Flood events have increasingly affected urban areas in the southwestern Amazon, raising concerns about the exposure of cities located along major river floodplains. This study investigates the March 2024 flood in Rio Branco, Acre—one of the largest on record—with the objective of reconstructing the event and evaluating how hydraulic flood severity translates into spatial patterns of hazard to people in an expanding Amazonian city. We hypothesize that severe flood impacts are primarily structured by the spatial overlap between hydraulic intensity and urban occupation of the floodplain rather than by detectable long-term trends in hydroclimatic variables. To test this hypothesis, the study integrates long-term hydrometeorological time series analysis (1981–2024), two-dimensional hydrodynamic modeling using HEC-RAS, and event-based flood hazard mapping. Rainfall and discharge records do not exhibit statistically significant long-term monotonic trends; however, river stage shows a significant declining trend (–0.0301 m yr -1 ; p = 0.0101) together with pronounced interannual variability and clustering of extreme events, particularly during La Niña phases. The hydrodynamic simulation reproduced the observed flood dynamics with high accuracy and indicates that 27.52% of the analyzed urban area falls within the extreme hazard-to-people class (HR), largely coinciding with densely populated neighborhoods located along the Acre River floodplain. These findings demonstrate that severe flood impacts in Rio Branco are primarily determined by the spatial overlap between hydraulic flood severity and patterns of urban occupation within the floodplain.
Mantovani et al. (Sun,) studied this question.