Turbulence structure and concentration exchange property in compound open-channel flows with emergent trees on the floodplain edge

Abstract

There are aquatic plants, trees, and shrubs at the floodplain edge in natural rivers. In particular, spanwise profile of streamwise velocity component is influenced by the drag force of the trees, and consequently, the streamwise velocity decreases locally behind trees and near the junction between the main channel and the floodplain. Furthermore, concentration exchanges such as nutrients between the main channel and the floodplain differ from those observed in the non-tree compound open-channel flows. It is thus very important to investigate these hydrodynamic properties and turbulence structure, considering the emergent tree effects in river engineering and eco-hydraulics. So, in the present study, turbulence measurements by 3D acoustic Doppler velocimetry were conducted in a wide laboratory flume, in which several cylinders are placed as tree models. Three-dimensional large eddy simulation was also carried out to simulate concentration transport properties in these complex flows and it is found that there is a maximum exchange rate of concentration between the main channel and floodplains.

Keywords:

• Emergent trees
• turbulence
• compound channel
• secondary currents
• mass and concentration transports

Acknowledgements

The authors thank graduate students, Mr Sohei Suzuki and Mr Kota Itai, for their supports of turbulence measurements in environmental hydraulics laboratory in Kyoto University.