Aree urbane e acque meteoriche. Un approccio integrato per la pianificazione della città resiliente

Abstract

Urban pluvial flooding is stressing urban areas with increasing frequency, becoming a factor of great concern. Soil sealing resulting from urban development is one of the main reasons for changes in natural hydrological processes and related recurring failures of urban drainage systems, especially during heavy rainfall events. Today, several studies are looking at the concept of urban resilience as a new paradigm, for a better integration of issues of water and flood risk with urban planning. Resilience is viewed as a way to tackle risk, showing bonds with its different sections, among which the flood hazard. It is broadly agreed that spatial planning, by incorporating Sustainable urban Drainage Systems (SuDS) within tools and polices, helps to build urban flood resilience. In particular, SuDS, as alternative strategy for surface water management, could potentially address anthropogenically generated hazard, thanks to the water-flow regulating service and benefits they provide. This research explores the relationship between the risk of urban pluvial flooding, resilience and urban planning. Particularly, the concept of resilience is clarified in order to highlight how it contributes to both analyse urban systems by adding levels of knowledge, and steer planning and policy approaches towards the mitigation of pluvial flood risk. By applying a research methodology based on the use of EPA-Storm Water Management Model (SWMM), the main aim of the thesis was to define a proper methodology and to build-up an analytical tool in order to analyse and assess the urban system s response to rainfall events, and to be used during and for purposes of the planning process. The proposed methodology is open to be flexibly applied to aptly handle the previous issues. Accordingly, the purpose was twofold: to assess the impact of masterplan in terms of increase of flow peak releases from urban catchments concerned by planned urban developments; to examine the urban system s reaction to rainfall and to evaluate how the response is affected by SuDS implementation at the catchment level. Case study areas were selected in the cities of Catania and Avola, in Sicily, for which masterplans design has been recently proposed by local planning authorities. Simulation of scenarios were carried out for a number of design storm events of selected return periods. Input parameters for the modelling were derived from urban analyses and hydrologic analyses and processing. Firstly, the methodology was based on the comparison between pre- and post-development catchment release scenarios and was applied to a case study catchment in the southern part of Catania. The study showed the need of careful consideration of the hydraulic invariance principle in land use planning practices. In particular, a set of flow release restrictions were determined for new areas of development, achieving the condition of unvaried flow peaks at the sub-catchment level, for different return periods of the storm-water event. Secondly, the methodology was applied to selected urban catchments in the centre of Avola. SWMM was used to track the quantity of runoff generated within each sub-catchment, and the flow rate and flow depth of water in each pipe in order to profile the system response to rainfall-runoff simulation. A dual-drainage approach was used to simulate the interaction between the minor and the major drainage systems and to obtain local flood characteristics to be mapped. Moreover, different effectiveness of selected SUDS measures were demonstrated in terms of improved water-flow regulation service and flood hazard mitigation, by comparing scenarios of pre- and post-implementation. Thesis discussion reflects the need for planning emphasis on mitigation and translating the understanding about risk, resilience and sustainable drainage into decisions via effective policy mechanisms. Suitable tools are needed to encourage a drainage-sensitive urban development and retrofitting.