Entrainment by bottom currents in Lake Erie

Abstract

Previous studies of the central basin of Lake Erie have indicated, on the basis of lakewide budgets of heat and dissolved oxygen, that the thickness of the hypolimnion and the interaction of the hypolimnion with the overlying fluicl are important factors governing the dissolved oxygen concentration in the near-bottom water. Data collcctcd during an intensive field program in 1979 contain an example of an event during which both the thickness and temperature of the hypolimnion incrcasc due to an erosion of the thermocline from below. This thickening of the hypolimnion requires two conditions. First, the thermocline region, or mctalimnion, must be thick enough so that the effects of the energetic surface mixing processes are confined to the upper portion of the thermocline.And second, the currents in the thin hypolimnion must be strong enough to entrain overlying metalimnion water down into the hypolimnion.An analysis of the current meter data suggests the source of turbulent energy driving the mixing is shearing stresses at the bottom. During the entrainment event, the contribution to bottom dissolved oxygen supply is about 10% of the daily demand.

A model of the vertical mixing in Lake Erie in summer

“…where ub is the daily average scalar speed from a current meter at the site (3.3 m off the bottom) and C,= 1 x 10-3-a value which compares favorably with the estimates in Ivey and Boyce (1982) and the suggested values of Sternberg (1972). Typical values of (&)b are then of the order of 0.2 cm.…”

“…We chose to simulate the month from mid-July to mid-August during the field year 1979 for several reasons: the lake was strongly stratified during that period, while at the same time there were storm events which drove strong mixing events in the UML; Ivey and Boyce (1982) had shown that changes in the BML properties were predominantly due to vertical mixing; the timespan encompassed a period where the largest increases in BML thickness were observed over the summer period; and the late summer is important as the bottom water may become anoxic at this time.…”

“…Consistent with our approach for the UML, however, we use an integral-type slab model for the BML in Lake Erie. From an examination of current meter observations, Ivey and Boyce (1982) suggested that shear production at the top of the BML was unlikely to be important and that the main source of turbulent kinetic energy driving entrainment was shear production at the bottom. The base of the UML, where we have argued shear production may be important, does not coincide with the top of the BML in our simulations.…”

“…The thickness of this BML, or hypolimnion, and the interaction of this layer with the overlying fluid are thought to be important factors in governing the dissolved oxygen concentration in the bottom waters. Ivey and Boyce (1982) showed, by an analysis of field observations, that for certain periods vertical mixing was more important than horizontal advection in determining the temperature and the dissolved oxygen concentration in the BML in the interior of the central basin. Their analysis suggested that, at times, the BML was energetic enough to entrain overlying water down.…”

Estimates of phosphorus entrainment in Lake Mendota: a comparison of one‐dimensional and three‐dimensional approaches

“…Previous publications address the limitations of (Stauffer 1985(Stauffer , 1993 and justifications for (Patterson et al 1984) this assumption. A number of recent studies, however, show spatial heterogeneity in turbulent events (EtemadShahidi and Imberger 2001, Saggio and Imberger 2001, MacIntyre et al 2002, Boegman et al 2003, MacIntyre et al 2006, Na and Park 2006; and yet others connect dynamic physical processes to spatial heterogeneity in biological and chemical variables (Ivey and Boyce 1982, Stauffer 1985, MacIntyre and Melack 1995, MacIntyre and Jellison 2001, Eckert et al 2002, Chao et al 2006, Marce et al 2007). …”