Current Issues on Modeling Extreme Inflows in Stormwater Systems
The increasing availability of different numerical models to simulate flow in storm sewers has raised several questions on the applicability of these models in the simulation of extreme inflow conditions. These issues include accurate modeling and prediction of bores, flow regime transition into pressurized flow and the possibility of air pocket entrapment and the subsequent effect on the system.
With regards to bore description, the issue is not limited to the choice of equations for the problem solution, but includes the selection of numerical schemes that will ensure the correct bore propagation speed. It will be shown that the selection of the variables used to solve the mass and momentum equations has an impact in the solution of discontinuous flows. It will also be seen that numerical models without shock-capturing capabilities have limited ability to model flow discontinuities.
Flow regime transition models can be divided into shock-fitting or shock-capturing approaches, each group of approaches with particular strengths and limitations. Due to the popularity of Preissmann slot-based models, including the soon-to-be released SWMM 5, a more detailed discussion on the issues of this approach is provided. Some new model improvements to shock-capturing approaches are presented, including a new conceptual model for simulating sewer surcharging referred to as the Decoupled Pressure Approach.
Finally, some of the current challenges in the incorporation of the air phase in the computation of the rapid filling pipe problem are considered. These challenges include the limitations of the one-dimensional framework, difficulties of dealing with boundary conditions for the air phase and the possibility of incorporating the air phase in either a two-phase flow or a sub-model framework.
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