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- Title
Influence of Channel‐Spanning Engineered Logjam Structures on Channel Hydrodynamics.
- Authors
Müller, S.; Follett, E. M.; Ouro, P.; Wilson, C. A. M. E.
- Abstract
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. 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 logjams with a lower gap, the downstream flow field at logjam height was dependent on logjam design. Porous ELJs allowing flow through the structure, for instance, generated smaller, weaker streams which influenced the flow field. Independent of the logjam design, the flow field recovered to its original, undisturbed flow field at nearly the same downstream distance. Our study highlights the flow alterations associated with different physical logjam designs and raises potential secondary impacts on the riverine ecosystem such as local scouring, sediment mobilization, and trapping as well as the enhancement in habitat complexity. Key Points: We experimentally investigated the impact of six non‐porous and porous engineered logjams on upstream and downstream channel hydrodynamicsMain gap below logjams produced a wall jet, with maximum jet velocity dependent on channel blockageHighly turbulent near wake observed for non‐porous and short logjams, with long porous jams characterized by lower turbulent kinetic energy
- Subjects
DOPPLER velocimetry; HYDRODYNAMICS; STRUCTURAL engineering; FLOW velocity; STREAM restoration; FLOW visualization; TURBULENT mixing
- Publication
Water Resources Research, 2022, Vol 58, Issue 12, p1
- ISSN
0043-1397
- Publication type
Article
- DOI
10.1029/2022WR032111