Three-dimensional Numerical Model of Driftwood Catching by Bridge Piers and Elucidation of Dependency on DRI (Driftwood Richardson Number)

Background: Prediction of driftwood behavior in rivers is an important task from the viewpoint of water disaster mitigation. Driftwood deposition near the bridge piers is an important phenomenon that causes bridge failures and flooding caused by clogging of river channels. The purpose of this research is to develop a numerical model to reproduce this phenomenon and to examine the dominant parameter of driftwood deposition.

Computational Model: Since capturing near the bridge piers however happen three dimensionally, we have constructed a "3D-3D model" that solves both in liquid and solid phases in three dimensions. The collision between driftwood pieces is evaluated by DEM (Discrete Element Method).

Comparison between experiment and numerical calculation

For the verification of the model, experiments were performed in a straight channel of 10 m long and 20 cm wide. The channel slope was 0.001. Cylindrical wood pieces were used for driftwood model. For the model of bridge piers, rectangular wood pieces of 1 cm square were used. The computations were carried out under the same conditions of the laboratory tests.

　Figure 1 Laboratory experiment(DRI=4.32)　 Figure2 Computation(DRI=4.32)

Figure 2 shows an image in the present experiments and Figure 2 shows a snap shot of the computation. The water flow is heading from the left to the right. The present computational model successfully reproduced the capturing of driftwood by bridge piers and the three-dimensional features of stacking.

Influence of DRI: Kimura et al. (2018) pointed out that the three-dimensional feature of driftwood deposition is dominated by the Driftwoods Richardson number (DRI). In this research, experiments and simulations were carried out by changing DRI. In the experimental results with larger DRI, the three-dimensional behavior of the deposition becomes weak and the trapping rate increased. The reason is that when the DRI is large, driftwood settles two-dimensionally and cannot pass three the bottom of the previously settled driftwood. However, if DRI is small, the "slip phenomenon" frequently occurs. A similar tendency was simulated well by the present numerical model.

References:

Kimura & Kitazono (2018): Studies on driftwood motions around obstacles by laboratory and numerical experiments, RiverFlow 2018