Hydraulic structures have a large impact on the amount of
dissolved gases in a river system. Even though the water
passes over the structure for only a short time, the water
flowing over a spillway or weir entrains air bubbles, creating
significantly more air-water surface area for gas transfer.
In addition, the high turbulence that occurs at most hydraulic
structures will increase the transfer rate coefficients. The
same quantity of gas transfer that normally would occur in
several miles in a river can occur at a hydraulic structure.
The transfer of oxygen from the atmosphere to the water
is often of interest, therefore it seems logical to directly
measure oxygen transfer. However, there are some problems
associated with the measurement of dissolved oxygen (DO)
concentration. If the DO level is close to saturation (within
approximately 2.5 mg/.e) , the tremendous uncertainty associated
with the current measurement techniques makes the estimation
of gas transfer useless (Gulliver and Wilhelms, 1992). Also,
if the reservoir is stratified, it is difficult to predict
withdrawal from the various layers with the required
precision, and usually impossible to sample at the spillway
crest (Gulliver and Rindels, 1993). Because the required
field conditions for accurate DO measurement often do not
occur, other measurement techniques, such as the tracer
technique are used.
Hibbs, David E.; Gulliver, John S.; McDonald, John P..
Methane Sampling Technique and the Measurement of Plunge Pool Impact on Gas Transfer Rates at Low-Head Hydraulic Structures.
St. Anthony Falls Hydraulic Laboratory.
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