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HomeMy WebLinkAboutSW3230601_Response To Comments_20230817 4 Ilk PROPOSAL L September 15, 2023 landscape architect NC DEQ J.DAVIDLEDFORD, RLA Post Construction Stormwater Program c/o Brianna Holland 2730 Maiden Highway 512 N. Salisbury Street, Office 640E Lincolnton, NC 28092 Raleigh, NC 27604 Cell 704 530 7880 jdavidledford@yahoo.corn RE: Denver Fitness Facility - SW3230601 N. NC 16 Business Hwy. Denver, NC Brianna, This is a cover letter for the Request for Additional Information that we received from you office last week. The requested information has been addressed with revised forms, plan sheets, and calculations. In the following pages is the comment list of requested information along with a response in red to the items addressed and the location to find them as needed. I have also enclosed hard copies as requested in the list for your use in the review of the project. If you have any additional questions or comments during your review, feel free to contact me at any time. Thank You, �I ii F„ 46 David Ledford REVIEW COMMENT RESPONSES: 1. Supplement-EZ Note: On the Drainage Area Information Table,the Entire Site Column is for the information that relates to the entire site's drainage area, while the Drainage Area 1 Column is for the drainage area treated by the SCM. a. Entire Site Column: Please fill out lines 6-7, 11 and revise Line 8 per the comment below. Data inserted into Entire Site Column for Lines#6-7, and #11. Line#8 was revised per the offsite BUA information mentioned below. b. Drainage Area 1 Column: i. Line 8—Unless the offsite BUA is being diverted, please include this in the total BUA.This is the BUA that will be treated by the sand filter. Runoff from off- site areas or existing development that is not bypassed shall be included in the sizing of on- site SCMs at its full built-out potential per 15A NCAC 02H .1003 (3) (b). Revised Off-Site BUA to be included in Total BUA on Line#8 of Supplement-EZ. c. Drainage Areas, Line 11 shows an offsite BUA of 15,325 SF, but the Application Section IV, 10 lists 15,328 SF, please revise this or the Application for consistency. Revised Off-Site BUA to 15,328 SF inserted on Line#11 of Supplement-EZ. d. Drainage Areas, Line 21: In the provided calculations, it appears the "Simple Method"was utilized to calculate the design volume, please revise. Revised Calculation Method listed to Simple Method on Line#21 of Supplement-EZ. 2. Plans a. Per 15A NCAC 02H .1042 (2) and Section IV, 8n of the Application,the inverts and diameters of all pipes in the stormwater collection system, excluding driveway culverts must be provided. It is noted that most pipes are labeled, but please ensure that all have been labeled on the plans. Missing invert and diameters of pipes on plan have been updated with notes and labels to show requested information. b. SCM DETAILS i. Please provide a detail of the sand filter in plan view at a scale of one inch equal to 30 feet or larger per 15A NCAC 02H .1042 (2) (h). Detail of Sand Filter in Plan View was provided at a Scale of 1"= 20' on Sheet D6. ii. The plan details shall include dimensions, side slopes, and elevations with a benchmark for clean-out per 15A NCAC 02H .1042 (2) (h) (i).These were not included in this cross-section, please revise from a generic sand filter detail to a site-specific, to scale cross-section with this information included. Among the dimensions, please ensure that values that are specifically asked for in the Sand Filter portion of the Supplement-EZ are included so they can be verified. Include the elevation of the SHWT provided in the soils report in the cross-section to verify the separation requirements are met(MDC 1). Include the maximum ponding depth to ensure requirements are met (MDC 4). Cross-Section Detail of Sand Filter was provided at a 1"=10' Horizontal Scale and 1"=5' Vertical Scale per discussion to clearly show Sand Filter components with requested information. Cross- Section Detail has been included on Sheet D6. iii. The plan details shall include all conveyance devices, including inlet device, bypass structure, pretreatment area,flow distribution device, underdrains, outlet device, and energy dissipater. Please include the elevations and dimensions of all conveyance devices. Plan Details and Charts revised to show all conveyance measures along with notes for size and dimensions of each on Sheets D1 & D6. c. The Rip Rap Outlet Protection Table on Sheet D1 does not include the class of the rip-rap, which needs to be included to ensure inlet protection (required per 15A NCAC 02H .1050 (4). Please include the class (that was provided in the calculations) on the plans. Rip Rap Size Column added to Rip Rap Outlet Protection Table on Sheet D1. 3. Calculations a. Please provide calculations (such as a stage storage table for both sand chamber and sediment chamber)that demonstrate the volume and surface areas of the sediment and sand chambers meet the requirements listed in 15A NCAC 02H .1056 (3). Stage Storage Charts for both the sediment and sand chamber have been provided for the calculation packet and they are also shown on Sheet D-6. b. When providing volume calculations (per the previous comment), please provide the elevation at which the required treatment volume is met.The ponding depth associated with this volume will be used to verify the appropriate elevation of bypass devices per Sand Filter MDC 3. The required treatment volume elevation is the 799.50' elevation. At this point the provided Vt of 3,588 CF is right at the required Vt of 3,572 CF.There is an asterisk on the cumulative volume value on the 799.50' contour elevation line that references the note at the bottom of the chart to show treatment volume has been met for the Vt of 3,572 CF.The elevation has also been labeled on the Sand Filter details on Sheet D6. c. Per 15A NCAC 02H .1050 (4), provide engineering calculations showing that: i.The inlets of SCMs are designed to protect the SCM from erosion resulting from stormwater discharges. The inlets to the SCM have designed riprap energy dissipator that have been sized based on proposed flow in the pipe per the initial pipe size calculations and riprap energy dissipator sizing charts.The NC DEMLR Erosion Control Manual states the following as a definition and purpose of riprap outlet energy dissipator:A structure designed to control and prevent erosion at the outlet of a channel or conduit by reducing the velocity of flow and dissipating energy.With this in mind, I would think the dissipator would be sufficient. However, I have noted on the plans that during the stabilization of any outlet area downstream disturbed is to be lined with NAG SC/150 liner as additional protection until stable. ii.The outlets of the SCM do not cause erosion downslope of the discharge point during the peak flow from the 10-year storm event. Including the same as response above. I have also calculated the downstream ditchline stabalization showing the proposed CFS flow of the pipe to determine the Stable condition with the NAG SC/150 liner until stabilization has been achieved. iii. Provide time of concentration calculations so that the ten-year storm intensity can be verified. Ten Year Storm Intensity was derived from Table 8.03c Intensity Duration Frequency for Use with the Ration Method in the NC DEMLR Erosion Control Manual Chapter 8—Appendices. I used the lowest time of concentration value of 5 Min. on the Charlotte Area Chart which resulted in a Intensity of 7.26" Hour/ 10 Yr. Storm Event.That should cover the "frog stranglers" as we call them on a bad day. d. Provide engineering calculations exhibiting that stormwater outlets are designed so that they do not cause erosion downslope of the discharge point during the peak flow from the 10-year storm event per 15A NCAC 02H .1003 (5). Show with the calculations that the flow rate of water does not exceed the maximum permissible velocity for the condition and type of soil and groundcover over which the water is flowing.The proposed swale and primary overflow system outlet to an existing headwall and culvert. Please show the outlet of the existing system and demonstrate that erosion is not caused. Including the same response above and calculations, I have also provided a spillway stabilization calculation showing the proposed CFS flow of the spillway to determine the Stable condition with the NAG SC/150 liner until stabilization has been achieved. The existing outlet and cross-pipe location with size from the existing headwall has been show under Hwy. 16 on the Stormwater Plan Sheets C4&C5.The dissipated volume has been achieved prior to the headwall with the installation of the above measures. e. Per the calculations,the provided average surface area for the sand chamber is 1,196 SF.The underdrain drawdown calculation pipe sizing the sand filter surface area appears to be for half of the surface area of the sand chamber (648 SF) and therefore calculations need to be updated for the full area. It seems like the calculations may have been cut-off from what was intended. The average surface area elevation of the sand chamber is at a point half the ponding depth of the sand chamber. With the 3:1 side slope of the sand chamber, this in turn reduces the surface area of the sand filter surface to 582 SF per the current design.The 648 SF area that I had listed initially was during the first iteration of the design and I forgot to revise. With the infiltration calculation based on the sand surface area, I have revised my calculation with the use of the 582 SF to find underdrain pipe size. Calculation Sheet provided for the Calculation Packet. 4. Provide PDFs of all revisions, 2 hardcopies of revised plan sheets, 1 hardcopy of other documents, and a response to comments letter briefly describing how the comments have been addressed. PDF's, Hardcopies, and Response Letter provided as requested. 5. PDFs must be uploaded using the form at: https://edocs.deq.nc.gov/Forms/SW-Supplemental- Upload Please ensure that all documents that are part of the package are uploaded to the electronic repository. In the previous electronic submittal, Sheet D1, which contains the rip rap protection sizing table,was not included. PDF's provided as requested. a. The hard copy of the soils report provided includes a figure showing the bore locations, but the PDF uploaded to the electronic repository does not include this. Please revise the electronic copy so that the bore locations are apparent. PDF's provided as requested. b. While not required to be addressed,the following items were noted: -The Application Section I, 3 (Directions to project) reference Clark Tire &Auto, but searching for Clark Tire &Auto does not lead to relevant results. (It looks like it may be a Mr. Tire now.) An alternate description of the directions to the project is recommended. Description of Project Directions has been revised on Application Section I, 3 for name change of tire store. -Sheet C5 shows an Emergency Spillway invert of 880.00', please revise to 800.00'for consistency. Emergency Spillway Invert revised to 800.00' on Sheets C5 & C6. - For the clean-outs, it is recommended to specify a PVC pipe that has glued clean-out fittings with screw type caps. It is crucial that the cap be secure so that the stormwater will not leave the sand filter via the pipe rather than passing through the sand media as intended. In addition,the ends of each underdrain pipe should be capped to prevent clogging of the underdrain system. PVC pipe that has glued clean-out fittings with screw type caps have been label on the clean- outs in the Sand Filter Chamber.Also, I have shown the top inert being 6" above the proposed sand surface elevation for easy location and any maintenance. - Recommend providing a legend or more labeling identifying components of the plan sheets per the Application, Section IV, 8d. Additional Plans and Details Notes were added to clarify plan components per request. -The planting plan indicates that 18 "swing low" distylium will be planted by the primary overflow system. Recommend considering potential issues of access to the primary overflow system if maintenance is needed. Plantings are required by County UDO requirements. If issues arise in access for maintenance, then the plants will be removed to access structure and then replaced after maintenance has been completed.