 Let me explain how this rain garden works. Step one, runoff from the street enters the rain garden from up here through this opening where it fills this first vegetated chamber. After this top chamber is filled with runoff, the vegetation slows and infiltrates that runoff. When this chamber becomes full, there's a weir here, as you can see, which is simply a slot that allows the water to flow into this bottom chamber if necessary. When both of these rain gardens chambers are filled with runoff, the excess water, if there is any, flows into this drain that's connected with a normal subterranean stormwater drainage system.