## 4 Results
### Comparison of Flood Impact Severity Between 2009 and 2020  
A comparative analysis of the impact severity for each event reveals both similarities and differences, shaped by environmental characteristics, localized land use changes, and the magnitude of the water discharge. In the sector of the built environment (Figure 14), the severity classes observed in 2020 are notably lower than those recorded in 2009 near the delta of the Lilas River, despite the higher water discharge and water stage associated with the 2020 flood (Figure 18). This discrepancy is primarily attributed to substantial changes in the morphology of the river channel after the 2009 flood, as the riverbed experienced significant widening, increasing its discharge capacity. Furthermore, it is important to highlight that in 2009, several buildings situated along the river channel and near its estuary were partially or fully destroyed by the flood (Figure 19). As a result, these structures were removed in the period after the 2009 flood and were no longer present in 2020 to be impacted by the floodwaters, contributing to the reduced severity values in the built environment during the latter event. Conversely, higher severity classes were recorded in 2020 near the settlements of Vassiliko, Filla, and Afrati compared to 2009. This increase is attributed to the greater volume of river discharge of the 2020 event, causing significant erosion, which exerted considerable pressure on the bridges in these areas, leading to structural damages and, in some instances, partial collapses (Figures 18 and 19). Figure 18: Drone view of the morphological changes and the riverbed widening from the flood of 2020. Within the context of the natural environment (Figure 15), a significant increase in the impact severity was recorded in the area extending from Agios Nikolaos to Afrati and along the Afrati-Pissonas road during the 2020 flood. The higher water stage and heightened discharge substantially influenced the river's geomorphology, resulting in significant deposition within the riverbed and extensive soil erosion along the riverbanks. Such geomorphological impacts were absent during the 2009 flood, primarily due to the lower discharge observed during that event (Figure 20). Regarding mobile objects and humans, no impact data were recorded for the 2009 flood event. However, we know that there were no fatalities in the Lilas catchment, indicating that the 2020 flood had a higher severity. Consequently, the available data and observations pertain exclusively to the 2020 flood (Figure 16). Notably, high severity classifications were observed near the Afrati Bridge, attributed to the presence of a business operating large trucks and bulldozers, which were swept away by the floodwaters (Figure 21). Figure 19: The riverbed and estuary morphology of the Lilas River prior to the 2009 flood (**up**), as well as the buildings destroyed during the 2009 flood due to the widening of the riverbed (**down**). Figure 20: A downstream view of the Vassiliko bridge, illustrating the estimated maximum water levels during the 2009 and 2020 flood events (indicated by white and yellow lines), based on the high-water marks identified during post-flood surveys. The blue arrow indicates the river flow direction. It is important to highlight that no impact data for mobile objects located near the river and its estuary could be recorded. This was due to these objects being carried away by the flood and transported into the sea, thereby rendering aerial observations using UAVs ineffective for their documentation. In the domain of the human population, low severity impact values were observed in the areas of Agios Nikolaos, Lefkadi, and Filla. It is important to emphasize that these impact records correspond to specific sections of the basin for which we were able to obtain data pertinent to this domain, ensuring they could be georeferenced and verified for accuracy and reliability.