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Home My WebLink AboutSW5210202_SW Report_20210224 FRANKLIN COUNTY EMERGENCY COMMUNICATIONS STORMWATER IMPACT ANALYSIS STATEMENT FEBRUARY 9, 2021 PREPARED BY: Brandon McLamb, PE 5410 Trinity Road, Suite 102 Raleigh, North Carolina 27607 919.866.4935 phone 919.859.5663 fax brandon.mclamb@timmons.com www.timmons.com Franklin County E911 Louisburg, NC Project Number: 46616 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 & Anti-Floatation Calculations Pre/Post Drainage Area Calculations HGL Calculations Section 4: Stormwater Compliance Pre/Post Stormwater Maps Nutrient Loading Franklin County E911 Louisburg, NC Project Number: 46616 Existing and Proposed Conditions The existing site is partially developed with the . The proposed 911 center and parking lot will be added to the south of the existing building. The overall site is approximately 28.16 acres. The intended use is for a 911 emergency call center. This is the reason for the 83 foot offset from the existing parking lot. The proposed increase in impervious area for the project site is 0.59 acres. Pertinent Onsite and Offsite Drainage Conditions The site is about halfway developed and the other half is wooded and contains wetlands. The portion of the site allocated to the building is undeveloped and wooded. The site contains two large drainage areas, Catchment 2 drains to the north to a stream running along the northern edge of the property and leaves the site on the north east corner. Catchment 1 drains to the south into a stream and the stream then runs west to east and leaves the site at the middle of the eastern edge. There is a very small third drainage area called Catchment 3 that leaves the western portion of the site. Anticipated Stormwater Impacts & Design Criteria The additional impervious will be require a stormwater control measure, which will be a wet pond located behind the proposed building and parking lot. The wet pond will also help with nutrient loading, TSS, and peak flow attenuation. Soils Series and Flood Hazard Areas The site is located in the Tar River Basin. Soils on the site are mostly silt loam, hydrologic soil group D, based upon the Franklin 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 building and parking lot. Calculation Method The SCS method was used to calculate pre and post development flows and preform 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 The wet pond will capture and treat approximately 1.44 acres, of which approximately 0.61 acres are impervious. The wet pond will reduce flow downstream which would otherwise be increased from the added impervious. There are three analysis points on this site, Catchment 1 is located to the north east and Catchment 2 to the east, and Catchment 3 is toward the west. Drainage area A will contain the bioretention pond and will reduce runoff to below pre-development levels for the 1 and 10 year storm events. Catchments 2 and 3 will not contain a stormwater control measures or changes in drainage area or impervious so they will remain the same from the pre to post flows. See the tables below for pre-development and post-development conditions for each analysis point. www.timmons.com Franklin County E911 Louisburg, NC Project Number: 46616 Pre/Post Summary Tables Site Analysis Point A Site Condition 1 Year (cfs) 2 Year (cfs) 10 Year (cfs) Pre-Development A 16.90 23.66 43.11 Post-Development A 15.23 21.31 40.87 Site Analysis Point B Site Condition 1 Year (cfs) 2 Year (cfs) 10 Year (cfs) Pre-Development B 23.50 31.53 53.90 Post-Development B 23.50 31.53 53.90 Site Analysis Point C Site Condition 1 Year (cfs) 2 Year (cfs) 10 Year (cfs) Pre-Development C 0.37 0.98 1.80 Post-Development C 0.37 0.98 1.80 www.timmons.com Franklin County E911 Louisburg, NC Project Number: 46616 www.timmons.com SITE Zpv!dsfbufe!uijt!QEG!gspn!bo!bqqmjdbujpo!uibu!jt!opu!mjdfotfe!up!qsjou!up!opwbQEG!qsjoufs!)iuuq;00xxx/opwbqeg/dpn* Franklin County E911 Louisburg, NC Project Number: 46616 www.timmons.com Zpv!dsfbufe!uijt!QEG!gspn!bo!bqqmjdbujpo!uibu!jt!opu!mjdfotfe!up!qsjou!up!opwbQEG!qsjoufs!)iuuq;00xxx/opwbqeg/dpn* Franklin County E911 Louisburg, NC Project Number: 46616 www.timmons.com Zpv!dsfbufe!uijt!QEG!gspn!bo!bqqmjdbujpo!uibu!jt!opu!mjdfotfe!up!qsjou!up!opwbQEG!qsjoufs!)iuuq;00xxx/opwbqeg/dpn* Franklin County E911 Louisburg, NC Project Number: 46616 www.timmons.com Franklin County E911 Louisburg, NC Project Number: 46616 www.timmons.com Franklin County E911 Louisburg, NC Project Number: 46616 www.timmons.com DC!4 QJQF!5 EJ!3 QJQF!6 DC!5 QJQF!7 EJ!2 QJQF!4 DC!3 QJQF!3 DC!2 QJQF!2 FY/!EJ Franklin County E911 Louisburg, NC Project Number: 46616 www.timmons.com Franklin County E911 Louisburg, NC Project Number: 46616 www.timmons.com