Mannings n Coefficients
Manning’s n Coefficient Table
| Lined Channels | Manning’s n | ||
|---|---|---|---|
| Concrete, with surfaces as indicated | Formed, no finish | 0.013 - 0.017 | |
| Trowel finish | 0.012 - 0.014 | ||
| Float finish | 0.013 - 0.015 | ||
| Float finish, gravel on bottom | 0.015 - 0.017 | ||
| Gunite, good section | 0.016 - 0.019 | ||
| Gunite, wavy section | 0.018 - 0.022 | ||
| Concrete, bottom float-finished, sides as indicated | Dressed stone in mortar | 0.015 - 0.017 | |
| Random stone in mortar | 0.017 - 0.020 | ||
| Cement rubble masonry | 0.020 - 0.025 | ||
| Cement rubble masonry, plastered | 0.016 - 0.020 | ||
| Dry rubble (riprap) | 0.020 - 0.030 | ||
| Gravel bottom, sides as indicated | Formed concrete | 0.017 - 0.020 | |
| Random stone in mortar | 0.020 - 0.023 | ||
| Dry rubble (riprap) | 0.023 - 0.033 | ||
| Brick | 0.014 - 0.017 | ||
| Asphalt | Smooth | 0.013 | |
| Rough | 0.016 | ||
| Wood, planed, clean | 0.011 - 0.013 | ||
| Concrete-lined excavated rock | Good section | 0.017 - 0.020 | |
| Irregular section | 0.022 - 0.027 | ||
| Unlined Channels | Manning’s n | ||
| Earth, uniform section | Clean, recently completed | 0.016 - 0.018 | |
| Clean, after weathering | 0.018 - 0.020 | ||
| With short grass, few weeds | 0.022 - 0.027 | ||
| In gravely soil, uniform section, clean | 0.022 - 0.025 | ||
| Earth, fairly uniform section | No vegetation | 0.022 - 0.025 | |
| Grass, some weeds | 0.025 - 0.030 | ||
| Dense weeds or aquatic plants in deep channels | 0.030 - 0.035 | ||
| Sides clean, gravel bottom | 0.025 - 0.030 | ||
| Sides clean, cobble bottom | 0.030 - 0.040 | ||
| Dragline excavated or dredged | No vegetation | 0.028 - 0.033 | |
| Light brush on banks | 0.035 - 0.050 | ||
| Rock: Based on design section | 0.035 | ||
| Based on actual mean section | Smooth and uniform | 0.035 - 0.040 | |
| Jagged and irregular | 0.040 - 0.045 | ||
| Channels not maintained, weeds and brush uncut | Dense weeds, high as flow depth | 0.08 - 0.12 | |
| Clean bottom, brush on sides | 0.05 - 0.08 | ||
| Clean bottom, brush on sides, highest stage of flow | 0.07 - 0.11 | ||
| Dense brush, high stage | 0.10 - 0.14 | ||
| Highway Channels and Swales with Maintained Vegetation | Manning’s n | ||
| (Values shown are for velocities of 2 and 5 fps) | |||
| Depth of flow up to 0.7 ft. | |||
| Bermuda grass, Kentucky bluegrass, buffalo grass | Mowed to 2 in. | 0.07 - 0.045 | |
| Length 4 to 6 in. | 0.09 - 0.05 | ||
| Good Stand, any grass | Length about 12 in. | 0.18 - 0.09 | |
| Length about 24 in. | 0.30 - 0.15 | ||
| Fair stand, any grass | Length about 12 in. | 0.14 - 0.08 | |
| Length about 24 in. | 0.25 - 0.13 | ||
| Depth flow 0.7 ft. to 1.5 ft. | |||
| Bermuda grass, Kentucky bluegrass, buffalo grass | Mowed to 2 in. | 0.05 - 0.035 | |
| Length 4 to 6 in. | 0.06 - 0.04 | ||
| Good Stand, any grass | Length about 12 in. | 0.12 - 0.07 | |
| Length about 24 in. | 0.20 - 0.10 | ||
| Fair stand, any grass | Length about 12 in. | 0.10 - 0.06 | |
| Length about 24 in. | 0.17 - 0.09 | ||
| Gutters | Manning’s n | ||
| Concrete gutter | Troweled finish | 0.012 | |
| Asphalt pavement | Smooth texture | 0.013 | |
| Rough texture | 0.016 | ||
| Concrete gutter with asphalt pavement | Smooth | 0.013 | |
| Rough | 0.015 | ||
| Concrete pavement | Float finish | 0.014 | |
| Broom finish | 0.016 | ||
| For gutters with small slope, where sediment may accumulate, increase all above values of n by | 0.002 |
(Source: Searcy 1973.)(Source: U.S. Soil Conservation Service 1986.)
- The n values are a composite of information compiled by Engman (1986).
- Includes species such as weeping lovegrass, bluegrass, buffalo grass, blue grama grass, and native grass mixtures.
- When selecting n, consider cover to a height of about 0.1 ft. This is the only part of the plant cover that obstructs sheet flow.