Wide-swath satellite altimetry reveals the 2024 Porto Alegre extreme flood was intensified by backwater effect across choked river section

In May 2024, the city of Porto Alegre, southern Brazil, experienced the worst flood in recorded history. Extensive damage occurred despite large flood control measures, such as levees and floodwalls, implemented after the last great flood, in 1941. Wide-swath satellite altimetry, as provided by the NASA/CNES SWOT mission, revealed distinct flooding patterns. In the braided Jacuí river, flooding was aggravated by the backwater effect across a choked section. The wider Guaíba River experienced smaller water levels and slopes. Stream gradient increased dramatically, from 0.4 cm/km to 15 cm/km. We have validated the satellite maps with in situ GNSS hydrographic surveys as well as three water level stations employing GNSS Interferometric Reflectometry. Results suggest flood control could be improved in the north region of the city. Levees failed where water levels were a few meters higher than that experienced at the same time in other regions of the city.