Log jam formation at bridges and the effect on floodplain flow: A flume experiment

During localized torrential rains, a large amount of driftwood can be entrained into the river leading to decreased discharge capacity and increased water levels. In August 2012, driftwood accumulated at the bridge in Shizugawa River in Uji, Japan and a house was washed away by the floodplain flow. For disaster prevention, it is important to know the flow characteristics of the flow floodplains. However, detailed information to accurately characterize the floodplain flow around bridges is usually not available. In this study, three kinds of flume experiments were performed. First, we conducted the driftwood accumulation and porous board tests at the model bridge to evaluate backwater rise and blockage ratio of driftwood accumulation (to further conduct PIV measurements). After a large amount of driftwood accumulated at the bridge, the water overflowed the banks and floodplain flow occurred. Next, we examined the effect of the surface hydraulics on driftwood accumulation process by PIV. Then, we measured the velocity of floodplain flow by PIV, in which the accumulation was modeled using a porous board. The results revealed the effect of driftwood accumulation on floodplain flow. The obtained velocity distribution of floodplain flow is useful to estimate the erosion damage location by floodplain flow.