Flood tragedy in South Brazil partly human‑caused, specialists say
The tragedy that affected 446 municipalities in Rio Grande do Sul, South Brazil, was partly caused by the actions of people, who built houses in flood areas and failed to properly maintain containment dykes and flood barriers.
Such is the assessment of Professor Roberto Reis, from the postgraduate program in Ecology and Evolution of Biodiversity at the Pontifical Catholic University of Rio Grande do Sul (PUC-RS). Those dykes and barriers, he added, were built in the 1970s and were never properly looked after. “The planet is to blame for the flood. But the state and city administrators are to blame for the tragedy,” he remarked.
In an interview with Agência Brasil, Professor Reis pointed out that state capital Porto Alegre is a floodplain area characterized by the confluence of rivers on the shores of Guaíba lake. “It’s a natural formation. We simply shouldn’t have built in an area that floods periodically.”
Floods occur in Porto Alegre every two or three years, he noted, but this time it was extremely severe. “It’s never been so high.” The level of the water in the lake cannot be stopped from rising, he went on to say. “We can, however, prevent flooding by operating the containment dykes and everything else properly.” Maintaining or rebuilding the dykes and dams on the state’s rivers is the way out to prevent further tragedies, he argued.
Professor Reis recalled that, in September last year, Rio Grande do Sul had to deal with a large-scale flood. “That’s when we saw that the floodgates and some of the dykes weren’t working. It was time to fix them. It was a mega-flood. The big one has come now. We should have sorted it all out in September. I hope they learn their lesson this time, because we’re paying dearly.”
Excessive rain caused by climate change, he added, is a natural phenomenon. “Every so often, extreme rains cause floods,” he noted. This time, however, the state experienced its heaviest flood in history, which he attributes partly to climate change caused by excess carbon dioxide in the atmosphere. The rest, in his view, is man’s doing.
Rainfall
According to Professor Rodrigo Paiva, from the Hydraulic Research Institute (IPH) at the Federal University of Rio Grande do Sul (UFRGS), the main cause of the flooding in Guaíba lake is the record-breaking rainfall observed in its basin since late April and early May—800 millimeters in some locations.
The expert told Agência Brasil that rivers flooded quickly in the mountains, as steep valleys carry large amounts of water at a high speed. “In some cases, rivers have risen 20 meters in less than a day. This wreaked havoc, for example, in the Taquari Valley, yet again.” The region was assailed by massive floods in September 2023.
After reaching the lowlands, Professor Paiva said, this water spreads over the floodplains and flows at a slower rate. “That’s why the water from all those rains takes a few days to reach lake Guaíba down in the city.” Since May 5, the lake has reached a record 5.3 meters.
In addition to the lake, the region is home to the Lagoa dos Patos, he noted. Because these bodies of water have a large surface area, they are subject to the winds. “When we have a southerly wind, it can dam up this water and raise it by around 20 centimeters, or even more when the wing is stronger. This also helps a little with the flooding of lake Guaíba, even though the main factor is the large rainfall.”
Duration
Another key aspect of the phenomenon is its duration, Professor Paiva noted. The long time it takes for the lake to resume its lower levels stems from difficult flow in these lowland rivers, especially the Jacuí. “The water is virtually stagnant in those floodplains.”
On the other hand, it can be intriguing, the specialist said, because, “if we didn’t have these floodplains, which already act as a natural reservoir that attenuates the floods, perhaps the water level would have risen much more in Porto Alegre, and much more quickly too.”
The consequences could also be worse, the professor admitted, because Porto Alegre’s metropolitan region is located in a very low-lying area, affecting cities such as Eldorado and Canoas. “The flooding reaches significant levels—one or two meters deep in some places—but the water doesn’t flow as quickly. In the valley, on the Taquari river, since it’s deeper and steeper, the flow can be devastating, wrecking homes and sweeping things away,” he argued.
