Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA.
Laboratory of Hydraulics, Hydrology and Glaciology, ETH Zurich, Zurich, Switzerland.
Sci Rep. 2021 Apr 21;11(1):8644. doi: 10.1038/s41598-021-87892-7.
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.
木材是河流生态系统的组成部分，越来越多的使用原木放置的恢复项目。本文利用物理模型试验探讨了木材位置和淹没深度（流量）如何影响尾流结构，进而影响生境。我们观察到位于河道中心的外露原木产生了卡门涡街（VS），而淹没原木则没有形成 VS，因为来自原木上方的流动进入尾流（扫流）抑制了 VS 的形成。因此，与淹没原木相比，位于河道中心的外露原木产生了十倍以上的更高湍流动能。此外，随着原木长度的增加和淹没深度的降低，时均速度和湍流水平的空间变化也随之增加。淹没原木和放置在河道边缘的原木产生了更大的速度亏缺和更长的回流区，这两者都可以增加尾流中的停留时间和有机物和养分的沉积。研究结果表明，原木尺寸和淹没程度的变化可作为一种工具，根据不同鱼类的喜好来改变生境适宜性。为了使河流的生境多样性最大化，我们建议进行多样化的大木材放置。
