Design of open channel measurement system is 'textbook' when the flow is moderate, for example from 5 to 200 l/s, and there is an appropriate length of channel available.

For high flows, care must be taken to avoid unnecessary construction costs.

It is at low flow that some of the most challenging problems emerge.   It is at low flow, accuracy of head measurement is essential.   And at low flow the tendency to foul is at its worst.