Quantitative geomorphic analysis to evaluate flood hazards

The aim of this study is to evaluate flood hazards through the use of quantitative geomorphic methods, which were already applied to estimate mass movement hazards and accelerated erosion hazards in some Italian areas. The study area is the Fiume Ombrone drainage basin (Central Italy), where wide zones of the main valley floors have been recently flooded. In the last decade of the past century the frequency of these hazardous events has increased. Firstly, historical data and geomorphol ogical survey provided maps of flooded areas. Successively, a map overlay was performed between these maps and each thematic map representing the main factors of flood hazard (topography, surface drainage, climate, land use) expressed by suitable parameters. Map overlay allowed to weight the influence of each category of factors on the occurrence of flood events. To evaluate the effects produced by the simultaneous influence of different factors, the thematic maps of these factors were overlaid, in turn, on each map of flooded areas and the attributes of the different factor types were combined. Results obtained allowed the identification of zones prone to flood hazard and the evaluation of their hazard index. After this method, different maps of flood hazards were obtained, which show the spatial variation of the hazard index for flood events with different recurrence intervals.