NDS 421 Green Plastic Spring Loaded Pop-Up Drainage Emitter w/ Elbow for 4" Drain Pipes, water is discharged safely when installed away from structures, erosion-prone landscapes, and poor drainage areas. The spring-loaded cap pops up to 1 in. high when water pressure builds to allow water to be dispersed at up to 56 GPM, then automatically retracts once the flow diminishes. Cap stays closed when not in use to prevent debris and rodents from entering and potentially clogging the drainage system. Weep hole at bottom of elbow drains out water left in pipes to prevent stagnation or freezing in cold weather. The Pop-Up Emitter with Elbow connects to 4 in. sewer and drain pipe and 4 in. triple-wall pipe for a soil-tight. fit. Cap and cover made of high-density polyethylene (HDPE); elbow is made of polyvinyl chloride (PVC).