Measurements of Oxygen Transfer at Spillways and Overfalls

Abstract

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).