Sustainable Drainage Systems as an adaptation strategy to climate change consequences: a case study in Northern Italy

Urban sprawl and climate change effects are definitely the leading cause of flooding phenomena within the urban environment. With its countless number of cities characterized by a high degree of hydro-geological hazard, the Italian territory often deals with the inability of traditional sewer systems to manage storm water in a sustainable and effective manner. Worldwide, integrated strategies that involve the use of Sustainable Drainage Systems (SuDS) to reduce the surface runoff generation, among a multiplicity of benefits, appeared more and more popular since the early 2000. An assessment in terms of stormwater management benefits deriving from the implementation of SuDS technologies in a 290 ha area belonging to Sesto Ulteriano (Milan, Northern Italy), was presented to quantitatively measure SuDS ability as an adaptation strategy to climate change. The actual configuration of the drainage network and the SuDS project proposal which provides a retrofitting of about the 10% of the impervious surface using different techniques (rain gardens, trench drains, pervious parking lots and cisterns-planters), was simulated using the SWMM5 model and was here tested under event scale rainfall time series. Statistical analyses on historical rainfall data sets typical of the study area, covering together the timeframe from 1858 to 2019, provided identification of temporal trends in precipitation extremes in order to design potential climate scenario within a 30 years future time window. In particular, precipitation trend analysis results suggested that a clear percentage increase in 24 hours (26%), 1 hour (4%) and 30 minutes (13%) rainfall extremes was registered for different stations surrounding the case study area since early 1900. Conversely, cumulative annual rainfall displayed more evidently a decrease (5.50%). Starting from these results and assuming a short rainfall events intensification on one side and long term precipitation reduction on the other side, design hyetographs under current (2020) and potential future climate condition (2050) were derived. Current and future rainfall scenarios were used to force the combined hydraulic-hydrological model of the urban drainage network, both accounting for a “traditional” scenario, where no SuDS practices are considered, and a “SuDS” scenario. Results were assessed in terms of reduction of maximum discharge and total volume discharged from the combined sewer overflow of the network. Both under current and future potential climate conditions, the implementation of SuDS techniques resulted in a notable reduction of both maximum discharge and total volume. As an example, with reference to short rainfall events and for return periods T= 5 years, the maximum discharge and total volume reduction, in the case of the current scenario, is respectively about 80% and 75%. However, in the case of the future potential climate scenario, the reduction percentages reduce to 70%, showing, at the same time, how SuDS can be valuable used to adapt to climate change conditions but that the resilience they provide in terms of stormwater management issue would be much more sensitive to climate input in the next future.