# Flood Monitoring Based on the Study of Sentinel-1 SAR Images: The Ebro River Case Study
## 4 Results and Discussion
### Photointerpretation with Orthophoto Results  
By analyzing the historical data of flows and levels in 2018, we can identify when the maximum annual flows of the Ebro were reached in the area of study. Specifically, they were reached in Novillas on 14 and 15 April at the Pina dam located downstream of the town of El Burgo de Ebro. In the villages between these two points, the maximum flow of the flood occurs between these two dates, migrating according to the direction of the river flow. In view of the data, it can be interpreted that the maximum flood day for the Ribera Alta was 14 April and that a comparison is possible between the RADAR image of 13 April and the orthophotography of the Ribera Alta. A comparison to determine both the flood areas was calculated through the result of the calibration threshold methodology and through photo interpretation of the flooded areas, and also calculation of the flooded surface for a specific day. Therefore, the areas in which the presence of water was perceived as flooding the fields were digitized, generating a vector layer on the orthophotography of the Ribera Alta. At the same time, the Ebro river was also digitized using the PNOA as a base map. For that purpose, the ArcGIS version 10.5 program was used to create the shape layers of the flood area and the Ebro river. In this way, the permanent river channel areas were discriminated from the temporarily inundated areas. The flood areas of the RADAR image and the orthophotography are combined in Figure 15. In this way, common flood areas were obtained. Figure 15: General map of comparative flooding between the RADAR image of 13 April 2018 and the orthophotography provided by the CHE on 14 April 2018 in Ribera Alta. To establish the subsequent comparison, both layers were evaluated to calculate the flooded area overlapping the extent of flood areas mapped from both data sources (Table 5). It should be noted that in the RADAR image, the maximum flood areas are located in the upper part of that section of the river, and that in the orthophotography, the maximum flood is in the lower part of the same study area. Considering flood mapping obtained from Sentinel-1 on 13 April 2018, and that flood mapping obtained from orthophotography one day after on 14 April 2018, the fast evolution of the flood areas is clear. The variability in the value of the flood area is mainly due to the fact that the quality of the RADAR image, which needed numerous treatments, was reduced. This generates a loss of resolution quality that, together with the fact that there were intensity values that were not perceived as flooded areas (tree areas close to the riverbed, mainly), causes a reduction in the calculated area. In October 2007, the European Directive 2007/60/CE was approved. This directive highlights the need to create flood mapping to delineate areas with greater exposure to these events and to generate hazard and flood risk maps. In accordance with the European Directive 2007/60/CE, Spain's water governance system has made it possible to guarantee security and flood management. The Flood Risk Management Plans are based on the flood hazard maps (calculation of the flood zone) of the SNCZI for the 10, 50, 100, and 500 year return periods. SNCZI is the official public source for flood hazard information produced by MITECO [66]. The tools used by Spain's government for the creation of the layers showing the flood return periods take into account the following: Hydrological studies that define the flows associated with the return periods considered in each moment; geomorphological studies, including a field cabinet analysis, LIDAR, or orthophotography images; historical studies; and a combination of the previous two and hydraulic studies, such as the generation of two-dimensional models [67]. However, they do not include SAR images as available base information to develop or validate the flood hazard maps.