Abstract
The supply of oxygen into water by reaeration and subsequent transfer process is one of the most significant self-purification processes in natural streams. Thus, the purpose of this paper is to investigate the transfer process of dissolved oxygen by turbulent diffusion in a natural stream.
Velocity during 300 seconds with 1 second interval at the maximum-velocity point was measured and analyzed based on the Taylor's turbulent-diffusion theory modified by the assumption that Lagrange's velocity in natural streams depends on the velocity of flow direction in each section.
The results are summarized as follows: 1) the strongest correlation is observed between velocity and the strength of turbulence, 2) the turbulent diffusion process is accelerated by subsurface flows which consist of the periodical long wave and the short wave of auto-correlation function, and 3) a clear and accurate linear relationship exists between turbulent diffusivity and mixing rate depending on the length of residence time.
Auto-correlation function of subsurface flows and residence time are strongly influenced by discharge and topographical conditions such as water depth, river width and head. The turbulent diffusion process is, therefore, thought to be, the multiple effects of these factors in natural streams. Consequently, conservation and effective use of the self-purification process in natural streams is possible when erosion control techniques incorporate these aforementioned factors skillfully into minor bed fishway and so on.
