Responses of bed morphology and congestion patterns with driftwood on sand bars

The numerical simulations and the flume experiments were conducted for investigation of driftwood deposition patterns and the channel bed deformations. In the study, we generated an alternating bar under the 1.1 liter/s inlet flow discharge and the 0.005 m/m channel slope. The experimental and computational flumes are 12 × 0.4 m² and 5 × 0.4 m², respectively. The uniform grain size was 0.51 mm. As for the computation, we used the 2D (two-dimensional) depth-averaged flow model, called Nays2DH of the iRIC software to calculate water flows. Nays2DH is a Eulerian model for water flows. For the driftwood dynamics, a Lagrangian type driftwood model as connected spherical shaped particles is coupled with Nays2DH. In the experiments and the simulations, we considered the root wad effect by using a cross shape of root wad and a larger size for a particle at a head of driftwood, respectively. As for the simulations for wood collisions, we used a discrete element method (DEM). Through the study, we obtained the bed profile before/after the driftwood supplies in the laboratory test. The results showed that the speed of bar migration is locally changed by wood jams because the wood jams disturb the water flow. The wood jams can deform alternating bar into mid-channel bar when it is generated at the centre part of the channel. In addition, the smaller time interval of wood supply increases the wood collisions and decreases the number of stored wood pieces. Those results would be informative to study the morphodynamics with driftwood in rivers.