## 2 Study Area and Flood Events  
### Study Area  
The catchment of the Lilas River extends to 249 km2 in the administrational area of two municipalities on the island of Evia in the eastern part of mainland Greece (Figure 1). The most important residential areas are the Vassiliko town and the Lefkadi, Filla, Afrati, and Pourmos settlements, aggregating together with other smaller settlements, with a total population of approximately 16,500 people [19]. The larger part of the catchment is characterized by natural vegetation, mostly forested land. However, a significant portion (mostly the middle and downstream parts) is used for agriculture. The plain of the Lilas River (so-called Lelantine Plain), formed near the outlet of the catchment, has been famous for centuries for its agricultural value and has even been contested among different city-states in Ancient Greece. The Lilas catchment drains the south and southwest slopes of the Dirfi mountain (1.743 m a.s.l.) and other Messapios mountains, initially through rugged morphology that gradually shows lower slopes toward lower elevations. The drainage network has an area of 259.19 km2 and presents a higher density upstream, as in the lower elevations the plain and the main channel have been subject to flooding in the past (Figure 2)  
Figure 1: Map of Evia Island in the eastern part of mainland Greece. The red frame indicates the study area affected by flash floods in 2009 and 2020. Previous events in the area occurred on 19 September 1924, 23 December 1952, 10 October 1953, and 6 November 1961. Past flood events damaged mostly agricultural land, a few residential buildings in and around Vassiliko town and Lefkadi settlement, and several road network elements, including bridges. In Figure 3, detailed maps illustrate the distribution and types of land use within the Lilas River basin during two distinct periods: before the 2009 and before the 2020 floods. These visual representations allow for a comparative analysis of the basin's landscape over time. The dominant features of the area include extensive natural vegetation and agricultural land, which constitute the majority of the land cover. Additionally, there is a relatively smaller area of urban development concentrated near the basin's outlet, where the river meets its endpoint. Upon closer examination and comparison of the data derived from the CORINE Land Cover datasets from 2009 and 2018, it becomes evident that there have been no substantial changes in the patterns of land use over the years, as outlined in Table 1. In both examined years, the majority of the drainage basin was occupied by \"transitional woodland-shrub\", accounting for 18.341% of the area in 2006, which increased to 20.123% in 2018. The second most significant land cover category within the drainage basin was \"sclerophyllous vegetation\", which constituted 19.383% in 2006 but exhibited a slight decrease to 19.093% in 2018. The category with the smallest land cover percentage, standing at 0.01% for both years, was \"industrial or commercial units\". Furthermore, it is noteworthy that \"bare rocks\" were identified as a new land cover category in 2018, occupying 0.120% of the area, whereas they were absent in 2006. This stability suggests that the region's primary uses--natural ecosystems, farming areas, and limited urban infrastructure--have remained largely consistent, without significant transformations or disruptions. This continuity provides a foundation for understanding the environmental and anthropogenic factors influencing the basin during the two examined periods. Figure 2: (**a**) Map showing the flood-affected Lilas catchment, along with its drainage network and the elevation reaching over 1730 m a.s.l. in the wider mountainous area comprising the Messapios and Dirfi Mts located north of Lilas. (**b**) Map illustrating the lower part of the Lilas catchment, which was mainly affected by the 2009 and 2020 flash floods.