Manning's roughness coefficient

Page last updated: Friday, 10 July 2020 - 9:17am

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, 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.

Contact information

John Simons
+61 (0)8 9083 1128
Paul Galloway
+61 (0)8 9083 1127