Failed flap valves allowed site flooding, fitting Guvnu valves to existing flanges solved the problem.

At a container port the surface drainage attenuation was filled by the rising tide water. The original design specified four double hung flap valves. The port had replaced these flap valves at least twice.

In coastal exposed sites, flap valves are unlikely to survive without high levels of maintenance.

The usual failure path is grit in the hinge pins. Wave and currents will continually move the flap when it is submerged. The pins are worn away rapidly and the flap simply drops off, leaving the frame and a fully open pipe to the attenuation.

HDPE flaps and even, metal flaps will be lifted in high winds and this can shatter and crack cast flaps as seen in the video.

Guvnu valves using the Inline rubber valves  were easily fitted within the tidal window.