Transport Dynamics of Large Wood in a Stream Channel Affected by Spur Dikes

Abstract

AbstractThe damage to river engineering structures induced by large‐wood (LW) jams is one of the significant problems in river management. For example, typical bridge damage in alluvial areas in Japan, such as wash away, is often caused by the accumulation of large wood materials. The densely transported large wood pieces are an important factor in the accumulation process. We investigated the influence of local flows generated by hydraulic structures on the initial stage of the concentration of large wood pieces. First, we experimentally investigated the flow structures generated by spur dikes in a 0.6 m wide channel and the trajectory of large wood pieces. Then, we numerically investigated the influence of external force on the transport mechanisms of large wood pieces. We employed a simplified Basset‐Boussinesq‐Oseen model to evaluate each term of the momentum equation of the particles and calculated the trajectories of the wood pieces previously observed experimentally. The results showed that the transported large wood pieces were concentrated in low‐vorticity regions of the water surface. This tendency was more prevalent in short pieces (10 mm) than in long pieces (40 mm) because the influence of the relatively small‐scale patterns of the pressure gradient of the surrounding water surface might be spatially averaged for long pieces. This tendency has not been explicitly elucidated in previous works; thus, this knowledge is expected to be key in controlling LW transport, such as detecting the possible regions where LW jams occur or designing efficient inlet sections of LW retention facilities.