Flow and wake characteristics associated with large wood to inform river restoration

Isabella Schalko,Heidi M Nepf,Ellen Wohl

doi:10.1038/s41598-021-87892-7

Isabella Schalko, Heidi M Nepf + Show 1 more

Open Access

https://doi.org/10.1038/s41598-021-87892-7

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Abstract

Wood is an integral part of a river ecosystem and the number of restoration projects using log placements is increasing. Physical model tests were used to explore how the wood position and submergence level (discharge) affect wake structure, and hence the resulting habitat. We observed a von-Kármán vortex street (VS) for emergent logs placed at the channel center, while no VS formed for submerged logs, because the flow entering the wake from above the log (sweeping flow) inhibited VS formation. As a result, emergent logs placed at the channel center resulted in ten times higher turbulent kinetic energy compared to submerged logs. In addition, both spatial variation in time-mean velocity and turbulence level increased with increasing log length and decreasing submergence level. Submerged logs and logs placed at the channel side created a greater velocity deficit and a longer recirculation zone, both of which can increase the residence time in the wake and deposition of organic matter and nutrients. The results demonstrate that variation in log size and degree of submergence can be used as a tool to vary habitat suitability for different fish preferences. To maximize habitat diversity in rivers, we suggest a diverse large wood placement.

Highlights

For emergent logs placed at the channel center, a von-Kármán vortex street (VS) was formed, characterized by the periodic detachment of vortices from alternating ends of the log (Fig. 4a; note that only one vortex is illustrated as the dye was injected on one side)
No VS was formed for submerged logs, due to the suppression of vortex formation by the flow passing over the top of the log and into the wake
The formation of a VS elevated the turbulence within the wake by a factor of 10, compared to the turbulence observed without VS formation

Summary

Introduction

Log placement increased the spatial variability in the flow[27,28], but our results demonstrated that variation in log size and positioning can significantly alter the velocity and turbulence in the wake, in some cases by an order of magnitude. For logs at the channel center, the wake length Lw scaled with the log length and was longer for submerged logs with Lw,s = (9 to 15) L compared to Lw,e = (4 to 10) L for emergent logs (Fig. 5a,c,e; Table 1).

Results

Conclusion