Wiktionary
flap gate
alt. A device that allows water to flow in one direction only through a culvert; it is used especially to drain surface water from coastal marshes at low tide n. A device that allows water to flow in one direction only through a culvert; it is used especially to drain surface water from coastal marshes at low tide
Usage examples of "flap gate".
They both looked our way, then the cashier lifted the flap gate in the bar and headed toward us.