16.10.140 Manning equation.
The hydraulic capacity of storm sewers shall be determined using Manning’s Equations:
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V=(1.486 R (2/3) S (1/2)) / (n) |
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V = |
mean velocity of flow in feet per second |
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R = |
the hydraulic radius in feet |
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S = |
the slope of the energy grade line in feet per foot |
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n = |
roughness coefficient |
The hydraulic radius, R, is defined as the cross sectional area of flow divided by the wetted flow surface or wetted perimeter. Typical “n” values and maximum permissible velocities for storm sewer materials are listed in Table 3. Roughness coefficient (n) values for other sewer materials can be found in standard hydraulics texts and references.
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Material |
Manning’s n |
Desirable Maximum Velocities |
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Closed Conduits |
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Concrete |
0.013 |
15 f.p.s. |
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Vitrified Clay |
0.013 |
15 f.p.s. |
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Brick |
0.015 |
15 f.p.s. |
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Cast iron |
0.013 |
15 f.p.s. |
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Circular Corrugated Metal Pipe, Annular Corrugations – 2 2/3 x 1/2 in. |
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Unpaved |
0.024 |
7 f.p.s. |
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25% paved |
0.021 |
7 f.p.s |
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50% paved |
0.018 |
7 f.p.s |
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100% paved |
0.013 |
7 f.p.s |
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Circular corrugated metal pipe, helical – 2 2/3 x 1/2 in., unpaved corrugations |
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12" |
0.011 |
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18" |
0.013 |
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24" |
0.015 |
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36" |
0.018 |
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48" |
0.020 |
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60" or larger |
0.021 |
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Corrugated polyethylene, smooth interior pipe |
0.012 |
15 f.p.s. |
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Concrete culverts |
0.013 |
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Open channels |
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Concrete, trowel finish |
0.013 |
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Concrete, broom or float finish |
0.015 |
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Gunite |
0.018 |
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Riprap placed |
0.030 |
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Riprap dumped |
0.035 |
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Gabion |
0.028 |
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New earth (uniform, sodded, clay) |
0.025 |
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Existing earth (faintly uniform, with some weeds) |
0.030 |
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Dense growth of weeds |
0.040 |
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Dense weeds and brush |
0.040 |
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Swale with grass |
0.035 |
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[Ord. 12-1-97C § 11(A).]