Thermally-Driven Exchange Flows in Regions of Vegetation

Massachusetts Institute Of Technology, Cambridge MA

Investigators

Abstract

0509658 H. M. Nepf Spatial heterogeneity in water temperature, and thus density, will drive horizontal flows that carry fluxes between chemically distinct regions of a surface water body. The spatial gradients in water temperature may arise from differences in water depth or differences in light penetration. For example, a spatially uniform heat flux causes the temperature within a shallow region to rise and fall more rapidly than an adjacent deep region. It is now well recognized that the resulting buoyancy-driven exchange between the littoral and pelagic zones plays an important role in setting lake-scale chemistry and ecology. The presence of emergent and submerged aquatic vegetation is a common feature of the littoral region, yet only a handful of studies have considered the impact of vegetation on littoral-pelagic exchange. Because vegetation provides significant drag, it can modify the thermally-driven exchange current. Indeed, a scale analysis presented in the proposal shows how vegetation can control the rates of exchange. In addition, by partially or fully shading regions of the water column, the presence of both emergent and submerged vegetation can produce temperature gradients that drive flow between the vegetated region and the open water. Because a distinct bio-chemical environment often exists within the vegetated zone, these exchange flows provide an important hydrodynamic link between biologically and chemically distinct regions of surface water. To fully evaluate the impact of these flows on lake-scale chemistry and ecology, one must be able to predict their frequency, duration and spatial footprint. The proposed project will provide this predictive ability. A series of laboratory experiments have been designed to observe the formation and magnitude of thermally-driven exchange flows between open water and water with vegetation of different morphology. The velocity field will be measured using digital particle imaging velocimetry (DPIV). Both emergent and submerged vegetation canopies will be considered. Further, the depth of heat penetration will be varied using water colorant to block more deeply penetrating wavelengths. The results of the proposed work will enable researchers and lake managers to make better evaluations of how changes in the littoral zone, e.g. due to land development, may impact the hydrodynamic link between the littoral and pelagic regions, and thus may impact the lake-scale nutrient budget and ecology.