Relationship between surface velocity divergence and gas transfer in open-channel flows with submerged simulated vegetation

M Sanjou,T Okamoto,I Nezu

doi:10.1088/1755-1315/35/1/012016

Abstract

Velocity and gas concentration measurements were carried out to reveal gas transfer phenomena in open-channel turbulent flows with flat bottom and submerged vegetation bottom conditions. A large-scale coherent vortex appears near the vegetation top due to shear instability, and the submerged vegetation was found to promote gas transfer beneath the air- water interface. Furthermore, we revealed a great dependency of gas transfer on vegetation density. The present measurement results propose a new surface divergence model with wide generality, connecting reasonably the gas transfer velocity and the surface divergence intensity in open-channel flows, irrespective of bottom roughness conditions.

Highlights

IntroductionSurface velocity divergence is closely related to coherent motions induced by bottom turbulence
Surface velocity divergence is closely related to coherent motions induced by bottom turbulence.Instantaneous upward and downward motions occur together with the strong surface divergence, and it results in periodic renewal of the dissolved gas situation in the free surface
It was found that a proportionality coefficient of the original SD model depends on the water depth

Summary

Introduction

Surface velocity divergence is closely related to coherent motions induced by bottom turbulence. It is generally well known that natural rivers are composed of various bottom roughness features such as submerged vegetation, gravel, sand waves, etc. They contribute to produce greater turbulence compared with flat bottom conditions. Practical prediction methods of the gas transfer rate are modelled by friction velocity. With development of video recording devices and particle image velocimetry (PIV) techniques, the horizontal velocity components could be obtained in the free surface, and it is possible that the SD model will be the most useful method to predict the gas transfer rate in rivers. Published under licence by IOP Publishing Ltd doi:10.1088/1755-1315/35/1/012016 be used as a proper velocity scale In both cases of smooth and vegetated bed conditions, the modified. SD model is proved to be useful without a dependency on the bulk mean velocity and depth

Experimental procedure

Results in flat-bottom open-channel flows

Gas transfer in vegetated-bottom conditions

Conclusions