SW6190504_SW6190504 Response Letter_7/22/2019

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July 22, 2019
North Carolina Department of Environmental Quality
Division of Energy, Mineral and Land Resources
Stormwater Program
512 N Salisbury Street, Raleigh, NC 27699
Reference: Request for Additional Information
Stormwater Permit No. SW6190504
Hoke County Cooperative Extension Office
Hoke County
Attn: Corey Anen
The following are the requests for additional information, clarifications and our responses for the above
referenced plan:
COMMENT:
The Supplement EZ form states that the elevations of the seasonal high water table for
infiltration basins one and two are 245 and 241 respectively. These numbers appear to be
calculated using information from borings that occurred outside of the infiltration areas.
The submitted geotechnical evaluation states on Pages 8-9 that the seasonal high water
table was indicated at 3-3.5 feet below the existing ground surface. Based on the existing
conditions survey and the boring locations indicated in Figure 1 of the geotechnical
report, this indicates that the seasonal high water table in the area of the proposed
infiltration basins is significantly higher than shown in the Supplement EZ form and the
required 2 foot separation will not be achieved (Infiltration MDC 2).
RESPONSE:
We tasked the Geotechnical Engineer with re -visiting the site and providing a new evaluation of SHWT
at the specific locations of SCM 1 and SCM 2. This letter is included with this submittal. The SHWT
elevation at Infiltration Basin #1 {SCM 1) was verified at 3.5' below existing grade at that location. This
required a slight redesign of SCM 1 for basin bottom to be 0.5' higher to provide minim 2' separation.
The SHWT elevation at Infiltration #2 (SCM 2) was found to be greater then 5' below existing grades,
and no revisions to design were necessary.
2. COMMENT:
Please provide calculations that show that the inlet and outlet protection for the SCMs is
sufficient to prevent erosion during the 10-year storm (General MDC 4).
RESPONSE:
Copies of Inlet protection for storm drainage entering SCM 1 and 2 are provided. There is no release of
stormwater from either basin for a 10 year design storm.
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NORTH CAROLINA 600 GERMANTOWN ROAD RALEIGH, N.C. 27607 • 919-851-1980 FAX [phone]
SOUTH CAROLINA 500 LAWAND DR., SUITE 103 COLUMBIA, S.C. 29210 • 803-731-5550 FAX [phone]

3. COMMENT:
Please add a detail of the spillway for infiltration basin one and a cross sectional detail
showing the flared end section outlet for infiltration basin two. These details should show
the proposed design and elevations.
RESPONSE:
New Section details A -A, and B-B have been added to the Stormwater Detail Sheet C7.3.
4. COMMENT:
The Supplement EZ form lists the drain time for both infiltration basins to be 4 hours. No
calculations were submitted to confirm this. Please provide these calculations and update
the Supplement EZ form as required.
RESPONSE:
Infiltration draw down calculations are included with this submittal. Modeled depth of ponding is
approximately 8" in each basin. In previous calculation we utilized a 2"/br infiltration for drawdown,
however with this submittal we revised the infiltration rate to 1"/hr to better coordinate with safety
factor used when routing the storms in Hydoflow.
S. COMMENT:
Provide pdfs of all revisions, 2 hardcopies of revised plan sheets, and 1 hardcopy of other
documents. Pdfs must be uploaded using the form at
RESPONSE:
Noted
Please call if you have any questions, 919-851-1980
Sincerely,
BOBBITT DESIGN BUILD, INC.
Blair Pittman, PE