Connectivity-based assessment of trajectories and channel linkage of rainfall-triggered mass movements

Identifying the trajectories followed by mass movements, especially when they evolve into debris flows, is essential for producing hazard maps and for understanding the sediment inputs to channels associated with these processes. Hydrosedimentological connectivity makes it possible to estimate the transfer potential of material mobilized in source areas toward targets of interest, such as the drainage network, while also indicating possible preferential pathways. Because the trajectories of sediments and mobilized material are conditioned by topography and surface runoff, structural and functional elements of connectivity can serve as a proxy to interpret the dynamics and routes of mass movements. This study evaluates connectivity along the scars of mass movements, both connected and not connected to the channel network, triggered by an extreme precipitation event in the municipality of Angra dos Reis (Rio de Janeiro State), Brazil, in 2023. To this end, we analyze: (i) structural connectivity, represented by the Index of Connectivity (IC), and (ii) structural and functional connectivity, represented by the Index of Hydrosedimentological Connectivity (IHC). Differences between connected and disconnected scars were examined using statistical tests, including assessments of normality and between-group comparisons using Student’s t-test and the Mann–Whitney U test, applied to scar-level statistical metrics (mean, median, standard deviation, maximum, range, and variance), according to the data distribution. Effect magnitudes were quantified using Cohen’s d and r (rank-biserial). The results indicate that both indices were able to capture mass-movement trajectories, highlighting preferential sediment-transfer pathways. IC and IHC values show a significant difference between connected and disconnected scars. The approximately normal distribution observed for the IHC scar statistics (mean, median, and standard deviation) suggests control by multiple compensatory processes, whereas the non-normality of these statistics for IC, contrasted with the normality of maximum IC values, may indicate a stronger influence of local controls. In addition, IHC values for the scars show consistently high effect sizes for central metrics (mean, median, and variance), whereas IC values for the scars tend to show more pronounced effects in extreme values and in the overall connectivity range. Taken together, these results reinforce the potential of IC and IHC as useful indices to evaluate the trajectories of mass movements triggered by intense rainfall and their associated sediment delivery to the drainage network, as well as to support hazard-mapping analyses.