An integrated approach of ground and aerial observations in flash flood disaster investigations. The case of the 2017 Mandra flash flood in Greece

On November 15, 2017, a high intensity convective storm, reaching 300 mm in 13 h in the core zone of the event, hit the western part of the region of Attica in Greece, causing a catastrophic flash flood in the town of Mandra and a tragic loss of 24 people, making it the most deadly flood in the country, in a period of 40 years. The research team surveyed the area during and after the flood using a combination of systematic ground and aerial observations with the aid of an unmanned aerial vehicle (UAV), aiming to reconstruct the basic physical and hydrological characteristics of the flood and its impacts. The analysis produced detailed flood extent and depth maps that provided a comprehensive description of the physical characteristics of floodwaters across the inundated area. Peak discharge was estimated, using a UAV-derived digital surface model, at two locations, corresponding to the two main tributaries and indicated an impressive hydrological response, between 9 and 10 m(3)/s/km(2). Impact analysis on the basis of these observations showed an extensive diversity, including effects in geomorphology, vegetation, buildings, infrastructure and human population. Analysis of meteorological, botanical and geomorphological evidence lead to the conclusion that this flash flood was a very rare event. Results demonstrate that the combination of aerial and ground observations allow an enhanced and holistic reconstruction of a flash flood and its impacts with high accuracy, leading to the conclusion that the approach used has a significant potential in many aspects of flood disaster investigations.