Studies on Driftwood Motions around Obstacles by Laboratory and Nnumerical Experiments

From the engineering point of view, prediction of driftwood motions around hydraulic obstacles in rivers is important. We carried out laboratory experiments to understand the driftwood behaviour around grid like obstacles and found out that there are two different patterns of the capturing process of driftwood: 2D (two-dimension) type and 3D (threedimensional) type. We proposed the governing parameter Driftwood Richardson Number for classifying the types of the driftwood capture. A numerical model to simulate driftwood motions based on the coupling of a Euler type three-dimensional flow model and a Lagrange type twodimensional driftwood model (3D-2D model) is proposed to analyse the driftwood behaviour around obstacles. The numerical model could predict well the flowing pattern of driftwood affected by the secondary current of the first kind in a meandering open channel. The numerical results with obstacles showed that the present 3D-2D type model is applicable only if the driftwood Richardson number is larger than 10, in which the driftwood capturing takes place in the 2D type.