Observations of turbulence and mean flow in the low-energy hypolimnetic boundary layer of a large lake

Abstract

Near-bed measurements are reported for both mean flow and turbulence structure in the deep hypolimnetic waters of Lake Michigan (55 m depth) during stratified and unstratified periods to determine validity and restrictions of the expected law-of-the-wall (LOW) behavior. Near-bed currents were weak (U50 = 3, 16 cm s−1 for mean, maximum currents respectively at 50 cm elevation), dominated by subinertial energy across all seasons, and showed little seasonal variation in spite of the strong seasonality to wind forcing. Velocity structure for wave-free conditions showed strong log-linear trends within 1 mab, with over 98% of the 2152 velocity profiles producing significant log-linear fits within the bottom meter and a strictly logarithmic velocity profile extending to only 66 cmab on average (Cd 50 = 0.0052; zo = 0.0015 m). Stratification was dynamically unimportant to mean flow and turbulence, but fitted log-linear length scales suggest that deviations from strictly logarithmic velocity structure may be explained by flow unsteadiness. Turbulent quantities measured within 1 m of the bed including dissipation, turbulent kinetic energy, and turbulent length scales followed LOW expectations in the mean, but individual estimates deviated by several orders of magnitude. The observed deviations from LOW turbulent structure were found to be correlated with the log-linear length scales fit to mean velocity profiles and were consistent with the effects of flow unsteadiness.