Incorporating the Effects of Entrapped Air in SWMM5+ Mixed Flow Modeling

Sazzad Sharior, Abdulmuttalib Lokhandwala and Ben R. Hodges, University of Texas at Austin, TX, USA


In stormwater sewer and storage tunnels, the transition from free surface to pressurized flow, known as “mixed flow”, poses significant operational challenges due to the potential of air entrapment. Entrapped air along with a lack of ventilation has been attributed to disastrous events such as pressure surges, structural damages, manhole cover blowouts, and episodic urban geysers. Existing mixed flow models predominantly focus on water-phase transitions, often neglecting entrapped air’s impact, leading to flawed simulations. Here, we developed an air entrapment model, which detects and models the effects of entrapped air pockets on the free surface flow. The effect of air is modeled using the ideal gas law and air ventilation from a pocket is modeled using an orifice equation. The air entrapment model is built on top of the SWMM5+ hydraulic solver, where the flow is modeled using the Saint Venant equations and the mixed flow transition is handled using the Dynamic Preissmann Slot (DPS) algorithm. The new air entrapment model is validated against rapid filling laboratory experiments. The model demonstrated the drastic difference in simulation results when considering the effects of air pockets. Also, this difference is negligible when the system is properly ventilated.

 Click here to download a static PDF version of the presentation.

Permanent link: