Red Valve Company’sFirst Flush Valve for stormwater metering is a simple rubber thimbleinstalled at the end of stormwater lines or in CSO diversion chambersto prevent street and sewage plants from flooding. The valve isdesigned to control the “first flush” of a rainstorm by enabling anoperator to store 30-80 percent of the stormwater in its system. TheFirst Flush Valve is a “passive device” requiring no external control,power source, or actuation.  An opening at the bottom of Zone 1allows small size debris (stones, silt etc.) to pass throughcontinuously. Zone 2 is the holding area which retains the heavy “firstflush” downpour. As flowrates increase, Zone 3’s rectangular opening(size is calculated to regulate required flow) permits discharge at acontrolled rate.  At the top, Zone 4 allows for full flow todischarge when the system is surcharged and also permits escape offloatables. The First Flush Valve is available in sizes six inches to72 inches.