Counterfactual Analysis Applied to Flood Risk in Relation to Climate Change

Traditional flood estimation relies on statistics; however, climate change uncertainty can lead to the underestimation of potential extreme events. Therefore, this study proposes a methodology for analyzing local river flood sensitivity using the concept of “counterfactuals.” This involves tracking potential catastrophic floods using a previously recorded extreme event in the area and assessing its impact. The application is demonstrated by evaluating the flooding of the Onyar River caused by the Glories storm in 2020 in an urban area of Girona, Spain. The hydraulic model was built using IBER 3.1, and counterfactual scenarios were created by considering changes in three main parameters of the original hydrograph: maximum flow, number of peaks, and duration. The results show that the magnitude of the peak flow is the primary parameter conditioning the flood’s effect in Girona, and six equations have been obtained that relate the flood effects to peak flow and affected area.