AERATION PERFORMANCE OF LOW DROP WEIRSa Discussion

In open chutes, supercritical turbulent flows are often characterized by large quantities of air entrained across the free surface (Fig. 4). The high level of turbulence and the air entrainment process enhance the air-water transfer of atmospheric gases and modify the downstream water quality. For example, the downstream dissolved oxygen content is significantly higher (e.g., Rindels and Gulliver 1989). In some cases, nitrogen supersaturation could cause ‘‘gas bubble disease’’ and high fish mortality for salmonids and steelheads, as in the Columbia and Snake rivers (in the United States) (Boyer 1971; Smith 1973). Some researchers have proposed empirical correlations to predict the oxygen content downstream of chutes (e.g., Department of Environment 1973). Recently, the discusser’s work (Chanson 1995, 1997) emphasized that the aeration performances of chutes are directly