Methane Sampling Technique and the Measurement of Plunge Pool Impact on Gas Transfer Rates at Low-Head Hydraulic Structures

Abstract

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/l), 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.