SW6211206_Design Calculations_20220314

BAYWOOD TERRACE SUBDIVISION
SANDEROSA ROAD, EASTOVER
STORMWATER NARRATIVE
STORMWATER CALCULATIONS
11-28-21
Revised 1-03-21
Revised 1-19-22
Revised 2-24-22
Revised 3-10-22
PROJECT DESCRIPTION
Baywood Terrace is a proposed 22-lot single-family subdivision on a 10.35 acre tract off Sanderosa
Road in Cumberland County. The property is bounded by Sanderosa Road (SR 1833) to the south,
vacant farmland to the east, one single-family property and vacant land to the west and the Stone
Creek single-family subdivision to the north. The entire site is cleared with reasonably well -
established ground cover. The site is extremely flat, slopes from north to south in the direction of
Sanderosa Road, and ranges in elevation from approximate elevation 143 at the northernmost
property line to approximate elevation 140 at Sanderosa Road. There are no existing impervious
surfaces on the site.
STORMWATER MANAGEMENT PLAN
The property will be developed with one cul-de-sac street using NCDOT standards for a residential
collector subdivision road. This standard calls for a 50-foot right-of-way with a 20-foot wide
pavement section and side ditches. The topography of the site dictates that the side ditches will have
an extremely flat longitudinal slope with the bottom of the ditch being approximately 18 inches
below the edge of pavement. All of the lots will be graded for slab -on -grade home construction with
approximately one-half of each lot draining to the rear (away from the roadway ditch) and one-half
draining to the front (towards the roadway ditch). A typical house footprint and driveway section
(see detail on sheet SP6) was used to calculate expected runoff from each lot once fully developed.
For the purposes of the stormwater design, it was assumed that all of the impervious surfaces on each
lot will drain towards the roadside ditch.
The site is nearly 100% Candor Sand according to the Cumberland County Soil Survey. These are
Group A soils with a high -to -moderately -high capacity to transmit water. Virtually the entire site is
suitable for septic systems per a report by Southeastern Soil & Environmental Associates.
PART A — STORMWATER CALCULATIONS FOR ROADWAY DITCHES
Use the Rational method to determine runoff amounts to the roadway ditches and Manning's
equation to determine stormwater velocity based on the ditch section.
Area of the pavement is 22,212 sf with one-half flowing to the ditch on the western side of the
roadway and one-half flowing to the ditch on the eastern side of the roadway. Lots 1-11 will drain
the ditch on the eastern side and lots 12-22 will drain to the western side. Total area of lots 1-I 1 is
190,391 square feet or 4.37 acres. Total area of lots 12-22 is 207,725 square feet or 4.77 acres. Per
the typical lot development detail on sheet SP6, each lot will have 3,540 square feet of impervious
area draining to the roadway ditch. Total impervious area upon full development of the subdivision

is (22)(3540) + 22212 = 100,092 sf = 2.30 acres. Percent impervious = 2.30/10.35 = 22.2% = low
density project.
Calculate total flow and velocity for the 10-year storm in the western roadwa ditch:
Impervious = 11 lots x 3540 square feet per lot = 38,940 sf plus 1/2 of roadway -- 11,106 sf — 50,046
sf = 1.15 acres
Total area draining to western roadway ditch = 103,863 sf (see sheet SP5a) + 11,106 sf = 114,969 sf
= 2.64 acres
Pervious area — 114969 — 50046 = 64,923 sf = 1.49 acres
Weighted C = 50046/114969(0.95) +64923/114969(0.10) = 0.47
Determine precipitation intensity for 10-year storm using precipitation frequency data server at
http://hdsc.nws.noaa.gov/hdse/pfds/orb/nc pfds.html.
Longitude and latitude of approximate center of property is 35-04-03N,
78-45-46W.
10-year (5 minute time of concentration, 24-hour duration) = 7.96 inches per hour
Qio = CIA
Qio = (0.47)(7.96)(2.64) = 9.9 cfs
Ditch varies in elevation from 143 to 141 over an approximate length of 1007 linear feet.
Longitudinal slope of roadway ditch = (143-141)/1007 = 0.2%
Calculate total flow and velocity for the 10-year storm in the eastern roadway:
Impervious = 50,046 sf = 1.15 acres
Total area draining to eastern roadway ditch = 95196 sf (see sheet SP5a) + 11,106 = 106,302 sf =
2.44 acres
Pervious area = 106302 - 50046 = 56,256 sf = 1.29 acres
Weighted C = 50046/106302(0.95) + 56256/106302(0.10) = 0.50
Determine precipitation intensity for 10-year storm using precipitation frequency data server at
http://hdsc.nws.noaa.gov/hdsc/i)fds/orb/nc i)fds.html .
Longitude and latitude of approximate center of property is 35-04-03N,
78-45-46W.
10-year (5 minute time of concentration, 24-hour duration) = 796 inches per hour
Qio = CIA
Qio = (0.50)(7.96)(2.44) = 9.7 cfs
Ditch varies in elevation from 143 to 141 over an approximate length of 1007 linear feet.
Longitudinal slope of roadway ditch = (143-141)/ 1007 = 0.2%
Using Manning's Equation by trial and error, solve for area A
Use 0.03 for Manning's n
If A = 5.47 sf, then P = 9.11'
R=A/P=0.60
Qio= 1.49/0.03(5.47)(0.602/3)(0.002)1/2
= 8.6 cfs
If A = 6.34 sf, then P = 11.93'
R=A/P=0.53
QEo= 1.49/0.03(6.34)(0.532/3)(0.002)1/2
= 9.2 cfs
If A = 7.32 sf, then P = 13.16'
R—A/P=0.56

QEo= 1.49/0.03(7.32)(0.562/3)(0.002)112
= 11.0 cfs
By interpolation for 9.9 cfs, A = 6.47
Q=VA
V = Q/A
= 9.9/6.47 = L5 fps therefore non -erosive
Calculate flow and velocfty for the 10- ear storm for the shoulder area Sanderosa Road:
Distance from start of water line construction at Wild Pine Drive to inlet side of pipe at entrance to
Baywood Terrace = 900'. Per Section A -A, shoulder of roadway is 6' wide at 8% slope, then 18" to
silt fence location at 4:1 slope. Shoulder section area = (7.5)(900) = 6,750 square feet. Sanderosa
Road is 2V of asphalt. Area of one-half of asphalt roadway = (21/2)(900) = 9,450 square feet.
Total drainage area = 6750 + 9450 = 16,200 sf = 0.37 acres.
By judgment use C = 0.60 for bare soil shoulders during water line construction. Weighted C =
9450/16200(0.95) + 6750/16200(0.6) = 0.80
Qio = CIA
Qto = (0.80)(7.96)(0.37) = 2.4 cfs
Shoulder ranges in elevation from approximate elevation 142 at Wild Pine Drive to approximate
elevation 140 at the Baywood Terrace entrance. Longitudinal slope = 2/900 = 0.22%
Using Manning's Equation by trial and error, solve for area A
Use 0.03 for Manning's n
If A = 5.5 sf, then P — 11.64'
R=A/P=0.47
Qio= 1.49/0.03(5.5)(0.472/3)(0.0022)1/2
= 7.5 cfs
If A = 2.0 sf, then P = 5.12'
R=A/P=0.39
Qio= 1.49/0.03(2.0)(0.392/3)(0.0022)1/
= 2.5 efs
This represents an area from the edge of the 8% shoulder slope to the silt fence. Horizontal
distance = 4' of water flowing in the "ditch" created by the silt fence.
V=Q/A
= 2.5/2.0 = 1.24 fps therefore non -erosive velocity