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