SW3230608_Stormwater Narrative_20230925

Stormwater Narrative
Stoneglen is a proposed 31-lot single family subdivision located on a 12.4-acre parcel off W.
Peeler St. in Granite Quarry, NC. The project is a high-density project and the storm water
runoff that is generated by the development will be treated as required by NCDEQ including
treating the runoff from the first one inch of rainfall for 85% TSS removal. The project will be
required to comply with Town of Granite Quarry requirements to detain the post-development
1-year and 10-year storm event runoff rates to less than pre-development runoff rates. In
addition, the SCM will be designed to have minimum 6 inches of freeboard during the 50-year
storm event. The piped storm drainage system has been designed to convey the runoff from
the 10 yr. storm event while ensuring that the hydraulic grade line is minimum 6"below the
top of storm structures while accounting for the tailwater of the proposed sand filter SCM.
Stormwater improvements include a piped stormwater collection system and an on-site
detention and water quality SCM consisting of a combination dry pond and sand filter.
Sizing of the SCM has been completed per the NCDEQ Stormwater Design Manual.
The predevelopment conditions for the site included on-site and off-site drainage that drain in
a westerly direction across the site. The pre-development drainage area for this site is 23.32
acres including 10.92 acres of off-site drainage.
Post development, 11.52 acres of runoff will be captured in two clear water diversion ditches
and routed around the site and bypass the SCM. The remaining 11.8 acres, which includes all
new impervious, will be routed to the proposed SCM. The post-development discharge point for
all stormwater runoff is the same as the predevelopment discharge point.
Drainage Basins
Pre-Development
The predevelopment conditions for the site included on-site and off-site drainage that drain in
a westerly direction across the site. The pre-development drainage area for this site is 23.32
acres including 12.92 acres of off-site drainage. The point of analysis for this project is
located at the southwest side of the parcel.
The pre-development model consists of three drainage basins, one for the on-site drainage and
two off-site drainage basins. These drainage basins are bound by topography onsite and the
site boundary. The pre-development cover condition for the subject parcel will be modeled as
100% wooded. There is no existing built upon area (BUA) located within the project
boundary. This will ensure a conservative pre-development flow rate, and volume. The off-
site drainage areas consist of 1-acre residential parcels. Note that the off-site drainage is
partially diverted around the site by existing clean water ditches. Time of Concentration
("Tc") for each basin has been calculated. Tc path for off-site flow ends at the property line of
the parcel.
Post-Development
The post-development model consists of four drainage basins. 11.52 acres of on-site drainage,

which includes all impervious, will be directed to the new SCM. A 0.28 acre basin of on-site
drainage comprised of the pond embankment and nearby grassed areas, which has no
impervious, will surface drain to the point of analysis. The two off-site basins will be directed
around the site via existing clean water ditches and additional piped storm drainage systems.
The same point of analysis is used for pre-development and post-development. Tc path for
off-site flow is unchanged from pre-development. All on-site "Tc"paths for on-site flow are
assumed to be 5 minutes in the post-development. Cover conditions for the entire on-site
parcel were modeled as 1/-acre residential districts as per Soil Conservation Service TR-55
method.
The at-grade sand filter SCM has been sized to treat the runoff volume from the 1-inch storm.
Runoff volume has been determined using the Simple Method. Size and storage volume for
the sand filter has been designed as per the NC DEQ Stormwater Design Manual.
Built upon areas for the project include:
1) BUA as detailed in the construction plans including road, sidewalk, curb & gutter, etc.
2) In addition, allowances for built upon areas for each parcel (Houses, driveways, hardscape
etc.) were calculated as detailed in the report. Maximum built upon area for each parcel
was calculated as 3,600 SF.
Calculation Methodology and Assumptions
• Rainfall Data is taken from NOAA Atlas 14.
• HydroCAD 10.00-22, is used to produce the pre-development and post-development
runoff hydrographs.
• Soil Data is from the NRCS web soil survey. Soil types for each subbasin are calculated
and composite curve number created from this data. The site is completely composed
of"HSG C" soils.
• A composite SCS curve number was calculated for the pre-development condition and
the proposed post-development drainage area based on cover conditions and existing
soils.
• Time of Concentration is calculated based on the SCS (NRCS) TR-55 methodology. In
the pre-development all overland flow occurs in 100'.In the post-development condition
all developed areas are assumed to have times of concentration of 5 minutes.
• Storm drainage pipe capacity calculations have been completed utilizing Hydraflow
Hydrographs Extension for Autodeck Civil 3D (2021). The calculated tailwater for the
10-year storm(as per HydroCAD model)has been utilized as the tailwater condition for
pipes draining to the SCM.
• Inlet gutter spread calculations have been completed utilizing Hydraulic Toolbox 5.2.

Results Summary
Predevelopment Peak Flow Storm Event
100-
1-Year 10-Year Year
Drainage Areas (cfs) (cfs) (cfs)
Off-Site (Subbasin 1) 2.92 8.25 13.62
Off-Site (Subbasin 2) 5.06 13.31 21.3
On-Site (Subbasin 3) 4.92 17.9 32.02
Total Peak Runoff 12.84 39.09 66.44
Post-Development Peak Flow Storm Event
100-
1-Year 10-Year Year
Drainage Areas (cfs) (cfs) (cfs)
Off-Site (Subbasin 1) 3.04 10.06 14.09
Off-Site (Subbasin 2) 5.5 15.81 22.93
On-Site 2 (Bypass) 0.3 0.86 1.37
On-Site 1 (Subbasin 7)
Unattenuated Flow 15.4 35.11 52.07
Attenutated Flow 3.33 13.96 29.27
Total Runoff 10.14 37.21 65.47
Post Development Increase (cfs) -2.7 -1.88 -0.97
The proposed stormwater management plan for the Stoneglen Subdivision meets the design
criteria as required by the Town of Granite Quarry and the NC DEQ. As required by the
Town of Granite Quarry, the post-construction runoff is less than the pre- construction runoff.
As per NC DEQ requirements the sand filter is sized to store and treat the water quality runoff
volume.
If the development is built as proposed within this report, all applicable design criteria will be
met. However, modifications to the proposed development that change assumptions made in
this report will invalidate this report.