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Copernicus Sentinels ally to monitor floods in Northern Italy

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Italy has been victim of days-long flooding and high-tides, owing to the severe weather of the past weeks. Although forecasts do not promise an improvement just yet, the Copernicus Sentinels have teamed up to help monitor the situation.

From  mid-November Italy has experienced particularly large amounts of rain, or snow, respectively, in the Southern Alps, which led to wide-spread flooding in the lowlands between the Alps and the Mediterranean Sea.
In the Laguna of Venice, the situation was aggravated by southern winds (the so-called Scirocco wind coming in from the Sahara) that pressed additional water into the laguna, leading to record high water levels in and around Venice.

Optical satellite instruments such as those on the Copernicus Sentinel-2 satellites cannot see through clouds, which, for instance, covered Northern Italy most of the time during the first half of November 2019. Cloud-free scenes over Northern Italy from Copernicus Sentinel-2 are only available for 26 October and 10 November 2019.

However, the two advanced radar instruments aboard the Sentinel-1A and Sentinel-1B satellites of the European Union's Copernicus programme penetrate through clouds and are able to provide data regardless of cloud cover and sun illumination. The reflection of radar waves on the ground (called backscatter) is particularly sensitive to humidity and water. This renders Copernicus Sentinel-1 a prime constellation of satellites to detect, map and monitor floods.

Researchers of the Department of Geosciences at the University of Oslo have compared Copernicus Sentinel-1 data from before (3, 5, and 9 October 2019) with those during the floods (of 15 and 17 October). They were thus able to detect the areas that were under water due to flooding. The rare cloud-free Copernicus Sentinel-2 data could not be used to map areas under water, but they did highlight the increasing water flow in the rivers.

"The combined Copernicus Sentinel-1 radar images show the flooded areas in red", explains Andreas Kääb,  from the University of Oslo. "Since radar satellites such as Copernicus Sentinel-1 observe Earth's surface from an oblique angle, meaning by looking from the side, they cannot see into the narrow streets of Venice to detect the devastating flooding that struck there. The radar signal is mainly dominated by backscatter from the buildings. However, the flooding in the laguna wetlands surrounding Venice becomes well visible in the combined Copernicus Sentinel-1 images, and is a good indicator of the exceptionally high water level in the laguna.

"In the past, optical images such as those from Copernicus Sentinel-2 were not as useful for flood-mapping as Copernicus Sentinel-1 images, since the heavy rain falls that lead to flooding come together with a cloud-covered sky. However, with the increased revisit, more and more Copernicus Sentinel-2 images are being used for flood-mapping around the world. In this case, the few cloud-free Copernicus Sentinel 2 scenes during the rise of the flood illustrate well the rising flow in the rivers from the Southern Alps towards the Mediterranean Sea. We obtained the Copernicus Sentinel-1 radar images within a few hours after their acquisition, and were thus able to analyse them almost in real-time." Concluded Professor Kääb.

The Copernicus Emergency Management Service (EMS) is regularly using the techniques illustrated here for flood mapping. Related operational activities are also currently ongoing for Northern Italy.

About the Copernicus Sentinels

The Copernicus Sentinels are a fleet of dedicated EU-owned satellites, designed to deliver the wealth of data and imagery that are central to the European Union's Copernicus environmental programme.

The European Commission leads and coordinates this programme, to improve the management of the environment, safeguarding lives every day. ESA is in charge of the space component, responsible for developing the family of Copernicus Sentinel satellites on behalf of the European Union and ensuring the flow of data for the Copernicus services, while the operations of the Copernicus Sentinels have been entrusted to ESA and EUMETSAT.