Regional-scale mapping of flash flood susceptibility using a simple and operational approach

Flash floods are among the most difficult natural hazards to deal with because they are usually associated with short, intense, and localised rainfall events in small, ungauged catchments. Their frequency and associated impacts have increased over the last few decades due to climate change impacts on extreme precipitation and land-use changes. The Northern Apennines River Basin District (Autorità di bacino distrettuale dell’Appennino Settentrionale - ADAS in Italian), which covers much of the river basins in Tuscany and Liguria, is particularly prone to such hazards due to its steep topography and proximity to the Tyrrhenian coast. Indeed, many flash flood events have occurred recently in these regions.

Because systematic observations of flash floods are scarce, especially in small river basins, regional-scale approaches are essential to support decision-making and to identify susceptible areas. In this context, the ADAS developed a rapid assessment procedure, known as the “Metodo Arno”, to map flash flood-prone areas at the management-district scale, which has been recognised as a best practice and adopted at the national level. Recently, a joint agreement between ADAS and the Department of Civil and Environmental Engineering of the University of Florence has been signed to further improve and harmonise the method.

In its recent version, the “Metodo Arno” is a multi-criteria flash flood susceptibility index derived from three indicators: the time of concentration and the average curve number of the river basin, and the return period associated with 50 mm in an hour. These three indicators are selected because they represent, respectively, the subbasin's response time, its propensity to generate runoff, and the frequency of extreme events. The indicators are normalised to a common scale and then combined using Simple Additive Weighting (SAW) to map the Flash Flood Potential index. All the river basins in the ADAS were delineated from a 10-meter-resolution Digital Elevation Model, with a minimum watershed size of 5 km². The time of concentration for each river basin was extracted using the Lekan software, which applies the more commonly used formulas in the literature. The return period associated with 50mm/h was estimated using local Intensity-Duration-Frequency curves. The curve number was estimated by combining information on the hydrological soil group and the Corine Land Cover. 

A flash flood database has been generated using an automated online news search with keywords to identify locations where past flash flood events occurred. The database is used to validate the procedure. The results show that the approach can map river basins prone to flash flooding. Future steps will focus on assessing the impact of climate and land-use change on the Flash Flood Potential Index.