Wyre Council

Basic guide to calculating falls and gradients for drainage

Pipe Gradients

All above ground 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 a gradient is too steep i.e. steeper than 1 in 40, the liquid may run faster than the solids in the sloping foul water pipe thus leaving the solids stranded, which could then block the pipe.

If the gradient is not steep enough, i.e. less than 1 in 110, then the pipe could still 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. The diagram below show pipe fall and distance.

Fall in drainage pipe A gradient may be defined as fall divided by distance.

GRADIENT = FALL / DISTANCE

For example is a 24 metre section of drainage pipe has a fall of 0.30 metres, calculate the gradient.

Gradient = 0.30 / 24

Gradient = 0.0125

This can be converted into a gradient written as a ratio or 1: some number.

Gradient = 1 / 0.0125 = 80

Gradient = 1 in 80

The above formula may be rearranged for Fall if the gradient is known:

FALL = GRADIENT X DISTANCE

For example, calculate the fall in a 50 metre section of foul water pipework if the gradient is to be 1 in 80.

A gradient of 1 in 80 is converted to a number instead of a ratio.

1 / 80 = 0.0125

Fall = Gradient x Distance

Fall = 0.0125 x 50

Fall = 0.625 metres or 625mm.

The previous diagram may be completed by adding a pipe gradient.

Fall and gradient in drainage pipe

Invert Levels

The Invert Level of a pipe is the level taken from the bottom of the inside of the pipe as shown below.

Invert level of pipe The level at the crown of the pipe is the Invert level plus the internal diameter of the pipe plus the pipe wall thickness. It may be necessary to use this in calculations when level measurements are taken from the crown of a pipe.

Manholes

A manhole or access chamber is required to gain access to a drainage system for un-blocking, cleaning, rodding or inspection. A typical manhole is shown below.

Brick built manhole

Manholes may be manufactured from masonry or precast concrete. Sometimes several precast concrete rings are used to form a manhole which speeds up the on-site construction process. Normally deep manholes below 1.0 metre in depth require step irons to assist access for a workman.

Manholes and access chambers are also manufactured in PVC. An access chamber is not usually large enough to admit a person but is suitable for access by cleaning rods or hose and they are used for domestic applications, a common size of plastic access chamber is 450mm diameter. For the domestic market plastic, fibreglass or galvanised steel lids may be used but cast iron lids are required where traffic crosses.

Brick built manhole and plastic manhole building items

A back drop manhole is used in areas where the surface level slopes as shown below.

If the undergroung sewer pipe is to stay below ground it must follow the average gradient of the slope. This invariably means that the pipe gradient becomes too steep, resulting in the solids being left stranded in the pipe therefore causing a blockage.

To overcome this problem the back drop manhole was developed, as shown below.

Sewer on a sloping pipe An easier way to construct a back drop manhole is to use an internal vertical section of pipe as shown below. 

Backdrop manhole with internal outlet pipe

For additional information and guidance please see the following document. 

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