Documentaion and analysis of the effects of natural disasters: The dynamics of the flood zone in the river Nilea in Northern Euboea

The objective of the present study is to investigate the hydrological and environmental impacts of flooding phenomena in the Nileas River basin in Northern Evia, with particular emphasis on the damage caused by the storms Daniel and Elias, as well as the consequences of wildfires and the disease affecting plane trees. The comparison with Cyclone Zorbas highlights the differences in the flood response of the area to recent extreme weather events. Although the storm Elias brought less precipitation compared to storm Daniel, the floods were more intense, indicating a significant change in the behavior of the floodplain due to vegetation loss and the accumulation of sediment. The destruction of vegetation, particularly of plane trees, due to the disease Ceratocystis platani and wildfires, resulted in a significant reduction in the soil's natural capacity to retain water, leading to increased runoff and erosion. The study includes detailed field surveys conducted in the study area, documenting changes in topography and hydrological conditions following each catastrophic event. Additionally, the Rational Method was applied to calculate runoff during intense rainfall events, providing critical data for understanding the hydrological impacts of vegetation loss and the alterations that have occurred in the landscape. The analysis of Geographic Information Systems (GIS) constitutes a key component of the study, offering a comprehensive mapping of the physiographic characteristics of the area. Using GIS tools, changes in vegetation, areas at high flood risk, and land-use changes were mapped. This analysis enhances the ability to predict flood events and identifies areas requiring immediate intervention. Subsequently, the study simulates flood conditions using the HEC-RAS model and satellite imagery from Sentinel-1, analyzing the increased vulnerability of the area following the recent wildfires. The results of the simulation reveal the significant impact of the loss of plane trees on the hydrological behavior of the basin, which, combined with erosion and sediment accumulation, increases the risk of flooding even with lower rainfall amounts. The paradox of recent floods, where smaller amounts of rainfall resulted in a greater flood response, underscores the need for targeted preventive measures and restoration strategies. Water management, vegetation restoration, and the development of practices to mitigate environmental impacts are vital for the sustainability of the area.