Driftwood disasters often happen and give us severe damages in the town with rivers near mountain areas. We have developed a code for 3‐D simulations with free‐surface flow including a lot of pieces of driftwood and solid structures of closed dam, driftwood trap device and river bed. The code is based on the cumulant Lattice Boltzmann Method (LBM) coupled with the discrete element method (DEM). The free‐surface is described by solving the conservative Allen‐Cahn equation of the phase‐field model. We have implemented an octree Adaptive Mesh Refinement (AMR) method to assign high‐resolution mesh around driftwood and near structures and the computational efficiency has been greatly improved. The simulations have been executed on supercomputers equipped with multiple GPUs. Two validation tests for a small‐scale experiment show good agreements with the experimental results. For a real closed dam with a driftwood trap device, we examine flows including 1,000 pieces of driftwood with several parameters. It is found that the number of trapped driftwood strongly depends on the driftwood length and more than 90% driftwood is captured when the length is greater than 75% of the trap device gap. We can show the driftwood distribution under the water and the angle to the water surface is smaller for longer driftwood. It is found that the dammed up depth in front of the trap device has a strong correlation with the porosity and the width of trapped area. We have successfully simulated free‐surface flows including driftwood and understood the process of driftwood trapping. It is expected that the code is applied to more real situations and other experimental cases to examine the effectiveness for driftwood trapping.