Typical values for Manning's coefficient (n) for different waterways
- bare soil waterways
- grassed, constructed waterway, in sand to fine gravel soils
- grassed, constructed waterway, with pasture species
- grassed, constructed waterway, in stiff (low plasticity) clay and coarse gravel soils
- minor natural streams <30m wide
- major natural streams >30m wide
Table 1 Typical values for Manning's coefficient (n) for bare soil waterways
| Soil | Minimum | Design | Maximum |
|---|---|---|---|
| Fine sand colloidal | — | 0.020 | — |
| Sandy loam non-colloidal | — | 0.020 | — |
| Loam | — | 0.020 | — |
| Fine gravel >2mm | — | 0.020 | — |
| Coarse gravel <60mm | — | 0.025 | — |
| Low plasticity (stiff) clay | — | 0.025 | — |
| Soils with stony surface — rounded | — | 0.035 | — |
| Soils with stony surface — angular | — | 0.040 | — |
Table 2 Typical values for Manning's coefficient (n) for grassed, constructed waterway, in sand to fine gravel soils
| Depth of flow | Minimum | Design | Maximum |
|---|---|---|---|
| Average depth of flow is 2 or more times grass height | 0.025 | — | 0.030 |
| Average depth of flow is 1 to 2 times grass height | 0.030 | — | 0.040 |
| Average depth of flow is similar to grass height | 0.045 | — | 0.070 |
| Average depth of flow is less than one half grass height | 0.070 | — | 0.120 |
Table 3 Typical values for Manning's coefficient (n) for grassed, constructed waterway, with pasture species
| Depth of flow | Minimum | Design | Maximum |
|---|---|---|---|
| Average depth of flow greater than height of grass | — | — | — |
| Low grass (<250mm) | 0.025 | 0.030 | 0.035 |
| Tall grass (250–500mm) | 0.030 | 0.035 | 0.050 |
Table 4 Typical values for Manning's coefficient (n) for grassed, constructed waterway, in stiff (low plasticity) clay and coarse gravel soils
| Depth of flow | Minimum | Design | Maximum |
|---|---|---|---|
| Average depth of flow is 2 or more times grass height | 0.030 | — | 0.035 |
| Average depth of flow is 1 to 2 times grass height | 0.035 | — | 0.045 |
| Average depth of flow is similar to grass height | 0.050 | — | 0.075 |
| Average depth of flow is less than one half grass height | 0.075 | — | 0.125 |
Table 5 Typical values for Manning's coefficient (n) for minor natural streams <30m wide
| Stream type | Minimum | Design | Maximum |
|---|---|---|---|
| Straight bank, full stage, no rifts (shallow stony sections) | 0.025 | 0.030 | 0.033 |
| Straight bank, full stage, no deep pools, some weeds | 0.030 | 0.035 | 0.040 |
| Winding bank, some pools and shoals | 0.033 | 0.040 | 0.045 |
| Winding bank, some pools, shoals, weeds and stones | 0.035 | 0.045 | 0.050 |
| Light shrubs and trees – natural vegetation | 0.040 | 0.060 | 0.080 |
| Medium to dense shrubs and trees – natural vegetation | 0.070 | 0.100 | 0.160 |
| Scattered shrubs, grasses and weeds – degraded natural vegetation | 0.035 | 0.050 | 0.070 |
Table 6 Typical values for Manning's coefficient (n) for major natural streams >30m wide
| Stream type | Minimum | Design | Maximum |
|---|---|---|---|
| Regular cross-section with no boulders or shrubs | 0.025 | — | 0.060 |
| Irregular and rough cross-section | 0.035 | — | 0.100 |
Reference material
Ball, J, Babister, M, Nathan, R, Weeks, W, Weinmann, E, Retallick, M, & Testoni, I, (Editors), 2019, Australian Rainfall and Runoff: A Guide to Flood Estimation, Commonwealth of Australia
Schwab, GO, Fangmeier, D, Elliot, W, & Frevert, R 1992, Soil and water conservation engineering, 4th edn, John Wiley and Sons Inc., Toronto, Canada.