Figure 3. A rectangular flap gate at the end of a channel (for example, ditch) in closed position (left, at high tide) and angled in open position (right, at low tide, conveying stormwater runoff).
Schematics: (l) Closed rectangular flap gate at outfall, submerged halfway with water outside it. (r) Same outfall. Flap gate open at bottom & discharging to water body below.