From Flood Protection
Check Element check elements are especially suited for the prevention of flooding caused by high downstream water levels. The check elements are equipped with flexible elastomeric covers (or flaps) that act as seals. The most important feature of the Check Element is their modular construction. They are well suited for new designs or can be easily retrofitted to accommodate existing structures.
The Check Element is equipped with a flexible elastomeric cover mounted over the discharge opening that acts as a seal during normal water level conditions. If the downstream water level rises, the flap presses against the sealing edge of the discharge opening so that the water from the receiving stream cannot flow back into the sewer or tank system. If the up-stream water level rises, the hydrostatic pressure opens the elastomeric cover, allowing the water to flow. The Check Element has a very low hydraulic resistance so as not to adversely affect the upstream hydraulic grade line.