Field observations over the last decade suggest that mixing near basin boundaries exceeds that in the interior by an order of magnitude or more and largely determines the basin-averaged mixing rate. In the ocean, boundary mixing has been attributed to critical reflection of internal waves on topography and internal waves generated by internal tides flowing over rough topography. In lakes, boundary mixing is due to critical reflection and turbulence generated by bottom currents from seiches. Although much recent work has focused on boundary mixing processes, there has been little work on the ultimate fate of the mixed fluid. In this study, field experiments will be conducted to determine how fluid mixed at the boundary of a stratified water body is transported to the interior.
Experiments will be conducted in two lakes in Iowa: Ada Hayden Lake, near Iowa State University and West Okoboji Lake, which is larger and deeper. The objectives of the proposed work are to 1) classify the response of lakes to forcing according to the properties of the resulting internal wave field and mean currents, 2) identify intrusions and relate the turbulence and mixing to the forcing, and 3) to track the intrusions as they evolve. The lakes differ in size and one hypothesis that will be tested is that the magnitudes of potential energy change and energy supplied for mixing depend on the size of the lake but the ratio of these rates (called the mixing efficiency) will be larger on the sloping sides of the lake than along the flat bottom. Measurements of currents and temperature will be made and the spectra of internal waves will be calculated. Intrusions will be tracked with a fluorescent dye and microstructure measurements will be made throughout the dye patch.
Broader impacts of the proposed work include the training of graduate and undergraduate students, outreach to a rural school near Iowa State University, and outreach based out of the Iowa Lakeside Laboratory. This will a new summer class on physical limnology at the Lakeside Laboratory as well as non-credit courses for local residents and K-12 schools. Graduate students in the PI's research group will both conduct the experiments for the proposed research and participate in the outreach activities. Requested equipment will enhance the infrastructure for research. The PI's research group has broadened participation of underrepresented groups and disseminated results to the public through its past activities. Results from this project will affect and connect oceanography, limnology, and engineering; in particular, the outreach will increase awareness of local and national water quality issues.
Normal 0 false false false EN-US X-NONE X-NONE Our research group conducted field experiments in two Iowa lakes to improve the understanding of how heat, nutrients, and contaminants are transported in lakes. By using lakes as a ‘natural laboratory’ we can address questions from oceanography more easily than in the ocean. Previous researchers have shown that much of the turbulence and mixing in lakes and the ocean occurs near the sloping sides, but fewer studies have investigated the fate of the mixed fluid. We monitored water temperature at many depths, measured turbulence, and tracked a fluorescent dye injected near the side of the lake. One experiment provided the first measurements of the transport of fluid mixed by strong turbulence at a lake boundary (see the images below) and allowed a mathematical model of the transport to be developed. In another experiment, the dye moved 900 m in about 29 hours, but elevated mixing and turbulence at the boundary did not occur as in the other experiment, even though previous research by others suggested that it should have. This result suggests that previous criteria based mainly on the wind speed should be rethought. These experiments provide an example of tracking mixed fluid in natural flows such as lakes and oceans, and they can help to answer questions about the transport of heat, sediment, and other material. The project involved the training of three graduate students; Dr. Danielle Wain has continued in academia, while Michael Kohn works for the U.S. Geological Survey and Joshua Scanlon works for a consulting company. The project also supported five undergraduates who have continued in technical areas, such as working for NASA, the U.S. Army Corps of Engineers, and consulting companies. We have disseminated results to other scientists through one journal article so far and several conference talks. We have also disseminated results to the public through articles in three newspapers, a blog, a web site with data from the lake, a newsletter to residents of Ames, Iowa, and a talk to the public in Milford, Iowa. During the work at West Okoboji Lake, a press release on the project notified the residents of the Okoboji area about the project. The Iowa Great Lakes attract many tourists; at the peak, 600-800 boats per hour past between West and East Okoboji Lakes during the summer. We also conducted outreach to Girl Scouts and elementary schools; students conducted small experiments on mixing in natural flows. In 2010 and 2011, we helped a consulting firm with outreach to high school students interested in engineering in particular and higher education in general. 000198556
