Automatic floodgate

It is not buoyancy that activates that gate. It is uneven pressure on opposites side of the gate.

As shown in the graphic, with very little water the gate would hang in an open position. As the water level rises higher on the upstream side (right) the water will push the gate open allowing more water flow. That is not buoyancy. In the graphic, the "float" just barely come into play until the gate is almost totally open already. In a static situation, with no water flow in either direction, no matter how high the water is, if it is the same level on both sides, the gate will hang down almost like it would with no water at all.

When the water level on the downstream side (Left) is higher upstream it begins to close the gate, preventing water to flow upstream, but also preventing down stream flow.

The gates may work well, but the graphic does not properly represent the action of the gates.

Automatic floodgate - GIF