Underflow gates are vital for controlling water flow in irrigation canals. The three main types of underflow gates — vertical, radial, and drum gates — serve different purposes while ensuring effective flow management. Vertical gates move up and down, generating a free-flowing water jet; radial gates pivot to regulate the flow; and drum gates rotate for precise adjustments. The flow through these gates is influenced by downstream conditions, resulting in free or drowned outflow.

Free and Drowned Outflow

In free outflow conditions, water exits as a supercritical jet exposed to the atmosphere. The flow rate in this regime depends on the height of the gate opening and a reference velocity, both of which are adjusted by a discharge coefficient. This coefficient accounts for the effects of gate geometry and fluid dynamics, playing a critical role in determining flow behavior.

Drowned outflow occurs when downstream conditions, such as an obstacle, raise the water depth, creating a turbulent water mass over the jet. This submergence reduces the effective head and requires an adjusted discharge coefficient to reflect the new flow conditions. Flow characteristics for drowned outflow are depicted as lower curves on discharge coefficient plots, representing typical operational values for submerged flow. Drowned outflow provides controlled discharge under variable downstream conditions, reducing jet velocity, minimizing erosion, stabilizing water levels, and preventing excessive aeration. It is beneficial for energy dissipation, flow regulation, and maintaining steady hydraulic conditions in irrigation and flood control systems.

Influence of Depth Ratios on Flow

The relationship between upstream and downstream water depths is crucial for understanding flow through underflow gates. When the downstream depth (y₃​) equals the upstream depth (y₁​), there is no effective head to drive the flow, causing the discharge coefficient to drop to zero and the flow to stop. As the ratio y₃/a (downstream depth relative to a reference dimension) decreases for a given upstream depth, the discharge coefficient increases until reaching a maximum value, reflecting optimal flow conditions. These dynamics highlight the importance of accurate gate operation in regulating water flow efficiently.