The Free On-line Aquaculture Dictionary

Friction Loss - Water
Pipes Channels

1. Pipes carrying water (smooth bore pipes)

   A spreadsheet is available for download here which will perform the friction loss calculation for you.

To calculate the flow down a pipe, knowing the headloss : Q = 0.00045 x D2.69 x H0.56

Where Q = flow in litres per second

             D - Internal diameter of pipe in millimetres

             H = Hydraulic gradient = Metres headloss of pipe (h)

                                                        Metres of pipelength (L)

Changing this equation round, the headloss can be calculated if the flow required is known;  

This is fine for individual lengths of pipe with no fittings, but pipe system are rarely like this, the table below gives an equivalent pipe length to each fitting, and by adding the sum of all the “equivalent lengths” form the fittings and the total pipe length, the above equation can be used to calculate the friction loss (or headloss):


Equivalent Pipe Length

e.g. 30D = 30 x diameter of pipe

90 degree elbow


45 degree elbow


T straight through


T through side


Swept 90 bend


Open gate valve


Open globe valve


Full bore non return valve


Butterfly valve


There are also other methods for calculating friction losses in pipes but the above is the simplest. The headloss figure that is gained form the above calculation can then be used to specify pumps, or the height of the header tank required.




2. Channels



The amount of water which flows down a channel is a function of the cross sectional area, the wetted perimeter, the water velocity, the gradient of the channel and the coefficient of roughness of the channel.





Coefficient of roughness of common materials



Glazed and very smooth surfaces



Cement plaster, iron and other smooth pipes in good condition



Concrete sewers >1500mm diameter



Concrete sewers 600 - 1500mm diameter



Brick sided, concrete based channels



Coarse brick, earth in good order channels



Earth canal in reasonably good condition, free of stones and weed



Earth canal in poor condition, with some stones and weed






Calculation  Q = (1/n x ((A x WP)2/3) x S1/2) x A


Q = Flow rate m3/sec

n = Coefficient of roughness  (see above)

A = Cross sectional area of channel (m2)

WP = Wetted perimeter of channel (m)

S = Gradient of channel (m). Metres per 100m. e.g. if the channel falls

       0.5m every 100m, S=0.5


See also Channel, Limiting velocity and Scouring velocity for other design criteria which must be considered.