Flood proofing measures cost-efficiency analysis for hydraulic risk mitigation in an urbanized riverine area.

Flood Risk Management Plans, established by European Directive 60/2007, implemented in Italy with D.L. 49/2010, provide for the priority implementation of structural and non-structural interventions in order to contain the risk of flooding in areas where there is a potential risk deemed significant; with the reduction of potential negative consequences for human health, the territory, assets, the environment, cultural heritage and economic and social activities. For a long time, flood prevention has been entrusted to structural interventions, which allow the flood managers to control and contain floods. In many geographical areas, however, the same interventions are not possible, either because of territorial constraints, or because of high costs, and are often not sustainable, especially in relation to a reasonable comparison between costs and benefits. This comparison then requires a correct assessment of the damages resulting from a flood which is often quite complex, as these depend both on the characteristics of the flood, in terms of water tie, current speed, possible presence of sediments carried by the flood, but above all by the characteristics of vulnerability and exposure of the assets located in the area at risk. In assessing hydraulic risk, it is essential to estimate flood damage, in order to identify the interventions that can be carried out as part of a costbenefit analysis. Through the following analysis we try to identify the cost and benefit components corresponding to each of the proposed solutions, in order to provide knowledge bases and significant interpretative elements for their judgment and feasibility. It is necessary to identify the degree of risk (hydraulic) in the current situation and after the implementation of the interventions, and which is the most convenient viable alternative solution. In the case of flood mitigation interventions, the costs and benefits can be characterized as follows: first of all, the direct costs, which coincide with the infrastructure costs aimed at mitigating flood interventions or to protect the inhabited centers and infrastructures, generally from floods. Secondly, in connection with these costs, the most important benefits are the defence of settlements and buildings against the danger of flooding. In this sense, an approximation of the (direct) benefits of the works for the hydraulic arrangement is given by the damages deriving from flood events which are avoided thanks to the aforementioned interventions. Therefore, the cost-benefit analysis makes it possible to assess in monetary terms the positive impacts (benefits) and negatives (costs) of the proposed policy actions, in order to calculate the net benefits deriving from the implementation of measures to mitigate the risk.