Flap gates at the downstream end of pipes that discharge into streams, lakes, or other drainages are not only susceptible to infestation but are also often difficult to inspect. If a flap gate becomes fouled with zebra mussels and associated debris so that it does not close, flood waters will enter protected areas at high river stages.
Facility operators should carefully inspect flap gates at least once a year when water temperature is greater than 12ºC. The inspection should include the outer portion of the gate, the hinges, and the downstream end of the pipe. If not observed in these areas, zebra mussels are unlikely to exist farther up the pipe. If the flap gate is underwater or in a difficult area to inspect, attach a section of polyvinyl chloride (PVC) pipe or plate (an appropriate test substrate) and a concrete or ceramic tile to a nylon rope or cable in a protected area near the flap gate. Zebra mussels will attach to the test substrate which can be easily pulled out of the water and inspected.
Zebra mussels can be removed from the flap gate and adjacent piping with a wire brush, high-pressure water, scrapers, or by other physical means. Surfaces can be coated with nontoxic foul-release paint with a slick surface that zebra mussels do not remain attached to during high flow. Alternatively, toxic antifouling paints could be applied.