What is the primary purpose of weirs and flumes in wastewater flow measurement?

• A. They are used to aerate water
• B. They are hydraulic structures that change the flow so that the water level indicates the flow rate
• C. They are used to filter solids
• D. They are used to increase pressure in pipes

Answer: (B)

Weirs and flumes are used to measure wastewater flow by forcing the water through a controlled hydraulic element so the flow rate can be inferred from a measurable water level. A weir creates a known crest, and the amount of water that flows over it produces a head (height above the crest) that is related to discharge by a standard rating curve. A flume constricts the channel, speeding up the flow and creating a predictable head relationship at a specific location (such as the throat or a monitored point). In both cases, the varying flow is converted into a measurable water level, which can then be translated into flow rate with calibration. They aren’t used to aerate water, filter solids, or increase pressure in pipes.