Logarithmic velocity structure in the deep hypolimnetic waters of Lake Michigan

Abstract

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 are Cd1m = 0.004 and z0 = 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, 0.07 u* / 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 m2s−1.

How are coastal benthos fed?

“…In this summertime study under mostly stratified conditions, vertical turbulent diffusion coefficients ( K z ), derived from 234 Th and 90 Y fluxes by Waples (), averaged 1.8 × 10 −3 m 2 s −1 (range: 3.5 × 10 −4 m 2 s −1 to 6.6 × 10 −3 m 2 s −1 ) over the water column and correlated positively with onshore advection. These K z values were ∼ 100× greater than those found by Edwards et al () in Lake Erie, but in good agreement with boundary mixing rates observed by MacIntyre et al () in Mono Lake, Lorke et al () in Lake Constance, and more recently by Troy et al () in Lake Michigan.…”

Ice‐Free Radiative Convection Drives Spring Mixing in a Large Lake

“…A 15‐day (5–20 April 2018) mooring deployment and microstructure campaign was used to obtain high‐resolution measurements of mean flow, stratification, and turbulence in Lake Michigan at a 55‐m deep site located near Milwaukee, Wisconsin, USA (43 ° 04 ' 27 ″ N,87 ° 45 ' 14 ″ W; Figure ), a location for which we have previously reported both hydrodynamic (Cannon & Troy, ; Troy et al, ) and biogeochemical conditions (Mosley & Bootsma, ; Shen et al, ). An instrumented tripod equipped with two acoustic current profilers and a thermistor string was deployed throughout the experiment, while vertical profiles of velocity shear and temperature were collected during three separate profiling days (5, 12, and 20 April).…”

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

“…The location is offshore of the thermocline‐shelf intersection in the summer, and, with an offshore distance of 15 km, this site is situated at the outer edge of the lateral coastal boundary layer (Rao and Schwab ). Details of the summer deployment and field site were presented in Troy et al () and are briefly summarized here again for convenience. A large tripod with three mounted acoustic current profilers and an attached thermistor chain was deployed at the field site for the entirety of this period (Fig.…”

“…Using u * and z o as fitting parameters, logarithmic profiles (Eq. ) were fit to the all wave‐free mean velocity profiles collected during this experiment (2152 bursts over 290 d) by following the optimization algorithm outlined in Troy et al (). From these unmodified log law fits, a median z o of 0.0015 m was estimated over all profiles with fitted friction velocity uncertainties less than 15%.…”

“…Measurements were collected during an 11-month field study using a burst-sampled highresolution acoustic profiler that resolved near-bed (10-85 cmab) currents over a mussel-inhabited substrate. In a related work (Troy et al 2016), we reported on the mean logarithmic structure observed during the summer months of the deployment, but did not report on turbulence characteristics or determining factors that limit LOW applicability on individual profiles due to a lack of near-bed stratification measurements and the relatively constant forcing conditions (stratified; weak wind forcing) during those 3 months. This manuscript presents a nearly full-year characterization of nearbed hypolimnetic currents and turbulence at a deep-water site in Lake Michigan, including causes for deviation from expected LOW behavior and the potential application of modified LOW prescriptions for mean flow and turbulence.…”