Capabilities of satellite radar measurements to map large-scale flooding: Satellite flood mapping

In October 2023 the German coasts of the Baltic Sea experienced a rare storm flood event. Significant infrastructure damages have been caused directly at the coast and further inland. Furthermore, some of the coastal protection infrastructure did not withstand the water pressure and/or duration of the flood everywhere and failed in parts. As a consequence, some protected low-elevation areas got flooded as well. There the retreat of water is hampered and they remained flooded even after the Baltic Sea water levels returned to normal. In terms of risk these areas require a different attention than places directly at the coastline.

While locally the extent of the flooding has been witnessed and recorded in detail, assembling the big picture in a timely fashion across the landscape is be difficult. Space-borne remote sensing can be of service here. In this study I demonstrate the potential of detecting and mapping the flooding of land surface with space-borne synthetic aperture radar images and methods of change detection as well as image segmentation. While for the October 2023 flood, space-borne radar images have only been recorded 18 to 20 hours after the peak of the flood and when the open sea water level had already returned to normal, a large area evidently remained flooded and could be mapped. Among them are several large flooded areas along the shores of the Schlei estuary up-river to the town of Schleswig and much further east between the mainland and the Darß peninsula. Notable is as well that even 12 days later, on November 2, a not negligible part remained flooded.

In this study, I augment the detections of flooding with complementary and open spatial data that provide information on elevation and infrastructure. The methods presented are not new, but show a general applicability and large potential if exploited. With rising sea levels, storm floods will likely occur more often with such severity. At the at the same time we may expect the means of satellite observations to improve in the future.