Proposal of a flash flood impact severity scale for the classification and mapping of flash flood impacts

Flash floods cause some of the most severe natural disasters around the world. The extensive diversity and discontinuity of flash flood impacts, which are controlled mostly by surface properties, lead to major difficulties in obtaining a holistic appraisal and a realistic overview of flash flood effects and their severity and make predicting future impacts a significant challenge. Current practices of describing flood impact severity use a limited set of criteria and assign a qualitative characterization (e.g. major, catastrophic, etc.) to each event. The present study proposes an approach that provides a coherent overview of these effects through the classification of impact types and severity and for the first time mapping their spatial extent in a continuous way across the floodplain. To this end, the flood effects are grouped into 4 categories depending on the affected elements, namely: (i) impacts on built environment (ii) impacts on man-made mobile objects, (iii) impacts on the natural environment (including vegetation, agriculture, geomorphology, and pollution) and (iv) impacts on the human population (entrapments, injuries, fatalities). Each of the four above categories is classified in a system of 10 severity classes that are defined with a logical order of increasing importance of damages forming a severity scale. The scale's application is tested on a catastrophic flood event of 2017 in Mandra, Greece, with a welldescribed and wide range of impacts, including 24 fatalities. The application allowed the development of high-resolution impact-severity maps that revealed interesting damage patterns and highlighted high severity areas. The resulting maps offer insights on future impacts and indicate a potential to provide the groundwork for targeted prevention measures as well as correlations of impact severity with hydrological aspects of flooding.