Risk-based design of hydraulic infrastructures for flood risk mitigation using cost-benefit analysis

Abstract

Traditional infrastructure design methodologies for flood risk mitigation based on the concept of return time are now challenged by climate change, increased public attention towards the social and environmental impacts of this type of works and ultimately by the relevant financial costs required. While the use of appraisal tools such as Cost-Benefit-Analysis (CBA) is largely invoked to justify and prioritize large flood mitigation initiatives at a river basin scale (EU Flood Directive, 2007), the use of CBA as a design tool is far less widespread and accepted. This explains the favorable reception and the subsequent unding by the Italian Ministry of the Environment of the RIDES project, a two-year (2020 – 2022) research initiative carried out by three Italian Universities to develop and demonstrate risk-based methodologies for the optimal choice of structural interventions to mitigate hydraulic risk. The core of the methodology is a costbenefit analysis, which unlike traditional design methodologies uses the flood flow rate as the decision variable - instead than as a parameter– of an optimization problem where the objective function Is the maximization of net benefits, i.e. the difference between the benefits generated by the infrastructures in terms of reduction of expected damage (NRC. 2000, Tsakiris 2014) and the internal (investment, operation and maintenance) and external (environmental) costs to build the infrastructure. This paper summarizes the main outcomes of the project concerning the design methodology; however, the project also included an extended survey of the most widespread structural interventions in Italy and their classification by area of application (urban, peri-urban, river, mountain) and by type of flood event (urban flood, river flood, pluvial flood) that was carried out using the RENDIS database. In addition, the project has produced a probably unique nation-wide database of investment cost appraisals for the construction or implementation of many types of structural elements such as embankments, concrete channels, gabion mattresses, large-section excavation, whose combination gives rise to mitigation infrastructures such as levees, large off-stream retention tanks etc. Costs of the more complex elements such as channels and embankments were appraised by typifying the element and developing an analytical assessment based on its geometry and on the prices of the inputs required (labour, machinery, materials). These results are available for the ten largest Italian regions and highlight some significant differences in unit prices, due to different costs of some inputs.