Transport of driftwood during heavy rainfall events can intensify the flood hazard. Driftwood accumulates at a bridge and backwater rises. Accumulated logs sink against the buoyancy force and the whole depth region is blocked. However, the mechanism governing the sink motion of accumulated logs at bridges is not fully understood. In this paper, we conducted the driftwood accumulation experiment at a bridge with a single pier to examine the formation and growth of log jam. The observations revealed that the accumulation process could be divided into four phases. Backwater rise was compared with the porous board tests to evaluate the blockage ratio of log jam. Next, we examined the effect of the velocity field on the accumulation process after a large number of logs accumulated at bridges. Simultaneous measurements of the instantaneous flow velocity below the log jam and the instantaneous motion of the incoming logs was conducted by PIV. Then, the effects of the approach flow velocity, channel width and model log length on the log jam formation were examined. These results revealed the governing parameter of the accumulation process (sink motion of logs) and the effect of the driftwood carpet shape.

暴雨期间浮木的运输会加剧洪水的危害。浮木堆积在桥梁上，死水上升。累积的原木逆着浮力而下沉，整个深度区域被阻塞。但是，控制桥上堆积的原木的下沉运动的机制还没有被完全理解。在本文中，我们在一个单墩桥上进行了浮木堆积实验，以检查原木堵塞的形成和生长。观察结果表明，积累过程可以分为四个阶段。将回水上升与多孔板测试进行比较，以评估原木堵塞的堵塞率。接下来，我们检查了速度场对桥梁上大量原木堆积后堆积过程的影响。PIV可以同时测量低于原木堵塞的瞬时流速和进入原木的瞬时运动。然后，研究了进近流速，通道宽度和模型对数长度对原木堵塞形成的影响。这些结果揭示了积累过程的控制参数（原木的下沉运动）和浮木地毯形状的影响。