Incipient motion of large wood in river channels considering log density and orientation

The incipient motion conditions of large wood were tested in flume experiments using 48 handmade cylindrical logs of different lengths, diameters and densities. Their orientation to flow determined the motion: parallel logs slid, while oblique or transverse logs rolled. Log orientation, channel bed roughness, and log diameter had significance influence on entrainment depth. The logs’ density strongly influenced motion for logs parallel to flow. Logs oriented parallel to flow needed large entrainment depth to entrain than similar logs oriented oblique or transverse. Logs denser than water had high correlation with buoyancy, suggesting initial movement by floating, followed by either sliding or rolling downstream along the bed. From the modelled datasets, our results guided by the goodness of fit demonstrate that the floatation thresholds taken from global datasets should be applied with caution under different bed roughness conditions.