Influence of Channel-Spanning Engineered Logjam Structures on Channel Hydrodynamics

Nature-based solutions to flood risk management, such as engineered logjams (ELJs), contribute to the reintroduction of wood in rivers. As part of stream restoration, and utilized in tributaries, ELJs increase upstream water levels, causing the flow to spill onto surrounding floodplains, resulting in the desynchronization of peak flows in a river network. To understand the effect of ELJs on local river hydrodynamics, we experimentally investigate the flow field upstream and downstream of six ELJs, using acoustic Doppler velocimetry and flow visualization. We consider channel-spanning structures designed with a gap (b₀) underneath, allowing unhindered baseflow. Our results revealed that upstream of the logjams, flow diverted toward the lower gap, creating a primary jet exiting underneath the structures, whose strength depends on the physical logjam design. Maximum jet velocities remained constant until a downstream distance of 4b₀ for all logjams. The upper wake was structure-dependent, with logjam structures allowing distinct internal flow paths generating secondary jets, which influenced near wake decay (x < 4b₀) and turbulent mixing. The highest turbulence in the near wake was found for the non-porous and short, porous logjam designs, while the upper wake of all long, porous logjams was characterized by low turbulent kinetic energy levels. Far wake decay (x > 4b₀) was self-similar for all logjams and resulted in near flow recovery at downstream streamwise distances greater than 35b₀. ELJs are likely to enhance bed shear stress, increasing the risk of local scour and sediment mobilization. Our study expands the current knowledge of ELJ hydrodynamics and highlights potential implications for the riverine ecosystem.