Hydraulic Design of Highway Culverts - DOT On-Line Publications

Manning’s n value for each material and the perimeter of the pipe composed of each material.
The method assumes a constant Manning’s n value for each material (no variation with size or
flow velocity). In addition, the perimeters should be adjusted for partly full flow. The method
ignores the dynamic interaction between the flow prisms affected by each roughness.
A better method is based on the assumption that the conveyance section can be broken into G
parts with associated wetted perimeters (p) and Manning’s n values.
Each part of the conveyance section is then assumed to have a mean velocity equal to the
mean velocity of the entire flow section. These assumptions lead to Equation (36).
⎡
⎢
n = ⎢
⎢
⎣
G
∑
i=
1
( p
i
p
n
1.
5
i
0.
67
)
⎤
⎥
⎥
⎥
⎦
n is the weighted Manning’s n value
G is the number of different roughness' in the perimeter
p1 is the wetted perimeter in ft. influenced by the material 1
p2 is the perimeter influenced by material 2, etc.
n1 is the Manning’s n value for material 1, n2 is for material 2, etc.
p is the total wetted perimeter, ft
Example: Compute the Manning’s n value for a 6 ft. diameter corrugated metal pipe with 5 by 1
in annular corrugations, and a smooth lining over 40 percent of the perimeter.
1. Determine the Manning’s n for the 6 ft corrugated metal pipe with 5 by 1 in corrugations.
n = 0.026 (Figure B-3)
2. Determine the Manning’s n for smooth lining.
n = 0.013 (assume concrete lining)
3. Determine the relative perimeters composed of each material.
p = D = (3.14)(6) = 18.84 ft (total wetted perimeter)
p1 (corrugated) = (0.60)(18.84) = 11.30 ft
p2 (smooth) = (0.40)(18.84) = 7.54 ft
4. Use Equation (36) to calculate the Manning’s n value
1.
5
1.
5
⎡ ( 11.
30)(
0.
026)
+ ( 7.
54)(
0.
013)
⎤
n = ⎢
⎥
⎣
18.
84
⎦
=
0.
021
0.
67
206
(36)