Upstream of a flume the water is _______.

• A. Fast Moving and Shallow
• B. Calm and Shallow
• C. Slow Moving and Deeper
• D. Turbulent and Deep

Answer: (C)

In open-channel flow through a flume, the water upstream of the constriction tends to be in subcritical flow: it is relatively deep and moves slowly. Why this happens is tied to continuity and energy. The upstream cross-sectional area is larger, so for a given discharge the velocity must be lower. At the same time, the water depth increases to carry the same amount of water, creating a calmer, deeper pool just before the throat. As the water enters the narrower throat, it speeds up and depth shallows to pass through the constriction, which is why upstream is described as slow moving and deeper. The other descriptions imply conditions you’d more likely see at or after the constriction, not upstream.