CHANNEL FLOW MANAGEMENT

Method

Measuring the velocity of liquid in large, open channels can be challenging. However, by introducing a flow restriction, such as a V-notch weir, you can accurately measure flow rates even for extremely large volumes.

A V-notch weir is essentially a wall placed in the flow path, featuring a precisely cut ‘V’ notch through which the liquid flows. As the liquid approaches the weir, it accumulates until it reaches the base of the ‘V’ and begins to flow through. By knowing the angle of the V, you can accurately calculate the flow rate based on the height of the water column passing through the notch.

To measure this height, you can use an ultrasonic or radar-level sensor positioned above the weir, aimed at the centre of the restriction, or a submersible sensor placed at the base.

Advantages

• Highly accurate
• Suitable for small to extremely large flow rates in open channels

Disadvantages

• Can be costly and challenging to install

Considerations

This method isn’t limited to V-notch weirs; a restriction with a flat top, such as a flat-topped or rectangular-notch weir, can also be used. While these weirs accommodate larger flow volumes, they tend to be less accurate at lower flow rates. Additionally, ensure the sides of the restriction are high enough to avoid overflow in unusual flow conditions, but not so high as to risk flooding.