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One of the most used 1D tools to model collection systems is the Storm Water Management Model (SWMM). Solving the full form of the Saint Venant equations, this model represents typical unsteady flow conditions in sewer systems. However, it may be insufficient to address fast transient flow conditions that can be present during extreme events or unscheduled operational conditions. SWMM version 5.1.013 was implemented the Preissmann slot as an alternative method to handle pressurization. To date, no studies were found analysing the applicability of SWMM to represent closed-pipe fast transient flows applying the Preissmann slot pressurization algorithm. Therefore, the present work investigates the ability to model different fast transients conditions in SWMM under various spatial and temporal discretizations along with variations in the Preissmann slot algorithm. Using an alternative implementation of the SLOT pressurization algorithm, along with artificial spatial discretization and routing time-steps estimated by the Courant stability condition, it is shown that SWMM is capable to perform satisfactory certain types of closed pipe transient simulations. [ABSTRACT FROM AUTHOR]

The City of Columbus, Ohio is implementing a tunnel system to reduce the number of episodes of combined sewer overflows into the Scioto River. The tunnel systems provide relief to the existing Olentangy Scioto Interceptor Sewer. Two new tunnels being implemented are the OSIS Augmentation and Relief Sewer (OARS), in service since July 2017, and the Lower Olentangy Tunnel (LOT) that is planned to be in service in 2025. The performance of these tunnels in respect to high inflow conditions was investigated with the use of the HAST mixed flow model and the OpenFOAM CFD model to determine the magnitude of surges, the possibility of air pocket entrapment, air-water surging, and the consequences of uncontrolled air pocket releases through shafts. Inflows into the systems were obtained from a calibrated collection system SWMM model. Modeling results quantified surging in the tunnel dropshafts and their mitigation from built-in surge control chambers. HAST simulations also pointed to locations where air pockets could form. These results were used in OpenFOAM to determine the effects of uncontrolled air release through the shaft that links the two tunnels. It was shown that proper ventilation at the shaft will mitigate the growth of air phase pressure to damaging levels. [ABSTRACT FROM AUTHOR]