Boundary and Estimation of the Flood Development Velocity Using Unmanned Aerial Vehicle (UAV)-Derived Imagery and Ground Observations: The Case of the 2017 Mandra Flash Flood in Greece. Bulletin of the Geological Society of Greece, Sp. Pub. 7, 15th International Congress of the Geological Society of Greece, Athens, 22-24 May, 2019.

The UAV allowed the collection of aerial photographs in an extended area during flood flows despite that a large portion of it was inaccessible due to road closures and safety issues. This way a large part of the useful data was captured in short time during and after the flood. Especially in the case of flash floods which are characterized by a rapid rise and withdrawal of floodwaters, it is virtually impossible for a field survey team to reach different parts of the inundated area and make ground observations during the actual phenomenon. As flood effects and flood marks of all kinds (e.g. high water marks, flood deposits) were removed during cleanup efforts in the days following the disaster, there was limited time to cover the whole affected area with detailed ground observations. In this case, the integration of aerial observations in post-flood analysis provided guidance to intensify ground observations at key locations where it was needed, in time before crucial evidence disappeared. This work demonstrated that the combination of aerial and ground observations allowed an accurate reconstruction of the 2017 flood, contributing to flood extent and water depth determination, peak discharge estimation and detailed mapping of impacts. The approach is considered to have several advantages connected with the collection of data during flash flood investigations and is capable of providing a holistic overview of multiple aspects of a flood that can be valuable to both science and civil protection. Aerial observations can act as an extra point of observation that accompanies the traditional ground-based field survey and fits well in the opportunistic nature of this type of studies.