Dynamics of Turbulence in Low-Speed Oscillating Bottom-Boundary Layers of Stratified Basins

This paper focuses on the impact of an oscillating low-speed current on the structure and dynamics of the bottom-boundary layer (BBL) in a small stratified basin. A set of high-resolution current profile measurements in combination with temperature-microstructure measurements were collected during a complete cycle of the internal oscillation ('seiching') in the BBL of Lake Alpnach, Switzerland. It was found that even a relatively long seiching period of 24 hours significantly changed the form of the near-bottom current profiles as well as the dynamics of the turbulent dissipation rate compared to the steady-state law-of-the-wall. A logarithmic fit to the measured current profiles starting at a distance of 0.5 m above the sediment led to inconsistent estimates of both friction velocity and roughness length. Moreover, a phase lag between the current and the turbulent dissipation of 1.5 hours and a persistent maximum in the current profile at a height of 2.5 to 3 m above the sediment were observed. The experimental findings were compared to the results of a k-epsilon turbulence model and showed good agreement in general. Specifically, the inconsistent logarithmic fitting results and the observed phase lag were reproduced well by the model.

Variable sediment oxygen uptake in response to dynamic forcing

Alfred Johny Wüest

Seiche-induced turbulence and the vertical distribution of dissolved oxygen above and within the sediment were analyzed to evaluate the sediment oxygen uptake rate (J(O2)), diffusive boundary layer thickness (delta(DBL)), and sediment oxic zone depth (z(max)) in situ. High temporal-resolution microprofiles across the sediment-water interface and current velocity data within the bottom boundary layer in a medium-sized mesotrophic lake were obtained during a 12-h field study. We resolved the dynamic forcing of a full 8-h seiche cycle and evaluated J(O2) from both sides of the sediment-water interface. Turbulence (characterized by the energy dissipation rate, epsilon), the vertical distribution of dissolved oxygen across the sediment-water interface (characterized by delta(DBL) and z(max)), J(O2), and the sediment oxygen consumption rate (R(O2)) are all strongly correlated in our freshwater system. Seiche-induced turbulence shifted from relatively active (epsilon = 1.2 x 10(-8) W kg(-1)) to inactive (epsilon = 7.8 x 10(-12) W kg(-1)). In response to this dynamic forcing, delta(DBL) increased from 1.0 mm to the point of becoming undefined, z(max) decreased from 2.2 to 0.3 mm as oxygen was depleted from the sediment, and J(O2) decreased from 7.0 to 1.1 mmol m(-2) d(-1) over a time span of hours. J(O2) and oxygen consumption were found to be almost equivalent (within similar to 5% and thus close to steady state), with R(O2) adjusting rapidly to changes in J(O2). Our results reveal the transient nature of sediment oxygen uptake and the importance of accurately characterizing turbulence when estimating J(O2).

Amer Soc Limnology Oceanography2010

Intermittent oxygen flux from the interior into the bottom boundary of lakes as observed by eddy correlation

Bernhard Wehrli, Alfred Johny Wüest

Turbulent oxygen transport from the overlying stratified water column into the bottom boundary layer (BBL) on the slope of a medium-sized lake was investigated using the eddy correlation (EC) technique. The seicheinduced oscillatory flow of the BBL, with a period of similar to 1 d, was identified as the mechanism driving turbulent oxygen transport. Sporadic short- term EC vertical oxygen fluxes exceeded the sedimentary oxygen uptake of 13 +/- 2 mmol m(-2) d(-1) calculated from sediment oxygen profiles by more than a factor of three. The average EC flux over half of a seiching period was 9.2 mmol m(-2) d(-1) similar in range to the flux into the sediment; however, these two fluxes do not have to coincide spatially and temporally. The EC oxygen flux was only significant when the deep basin-scale currents exceeded a velocity of 2 cm s(-1) and the corresponding bottom shear was sufficient to produce active turbulence. Below this threshold, decaying turbulence resulted in oxygen fluxes lower than 3.5 mmol m(-2) d(-1), with an even lower average flux of 0.8 mmol m(-2) d(-1) observed during reversals of the seiching. At low velocities, the weak turbulence is insufficient to transport dissolved oxygen through the stratified top of the BBL ( stability N(2) approximate to 2.4 x 10(-4) s(-2)), even though turbulence was found in the inertial subrange and periodical bottom convective mixing was still present. The EC technique provided valuable data on the temporal variability of oxygen transport related to the BBL hydrodynamics and flux pathways.

Amer Soc Limnology Oceanography2008

Breathing sediments: The control of diffusive transport across the sediment-water interface by periodic boundary-layer turbulence

Alfred Johny Wüest

We performed combined in situ measurements of bottom boundary-layer turbulence and of diffusive oxygen fluxes at the sediment-water interface in a medium-sized mesotrophic lake. The turbulence was driven by internal seiching with a period of 18 h. This periodic forcing, a prominent feature of enclosed water bodies, led to distinct deviations of the structure and the dynamics of the bottom boundary layer from the classical law-of-the-wall theory. A major feature was a phase lag between the current velocity and the turbulent energy dissipation of approximately 10% of the seiching period (1.5-2 h). The oxygen flux into the sediment was controlled by the diffusive boundary layer, the thickness of which varied between 0.16 and 0.84 mm during the course of a seiching period, and was strongly affected by the periodic bottom boundary-layer turbulence. The rate of dissipation of turbulent energy in the bottom boundary layer allowed us to define the Batchelor length for dissolved oxygen, which quantifies the smallest scales of oxygen fluctuations and provides an appropriate scaling for the diffusive boundary-layer thickness and the corresponding oxygen fluxes. An analysis of the governing time scales revealed the importance of turbulence in controlling the small-scale spatial heterogeneity of the diffusive fluxes. Higher turbulence causes the diffusive boundary layer (DBL) to follow the sediment topography more smoothly, resulting in an increased area-averaged flux due to the greater effective surface area.

Amer Soc Limnology Oceanography2003