00-Comment Response-CH Fire Station Request for Add Info 0514 2024
May 14, 2024
Timmons Group
5410 Trinity Road, Suite 102
Raleigh, NC 27607
RE: Castle Hayne Fire Station – Request for Additional Information
Dear Will Atlman:
We have reviewed the permit application and submittal package for the above referenced project and we offer the
following comments requesting additional information:
1. Per Section 2.3.2.1 and Table 2-4 of the New Hanover County Stormwater Design Manual, items 5, 8, 11, and
16, please indicate on the plans the (non) presence of 404 wetlands; the soil types with hydrologic group; a
summary table of the proposed and existing built upon area; and delineation of offsite and onsite drainage
contributing to the southern open channel and driveway culvert.
Response: Refer to the Site data table on the Cover Sheet (C0.0) that has been updated to include non
presence of wetlands and the HSG for this site. The Stormwater Compliance plan (C4.1) shows a summary
table of the proposed and existing built upon area; and delineation of offsite and onsite drainage contributing
to the southern open channel and driveway culvert.
2. Per Section 2.3.2.2 and Table 2-5, items 2, 5, 6, 11 and 12, please include in the narrative the proposed phasing
of the development; required calculations as an appendix for the southern open channel and culvert; NOAA
Atlas 14 rainfall data; a list with discussion of coordinating permits; and a discussion on easement requirements.
Response: The SIA narrative has been updated to include the phasing of the development, refer to Project
Phasing section. A list with the coordinating permits and easement requirements has been added to the Project
Data section of the SIA narrative. NOAA rainfall data and open channel and culvert calculations are included
in the Stormwater Calculations section.
3. Per Section 2.3.2.3 and Table 2-8, please provide the culvert calculations for the design flows indicated in item
#1 as an appendix to the narrative.
Response: The culvert calculation has been included in the Stormwater Calculations section.
4. Per Section 2.3.2.3 and Table 2-9, please include the open channels calculations as indicated in chapter 4 as an
appendix to the narrative.
Response: The open channel calculation has been included in the Stormwater Calculations section.
5. Per Section 2.3.2.3 and Table 2-10, items 5, 6, and 9, please include the stage discharge data for the draw down
siphon and it’s outfall piping and stage discharge data for the clogged scenario; provide detailed computations
for the 100 year 24-hour storm assuming the primary spillway is not functioning; supporting calculations for
the emergency spillway with required freeboard as indicated in Section 5.2.4.
Response: The clogged scenario has been completed. A variance has been added to this submittal due to site
conditions. The stage discharge data for the pond for both the clogged and regular scenarios can be found in
the hydraflow pond report. The emergency spillway calculations are included in the pre/post calculation
section in the pond report. A variance is being requested for some requirements. See attached Variance Letter.
6. The draw down siphon as detailed in the plans only discharged through the 4” pipe leading to the Stormfilter.
The pond calculations indicate the discharge is via the 15” culvert. Please clarify.
Response: There is a 4” line that takes the water quality volume through the stromfilter. The discharge pipe
for the rest of the water is 15”. The hydraflow calculations have been updated to show a 15” outlet pipe.
7. Per Section 2.3.2.5, please submit the executed operation and maintenance (O&M) plan and agreement. The
form can be downloaded at https://www.nhcgov.com/DocumentCenter/View/465/Stormwater-
Maintenance-Form-PDF.
Response: The O&M is attached.
8. Per Section 2.4.2 of the New Hanover County Stormwater Design Manual, please provide an executed
Designer’s Plan and Property Owner’s Plan Certification on the cover page of the plans. These forms can be
downloaded at https://www.nhcgov.com/DocumentCenter/View/3398/NHC-Designer-Plan-Certification
and https://www.nhcgov.com/DocumentCenter/View/3396/NHC-Property-Owner-Plan-Certification.
Response: The Certifications have been added onto the Cover Page, refer to C0.0.
9. Per Section 3.1 and 3.6.3, the rational method shall be used for pipe systems, culverts and inlets and the NRCS
method for storm volumes and type III rainfall distribution shall be used. The modified rational method used
in the pond calculations are not acceptable.
Response: The calculations have been updated to utilize the SCS method as requested above. Refer to the
updated SIA showing the updated calculations.
10. Per Section 3.5, please use the frequency factor with the revised calculations that rely on the rational method. It
appears these factors were not used in the pipe calculations as the composite C is the same for the 25 and 100
year storm events.
Response: The frequency factors have been added to the 25 year storm and 100 year storm events. Refer to the
C’s and A’s spreadsheet in the SIA showing the updated C- Values.
11. Per Section 4.4.2, the minimum sized pipe is 15” but the driveway culvert is 10”. The water quality outfall is not
subject to the 15” minimum size as it’s intended to meet State stormwater requirements.
Response: Refer to the attached calculation. A 10” pipe is sufficient and needed for the elevations and cover
requirements. Refer to Variance for full explanation.
12. The thickness of the rip rap for the emergency spillway, the details for the southern open channel and the
transition from DIP to PVC for the draw down siphon could not be found in the plans.
Response: The transition from the siphon to the box can be found on the permanent reinforced concrete
outlet structure detail. The rip rap emergency spillway detail has been updated to show the thickness.
Refer to C4.4.
13. Per Section 3.6.1, the CN values for the Leon soils present on the site shall be based on D soils. Consideration
should also be given to D soils when selecting the “c” factor when using the rational method.
Response: Comment noted. Calculations have been updated to take into account the change of Leon soils
from A to D.
14. The primary riser top elevation in the calculations indicate 19.81’ but the plans indicate 19.83’. Please clarify.
Response: The calculations have been updated, showing the primary riser top at 20.50’.
15. The energy dissipator calculations indicate a 4” outfall pipe but the plans indicate 8”. Please clarify.
Response: Energy dissipator calculation for the outfall pipe has been updated to 8”. The 4” is for the phase 1
skimmer that is not attached to the riser.
16. The proposed grading will result in ponding along the eastern property line and north of the Castle Hayne
Road driveway. What measures are proposed to eliminate or reduce the standing water in these areas?
Response: Grades have been updated north of the Castle Hayne Road driveway to provide positive
drainage to DI-1. It is our understanding that topo naturally flows south and that water will naturally flow
towards the existing ditch on the southern property boundary. If all water doesn’t flow to the ditch, it will
infiltrate.
17. Please send, as individual PDF files, the comment responses, consolidated plans and narrative report from
native PDF digitally with the resubmission.
Response: Comment noted.
These items must be addressed before the review and approval of the project can be completed.
The following items were noted during the review. These are only suggestions and recommendations intended for the
designer’s consideration and will have no bearing on the issuance of the stormwater permit.
1. Several of the roof drain connections to the storm sewer system results in a change of flow direction greater
than 90 degrees. Reconfiguration of the roof/storm system is recommended to avoid this situation.
Response: Comment noted, the plans will be updated as they can, but storm elevations are tight on this site,
which may limit the above.
2. It is recommended additional spot elevations be added to the grading plan to indicate changes in grade,
transition from catch to spill curb and edge of hard surfaces (parking lots, dumpster pads, along ADA
complaint paths, between buildings, etc) to assist the grading contractor with ensuring all impervious areas are
directed to the ponds.
Response: Comment noted
Please be advised, the area of disturbance will need a separate Sedimentation & Erosion Control Permit and a
stormwater permit issued by the NC Department of Environmental Quality will also be required.
If you have any questions regarding the foregoing information, please contact our office.
Sincerely,
Digitally signed by Galen M Jamison, PE DN: C=US, E=gjamison@nhcgov.com, OU=New Hanover County Engineering, CN="Galen M Jamison, PE" Date: 2024.05.14 11:19:43-04'00' Galen M Jamison, PE
Chief Project Engineer
New Hanover County
Galen M Jamison, PE