Glass-reinforced plastic (GRP) flap gates, also known as composite flap gates, are primarily composed of unsaturated polyester resin and glass fiber. These flap gates are a type of one-way valve installed at the outlet of drainage pipes along rivers. When the river level is higher than the outlet of the pipe and the pressure is greater than the pressure inside the pipe, the flap panel automatically closes to prevent river water from backflowing into the drainage pipeline. They are suitable for overflow and check valve applications in various pipelines and water channels for water supply, drainage, and wastewater treatment projects, and are used at outlets for one-way water flow.