SW5240201_Stormwater Narrative_20240723

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TIMMONS GROUP Suite 102
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July 23, 2024
NC Dept of Enviro. Quality
Raleigh Regional Office
Attn: Brianna Holland
512 N. Salisbury Street
Office 625 Y
Raleigh, NC 27604
Subject: Wrenn Farms (SW5240201) Stormwater Narrative
County: Franklin
City: Youngsville
Dear Ms. Holland,
Wrenn Farms is a proposed 183 lot, townhome subdivision located on NC HWY 96 in Franklin County. The
runoff from the project discharges to Richland Creek to the south which is part of the Neuse River Basin —
Phase II ETJ Area. The Town of Youngsville is not named a Nutrient Sensitive Water area; however, the site
is in a Tipped County which requires post-construction stormwater permitting through NCDEQ to confirm
compliance with Neuse Basin Rules, 15A NCAC 02H .1017. The subdivision will be constructed as one
project with one proposed Wet Pond. The proposed wet pond, SCM 1, will also treat the future Wrenn
Apartments site and small portions of the adjacent subdivision, Wrenn Villas. SCM 1 will be constructed as
part of the subdivision improvements and will reduce the post-development peak flows to pre-development
peak flow conditions for the 10-year and 25-year, 24-hour storm events. Additionally, this project proposes
85% Total Suspended Solids removal and treatment of the required Water Quality Volume per NCDEQ
requirements. The project will also reduce Total Nitrogen export by 30% and buy down to 3.6 lbs/ac/yr per
Neuse Nutrient Strategy: Stormwater, 15A NCAC 02B .0711 and Neuse Stormwater Rule, 15A NCAC 2B
.0235.
Pre-development and post-development hydrologic analyses were completed using the NRCS (SCS)
method. Drainage area maps and CN and Tc calculation sheets are provided in the SIA (Stormwater Impact
Analysis). There are 3 drainage areas in pre-development conditions which are named by their point of
analyses, 'POI 1', 'POI 2', and 'POI 3'. A large portion of'POI 1' is offsite area that only briefly drains through
the site. There are 3 drainage areas in post-development conditions as well which are named by their point
of analyses, 'POI 1', 'POI 2', and 'POI 3'. 'POI 1' is broken into multiple sub-areas to better denote the
treatment type of each and whether the area is onsite or offsite. All the onsite BUA for the project is located
within 'POI 1 - ONSITE TO SCM 1' and drains to SCM 1. There are 2 small offsite areas located within 'POI
1 —OFFSITE TO SCM 1' that drain to SCM 1 as well. For the purpose of this analysis, any offsite areas that
drained to SCM 1 were considered to be 100% impervious.
A hydraulic analysis was completed using Hydraflow modeling software to route these storm events through
the proposed SCM and outlet structures. Results of this analysis are provided in the SIA.
The NC Stormwater Nitrogen and Phosphorus (SNAP) Tool was used to demonstrate compliance with
Neuse Nutrient Strategy: Stormwater, 15A NCAC 02B .0711 and Neuse Stormwater Rule 15A NCAC 2B
.0235. The results of the SNAP tool showed a 42.42% reduction in Total Nitrogen and show that the project
requires 7 lb/yr of nitrogen offset credits. The workbook is provided with this submittal.
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