Experimental study of density induced plunging flow into reservoirs and costal regions

Abstract

The inflow entering a lake or reservoir is probably not precisely of the same density as the lake or reservoir. If the density of the inflow is less than the receiving water, the inflow will float on the surface. If the density of the inflow is greater than that of the receiving water, the inflow sinks below the surface, forming a density current. This phenomenon is referred to as "plunging" and the location where the inflow dives below the surface as the "plunge point" or "plunge line." The plunging phenomenon can be defined as the transition from open channel flow to a stratified underflow. Information is available on underflows (Ashida and Egashira, 1977; Ellison and Turner, 1959; Turner, 1973) but does not include the plunging process itself. The plunging flow can entrain some of the ambient water, thus changing the character of the inflow itself. In Minnesota plunging flow can occur in the late summer and fall when lakes are still warm from the heat absorbed in the summer and the inflowing streams are cool. It also occurs after cold weather spells in midsummer. Plunging flow can also occur if the excess density of the inflow is produced by suspended material or dissolved substances. The plunging phenomenon is not fully understood even when the receiving water is of uniform density. The situation becomes more complex when a plunging flow enters a stratified lake or reservoir where the plunging flow will entrain ambient water of increasing density as it sinks. Not only will the rate of dilution determine to which depth the underflow will sink in a stratified reservoir, lake or impoundment (Elder and Wunderlich, 1972), but it is a parameter which controls the location of the plunge line. One-dimensional unsteady water quality models require knowledge of plunging flow to be able to predict into which layer in a stratified lake or reservoir an inflow will enter.

Akiyama and Stefan (1987) and Farrell and Stefan (1986) provide literature reviews of previous work on plunging flows. Akiyama and Stefan (1987) list previous studies indicating that the dilution rate of plunging flows ranges from 0 to 500% in field studies and 0 to 200% in laboratory studies. These reviews, however, indicate that most of the previous experimental and analytical work dealt with sloping parallel sided (two-dimensional) or unconfined sloping configurations. Akiyama and Stefan (1987) conducted experiments and developed an integral type analysis of plunging in a mildly diverging horizontal channel.Farrell and Stefan (1986) used numerical analysis to solve the entire flow field driven by a plunging flow in a 2-D reservoir. The geometrical configurations modeled were either mildly diverging horizontal channels or parallel sided sloping channels, which lend themselves to two-dimensional analysis. Negatively buoyant flow in a strongly diverging channel where the inflow separates from the sidewalls has not been the subject of laboratory or analytical studies, probably due to the three-dimensional nature of the flow field. In order to provide information on plunging flow an experimental study dealing primarily with strongly diverging channels (Fig. 1-1) was conducted at St. Anthony Falls Hydraulic Laboratory_ This type of flow can be related to previous analytical and experimental studies which deal with diffuser flow, jet flow, and stratified flow. Previous work done in these three subject areas will be related to the experimental results presented in the body of this study. In addition, a set of field measurements will be reported and related to laboratory experimental results.