Near-Bottom Turbulence in the Deep Hypolimnetic Waters of a Large Stratified Basin

Turbulence measurements from a unique year-round high-resolution ADCP and microstructure data set taken in Lake Michigan highlight a sluggish bottom boundary layer driven by sub- and near-inertial processes. These currents show only weak seasonal variation in spite of the strong seasonality to the wind. Logarithmic velocity structure is found within the bottom 1 m of flow over 90% of the time, in spite of the weak currents, with 1m drag coefficient and roughness values of 0.005 and 0.0021m, respectively. The thickness of the logarithmic layer is found to be about 15% of the estimate Ekman layer thickness. The drag and roughness values show some flow speed dependence, with lower values at higher flow speeds, and large deviations from canonical values at low flow speeds. Turbulent kinetic energy dissipation values estimated from -5/3 spectral fitting and structure function fitting agree well with one another, but only approach expected law-of-the-wall estimates for the highest flow speeds observed during the mooring deployment. Vertical structure of dissipation suggests a scale limitation, in spite of the lack of near-bed stratification (as characterized by the turbulent Froude number), which may be due to higher-frequency processes driving the boundary layer currents. Near-bed turbulent diffusion coefficients vary between 10^-4 and 10^-2 m²s^-1 , which has important implications for the abilities of invasive bivalves to filter the hypolimnetic waters. Turbulent microstructure measurements taken during the deployment highlight the weak turbulence above the turbulent bottom boundary layer that serves as a bottleneck to the benthic delivery of dissolved and settling materials.