Flood hazard analysis for river systems

Flood hazard assessment and mapping is a necessary step to define flood risk reduction strategies and to develop risk management plans. Anyway, in Italy, and in particular in Lombardy Region, legislation provides only vague indications on how to assess flood hazard, therefore the definition of risk is lacking in a scientific basis, and wide space is left to subjectivity and to approximate analyses. This PhD research aims to improve the topic presenting an approach for flood hazard analysis and mapping that fits the Lombardy Region legislative framework, but introduces a level of experimental modelling. The approach has been applied on an area located in the medium Valtellina (Alps, northern Italy) – 26 km2 wide – and makes use of advanced flood modelling tools, in order to support the development of Emergency Plans and to provide suggestions to deepen the analyses required for Urban Planning. Hydrologic and hydraulic conditions of the site are quite complex, and data availability is not optimal. Therefore, several modelling strategies (1D, 2D and combined 1D2D approaches) and three software packages (SOBEK, FLO-2D and FloodArea) were tested and results were compared and discussed. Lots of efforts have been spent in trying to define an accurate topographical description: a TIN was constructed from available 3D cartography and cross sections profiles, then converted into a DEM. Institutional values of peak discharges for the return times of 20, 100 and 200 years were used to construct input hydrographs. Roughness coefficients were set according to literature tables and available local studies, and their influence on models behaviour was tested through sensitivity analyses. Difficulties related to some of the models and/ or verification of inappropriate results led to exclude two software packages and to select SOBEK 1D2D as the most suitable tool for flood modelling in the study area. Results were converted into hazard maps useful for both the purposes of Civil Protection and Urban Planning, basing on an innovative method, including an expression of uncertainty. Most of the complexities of the issue are analysed and discussed, referring to a wide literature background, which the research will contribute to enrich.