Dynamics of mixed bottom boundary layers and its implications for diapycnal transport in a stratified, natural water basin

Abstract

Abstract. Here we report on two field experiments from Lake Alpnach (surface area: 4.8 km2; maximum depth: 34 m) that were designed to study the process of boundary mixing and to estimate its efficiency, the ratio between the turbulent kinetic energy converted into potential energy and dissipated into heat, for diapycnal tracer transport. Lake Alpnach is particularly suited for this purpose because it is known from earlier experiments that (1) its currents follow a regular oscillatory pattern, associated with basin-wide standing internal waves (seiches, period •0.5 days and I day, respectively), and (2) diapycnal tracer transport is mainly caused by boundary mixing. During one of the experiments reported here, strong seiches were excited regularly and damped on a timescale of the order of 3 days; during the other experiment seiching motion was comparably weak. If seiching is excited regularly, we find a persistent well-mixed bottom layer of 4-5 m height at the deepest part of the lake. In the absence of regular seiching the layer disappears within 10-20 days. On the sloping bottom boundaries the well-mixed layers (1) are of much more transient nature, (2)exhibit different thermal structure, and (3) decrease in thickness toward shallower '• depth inversely proportionally to the stability at the same depth in the lake interior. Their reduced thickness is possibly the result of repeatedly occurring intrusions of boundary mixed water masses that are observed to extend horizontally •0100-200 m into the lake interior. As a consequence of repeated generation of intrusions, mixing on the sloping boundaries is expected to be considerably more efficient compared to mixing over fiat bottom boundaries. The observed mixing efficiency, the ratio of the rate of change of potential energy below depth z caused by turbulence to the energy loss by bottom friction below depth z, increases indeed from 0.01 q-0.01 in the deepest well-mixed layers to 0.15 q-0.04 in the upper pycnocline.

Shear-induced convective mixing in bottom boundary layers on slopes

“…Owing to its pronounced seiching, Lake Alpnach has been the subject of numerous previous studies, emphasizing the structure and dynamics of the internal seiching (Münnich et al 1992;Wüest et al 2000), the seiche-induced BBL (Gloor et al 1994;Lorke et al 2002), as well as the control of sediment-water exchange by BBL turbulence (Lorke et al 2003).…”

“…The height of such well-mixed layers varies between a few meters in lakes and reservoirs (Gloor et al 2000;Hondzo and Haider 2004;Lemckert et al 2004) to several tens of meters in oceans (Caldwell 1978;Lentz and Trowbridge 1991). The turbulent kinetic energy (TKE) required to generate and maintain such mixed layers is usually assumed to be produced by the bottom friction of basin-or large-scale currents (Fricker and Nepf 2000;Wüest et al 2000), shoaling and critical reflection of high-frequency internal waves (Thorpe 1997;Imberger 1998), or the interaction of large-scale currents with rough topography (Rudnick et al 2003).…”

Poincaré wave–induced mixing in a large lake

“…The difficulty results from the very low diapycnal diffusivity characteristics of the stratified flow, which is disrupted by intermittent strong mixing events (Ivey et al 2008). Therefore, the energy-flux paths are usually estimated through indirect basin-scale approaches, such as the heat budget method (Gloor et al 2000;Ravens et al 2000;Wü est et al 2000), or by estimating the energy budget of basin-scale internal waves (Antenucci et al 2001;Boegman et al 2005;Shimizu and Imberger 2008). These methods show that the wind energy entering the lake is transferred to basin-scale internal waves before cascading to smaller scale nonlinear internal waves and shear instabilities and being ultimately lost to dissipation and diapycnal mixing (Imberger 1998).…”

Strong vertical mixing of deep water of a stratified reservoir during the Maule earthquake, central Chile (Mw 8.8)

“…Questions arise whether or not this was the only mixing mechanism excited by the earthquake. The excitation of surface and internal waves is a plausible hypothesis for also explaining the observed increase in vertical mixing, as the benthic boundary is usually energized by shear-induced turbulence or by internal wave breaking [Gloor et al, 2000;Boegman et al, 2003;Lorke et al, 2005]. To preliminarily define whether the excitation of surface and internal waves by the earthquake is possible, it is considered that only those waves with natural frequencies near the earthquake frequency are likely to be excited, thus neglecting nonlinear energy transfers among waves as a mechanism of excitation [de la Fuente et al, 2010].…”

“…To put in context this change in potential energy per unit of area of bed, about 1% of the total energy in the internal wave field of a stratified lake may be used in raising the potential energy [Gloor et al, 2000;Shimizu and Imberger, 2008], and the total energy per unit of area in the internal wave field induced by typhoons passing over Lake Biwa (Japan) was estimated in 0.52 kJm −2 [Shimizu et al, 2007]. Therefore, typhoons on Lake Biwa may induce vertical mixing equivalent to 0.005 kJm −2 (1% of the total energy in the flow) in a timescale of days, while Maule earthquake produced mixing two order of magnitude larger in a timescale of minutes.…”