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Home My WebLink AboutAddInfoRequest_amended- Durham TechEnergy, Mineral and Land Resources ENVIRONMENTAL QUALITY August 17, 2018 Mr. Vincent J.Chirichella, PE Coulter Jewell Thames, PA I I I W. Main St. Durham, NC 27701 Subject: Request for Additional Information Stormwater Permit No. SW5180701 Durham Tech Facilities Building — Durham, NC Durham County Dear Mr. Chirichella: ROY COOPER Governor MICHAEL S. REGAN Secretary WILLIAM E. TOBY VINSON, JR. Interim Director The Division of Energy, Mineral and Land Resources (DEMLR), received a Stormwater Management Permit Application for the subject project on July 17, 2018. A review of that information has determined that the application is not complete. The following information is needed to continue the stormwater review: 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. 2. Please resubmit the supplemental form with the following updates: a. Attach a sealed copy of page 1 with a wet signature. b. The "Compliance with the Applicable Stormwater Program" section should list the Phase II and Jordan Lake Rules. c. Under General MDC 5, the weir in the sand filter that allows larger storms to overflow would be considered a bypass. d. Under General MDC 12, please state that there is an O&M Plan that complies. 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". 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. 4. Please show the invert of the pipe into the sand filter on the details. 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. 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. 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. 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 ProjectSubmittal 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. 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. 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. If you have any questions concerning this matter please feel free to contact me at Corey. Anen(cncdenr.gov or (919) 707-3649. Sincerely, Nothing Compares. State of North Carolina I Environmental Quality I Energy, Mineral and Land Resources 512 N. Salisbury Street 1 1612 Mai Service Center I Raleigh, NorthCaroiina27699-1612 919 707 9200 ec: Raleigh Regional Office Tiffany R. McCormack, EI Corey Anen Environmental Engineer Stormwater Program Nothing Compares. State of North Carolina I Environmental Quality I Energy, Mineral and Land Resources 512 N. Salisbury Street 1 1612 Mai Service Center I Raleigh, NorthCaroiina27699-1612 919 707 9200