Simply put, weir boxes are a tool used to direct open channel flow for measurement purposes. A weir box usually contains a weir plate at the end which contains a notch in the shape of the letter 'v'. Water is forced over the notch in the weir plate and allows you to take accurate measurements, as well as water samples.
Whats a weir Box?
: a wooden or concrete box oblong in shape and open at both ends which is set lengthwise in a canal and in which a weir for the measurement of irrigation water is set crosswise.
What is a weir plate?
When used for flow measurement, a weir plate is a barrier across a body of water over which water flows. Spanning the full width of the channel or body of water, the shape of a weir plate generates a known level-to-flow relationship.