Red Valve Company’s First Flush Valve for stormwater metering is a simple rubber thimble installed at the end of stormwater lines or in CSO diversion chambers to prevent street and sewage plants from flooding. The valve is designed to control the “first flush” of a rainstorm by enabling an operator to store 30-80 percent of the stormwater in its system. The First Flush Valve is a “passive device” requiring no external control, power source, or actuation. An opening at the bottom of Zone 1 allows small size debris (stones, silt etc.) to pass through continuously. Zone 2 is the holding area which retains the heavy “first flush” downpour. As flowrates increase, Zone 3’s rectangular opening (size is calculated to regulate required flow) permits discharge at a controlled rate. At the top, Zone 4 allows for full flow to discharge when the system is surcharged and also permits escape of floatables. The First Flush Valve is available in sizes six inches to 72 inches.