SW6230807_Response To Comments_20240327

-I FA
HARRISON FRENCH
& ASSOCIATES, P 1.I.C
15 March 2024
Brianna Holland
Environmental Engineer
DEMLR Post-Construction Stormwater Program
RE: Love's Travel Stop (Store 412)
Dunn, NC
HFA Project 03-22-30004
Dear, Brianna Holland
Subject: Request for Additional Information
Stormwater Permit No. SW6230807
Harnett County
1. Previous Comments Unresolved:
a. Per Sheet C7.7, the post-development drainage area is 9.63 ac, but the drainage area utilized in
the calculations provided was 9.55 ac.
Response: The Post-Development Drainage Area has been revised. The drainage area being served by the
SCM is 9.55 acres.
Current Comment: Ensure consistency throughout the package. Each of the following items is meant to
provide information pertaining to the drainage area served by the SCM, outlined in red to ensure clarity.
These areas should be consistent throughout the package. Currently there are contradictions in the
package such as the drainage area provided on sheet C7.7 (9.63 ac), Supplement-EZ (8.32 ac), and the
drainage area provided in the calculations (9.55 ac).
Response: The drainage areas have been revised on the EZ form to be consistent with the Stormwater
Management Application.To further explain the different areas:The 9.63 ac includes the offsite work dealing
with the right-turn lane, this area drains away from the site during pre- and post-development conditions
and is not being treated by the on-site SCM. The 9.55 ac is the correct area being treated by the SCM and
the area being used for all calculations.
b. Per 15A NCAC 02H .1042 (2) (h) (i), the plan details shall include labeled side slopes and a
benchmark for clean-out.
Current Comment: The revised plan details include additional labeled slopes, however, to ensure
compliance with General MDC 2("Side slopes of SCMs stabilized with vegetated cover shall be no steeper
than 3:1"), please label the side slopes of the SCM. This will include the side slopes of both sides of the
dam embankment.
Response: The side slopes of both sides of the SCM have been labeled on the Detention Plan & Profile,
sheet C7.8.The slopes stabilized with vegetated cover were designed at a 3:1 slope.
A benchmark for cleanout was not provided on the SCM detail sheets, please provide. Please refer to
Chapter C-3 of the Stormwater Design Manual, Figure 2: Wet Pond Example: Cross-Section View 1, for
an example figure of noting sediment storage depth.
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Response: The cleanout benchmark has been added to the Detention Plan & Profile, sheet C7.8. The
elevation for the top of sediment storage for the Main Pool is 231.50, refer to sheet C7.8. Forebay 1 elevation
for the top of sediment storage is 235.50, refer to sheet C7.9. The north end of Forebay 1 has additional
sediment storage for runoff coming from the roof drains.Forebay 2 elevation for the top of sediment storage
is 232.50, refer to sheet C7.9.
c. Per Wet Pond MDC 5 (b), the forebay entrance shall be deeper than the forebay exit.
Current Comment: The cross-section detail provided on Sheet C7.9 does not indicate compliance with
this minimum design criteria, nor does the plan view of the SCM. For a visual representation of this MDC,
please refer to Chapter C-3 of the Stormwater Design Manual, Figure 2: Wet Pond Example: Cross-
Section View 1.
Response: For Forebay 1 the depth at the entrance is 3.5ft and 1ft at the exit. For Forebay 2 the depth at the
entrance is 6.5ft and 1ft at the exit.
These depths are taken with the direction of flow from the Influent Stormwater to the Main Pool.
d. Provide engineering calculations demonstrating the inlets of the wet pond are 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 downslope of the discharge point during the peak flow from the 10-year
storm event. Rip-rap apron sizing calculations should be provided, and the class and dimensions of the
rip-rap aprons should be specified on the plans.
Current Comment: Per General MDC 4: The inlets of 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 downslope of the discharge point during the peak flow from the 10-year storm event as
shown by engineering calculations.
Per 15A NCAC 02H .1003 (5), stormwater outlets 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.
To ensure protection from erosion, separate engineering calculations are required for each SCM
inlet, SCM outlet, and stormwater discharge point. The site-specific conditions of each structure (ex:
tailwater condition, discharge velocity, pipe diameter, etc) must be accounted for in the calculations to
determine the appropriate size and class of outlet protection. In the last submittal, only one calculation
was provided (for the outlet of the site). One class calculation was found, but the class did not appear to
be indicated on the plans. Please provide site-specific calculations for each protection location. Once this
analysis is completed, protection which corresponds to the calculated dimensions (length, width,
thickness) and class must be stated on the plans.
Response:The inlet protection calculations(See Appendix H in the attached Drainage Report)for each inlet
of the pond as well as the pond outlet have been provided. The riprap size for two of the inlets is D50=0.25ft
or 3-inches (Class 1) while the riprap for the remaining inlet and the outlet pipe was calculated to be
D50=0.50ft or 6-inches (Class 2). However, for ease of construction and to provide maximum erosion
protecting all the pond inlets will be Class 2, 6-inch stone. The sizes for each of these erosion protection
devices have been added to the plans. Please refer to sheets C7.0 AND C 7.1
e. Supplement-EZ, Drainage Areas Page, Entire Site, Line 21: The Rational Method is not a
calculation method for the design volume, it is a method for calculating peak flow. The calculation method
that coordinates with the minimum required treatment volume provided is the Simple Method (Schueler
1987), which was found in the calculations.
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Current Comment: This was unresolved. This is required per General MDC 1.
Response: This has been revised.The calculation method used for the design volume is the Simple Method
as shown in the Drainage Report.
f. Per the Supplement-EZ, the bottom of sediment storage for the main pool is 231. Using this area
in the stage storage table overestimates the volume that will be stored within the main pool. It is unclear
what the storage elevation for the forebay is, so this comment may apply to the forebay stage storage
calculations as well. If 231.5 is the bottom of the pond/top of sediment elevation for the main pool, please
provide the surface area at this elevation so that volume calculations can be verified.
Current Comment: Revise the stage storage calculations as including the sediment storage in the volume
calculations overestimates the volume provided, per Wet Pond MDC 3. Note that the value provided in
the calculations does not correspond to the value provided in the Supplement-EZ, and the package will
need to be revised for consistency.
Note that revising the volume will result in a change to the average depth.
ii. Note that the forebay storage depth was not clarified.
Response: The main pool and forebay volumes have been revised in the plans, see sheets C7.8, C7.9 and
C7.91.The volumes noted on the plan do not take into account the sediment storage area.The stage storage
for the pond and forebays are shown in the Drainage Report.
2. Consistency:
a. The application submitted on 8/29/23 was modified to include an additional page. Due to
inconsistencies in the page breaks, inserting the page provided in the 12/11/23 into the Application leads
to conflicting information within the package. (Example, Section IV, 4, 7, 8, 10) To ensure a clear and
consistent presentation, the application's page breaks may need adjustment before revised pages can be
seamlessly integrated (recommended). Alternatively, with the permission of the consultant and applicant,
I can void the conflicting information provided in the previous application. Indicate in the response to
comments letter which option to proceed with.
Response: A new application has been sent to the owner for signature and will be resubmitted along with
this project to avoid any further confusion.
3. Supplement-EZ:
1x original Supplement-EZ Form and 1x hard copy with the following corrections made: (This item is
required per Sections V& VI, 3 of the Application)
a. Drainage Areas Page, Line 12 — For consistency, the pond's surface area should either be
excluded from the BUA breakdown on this line, or included in the total BUA (Drainage Areas Page, Line
8).
Response: The pond's surface area has been excluded from the BUA breakdown.
b. Drainage Areas Page, Line 19—The provided calculations list the design storm as 1 inch. (Refer
to page 4 of the calculations provided by HFA dated 12/4/23.) Revise for consistency. (Required per
General MDC 1)
Response: Line 19 has been corrected to show the design storm depth, RD = 1.0-in. The previously shown
design storm was in reference to the 10.1-in rainfall for the 100-yr storm event.
c. Wet Pond Page, Line 28 - The average depth of the main pool shall be three to eight feet below
the permanent pool elevation per Wet Pond MDC 2. Therefore, 8330.00 ft is not acceptable.
Response: The average depth of the main pool is 7.5ft, this depth does not include the 0.5 ft for sediment
storage.
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d. Wet Pond Page, Line 32—The volume of the forebay presented on this line does not correspond
with either the calculations or the plans. Revise the package as needed for consistency. Note that per Wet
Pond MDC 2, the volume for the forebay shall be 15 to 20% of the main pool volume. If the pond has more
than one forebay, then the total volumes of the forebays shall be 15 to 20% of the main pool volume.
Response:The minimum required Forebay volume was calculated at 8,231 ft3.As such the proposed design
provided a combined Forebay volume of 9,054 ft3, this volume does not account for the sediment storage
for each forebay. Forebay 1 provides a volume of 3,829 ft3 while Forebay 2 provides a volume of 5,225 ft3.
e. Wet Pond Page, Line 34 — Revise this value to correspond to the value provided in the O&M
Agreement.
Response: This value has been updated to correspond with the O&M form. In addition, the units on the EZ
form are in feet to match the O&M form.
f. Wet Pond Page, Line 43 —This is the depth of the bottom of the forebay at the entrance and the
exit. Please revise to match the plans.
Response: The depth of each forebay has been added as (a) for Forebay 1 and (b) for Forebay 2. This
information is also shown on sheets C7.8 thru C7.91.
g. Wet Pond Page, Lines 32-34, 43-44 — As the elevations and geometry of the two forebays are
distinct, please provide information for each.
Response: The information for both forebays has been added to the EZ form and it corresponds with the
O&M form.
4. Plans:
2x signed & sealed hard copies of a plan sheet are required per 15A NCAC 02H.1042 (2)(g) and per
Section VI, 8 of the Application
a. It appears that upon resubmission of Sheet C9.3, the trash rack on top of the riser structure was
removed. While it is noted that side wall trash racks and pipe guard grates are included, a top-mounted
trash rack is still required. Per Wet Pond MDC 10, a trash rack or other device shall be provided to prevent
large debris from entering the outlet system.
Response: The trash rack will still be used for the outlet structure.The information for the riser structure as
well as the trash rack detail has been moved to sheet C9.2.
b. Due to changes that were made to the pond grading and geometry, the planting plan needs to be
revised. A planting plan is required for the vegetated shelf per 15A NCAC 02H .1042 (2) (i).
Response: The landscape plan has been updated to account for the changes in the grading and geometry
of the pond.
5. Calculations:
1x signed, sealed, and dated hard copy of calculations with the following corrections made: (This item is
required per Section VI, 7 of the Application)
a. The drawdown time could not be verified as the calculation method (including equations and
assumed coefficients)was not provided. This is required to ensure compliance with Wet Pond MDC 7.
Response: The Orifice Coefficient for the drawdown calculations is 0.6 when the orifice is fully submerged.
Due to the orifice not being fully submerged at the first stage-storage the V-notch weir equation was used.
The Weir Coefficient of 1.03 for 45° was used for this calculation due to the proposed circular orifice. The
equations used for these calculations have been added to the Drainage Report.
6. O&M Agreement Form:
1x original, signed&notarized hard copy of the O&M Agreement Form with the following corrections made:
(This item is required per 15A NCAC 02H.1042(2)(1) and per Sections V& VI, 3 of the Application).
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a. The pond was revised to include multiple forebays, please provide information for the additional
forebay in the O&M Wet Pond Summary, as the sediment removal/clean out depths are not the same in
each forebay.
Response: The O&M form has been updated to include information for both proposed Forebays.
7. MDC Requirements:
a. The depth of sediment stored in the forebays could not be confirmed from the calculations or the
plan sheets. Per Wet Pond MDC 3, the forebay and main pool shall have a minimum sediment storage
depth of six inches.
Response: The main pool and the forebays have the required 6-in sediment storage, refer to sheets C7.8,
C7.9 and C7.91.
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.
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:
For FedEx/UPS: Brianna Holland
512 N. Salisbury Street, Office 640E Raleigh, NC 27604
ii. For USPS:
Brianna Holland
1612 Mail Service Center Raleigh, NC 27699-1612
ii. 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.
NOTE: Hard copies should not be sent to a Regional Office. Doing so will delay the review process and
the submission package may be lost while being sent from the Regional Office to me in the Central Office.
While not required to be addressed, the following was noted:
- Temporary erosion control measures are not reviewed as part of this permit,so generally, sheets like C4.2,
Erosion Control Phase II are not retained in the final set of permitted plans. Only the permanent, post
construction state is considered within this permit. It is recommended to present information such as the
easement and maintenance access on a plan sheet that reflects the permanent conditions of the site.
Response: Noted.
- A drawdown orifice should have a turned-down elbow in order to prevent trash or other material floating on
the surface from clogging the pipe.
Response: The drawdown orifice will have a hooded trash screen to prevent trash from clogging it.
- Emergency overflow spillways should be designed with hardened materials to prevent structural failures.
Response: The emergency spillway will have reinforced turn to prevent erosion.
- The Application Section II 1 b- 1 a states that this project is"new"and not a"modification to an existing state
stormwater permit." Therefore, do not select "not started" because this means that this application was a
modification to an existing permit where the construction was not started.
Response: This has been corrected on the application.
- The Supplement-EZ, Wet Pond Page, Line 53 has a typo.
Response: The typo has been fixed.
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To ensure a fair, timely, and transparent review process, any disagreement with provided comments will
need to be addressed in writing as part of the requested information resubmission, within the required
resubmittal window. The merits of the disagreement will be reviewed during the next scheduled technical
review and, if applicable, will be addressed in further requests for additional information. If the resolution
provided by the reviewer is found to be unsatisfactory, the applicant can agree in writing to a longer review
period,during which the dispute will be brought to the technical review committee and a final determination
will be made.
Due to staffing limitations and to ensure an efficient project review, requests for individual meetings to
discuss provided comments are decided at the discretion of the reviewer. We encourage you to carefully
review the written feedback provided and the applicable
rules. The NCDEQ Stormwater Design Manual provides additional insight into design criteria. Any
questions or further requests for clarification should be submitted in writing in the requested information
resubmission,within the required resubmittal window.We appreciate your understanding and cooperation
in this streamlined process.
Please note that this request for additional information is in response to a preliminary review. The
requested information should be received by this Office prior to April 5, 2024, or the application will be
returned as incomplete. The return of a project will necessitate resubmittal of all required items, including
the application fee. Please reference the State assigned project number SW6230807 on all
correspondence.
Sincerely,
• 15 2021
Diego A. Garcia, P.E., CFM
Civil Engineer
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