Conceptual Model of the Debarking Process of Large Wood in Rivers

When transported in rivers by short‐term episodic events such as floods and debris flows, large wood interacts with the flow, sediment, and river boundaries, leading to physical degradation such as weathering, abrasion, and breakage. This degradation, manifested as mass loss and structural alteration, weakens wood stability, increases mobility, and diminishes its geomorphic and ecological functions. However, this process remains largely overlooked when assessing wood transport and accumulation. Here, we introduce laboratory tumbling experiments to investigate the physical degradation of fresh large wood, focusing on mechanical bark removal during fluvial transport. Fifteen tests were conducted under varying flow velocities and water volumes, with three White Willow samples in each test. Wood debarking causes changes in the volume of wood samples during the experiment. Therefore, Structure‐from‐Motion photogrammetry was employed to quantify changes in wood volume and thereby evaluate the debarking process. Results revealed that debarking involves two coupled processes: (a) expansion due to mechanical cell‐wall breakdown, increased porosity, and hydration, and (b) abrasion caused by friction and repeated contact. Three debarking modes were identified under varying hydraulic conditions: abrasion‐dominated, expansion–abrasion, and expansion-dominated. These modes correspond to distinct transport regimes: congested (sliding–rolling), semi‐congested (sliding–rolling–floating), and uncongested (floating). The maximum expanded volume increased with water volume, while abraded volume first increased then decreased due to reduced wood‐chamber interactions. Under different water‐volume conditions, increasing flow velocity decreased the maximum expanded volume and increased the abraded volume, with minor variations in the mean values, but relatively large standard deviations. A conceptual model integrating both expansion and abrasion processes is further proposed to describe physical wood debarking during fluvial transport. This study provides first experimental evidence and conceptual framework for understanding large wood degradation in rivers.