An integrated framework for evaluating large wood recruitment from hillslopes to channel network in forested mountain catchments

Large wood (LW) is a key factor influencing the physical, chemical, environmental, and biological characteristics of low-order mountain stream systems. LW recruitment is controlled by several physical processes, including debris flows, shallow landslides, streambank erosion, and windthrow, and it can significantly increase hazards to downstream populations and infrastructure during extreme events. Quantifying LW recruitment is particularly challenging due to the diversity of potential source areas and mobilization processes. 

Accurate quantification requires an integrated approach that accounts for LW recruitment from hillslopes mobilized by shallow landslides, from headwater hollows affected by debris flows, along the channel network through streambank failures, and during downstream transport. This study combines a physically based and probabilistic slope stability analysis, several empirical relationships for debris-flow initiation/propagation, a spatially distributed sediment connectivity index, and a simplified one-dimensional hydraulic model to simulate channel widening and downstream LW transport. Input parameters were derived from analyses of forest stand characteristics, soil and lithological properties, intensity–duration–frequency curves, and digital elevation model.

The proposed approach identifies critical channel stretches and crossing infrastructures that are most prone to obstruction by floating recruited LW. The model was applied to a small mountainous headwater catchment in the Northern Apennines, characterized by a dense forest cover and a high susceptibility to shallow landslides and debris flows, particularly in late spring and early autumn. Results indicate that the estimated LW volumes are comparable to those measured through field surveys, demonstrating the robustness of the proposed methodology. Because the approach relies on commonly available data, it represents a valuable tool for forest planning and management, for assessing the impacts of natural and anthropogenic forest disturbances (e.g., diseases, fires, clear-cutting, or clearing), and for supporting the optimal placement of in-channel wood retention structures.