SW6230606_Stormwater Narrative_20230906

Detailed Narrative
Project Description
This is a project to develop a 12,200 SF Emergency Service. The project is located at
13151 NC HWY 27 W, Broadway,NC 27505 on a lot of parcel size 2.49 ac. The site lies
within the jurisdiction of Harnett County with the zoning district of RA-20R.
The property is part of Falcon Crest Development. The project lies in FEMA Floodplain
Zone X, area of minimal flood hazard per FIRM Panel# 3710958600J dated Oct 3, 2006.
Neither USGS 7.5-minute quadrangle map (Olivia Quadrangle NC 7.5-Minute Series)
nor Soil Survey of Harnett County,North Carolina(Sheet 9 of 14)by USDA Soil
Conservation Service has jurisdictional features to the property. Also, S&EC (Soil &
Environmental Consultants, PA) of 8412 Falls of Neuse Road, Suite 104, Raleigh,NC
27615 believes that there are no jurisdictional features on property per site visit
conducted on 07/01/2022. The property lies in the Cape Fear River basin. The project lies
outside of a water supply watershed.
As part of the development, approximately 2.15-Acres of area will be disturbed with 0.73
acres of additional impervious (proposed building,parking spaces and sidewalk)will be
added to the parcel. The total disturbed area includes the 1.57 acres of development area
draining towards the Southwest side of the property. A small area of 0.58 acres drains
along Dunrovin Ln, where the improvement will not increase the runoff. Thus, for the
calculation purpose only 1.57-acre area as on Annex A are considered.
Drainage area
The site has a high spot near NE corner of the property. Most of the existing water
naturally flows towards the SW corner of the property. Two 1' deep and 3:1 side slope,
trapezoidal swales were proposed to collect the runoff for the treatment before releasing
near the SW corner of the property. A bioretention cell will serve the purpose of a
Stormwater Control Measure(SCM),by treating the runoff from the developed area(the
detail of the drainage area could be seen on Drainage Area Map, attached to this report).
The area is graded in such a way that the pre and post development drainage area remain
the same (1.57 acres) for a point near the outlet of the bioretention. This point is
considered as Point Of Analysis (POA) for the purpose of runoff calculations. Further, all
the impervious surface within the subject parcel is collected and treated to the SCM. The
SCM itself has a drainage area of 1.45 acres.
Bioretention Cell
A 2,850 SF Bioretention cell with IWS is proposed as a Stormwater Control Measure
(SCM). The bioretention cell is designed with 2.5' of media depth. PVC underdrain pipes
will be placed at the bottom of the cell and are connected to a 2'x2' concrete riser. The
riser will eventually discharge the collected water at the outlet pad via a 15"RCP pipe.
The proposed bioretention cell without IWS can achieve various pollutant removal. It can
reduce 40%total nitrogen, 45% of total phosphorus and 85% of suspended solids. A riser
with a 15"RCP outlet is proposed to discharge the outflow near the SW corner of the
property.

SHWT is identified at 43inch below surface during the storm water soil evaluation
carried out by"Soil & Environmental Consultants, PA" of 8412 Falls of Neuse Road,
Suite 104, Raleigh,NC 27615 (see included report),with a report titled"Detailed storm
water soils evaluation for Benhaven, 13151 NC Hwy 27, Broadway,NC.". Existing
ground elevation at the test location is 278', thus the existing SHWT is estimated at
274.42 fmsl, which is more than 2' below proposed bottom of bioretention cell (276.67').