Jefferson Street was experiencing heavy flooding at the intersection of 16th Ave. N. and Jefferson Street, where water covered the entire width of Jefferson Street during a significant rainfall event. This area is within Metro Nashville’s combined sewer system where storm water and raw sewage is conveyed to the wastewater treatment system via the same pipe system. There is no stream or storm water system in which to discharge the collected storm water within this drainage basin, thus the engineering challenge was to capture as much storm water as possible to prevent flooding, while preventing the surcharge of the combined sewer. The only outfall point was the combined sewer that extended under Interstate 40, thus the downstream pipe system could not be enlarged. In addition, the project included replacing the aging water distribution pipes within the project limits. The solution to minimize the potential of surcharging of the combined sewer was to purchase property at 16th Ave. N. and construct a 55,600 cubic foot underground detention system using precast concrete structures. A structure was designed at the outfall of the underground detention system to combine the storm water with sewage before entering the pipe under Interstate 40. This combination structure was designed to bypass storm water leaving the detention system in the event the pipe under I-40 could not convey the combined flows. A small park was constructed over the underground detention to add green space in the neighborhood.