Monitoring the 2020 storm Gloria in the Ebro Delta (Western Mediterranean) using Earth Observation data

Coastal hazards and extreme events are increasing in frequency and severity due to climate change, making the littoral zone even more vulnerable and requiring continuous monitoring for its optimized management. The Ebro Delta ecosystem, located in the NW Mediterranean, was subject to storm “Gloria” in the winter of 2020, the most severe coastal storm registered in the area in decades. In this study, Landsat-8 and Sentinel-2 satellites were used to monitor flooding impact and water quality status, including chlorophyll-a, suspended particulate matter, and turbidity in different time slots to evaluate pre-, syn-, and post-storm scenarios. Image processing was carried out using the ACOLITE software and the on-the-cloud Google Earth Engine platform for water quality and flood mapping, respectively, showing a consistent performance for both satellites. This cost-effective methodology allowed us to characterize the main water quality variation in the coastal environment during the storm, as well as to detect a higher flooding impact (7311 ha) compared to the one registered 3 days later by the Copernicus Emergency Service (3672 ha) regarding the same area. Moreover, the time series revealed how the detrimental impact on water quality and turbidity conditions was restored two weeks after the extreme weather event, and no phytoplankton blooms appeared during the study period neither in the Ebro Delta nor in adjacent regions. These results, obtained within the EuroSea project (H2020 grant agreement No 862626), demonstrate that the used workflow is suitable for monitoring extreme coastal events in the Ebro Delta using open satellite imagery at 10-30 m spatial resolution and four-day revisit time, thus providing valuable information for early-warning to facilitate timely assistance and hazard impact evaluation. The integration of high-resolution remote sensing tools into ecological disaster management can significantly improve current monitoring strategies, supporting decision-makers from the local to national level especially in prevention, adaptation measures, and damage compensation.

Keywords: Remote Sensing; Water Quality; Flooding; Coastal Management; Sentinel-2; Landsat-8.