
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 D^{2.69} x H^{0.56}

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):

Fitting

Equivalent
Pipe Length
e.g. 30D = 30 x diameter of pipe

90 degree elbow

30D

45 degree elbow

20D

T straight through

16D

T through side

60D

Swept 90 bend

48D

Open gate valve

9D

Open globe valve

275D

Full bore non return valve

6D

Butterfly valve

20D

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

0.010


Cement plaster, iron and other smooth pipes in good
condition

0.011


Concrete sewers >1500mm diameter

0.012


Concrete sewers 600  1500mm diameter

0.013


Brick sided, concrete based channels

0.015


Coarse brick, earth in good order channels

0.020


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

0.025


Earth canal in poor condition, with some stones and
weed

0.030





Calculation Q
= (1/n x ((A x WP)^{2/3}) x S^{1/2}) x A
Where:
Q = Flow rate m^{3}/sec
n = Coefficient of roughness
(see above)
A = Cross sectional area of channel (m^{2})
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.



