Model for Total Dissolved Gas Supersaturation from Plunging Jets in High Dams

Abstract

The total dissolved gas (TDG) supersaturation that results from dam spillage may cause adverse effects, including increases in the risk of gas-bubble disease and mortality in fish. The accurate prediction of TDG levels is necessary in the exploration of measures for ameliorating the effects of TDG supersaturation. Based on an analysis of the mechanisms of hydropower projects with a plunging jet that produces high TDG levels, the process of TDG generation is divided into three stages. In Stage 1, TDG levels return to normal during jet spillage in air; in Stage 2, gas is dissolved in the stilling basin under high pressure; and in Stage 3, the TDG is abruptly released at the outlet of the stilling basin. According to previous research on Stage 1, the TDG level of water entering stilling basins can reach 100%. Experiments were carried out to estimate the TDG levels in Stage 2 under different pressures and retention times, and these experiments indicated that a TDG level above equilibrium saturation (ΔG0) displays a linear relationship with the average pressure (ΔP) and a negative exponential relationship with retention time (tR). Experiments were also conducted using physical models of the Songta and Yangfanggou dam projects in China to develop a method for estimating the retention time in stilling basins. The resulting formula for estimating the retention time is a function of the water depth in the stilling basin (hk), length of the stilling basin (l), distance between the toe of the dam and impact point of the jet (l0), and the dimensionless number at the stilling basin outlet λ. For Stage 3, in which the abrupt release of TDG occurs, field measurements were used to determine the values of the parameters used in the abrupt release expression contained in the model. By combining the results for the three stages, a predictive model of TDG levels was obtained. TDG observations collected at six different hydropower projects in China were used for validation. Substantial agreement between predictions and measurements was found. This work may provide a scientific basis for the production of precise predictive models of TDG levels, and it has considerable application value in assessing the effects of TDG and minimizing the risks posed by elevated TDG levels to aquatic life.