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

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 along‐channel sections of density, velocity, and dissipation rate of turbulent kinetic energy ∈ 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. We estimate water from below 150 m, 3 times the sill depth, is drawn into the Narrows on a 3‐m flood tide. Once in the Narrows the flow remains stratified until it passes a 50° bend where a strong secondary circulation overturns the 50‐m‐deep water column and generates intense turbulent mixing. Cross‐channel velocities of up to 0.4 m s−1 are observed, and maximum values of ∈ exceed 10−3 W kg−1. Upon leaving the sill, stratification is reestablished, and turbulence decays. A similar set of sequences occurs on ebb, except that the outflow bypasses the flood inflow channel and instead discharges into Colvos Passage, the third branch of the triple junction. Colvos Passage ultimately discharges the ebb effluent back into Main 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 m2 s−1.