In open channel flow measurement, which parameter must be measured to determine flow rate?

• A. Speed of the water
• B. Weight of the water
• C. Color of the water
• D. Height (depth) of the water

Answer: (D)

In open-channel flow, how much water passes a cross-section (the discharge) comes from how wide and deep the water is (the cross-sectional area) and how fast it moves. The depth of the water defines that cross-sectional area for the channel shape, so knowing the height of the water at a section lets you determine A. Once you have the area, you can use a velocity measurement or a rating curve that links depth to discharge to get the flow rate. The other options—color, weight, or simply “speed”—don’t directly give the discharge without additional information, whereas depth provides the necessary basis to relate area to flow.