Tuesday, September 10, 2024
1:30 PM - 2:00 PM
Location Name
Using 2D Modeling to Evaluate Stormwater Sewer Alignment Alternatives in Nashville, Tennessee

The City of Nashville, Tennessee is experiencing population growth which requires new housing developments. The rapid development in the North Nashville neighborhood plus the aging and undersized combined sewer system is leading to multiple combined sewer overflows (CSOs) into the Cumberland River every year. Metro Nashville Water – Clean Water Nashville (CWN) was given a consent decree from the Environmental Protection Agency to stop all CSOs from this drainage basin by 2028. Hazen and Sawyer (Hazen) are working with CWN to design a separate sanitary and stormwater sewer system. To assist with the design, Hazen is using ArcGIS Pro and PCSWMM to create both a 1D model and a 2D model to validate the hydraulics of the new stormwater sewer design. The 2D model uses a 2D surface mesh to accurately represent surface runoff of different design storms. 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. Initially, the existing combined sewer (ECS) was evaluated as a storm sewer. The results showed the ECS is undersized and does not meet the design criteria. Using the ECS, three main trunks were identified: Upper Main Trunk (UMT), North Trunk (NT), and South Trunk (ST). A new storm sewer system was designed, and modeled, and the 2D results found the design to be unconstructible. The new design still used the same trunks as the ECS, which allowed for different storm evaluations to try and make the design more constructible. The first evaluation of the UMT was reusing most of the UMT ECS by only having ST connect to it. The client would not have to buy multiple properties along the UMT. NT would bypass the UMT through the Buchanan Street Relief (BSR) and discharge into either the lower main trunk or overflow into a new open channel that would discharge to the Cumberland River. Some evaluations investigated upsizing the UMT and having both NT and ST connect to it. The main NT evaluations were based on reusing a portion of the NT ECS. To reuse the NT ECS, multiple inline storages were placed upstream to allow the system to handle the flow. Other evaluations looked at repurposing that same NT ECS as sanitary sewer and rerouting the storm sewer alignments. The new alignments could meet the design criteria without using inline storage. The pipes and manholes were deeper than the first NT evaluation. The main evaluation performed on ST was adding an offline storage tank on an empty lot which allowed the flow to meet the design criteria in the ECS UMT. Using the upsized UMT, the pipe sizes in ST could be significantly reduced since the ST pipes were not acting as inline storage before connecting to the ECS UMT. There are different impacts on the constructability, costs, and depths to the three main trunks depending on which evaluation combination was used.