SW6191001_SW Report_20200728

DRAINAGE DESIGN NARRATIVE & CALCULATIONS
Yard Storage Inc Request
July 2020
Engineer Firm
Castle Keep
22 Long Ridge Ct
Willow Springs, NC 27592
919-796-8561
Project Description
The purpose of the project is to construct gravel parking area for recreation vehicles. Approximately
1.7 acres will be disturbed during this construction period. The site is 2.2 acres located in Buckhorn
Township, Harnett.
Site Description
The site has rolling topography with slopes generally 2 to 7%. Slopes steepen to 10% in the northeast
portion of the property where the proposed stormwater basin will be located. The site at present is
predominantly stripped off of vegetation, rest of the areas are covered with short grass and woody
vegetation at north boundary. There is no evidence of significant erosion under present site
conditions.
Adjacent Property
Land use in the vicinity is residential. The land immediately to the north and east has been
developed for residential use. Area to the north is predominantly undeveloped. The existing ditch
along the gravel drive will receive the overflow from the sand filter. Sediment control measure will
be taken to protect the existing ditch and the adjacent residential properties.
Soils
The subsurface is sandy loam, 10 inches thick. The subsoil consists of clay, 45 inches thick. Below 55
inches is a layer of loam. Soil morphological characteristics indicating a seasonal high-water table
(SWHT) were not observed within 120 inches below surface. Infiltration rates (permeability) range
from 0.2 in/hr in the clay layer to as high as 2 in/hr in the loamy sand.
Proposed SCM
Open bottom sand filter system is proposed to capture stormwater runoff from all disturbed areas.
Volume in excess of the storage capacity will outlet via drain pipe and be dissipated by riprap pad.

Stormwater Calculations
Total Area = 96,333 sq.ft = 2.21 acre
Impervious Area = 72,000 sq.ft = 1.65 acre
Impervious fraction, IA = 1.65 / 2.21= 0.747
Runoff Volume Calculation:
Rv=0.05*0.91A
Where: Rv = Runoff coefficient (unitless)
IA = Impervious fraction (unitless)
Rv = 0.05 + 0.9(0.747)
Rv = 0.722
Dv = 3630 * RD * Rv* A
Where: Dv = Design volume (cu.ft)
Ra = Design storm depth (in)
A = Drainage area (ac)
Rp = tin
Dv = 3630 * 1 * 0.722 * 2.21
Dv = 5,792.10 cu.ft
Forebay and Sand Filter Calculations
Treatment Volume
Tv = 7S% of Design Volume, Dv
= 0.75 x 5,792.10 cu.ft.
Tv = 4,344.08 cu.ft
Sand filter Sizing
Sv = 50% of Treatment Volume, Tv
= 0.50 x 4,344.08
Sv = 2,172.04 cu.ft.
Use pond depth over sand = 4 ft. < 6 ft. max.
SA = 2,172.04/4
SA = 543.01 sci t.
Area provided = 544 sq.ft.

Forebay Sizing
Fv = 50% of Treatment Volume, Tv
= 0.50 x 4,344.08
Fv = 2,172.04 cu.ft.
Use forebay depth = 4 ft.
FA = 2,172.04/4
Fa = 543.01 sq.ft.
Area provided = 550 sq.ft.
Outlet Pipe Design
Description of Surface
Runoff Coefficient, C
Area, ac
C x A
Gravel
0.85
1.65
1.40
Basin
1.0
0.22
0.22
Total CxA
1.62
Composite C
0.87
Time of Concentration, Tc
Maximum slope length = 287 ft overland flow
Average slope = 2%
Location: Holly Springs, North Carolina
Surface Manning's, n = 0.011(from Table 8.03a)
Using Kinematic Wave Equation
Tc (overland) = 1.60 minutes (for 100 ft)
V = 20.3282 x 51/2
V = 2.87 minutes
V _ 1.49 Rz/3S1/2
n
R = 0.44
V = 17.05 fps
Channel Length = 466 ft
Tc (channel) = L / (60 x V)
Tc (channel) = 0.46 minutes
Tc = Tc (overland) + Tc (shallow concentrated) +Tc (channel)
Tc = 1.45 + 2.87 + 0.46
Tc = 4.78 minutes
Use Tc = 5 minutes
Rainfall Intensity, I
I = 7.55 in/hr (from Table 8.03C) using a 10-year storm, 5-minute duration.
-.G .4 ,0

Peak rate of Runoff, Q10
Q10 = CIA
Q10 = 0.87 x 7.55 in/hr x 1.62 acre
Q10=10.64cfs
Length = 76.5 ft
Slope, So = 0.01 ft/ft
Allowable Headwater Depth, AHW = 6.62 ft
Culvert type: PVC headwall
Ke = 0.2
n = 0.012
Tailwater (TW) = 1,25 ft
Z�4 '4

OUTLET CONTROL, CIRCULAR CONCRETE PIPE
- 2000
Boo
20
Ito
too
K
.0s
72
-ss
-60
- 34
+s p='n
42
2t
SP� Ls
—.F.•. *. .r.. ". a.w ra....
HEAD FOR
CONCRETE PIPE CULVERTS
FLOWING FULL
n-0.022

I\LLT CONTROL, CIRCULAR R (- DVE PIPE
r' fa
L
r
r _ _
HEADWATER DEPTH FOR
Q DrTrRi. GATED HOPE GUhVERTa
WITH MLET CONTROL
��A

C RITC AL DEI' F11. CIRCULAR WIPE
CHART 4
s
S �+' 14 ~ E
�t :Mlgt 4.�:kk7 3a �� •.�k
i
�- 70 y�j •p SO to 70 so to '00
QrSC�R6E-0-tr5
I I E i •�
s�
i st cfir"n Top Or P yr r
30 too too ioo ` Z3 5-00 coo too •oo too too*
v �
�4
i I
� 1
• � � 1 I
* .__. f � 4 _!• - aj Ci�w: fit CL[DTOP OfFt/i
F L--lflj
3000 s000
DjSC"+RLE - Q -CFS
GRM CAL DE PTH
CIRCULAR PIPE

-
(D
3Lo
Z
LLJ
qq0.
k
%
w
AA,*O,3A
CO
z \
„
Ln
jLj
s
w Q
k
0 0
»
■
ww
Z
it
0| � �
2
s
d
LLA% Lai4
\ 22
6|-
-
_
E
$
C4
Is
,
q
7
■
©
+cv
m
a
■
w
2
I.
z
4 2 z
e
0
u
LO
00
di q gLLI
o
2
R
z
9
_
o
®
W w
_Or
M cr
6
� w w ® k
2 � « w
S
«
3
2 q E 2
U
� k
©
N
L
■
0
k k
w
x
'
k
d
§
0
cr
%E
k
�
o ±
g.
R
,�z>

( t"FLETCON"FROL, CIRCULAR CONCRETE f 1PE
RECOMMENDED tf. NNENG'S n-VALUES
T%pecft'atwlok
W.111 rkwriptian
IT
C'xxxctc Pipe
Smtxuh oatll+
Ei i1IEt - 0.01
COM--rctc Boxes
SnuxiGh u alL,
0.012 - 0-01
C'�-tttt;xtcd WW
' , s t"� ' is'lch
o- (12r' - 0.027
Pip, and Boxc>. _'granular or
eorrugatin"nN
llelwA Pipc
h b% I inch
0.022 - 0.025
corrui!atiom
5 b� I inch
0.025 - 0.026
corruzations
? br I ittch
0.027 - 0.028
conueatinns
6 hv _' inch
0.033 - 0.035
Smoutai )^Banc
q br _' `= inch
0.0113 -0.0i7
titructuraf plate
Corrugated Metal
2 _' _, by 12 inch corrueatiom
0.012 - 0.024
Pipe
T
Spiral Rib Wtal
Saxxith %:ells
0-012-0.01 z
(StM or Alum.l
PVC
Snxwth itsicnot
G -010 - 01.012
i'ol velhviene I PE or HDPE i
Srnmuh inwnku
0 011 - 0 it l
C'ormg- alcd PE or 11DPE
C orrugatcd antcrior
0.0i2 - i1.026
Ncitc J _Th%- %rduw, irwlicatcd in this table arc recx*mmendecl 'arming-s -n" deign values. Actual
field ►alues amy card depew ing on the effects of abrasion, corrosion. deflection. and joint
conditions. Cuncn-tc pipe with pcx.+r joints and dctenoraled halls may ha%a "n" +allucs of 0.014
to O.01:t_ Comtgatcd metal with join and %aill problems may also hat c .hugger "n' ►aluc-S. and in
at&&iiom uny ciPivicatY shape claauzves v►hirh cc uJd ad--crklti .al%-ct llsc -eneral hydrJulic
rlsatac-tcristics ofth- culat-rt. `
Note _':Fore further infumaation con:emin-, %twining n %aloe, Cor,cJccfcd c.mduits ,:k1nNult
Hydraulic Design of Hizhua% Culccrts. Federal Highwar Aduministralion. HDS No. 5. page 163_
Riprap Outlet Protection for Outlet Pipe
Q10 = it cfs
Pipe Diameter = 15 inches
Tailwater = 1.25 ft
Length of apron = 23 ft (Figure 8.06b)
D50 = 0.25 ft or 3 inches
Depth of rprap (laid flush with ground surface) = 1.5 x dmax = 4.5inches; Use 1 ft
Upstream apron width = 3 x pipe diameter = 3.75 ft
Downstream apron width = apron length + pipe diameter =24.25 ft.
-4e--'0

E�D
o,aes rf14r«--- La
f
f
3 1
a
13
r 10
v 15
aschalgo tttAdsftj
GufveS rraY rlct Z* callapotatcc:
Figure 0.06b .Cv5gn of o e.:f p'o?;Cf of bcm a to rmf p o %jwr 2 S.i+ ni�a' r :r�vakw COn{ Ofl S>.2 Sda na9Elj

Sand Media Specification
The media in the sand filter shall be cleaned, washed, course masonry sand such as ASTM
C33 or the equivalent. The sand particles shall be less than 2 mm average diameter. The
entire surface area of the sand chamber must contain sand.
Maintenance of Media
The sand filter shall be maintained in a manner that results in a drawdown of at least two
inches per hour at the sand surface.
Clean -out Pipes
At least one clean -out pipe shall be provided at the low point of each underdrain line. Clean
out pipes shall be capped.
Access to Underground Sand Filters
It is recommended to provide access to underground sand filters that applies with OSHA
regulations.
Maintenance
Sand filters shall be inspected at least once a quarter and shall be maintained as needed to
remove surface sediment accumulation, trash, debris, and leaf litter to prevent the filter from
clogging. Sediment should be cleaned out of the forebay/sedimentation chamber when it
accumulates to a depth of more than 6 inches. During inspection, structures such as outlets
and flow diversions should be checked at least annually for damage or degradation.
Maintenance Plan
1. Manage the drainage area to reduce the sediment load to the sand filter.
2. Clean out the sedimentation chamber or forebay whenever sediment depth exceeds six
inches.
3. At least once a year, skim the sand media.
4. Replace the sand filter media whenever it fails to function properly after maintenance.
5. Inspect the sand filter at least quarterly. Any problems that are found shall be repaired
immediately. �t;era+►�.■r„r�
o �� C ARoO4..
=o. �o�ESSlpy••.9 �'..
r' Q SEAL 9�
9519