The oxygen transfer across the air-water interface at a spillway or
overfall is an important dissolved oxygen source or sink in a river-reservoir
system. Normally many river miles are required for a significant air-water
transfer of oxygen to occur, but at a spillway this same oxygen transfer may
occur in the short residence time at the spillway/weir. The primary reason
for this accelerated oxygen transfer is that air is entrained into the flow,
producing a large number of bubbles. Air bubbles greatly increase the
surface area available for gas transfer. In addition, the bubbles are
transported by the flow to various depths downstream of the structure,
increasing gas transfer and the possibility of supersaturation due to an
increased saturation concentration at higher pressures. This is not a problem
with oxygen, but in the case of dissolved nitrogen this supersaturation may
cause fish mortality by nitrogen gas bubble disease. The results of this study
are limited to oxygen but can be applied to transfer of any chemical for
which transport is controlled by the water side of the interface using
procedures described in Gulliver, Thene, and Rindels (1989).
Rindels, Alan J.; Gulliver, John S..
Measurements of Oxygen Transfer at Spillways and Overfalls.
St. Anthony Falls Hydraulic Laboratory.
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