Scour Around Large Wood with Rootwads: An Experimental and Computational Study

Fallen trees in river corridors play a critical role in fluvial hydraulics. In this study, we investigated the impact of a porous rootwad on flow and sediment transport around a single log. We conducted flume experiments with two structures: one consisting of a single log and the other with an added porous rootwad, both placed on an erodible bed. These experiments aimed to determine how the rootwad influences flow dynamics and morphological patterns. We also performed numerical simulations with the rootwad represented with different approaches, i.e., fully resolved, the solid model, and the porosity model. The porosity model and the fully resolved rootwad captured the extent of scour and deposition reasonably, while the solid rootwad overestimated the scour depth. The single log simulation also effectively captured the contraction scour depth at the edge of the structure. Our study found that the addition of a rootwad significantly increases hydrodynamic and morphological variability in both experiments and simulations. The presence of a rootwad can induce tunneling flow beneath the trunk. It can also help to stabilize the log on the bed. In contrast, a single log without a rootwad may experience sweeping flow recirculation, leading to sediment deposition at the trailing edge. As the scour hole deepens, the log might sink into the bed and be buried, reducing its effectiveness.