Rain Gardens & Bio Retentions are used to retain and filter storm water. Rain gardens/ Bio Retention Basins have a well-drained soil profile with a pipe in the bottom to slowly discharge water and an overflow spillway for larger rain events. The profile typically consists of gravel, sand, soil, a thin layer of mulch and then plantings on the surface. The plants that are chosen can tolerate being submerged in water for short periods of time.