River basin planning: from qualitative to quantitative flood risk assessment: the case of Abruzzo Region (central Italy)

Flood risk assessments are becoming an essential tool for a rational decision-making in river basin planning throughout Europe. In order to comply with the prescriptions of the Floods Directive, quantitative approaches, based on the estimation of the expected annual damage as a risk indicator, should increasingly be used for flood risk mapping in addition to the traditional qualitative methods. In this paper, a comparative application of the two methodologies to the river basins of the Abruzzo Region (central Italy) was performed for evaluating their strengths and weaknesses. The analysis was limited to direct damage estimation. The results showed that qualitative mapping could not be considered a fully effective tool for the identification of appropriate flood risk management strategies due to its limits in representing the real flood risk scenarios, mainly related to the use of a coarse hazard classification. Therefore, if it could be used at the basin scale for a preliminary identification of high-risk areas, it should be integrated with a more analytical assessment of the expected losses with quantitative models. However, a sensitivity analysis on the quantitative methodology showed that the large uncertainties inherent in damage modelling (with difference factors ranging from 2 to 10) may hinder the reliability of the estimates, with possible repercussions on the results of economic appraisals. Two case studies on the implementation of flood risk reduction measures confirmed that damage model uncertainty had huge influence on the results of cost–benefit analyses, with net present values switching from negative to positive according to the selection of the damage functions and the economic values of the exposed assets.