| The density of the water in a river discharged into a reservoir or lake can be greater than the receiving water due to temperature difference, dissolved or suspended materials. In such cases, the inflow sinks beneath the receiving water surface, entrains ambient water, and forms gravity currents along the bottom. Such flows are also called plunging flows.;Of particular interest is the plunge condition, i.e. the location where plunging flows occur or, in dimensionless form, the densimetric Froude number at the plunge point. The classic theoretical treatment is due to Akiyama and Stefan (1984) in which a two-control-volume framework was designed for analytical purposes. In this research, we showed that this framework is not perfect in light of physical principles and use of this framework leads to unforeseen paradoxical plunge condition.;A new and generalized theoretical framework for the analysis of plunging flows was developed in this research. This new framework bears on the understanding of the initial mixing coefficient, which is defined as the ratio of the entrained ambient flow rate to the total inflow rate. We also used an energy argument to show that the initial mixing coefficient does not exceed approximately 0.4 for laterally confined plunging flows which agrees with reported values in the literature. Numerical simulations were carried out for plunging flows on different bottom slopes and inflow rates. The characterizing dimensionless parameters for plunging flows were derived from the simulation results. Good agreement is found between the simulation results, reported experimental data, and proposed theory.;To extend the knowledge gained to this point to the field observations, we found that the plunging flows in the field could be different from those in current theoretical and laboratory scale modeling. In particular, two unusual and speculated observations drew our attention: Fleenor and Schladow (2000) reported an exceptionally high initial mixing coefficient and Best et al. (2005) observed a shifting plunge line and pulsing underflows in the field. In this research, we developed a theoretical model, an interplay between the flow momentum upstream of the plunge line and the turbulent Rayleigh-Taylor instability, to offer rational explanations for these two special observations. |
