The Potential for Green Infrastructure Practices to Reduce Combined Sewer Overflows as Examined in Nashville, Tennessee

The Nashville downtown, as in many cities, is drained by a combined sewer suffering from overflows. This project was performed as part of an overall evaluation of the use of Green Infrastructure (GI) to reduce the runoff entering the system. Continuous simulations were performed with 36 years of local rainfall and potential evapotranspiration data. Prototype bioretention cells, pervious pavements, green roofs and tree planters were modeled using the Environmental Protection Agency Storm Water Management Model (SWMM). SWMM was used to model the GI prototypes. Rainfall, surface evaporation, infiltration, transpiration, deep percolation, runoff and runon were considered. To test sensitivity and create design curves, a full range of models was created by varying media depths, ratio of runon area to facility area, deep percolation rates, media and plantings. A total of 103 prototype models were run and evaluated for the volume of runoff. The modelling results were realistic and showed clear trends in runoff volume. The GI practices were found to significantly reduce volume of runoff. Runoff volume was sensitive to deep percolation rate and runon area, and less sensitive to media and planting characteristics. Design curves were created based on the continuous modelling so that designers could properly size GI facilities to consider the full range of flows, rather than on a single event flow or rule of thumb.