Open channel flowmeters perform flow measurement of liquids that are open to the atmosphere at some point in the flow measurement path. The liquid may be entirely open to the atmosphere, or may be contained within a closed pipe that is not full of liquid and only open to the atmosphere at the installation point of the flowmeter itself.
Regardless of which type of open channel flow measurement techniques are used, level measurement must be used in combination with velocity measurement, and a flow computer using the manning equation of open channel flow to calculate the true flow rate of any wetted channel (pipes, streams, etc).