Empirical flash flood vulnerability functions for residential buildings

The estimation of flood losses is crucial for the quantitative estimation of risk and the cost–benefit analysis of risk mitigation and prevention measures. Flood losses are usually estimated by means of vulnerability functions, which are often developed in a synthetic way by experts, since loss datasets collected after events are sparse or lacking and mostly refer to slow riverine inundations. Simple univariable empirical vulnerability functions are recognized as a more valuable tool with respect to synthetic curves developed for different contexts. The aim of this work is to derive empirical vulnerability functions for a flash flood event which occurred in Livorno (Italy) in September 2017, based on the hydrologic–hydraulic reconstruction of the flood and observed losses to residential properties. The hydrologic processes are reproduced with a distributed hydrologic model, and the flood propagation is simulated with a full 2D model. Many open GIS data are used and combined with virtual on-site inspection in the study area to analyze building exposure. Different regression curves are tested to fit the data and obtain damage curves. The results show (i) a poor correlation between relative or absolute losses and flow velocity, (ii) a low correlation between relative losses and water depth, thus confirming the existing literature. More significant correlation is found between absolute losses and water depths. The curves here derived provide a first attempt to develop damage functions for flash flood in Italy, which could be applied in similar urban and morphologic contexts.