Growing challenges of rapid filling conditions in urban hydraulic systems

Stormwater management intending to minimize environmental impacts created by runoff discharges to receiving water bodies in urbanized areas is a complex task. Accurate modeling of inflows within these systems is a key point for its design and verification in a broad range of conditions. Traditional hydrodynamic models like SWMM and HEC-RAS have an unsteady formulation that solves the Saint-Venant equations in a network numerical methodology of links and nodes. This solution well-represents typical stormwater conditions. However, in the last years, complex and multiple time-varying inflow hydrographs created by strong convective storms associated with growing extremes and the use of intricate system geometries can lead to rapid filling conditions of the collection systems. These conditions have the potential to cause problems such as pressurization of conduits, entrapment of air pockets within the pipes, pressure surges, and even the water hammer transient phenomena. These conditions are not normally captured by traditional models and may under-represent the sudden changes in sewer flow conditions and yield significant flow continuity errors accompanied by numerical instabilities. Moreover, the lack of measured data of these systems under extreme events normally prevents calibration and may give a false perception of the model's reliability and its boundaries of effectiveness. In this light, the path to coping with these issues involves the comprehensive education of engineers, decision-makers, and other stakeholders to acknowledge the usefulness and limitations of available models, as well as robust investment in data collection, analytics, and smarter application of tools that can help practitioners to cope with hydraulic transients in sewer systems. Using SWMM as an example, these tools can range from analysis (e.g. McDonnell et al. 2017) model plugins that recommend models setups based on expected dynamic flow conditions (e.g. Pachaly et al. 2018), or even more complex numerical models (e.g. Hodges et al. 2019). This paper presents results ongoing on data gathering and modeling experience of these systems by members of the research network "International Cooperation for the Analysis and Research on Urban Hydraulic Systems" (ICARUHS).

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