A very different style of variable-area flowmeter is used extensively to measure flow rate through
open channels, such as irrigation ditches. If an obstruction is placed within a channel, any liquid
flowing through the channel must rise on the upstream side of the obstruction. By measuring this
liquid level rise, it is possible to infer the rate of liquid flow past the obstruction.
The first form of open-channel flowmeter is the weir, which is nothing more than a dam
obstructing passage of liquid through the channel. Three styles of weir are shown in the following
illustration; the rectangular, Cippoletti, and V-notch