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Home My WebLink AboutSW4230801_Stormwater Narrative_20231208 ACC PUBLIC SAFETY TRAINING CENTER STORMWATER IMPACT ANALYSIS STATEMENT • � 'i ter'. � I + ,''��" • ll' .�- -�..•. �`r r 1 x. F ,11*77 DECEMBER 8, 2023 PREPARED BY: = z a SEA y s Garrett Frank, PE, PLA = 5 80 5410 Trinity Road, Suite 102 �• s Raleigh, North Carolina 27607 919.866.4503 phone 919.859.5663 fax Garrett.Fran k(aD-timmons.com .••S`sue www.timmons.com TIMMONS GROUP YOUR VISION ACHIEVED THROUGH OURS. ACC Public Safety Training Center Burlington, NC Project Number:49685 Table of Contents Section 1: Site Analysis and Narrative Existing and Proposed Conditions Pertinent Onsite and Offsite Drainage Conditions Anticipated Stormwater Impacts & Design Criteria Soils Series and Flood Hazard Areas Proposed Stormwater Management Practices Calculation Method Summary of Results Pre/Post Summary Tables Section 2: Environmental Maps Soil Map FEMA Map USGS Quad Map Section 3: Stormwater Calculations Wet Pond Sizing Calculations Pre/Post Drainage Area Calculations HGL Calculations Rip Rap Outlet Protection Design Section 4: Stormwater Compliance Maps Pre/Post Stormwater Maps ACC Public Safety Training Center Burlington, NC Project Number: 49685 Section 1 : Site Analysis and Narrative Existing and Proposed Conditions The existing site is undeveloped and previously used for mining. The proposed development will include an administration/classroom building, 400'x400' asphalt driving pad, pavilion, and training tower. The site work will include asphalt paving, concrete paving, grading, erosion control, and utility work. The overall site is approximately 57.49 acres. The total lease agreement is for 90.43 acres. The proposed increase in impervious area for the project site is 8.00 acres. Pertinent Onsite and Offsite Drainage Conditions The site is undeveloped with vegetation and trees. The site and surrounding areas drain to Boyd's Creek to the west of the site. Anticipated Stormwater Impacts & Design Criteria The additional impervious will require a stormwater control measure to peak flow control caused by the increase in impervious area. This project will provide a wet pond SCM located west of the driving pad. The wet pond is sized to treat the proposed additional impervious area as well as the indoor shooting range alternate. Soils Series and Flood Hazard Areas The site is located in the Cape Fear River Basin. Soils on the site are mostly Udorthents and Vance Sandy Loam. hydrologic soil group C, based upon the Alamance County Soils Survey. A review of Flood Hazard Areas based on the published maps and the NC Flood plain Mapping Information System indicate that there are no flood zones on site. Proposed Stormwater Management Practices There will be a proposed wet pond which will accept flow from the proposed administrative/classroom building, asphalt driving pad, the future indoor shooting range, and parking lots. Calculation Method The Discrete NRCS Curve Number Method was used to calculate pre and post development flows and perform routing. The rainfall frequency data was collected from NOAA Atlas 14. The Rational Method was used to calculate the hydraulic grade lines in the storm network to confirm the pipes can meet capacity. Summary of Results Site Nitrogen and Phosphorus Loading Requirements: The site lies within the Boyds Creek watershed which is located within the Jordan Reservoir Watershed. The nitrogen and phosphorus must be calculated per the state procedures and approved by the stormwater reviewer. This export will be calculated in pounds per acre per year (lb/ac/yr). Any activity in the Jordan delivery zone 030050 shall not exceed 1.43 Ib/ac/yr for nitrogen and phosphorus. After stormwater treatment on site, the total nitrogen load is 3.42 Ib/ac/yr and the total phosphorus load is 0.57 Ib/ac/yr. There is no nutrient buydown required to meet the nitrogen and phosphorus loading requirements. The total loading was calculated for the current project area and the future ACC project area (total leased area) of 90.43 acres. .Gee www.timmons.com TIMMONS GROUP Yl U_.I I]I VA N A_I I IL'I L V 1-1-U--- ACC Public Safety Training Center Burlington, NC Project Number: 49685 Pre/Post: The wet pond will capture and treat approximately 21.24 acres, of which approximately 8.29 acres is impervious. The wet pond will reduce flow downstream which would otherwise be increased from the added impervious. There are three analysis points located on-site where the proposed storm network daylights into the existing ditches and Boyd's Creek. Drainage area 1 contains the wet pond and will reduce runoff to below pre-development levels for the 1, 2, and 10 year storm events by treating upstream runoff from the proposed development. Drainage areas 2 and 3 will reduce runoff to below the pre-development levels for the 1, 2, and 10- year storm events by directing flow to drainage area 1. See the table below for pre-development and post- development conditions for analysis point 1, 2 and 3. www.timmons.com TIMMONS GROUP ACC Public Safety Training Center Burlington, NC Project Number:49685 Section 1 : Site Analysis and Narrative Pre/Post Summary Tables Site Anal is Point 1 Site Condition 1 Year cfs 2 Year cfs 10 Year cfs Pre-Development 10.96 17.02 34.63 Post-Development 8.23 12.87 23.20 Site Anal is Point 2 Site Condition 1 Year cfs 2 Year cfs 10 Year cfs Pre-Development 3.520 5.278 10.36 Post-Development 1.732 2.628 5.193 Site Anal is Point 3 Site Condition 1 Year cfs 2 Year cfs 10 Year cfs Pre-Development 15.66 24.17 48.79 Post-Development 14.62 22.18 43.86 www.timmons.com TIMMONS GROUP YOUR VISION AC-LVfD TIROUGN OURS. ACC Public Safety Training Center Burlington, NC Project Number:49685 Section 2: Environmental Maps Soil Map www.timmons.com TIMMONS GROUP YOUR VISION AC-LVfD TIROUGN OURS.