In this study, mechanisms of woody debris transport on the unsteady flow around a group of spur dikes were investigated by performing a flume experiment with the Particle Tracking Velocimetry (PTV) technique for physical models of woody debris and Particle Image Velocimetry (PIV) technique for particles of neutral buoyancy. The particle based numerical model that treats woody debris as combined particle elements was additionally applied in order to investigate the details of the debris motions. The experimental results show that the woody debris concentrated to the regions where the vorticity was relatively low. Meanwhile, woody debris diffused while they were transported donwstream as their length is longer. The calculation results imply that the diffusion of woody debris was caused by the filter-like effect by their spatial features.
