All rubber duckbill check valves are an excellent alternative to flap valves for these outfall applications.

In the photos above, it can be seen how the 600mm duckbill valve on Llansteffan Beach has passively cleared itself after being half-buried in tightly packed sand. The elastomer construction is similar to a truck tyre and delivers a long service life with no required maintenance. Currently, there are valves in service since 1984 and we have many marine installations that are over 25 years in service.

In addition to self clearing, there is no corrosion in salt water, no freezing in winter and no sticking open or shut, no metal theft and no unsightly lubricating hydrocarbons are deposited into the water.

Duckbill valves operate on differential pressure, forward flow opens the valve and reverse flow/backpressure seals the valve. The passive valve operation requires no external energy, just 25-40mm of water head differential can open the valve, draining the line down to low tide level.

Submerged operation is slightly more efficient than free discharge, as the valve operates on differential pressure, it will continue to discharge when submerged, as long as the level inside the pipe is higher than the receiving water.

This means that the duckbill valves are never "tide locked" and maximum flows are obtained from upstream attenuation on each tide cycle. The valves are flowing whenever the level inside the pipe is higher than the level outside.