This work compares several alternative methods of pressure-driven analysis (PDA) implemented within the code base of the EPANET water distribution system modeling software. The resulting code includes a direct method, the original and a newly modified version of the inverse method currently used by EPANET 2.2, and two recently published methods (the global gradient approach-based PDA and the active set method). The latter four methods solve PDA problems using an inverse flow–pressure relationship, whereas the direct method uses the original relationship. The alternative methods were extensively tested, and their performance was compared over several case study water distribution networks of vastly different sizes and complexities. The results showed that all of the new inverse methods are equally efficient and reliable, whereas the direct method is less reliable by having a higher frequency of failing to converge.
<p>Design and management of combined sewer overflows (CSO) have been, so far, mainly based only on complying a fixed dilution rate of wastewater in stormwater during rain events. This poses serious environmental issues since the definition of the acceptable dilution (i.e. overflows are usually designed for activation when Q > ~5Q<sub>mw</sub>, the latter being the mean dry weather wastewater discharge) does not consider the characteristics of the upstream urban catchment nor those of the receiving water body. Accordingly, more recent regulations started enforcing limits also on the yearly number of overflows or increasing dilution coefficient.</p><p>Overflows activation frequency and discharged volumes of pollutants may depend on the upstream catchment features as well as on the precipitation regime. The great variability of these factors could make the impact on the receiving water body of similarly designed overflows to be quite different.</p><p>In this study, the behaviour of a CSO with fixed dilution rate placed at the outlet of urban catchments with same size, but different hydrological and urbanistic characteristics, has been simulated through SWMM multi-scenario simulations. The considered hydrological parameters were catchment imperviousness, width, slope, routing Manning coefficient and depression storage for both pervious and impervious surfaces. Urbanistic characteristics of the catchment affecting the combined sewer hydraulic regime were studied by changing the density of population, the imperviousness, and the mean per capita wastewater discharge.</p><p>After defining realistic ranges for each parameter, the time series of discharged overflows have been calculated for all the combinations of the variable catchment parameters, corresponding to 20 years long precipitation time series of a single rain gauge.</p><p>The obtained results indicate that CSOs impact on the receiving water body, strongly depends on the characteristics of the upstream urban catchment. Therefore, such characteristics should be considered in CSO design and management.</p>
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