Measurement of Lateral and Wake Flows Associated with Stream-scale Willow Patches

Shrubs and bushes often grow forming distinct patches on channel bars, river banks and floodplains. While understanding on the influence of such flexible woody vegetation on plant-scale drag and patch-scale flow resistance is rapidly advancing, reliable description of the mixing and transport processes requires additionally quantifying the modifications in the turbulent flow structure. Caused by the challenges in flow measurements with such flexible, dynamically waving vegetation, riparian shrubs and bushes are typically simulated as rigid cylinders in a uniform distribution. However, this representation misses the flexibility-induced reconfiguration and streamlining that control the response to flow.

The aim of this paper is to investigate the relationships between physically-based willow patch properties and the lateral and wake flow characteristics at natural scale. We conducted experiments in a fully controllable outdoor channel with a bottom width of 3 m, water depths of 0.8 – 1.0 m and wetted surface widths of 5.7 – 6.2 m. The banks grew low grasses while the sandy bottom grew two 4 m long and 1.5 m wide willow patches planted three years earlier. The willows were maintained by coppicing, which resulted in rigid, unfoliated main stems below the experimentally obtainable water depths. To simulate young willow saplings with submerged foliage, flexible willow branches were attached to the rigid main stems of one patch just before the start of the experiments while the other patch was kept unfoliated for comparison.

The differences in turbulent flow field between the foliated and unfoliated patches were determined under two hydraulic boundary conditions with the flow rates of 1 m³/s and 3 m³/s. The vertical flow distributions were measured within the patches and in the regions extending upstream, downstream and laterally from the patches using Acoustic Doppler Velocimetry. The reach-scale surface flow distributions were recorded with Large-Scale Particle Image Velocimetry. The vertical distributions of the foliage and woody stems were determined using a combination of manual surveys and sophisticated techniques, including hemispherical digital photography, while the streamlining and reconfiguration were recorded with underwater photography. The paper reports the differences observed between the flexible foliated and rigid unfoliated patches, considering the effects of reconfiguration. We analyze the dependency of the lateral shear layer and wake flow structure on the physically-based vegetation properties at the patch scale. In the future, the natural-scale data is expected to aid in developing the numerical modeling of shrub-vegetated flows.