## 4 Results
### Application of the FFISS in the Study Area
Even greater impacts were observed on the geomorphology, including the deposition and erosion of soil material with a thickness of less than and greater than 3 m, mainly on the riverbed and along the riverbanks, respectively. This resulted in changes to the morphology of the riverbed and the river delta (Figure 11). Projecting the UFIEs as points in the GIS allowed the compilation of thematic maps for each impact category, as well as severity maps that illustrated the range of severity classes found across the flooded area. Table 3 shows the number of UFIEs identified by category and severity class for both flash floods. Furthermore, Figures 12 and 13 provide an in-depth visualization of the frequency distribution of the severity classes. A detailed analysis of Figure 12 reveals notable trends in the built and natural environments for the years 2009 and 2020. In the built environment, the year 2020 recorded the highest frequency of severity in Class III, with a total of 104 events. This represents a significant concentration of low impacts within this category. Conversely, in 2009, the most frequent severity level in the built environment was observed in Class V, with 15 recorded events, highlighting a stark difference in the distribution of more extreme impacts between the two years. Turning to the natural environment, the patterns of the severity distribution also present noteworthy differences. In 2020, the highest frequency of severity was observed in Class IV, with a total of 116 recorded events, indicating a substantial number of higher-impact occurrences. Similarly, in 2009, the natural environment also exhibited its highest frequency of severity in Class IV, though with a comparatively lower count of 57 events. This consistency in the Class IV dominance, despite the difference in magnitude, underscores a persistent vulnerability of the natural environment over the analyzed years. Figure 12: Graphical representation of the frequency of occurrence of the severity classes for the built environment in 2009 and 2020 (**a**) and the natural environment in 2009 and 2020 (**b**). Figure 13: Graphical representation of the frequency of occurrence of severity classes for the mobile objects (**a**) and human population (**b**) in 2020. In Figure 13, in the analysis, it is observed that in the year 2020, the highest frequency of severity classes for the sector of mobile objects is recorded in Class IV, with a total of 27 events. This indicates a significant concentration of impacts within this specific severity class for mobile objects during that period. Similarly, for the human population, the highest frequency of severity is recorded in Class III, with a total of 48 events, highlighting the moderate level of impact experienced in this category during the 2020 flood. It is important to note that no impact data were recorded for either the mobile objects sector or the human population during the flood event of 2009. This absence of data can be attributed to the lack of precise georeferenced information for these categories during that period. Overall, 886 unique flood impact elements (UFIEs) were identified. We identified 187 and 699 UFIEs for the 2009 and the 2020 events, respectively. Most of them were found in the area of the Lilas River delta, upstream of Vassiliko town and downstream of Filla settlement. Below, the maps of the impact severity spatial distribution are presented for both floods, including the impacts on the built environment (Figure 14), the impacts on the natural environment (Figure 15), and the impacts on mobile objects and the human population (Figure 16). In addition, we compare the maximum impact severity between 2009 and 2020 (Figure 17). Figure 16: Maps of the maximum flash flood impact severity for mobile objects (**a**) and the human population (**b**) only in the 2020 event. Figure 15: Maps of the maximum flash flood impact severity for the natural environment in 2009 (**a**) and 2020 (**b**).