Flume Experiments on the Interactions of Fluid Flow and Stable Wood Debris in Rivers: Effects of Root Size, Shape, Porosity and Log Orientation on Pool Formation and Wood Burial

The interaction of fluid flow and stable wood debris in rivers creates important complex local streambed relief, such as the large pools that provide fish habitat and the sediment accumulations that bury and further stabilize wood debris. The spatial pattern of fluid flow is the driving force for sediment transport and changes in streambed relief in rivers. We use an annular flume to characterize the fluid flow field around wood debris and to examine the patterns and mechanisms of local scour and deposition. Model trees used in the experiments have a range of root sizes, shapes and porosities that correspond to natural wood debris. Preliminary experiments involve an immobile bed and individual stationary trees placed in a range of flow depths, flow velocities and orientations relative to flow. Fluid flow patterns on the bottom boundary layer, where bedload sediment transport occurs, are visualized using potassium permanganate crystals. Fluid flow in the water column is measured using powdered fish scales illuminated by light sheets at different planes in the flow. When the wood debris is oriented with the root pointing upstream we observe the formation of horseshoe vortices and a bow shock that create a crescent-shaped scour zone. When the orientation of wood debris relative to flow changes, there are drastic changes in the magnitude and location of vorticity, scour and depositional zones. We find that as root diameter increases or as root porosity decreases the spatial extent of the scour zones increases. When root porosity decreases the depositional zone expands and moves closer to the roots, potentially increasing the stability of the wood from burial of the roots. Future experiments are planned with a mobile sediment bed to observe the feedbacks between fluid flow, vorticity, and sediment transport that deposit sediment and excavate pools around wood debris.