A weir is defined as which of the following components used to calculate flow rate?

• A. Wall or plate, depth of flow used to calculate flow rate
• B. A device to filter solids
• C. A sensor for dissolved oxygen
• D. A tank for mixing

Answer: (A)

A weir is a barrier across a stream or open channel that creates a known crest for measuring flow. The key idea is that the discharge (flow rate) is determined from the height of water (the head) above that crest. By reading how deep the water is upstream of the weir, you can apply a standard relationship to estimate how much water is flowing per unit time. The wall or plate with the crest defines that measurable head, which is why this option best describes a weir used to calculate flow. The other items—something that filters solids, a sensor for dissolved oxygen, or a tank used for mixing—do not establish a known head-for-discharge relationship and thus aren’t used to calculate flow rate in the same way.