Energy Dissipation
in Extreme Tidal Environments

Burkard Baschek1,2) and David Farmer1,3)

1) Institute of Ocean Sciences, Sidney, Canada
2) now at University of California at Los Angeles, USA
3) now at Graduate School of Oceanography, Rhode Island, USA

 

Introduction

In some narrow channels along the coast of British Columbia (Figure 1) spectacular rapids are generated by powerful tidal currents sometimes exceeding 13 knots (6.7 m/s), creating supercritical flow and subsequent hydraulic jumps with standing waves (Figure 2).

Figure 2: Standing wave in the Nakwakto Rapids.
Figure 1: Location of some tidal rapids in British Columbia. Figure 3: Whirlpools in the wake of Tremble Island, Nakwakto Rapids.

Boundary layer separation from irregular shorelines can generate intense shears with whirlpools as their surface expression (Figure 3). In these environments, fresh surface water from the extensive inlets of the mainland coast and the saltier water from the Pacific Ocean is mixed thoroughly throughout the water column. In addition, extremely energetic eddies draw gas bubbles to great depth enhancing aeration of the water.

In May 2000, we participated in a film shoot for the Northlight production "Maelstrom" (Discovery Channel), providing us with an opportunity to visit and take measurements in some of the more extreme environments at  the coast of British Columbia.  We used a self-recording conductivity-temperature- pressure (CTD) sensor, a camera, a sensitive atmospheric pressure sensor and a GPS recorder, all operated from a small boat.  Brief but intriguing measurements were acquired in Dent Rapids,Arran Rapids, and Nakwatko Rapids (Figure 1), the latter experiencing the strongest currents of any navigable channel in the world. The drifting boat and GPS were used as a Lagrangian current sensor, the CTD to verify the thoroughness of tidal mixing and the atmospheric pressure sensor to measure the changes in sea surface elevation which leave such a striking visual impression on visitors.

  

Skookumchuk

The Skookumchuk tidal rapids (Sechelt Rapids) which are located about 100km north of Vancouver are well known for its standing waves. Kayakers from all over the world travel to these rapids to surf and play in the waves. The current speed sometimes exceeds 15 knots creating strong turbulence and eddies downstream of the waves.

Figure 5: Kayakers surfing in standing wave.
Figure 4: Kayakers at Skookumchuk. Figure 6: Kayakers waiting in a back eddy.

Nakwakto Rapids

The Nakwakto Rapids (Figure 7) are located in a remote wilderness location on the mainland side north of Port Hardy (Figure 1). At ebb tide, the current speed reaches up to 6 m/s (Figure 10) and the impressive sea surface slope along Turret Rock, an island in the middle of the rapids, which is also called Tremble Island, is in the order of 2 m (Figures 8, 11).

Figure 8: Tremble Island.
Figure 7: Nakwakto Rapids. (From: Beyond Nakwakto Rapids, Douglass and Hemingway-Douglass) Figure 9: Wake of Tremble Island.
Measurements were carried out with GPS, airpressure sensor, and CTD with which it was possible to determine the surface current speed and sea surface slope in the rapids and the density profiles on both sides of the rapids.
Figure 10: The central plot shows Nakwakto Rapids with ship tracks. The graphs on the side show the current speed along the ship tracks.
The kinetic energy of the water increases with the surface current speed, reaching it's maximal value close to Tremble Island, while the potential energy drops with the sea surface slope. The sum of both is constant in the upper part of the rapids and drops dramatically in the region of high turbulence and eddy activity downstream of Tremble Island (Figure 11a)
Figure 11: a) Aerial view of Nakwakto Rapids with ship track. The blue dots show the position of the measurements in the lower graphs. b) Sea surface slope. c) Potential and kinetic energy as well as the sum of both.
CTD profiles from both sides of the rapids show that the fresh surface layer on the upstream side and the denser water beneath it are mixed vigorously in the rapids. Also a denser water mass which was was not detected with the CTD on the upstream side is mixed into the water column. A comparison of the loss in potential and kinetic energy with the energy needed for mixing shows that the mixing efficiency is less than 4%.
Figure 12: a) Nakwakto Rapids. b) Salinity, c) temperature, and d) density profiles of the upstream and downstream side of the rapids.

Arran and Dent Rapids

Also at Arran Rapids (Figure 1) the flow is characterized by flow seperation processes (Figure 13) and subsequent eddy formation due to the strong horizontal shear (Figure 14).

Figure 13: Flow separation at Whirlpool Point.  Figure 14:  Eddy in Arran Rapids. Gas bubbles are visible in the core. Figure 14: Steller's Sea Lions close to Arran Rapids.
CTD measurements on the upstream side of the rapids (Figure 16) show a fresh and warm surface layer of Bute Inlet water and a denser water mass underneath it. On the downstream side the two water masses are mixed thoroughly throughout the water column showing the importance of mixing in the rapids.

Figure 16: a) Dent Rapids. b) Salinity, c) temperature, and d) density profiles of the upstream and downstream side of the rapids. 

Conclusions

Measurements of current speed, sea surface slope, and density show the importance of extreme tidal flows for the modification of water masses in a coastal enviroment.