THE EFFECTS OF WOOD DEBRIS ON STREAMBED MORPHOLOGY: SPATIAL PATTERNS OF FLUID FLOW AND SEDIMENT TRANSPORT AROUND INDIVIDUAL PIECES OF WOOD DEBRIS

The interaction of wood debris, fluid flow, and sediment transport in rivers creates local streambed morphology, such as large pools that are important fish habitats and sediment deposits that bury and stabilize woody debris. We used two sets of flume experiments and field measurements on a gravel-bedded river to investigate how wood geometry and orientation affect local flow and scour pool formation. For the first set of experiments, fluid flow patterns on the bottom boundary layer, where bedload sediment transport occurs, were visualized using solid dye crystals. The second set of experiments and the field measurements investigated streambed morphology around wood debris, as well as distributions of wood debris geometry and orientation in nature. We find that: 1) the presence of roots on wood debris leads to greater areas of flow disturbance and sediment scour; 2) the size of scour pools and flow disturbances around wood debris increase as the root cross-sectional area, oriented orthogonal to flow, increases. A better understanding of the underlying sediment physics and fluid dynamics around naturally occurring wood debris in rivers can lead to improved hazard prediction of mobile wood during floods and to improved design criteria for engineered logjams placed to protect infrastructure and improve aquatic habitat.