Turbulence In Low Energy Lacustrine Environments At The Sediment-Water Interface

Abstract

The sediment-water interface is an important region where biological, chemical, and physical processes occur. Fluid flow determines the ability of organisms to utilize the bottom sediments for food, shelter, and reproduction, the amount and rate of mass transfer between the sediment and bulk fluid, and the deposition, re-suspension and transport of sediment. Field experiments carried out in three medium sized basins, one a heavily polluted lake and the other two drinking water reservoirs, are used to describe typical turbulence levels during the mid to late stratified season (July through October) in a low energy lacustrine bottom boundary layer. Using these measurements, a turbulence chamber was developed for use in turbulent scalar flux studies. The chamber was characterized using particle image velocimetery (PIV), characterizing the near bed turbulence and dissipation levels. The chamber successfully reproduces the range of turbulent energy levels observed in the field, allowing a more direct comparison of results obtained in field and laboratory studies of scalar fluxes. Results from the two reservoirs were used in laboratory cohesive sediment erosion and resuspension tests of a mono-disperse kaolin clay and natural sediment cores obtained from various bottom and shore locations in one of the reservoirs. This testing showed the small bed stresses typical of the lacustrine bottom boundary layer were not sufficient to erode or resuspend significant quantities of sediment. Higher stress levels caused erosion and resuspension, but it was heterogeneous in nature and appeared to be tied to flow structures associated with the facility used to carry out the experiments. Even at higher stress levels, very short settling times (only 1-2 hours) were needed for observable erosion to occur. A method for estimating erosion utilizing images of the sediment-water interface and tracking the interface as an intensity peak over time was developed. Initial results show this is a reliable means to gauge the sediment-water interface position when optical access is available.