SW3191201_Stormwater Management Narrative_12/5/2019

Post -Construction Stormwater Management Narrative
Circle K Denver (Unity Church Rd)
11-19-19
Proposed Development
Circle K is proposing to develop a 2.35+ acre portion of the three existing, undeveloped parcels with a new
Circle K Convenience Store and Gas Station with Car Wash, as well as supporting infrastructure. The
proposed site is located on the northeast corner of NC-16 Business and Unity Church Road, Denver, North
Carolina.
Post -Development Drainage Basins
The post -development condition contains four basins.
Post Drainage Area A captures runoff from most of the new impervious areas including the new building,
fuel canopy, car wash, the southern half of the parking and driveways, and all of the new access drive
along Unity Church Road. Post Drainage Area A routes runoff to an open sand filter and detention system
at the southwest corner of the site. The sand filter detention system discharges to an existing grate inlet
at the intersection of NC-16 Business and Unity Church Road. The sand filter detention system controls
discharge flowrates and provides water quality control for Post Drainage Area A.
Post Drainage Area B consists of a small undeveloped, wooded area east of the site minus the impervious
portion of the new access drive along Unity Church Road. Post Drainage Area B is entirely pervious, and
runoff is not treated for water quality. Runoff from Post Drainage Area B discharges east of the site to an
existing swale along Unity Church Road.
Post Drainage Area C Bypass captures runoff from the undeveloped wooded area east of the site as well
as part of the remaining developed pervious, landscaped areas east and north of the site. The runoff from
Post Drainage Area C Bypass discharges to a swale running along NC-16 Business which outfalls to the
existing grate at the intersection of NC-16 Business and Unity Church Road. Post Drainage Area C Bypass
is entirely pervious with the exception of 278 square feet of sidewalk.
Post Drainage Area D captures runoff from the northern half of the parking and driveway areas, as well as
a portion of the developed, pervious area at the northern side of the site. The runoff from Post Drainage
Area D is routed through a bioretention area and discharged to a swale running along NC-16 Business
which leads to an existing grate inlet at the intersection of NC-16 Business and Unity Church Road. The
bioretention area controls discharge flowrates and provides water quality control for Post Drainage Area
D.
Open Sand Filter Detention System
The sand filter detention system consists of two pond areas connected by an 18" pipe: a sediment
chamber and a sand filter pond. The detention system outfall is controlled by an outlet control structure.
The combined volume of the ponds is 19,823 cubic feet and is designed to detain and control runoff for
the 10-yr 24-hr storm. Runoff is treated for water quality in the sand filter pond. The size of the sand
filter was determined by calculating the runoff volume for Post Drainage Area A using the simple
calculation method provided by North Carolina Department of Environmental Quality Primary Stormwater
Control Measure (SCM) design criteria. The design volume was calculated based on a design storm depth
of 1.0" and the discounted volume was used to determine the minimum sand filter surface area. The
minimum surface area required is 854 square feet and the provided surface area is 1,683 square feet. The
sand filter media is specified as ASTM C33 or equivalent.

Bioretention Area
The bioretention area consists of a shallow open pond area over bioretention media. The detention
system outfall is controlled by an outlet control structure. The ponding depth in the bioretention area is
1.0' and is designed to detain and control runoff for the 10-yr 24-hr storm event. Runoff is treated by
filtering through the bioretention media. The size of the bioretention area was determined by calculating
the runoff volume for Post Drainage Area D using the simple calculation method provided by North
Carolina Department of Environmental Quality Primary Stormwater Control Measure (SCM) design
criteria. The design volume was calculated based on a design storm depth of 1.0". The design volume
was used to determine the minimum required bioretention media surface area. The minimum surface
area required is 1,240 square feet and the provided surface area is 1,755 square feet. The bioretention
media mix is specified as 75-85%coarse washed sand, 8-10%fines (silt and clay), and 5-10%organic matter
(such as pine bark fines). The bioretention surface will be sodded.