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In irrigation canals, underflow gates are used to regulate water flow at specific points. The three main types are vertical, radial, and drum gates. 

Vertical gates create a free-flowing jet by moving up and down; radial gates pivot, and drum gates rotate for precise control.

Flow through these gates depends on downstream conditions, resulting in free or drowned outflow.

Free outflow occurs when water exits as a supercritical jet exposed to the atmosphere. The flow rate depends on the gate opening height and a reference velocity, adjusted by a discharge coefficient.

When a downstream obstacle controls the depth, a turbulent water mass overlays the jet, creating a drowned outflow. The flow rate in drowned outflow is adjusted by modifying the discharge coefficient.

A series of lower curves on the plot represents typical values for drowned outflow.

When y₃ ​equals y₁, no height difference drives the flow, causing the discharge coefficient to drop to zero, stopping the flow.

For a given y₃/a ratio, the discharge coefficient increases with an increasing upstream depth until it reaches a maximum. This maximum decreases with a decreasing y₃/a ratio.