Experimental and numerical investigation of the protective ability of tree array and basal-clearance barriers against debris flow

Abstract

Debris flows pose a serious threat to human lives and properties, particularly in areas with increased human activity in mountainous regions. It is thus crucial to find effective ways to mitigate debris flows in mountainous regions. Forests encompass approximately 39% of low-altitude mountainous regions worldwide, making the tree arrays a potential eco-friendly solution against debris flows. Previous studies on flow-tree interactions have relied on field investigations, physical experiments, or simplified numerical simulations. Field investigations provide valuable real-world evidence, but comparisons between different sites are challenging due to variations in terrain morphology, tree species, and soil properties. Physical experiments offer controlled evidence but struggle to capture the complexity of flow-tree interactions. Additionally, existing experimental studies have not thoroughly examined tree arrangement and spacing. Numerical algorithms also face difficulties in accurately representing these complex interactions, limiting our understanding of the protective effects of trees. To address these limitations, this study introduces a comprehensive set of tools to generate numerical and experimental evidence of flow-tree interactions. Lab-scale experiments are conducted using a newly developed model forest device, and a GPU-empowered numerical simulator is developed to capture the complex flow-tree interactions. A segmentation and flow digitalization algorithm are implemented to establish a numerical-physical interface, enabling the comparison of physical evidence with numerical simulation results. Parametric studies are then performed to investigate the protective abilities of trees. Additionally, unique simulation cases involving interactions between flows and barriers with basal clearance are examined to demonstrate the generalizability of the developed numerical simulator. The experimental and numerical results demonstrate that tree arrangements significantly influence flow kinematics. The presence of bow shocks and momentum concentration in tree arrays can even increase the runout distance of flow. However, interactions between adjacent bow shocks lead to the dissipation of flow energy, resulting in diverse behaviours of flow under varying tree arrangements. Moreover, uprooted tree stems contribute to timber debris and the creation of wood-laden flows, intensifying the destructive nature of debris flow events. The tools developed in this study provide comprehensive evidence for investigating flow-tree interactions and emphasize the importance of tree arrangements and spacing. These findings offer new insights for the design and assessment of forests as natural barriers against debris flows.