The sprawling of urban areas combined with the intensification of extreme storm events increases the frequency of floods. The Milan metropolitan area, Italy, is a glaring example of this condition, where floods occur on average two times a year and a new approach for urban water management is required. In recent years sustainable urban drainage systems (SuDs) have been spreading in many cities for lo- cal stormwater runoff reduction, in order to reduce the risk of overloading receiving water bodies and sewerage systems. These natural-based techniques also present advantages in terms of water quality improvement and in some cases also provide benefits at the ecological level. For an optimal river basin management, it is necessary to adopt holistic approaches including river restoration measures and SuDs for the hydraulic risk mitigation and the water and environmental quality enhancement. Previous research analysed the impact of re-naturalization measures on a stretch of the Seveso river on the flood risk for the urban area in Milan (Italy). Particularly, a potential reduction of 16% on the extent of the flooded area for 10 years return period events was assessed. In this case study benefits of additional measures of urban retrofitting based on parking areas de-sealing were investigated. To assess the effects, a rainfall-runoff process and a 2D flow simulation using SWMM and HEC-RAS software respectively were used. Results show, in particular, that transforming only the 2% of the Bresso municipality paved surfaces into permeable ones would achieve a reduction of both the peak and the volume of stormwater runoff, equal to 6.5% and 7.6% respectively, for 10-year return period events. Moreover, a consequent reduction of 3 ha of flooded areas at the river watershed-scale was observed. Management of flood risk by a combination of river restoration and sustainable drainage systems is then confirmed to be a reli- able strategy for the transition towards water sensitive cities also in very dense urban contexts as in the Milan area.
environmental quality enhancement, flood risk mitigation, river contract, river restoration, spatial planning, sustainable drainage systems (SuDs)
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