THE IMPORTANCE OF ASPIRATION AND CHANNEL CURVATURE IN PRODUCING STRONG VERTICAL MIXING OVER A SILL

On the basis of observations of the time-dependent, tidally forced flo w over a long sill we find that aspiration and channel curvature set t he flow structure and condition the flow to allow intense vertical mix ing. Aspiration reduces the potential energy of the water column by th inning it while maintaining its density contrast. Channel curvature in duces a cross-channel circulation that can rapidly overturn a stratifi ed flow. Eighteen along-channel sections of density, velocity, and dis sipation rate of turbulent kinetic energy epsilon were collected in an d around the Tacoma Narrows of Puget Sound, a site suspected of drivin g 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 upl ifted, or aspirated, into the Narrows. We estimate water from below 15 0 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 degrees bend where a strong secondary circulation overturns the 5 0-m-deep water column and generates intense turbulent mixing. Cross-ch annel velocities of up to 0.4 m s(-1) are observed, and maximum values of epsilon exceed 10(-3) W kg-l Upon leaving the sill, stratification is reestablished, and turbulence decays. A similar set of sequences o ccurs on ebb, except that the outflow bypasses the flood inflow channe l 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 b y discharging the mixed product far from the source. Scaling of the cr oss-channel momentum equation suggests that, below a threshold value o f along-channel velocity, stratification should suppress secondary cir culation for a given vertical shear, radius of curvature and channel w idth. 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 an d mixing in the Narrows can produce in Main Basin to be 10(-3) m(2) s( -1).