Abstract: A filtration system for storm drain collection boxes includes a helical coil frame, open at both ends, which coil frame is attached at an exit end to a vertical exit side wall of the collection box with the exit end aligned with a collection box outlet opening. The opposite, inlet end of the coil frame is suspended from a ceiling of the collection box, and a filter fabric "sock", which is also open at both ends, is positioned surrounding the coil frame. The inlet end of the coil frame is positioned at a level below the ceiling of the collection box. The filter sock normally filters out silt, sediment and solid materials from storm water run-off before the water enters the storm drainage system but the open inlet end of the coil frame and sock acts as an overflow during flooding conditions to permit storm water to bypass the filtration provided by the filter sock while still filtering out solid materials via a wide mesh screen.