Vertical mass and momentum transport in open-channel flows with submerged vegetations

Abstract

The importance of flow and turbulence to the ecology of aquatic benthic organism has been widely reported. Aquatic vegetation is one of environmental variables that influence turbulence and the ecological condition of rivers. Aquatic canopies have the potential to greatly improve water quality through the removal of nutrients and trace metals. To describe the efficiency of this removal, we must be able to quantify the rate of vertical mixing. In fully submerged vegetation flows, coherent turbulent motions are generated near the vegetation edge and these large-scale eddies control the vertical exchange of mass and momentum. It is therefore important for river management to investigate hydrodynamic characteristics and coherent eddies in open-channel flows with vegetation canopies. Turbulence structure and transport mechanism of momentum in vegetated flows have been investigated intensively in the past decade.However, the effect of the submerged vegetation on the vertical mass transport and turbulent diffusion has not been fully investigated. Therefore, in the present study, continuous dye injection experiments were conducted to evaluate the mass transport structure in open-channel flow with rigid vegetation models by changing the vegetation density. A combination technique between PIV and planar laser-induced fluorescence (LIF) was developed by using two sets of CCD cameras, to measure the instantaneous velocity and concentration field simultaneously. The technique is capable of determining the turbulent scalar flux as well as the Reynolds stress, mean and fluctuating velocity and concentration fields. Consequently, the effects of coherent vortices on the vertical turbulent diffusivity were examined in detail.