The URL can be used to link to this page

Home My WebLink About20071114 Ver 1_More Info Received_20081112FREELAND and KAUFFMAN, INC. EMGmzws • L wscAPE ARQ'pwrg November 7, 2008 To: Annette Lucas, 401 Permitting Unit NCDENR - Division of Water Quality 2321 Crabtree Blvd., Suite 250 Raleigh, NC 27604 (919-715-3425) tg@[p g pnpi p NOV 1 2 2008 DENR - YVP:CtF? QUGu I l7' VVETLANDS AND STORM1ATER BRANCH Re: Shoppes at Oxford Oxford, NC Request for Revision to Stormwater Approval Associated with 401 Certification DWQ EXP No. 07-1114 - Granville County On behalf of our client, Oxford Retail Investment, LLC, we are requesting a revision to the stormwater approval associated with the 401 Certification for the above-referenced project. Please find enclosed the following information: • (1) copy of the Stormwater Management Report Addendum (including detailed narrative and calculations) (1) copy of the revised Grading and Drainage Plans The purpose of the stormwater plan change is to modify the outlet design from the detention pond to direct more water to the existing wetlands area in the post- development condition. The enclosed Stormwater Management Report Addendum provides detailed discussion and calculations of this change. Since the changes are made only after stormwater discharges from the primary pond, the original pond design and water quality treatment remains unaffected from the original design and approval. Should you have any questions or need additional information, please contact me directly at 864.672.3441 or mranks _fk-inc.com. Thank you for your assistance. Sincerely M ael Ranks 209 West Stone Avenue • Greenville. South Carolina 29609 • Telephone 864-233-5497 • Fax 864-233-8915 STORMWATER MANAGEMENT REPORT ADDENDUM Shoppes at Oxford Oxford, NC November 6, 2008 Prepared by: Freeland and Kauffman, Inc. Engineers - Landscape Architects 209 West Stone Ave. Greenville, SC 29609 864-233-5497 ic a" ?';• r k L `' TABLE OF CONTENTS 1. Narrative ..................................................................................................................................... 1-2 2. Stormwater Hydrograph Summary ............................................................................................3-4 3. Pipe flow Calculations ....................................................................................................................5 4. Win TR-55 Outputs ...................................................................................................................6-10 5. RipRap Apron Design on chart ............................................................................................. 11-12 6. Stormwater Hydrographs ......................................................................................................13-18 NARRATIVE D1 _o 1?- There is an existing wetland area which is located on the southern portion of this project. A single detention pond has been designed to collect stormwater runoff from the developed condition of this project and discharge it at appropriate pre-development flow-rates. However, the discharge line from the pond bypasses the existing wetlands area, and the result is that the wetlands area receives a reduced amount of stormwater runoff than it did in the pre-development condition. We are, therefore, proposing to modify the discharge from this project's detention pond to direct the majority of flow during the 2 and 10 year storm events toward the existing wetlands area. This should better approximate the pre-development hydrology of the wetlands. In order to accomplish this, we propose to install an additional 18" discharge line from the pond which will be directed directly to the wetlands. A small berm (6' width) has been added within the pond extension area (grassed swale). This berm will act as a weir to insure that the initial runoff from all storm events is first directed to the wetlands. Larger flow rates will spill over the berm and continue to discharge to the original location (which bypasses the wetlands). Supporting calculations are included in this report. A summary of the design calculations is as follows: CALCULATION SUMMARY 2-Year Storm Event: • Total drainage area to the wetland in pre-development condition = ±11.80 AC • 2-yr pre-developed runoff rate to wetland area = ± 17.27 cfs (SCS method, CNw 75, T? = 20 min.) • 2-yr pre-developed runoff volume to wetland area = ± 55,714 cf • Drainage area which direct-discharges into the wetland in post-developed condition = ±33 AC • 2-yr post-developed direct runoff rate to wetland area = ± 7.42 cfs (SCS method, CNw 79, Tc = 10 min,) • 2-yr post-developed direct runoff volume to wetland area = ± 19,345 cf • 2-yr pond discharge flow rate = ± 1.49 cfs • 2-yr pond discharge volume = ± 308,289 cf • The maximum flow rate out of the pond during the 2-year storm event is calculated to be 1.49 cfs. Therefore, the additional 18" RCP discharge line should convey all runoff during the 10- year storm event to the wetlands area, as the capacity of an 18" RCP at 0.5% slope = 9.66 cfs (manning's equation). 10-Year Storm Event: • Total drainage area to the wetland in pre-development condition = ±11.80 AC • 10-yr pre-developed runoff rate to wetland area = ± 39.10 cfs (SCS method, CNw 75, T? _ 20min.) • 10-yr pre-developed runoff volume to wetland area = ± 122,425 cf • Drainage area which direct-discharges into the wetland in post-developed = ±33 AC • 10-yr post-developed direct runoff rate to wetland area = ± 15.39 cfs (SCS method, CNw 79, Tc = 10 min.) • 10-yr post-developed direct runoff volume to wetland area = ± 39,983 cf • 10-yr pond discharge flow rate = ± 6.61 cfs • 10-yr pond discharge volume = ± 539,121 cf • The maximum flow rate out of the pond during the 10-year storm event is calculated to be 6.61 cfs. Therefore, the additional 18" RCP discharge line should convey all runoff during the 10-year storm event to the wetlands area, as the capacity of an 18" RCP at 0.5% slope = 9.66 cfs (manning's equation). 2 Hydrograph Summary Report Hyd. No. Hydrograph type (origin) Peak flow (cis) Time Interval (min) Time to peak (min) Volume (cult) Inflow hyd(s) Maximum elevation (it) Maximum storage (cult) Hydrograph description 1 SCS Runoff 17.27 2 726 55,714 -- --- -- Pre DA -WETLAND 2 SCS Runoff 7.42 2 720 19,345 -- - - -- Post DA -WETLAND 3 Manual 1.49 2 1244 308,289 - --- - - Pond Discharge Wetland flow calcs.gpw Return Period: 2 Year Friday, Nov 7 2008, 11:18 AM Hydraflow Hydrographs by Intelisolve 3 Hydrograph Summary Report Hyd. No. Hydrograph type (origin) Peak flow (cfs) Time interval (min) Time to peak (min) Volume (cuft) Inflow hyd(s) Maximum elevation (ft) Maximum storage (cuft) Hydrograph description 1 SCS Runoff 39.10 2 724 122,425 ---- ------ -- Pre DA -WETLAND 2 SCS Runoff 15.39 2 720 39,983 ---- ------ Post DA -WETLAND 3 Manual 6.61 2 862 539,121 ---- ------ ------ Pond Discharge Wetland flow calcs.gpw Return Period: 10 Year Friday, Nov 7 2008, 2:33 PM Hydraflow Hydrographs by Intelisolve q Worksheet for Circular Pipe - 1 Project Description Friction Method Manning Formula Solve For Discharge Input Data Roughness Coefficient Channel Slope Normal Depth Diameter Results Discharge Flow Area Wetted Perimeter Top Width Critical Depth Percent Full Critical Slope Velocity Velocity Head Specific Energy Froude Number Ma)amum Discharge Discharge Full Slope Full Flow Type GVF Input Data Downstream Depth Length Number Of Steps GVF Output Data Upstream Depth Profile Description Profile Headloss Average End Depth Over Rise Normal Depth Over Rise Downstream Velocity Upstream Velocity SubCritical 0.010 0.00500 ft/ft 1.50 ft 1.50 ft 9.66 ft/s 1.77 ftz 4.71 ft 0.00 ft 1.20 ft 100.0 % 0.00524 ft/ft 5.46 ft/s 0.46 ft 1.96 ft 0.00 10.39 ft'/s 9.66 ft/s 0.00500 ft/ft 0.00 ft 0.00 ft 0 0.00 ft 0.00 ft 0.00 % 100.00 % Infinity ft/s Infinity ft/s Bentley Systems, Inc. Haestad Methods Solution Center Bentley FlowMaster [08.01.066.001 1117/2008 11:01:43 AM 27 Slemons Company Drive Suite 200 W Watertown, CT 06795 USA +1-203-755-1666 Page 1 of 2 J WinTR-55 Current Data Description Identification Data --- User: Prakash Date: 10/28/2008 Project: Units: English SubTitle: Areal Units: Acres State: North Carolina County: Granville Filename: G:\Projects\Wal-Mart\Oxford NC\Documents\SWM Design\TC-Wetland.w55 --- Sub-Area Data --- Name Description Reach Area(ac) RCN Tc ------------------------------------------------------------------------------ Wetland-Pr 11.79 75 0.335 Wetland-Po 3.3 79 0.162 Total area: 15.09 (ac) --- Storm Data -- Rainfall Depth by Rainfall Return Period 2-Yr 5-Yr 10-Yr 25-Yr 50-Yr 100-Yr 1-Yr (in) (in) (in) (in) (in) (in) (in) -------------------------------------------------------------------------------- 3.5 9.7 5.5 6.3 7.1 7.9 3.0 Storm Data Source: Granville County, NC (NRCS) Rainfall Distribution Type: Type II Dimensionless Unit Hydrograph: <standard> WinTR-55, Version 1.00.08 Page 1 11/7/2008 11:02:17 AM 6 Prakash Granville County, North Carolina Storm Data Rainfall Depth by Rainfall Return Period 2-Yr 5-Yr 10-Yr 25-Yr 50-Yr 100-Yr 1-Yr (in) (in) (in) (in) (in) (in) (in) -------------------------------------------------------------------------------- 3.5 4.7 5.5 6.3 7.1 7.9 3.0 Storm Data Source: Granville County, NC (NRCS) Rainfall Distribution Type: Type II Dimensionless Unit Hydrograph: <standard> WinTR-55, Version 1.00.08 Page 1 11/7/2008 11:02:17 AM Prakash Granville County, North Carolina Sub-Area Summary Table Sub-Area Drainage Time of Curve Receiving Identifier Area Concentration Number Reach (ac) (hr) ------------------------------------------------------- Wetland-Pr 11.79 0.335 75 Wetland-Po 3.30 0.162 79 Total Area: 15.09 (ac) Sub-Area Description WinTR-55, Version 1.00.08 Page 1 11/7/2008 11:02:17 AM 8 Prakash Granville County, North Carolina Sub-Area Time of Concentration Details Sub-Area Flow Mannings's Identifier/ Length Slope n ------------ (ft) ---------- (ft/ft) -------- ----------- Wetland-Pr SHEET 100 0.0330 0.400 SHALLOW 677 0.0450 0.050 Wetland-Po SHEET 100 0.0350 0.170 SHALLOW 300 0.0483 0.050 End Wetted Travel Area Perimeter Velocity Time (sq ft) (ft) (ft/sec) (hr) -------------------------------------- 0.280 0.055 WinTR-55, Version 1.00.08 Page 1 Time of Concentration 0.335 0.138 0.024 Time of Concentration 0.162 11/7/2008 11:02:17 AM 9 Prakash Granville County, North Carol ina Sub-Area Land Use and Curve Number Details Sub-Area Hydrologic Sub-Area Curve Identifier Land Use Soil Area Number ---------------- ------------------------------ ------ Group ----------- (ac) ----------- ------ Wetland-PrPaved parking lots, roofs, driveways C .41 98 Woods - grass combination (good) C 7.12 72 Woods (poor) C 4.26 77 Total Area / Weighted Curve Number 11.79 75 Wetland-PoPaved parking lots, roofs, driveways C .41 98 Woods - grass combination (good) C .63 72 Woods (poor) C 2.26 77 Total Area / Weighted Curve Number 3.3 79 WinTR-55, Version 1.00.08 Page 1 11/7/2008 11:02:17 AM (0 t . 'P. ----IT t• La= 9f I N a ro a 0 N ? 3? CASE fa,? 1000 Figure 4.5-2 Design of Riprap Apron under Minimum Tailwater Conditions (Source: USDA, SCS, 1975) Discharge (0 /sec) Curves may not be extrapolated. f ,_,' _ ?.? ? ?. ? FREELAND and KAUFFMAN, INC. ENGINEERS • LANDSCAPEARCHITECTS 209 West Stone Avenue Job: Shoppes at Oxford, Oxford, NC Job No.: Date: 11 /6/2008 Greenville, South Carolina 29609 864-233-5497 Computed By: PCK 864-233-8915 Checked By: MCR Calculation for: RIP-RAP APRONS DS #44 Q10 (CFS) = 6.61 PIPE DIA (IN) = 18 LA (FT) = 9 We (FT) = 4.5 (3 * PIPE DIAMETER) W end = 8.1 (Wc + (0.4 * LM D 50 (IN) = 6 D MAX (IN) = 9 (1.5 * D50) DEPTH (IN) = 13.5 0.5 * D MAX) I Z- Hydrograph Plot Hydraflow Hydrographs by Intelisolve Friday, Nov 7 2008, 11:22 AM Hyd. No. 1 Pre DA -WETLAND Hydrograph type = SCS Runoff Peak discharge = 17.27 cfs Storm frequency = 2 yrs Time interval = 2 min Drainage area = 11.79 ac Curve number = 75 Basin Slope = 0.0% Hydraulic length = 0 ft Tc method = USER Time of conc. (Tc) = 20 min Total precip. = 3.50 in Distribution = Type II Storm duration = 24 hrs Shape factor = 484 Hydrograph Volume = 55,714 cuft Pre DA -WETLAND Q (cfs) Q (cfs) Hyd. No. 1 -- 2 Yr 18.00 15.00 12.00 9.00 6.00 3.00 0.00 18.00 15.00 12.00 9.00 6.00 3.00 0.00 0 2 5 7 9 12 14 16 19 21 23 26 Hyd No. 1 Time (hrs) 1 13 Hydrograph Plot Hydraflow Hydrographs by Intelisolve Friday, Nov 7 2008, 11:22 AM Hyd. No. 1 Pre DA -WETLAND Hydrograph type = SCS Runoff Peak discharge = 39.10 cfs Storm frequency = 10 yrs Time interval = 2 min Drainage area = 11.79 ac Curve number = 75 Basin Slope = 0.0% Hydraulic length = 0 ft Tc method = USER Time of conc. (Tc) = 20 min Total precip. = 5.50 in Distribution = Type II Storm duration = 24 hrs Shape factor = 484 Hydrograph Volume =122,425 cuft Q (cfs) 40.00 30.00 20.00 10.00 Q (cfs) 40.00 30.00 20.00 10.00 0.00 ' - ' ' ' ' 0.00 0 2 5 7 9 12 14 16 19 21 23 26 Hyd No. 1 Time (hrs) Pre DA -WETLAND Hyd. No. 1 -- 10 Yr 3 q Hydrograph Plot 2 Hydraflow Hydrographs by Intelisolve Hyd. No. 2 Post DA -WETLAND Hydrograph type = SCS Runoff Storm frequency = 2 yrs Drainage area = 3.30 ac Basin Slope = 0.0% Tc method = USER Total precip. = 3.50 in Storm duration = 24 hrs Friday, Nov 7 2008, 11:22 AM Peak discharge = 7.42 cfs Time interval = 2 min Curve number = 79 Hydraulic length = 0 ft Time of conc. (Tc) = 10 min Distribution = Type II Shape factor = 484 Hydrograph Volume = 19,345 cuft Q (cfs) 8.00 Post DA -WETLAND Hyd. No. 2 -- 2 Yr 6.00 4.00 2.00 Q (cfs) 8.00 6.00 4.00 2.00 0.00 ` 0.00 0 2 5 7 9 12 14 16 19 21 23 26 Hyd No. 2 Time (hrs) 157 Hydrograph Plot Hydraflow Hydrographs by Intelisolve Friday, Nov 7 2008,11:22 AM Hyd. No. 2 Post DA -WETLAND Hydrograph type = SCS Runoff Peak discharge = 15.39 cfs Storm frequency = 10 yrs Time interval = 2 min Drainage area = 3.30 ac Curve number = 79 Basin Slope = 0.0% Hydraulic length = 0 ft Tc method = USER Time of conc. (Tc) = 10 min Total precip. = 5.50 in Distribution = Type II Storm duration = 24 hrs Shape factor = 484 Hydrograph Volume = 39,983 cuft Post DA -WETLAND Q (cfs) Hyd. No. 2 -- 10 Yr Q (cfs) 18.00 15.00 12.00 9.00 6.00 3.00 0.00 18.00 15.00 12.00 9.00 6.00 3.00 0.00 0 2 5 7 9 12 14 16 19 21 23 26 Hyd No. 2 Time (hrs) 4 1(0 Hydrograph Plot Hydraflow Hydrographs by Intelisolve Friday, Nov 7 2008, 2:36 PM Hyd. No. 3 Pond Discharge Hydrograph type = Manual Peak discharge = 1.49 cfs Storm frequency = 2 yrs Time interval = 2 min Hydrograph Volume = 308,289 cult Pond Discharge Q (cfs) Hyd. No. 3 -- 2 Yr 2.00 1.00 0.00 Q (cfs) 2.00 1.00 0.00 0 10 19 29 39 48 58 68 77 87 97 Hyd No. 3 Time (hrs) 1 1-7 Hydrograph Plot Hydraflow Hydrographs by Intelisolve Friday, Nov 7 2008, 2:36 PM Hyd. No. 3 Pond Discharge Hydrograph type = Manual Peak discharge = 6.61 cfs Storm frequency = 10 yrs Time interval = 2 min Hydrograph Volume = 539,121 cuft Pond Discharge Q (cfs) Hyd. No. 3 -- 10 Yr Q (cfs) 7.00 6.00 5.00 4.00 3.00 2.00 1.00 0.00 7.00 6.00 5.00 4.00 3.00 2.00 1.00 0.00 0 10 19 29 39 48 58 68 77 87 97 Time (hrs) Hyd No. 3 2 le