This study conducted flume experiments that simulated the initial entrainment of single woody debris in a moving channel bed. Woody debris of different lengths, diameters and orientations to flow direction were used to explore their interactions with the moving channel bed. In addition, their mode of transport and variation of the flow
field were investigated. Results of test were compared to the initial entrainment of woody debris on a fixed channel
bed. For the sake of comparison, this study was divided into three parts. In the first part, laser scanning was used to compare channel topography before and after woody debris movement. The second part used particle tracking velocimetry(PTV) to establish the fields of surface flow and velocity. The final part assessed the stability of the woody debris stability factor. The results of these experiments indicate that woody debris orientation influences the flow field, flow velocity and the final bed topography. Topography changes influenced the mode of wood transport and their entrainment threshold. Woody debris oriented parallel to flow had two modes of transport; rotation and sliding to downstream.Under oblique and transverse placements, woody debris movement can also be divided into two; rotation and sliding two modes and rotation is majority. A notable difference between the fixed and the moving bed channel was that entrainment of woody debris was primarily initialized by buoyancy in the fixed bed, whereas both buoyancy and dragforce initiated movement in the moving bed channels. Finally, our results demonstrated that woody debris in a moving bed channel needed larger discharge to initiate movement than that in fixed bed channels.
Key words: woody debris, wood entrainment, mobile bed, flume experiment.