Influence of Channel‐Spanning Engineered Logjam Structures on Channel Hydrodynamics

• Published in: Water resources research 2022-12, Vol.58 (12), p.n/a
• Main Authors: Müller, S., Follett, E. M., Ouro, P., Wilson, C. A. M. E.
• Format: Article
• Language: English
• Subjects: acoustic Doppler velocimetry; Aquatic ecosystems; Base flow; Bottom stress; Catchment area; Decay; Design; Distance; Doppler sonar; Downstream; Energy levels; Environmental restoration; Environmental risk; Flood management; Flood risk; Floodplains; Floods; Flow alteration; Flow measurement; Flow paths; Flow velocity; Flow visualization; Fluid flow; Fluid mechanics; Flumes; Groundwater; Height; Hydrodynamics; Internal flow; Kinetic energy; leaky barrier; Local flow; Log jams; logjam; natural flood management; Open channels; Reintroduction; Restoration; Risk management; River channels; River networks; River restoration; Rivers; Scour; Sediment; Shear stress; Streams; Structures; Synchronization; Tributaries; Turbulence; Turbulent kinetic energy; Turbulent mixing; Upstream; Velocimetry; Velocity; Water levels
• ISSN: 0043-1397; 1944-7973
• DOI: 10.1029/2022WR032111

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) 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. Plain Language Summary Engineered logjams (ELJs) with a lower gap are a nature‐based solution for flood risk management and river restoration. Channel‐spanning wooden logjams increase upstream water levels, causing the flow to spill onto surrounding floodplains, slowing down surface and ground water through the catchment. Using experimental flow velocity measurements in a laboratory open channel flume, we investigated the local flow field upstream and downstream of six ELJs. We demonstrate that the flow blockage caused by ELJs resulted in an increase in upstream flow depth, with a lower velocity at logjam height, and higher velocity at gap height which extended into the downstream region. While this high‐velocity stream was present for all logja