RESPONSE: Output from the Stormwater Modeling program is attached. The
calculated depth and surface elevation for each pond is included in the various
year storm event sections of the Stormwater Analysis Report. A summary sheet is
also included in this package.
3. Prior Comment 4—"...You will either need to provide a stage-storage table for each
bioretention cell or calculate the storage volume as the bottom surface area of the
bioretention cell multiplied by the ponding depth."Neither option appears to be provided.
Per the note on the Supplement-EZ Form, the first option(calculating the provided
storage volume using the cross-sectional areas of the SCMs) appears to be used for
determining the volume of the SCMs, however we require stage-storage tables for the
SCMs to verify these values. If you do not wish to provide stage-storage tables, you can
calculate the provided design volume in a simplified manner(multiplying the bottom
surface area of the SCM by the ponding depth).NOTE: Per the information shown in the
calculations,the surface area of the soil media is shown to be 19,588 sf for SCM A and
2,500 sf for SCMs B3, B4, &B5 whereas the Supplement-EZ Form indicates that the
cross-sectional area of the bottom of the SCMs is larger, 22,556 sf for SCM A, 4,656 sf
for SCM B3, 4,656 sf for SCM B4, and 3,783 sf for SCM B5 (bioretention media should
be placed below the entire bottom of the SCMs). Please revise as needed.
RESPONSE: There is a stage/storage table for each pond in the "Model Input"section
of the Stormwater Analysis Report. A summary sheet is also included in this package.
4. Prior Comment 6.b.ii.2. —"...The entire site column is an accounting of the entire project
area(similar to how the drainage area columns are an accounting of the drainage area to
the SCMs. When asked for the `drainage area' of the entire site, it is asking for the
project area." The project area as shown in Section IV, 7 of the Application is shown as
17.17 ac whereas Line 6 of the Entire Site Column only accounts for 15.16 ac (off-site
areas draining onto the project area are not counted as part of the project area in the
Entire Site Column). Please ensure that the entire project area is accounted for in the
Entire Site Column.
RESPONSE: The project area is assumed to be the limits of disturbance, which is 15.16
acres. The application has been updated to show this value.

5. Prior Comment 6.b.iii.2. —"Line 10—This item should be a sum of the items in Line 12"
For SCM A, the sum of the items of Line 12 (25,027 sf, 1,733 sf, 25,222 sf, 31,897 sf, &
153,878 sf) add up to 237,757 sf whereas Line 10 is shown as 237,759 sf. Please revise as
needed.
RESPONSE:Line 10 was adjusted to match the sum of the individual components.
Please note that the value of Line 10 may not match exactly with the total values shown
on sheet CE602 due to rounding error. The totals shown on sheet CE602 are a sum of
the non-rounded individual area values calculated directly from CAD, whereas the totals
shown on the Supplement-EZ form are a sum of the rounded individual area values
entered manually as whole numbers into the spreadsheet.
6. Prior Comment 6.c.ii. —"Line 18—Please include the SHWT elevation. If the SHWT was
not encountered above a certain elevation, please list the lowest boring elevation that the
SHWT was not encountered at. For example, if the boring terminated at elevation 100'
without encountering the SHWT, you can answer `100' or `<100' for this item." The
provided SHWT values use the greater than sign">" instead of the less than sign"<".
Please revise.
RESPONSE: The greater than sign "> "was changed to the less than sign "< ".
7. Prior Comment 6.c.iii. —"Line 22— See earlier comment."Please refer to earlier
comment about the design volume of the SCMs.
RESPONSE: There is a stage/storage table for each pond in the "Model Input"section
of the Stormwater Analysis Report. A summary sheet is also included in this package.
The design volume of the SCM was taken as the volume at 1 foot depth.
8. Prior Comment 6.c.v. —"Line 30... —This value... does not correspond to the plans or
other submittal documents."Please refer to earlier comment about the revised grading
plans.
RESPONSE: The bottom elevation of the three ponds B3, B4, &B5 are all 266. The 266
contour is now shown and labeled in all three ponds. The bottom elevation is also
defined on sheet CG204. The bottom of Pond A is 255 as is shown on sheets CG103 and
CG205.
9. Prior Comment 6.c.vii. —"Line 43 —Please clarify how grass/gravel will be used as
pretreatment for the bioretention cells." Please clearly show(or direct the reviewer's
attention to)the gravel areas for the SCMs in the plans. It is noted that the detail on plan
sheet CG502 indicates that 8" of gravel are provided"where shown on plan view"
however the plan view of the SCMs do not appear to show this gravel area. Please revise
if needed.
RESPONSE: The plans show a double line at the asphalt edge where curb is proposed.
The callout was referring to the areas that do not show a double line (no curb). For

clarity, the callout on the cross section of the ponds was revised to specify gravel in
locations "where runoff sheet flows from pavement"
10. Please specify the pond bottom elevation in the outlet structure detail for SCMs B3, B4,
& B5.
RESPONSE: The pond bottom elevation of 266.0 has been added to the detail.
11. 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.
RESPONSE:PDF's have been uploaded and hardcopies are enclosed.
a. PDFs must be uploaded using the form at:
https://edocs.deq.nc.gov/Forms/SW-Supplemental-Upload
b. Hard copies must be mailed or delivered to the following address:
i. For FedEx/UPS:
Jim Farkas
512 Salisbury Street, Office 640M
Raleigh,NC 27604
ii. For USPS:
Jim Farkas
1612 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.
If you have any questions about the proposed responses, please feel free to contact Lorraine
Roberts with Exp Federal at [e-mail].
Thank you,
Lorraine Roberts
Director of Infrastructure
Exp Federal, Inc.