Geomorphic change in the Guarda Mor River resulting from the extreme event of 2024

The understanding of fluvial erosion processes during extreme rainfall events is often limited by the lack of detailed pre-event topographic, sedimentological, and hydrological data. This study explores the 2024 extreme rainfall event in the Guarda Mor River experimental basin, Southern Brazil, based on a comparison of hydrological and topographic information obtained before, during, and after the event. The event, driven by a rainfall volume of 435 mm in 31 hours and an estimated peak discharge of 700 m3s−1, generated a unit stream power capable of greatly exceeding channel stability thresholds. To evaluate the impact of flow on fluvial erosion, pre- and post-event topographic and grain size data were compared, focusing specifically on a 130 m reach upstream of the monitoring section. Additionally, the hydrological study compared the hydraulic behavior of the channel using velocity and water level data from the basin’s existing monitoring program. The approach combined: (i) geomorphological change detection via DEM of Difference (DoD); (ii) characterization of the bed sedimentary restructuring via surface and subsurface grain size analysis; and (iii) changes in hydraulic geometry and flow resistance. The results reveal a complete “geomorphic reset” of the channel. The DoD quantified a substantial morphological reconfiguration, including an average channel widening of 24% and a transition from upstream erosion (max. −0.73 m) to massive downstream deposition (max. +1.19 m). Sedimentologically, the event energy was sufficient to break the pre-existing armoring layer, reducing the armoring ratio from 7.9 to 4.9 and causing intense winnowing of subsurface fines. This revealed a striking hydro-geomorphic duality: despite significant coarsening of the surface layer (D50 increased from 192.5 mm to 249.6 mm), the structural reorganization led to a “hydraulic smoothing” phenomenon. Flow resistance decreased drastically (Manning’s n dropped from ≈ 0.040 to ≈ 0.020), resulting in higher flow velocities and a transition from a subcritical to a critical/supercritical regime (Fr ≥ 1). The event redefined the channel into a new, hydraulically more efficient state, in which the erosive process altered fluvial morphology, bed configuration, and roughness to accommodate the occurring discharges, magnitudes that, apparently, tend to become more frequent. However, due to the breaking of the armoring, the bed is now in a state of latent instability and greater sensitivity to future events.