Logarithmic velocity structure in the deep hypolimnetic waters of L ake M ichigan

The characteristics of the bottom boundary layer are reported from a Lake Michigan field study carried out in deep hypolimnetic waters (55 m depth) during the stratified period (June–September 2012). The sandy substrate at the measurement site was densely covered with invasive quagga mussels (mean size: 1.6 cm; mean density: 10,000 mussels m −2 ). The measurements reveal a sluggish, compact bottom boundary layer, with flow speeds at 1 mab less than 5 cm s −1 for most of the period, and a dominance of subinertial energy. A downwelling event caused the largest currents observed during the deployment (10 cm s −1 at 1 mab) and a logarithmic layer thickness of 15 m. In spite of the weak flow, logarithmic profile fitting carried out on high‐resolution, near‐bed velocity profiles show consistent logarithmic structure (90% of profiles). Flow was dominated by subinertial energy but strong modified by near‐inertial waves. Fitted drag coefficients and roughness values areC d 1 m= 0.004 andz 0  = 0.12 cm, respectively. These values increase with decreasing flow speed, but approach canonical values for 1 mab flow speeds exceeding 4 cm s −1 . The estimated vertical extent of the logarithmic region was compact, with a mean value of 1.2 m and temporal variation that is reasonably described by Ekman scaling, 0.07u */ f , and the estimated overall Ekman layer thickness was generally less than 10 m. Near‐bed dissipation rates inferred from the law of the wall were 10 −8 −10 −7 W kg −1 and turbulent diffusivities were 10 −4 −10 −3 m 2 s −1 .