Current Issues on Modeling Extreme Inflows in Stormwater Systems

Abstract

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.

Pressure Transients due to Compression of Trapped Air in Rapidly Filling Sewer Storage Tunnels

“…One of the implications of this dimensionless presentation is that it could be combined with the results of a single phase flow analysis in order to make prototype predictions. A single phase flow model (such as that proposed by Vasconcelos et al, 2006a;2006b) must be capable of predicting the location and volume of trapped air pockets. At the instant of air entrapment, the conditions associated with bore propagation would be known from the simulation, specifically the water depth in front of the bore.…”

“…Although the assumed flow configuration is not particularly relevant to large sewer systems, it is possible to envision filling scenarios that result in air compression, so the possibility of significant transient pressures should be considered. A version of the numerical model developed by Vasconcelos et al (2006a;2006b) to simulate the filling of nearly horizontal conduits has been applied to the design analysis of proposed combined sewer overflow storage tunnels (Lautenbach et al, 2008). In a number of simulations, the model predicts that a large volume of air will be trapped during the filling process.…”

A New Riemann Solver for Modelling Bridges in Flood Flows -- Development and Experimental Validation

“…This enables the transition between free surface and pressurised flow to be captured via the Preissmann slot concept [11]. However, this also means that the method suffers from the well documented limitations of the Preissmann slot concept as discussed by Vasconcelos, Wright and Roe [12] and Malekpour and Karney [13] , which the authors also acknowledge in their work. Furthermore, the method requires further work in order to be suitable for generalised treatment of linear features, as it can only currently simulate pressurisation without overtopping.…”

Shock-Fitting Approach for Calculating Air Pocket Entrapment Caused by Full Obstruction in Closed Conduit Transient Flow

“…The Preissmann slot model, exemplified by the work of Cunge and Wegner (1964), has been used to simulate these transient flows; however, this model is unable to predict subatmospheric pressures. Furthermore, this hypothetical slot can result in spurious oscillations as shown by Trajkovic et al (1999) and Vasconcelos et al (2006). The two-components pressure approach (TPA), an alternative to the Preissmann slot, was proposed by Vasconcelos et al (2006) and further studied by Vasconcelos and Wright (2007) and Vasconcelos and Marwell (2011) to overcome the inability of the Preissmann slot model in calculating subatmospheric pressures.…”

“…Furthermore, this hypothetical slot can result in spurious oscillations as shown by Trajkovic et al (1999) and Vasconcelos et al (2006). The two-components pressure approach (TPA), an alternative to the Preissmann slot, was proposed by Vasconcelos et al (2006) and further studied by Vasconcelos and Wright (2007) and Vasconcelos and Marwell (2011) to overcome the inability of the Preissmann slot model in calculating subatmospheric pressures. This approach can capture the relevant experimental data with a good agreement; however, it is subject to oscillatory behaviors, particularly, in presence of high pressure wave speeds (Bousso et al 2013).…”