Density Currents
Density differences often prevail between tributaries and receiving lakes and reservoirs. These inflows tend to plunge when their density is greater than the surface layer of the receiving system, entering as a density current (where temperature or salinity are responsible) or turbidity current (where the suspended solids concentration dominates). If the density of the inflow is less then the surface layer, the inflow will tend to travel along the surface, thus entering as an overflow (See Figure 1). If local mixing is inadequate in the region of the inflow to eliminate density differences, the density or turbidity current will plunge and travel along the sloping bottom as an underflow. These processes are accompanied by entrainment of ambient water. If a depth is encountered in a stratified system where the density of the underflow equals that of the water column, the neutrally buoyant density current separates from the bottom and intrudes into that layer as an interflow.
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Figure 1. Animation of density currents (overlows, underflows and interflows)
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A number of investigators have reported in the literature the occurrence of density currents. Plunging inflows were documented for Wellington Reservoir, Australia associated with salinity pollution. Most often the phenomenon is attributable to seasonal temperature differences; e.g., tributaries are often cooler than surface waters of receiving lakes and reservoirs in late summer and fall in north temperate climates.
The plunging inflow phenomenon can be a concern for public health and water quality issues associated with contaminates and pollutants that may be carried in density currents. These usually are transient events, such as the chemical spill in Sacramento River (1991) that subsequently entered Shasta Reservoir, CA as a density current and the more common occurrence of turbid density currents in water supply lakes and reservoirs following runoff events. In Onondaga Lake, the plunging of its major tributaries may limit the availability of nutrient inputs for aquatic plant growth.
Onondaga Lake
Onondaga Lake and its major natural tributaries have elevated salinity levels. During times of low flow in tributaries, the salinity can be several times the salinity of lake water. This salinity difference can cause the contribute to these tributaries. We frequently see the affect of plunging in the profiles measured by the robot at South Deep. Often we can anticipate the
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Figure 2. Density long-term density seasonal density difference (inflow density minus lake surface density)
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depth to which Onondaga Creek will plunge by comparing its density to the water column's density.
Since inflows are major sources of nutrients to the lake, the location where they enter may affect how these nutrients influence aquatic life. Presently Onondaga Lake has an abundance of nutrients, so limiting external loading of nutrients to the lake is a priority in its cleanup. Since algae grow near the lake's surface, nutrients entering the water column near the surface will have a greater impact on their growth. The plot above (Figure 2) illustrates how differences in temperature and salinity affect Onondaga Lake's major inflows. We see that the outfall of Metro, tends to enter as an overflow or to plunge slightly. On the other hand, the tributaries tend to plunge for most of the year. This indicates that the Metro nutrient load would be relatively more available for algae growth then the tributary sources.
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Figure 3. Turbidity signatures of density currents (green line - before event; yellow line - after event )
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We can identify the effects of density currents from the robot profile data. For example, Figure 3 shows turbidity profile data for two different years (2002 and 2004) measured with the South Deep robot. Each plot contains two curves representing conditions prior to the density current event. The green line represent the antecedent conditions while the yellow illustrates the response of the system to the interflow event. Plots (a)-(c)
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Figure 4. Storm event in July 2004. The time-series plots show flow (a), creek turbidity, and creek temperature relative to the lake’s surface temperature (c). The profiles (d)-(f) show the measured turbidity and temperature at South Deep during the event and illustrate the occurance of an interflow at about 8 m.
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illustrate interflow events with mid-profile peaks in turbidity. Plot (d) illustrates an overflow event where the inflow temperature was warmer than the lake surface temperature (which is frequent occurrence in spring). The final plot (e) illustrates a condition were the lake was not stratified and tributary density was similar to that of lake water.
In Figure 4, we present an example illustrating an interflow occurrence during a runoff event in late July 2004. The top three time-series plots (a)-(c) are the Onondaga Creek conditions during the runoff event. The top plot (a) is the creek flow (Q) at Dorwin Avenue as measured by the USGS. Plots (b) and (c) are measured turbidity (Tn) and temperature (T) at the Dorwin Robohut (with lake surface temperatures added for comparison). In these plots we see that the turbidity increases in response to the increased flow.
Profile plots (d)-(f) in Figure 4 illustrates the effect of the runoff event on the turbidity profile, clearly demonstrating the existence of an interflow (e) with a turbidity maximum of over 30 NTU at 8 m. Plot (d) shows the antecedent condition prior to the event while plot (f) shows the post event profile, depicting a reduction in impact (e.g., settling and mixing).
Underflow events are often seen in the dissolved oxygen (DO) profiles near fall turnover. Onondaga Lake’s hypolimion remains anoxic from early summer until fall turnover in mid to late October. However, apparently anomalous DO profiles with nonzero DO occasionally occur in the bottom-most measurement, see Figure 5. This is characteristic of an underflow transporting oxygenated tributary (and perhaps entrained oxygenated surface) water to the bottom waters. Tributary temperatures are more responsive to changes in meteorological conditions than lake water. In the spring the lake is slowly warming from winter temperatures while tributary temperatures may be several degrees warmer responding to warmer air temperatures and greater solar heating. Thus in the spring, warmer tributary water will tend to overflow colder lake surface waters. Conversely, in the fall, tributaries will tend to cool more rapidly than lake waters in response to cooler air temperatures and reduced solar heating. Thus during the fall, tributaries tend to enter as an underflow.
In the plots that follow, we identify the depth and magnitude of maximum density gradient. This depth is described as the thermocline depth. A time-series plot showing the minimum and maximum densities during each profile is presented. Finally, the most recent profile's temperature (T), specific conductivity (SC), turbidity and density (calculated with and without salinity effects) are presented. Signatures of density currents can be seen where
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Figure 5. Example of underflow occurrence prior to fall turnover. Plot (b) shows increased dissolved oxygen (DO) in the near bottom characteristic of underflowing oxygenated inflow. The underflow continued until October 28 (c).
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there are subsurface maxima or minima in specific conductivity (SC) or when viewing the latest profile we might see a subsurface peak in turbidity. Late in the year, but prior to fall turnover, while the hypolimnetic waters are still anoxic, underflows maybe seen where the dissolved oxygen (DO) increases near the bottom. The salinity influence on the density may also be observed in the temperature profile, with temperature increasing at the lowest depths.
Onondaga Lake Density Plots
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