Floods are among the most devastating natural disasters. In 2018, some 400 such large-scale events occurred around the world, and in 2022 more than 57 million people were affected, resulting in more than €75 billion of damage.
Between 2023 and 2024, Saint-Omer in northern France found itself under water several times. The River Aa that runs through the town broke its banks, flooding the streets and surrounding countryside. Terrain elevation combined with repeated torrential rains to saturate soils and channel the rainwaters towards canals and river mouths, compounded by tides and sea level variations.
The image above is a combination of two acquisitions by the SWOT satellite one month apart of the town of Saint-Omer. The top panel above the white line shows the river in normal flow conditions, on 3 December 2023. The satellite image indicates that the river (in purple) is 50 metres wide and some 1.50 metres deep.
The bottom panel is composed from data acquired on 3 January 2024. We can see that the usually dry residential areas and croplands in the vicinity of the river are now flooded. The water level has reached as high as 3 metres above the river bed in places.
In the context of a changing climate, the latest IPCC report notes that increasing amounts of water are now stored in the atmosphere, bringing more rain and flooding. Understanding the different stages involved in a flood and finely characterizing risk zones and aggravating or mitigating factors should help reduce the impacts of such events on populations.
The French-U.S. SWOT mission, developed by CNES in partnership with NASA, measures water heights across the globe’s land surfaces. This unique capability allows it to precisely locate and measure water masses in rivers, lakes and reservoirs, as well as standing water that has recently collected in usually dry areas.
With an average revisit interval to a same point on the globe of 21 days maximum (depending on the latitude), SWOT satellite cannot provide real-time early warning of floods on its own. However, its data are a key complement to satellites with shorter revisit times that do not measure flood height and extent simultaneously.
These data are expected to help scientists improve flood prediction models and inform land planning and emergency response to keep populations safe in the event of flood warnings.
Oceanography data from the SWOT mission can be accessed via the AVISO platform and hydrology data at hydroweb.next.
To learn more, see our dedicated SWOT mission web page.
