Influence of Waves on Air-water transfer of low solubility gases

Abstract

Water quality of rivers and reservoirs or lakes is a main component of the quality of life in a state like Minnesota, where free surface waters are abundant. A major key to managing water quality is dissolved oxygen concentration. Low oxygen concentrations result in fish kills, gaseous . releases from sediments, and the release of toxic chemicals from sediments. The understanding of oxygen dynamics is therefore imperative in order to predict the impact of future developments along a river. One of the main ways of oxygen replenishment is the transfer of oxygen from air into the water through the water surface (reaeration). Reaeration in river reservoir systems. occurs most effectively through the turbulence of flowing water, turbulence and surface waves caused by the wind, and air entrainment caused by flow over hydraulic structures. Recent studies of air-water gas transfer and comparisons between large and small wind wave flume data have indicated the potential importance of waves in the gas transfer process. Experimental evidence is presented in this paper showing that. non-breaking deep-water gravity waves significantly influence gas transfer. Experiments were performed in a flume with a mechanical wave maker. Oxygen was used as a tracer gas, and the flume was chemically deoxygenated. Oxygen concentrations and wave characteristics were recorded over time at various locations along the flume. Horizontal mass transport was determined from conductivity measurements, using salt as a conservative tracer. Semi-empirical correlations of the transfer velocity and the wave characteristics were obtained. The transfer velocity is shown to increase proportionally to both wave height and frequency. Comparison against data reported in the literature indicates that approximately 50 percent of the observed gas transfer in wind wave flume experiments can be attributed to non-breaking waves. The presence of bubbles or bubble entraining breaking waves enhances the transfer velocity considerably. Visual correlation between breaking intensity and the transfer coefficient was very good, although the relation has not been quantified.