The Environmental Protection Agency issued a consent decree to Metro Nashville Water – Clean Water Nashville (CWN) to mitigate CSOs within the Schrader Lane, Benedict and Crutcher, and Boscobel combined sewer basins by 2031. To meet the consent decree, Hazen and Sawyer (Hazen) and CWN are working on a design to separate the combined sewer into separate sanitary and stormwater systems. Given the complexity of this project, Hazen selected to develop both a 1D and a 2D model for the Preliminary Engineering Report (PER) for each of the three combined sewer basins. To get an understanding of the existing hydraulics and surface conditions in each combined sewer basin the entire combined sewer system was modeled to act as a baseline condition for the proposed separate storm sewer. Both combined sewer models were developed in ArcGIS Pro and PCSWMM using updated survey information. The 1D model is delineated using stream paths and loads directly to either an inlet, catch basin, or area drain. While the 2D model uses a 2D surface mesh to accurately represent surface runoff. The 2D surface mesh is connected to all the storm inlets, via orifice links, which will allow the surface runoff to enter the storm sewer. The combined sewer was verified against existing flow monitoring data and historical rainfall data to confirm that both models have the correct hydraulic and hydrological conditions. When the verification was complete, a level of service evaluation was performed on the combined sewer system to identify areas where the combined sewer can be repurposed to the proposed separate storm sewer. Using both the 1D delineations and the 2D mesh, Hazen began to design a recommended design for the separate storm sewer which included many alternatives analysis for the design. The alternative analysis evaluates different routing options, repurposing existing combined sewer to storm through different rehabilitation options, storage and the estimated costs. While developing the various alternative analysis for the PERs, Hazen has identified several pros and cons for both the 1D model and 2D model for separating an entire combined sewer basin and why a 2D model is the better approach. Some of the topics that will be discussed are time to build each model, model resolution, model run times, and coordination with other firms.