SW6200701_Report (SW)_20201114

PROPER'ry OF: HONCr NAM
UP') I I t. T SbkTDCLI,-i--I%*4,'V,. V i,406"I'T C:OiIJ'VT-V'
STORMWATER CALCULATIONS
11-13-20
PROJEC'I'DESCRIPTION
The property boundary encompasses 92,625 square feet or 2.13 acres with approximately 235 linear
feet of frontage along NC Highway 210 in Harnett County. The project consists ofa 5,860 square
foot single -story building containing 3 lease spaces intended for mercantile or business uses. The
property is bounded by NC Highway 210 to the west, commercial properties to the south and
commercial and residential properties to the north and east. Approximately 25% of the site is cleared
with reasonably well -maintained grassed areas. The balance of the site is generally dense brush with
small trees. The site drops in elevation severely from the NC 210 right of way and ranges in,
elevation zoom approximate elevation 190 at the northernmost corner on the NC 210 right-of-way to
approximate elevation 160 in the southeast corner. An existing 36" steel pipe carrying runoff from
properties on the west side ofNC 210 outlets on the property. There are no existing impervious
surfaces on the site.
STORMWATER MANAGEMENT PLAN
Drainage on the project site is divided into basin and non -basin areas depending on whether or not the
area drains into a proposed sand Filter basin ror treatment. See a drainage area summary map on
sheet SP7 of the construction plans for specific pervious and impervious areas designated as basin
and non -basin. Stormwater -from the basin areas will be treated for quantity and quality in the new
sand filter basin to be located at the southeast corner of the site. Runoff entering the site through the
existing 36" steel pipe will not be included in basin or non -basin areas since the pipe will be extended
through the site to an outlet point downstream of the developed portion of the site. Off -site drainage
from the east entering an existing 12" drainage line will be piped through the site to an outlet point
downstream of the developed portion of the site. Other off -site drainage entering the site from the
east will connect with non -basin flows and will not be treated in the basin — resulting in no ofTsite
drainage being considered in the stormwater management plan.
The site contains, approximately 53% Blaney loamy sand, 35% Gilead loamy sand and 12% State fine
sandy loam. The portion of the site proposed :for the Phase One development consist almost
exclusively of the Blaney and State soils which are listed as Type B soils on Table 3 (Chapter 3) in
the BMP manual, The Gilead soils are listed as Type C. On -site soil tests by Southeastern Soil &
Environmental Associates show the seasonal high water table is at a depth of 35 inches at bore hole
42 (which equals approximate elevation 164.3).
1611,191
Determine pre -development curve number
Use SCS method to determine pre -development flows from the site. The grassed areas of the site are
generally State soils and the remainder ofthe site is a mixture of Blaney and Gilead soils. By
judgment curve number CN for the project site for pre -development conditions is 61 for the grassed
areas. Blaney soils CN = 48 and Gilead soils CN = 65 per Table 4 in the BMP manual,

Gilead soil areas = 0.35(92625) = 32,419 sf
State and Blaney soil areas � 0.65(92625)­ 60,206 sf
Grassed areas with CN of 61 = 0,25(92625) = 23,156 sf which leaves 37,050 sf of Blaney soils with
CN of 48.
Weighted CN = 23156/92625(61) -+- 37050/92625(48) --1- 32419/92625(65) = 57.2
Total pre -development drainage area = 92,625 sf = 2.13 acres
See routing results for pre -development flows.
Determine post -development curve numbers
Non -Basin areas:
Total non -basin pervious = 47,537 sf = 0.62 acres
Non -basin areas undisturbed in natural condition = 16,977 sf = 0.39 acres
Non -basin grassed and landscaped areas = 47537-16977 = 10,092 sf = 0.23 acres
There are no non -basin impervious areas.
By judgment use CN 39 for post -development grassed and landscaped areas and CN = 65 for
natural condition areas (since they are the Gilead soils).
Weighted non -basin post -development CN = 10092/27069(39) A- 16977/27069(65) = 55.3
Basin -areas:
Total basin pervious areas = 20468 4- 486 -1-510 .4-5389 4. 6791 = 3,3,644 sf = 01.77 acres
'Total basin impervious areas = 22364 (asph/curb) + 473 (dumpster) + 3215 (s/w) + 5860 (roof)
= 31,912 sf = 0.73 acres
Total basin areas = 33644 + 31912 = 65,556 sf = 1.50 acres
'total impervious surfaces basin and non -basin = 31,912 sf = 0.73 acres
% of project site that is impervious = 31912/92625 = 34.5%
By judgment use CN = 39 for good condition landscaped areas post -development,
Weighted basin post -development CN = 31912/65556(98) + 33644/65556(39) = 67,7
See routing report for results of routing the design storms.
Summary of the results are:
Pre -development:
Qi = 0.52 efs
Qio = 5.00 cfs
Qjoo = 12.66 efs
Post development flows:
Qi = 0.01 (non -basin) + 0.00 (basin) = 0.01 efs
Qio = 1.27 (non -basin) + 0.33 (basin) = 1.60 efs
Q i oo = 4.52 (non -basin) A- 8.29 (basin) = 12,81 cfs,
Maximum storm elevations in the basin:
1-year 169,79
10-year 170.54
100-year 1.70.96
WATER QUALITY VOLUME
Using the Simple Method, Rv = 0.05 + 0.91A
Total basin areas = 1,50 acres
Total basin impervious areas — 0.73 acres

IA = 0.73/1.50 = 0.49
Rv = 0.05 + 0.9(0.49)
0.49
Water quality volume to be treated = basin drainage area x F x 0.75 x Rv = (65556(1/12)(0.75)(0,49)
= 2,008 cf
Total volume in sand filter at elevation 171 = 9,556 cf (see routing report)
Sand filter must accommodate 50% of this volume in the sand chamber = 2008(0.,5) = 1,004 cf
Volume of sand filter between elevation 168 and elevation 170 = (720-1-1606)/2(2) = 2,326 cf OK
Check to see if the basin will hold 2 times the water quality volume in order to eliminate the need for
treatment of flows bypassing the pond.
Determine the volume required to capture the first 1 -inch of rainfall from the 10-year storm to be
drawn down over 5 days (then double that).
Using the Simple Method, Rv = 0.05 + 0.9 In.
Total basin areas = 1.50 acres
Total basin impervious areas = 0.73 acres
IA = 0.73/1.50 = 0.49
Rv 0.05 + 0.9(0.49)
0.49
Volume of runoff that must be controlled is:
V = 3 63 O(RD)(Rv)A where RD = I inch
� 3630(t)(0,49)(1.50)
— 2,668 cubic feet
(2668)(2) = 5,336 cubic feet
Storage available in basin elevation at 171.0 is 9,556 cubic feet therefore no treatment required for
any flows bypassing the basin.
Underdrain...Sizing
Determine underdrains needed to maintain drawdown rate of 2 inches per hour. Surface area at
elevation 170.54 is 2,269 sf.
Solve for Q
Q = 2.0(1/12)(1/3600)(2269) = 0.11 efs
With factor of safety of 10, underdrain design flow Q = 1.1 cfs
D = 16(QnJSo.5)3/8
= 16((Ll)(0,011)/(0.005)0.5) 3/8
= 8.25 inches of underdrain
Use 2 lines of 6" underdrain
PART B — EROSION CONTROL CALCULATIONS
See the skimmer basin areas summary on sheet SP2 of the construction plans.
TEMPORARY BASIN SIZING CALCULATIONS
Total contributory area draining to the proposed skimmer basin location is 51,492 sf = 1.18 acres
Storage required — 1.18(3600 cf/ac)
= 4,248 cubic feet
Construct basin approximately 30' wide by 80' long by 2' deep with 2:1 side slopes
Basin top = 30' wide by 80' long = 2,400 sf

Basin bottom = 26' wide: by 76' long = 1,976 sf
Volume in basin = (2400+1976)/2(2) = 4,376 cubic feet > 4,248 OK
Check surface area required --- 435 sr X Q i o where Qio = CIA
Determine precipitation intensity for I 0-year storm using precipitation frequency data server at
hitly l/hct -pws...qLv/11c�/Rrds/orJbL ng,_pfd Jitinf.
—sq— qqW_ - — -_�_ -
Longitude and latitude of approximate center of proposed infiltration basin is 35-13-26N,
78-56-15W.
I 0-year (5 minute time of concentration, 24-hour duration) = 7.88 inches per hour
By judgment use C = 0.60 for conditions during construction.
Q i o 0.60(7.8 8)(1.18)
5.6 efi s
Surface area = 43 5 (Q i o)
= 435(5.6)
= 2,436 sf
Surface area of basin is 30(80) = 2,400 sf — therefore size is adequate.
PART I) — RISER ANTI -FLOTATION CALCULATIONS
Riser R- I
Riser Structure is a 4'x4' box with 9" thick concrete base and 4" thick concrete top. See detail below.
Wnr, NJ. MUR PARTSTO M-
PV6 UMMV,1RAIN
5H I:Wx
LIM M roarl.--l'
br
12, RFV6P
MXV I"Do
TOP -OF-140
PLAN
4' COM TCI'
ALMM4 :94-x2ol-
5PILLKAY Mjwagy
PMHIMre NMf7 LDCK
Mf 4. INS 55*
SM
ELEV 166DO Sol
TOP SH
1610E &A% %%CX
MALLS
f5r-CITION
RISIER RI
2M P-W 14-4p, W "(11AIX
Weight of concrete = 150 pcf
Weight of water = 62.4 pcf
Weight of grout = 130 pcf
Rim of grate to invert = 170.5 -- 165.73 — 4.77'
Box wall height = 4.77 — (4/12) = 4.44'
Grout 3/12(4)1(4)(130) = 520 lbs
Concrete top = 5.33(5.33)(4/12)(150) -- 2(2)(4/12)(I 50) = 1,219 lbs
Grate (estimate) 225 lbs
Base = 4.83(4.83)(9/12)(150) = 2,625 lbs
Walls = (8/12)(4)(4.44)(2)(150) + (8/12)(2.67)(4,44)(2)(150) = 5,923 lbs

Total weight of riser = 520 + 1219 + 225 + 2625 + 5923 = 10,512 lbs
Estimate weight of soil columns over base projection at 125 lbs/cf
Soil weight = 5/12(4)(2.25)(4)(125) = 1,875 lbs
Total weight of riser plus soil = 10512 + 1875 = 12,387 lbs
Uplift = outside volume of box and base x weight of water
Uplift = 4.0(4.0)(5.88)(62A)+ (9/12)(4.83)(4.83)(62.4) = 6,962 lbs
Required safety factor = 1.5
Safety factor provided =12387/6962 = 1.78 OK
Size the pipe leaving DI-E I
Area draining to Cl- t is 8,153 sf of asphalt plus 585 sf of covered sidewalk plus 246 sf of pervious
surfaces.
Total area = 8,984 sf = 0.21 acres
Qio = CIA where C = 0.95 for asphalt, concrete and rooftop, C= 0.25 for pervious areas
Qio = (0.95)(7.88)(8738/43560) + 0.25(7,88)(246/43560)
= 1.5 cfs
Check capacity of 15" RCP at 0.50% slope using Manning's Equation,
Q= 1. 49/11AR2/3 S 1/2 where R = 1, 2 3 /3.93 = 0.31
� 1.49/0.013(1.23)(.31)2/3(0,005)1/2
= 4.6 cfs therefore 15" pipe is adequate
Size the pipe leaving CI-1
Area draining to CI-2 is 5,860 sf of roof = 0.13 acres
Q to = CIA where C = 0.95 for roof areas
Qio= 0.95(7.88)(5860/43560) + 1.5
Qio = 2.5 cfs therefore 15" pipe is adequate
Size the pipe leaving Tel I
mm
Area draining to Dl- I is 6,086 of asphalt area plus 1,085 sf of covered and uncovered sidewalk plus
281 sf of pervious surfaces.
Total area = 7,452 sf = 0.17 acres
Qio = CIA where C = 0.95 for concrete and 0.25 for pervious areas
Qio= 0.95(7.88)(6086-1--1085/43560) -1..(0.25)(7.88)(281/43560) + 2,5
Qio = 3.7 cfs therefore 15" pipe is adequate since pipe slope is 0,62%
Size the pipe leavin2 CI-2
Area draining to CI-3 is 8,567 of asphalt plus 619 sf of covered sidewalk.
Total area = 9,186 sf = 0.21 acres
Q i o = CIA where C = 0.95 for concrete and asphalt
Qio= 0,95(7.88)(9186/43560�) 4.3,7
Qio = 5.3 cfs
Check capacity of 18" RCP at 0.50% slope using Manning's Equation.
Q = 1.49/iiAR2/3S 1/2 where R = 1.77/4.71 = 0.3 8

= 1.49/0.013(1.77)(,38)2/3(0.005)�t/2
= 7.5 efs therefore 18" pipe is adequate
Check exit velocity V = Q/A
V = 5.3/1.77
= 3.0 fps non erosive velocity
Use 4' x 4' pad of 6" stone rip rap at outlet
Analysis of swale at end of 36" RCP
Without accurate watershed data for flows entering the 36" steel pipe at NC 2 10, assume 36" RCP is
flowing'/2 ftill during the 10-year storm. Slope on pipe is 2.77%.
Using Manning's Equation, Q=1.49/nAR2/3SI/2
Area of 36" RCP = 3.14(1.5)(1.5) = 7.07 sf
Area flowing '/2full = 3.54 sf
Wetted perimeter = 2(3.14)(1.5) = 9.42
Wetted perimeter flowing V2 -full = 4.71 If
R — A/P = 3.54/4.71 = 0.75
Q = (1.49/0.013)(3.54)(0.75)2/3(0.0277)j/2
Q = 55.8 efs
Use Manning's Equation to determine spread of 55.8 efs, in the downstream swale. Swale dimensions
are a 4' bottom with 3:1 side slopes and approximate slope of 4.5%.
Use n = 0.05 for grass -lined channel
By trial and error:
At V depth in channel, A = 7.0, P = 10.32, R = 7/10.32 = 0.68
Q = 1.49/0.05(7)(0.68)2/3(0.045)1/2
= 19.8 efs
At 15 depth in channel, A = 12.75, P = 13.49, R = 12.75/13.49 = 0.95
Q = 1.49/0.05(12.75)(0.95)2/3(0.045)1/2
= 77.9 cls
At 1.3' depth in channel, A = 10.27, P = 11.59, R = 10.27/11.59 — 0,89
Q = 1.49/0.05(10.27)(0.89)2/3(0.045)1/2
= 60.1 cfs
Width of spread in channel = 11.2'
Use 12' wide matting