Flume experiments on entrainment of large wood in low-land rivers

ABSTRACTFloating wood is increasingly recognized as a source of hazard during flood events, because of its potential to clog bridges and hydraulic structures. However, wood entrainment by water is not well understood and only few studies addressing this issue are published. This paper reports on laboratory tests that were performed in a straight flume with fixed bed on logs with different shapes, orientations and densities. The logs oriented parallel to the water flow moved by sliding at undisturbed water depths larger than their buoyant depths, which was due to significant lowering of the water level around the log. Our analyses demonstrate that the entrainment condition cannot be simply defined as a balance between bed friction and drag force. Instead, the flow around the logs should be taken into account, with inclusion of acceleration and streamline curvature. For reliable predictions more experimental data are needed, including the effects of a mobile bed.