Suspended sediment transport during the unprecedented flood in southern Brazil

The biggest Brazilian rainfall event occurred in May 2024 affecting millions of people. The damage caused includes several deaths and other social issues, billions in economic losses, and massive environmental devastation with more than 10,000 landslides and sediment settling in flat areas. In this context of extreme events, we aimed to estimate the suspended sediment transport in the Guaíba basin during this unprecedented flood. We used the model for large basins MGB-SED and daily precipitation to compute sediment erosion, transport and deposition. The model was calibrated considering the historical period, prioritizing the adequate representation of extreme events, resulting in KGE values higher than 0.4 in the main sediment stations. For the first time, our results provided sediment yield estimates for this event: 5 million tons of suspended sediment were delivered to Guaíba from April 27 to June 17, 2024. The Taquari River was the tributary that transported the most suspended sediment, reaching a peak of 554,500 tons on May 2, which is five times greater than the highest simulated peak in the historical period. After the event, a large deposition of coarse sediments in the lowland areas, silting up the rivers and islands was widely reported. Despite this, we observed from satellite images that the morphological changes (e.g. bank erosion and the appearance/ changes in sand banks) along the main channels were insignificant compared to the event scale. We conclude that, even in the face of the unprecedented sediment and water flow, these rivers demonstrate a bank stability condition and high suspended sediment transport capacity, even suggesting an equilibrium condition.