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CONSEQUENCES OF NON-LINEAR DENSITY EFFECTS ON BUOYANCY AND PLUME BEHAVIOR
Citation:
Frick, W E. AND D. J. Baumgartner. CONSEQUENCES OF NON-LINEAR DENSITY EFFECTS ON BUOYANCY AND PLUME BEHAVIOR. Presented at 2004 Ocean Research Conference, Honolulu, HI, February 15-20, 2004.
Impact/Purpose:
A main objective of this task is to combine empirical and physical mechanisms in a model, known as Visual Beach, that
â— is user-friendly
â— includes point and non-point sources of contamination
â— includes the latest bacterial decay mechanisms
â— incorporates real-time and web-based ambient and atmospheric and aquatic conditions
â— and has a predictive capability of up to three days to help avert potential beach closures.
The suite of predictive capabilities for this software application can enhance the utility of new methodology for analysis of indicator pathogens by identifying times that represent the highest probability of bacterial contamination. Successful use of this model will provide a means to direct timely collection of monitoring samples, strengthening the value of the short turnaround time for sampling. Additionally, in some cases of known point sources of bacteria, such as waste water treatment plant discharges, the model can be applied to help guide operational controls to help prevent resulting beach closures.
Description:
Aquatic plumes, as turbulent streams, grow by entraining ambient water. Buoyant plumes rise and dense ones sink, but, non-linear kinetic effects can reverse the buoyant force in mid-phenomenon. The class of nascent-density plumes begin as buoyant, upwardly accelerating plumes that subsequently reverse buoyancy, sinking to a more submerged trapping level or the bottom. Unlike double diffusion, where density adjusts to radiative or conductive temperature changes and salinity, the nascent-density effect is due to the mixing of plume and ambient fluids. A nascent-density example is a freshwater thermal plume discharged to freezing ambient fresh water, or to freezing brackish water up to a salinity of about 14psu. The highly buoyant plume rapidly becomes denser than the ambient fluid as the plume element temperature cools by entrainment. Once the mixture temperature falls below about 8C it becomes denser than the surrounding freezing water, decelerating its acquired upward velocity and ultimately sinking. If the equation of state were linear it would rise to an elevated trapping level or to the surface. The USEPA Visual Plumes software illustrates pertinent concepts with comparative examples.