# Manning’s Roughness Coefficient

## Manning’s Roughness Coefficient (n)

Manning’s Roughness Coefficient is a measurement of the average roughness of the wetted perimeter of a pipe or conveyance. This allows the Manning formula to correct for the effect of this roughness slowing down the water as it moves through the pipe or conveyance.

Typical values are given below;

 Channel Manning's n Values Min Normal Max Concrete or asbestos-cement pipe 0.011 0.013 0.015 Smooth walled helical spiral rib pipe 0.012 0.013 Corrugated metal subdrain 0.017 0.019 0.021 Plastic pipe (Smooth) 0.011 0.015 Plastic pipe (Corrugated) 0.024 Metal pipe or pipe arch with helically wound corrugations 0.024 Wrought iron 0.012 0.014 0.015 Cast iron 0.010 0.013 0.014 Steel pipe, welded 0.010 0.012 0.014 Brick 0.012 0.015 0.017 Vitrified clay sewer 0.011 0.014 0.017 Sanitary sewer, coated with sewage slime, with bends and connections 0.012 0.013 0.016 Concrete gutter, troweled finish 0.012 Asphalt pavement 0.016 Concrete lined channel 0.013 0.013 0.015 Grouted riprap 0.028 0.030 0.040 Bare soil 0.020 0.020 0.023 Rock cut 0.025 0.035 0.045 Grass, some weeds 0.025 0.030 0.033 Dense weeds or aquatic plants 0.030 0.035 0.040 Dense weeds, high as flow depth 0.050 0.080 0.120

##### Colebrook White Pipe Design Spreadsheet

This spreadsheet uses the Colebrook-White and Darcy-Weisbach formulas to calculate the flow conditions in a circular pipe acting under gravity.

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This spreadsheet uses the Manning formula to calculate the flow conditions in a circular pipe acting under gravity only.

#### Manning Open Channel Design Spreadsheet

This spreadsheet uses the Manning formula to calculate the flow conditions in an open channel acting under gravity only.

This spreadsheet calculates the design runoff flow for a site in accordance with the Flood Estimation Handbook.

This spreadsheet calculates the maximum flow from a specified linear drainage channel and checks whether the channel is sufficient for the specified site and storm conditions.

This spreadsheet calculates the requirements for a attenuation system and assists the user to design a suitable system.

This spreadsheet calculates the requirements for a soakaway system and assists the user to design a suitable system.

### Full Drainage Design Suite

Full drainage design suite (50% Discount) including 7 spreadsheets;

• Colebrook White Pipe Design
• Manning Pipe Design
• Manning Open Channel Design
• Linear Drainage Design
• Runoff Calculator
• Attenuation Design
• Soakaway Design