Durham Tech Facilities Building - 2018.09.12Response to comments

Coulter Jewell Thames, PA

MAIN OFFICE
111 WEST MAIN STREET
DURHAM, NC 27701
p919.682.0368 f919.688.5646

Planning for the Future

Corey Anen
Energy, Mineral Land Resources
512 N. Salisbury St.
1612 Mail Service Center
Raleigh, NC 27699

Subject: Response to Request for Additional Information
Stormwater Permit No. SW5180701
Durham Tech Facilities Building – Durham, NC
Durham County

To whom it may concern:

Coulter Jewell Thames has reviewed the request for additional information and has the
following responses to the comments provided.

1. The supplemental form states that the seasonal high water table elevation is lower than
326.75. There is no reference to this number in the geotechnical report. What was
the basis for this determination? Also, Sheet CD-9.0 states that the seasonal high
water table is assumed to be 336.25 based on a boring 135 feet away from the
sand filter. What is the justification for using this measurement rather than taking
a boring closer to the sand filter? This elevation would place the sand filter
below the groundwater table and would not meet Sand Filter MDC 1.
Response: The difference between 336.75 and 336.25 is simply a typo; our
apologies. The geotechnical report does not note an elevation for the seasonal
high water table (SWHT). Technically, the SHWT is not encountered in the
boring at a depth of >51”. As a conservative measure, the Engineer assumed the
depth to the perched water table, that was encountered in the boring, is the depth
to the SHWT. Due to the change in elevation between the boring location and the
location of the sandfilter, this depth is used at the location of the sandfilter to
assume an elevation for the SHWT. The bottom of the sandfilter is within the
theoretical SHWT elevation. A liner is proposed to prevent any drawdown of the
water table or any leaking of the treated water. Buoyancy calculations are
provided to ensure flotation of the facility will not occur.

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2. Please resubmit the supplemental form with the following updates:

a. Attach a sealed copy of page 1 with a wet signature.
Response: Included in resubmittal.
b. The “Compliance with the Applicable Stormwater Program” section
should list the Phase II and Jordan Lake Rules.
Response: Added to form.
c. Under General MDC 5, the weir in the sand filter that allows larger
storms to overflow would be considered a bypass.
Response: Revised on form.
d. Under General MDC 12, please state that there is an O&M Plan that complies.
Response: Added to form.
e. Sand Filter MDC 7 refers to the depth of sand above the underdrain pipe,
not the aggregate. Therefore, according the submitted plans this would
be 18” instead of 10”.
Response: Revised on form.

3. The supplemental form states that the method for dewatering the SCM for maintenance
is a drawdown orifice. Please show the drawdown orifice in the details on sheet CD-9.0.
Response: The drawdown orifice is not the intended method of dewatering. A pump is
proposed for maintenance dewatering.

4. Please show the invert of the pipe into the sand filter on the details.
Response: Invert is shown on detail 3/CD-9.0.

5. In accordance with General MDC 4, please provide engineering calculations that show
that the outlet will not cause erosion during the peak flow from the 10-year storm
event.
Response: HGL report is included in resubmittal. Report includes Outlet Protection
Analysis.

6. For the purposes of calculating peak flows, please use the discreet NRCS method, in
which impervious areas and pervious areas are routed separately. Weighting the curve
numbers of pervious areas and impervious areas results in artificially low peak flows
and runoff volumes.
Response: Peak Flow calculations are revised to separate different cover conditions and
remove weighted curve numbers.

7. Just to make sure you are aware, since the adoption of the MDCs into the stormwater
rules, it is required to use them to design SCMs for State Stormwater Permits, rather
than the equations in the old stormwater manual that were used to determine the sizing
of the sand filter. Specifically, it is required by Sand Filter MDC 2 that the storage in
the sand chamber and the sediment chamber are equivalent, whereas using the old
calculations often lead to the sand chamber being smaller than the sediment chamber.
As designed, the sand filter meets this requirement.
Response: Noted.
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8. 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:
https://edocs.deq.nc.gov/Forms/SW_Project_Submittal
Response: Resubmittal will be transmitted as requested.

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 September 16, 2018,
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 SW5180701 on all correspondence.
Response: Noted.

If you need additional time to submit the information, please submit your request for a time
extension to the Division at the contact below. The request must indicate the date by which you
expect to submit the required information. The Division is allowed 90 days from the receipt of
a completed application to issue the permit.
Response: Noted.

The construction of any impervious surfaces, other than a construction entrance under an
approved Sedimentation Erosion Control Plan, is a violation of NCGS 143-215.1 and is subject
to enforcement action pursuant to NCGS 143-215.6A.
Response: Noted.

If you have any questions concerning this matter please feel free to contact me at
[e-mail] or [phone].