Numerical Modeling of the Flow through a Bottom Slot Rack at a Combined Sewer Diversion Structure

Bottom slot racks are often used at combined sewer overflow (CSO) diversion structures to convey flows to the interceptor sewer. Once the capacity of the underdrain or downstream sewer system is reached, excess flows are conveyed to facilities such as storage tanks, deep tunnels, or directly to the CSO outfall. To maximize flow, a weir is often placed on the outfall sewer immediately downstream of the bottom slot. The hydraulic behavior of the diversion structure, including the inflow and outflow conduits, access manhole chamber, bottom slot, and overflow weir, is difficult to accurately represent in most one-dimensional (1D) collection system models. A particular challenge is accurately representing the flow structure and flow distribution between outfall and underdrain sewers, especially during the variable flow conditions experienced during rainfall events.In the present study, computational fluid dynamics (CFD) was used to analyze the complex flow structure within the diversion structure for several flow discharges representing underdrain free and pressurized flow conditions. Simulated flow distribution between overflow and underdrain conduits were validated against field measurements. The complex flow velocity and free-surface patterns within the diversion structure were analyzed. The model allowed identification of flow recirculation zones responsible for enhanced head losses leading to undesired reduction of the underdrain flow. This type of model could be used to investigate geometric modifications to minimize head losses while enhancing the underdrain flow. Model results could also be used to construct stage-discharge relationships and/or average flow discharge coefficients that could be readily implemented in 1D models to improve flow distribution between underflow and outfall conduits.