Mannings Equation

The headloss through a conduit for gravity flow is calculated using Manning’s equation, as given below;

 

 

Coefficients for Mannings equation are as follows;

Type of material Condition n
Salt glazed stone ware pipe Good

0.012

Fair

0.015

Cement concrete pipes (with collar joints) Good

0.013

Fair

0.015

Spun concrete pipes (with socket and spigot joint)

0.011

Masonry
Neat cement plaster

0.018

Sand and cement plaster

0.015

Concrete, steel trowled

0.014

Concrete, wood trowled

0.015

Brick in good condition

0.015

Brick in rough condition

0.017

Masonry in bad condition

0.020

Stone work
Smooth dressed Ashlar

0.015

Rubble set in cement

0.017

Fine, well packed gravel

0.020

Earth
Regular surface in good condition

0.020

In ordinary condition

0.025

With stones and weeds

0.030

In poor condition

0.035

Partially obstructed with debris or weeds

0.050

Steel
Welded

0.013

Riveted

0.017

Slightly tuberculated

0.020

With spun cement mortar lining

0.011

Cast iron
Unlined

0.013

With spun cement mortar lining

0.011

Asbestos cement

0.011

Plastic

0.011