Densely populated areas are frequently affected by floods, risking people's safety and economic activities. In Milan, Italy, the Seveso river crosses the urban area mainly in close conduits frequently flooding. The sprawling of urban areas combined with the intensification of extreme storm events increase the frequency of floods requiring pursuing a new approach on urban water management. The solutions must be sought not only on structural facilities directly on the river with large-scale dimensions: they present expensive construction and operation costs, and only give an apparent sense of security in a short period. It is necessary to identify natural-based strategies for the fluvial territory management taking a comprehensive view on watershed scale, moving from a traditional local and monothematic approach to a global and multisectoral towards water sensitive cities. This research aim is to assess some effects arising from the applications of river restoration and sustainable urban drainage techniques on a stretch of the Seveso river within Parco Nord, in particular through measures of parking de-waterproofing, improvement of river natural expansion and morphology re-naturalization and diversification of riverbanks and riverbed. To assess the effects, a 2D flow simulation using Hec-Ras and the recalculation of the river functionality index have been conducted. The results show benefits not only in raising better water and environmental quality, thanks to the enhancing of river functionality level, but also in risk mitigation, with the reduction of floodable areas, above all significatively for the storm event with a return period lower than 10 years. This research confirms the validity of the new approach and constitutes the first step towards the creation of a practical guide tool for the watershed management with similar characteristics to that of the Seveso river, to reach the European Directives' requests and to build up a strategy for adapting to climate change.

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 local 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 reliable strategy for the transition towards water sensitive cities also in very dense urban contexts as in the Milan area.

The traditional approach of stormwater management in urban areas frequently affected by floods has proven to be unsustainable for the growing urbanization scenario as well as not adapted to provide climate change resilience. In the spirit of water sensitive cities, a stormwater management change is necessary, moving from pipe-based conveyance system to solutions able to manage at its source by small scale structures sprawled over the entire watershed, in a collection of systems and practices known as SuDs (Sustainable Urban Drainage Systems). These techniques provide several benefits not only related to risk mitigation but also to water quality and ecosystem services enhancement that make cities resilient also in terms of climate change. Currently stormwater and land use regulations mostly define a flow-limit to reduce discharge on the sewer systems. In this way, however, the search for further benefits is not encouraged. Flow-limit restrictions are obtained through storage control systems designed with design-storm methods that neglects the effect of pre-filling reducing the storage capacity before a new rainfall event. This research compares different stormwater management solutions for a new developed area located in Bresso municipality, in the metropolitan area of Milan (Italy), within the Seveso River watershed. The proposed scenarios go from no intervention to completely avoiding discharge on the sewer network with the implementation of stormwater control structures (permeable pavements, green roofs, retention basins, among others). The storage facilities are designed with a traditional design method and then their performance is evaluated with rainfall-runoff and routing model continuous simulations, also used to compare the different scenarios. The simulations confirm that is possible to reach more restrictive goals beyond the flow-limit regulations using stormwater controls structures that also provide multiple benefits and have significant effects in terms of hydraulic risk mitigation at watershed level. Continuous simulation shows that the effect of chained rainfall events can however reduce controls performance and should be considered when designing.