Manning's roughness coefficient

Suggested values for the Manning's roughness coefficient for designing soil and water conservation earthworks, such as grassed waterways, grade banks and shallow relief drains.

The 'see also' links to the right are the pages that use this information.

Typical values for Manning's coefficient (n) for different waterways

Table 1 Typical values for Manning's coefficient (n) for bare soil waterways

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

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

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

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

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) or deep pools	0.025	0.030	0.033
Straight bank, full stage, no deep pools, some weeds and stones	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

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 [1], 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.

Links

Australian rainfall and runoff: Geoscience Australia [1]