Integration of numerical simulation (Morpho2DH) and fieldwork for a 2024 debris flow event in Santa Tereza, Rio Grande do Sul, Brazil

Debris flows are highly destructive landslide processes involving water, air, and sediments mobilizing by gravity. In 2024, the state of Rio Grande do Sul, southern Brazil, experienced the most extensive disaster in its history, with widespread mass movements and flooding, particularly in the Taquari-Antas Basin, one of the most important basins in the state. During this event, Santa Tereza recorded some of the largest debris flows.

The present study aimed to simulate a huge debris flow occurrence in Santa Tereza, integrating computational modeling and field survey observation. The debris flow was simulated using Morpho2DH (v. 2.1), a solver of iRIC software (v. 4.1) for unsteady horizontal two-dimensional bed deformation analysis.

Santa Tereza is characterized by a humid subtropical climate and steep terrain, which increases its susceptibility to landslides. During the 2024 extreme rainfall event, 411 properties were affected by landslides in the municipality. The studied debris flow traveled 1.5 km resulting in the destruction of two houses. Deposition occurred on an alluvial fan along the Marrecão Stream, a tributary of the Taquari River. The event was mapped by aerophotogrammetry by the Latitude/UFRGS research group, producing a high-resolution orthophoto.

Landslide initiation areas were defined based on orthophoto as two rupture polygons that converged into the channel and developed the debris flow. The digital elevation model used was from ALOS-PALSAR. Field observations indicated a maximum erosion depth of 2 m. Mean grain diameter of 0,001 m was obtained from granulometric analysis of eight in-situ samples. Vegetation parameters were set based on field data, assuming a density of 1 tree km^-2 and a mean vegetation height of 6.2 m. Post-event vegetation erosion depth was set to zero, reflecting the complete removal of vegetation cover observed in most of the affected area. The time step of 0.001 s was adopted. Remaining input data followed default model settings. Simulation tests indicated a total event duration of approximately 280 s, indicating high flow velocity and consistent with eyewitness accounts.

Model calibration was performed by comparing the simulated affected area and the flow route with orthophoto interpretations. The simulation estimated an affected area approximately twice as large as the visible scar mapped in the orthophoto, excluding the stretch above the Stream, which could not be calibrated. Despite the overestimation of the affected area, the model accurately reproduced the flow route. These results demonstrate that Morpho2DH can capture debris flow dynamics in Santa Tereza, and the conservative area estimates may be advantageous for disaster risk management applications.

Acknowledgements: This study was supported by FAPERGS under Grant Agreement No. 24/2551-0002124-8 (Call FAPERGS 06/2024).