## 2 Study Area and Flood Events
### The 2009 Flood  
On 11 September 2009, an exceptionally intense rainfall event hit the island of Evia, triggering flooding from several rivers, resulting in one casualty and leading civil protection authorities to take on search and rescue (SAR) operations. The area was declared to be in a state of emergency. After the event, coastal areas in the eastern and western parts of the island were devastated. The most severe impacts were recorded in the Lilas River catchment, including impacts on livestock farms and agricultural land, as well as buildings, roads, bridges, and other infrastructure. The total cost of the immediate recovery works was estimated at approximately EUR 12.4 M, while the total expenses for permanent rehabilitation and development of appropriate flood defenses surpassed EUR 21 M at the time [20].  
The 2009 flood was triggered by a high-intensity storm of limited spatial distribution, mainly centered above the central part of Evia Island. The Steni rain gauge recorded 376.8 mm in 18 h between the afternoon of 11 September and the morning of the next day. The Zarakes and Styra rain gauges recorded 56.6 mm and 51.8 mm, respectively (Figure 4) [21].  
The event underscored the area's vulnerability in terms of its infrastructure, vegetation, and human settlements. One significant impact was the collapse of a building's roof, likely due to structural instability exacerbated by prolonged water exposure and the force of floodwaters. The riverbed experienced substantial deposition, with sediment  
Figure 4: Map depicts the locations of the rain gauges of Steni, Seta, Zarakes, and Styra that provided rainfall measurements for the study area in 2009 and 2020.  
accumulation altering its natural flow dynamics. Low vegetation, integral to stabilizing the riverbanks and mitigating erosion, was unrooted, destroyed, and swept downstream, further aggravating the flood's intensity and impact. Erosion was widespread along the riverbanks, threatening their integrity and causing partial structural damage to properties near the watercourse. One house sustained heavy structural damage as erosion undermined its foundation, posing the risk of eventual collapse and making it unsafe for habitation. Beyond structural damage, the flood energy displaced vehicles, with reports of a car carried away by the forceful currents. This not only highlights the strength of the floodwaters but also points to the danger posed to lives and property during such events. Additionally, the flood transported debris, including branches and small to medium-sized objects, into the sea, contributing to marine pollution. The presence of such debris disrupts coastal ecosystems and raises concerns about the long-term environmental impacts of flood events. Collectively, these observations reveal the multifaceted nature of flood damage, encompassing infrastructure, ecology, and human safety. The event underscores the urgent need for integrated flood management strategies, including robust infrastructure, riparian vegetation restoration, and community preparedness to mitigate future flood risks and enhance resilience in the Lilas River floodplain (Figure 5).  
Figure 5: Typical views of the 2009 flash flood’s impacts on the natural environment and on several elements of the built environment (buildings, vehicles, etc.) in the flood plain of the Lilas River (from the river’s estuary and along the Lilas’ riverbed downstream of Vassiliko). (**a,c,d**) Damage to buildings due to intense erosion and undercutting phenomena, including exposure of the foundation (**a,d**) and total collapse of the building (**c**). (**b**) Flood deposits and debris within the riverbed after the completion of the destructive phenomena. (**e**) Vehicles were washed away by floodwaters. (**f**) Flood debris, comprising branches and small to medium-sized objects, observed in the sea after the flooding.