Calculating drainage fall and gradients

Drainage Pipework

All above and below ground horizontal drainage pipes should be laid to an adequate gradient.

Gradients from 1 in 40 to 1 in 110 will normally give adequate flow velocities.

A gradient of 1 in 80 is suitable for commencing calculations for pipe schemes.

If the gradient is not steep enough i.e. less than 1 in 110, then the pipe could block if the solids slow down and become stranded. The fall in a pipe may be defined as the vertical amount by which the pipe drops over a distance. The distance can be between sections of pipe or between manholes.

A gradient may be defined as fall divided by distance.

GRADIENT = FALL / DISTANCE

For example a 24 meter section of pipework has a fall of 0.30 meters the gradient = 0.30 / 24

gradient = 0.0125

This can be converted into a gradient written as a ratio

gradient = 1 / 0.0125 = 80

gradient = 1 in 80

If the gradient is known then the formula can be rearranged to calculate the fall

FALL = GRADIENT X DISTANCE

For example to calculate the fall in a 50 meters of pipework if the gradient is to be 1 in 80. A gradient of 1 in 80 as a number instead of a ratio is 1 / 80 = 0.0125

Fall = 0.0125 x 50

Fall = 0.625 meters (625 mm)

How we can help?

If you have issues with blocked drains, a soakaway or drainage field that has failed contact us at Tekneka Ltd onĀ  01772 846193 / 07968 143756 or fill in the contact form