Uncertainties in flood damage assessment under projected future extreme rainfall conditions: a case study in Northeastern Sicily

Floods are among the world’s most frequently occurring natural hazards, affecting more people than other natural disasters while causing enormous damage to the socio-economy, developments, and environment. Because of the increasing frequency of heavy precipitation events and storm surges, large areas are at increasing risk of inundation. Many countries, thus, are forced to spend millions of dollars every year to recover from the floods’ aftermath as well as on disaster prevention, mitigation, and adaptation. Over the last decades, these kinds of extreme events have presented a significant challenge in Europe, particularly in the Mediterranean region which experienced intense rainfall and flash floods. Many coastal urban areas in France, Italy and Spain have undergone severe damages and losses due to extreme rainfall events causing flash floods. This situation may further exacerbate due to the climate-change-driven impacts and intense human activities in the region.

As key components of risk assessment, modelling of hazard, vulnerability, and exposure are required to categorize the potential future damages and events. However, uncertainties in damage and risk estimation can be introduced from different sources such as model input data and model structure and parameters. Especially short-duration extreme events are often under-researched. Here, we focus on addressing uncertainties in the chain of multi-hazard risk assessment, particularly floods in the Mili and Santo Stefano di Briga Basins in the Northeastern Sicily. This study is a part of the “risk workflow for CAScading and COmpounding hazards in COastal urban areas” (CASCO) project, which aims to develop a framework to evaluate the damage as well as economic and human losses due to a series of several important natural hazards acting in a quick temporal succession: floods, earthquakes, tsunamis, heat waves, and landslides.

In this study, we use daily and sub-daily in-situ observations (2001 - 2022) and projected hourly rainfall from 8 ensemble runs of the EURO-CORDEX regional climate change projections under the RCP 8.5 scenario (2031-2060) to establish the intensity-depth-frequency (IDF) curves and drive a hydrological model for short-duration rainstorm events between 6 and 12 hours. The resulting flood depths, area, and velocities were obtained from 1D/2D hydrodynamic modelling. To model subsequent flood damages, we investigate different fragility curves in the literature relevant for Italian building classes. The exposure data are obtained from the newly developed European High-Resolution Exposure (EHRE) model (Nievas et al. 2023).

Our results show that in general, future rainfall extremes are projected to be more frequent and severe in the study area, leading to increasing flood hazard levels. As a consequence, damages in several areas are projected to increase as well. Overall damage estimation depends on the inputs at different stages of the modelling chain, which cause uncertainties and variability in the model estimations and resulting risk evaluations.  

Keywords: Extreme rainfall, Flood hazard and damage, Sicily, Uncertainty

Reference: 

Nievas, C. I., Kriegerowski, M., Delattre, F., Garcia Ospina, N., Prehn, K., Cotton, F. (2023): The European High-Resolution Exposure (EHRE) Model, (Scientific Technical Report STR ; 23/05), Potsdam : GFZ German Research Centre for Geosciences, 64 p. https://doi.org/10.48440/gfz.b103-23055