SW3210803_Stormwater Report_20211012

Cover1
Table of Contents2
Introduction3
Drainage Areas and Curve Numbers4
Pre-Development vs. Post-Development Flows Summary6
Time of Concentration9
Impervious Calculations12
Storm Drainage Chart (25-Year Storm)13
HW/D Calculations (100-Year Storm)14
Drop Inlet Calculations (100-Year Storm)18
Anti-Flotation (Outlet Structures)19
Rip-Rap Aprons20
Diversion Ditches21
Skimmer Sediment Basin23
On-Site Hydrology Output24
Routing Diagram
1-year, 24-hour Storm Event
2-year, 24-hour Storm Event
10-year, 24-hour Storm Event
25-year, 24-hour Storm Event
50-year, 24-hour Storm Event
100-year, 24-hour Storm Event
Drainage Area Maps
121
2

PMN
PMN
KJC
The proposed project is a 10 lot single family residential subdivision with that will be constructed on approximately 12 (+/-) acres of a 16
(+/-) acre parcel in Weddington, NC. The back side of the property contains a future NCDOT R/W, and therefore will not be included in
the development. The project site is located in the Catawba River Basin. The site will be accessed from Old Mill Road (SR-1392) with
a single entrance. The proposed area of disturbance consists of (+/-) 3 acres of grass field and wooded area. While this will ultimately
be a subdivision, only the roadway, permanent stormwater measures, and water line are being developed at this time. To the greatest
extent possible, disturbance has been limited to the roadway right of way and permanent pond. There is an existing pond on site that
will be retrofitted with a riser and outlet pipe to provide detention per the Town of Weddington regulations. Disturbance to the pond will
be limited to the installation of the outlet structure, emergency spillway, and slope inlet protection. There are jurisdictional wetlands
located on site that will be disturbed per the approved 401/404 permit. Disturbance to the jurisdictional features will be limited to the
greatest extent possible. The 401/404 permit is in progress.
The site will be developed to meet low density design criteria as required by NCDEQ Post-Construction Stormwater. Water detention
will be provided by installing a riser and outlet structure inside an existing pond on site. The outlet structure will be designed using the
pond's existing geometry to detain up to the 100-yr storm per Weddington regulations. Disturbance to the pond and surrounding
wetlands will be minimized as much as possible and limited to only what is needed to install the outlet structure, emergency spillway,
and slope inlet protection. Stormwater will be routed to the pond through vegetative swales. Part of the site will bypass the pond,
however the pond will over detain to account for this volume.
During construction, runoff will be minimized with the installation of a clean water ditch, which will result in site discharge points of less
than 1 acre. For this reason, all erosion control will be accomplished with silt fence and silt fence outlet devices. All pipe discharge
points will have rip-rap aprons.

PMN
PMN
KJC
Woods (Good condition)107640B55
Pasture/grassland (Good condition)151100B61
Existing Roadway6100B98
Pasture/grassland (Good condition)179470B61
Woods (Good condition)B55
Pasture/grassland (Good condition)B61

PMN
PMN
KJC
BUA in Private R/W16520B98
BUA on Lots56160B98
Woods (Good Condition)6530B55
Grassed Open Space (Good condition)263170B61
BUA in Private R/W6950B98
BUA on Lots28290B98
Grassed Open Space (Good condition)150330B61

6

100
0.24
0.04
u
672
0.073
100
0.24
0.05
u
380
0.045
386
3.28
9

100
0.24
0.03
u
498
0.064
100
0.24
0.04
u
170
0.14
250
2
10

100
0.24
0.05
u
380
0.045
386
3.54
11

16520
56160
6950
28290

CULVERT 1706.58705.737818CLASS V RCP 0.0134.330.358.3413.007.119.38
CULVERT 2694.80694.506018CLASS III RCP 0.0131.980.358.347.994.649.38
CULVERT 3694.00693.002915CLASS III RCP 0.0132.210.358.3413.0010.019.38
CULVERT 4687.43687.251818CLASS III RCP 0.0132.440.358.3411.306.399.38
13

14

15

16

17

07780-0013PMN
05/11/21PMN
08/04/21KJC
9.38
0.35
5
STANDARD DROP INLET w/ HORIZONTAL GRATE
Weir Eq.: Depth = \[Surface Q/(3.3*P)\]^(2/3)Orifice Eq.: Depth = \[Surface Q/(0.6*A*64.4^(1/2))\]^2
P of Std Drop Inlet Grate = 6.94 ft.Open Area of Std Drop Inlet Grate = 4.51 sq.ft.
SurfaceDepthDepth
DrainageFlow toInlet PipeOutlet Pipew/ Weirw/ OrificeRequired
Drop InletArea (acres)Inlet (cfs)Size (in)Size (in)Equation (ft)Equation (ft)Depth (ft)
1.98
2.44
18

1.25
9
6
4
19

CWD #21.0004.974.60.283
TDD 4 & 8 (EC STAGE 3)1.0001.952.80.283
CULVERT #11.50012.647.10.3104.5
CULVERT #21.5005.784.60.394.5
CULVERT #31.2506.4510.00.283.75
CULVERT #41.5007.126.40.394.5
20
POND OUTLET2.00013.815.80.4136

Trapezoidal Channel Computation Sheet
21

TrapezoidalChannelComputationSheet
22

0.932
3690
0.9
0.5
931
1014
1146
1285
1430
1582
1740
1904
2163
0.62.8
7.55
23

24

Type II 24-hr 1-Year Rainfall=2.93"
25

Type II 24-hr 1-Year Rainfall=2.93"
26

Type II 24-hr 1-Year Rainfall=2.93"
27

Type II 24-hr 1-Year Rainfall=2.93"
28

Type II 24-hr 1-Year Rainfall=2.93"
29

Type II 24-hr 1-Year Rainfall=2.93"
30

Type II 24-hr 1-Year Rainfall=2.93"
31

Type II 24-hr 1-Year Rainfall=2.93"
32

Type II 24-hr 1-Year Rainfall=2.93"
33

Type II 24-hr 1-Year Rainfall=2.93"
34

Type II 24-hr 1-Year Rainfall=2.93"
35

Type II 24-hr 1-Year Rainfall=2.93"
36

Type II 24-hr 1-Year Rainfall=2.93"
37

Type II 24-hr 1-Year Rainfall=2.93"
38

Type II 24-hr 1-Year Rainfall=2.93"
39

Type II 24-hr 1-Year Rainfall=2.93"
40

Type II 24-hr 2-Year Rainfall=3.53"
41

Type II 24-hr 2-Year Rainfall=3.53"
42

Type II 24-hr 2-Year Rainfall=3.53"
43

Type II 24-hr 2-Year Rainfall=3.53"
44

Type II 24-hr 2-Year Rainfall=3.53"
45

Type II 24-hr 2-Year Rainfall=3.53"
46

Type II 24-hr 2-Year Rainfall=3.53"
47

Type II 24-hr 2-Year Rainfall=3.53"
48

Type II 24-hr 2-Year Rainfall=3.53"
49

Type II 24-hr 2-Year Rainfall=3.53"
50

Type II 24-hr 2-Year Rainfall=3.53"
51

Type II 24-hr 2-Year Rainfall=3.53"
52

Type II 24-hr 2-Year Rainfall=3.53"
53

Type II 24-hr 2-Year Rainfall=3.53"
54

Type II 24-hr 2-Year Rainfall=3.53"
55

Type II 24-hr 2-Year Rainfall=3.53"
56

Type II 24-hr 10-Year Rainfall=5.16"
57

Type II 24-hr 10-Year Rainfall=5.16"
58

Type II 24-hr 10-Year Rainfall=5.16"
59

Type II 24-hr 10-Year Rainfall=5.16"
60

Type II 24-hr 10-Year Rainfall=5.16"
61

Type II 24-hr 10-Year Rainfall=5.16"
62

Type II 24-hr 10-Year Rainfall=5.16"
63

Type II 24-hr 10-Year Rainfall=5.16"
64

Type II 24-hr 10-Year Rainfall=5.16"
65

Type II 24-hr 10-Year Rainfall=5.16"
66

Type II 24-hr 10-Year Rainfall=5.16"
67

Type II 24-hr 10-Year Rainfall=5.16"
68

Type II 24-hr 10-Year Rainfall=5.16"
69

Type II 24-hr 10-Year Rainfall=5.16"
70

Type II 24-hr 10-Year Rainfall=5.16"
71

Type II 24-hr 10-Year Rainfall=5.16"
72

Type II 24-hr 25-Year Rainfall=6.15"
73

Type II 24-hr 25-Year Rainfall=6.15"
74

Type II 24-hr 25-Year Rainfall=6.15"
75

Type II 24-hr 25-Year Rainfall=6.15"
76

Type II 24-hr 25-Year Rainfall=6.15"
77

Type II 24-hr 25-Year Rainfall=6.15"
78

Type II 24-hr 25-Year Rainfall=6.15"
79

Type II 24-hr 25-Year Rainfall=6.15"
80

Type II 24-hr 25-Year Rainfall=6.15"
81

Type II 24-hr 25-Year Rainfall=6.15"
82

Type II 24-hr 25-Year Rainfall=6.15"
83

Type II 24-hr 25-Year Rainfall=6.15"
84

Type II 24-hr 25-Year Rainfall=6.15"
85

Type II 24-hr 25-Year Rainfall=6.15"
86

Type II 24-hr 25-Year Rainfall=6.15"
87

Type II 24-hr 25-Year Rainfall=6.15"
88

Type II 24-hr 50-Year Rainfall=6.94"
89

Type II 24-hr 50-Year Rainfall=6.94"
90

Type II 24-hr 50-Year Rainfall=6.94"
91

Type II 24-hr 50-Year Rainfall=6.94"
92

Type II 24-hr 50-Year Rainfall=6.94"
93

Type II 24-hr 50-Year Rainfall=6.94"
94

Type II 24-hr 50-Year Rainfall=6.94"
95

Type II 24-hr 50-Year Rainfall=6.94"
96

Type II 24-hr 50-Year Rainfall=6.94"
97

Type II 24-hr 50-Year Rainfall=6.94"
98

Type II 24-hr 50-Year Rainfall=6.94"
99

Type II 24-hr 50-Year Rainfall=6.94"
100

Type II 24-hr 50-Year Rainfall=6.94"
101

Type II 24-hr 50-Year Rainfall=6.94"
102

Type II 24-hr 50-Year Rainfall=6.94"
103

Type II 24-hr 50-Year Rainfall=6.94"
104

Type II 24-hr 100-Year Rainfall=7.77"
105

Type II 24-hr 100-Year Rainfall=7.77"
106

Type II 24-hr 100-Year Rainfall=7.77"
107

Type II 24-hr 100-Year Rainfall=7.77"
108

Type II 24-hr 100-Year Rainfall=7.77"
109

Type II 24-hr 100-Year Rainfall=7.77"
110

Type II 24-hr 100-Year Rainfall=7.77"
111

Type II 24-hr 100-Year Rainfall=7.77"
112

Type II 24-hr 100-Year Rainfall=7.77"
113

Type II 24-hr 100-Year Rainfall=7.77"
114

Type II 24-hr 100-Year Rainfall=7.77"
115

Type II 24-hr 100-Year Rainfall=7.77"
116

Type II 24-hr 100-Year Rainfall=7.77"
117

Type II 24-hr 100-Year Rainfall=7.77"
118

Type II 24-hr 100-Year Rainfall=7.77"
119

Type II 24-hr 100-Year Rainfall=7.77"
120

124

125