Flumes & Weirs

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Since their invention in the 1920’s, flumes have been used to provide an accurate and reliable method of measuring flow in an open channel.

Restricting the width of a channel dimension forms a flume.

This so that an accurate measurement of the cross sectional area of the flow can be made and also to ensure that the velocity of the flow is not affected by any downstream condition but is dependant only on the head (depth) upstream of the flume: –

Q = V x A

(Flow = Velocity x Area)

Flow measurement – accuracy

There is an increasing demand for accurate flow measurement on sewage works and trade discharges. For example in England and Wales the Environment Agency requires all discharges to watercourses which are in excess of 50m³/ day to be measured to within +/- 8% of the total daily flow. In many cases the ‘flow to full treatment’ (FTFT) can be measured and not the final effluent itself. The measuring installation must be provided with a certificate of conformity (to BS3680) and accuracy certificate, both provided by an independent specialist.

In Scotland SEPA require flumes to comply with BS3680.

Unfortunately many flumes do not comply with the tolerances of BS3680 and have to be replaced.

GRP (Fibre Glass) Flumes

For many years flumes have been formed using GRP either ‘cheeks’ or from one-piece units. These have been ‘back filled’ with concrete.

However GRP has a major drawback in that the concrete backfill does not bond to it and over a period of time the GRP starts to come away from the concrete resulting in a distorted flume.

Many exsisting flumes now have to be replaced in order to comply with Environment Agency requirements.

Venturi Channels

Pulsonic Technologies Venturi channels have been developed to ensure a high level of accuracy with long-term durability and ease of installation.

They can be supplied with an integral approach section or without (conventional flume) and manufacture in GRP or stainless steel – 304 grade as standard, – 316 grade is optional.

The most important feature of the Pulsonic Technologies Venturi Channels is that they are supplied with a rigid core (spacer) in the throat/ channel. The sidepieces are flexible and when the concrete pushes the flume sides up against the core ensuring dimensional accuracy and minimum distortion.

To ensure that the unit does not come away from the concrete, the outside faces either have multiplex fixing ties (stainless steel) or are specially treated (GRP).


Pulsonic Technologies provide a free design service, and provide installation instructions, drawings and head/flow data etc.

Three Options


Use where accuracy and surface finish of channel and invert are less important – large channels.


Use where surface finish of the channel and invert can affect accuracy – smaller channels.


Use to reduce width of existing channels and to increase approach velocity. (This will reduce the amount of sludge settling in the approach.

Full technical back-up

Pulsonic Technologies can provide a complete design service supported by 7 years experience in open channel flow measurement including: –

Head/flow calculations, installation drawings and instructions, site surveys and certification etc.