## 3 Methodology  
### Flash Flood Impact Data Collection  
Flood impact data for both flood events were collected from several sources, which varied from official damage reports from local, regional, and central authorities to imagery collected from daily and periodical press and online sources. More details are presented in Table 2. Regarding the evaluated imagery, the major portion of ground and aerial observations of impacts were collected during post-event field surveys conducted by the authors shortly after the completion of the destructive events. The field reconnaissance comprised the deployment of consumer-grade unmanned aerial systems (hereafter UASs). The collected disaster field data contained detailed descriptions of the damage location, type, and extent. Geo-tagged imagery was used in all cases. A UAS offers significant potential for improving the monitoring of diverse environmental parameters, including the collection of data on hydrogeomorphic events. Their ability to deliver high-resolution images and videos on demand makes them particularly valuable for studying flash flood events, which  
Figure 6: Typical views of the flash flood impacts along the Lilas River course due to the September 2020 event (**a–d**) and along the coastline (**e**,**f**) using UAV-derived imagery. (**a**) Erosion of the riverbanks and flood deposits and debris accumulated in the riverbed upstream of Villa settlement. (**b**) Large vehicles and machinery comprising trucks and bulldozers at a site close to Africa swept away by the floodwaters, along with erosion of the riverbank and collapse of a bridge in Africa settlement. (**c**) Erosion of the riverbank and destruction of protection works, along with erosion close to the bridge piers in Vassiliko village. (**d**) The bridge in Africa settlement suffered partial collapse. (**e**) Roads and plots flooded in Lefkadi settlement. (**f**) Erosion along the coastal zone of Lefkadi. occur on spatial and temporal scales that are often unsuitable for conventional monitoring methods. The research team employed a UAS (specifically, a DJI Mavic 2 quadcopter) with a flight duration limit of 20 min. The flight operations were strategically planned and executed to maximize coverage of the affected area and to gather comprehensive impact data, including from regions that were challenging to access [24,25]. These aerial surveys were complemented by detailed field surveys conducted on the ground, ensuring that both large-scale and localized impacts were documented with precision. This dual-method approach provided an unparalleled level of detail, enabling a nuanced understanding of the flood's effects on infrastructure, properties, and the environment. By integrating UAV technology, the data collection was significantly enhanced, capturing the spatial extent of damages in inaccessible or hazardous areas. The mapped data formed a robust foundation for understanding the varying degrees of the impacts across different locations, from urban settlements to rural landscapes and critical transport routes. Using the unique flood impact elements (UFIEs) scale provided a structured framework for categorizing and assessing the severity of damages, offering valuable input for disaster response planning and long-term mitigation strategies. This meticulous approach underscores the importance of rapid yet detailed data collection during the initial response phase, as it supports immediate recovery efforts and informs future resilience-building initiatives in flood-prone areas. The research process followed a structured approach that involved several key steps. Initially, fieldwork was conducted within the designated study area, where both ground-based and aerial observations were carried out. The aerial observations were facilitated using unmanned aerial vehicles (UAVs), which provided a broader perspective and detailed imagery of the region.