Hydrodynamic flood risk mapping of the march 2024 event in the acre river basin, southwestern amazon

Abstract

Flood events have become increasingly frequent and severe in the southwestern Amazon, raising concerns about the capacity of urban areas to cope with intensifying hydroclimatic extremes. This study investigates the March 2024 flood in Rio Branco, Acre—one of the largest on record—by combining hydrometeorological time series analysis (1981–2024), two-dimensional hydrodynamic modeling using HEC-RAS, and spatial risk mapping. Rainfall and streamflow data did not exhibit statistically significant long-term trends; however, the recurrence and magnitude of extreme events, particularly those associated with La Niña conditions, indicate a shift toward more intense hydrological regimes. The simulation reproduced the observed flood dynamics with high accuracy and revealed that over 25% of the urban area is exposed to extreme flood hazard, with critical overlap in socially vulnerable neighborhoods. These results demonstrate the compounded nature of flood risk, driven by both climate variability and urban expansion into flood-prone zones. The study concludes that traditional assumptions about flood recurrence intervals are no longer sufficient for urban planning and that robust modeling and risk mapping must be integrated into climate adaptation strategies in Amazonian cities.