Fast-Track Documentation of the Alterations on the Landscape, before and after a Natural Hazard—Case Study: North Euboea Greece before and after Storms Daniel and Elias

3.1. Description of the Study Area and the Successive Disasters North Euboea is an area with a rich topography, which allows the development of various species of flora and fauna in both mountainous and lowland areas. The study area is limited to the Nileas, Kireas, and Voudouros rivers, the latter formed by the confluence of the former two [19]. In recent years, this area has repeatedly suffered significant natural disasters [20]. Since 2017, a large part of the population of plane trees in Greece has been affected by the disease Ceratocystis platani. Ceratocystis platani is a fungus that affects species like Platanus orientalis in Greece and other parts of Europe [15]. Genetic analyses show a clear similarity to the fungus found in other countries. The disease causes significant mortality in trees and seems to spread through root contact with water and possibly via contaminated pruning tools [21,22]. To minimize the risk of the fungus spreading in the EU, several measures are proposed. These include certifying Platanus plants intended for planting, informing affected developing countries about adding firewood to timber regulations and establishing regulatory frameworks for machinery [23]. Specifically, the appearance of the disease in North Euboea seems to have affected the environment and the river currents [15,24]. However, further studies are needed to evaluate its impact fully. In 2021, a severe storm called Zorbas (total rainfall 114 mm [25,26]) hit the area (Figure 1). In 2021, a major catastrophic fire that hit North Euboea severely impacted the environment and the region’s ecosystems [16]. A portion of the total area was affected, impacting river flows and flooding. Water retention decreased and soil characteristics were altered, thus resulting in the increase of the volume and peak of stormwater runoff. After that, the area was affected by flooding phenomena due to two major storms (Daniel and Elias) (Figure 1), which hit North Euboea in September 2023 [17,27], creating new problems in the already burdened environment of the island [28]. The extreme weather event “Daniel” (total rainfall 314 mm [27,28]) affected Greece, Turkey, Bulgaria, and Libya from September 4th to 12th, 2023. In Greece, particularly in Thessaly and Euboea, it caused severe damage due to river overflows and heavy rainfall, resulting in floods and material damage [29]. The phenomenon was exacerbated by the storm “Elias” (total rainfall 261 mm [27,28]) at the end of September 2023, which caused further destruction, especially in Euboea, with floods, landslides and damage to infrastructure [30,31,32,33] (Figure 2) causing changes to the natural landscape, water flow, and the formation of the riverbed. In the narrowing of the river under the bridge in Figure 3, during the Zorbas storm, the water level was higher (by about half a meter) than during Daniel and Elias, even though the rainfall was of lower intensity. The analysis of flood plains in satellite images shows that the flood plains of Zorbas were more expanded than the flood plains of Daniel and Elias (Figure 4, Figure 5, Figure 6 and Figure 7 ). This occurred because, during Zorbas, the presence of plane trees slowed the water flow, preventing it from draining quickly. In the case of Daniel and Elias, the plane trees (and the forest) were absent. Although a much larger volume of water likely came through, it drained away quickly (along with debris and the riverbank slopes, expanding the active cross-section of the river). While Daniel and Elias caused significant damage and eroded the riverbanks, the floodplain area was smaller. 3.2. Description of the Mapping Approach Currently, much of the engineers’ research is done via computer using models and other computational tools. This capability, although largely promoting science, also harbors risks as engineers could not feel the spirit of the place (Latin: genius loci) [38] of the area of interest. One way for the researcher to confirm the validity of their study is through on-site field research of the area, taking insights from the inhabitants [36]. Acknowledging the risk of riverbed alterations, studies have been made [15,17] to quantify the conditions of the riverbed and highlight potential risks before the events of Daniel and Elias. To describe the initial (i.e., prior to the storm’s occurrence) soil conditions, the first step involves examining the representation of the study area on Google Earth and comparing maps from its archive. This gives a general idea of the changes over the years. Such images were also used to assess the landscape’s vulnerability in the Nileas watershed and to estimate the impact of the disease on plane trees. Historical maps also provided information on potential changes in the flora along the riverbeds during floods, indicating possible points of failure. Then, through Geographic Information Systems (GIS) [37], the characteristics and morphology of the Nileas river basin were analyzed. This analysis revealed features such as the basin’s area, the main watercourse length, and the ground slopes, which help further analyze the area’s hydrological data. On-site inspections were conducted in April 2023 and after the occurrence of Daniel and Elias storms [39,40], a second round of on-site inspections was also undertaken in November 2023. Visualizing these changes enhances our understanding of the river’s floodplain and its evolving dynamics, enabling more effective data processing to mitigate the impacts of such events. However, the on-site inspections were conducted on the ground without the aid of drones, which limited the scope of the observations. In this case study, we present the follow-up stage of the research, which involves mapping the ground in a three-dimensional (3D) model using drone and photogrammetry for a fast-track monitoring of the effects of the events. 3.3. Analysis Selected parts of the affected area were initially recorded through multiple consecutive images with a low-cost commercial drone. These photos were then processed through the online application KIRIENGINE, which uses photogrammetry to create three-dimensional models. The drone photos were uploadedto the online platform of KIRIENGINE and processed to create 3D models. Once the photos were imported, the application processed the data with photogrammetry and exported the 3D models, which enabled the comparison of similar views of the model with the affected area. The last step may also be viewed using the open-source software Blender [41]. The aforementioned procedure provides additional insight into the affected area and input for a rapid assessment, hence facilitating its study by the engineers. The selected areas for the composition of 3D models are near the village Kerinthos in North Euboea [42], depicted in Figure 8, an example of the results in position 5 is depicted in Figure 9, Figure 10 and Figure 11 and the results of positions 1–5 are depicted in the supplementary material. Figure 9 shows the area by the archive of Google Earth (before the storms Daniel and Elias) and Figure 10 and Figure 11 show the creation of 3D models in obj file viewer and Blender, respectively, by the same areas after the storms Daniel and Elias. 3.4 Results The key findings for the study area before and after storms Daniel and Elias are as follows:

• Even though storms Daniel and Elias were more intense than the Zorbas storm, the floodplain was smaller. The riverbed had significantly widened during the Daniel and Elias storms.
• Significant alterations were found within the riverbed width, especially at the lower altitudes of the basin. This is associated with the pre-existing conditions of local fauna and, in particular, the problems local plane trees face due to disease.
• A significant sediment accumulation was noticed in the delta of the Voudouros River. It is indicated that the landscape changes of the basin and the riverbeds have potentially led an increasein sediment transfer due to flood flows.
• Changes were found in the landscape along the river and the adjacent beach, all connected to the above-mentioned natural phenomena. Those changes are correlated with the basin’s behavior during flooding, which is at the forefront of this research andthe aesthetic, cultural and natural character of landscapes as perceived by locals and visitors.
• A final result that is secondarily associated with the research is the profound destruction of arable land and road collapses in several locations, an important strike towards local infrastructure.

Overall, the mixture of technologies that were utilized in combination with site visits offers the potential for a fast-track but detailed recording of landscape changes associated with flood events. This provides important feedback for potential future hazards in the area, e.g., in our case, the riverbed erosion, increased sediment flow, etc., andregarding measuring and characterizing local impacts.