The importance of aspiration and channel curvature in producing strong vertical mixing over a sill

Abstract

Abstract. On the basis of observations of the time-dependent, tidally forced flow over a long sill we find that aspiration and channel curvature set the flow structure and condition the flow to allow intense vertical mixing. Aspiration reduces the potential energy of the water column by thinning it while maintaining its density contrast. Channel curvature induces a cross-channel circulation that can rapidly overturn a stratified flow. Eighteen Mong-channel sections of density, velocity, and dissipation rate of turbulent kinetic energy e were collected in and around the Tacoma Narrows of Puget Sound, a site suspected of driving a strong vertical circulation in adjoining Main Basin. Rapid inflow to the Narrows on flood from one channel of a triple junction reduces dynamic pressure, allowing dense water from below sill depth to be uplifted, or aspirated, into the Narrows. Basin, enhancing the impact of mixing at the Narrows by discharging the mixed product far from the source. Scaling of the cross-channel momentum equation suggests that, below a threshold value of along-channel velocity, stratification should suppress secondary circulation for a given vertical shear, radius of curvature and channel width. Above the threshold velocity the magnitude of the cross-channel velocity is roughly consistent with predictions for unstratified flow. We estimate the maximum effective eddy diffusivity that aspiration and mixing in the Narrows can produce in Main Basin to be 10 -3 m 2 S --1.

Mixing processes and hydraulic control in a highly stratified estuary

“…These studies might incorporate similar measurement techniques to those used in the present study, but with sampling focused on the shallow areas to the inside of the bend, which would require the use of smaller vessels. The results would provide a better understanding of the mechanisms specific to mixing in the Fraser River estuary, and also allow comparisons with other regions where secondary flows appear to be important to the mixing climate, such as the Tacoma Narrows region of Puget Sound (Seim and Gregg, 1997), and San Francisco Bay (Lacy and Monismith, 2001). …”

“…A recent numerical study of mixing in a partially mixed estuary (MacCready and Geyer, 2001) indicates that mixing is most intense during the peak ebb, but more vertical salt flux occurs during the peak flood due to an extended along-channel length of the isopycnals. In the Tacoma Narrows section of Puget Sound, a large fjord-like estuary, channel curvature and the flow dynamics over a sill were found to produce strong vertical mixing (Seim and Gregg, 1997), reiterating the often observed importance of topographic features to mixing processes (e.g., Geyer and Canon, 1982;Farmer and Armi, 1999;Wesson and Gregg, 1994).…”

Structure, variability, and salt flux in a strongly forced salt wedge estuary

“…This transverse shear creates a lateral salinity gradient as well as lateral shear in along-channel velocity, with faster ebb velocities and lower salinities on the south side of the channel. The opposing lateral baroclinic pressure gradient might be expected to shut down the curvature-induced lateral circulation [Seim and Gregg, 1997;Chant, 2002]. Curvature-induced lateral circulation depends on deviations from the depth-averaged velocity [Kalkwijk and Booij, 1986;Geyer, 1993].…”

Oceanic Isopycnal Slope Spectra. Part II: Turbulence

“…At 700 m at Hawaii f/N ϭ 0.01, so a 10-m-thick patch could conceivably be 1 km long. Numerical modeling (Smyth et al 2001) and oceanic observations (Seim and Gregg 1997;Moum et al 2003) indicate that turbulence created by Kelvin-Helmholtz billows has a flattened aspect ratio, and it seems reasonable that the same will be true of convective instabilities.…”