Which flow measurement device uses the depth of flow to determine discharge?

• A. Parshall flume
• B. Venturi meter
• C. Weir
• D. Orifice plate

Answer: (C)

Measuring discharge from the height of water over a barrier is the key idea here. A weir works by creating a sharp edge across a channel so that the water must flow over it. The amount of water that passes each moment is determined by how high the stream is above the crest. That depth, or head, is what you read and then convert using a standard weir equation or rating curve to get the discharge.

This is different from devices that use pressure differences. Venturi meters and orifice plates rely on changes in pressure to infer flow, not the vertical depth of water over a crest. A Parshall flume does involve a constricted channel and uses measurements related to head differences to calculate flow, but the direct, straightforward read-the-depth-to-get-discharge relationship described here is characteristic of a weir.