20221688 Ver 2_North Bend BAA_Buffer Stormwater Calculations_20240808

Upland flow has been diverted away from the buffer. The only new conveyances through the buffer are riprap lined
ditches along both sides of the launch ramp. Highlighted sections below show the peak flow calculations for a 0.75in./hr.
storm for each ditch. Neither exceed 0.5cfs and were allowable with authorization per 15A NCAC 02B .0614 Sec. 8.b.ii.
Calculate swale with straw net as temporary liner
Q2 = 0.49 (5.34 in/hr) (0.0975 acre) = 0.26 cfs
Total Q2 = 1.71 + 0.26 cfs = 1.97 cfs 4 Qn = 0.128
Channel Design (Non -Erodible)
Channel Type: Trapezoidal, Equal Side Slopes
Dimensions: Left Side Slope 3.00:1
Right Side Slope 3.00:1
Base Dimension: 2.00
Wetted Perimeter: 4.04
Area of Wetted Cross Section: 0.96
Channel Slope: 5.5000
Manning's n of Channel: 0.0650
Discharge: 1.97 cfs
Depth of Flow: 0.32 feet
Velocity: 2.06 fps
Channel Lining: Straw Net
Freeboard: 0.50 feet
T = 62.4 pcf (0.32 ft) (0.055 ft/ft) = 1.09 psf < 1.45 psf OK
Area 3 - Drainage to natural swale east of parking area:
Area = 0.3129 Acre (16% impervious)
Weighted C - 0.0513 acre(0.9)+0.2616 acre(0.3) - 0.40
0.0513+0.2616 acre
Qio = 0.40 (7.12 in/hr) (0.3129 acre) = 0.89 cfs
QcB = 0.40 (0.75 in/hr) (0.3129 acre) = 0.09 cfs < 0.5 cfs OK
Note: QcB is calculated pursuant to 15A NCAC 02B .0614 (8)(b)(ii)
Area 6 - Rip Rap Channel 1 left side of ramp:
Area = 0.1239 Acre (35% impervious)
Weighted C - 0.0431 acre(0.9)+0.0808 acre(0.3) - 0.51
0.0431+0.0808 acre
Qio = 0.51 (7.12 in/hr) (0.1239 acre) = 0.45 cfs
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Rip Rap Channel 1 Design
Channel Type: Trapezoidal, Equal Side Slopes
Dimensions: Left Side Slope 2.00:1
Right Side Slope 2.00:1
Base Dimension: 1.00
Wetted Perimeter: 1.93
Area of Wetted Cross Section: 0.29
Channel Slope: 14.0000
Manning's n of Channel: 0.1040
Discharge: 0.45 cfs
Depth of Flow: 0.21 feet
Velocity: 1.53 fps
Channel Lining: Rip Rap
Freeboard: 0.50 feet
Qca = 0.51 (0.75 in/hr) (0.1239 acre) = 0.05 cfs < 0.5 cfs OK
Area 4A - Grassed Swale 2 west of parking area draining to Culvert 3:
Area = 0.1326 Acre (none impervious)
c = 0.30
Qio = 0.30 (7.12 in/hr) (0.1326 acre) = 0.28 cfs
Qz = 0.30 (5.34 in/hr) (0.1326 acre) = 0.212 cfs
Qca = 0.30 (0.75 in/hr) (0.1326 acre) = 0.030 cfs
Culvert 3 Design (15" CPP):
Design Parameters
Section
Shape:
Circular
Material:
HDPE
Diameter:
15.00
in
Manning's n:
0.0180
Number of Barrels:
1
Inlet
Inlet Type:
Square Projecting
Ke:
0.20
Inverts
Inlet Invert Elevation:
1056.000
ft
Outlet Invert Elevation:
1055.000
ft
Length:
30.000
ft
Slope:
3.33
%
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Culvert Calculation
Discharge:
Headwater Elevation:
Tailwater Elevation:
Downstream Velocity:
Downstream Flow Depth:
Critical Flow Depth:
Normal Flow Depth:
Flow Control Type:
0.28 cfs
1056.290 ft
1055.352 ft
0.99 ft/s
0.352 ft
0.205 ft
0.155 ft
Outlet Control, Gradually
Varied Flow
Area 48 — Grassed Swale 2 south of parking area draining to Rip Rap Channel 2:
Area = 0.2422 Acre (9% impervious)
Weighted c — 0.0223 acre(o.9)+0.2199 acre(o.3) _ 0.36
0.0223+0.2199 acre
Qio = 0.36 (7.12 in/hr) (0.2422 acre) = 0.62 cfs
Total Qio = 0.28 + 0.62 cfs = 0.90 cfs
Swale 3 Trapezoidal Channel Design
Design Peak Flow
= 0.90
cfs
Channel Slope =
1.58
percent
Max Top Width =
15.00
ft.
Min Bottom Width =
2.00
ft.
Side slope ratio =
3.00
: 1
Freeboard =
0.50
Soil classification =
non -cohesive
Soil d75 =
0.050
in.
Best Cond. Cover Factor =
0.750
Min retard. curve index =
4.44
Max retard. curve index =
5.60
NOTE: Grass design parameters estimated from retardance curves:
Best Condition Curve: C
Worst Condition Curve: D
Calculated Maximum Allowable Shear Stresses:
Soil (Tau -a) = 0.020 psf
Vegetative (Tau-va) = 4.200 psf
Top
Base
Flow
Channel
Side Slope Excavated
Width (ft)
Width (ft)
Depth (ft)
Depth (ft)
Ratio Area (sq ft)
-----------------------------------------------------------------------------
9.0
2.0
0.6
1.1
3.00 6.27
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Flow:
0.900
cfs
Depth:
0.601
ft
Area of Flow:
2.287
sq ft
Wetted Perimeter:
5.802
ft
Hydraulic Radius:
0.394
ft
Velocity:
0.394
ft/s
Critical Depth:
0.170
ft
Top Width:
5.607
ft
EGL:
0.604
ft
Calculate swale with straw net as temporary liner
Q2 = 0.36 (5.34 in/hr) (0.2422 acre) = 0.466 cfs
Total Q2 = 0.212 + 0.466 cfs = 0.68 cfs 4 Qn = 0.044
Channel Design (Non -Erodible)
Channel Type: Trapezoidal, Equal Side Slopes
Dimensions: Left Side Slope 3.00:1
Right Side Slope 3.00:1
Base Dimension: 2.00
Wetted Perimeter: 3.62
Area of Wetted Cross Section: 0.71
Channel Slope: 1.5800
Manning's n of Channel: 0.0650
Discharge: 0.68 cfs
Depth of Flow: 0.26 feet
Velocity: 0.97 fps
Channel Lining: Straw Net
T = 62.4 pcf (0.26 ft) (0.0158 ft/ft) = 0.26 psf < 1.45 psf OK
QcB = 0.36 (0.75 in/hr) (0.2422 acre) = 0.065 cfs
Area 5 - Rip Rap Channel 2 right side of ramp:
Area = 0.1782 Acre (66% impervious)
Weighted c - 0.1169 acre(o.9)+0.0613 acre(o.3) _ 0.69
0.1169+0.0613 acre
Qio = 0.69 (7.12 in/hr) (0.1782 acre) = 0.88 cfs
QTotal = 0.90 + 0.88 cfs = 1.78 cfs
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Rip Rap Channel 2 Design
Channel Type: Trapezoidal, Equal Side Slopes
Dimensions: Left Side Slope 2.00:1
Right Side Slope 2.00:1
Base Dimension: 1.00
Wetted Perimeter: 2.91
Area of Wetted Cross Section: 0.79
Channel Slope: 14.0000
Manning's n of Channel: 0.1040
Discharge: 1.78 cfs
Depth of Flow: 0.43 feet
Velocity: 2.25 fps
Channel Lining: Rip Rap
Freeboard: 0.50 feet
QcB = 0.69 (0.75 in/hr) (0.1782 acre) = 0.092 cfs
Total QcB = 0.030 + 0.065 + 0.092 cfs = 0.19 < 0.5 cfs OK
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