SW6160401_20200903 NCDEQ Stormwater Memo_20201002

4
September 03, 2020
Division of Energy, Mineral and Land Resources
North Carolina Department of Environmental Quality
217 W Jones Street
Raleigh, NC 27603
RE: PERMIT NO. SW6160401 - Stormwater Permit Modification
Dear Sir or Madam,
This request for a modification to permit no. SW6160401 is being submitted to update the
original permit that was filed and approved with project number PN749437 - Training Group
Headquarters. The original permit used assumed built -upon area (BUA) values for future
projects for two follow-on projects. The two follow-on projects since PN79437 are:
• PN79439 - Advanced Skills Training Facility
• PN81165 - Human Platform Force Generation (FORGE) Facility
The required water quality treatment volumes for the two follow-on projects covered under
this permit are also being updated with this modification. The PN79439 design is complete and
under construction. The PN81165 project is in design and the supporting calculations in this
modification for hydrology and hydraulics are for PN81165 only. The BUA values included in
this modification are for the entire permit and include values for all three projects.
The existing infiltration basin is designed as a regional stormwater management system and
permitted to incorporate all three projects above. The infiltration basin has a capacity of 18,128
cubic feet for treatment. The original permit submission uses conceptual designs for PN79439
and PN81165 to calculate a total of 415,817 square feet of BUA, which corresponds to a water
quality treatment volume of 8,360 cubic feet. Now that the designs for PN79439 and PN81165
are complete, the actual total BUA is 430,193 square feet. The required water quality treatment
volume for the actual BUA is 11,334 cubic feet.
The required water quality treatment volume for the three projects is 6,794 cubic feet below
the maximum capacity of the infiltration basin of 18,128 cubic feet. This corresponds to a BUA
value of 47,480 square feet. This permit modification will also permit the remaining water
quality volume capacity as future BUA. It is important to note that the DoD requirement for
treatment volume is based on a 1.8" rainfall event vs. the NCDEQ required 1.0" rainfall event.
The values in this permit and supporting calculations use the DoD requirement.
The maximum allowable BUA that corresponds to the maximum infiltration basin capacity of
18,128 cubic feet is 477,853 square feet. A summary is provided in Table 1 and detailed
Virginia5a400
Beach, VA 23462C L A R K N E X S E N
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Page 2 of 2
calculations for the water quality volume is included in Appendix F of the Stormwater Narrative
and Calculations.
Table 1 - BUA and WOV Summary
Permitted Values
Proposed Values
Revised Permit Values
BUA (sf)
Min. Treat.
BUA (sf)
Min. Treat.
BUA (sf)
Min. Treat.
Vol. (cu. Ft.)
Vol. (cu. Ft.)
Vol. (cu. Ft.)
415,817
8,360
430,193
11,334
477,853
18,128
The D-Area Campus infiltration basin collects a portion of the drainage basin that is shown on
the original permit documents. The drainage basin is associated with the composite outfall
shown on the original permit documents and matches the overall permit boundary. The basin
has a total drainage area of 28.53 acres and is summarized in Table 2 below. The drainage area
and BUA draining to the D-Area SCM and the portion that drains offsite are included in the
Table 2 values.
Table 2 - Total Drainage Basin Summary (Post Development)
Total Drainage Basin Area
Total Drainage Basin BUA
Acres
SF
Acres
SF
28.53
1,242,912
14.08
1 613,328
If you have any questions on the items submitted with this permit modification, please don't
hesitate to contact us. We look forward to working with your office to complete this project on
behalf of the Army Corps of Engineers and the Fort Bragg JFK Special Warfare Center and
School.
Nathan Jones, EIT
Civil Engineer EIT
[phone]
ivatnan.joriu�kW�Larknexsen.com
Attachments
Stormwater Narrative and Calculations
PN81165 FORGE Facility Stormwater Plans
Operations and Maintenance Agreement
Stormwater Application
Supplement - Infiltration Basin Parts I, II, II
4525 Main Street, Suite 1400
Virginia Beach, VA 23462
clarknexsen.com
Lewis Hutchins, PE
Senior Civil Engineer
[phone]
LUVVI�.i iuL�l III iS(cDcLa, Knexsen.com
Christopher T. Aebel, PE, CDT, ENV SP
Department head - Civil
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