KnightInlet

<div class="figure"> <img id="im" style="border: 0em;" width="500" src="ebbtideKI.jpg"> <p>Turbulence profiler drops made in the lee of the Knight Inlet sill during a strong ebb tide.</p> </div>

The flow over a sill in Knight Inlet, British Columbia, is a very spectacular and accessible example of an internal hydraulic flow. Internal hydraulic flows are important in both the atmosphere and the ocean because they greatly increasing the drag on currents over the stress due to boundary friction and linear waves.

My thesis, with Mike Gregg at UW/APL, dealt with three aspects of the flow over the Knight Inlet sill, all three of which are intertwined. - The flow in the lee of the sill is very three-dimensional - The time-dependence is strongly affected by the background stratification. - The turbulence near the sill is strong, but not strong enough to account for the loss of barotropic tidal energy.

Three-dimensionality of the flow in Knight Inlet

<div class="figure"> <img id="im" style="border: 0em; "src="<nop>kiimages/<nop>topo3d_new.jpg"><p> Three-dimensional flow over the Knight Inlet sill during flood tide. The viewer is looking west towards the ocean. There is a strong jet of water flowing towards the viewer (red) in the center of the channel and two counter-flows (blue) on the sides. </p> </div>

The flow near the sill in Knight Inlet is very three-dimensional. Large lee-vortices form behind the headlands each tide. These vortices are produced by sidewall friction and then appear to be amplified by baroclinic stretching in the lee wave. From a practical point of view, they strongly alias along-channel flow-fields in the lee of the sill, making two-dimensional budgets very difficult.

Klymak, J. M. and M. C. Gregg, 2001, The three-dimensional nature of flow near a sill. J. Geophys. Res., 106, 22,295—22,311.

You can get my version at (about 2 Mb): http://opg1.ucsd.edu/~jklymak/KnightInlet/paper1_pp.pdf

Or JGR's version (about 6 Mb): http://opg1.ucsd.edu/~jklymak/KnightInlet/KlymakGregg01_jgr.pdf

Time dependence of the flow

<div class="figure"> <img id="im" style="border: 0em; "src="<nop>kiimages/<nop>blsep39o.jpg"><p> Flow over Knight Inlet sill at peak ebb tide. Water is riding over the density layer in the lee of the sill. </p> </div>

We present data from a full tidal cycle in Knight Inlet, and demonstrate that while there is a delay in the growth of a lee wave during ebb tide, there is no delay during flood. We attribute this to the strong density gradient across the sill. Using a numerical model, we demonstrate how the lee wave during ebb can be suppressed by the addition of the density contrast.

Klymak, J. M. and M. C. Gregg, 2003, The role of upstream waves and a downstream density-pool in the growth of lee-waves: stratified flow over the Knight Inlet sill, J. Phys. Oceanogr., Vol. 33, No. 7, pp 1446—1461

A copy can be found here (2 Mb): http://opg1.ucsd.edu/~jklymak/KnightInlet/paper3JPO.pdf

Tidal energy budget in the fjord

<div class="figure"> <img id="im" style="border: 0em; "src="<nop>kiimages/<nop>xzaverageKrho.jpg"><p> Average diffusivities near the Knight Inlet sill. Data was binned by tidal phase and then averaged with each phase of the tide getting an equal weighting (to reduce biasing by extra sampling during strong tides). </p> </div>

We present the first systematic measurements of turbulence in Knight Inlet, and demonstrate how that turbulence is tied to features in the lee-waves and vortices discussed in the previous two papers. We then perform an energy budget on the Inlet by comparing energy lost from the barotropic tide to various sinks of energy near the sill. The first sink is radiating internal waves that appear to carry ~2/3 of the energy away from the sill region. The rest of the processes are near the sill and include wave breaking and shear-layer turbulence (~1/12), near bottom turbulence (~1/12), and energy into the headland vortices (~2/12).