Flow-Topography Interaction & Tidal Fronts

Burkard Baschek¹⁾ and David Farmer²⁾

^{Institute of Ocean Sciences, Sidney, Canada
1) now at University of California at Los Angeles (UCLA)
2) now at Graduate School of Oceanography, URI}

Introduction

The Fraser Estuary is located in British Columbia, Canada. The Fraser River brings fresh water into the Strait of Georgia (Figure 1). From there, it flows at the surface through Haro Strait and Juan de Fuca Strait into the Pacific Ocean. In return, dense water flows at the bottom of Juan de Fuca Strait into the estuary. Most of the mixing between both water masses takes place in Haro Strait, a region that is characterized by strong flow-topography interaction and several tidal fronts (Figure 2).

	Figure 1: Map of southern British Columbia, Canada.		Figure 2: Ebb (red) and flood (green) tidal fronts in Haro Strait.

Tidal fronts are a common feature of the coastal ocean. They are formed by the interaction of tidal currents with topography and can be defined as sharp transition zones between two water masses of different density and (tidal) current speed. At the sea surface, a prononouced front line of 1-20 m width can be observed (Figures 3,5). Due to the strong shear across the front, energetic eddies are formed that enhance the mixing between the two water masses on both sides of the front. This front extends into the water column to depths of typically 50-150 m and forms an interface that may be tilted due to density differences between the two water masses [Farmer et al., 2002] or due to the flow dynamics in the front [Baschek, 2003].

	Figure 3: Aerial view of tidal fronts in Haro Strait.

Types of Tidal Fronts

Tidal fronts are generated by the interaction of strong tidal currents with topography and are a common feature of the coastal ocean in mid latitudes.

Tidal Fronts can be formed by three different mechanisms:

• Type I: Inflow of dense water from an adjacent channel and plunging flow into intermediate depths;
  
• Type II: Flow separation processes past a headland, i.e. flow-topography interaction at a vertical boundary [Farmer et al., 2002];
  
• Type III: Flow over a shallow sill, i.e. flow-topography interaction at a horizontal boundary.
  

	Figure 4: Different types of tidal fronts: a) inflow of dense water from an adjacent channel; b) flow separation past a headland; c) flow over a shallow sill.

Example Boundary Pass

During ebb tide, the strong tidal flow in Spieden Channel forms two distinct front lines at Battleship Island as it enters Haro Strait (Figure 5). These two front lines meet as the denser water of Spieden Channel sinks rapidly under the slowly moving water of Haro Strait (Figure 6). At intermediate depths of 50-100m (Figure 5) the water then spreads and mixes with the resident water mass in Haro Strait.

	Figure 5: Tidal front in Spieden Channel.		Figure 6: Sketch of the processes in the Battleship Island tidal front. For explanations see text. The red rectangle marks the location of the measurments shown in Figure 7.

	Figure 7: Section through the tidal front at Battleship Island (Figure 5). a) Along-channel current speed [cm/s]; b) cross channel current speed [cm/s]; c) acoustic backscatter intensity [dB]; d) density [kg/m3].

References

L. Armi and D.M. Farmer(2002): Stratified flow over topography: Bifurcation fronts and transition to uncontrolled state. Proc. Roy. Soc. 458, A, 513-538.

B.Baschek (2003): Air-sea gas exchange in tidal fronts. PhD thesis, University of Victoria, Canada.

D.M Farmer, E.A. D'Asaro, M.V. Trevorrow, and G.T. Daikiri (1995): Three-dimensional structure in a tidal convergence front. Continental Shelf Res., Vol. 15, 13, pp. 1649-1673.

D.M. Farmer and L. Armi (1999): Stratified flow over topography: The role of small scale entrainment and mixing in flow establishment. Proc. Roy Soc., A 455, pp. 3221-3258.