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HomeMy WebLinkAboutSW6130501_HISTORICAL FILE_20130522STORMWATER DIVISION CODING SHEET POST -CONSTRUCTION PERMITS PERMIT NO. SW Cphj DOC TYPE ❑ CURRENT PERMIT ❑ APPROVED PLANS IUD HISTORICAL FILE C(❑,/ COMPLIANCE EVALUATION INSPECTION DOC DATE YYYYM M DD Dewberry Dewberry Engineers Inc. 2301 Rexwoods Drive, Suite 200 Raleigh, NC 27607-3366 919.881.9939 919.881.9923 fax www.dewberry.com May 22, 2013 Mr. Mike Randall Stormwater Staff Engineer NCDENR Division of Water Quality 512 N. Salisbury Street Raleigh, North Carolina 27604 Tel.: (919) 807-6374 Reference: Conferencing and Catering Center Fort Bragg, NC SWM Submittal (Standard Review) Dear Mr. Randall: Please see attached the Stormwater Management Review package for your review. We have included the following: • 1 Stormwater Management Permit Application, signed • 1 Bioretention Operation and Maintenance Agreement, signed • 1 Dry Extended Detention Basin Operation and Maintenance Agreement, signed • 1 Filter Strip, Riparian Buffer and Level Spreader Operation and Maintenance Agreement, signed • 2 Sets of Design Drawings • 2 Sets of Stormwater Management Reports • 1 Check for Standard Review ($505.00) Should you have any questions or need any additional information, please feel free to contact me via phone (919) 424-3740 or email (fvnorton@dewberty.com). Sincerely, Dewberry Engineers Inc. F. Virginia Norton, PE, LEED AP Project Manager P:\50057270\1)eliverables\Submittals\2013.05.09 - Permit Submittal (Slonmvater)\Documents 1. Stormwater Management Narrative 1.1. Project Description The Government plans to construct a Conferencing and Catering Center off Reilly Road at Fort Bragg, NC. The building is approximately 50,000 SF and will be built to support the growing requirements for meetings, conferences, social events, and recreational needs at Fort Bragg. The building will be constructed on a previously developed site bound by Reilly Road to the west, and woods on the remaining sides. 1.2. Site Description New work for this project will include construction of an approximately 50,000 SF building with associated site work including parking area, sidewalk, access drives, new utilities and new stormwater BMPs. New work will disturb approximately 10.5 acres. 1.3. Soils Characteristics According to the USDA Web Soil Survey, the soils in the area are classified as WgB (Wagram-Urban land complex, o to 8 percent slopes). See Appendix C for the USDA Web Soil Survey. 1.4. Stormwater Management The project area has a total built upon area of 6o%. The site has been split up into three designated drainage areas. The Upper Basin (1), the Lower Basin (2), and offsite flow (3). The offsite flow basin includes area that could not be captured in the BMPs, based on the topography of the site, and the overall layout of parking and building footprint. Every effort has been made to ensure the post development conditions in terms of stornwater runoff are improved from the existing conditions of the site. The upper basin (1) is being routed to two bioretention cells that are placed within a parking island. The bioretention cells have a pretreatment device that includes 8" of gravel and 3' of sod. The bioretention area is designed to remove 85% of TSS. The cells have been sized with the ultimate build out conditions in mind. Major design elements include: The bottom of the bioretention cell is separated by 2' from the recorded seasonal high water table • The cell is located at least 30 feet from surface waters and 50 feet from class SA waters • Ponding depth is 12 inches • Soil media in cell is specified to be homogeneous soil mix of 85-88 percent by volume sand, 8-12 percent fines, and 3-5 percent organic matter. The P-Index shall be between to and 3o and permeability between 1 to 2 inches per hour • 4 inch diameter perforated underdrains with sock have been provided with non perforated standpipe and cleanout The lower basin (2) is being routed to dual dry detention basins in series separated by a permanent baffle. The flow from the dry detention basin is routed through an outlet structure to a level ® Dewberry Fort Bragg I Conferencing and Catering Center) Stormwater Narrative 11 spreader/vegetated filter strip. The combined removal efficiency for these devices is 85%. Again, these devices are designed with ultimate build out in mind. Major design elements include: • 3a vegetated side slopes • 25% additional storage volume for sediment deposition • Seasonal high water table is at least 2 feet below the bottom of the basin • A sediment depth indicator • Emergency drain • Outlet structure with regular pipe out plus a normally closed 6 inch diameter emergency drain • Outlet structure is also equipped with a larger orifice at a higher elevation to serve as an overflow drain • A concrete level spreader with slope of zero across the surface • A 30 feet long x 30' wide vegetated filter strip which will be vegetated with sod 1.5. Summary of Stormwater Flow Rates Peak flow calculations (pre and post construction) were calculated using the Rational Method. The inflow hydrographs and volumes were calculated using the SCS method. The volume of treatment that must be controlled was calculated using the Simple Method. State -storage -discharge models were developed using Chainsaw Routing. All calculations methods are described in Section 3 of the North Carolina Division of Water Quality Best Management Practices Manual, latest edition. A summary of the pre and post construction flow rates are as follows: Summary of Flow Rates t year to year too year Pre Development Flow Rate (CFS) 32.6 49-5 62. Post Development Flow Rate (CFS) 36.9 56.0 70.5 Post Development Flow Rate with BMPs installed (CPS) 15.46 23.58 40.17 9 Dewberry Fort Bragg I Conferencing and Catering Center) Stormwater Narrative 12 Conferencing and Catering Center Fort Bragg, INC DISSIPATOR PAD A Dissipator Pad Calculations Objective: To ensure the flow velocity at the outlet of the crossing culvert is non -erosive for the receiving ditch Design Criteria: Dissipator Pad is selected based on Ofull from the culvert Source: NCDENR E&SC Manual section 8.06 Values listed below are from FlowMaster Calculations Culvert Size 18 INCHES. Culvert Sloe 0.005 FT/FT Ofull 7.43 US Vfo 4.20 FT/S Results from figure 8.06c Zone 1 Apron Material Class B Minimum Thickness 18 INCHES Minimum Apron Length g FT Minimum To Width 4.5 FT Minimum Bottom Width 10.5 FT Conferencing and Catering Center Fort Bragg, North Carolina DISSIPATOR PAD B Dissipator Pad Calculations Objective: To ensure the flow velocity at the outlet of the crossing culvert is non -erosive for the receiving ditch Design Criteria: Dissipator Pad is selected based on Qfull from the culvert Source: NCDENR EBSC Manual section 8.06 Values listed below are from FlowMaster Calculations Culvert Size 24 INCHES. Culvert Sloe 0.005 FT/FT QfU11 16.00 CFS All 5.09 FT/S Results from figure 8.06c Zone 1 Apron Material Class B Minimum Thickness 18 INCHES Minimum Apron Length 121 FT Minimum Top Width 6 FT Minimum Bottom Width 1 141 FT Conferencing and Catering Center Fort Bragg, North Carolina DISSIPATOR PAD C Flume Information: Approximate Length Approximate Width Approximate Depth Mannings n coefficient Equivalent Pipe Diameter Drainage Area Information Approximate Total Area 125 6 FT 6 FT 0.5 FT 0.013 for concrete 2.0 FT 0.13 AC 8.78 IN/HR Rational C Area ac Open Space 0.40 0.02 Impervious 0.90 0.11 Woods 0.25 0.00 Composite C 0.84 Peak Flow Calculations 25= 0.93 CFS 25= 0.31 FPS Dissipator Pad Calculations Objective: To ensure the flow velocity at the outlet of the crossing culvert is non -erosive for the receiving ditch Design Criteria: Dissipator Pad is selected based on Qun Source: NCDENR E&SC Manual section 8.06 Values listed below are from FlowMaster Calculations mn 0.93 CIFS mn 0.31 FT S Results from figure 8.06c ZONE 1 Apron Material CLASS B Minimum Thickness 18 INCHES Minimum Apron Length 12 FT Minimum Top Width 8 FT Minimum Bottom Width 1 12 FT Conferencing and Catering Center Fort Bragg, North Carolina DISSIPATOR PAD D Flume Information: Approximate Length Approximate Width Approximate Depth Mannings n coefficient Equivalent Pipe Diameter Drainage Area Information Approximate Total Area 125 6 FT 6 FT 0.5 FT 0.013 for concrete 2.0 FT 0.30 AC 8.78 IN/HR Rational C Area ac Open Space 0.40 0.14 Impervious 0.90 0.16 Woods 0.25 0.00 Composite C 0.67 Peak Flow Calculations 2s= 1.78 CFS 25=1 0.59 FPS Dissipator Pad Calculations Objective: To ensure the flow velocity at the outlet of the crossing culvert is non -erosive for the receiving ditch Design Criteria: Dissipator Pad is selected based on Qw„ Source: NCDENR E&SC Manual section 8.06 Values listed below are from FlowMaster Calculations f,n 1.78 CFS n,o 0.59 T/S Results from figure 8.06c ZONE 1 Apron Material CLASS B Minimum Thickness 18 INCHES Minimum Apron Length 12 FT Minimum Top Width 8 FT Minimum Bottom Width 1 121 FT Conferencing and Catering Center Fort Bragg, North Carolina DISSIPATOR PAD E Flume Information: Approximate Length Approximate Width Approximate Depth Mannings n coefficient Equivalent Pipe Diameter Drainage Area Information Approximate Total Area 125 6 FT 6 FT 0.5 FT 0.013 for concrete 2.0 FT 0.07 AC 8.78 IN/HR Rational C Area (ac) Open Space 0.40 0.01 Impervious 0.90 0.06 Woods 0.25 0.00 Composite C 0.80 Peak Flow Calculations 25- 25= Dissioator Pad Calculations Obiective: To ensure the flow velocity at the outlet of the crossing culvert is non -erosive for the receiving ditch Design Criteria: Dissipator Pad is selected based on Ctull Source: NCDENR EBSC Manual section 8.06 Values listed below are from FlowMaster Calculations ull ull Results from figure 8.06c ZONE 1 Apron Material CLASS B Minimum Thickness 18 INCHES Minimum Apron Length 12 FT Minimum Top Width 8 FT Minimum Bottom Width 1 12 FT