Experimental Investigation of Movement and Deposition of Woody-Debris Suspensions in Inclined Channel Tests

Chyan-Deng Jan, Le-Trang Nguyen 

Debris flows, which mobilize large volumes of water, sediment, and woody debris, pose significant risks to human communities and infrastructure. In wildfire-affected forested areas, the accumulation of woody debris in drainage channels is exacerbated, thereby increasing the potential for more hazardous debris flows. To examine the influence of woody debris on debris flow dynamics, an inclined channel test, allowing the observation of woody debris flow in an inclined channel and its deposition in a horizontal tank, was conducted with highly concentrated woody-debris suspensions composed of clay, silt, woody debris, and water. This study explores how variations in fine-sediment fraction (Cvf), woody debris proportion (Cvg), and woody debris size (Sw) impact flow behavior, including the entry speed (V0) into the horizontal tank, and deposition characteristics such as runout distance (LR), deposit width (WR), deposit thickness (HR), and final profiles on a 20o channel slope. To examine the influence of proportions and sizes of woody debris on the entry speed, an empirical equation is presented relating V0 to Cvf, Cvg and Sw using multiple linear regression analysis. The results indicate that a higher Cvf and Cvg yields smaller entry speeds, leading to shorter runout distances, thicker deposits, and wider deposit extents. The tests of larger Sw generate larger entry speeds, resulting in longer runout distances while producing thinner and narrower deposits. Empirical equations relating V0 to LR and WR are also provided to further demonstrate the influence of entry speeds on the deposit characteristics. Additionally, a strong correlation was found between inclined channel test parameters (e.g., entry speed, runout distance, and maximum deposit width) and rheological parameters (e.g., yield stress and viscosity), suggesting that rheological properties can be indirectly estimated from inclined channel tests. These findings offer valuable insights for improving the understanding and prediction of woody debris flow behavior, particularly in post-wildfire landscapes.