Effects of grain size on landslide–forest interaction

Forests play an essential but poorly-understood role in mitigating landslide runout by providing mechanical resistance and dissipating flow energy. Despite growing interest, existing models treat forests as friction modifiers or generic porous obstacles, and largely ignore how grain size controls retention and jamming. This study experimentally investigates and resolves the influence of grain size, slope angle, and tree spacing on landslide–forest interactions using reduced-scale flume tests with different granular materials. Results show that although forests can reduce flow mobility, preferential flow paths may develop along trees, leading to ineffective energy dissipation along the flow flanks. For fine-grained flows composed of sand, the deposition behaviour is governed by the normalised slope angle and the transverse blockage ratio. For coarse-grained flows composed of gravel, the deposition and retention are controlled by two distinct jamming mechanisms: frontal deposit-induced jamming and arching-induced jamming. Frontal deposit-induced jamming occurs in all jamming cases, whereas arching-induced jamming only develops when tree spacing is smaller than three times of the grain size. We capture this variety of phenomena within two phase diagrams for fine-grained and coarse-grained flows. The phase diagrams provide a direct screening rule for minimum tree density and slope condition required to ensure jamming for a given grain size distribution.