Using Computational Fluid Dynamics Modeling to Improve Flow Partition in a Combined Sewer Diversion Structure under Supercritical Flow

Bottom slot racks are used in combined sewer overflow (CSO) diversion structures to convey dry and wet weather flows through an underdrain conduit to the intercepting sewer. Once the capacity of the underdrain sewer system is reached, excess flows are conveyed to facilities, such as storage tanks and deep storage tunnels, or directly to the CSO outfall. Often a bottom weir or splash plate is placed on or near the overflow sewer to maximize flows into the underdrain and minimize splash-over flows.The design of these structures commonly relies on semi-empirical formulae, designer experience, and/or one-dimensional (1D) conveyance models. These approaches are not able to account for the effects specific geometric configurations have on localized head losses caused by such things as flow recirculation cells and air entrainment. This, in turn, decreases the amount of flow into the underdrain conduit.In this study, the hydraulic behavior of a diversion structure was initially evaluated within InfoWorks ICM against flow meter data. The structure was comprised of an access manhole chamber, bottom slot rack, splash plate, a steep inflow pipe, underdrain, and overflow conduits. The 1D conveyance model did not fully capture the complex flow behavior at the diversion structure, which was augmented by high inlet velocities inducing a “flow leap” over the splash plate (or overflow weir) even before the capacity of the underflow was exceeded. The highly unsteady and irregular free-surface patterns at the interior of the structure cannot be accurately represented by conventional 1D conveyance models. This limitation of 1D hydraulic models may result in instances of gross miscalculation of offline storage systems devised to handle excess flows.A computational fluid dynamics (CFD) three-dimensional model was used to evaluate the flow hydraulics at the diversion structure under several discharge rates representing free and pressurized flow conditions. This was compared against flow meter data. The CFD model was used to evaluate alternative geometric configurations of the splash plate within the CSO regulator structure to maximize flows through the slot rack and minimize overflows due to flow leaps. The resulting design is also intended to be represented in conveyance models more accurately and thus enhance their ability to calculate the flow partition between underflow and overflow conduits.