Natural Convection in Stratified Lakes and ReservoirsImpacts on Pollutant Transport, Oxygen Budget, and Nutrient DynamicsEPA Grant Number: R825428
Title: Natural Convection in Stratified Lakes and ReservoirsImpacts on Pollutant Transport, Oxygen Budget, and Nutrient Dynamics
Investigators: Schladow, S. G.
Institution: University of California - Davis
EPA Project Officer: Lasat, Mitch
Project Period: November 1, 1996 through October 31, 1999 (Extended to October 31, 2000)
Project Amount: $426,923
RFA: Exploratory Research - Water Engineering (1996) RFA Text | Recipients Lists
Research Category: Water , Land and Waste Management , Engineering and Environmental Chemistry
Natural convection is initiated in stratified water bodies when the surface water is cooled. The vertical fall of this cooled, denser water leads to the nightly formation of a mixed surface layer in lakes and reservoirs and the seasonal deepening of the surface layer that may culminate in complete lake overturn. In the shallow, near-shore regions, surface cooling results in a horizontal temperature differential between the shallow and the deep regions. This difference gives rise to a horizontal pressure gradient that will drive bottom currents away from the near-shore regions.
This project will study the mechanisms and some specific environmental impacts of the vertical transport and away-shore transport produced by natural convection. Specific issues are: 1) quantifying the oxygen transfer through the water surface due to convective motions alone. This is believed to be an important component of the oxygen budget in many systems, but one that is presently not accounted for. Without quantifying the convective effect, accurate estimates of algal productivity and lake oxygen demand are not possible; 2) quantifying the away-shore transport by differential cooling. Two consequences to be specifically addressed are: (a) the transport of contaminants (pathogens, herbicides, nutrients) to other parts of the lake, and (b) the ventilation of the sediments due to oxygen transport. Results will provide a basis for decisions that impact activities at the edges of lakes and reservoirs; 3) quantifying the process of seasonal convection in deep lakes that only occasionally mix to the full depth of the lake. The extent of convective mixing in these systems controls internal nutrient load. As many of the deep lakes of interest are oligotrophic and nutrient recycling is an important source of nutrients in such systems, this issue has important ecological consequences.
A combination of laboratory experimentation, field work and numerical simulation will be used to address the issues. The laboratory experiments will be directed toward the study of oxygen uptake under shear-free, convectively unstable conditions. A dedicated, tank has been designed and built for these experiments. Using a novel combination of advanced laser induced fluorescence (LIF) techniques for oxygen measurements, digital particle thermometry (DPT) for temperature measurements and particle image velocimetry (PIV) for velocity measurements, a quantified, two dimensional picture of the evolving oxygen, temperature and flowfield will be produced. Fast response microsensors will also provide temperature and oxygen data.
The field experiments will take place at two very different lakes: Clear Lake and Lake Tahoe, Calif. The experiments will include the deployment of high accuracy thermistor arrays to track the evolution and spatial extent of convectively driven flows. Acoustic Doppler velocity measurements will also provide direct confirmation of the velocity of the flow, and the turbulence characteristics.
Numerical simulations of both the laboratory experiments and the field experiments will be conducted using two-dimensional finite difference and finite element codes.