Re-think urban drainage following a SuDS retrofitting approach against urban flooding: A modelling investigation for an Italian case study

In the last decades, urban sprawl and soil sealing led to an increase of urban flooding phenomena. Sustainable Drainage Systems (SuDS) seem able to weaken stormwater-related criticalities, enhancing sustainability and city resilience.Relying on a detailed and feasible preparatory study of SuDS feasible retrofit design, based on a punctual identification of the areas suitable for retrofitting and the most appropriate combination of SuDS technologies, the reported research aimed to assess the effectiveness of a sustainable drainage approach in Sesto Ulteriano (Italy), an urban catchment suffering from stormwater management concerns. In particular, using the approach of a typical scenario analysis, this comparative modeling analysis involved SWMM5 for the assessment of the differences in the catchment hydrological behaviour between the mentioned specific and feasible SuDS retrofitting scenarios and a potential one, where non-specificity is considered for SuDS retrofitting location. Besides, the analyses focused on investigating how rainfall severity, areal extension and land use typical feature could influence the effectiveness of the sustainable redevelopment of the urban area.Results indicate that SuDS projects based on potential designs, which does not account for the feasible suds location, might result in a significant overestimation of the hydrological benefit. They showed, indeed, an improved hydrological performance, with average total volume reductions of the Combined Sewer Overflows up to over 70 % (retrofitting the 8.3 % of the catchment area), that is about 40 % higher than those obtained under the same areal extension by the feasible scenarios. Moreover, it was found that there could be an optimal SuDS retrofitting percentage above which additional hydrological benefits are undetectable. Land use, resulting in the variability in the degree of imperviousness necessarily associated to a variability in the retrofitting potential, also seemed to affect SuDS hydrological performance and for this reason should be included in an overall assessment. SuDS also proved to act successfully on the actual maximum percentage of nodes of the drainage network above a 0.7 filling degree threshold (about 52 % under 10-year return period rainfall) reducing it to 24 % with a feasible retrofitting involving the 8.3 % of the study area.