NDS 422G 4" Pop Up Drainage Emitter with Elbow, Green The pop-up emitter is opened by the hydrostatic pressure of water flowing through the drain pipe and as flow diminishes the emitter closes The special patented design allows the emitter to open with a negligible amount of pressure Since the emitter is closed during dry weather, debris and rodents cannot enter the drain pipes Fits 4-inch hub sewer and drain fittings Ideal for discharging water from retaining walls and street curbs