Table 2. Manning's n values recommended by HEC-RAS.
|
Type of channel and description |
Minimum
|
Normal
|
Maximum
|
A. Natural streams |
1. Main channels |
a. Clean, straight, full stage, no rifts or deep pools
|
0.025
|
0.030
|
0.033
|
b. Same as above, but more stones and weeds
|
0.030
|
0.035
|
0.040
|
c. Clean, winding, some pools and shoals
|
0.033
|
0.040
|
0.045
|
d. Same as above, but some weeds and stones
|
0.035
|
0.045
|
0.050
|
e. Same as above, lower stages, more ineffective slopes
and sections
|
0.040
|
0.048
|
0.055
|
f. Same as "d" but more stones
|
0.045
|
0.050
|
0.060
|
g. Sluggish reaches, weedy, deep pools
|
0.050
|
0.070
|
0.080
|
h. Very weedy reaches, deep pools, or floodways with
heavy stands of timber and brush
|
0.070
|
0.100
|
0.150
|
2. Flood Plains
|
a. Pasture no brush
|
|
1. Short grass
|
0.025
|
0.030
|
0.035
|
2. High grass
|
0.030
|
0.035
|
0.050
|
b. Cultivated areas |
|
1. No crop
|
0.020
|
0.030
|
0.040
|
2. Mature row crops
|
0.025
|
0.035
|
0.045
|
3. Mature field crops
|
0.030
|
0.040
|
0.050
|
c. Brush |
|
1. Scattered brush, heavy weeds
|
0.035
|
0.050
|
0.070
|
2. Light brush and trees, in winter
|
0.035
|
0.050
|
0.060
|
3. Light brush and trees, in summer
|
0.040
|
0.060
|
0.080
|
4. Medium dense brush, in winter
|
0.045
|
0.070
|
0.110
|
5. Medium dense brush, in summer
|
0.070
|
0.010
|
0.160
|
d. Trees |
|
1. Cleared land with tree stumps, no sprouts
|
0.030
|
0.040
|
0.050
|
2. Same as above but heavy sprouts
|
0.050
|
0.60
|
0.080
|
3. Heavy stand of timber, few down trees, little
undergrowth, flood stage below branches
|
0.080
|
0.100
|
0.120
|
4. Same as above but with flood stage reaching branches
|
0.100
|
0.120
|
0.160
|
5. Dense willows, summer straight
|
0.110
|
0.150
|
0.200
|
3. Mountain streams, no vegetation in channel, banks usually
steep, with trees and brush on bank submerged
|
a. Bottom: gravels, cobbles, and few boulders
|
0.030
|
0.040
|
0.050
|
b. Bottom: cobbles with large boulders
|
0.040
|
0.050
|
0.070
|
Type of channel and description
|
Minimum
|
Normal
|
Maximum
|
B. Lined or built-up channels |
1. Concrete |
a. Trowel finish
|
0.011
|
0.013
|
0.015
|
b. Float Finish
|
0.013
|
0.015
|
0.016
|
c. Finished with gravel bottom
|
0.015
|
0.017
|
0.020
|
d. Unfinished
|
0.014
|
0.017
|
0.020
|
e. Granite, good section
|
0.016
|
0.019
|
0.023
|
f. Gunite, wavy section
|
0.018
|
0.022
|
0.025
|
g. On good excavated rock
|
0.017
|
0.020
|
|
h. On irregular excavated rock
|
0.022
|
0.027
|
|
2. Concrete bottom float finished with sides of: |
a. Dressed stone in mortar
|
0.015
|
0.017
|
0.020
|
b. Random stone in morter
|
0.017
|
0.020
|
0.024
|
c. Cement rubble masonry, plastered
|
0.016
|
0.020
|
0.024
|
d. Cement rubble masonry
|
0.020
|
0.025
|
0.030
|
e. Dry rubble on riprap
|
0.020
|
0.030
|
0.035
|
3. Gravel bottom with sides of: |
a. Formed concrete
|
0.017
|
0.020
|
0.025
|
b. Random stone in mortar
|
0.020
|
0.023
|
0.026
|
c. Dry rubble rip rap
|
0.023
|
0.023
|
0.036
|
4. Brick |
a. Glazed
|
0.011
|
0.013
|
0.015
|
b. In cement mortar
|
0.012
|
0.015
|
0.018
|
5. Smooth steel surface |
a. Smooth steel surface
|
0.011
|
0.012
|
0.014
|
b. Corrugated metal
|
0.021
|
0.025
|
0.030
|
6. Asphalt |
a. Smooth
|
0.013
|
0.013
|
|
b. Rough
|
0.016
|
0.016
|
|
7. Vegetal lining |
0.030
|
|
0.500
|
Type of channel and description |
Minimum
|
Normal
|
Maximum
|
C. Excavated or dredged channels |
1. Earth, straight and uniform |
a. Clean recently completed
|
0.016
|
0.018
|
0.020
|
b. Clean after weathering
|
0.018
|
0.022
|
0.025
|
c. Gravel, uniform section, clean
|
0.022
|
0.025
|
0.030
|
d. With short grass, few weeds
|
0.022
|
0.027
|
0.033
|
2. Earth, winding and sluggish |
a. No vegetation
|
0.023
|
0.025
|
0.030
|
b. Grass, some weeds
|
0.025
|
0.030
|
0.033
|
c. Dense weeds or aquatic plants in deep channels
|
0.030
|
0.035
|
0.040
|
d. Earth bottom and rubble side
|
0.028
|
0.030
|
0.035
|
e. Stony bottom and weedy banks
|
0.025
|
0.035
|
0.040
|
f. Cobble bottom and clean sides
|
0.030
|
0.040
|
0.050
|
3. Dragline-excavated or dredged |
a. No excavation
|
0.025
|
0.028
|
0.033
|
b. Light brush on banks
|
0.035
|
0.050
|
0.060
|
4. Rock cuts |
a. Smooth and uniform
|
0.025
|
0.035
|
0.040
|
b. Jagged and irregular
|
0.035
|
0.040
|
0.050
|
5. Channel not maintained, weeds and brush |
a. Clean bottom, brush on sides
|
0.040
|
0.050
|
0.080
|
b. Same as above, highest stage of flow
|
0.045
|
0.070
|
0.110
|
c. Dense weeds, high as flow depth
|
0.050
|
0.080
|
0.120
|
d. Dense brush, high stage
|
0.080
|
0.100
|
0.140
|
|