Enhanced flow variability and morphological changes through individual wood placements on a gravel bed

Abstract

To design and evaluate river restoration projects with wood, it is crucial to understand how the placement of wood will alter flow and bed morphology. This information is still missing. To address this, flume experiments were conducted with a single log placed on a mobile bed with a uniform grain size. The effect of log diameter, log orientation to the flow, and log length on flow variability and bed morphology was studied for various initial flow conditions. Even a single log increased flow variability, herein defined as spatial variability in both velocity and turbulent kinetic energy. For each log placed at the channel center, the logs generated a symmetric scour pattern alongside and downstream of the log due to increased bed shear and turbulence generated by the log. A steady, adjusted cross-section was reached at the cessation of erosion, which occurred when the bed stress decreased below the critical shear velocity. Logs with larger diameter tended to erode more deeply, so that the final, adjusted cross-section was similar for different log dimensions. Changes in bed topography affected wake characteristics. Logs with small changes in bed topography generated longer zones of reduced velocity, which can increase the residence time of organic matter or nutrients in the wake region. Logs with big changes in bed topography created the deepest and longest scour holes, which may locally reduce stream temperature and improve fish habitat during summer.